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Interpretation of Mass Spectra Part 4

Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

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Page 1: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Interpretation of Mass Spectra Part 4

Page 2: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

ObjectivesTo describe the main features of EI, CI, ESI spectra of organic compoundsTo indicate how to identify the M+., MH+ or [M+nH]n+ ion and to suggest a possible molecular formulaTo review the major types of fragmentation mechanisms and how to recognise them in the mass spectra of different classes of compoundsTo give some practice in the interpretation of mass spectra of simple unknown organic samples

Page 3: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Relative Molecular Mass and Isotopes

Page 4: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Relative Molecular MassMany non-radioactive elements exist in more than one isotopic form. The lightest isotopes of the common light elements are usually the most abundant e.g. 1H, 12C, 14N, 16O, 32S, 35Cl, 79Br The masses of these isotopes, rounded to the nearest whole number, are used to calculate the nominal RMM of a compound. The monoisotopic RMM of a compound is that calculated using the accurate atomic weights of the most abundant isotopes of the constituent elements.

Page 5: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Relative Molecular MassAs the number of H atoms increases, the difference between the nominal and the mono-isotopic RMM slowly rises and the nominal RMM is no longer useful, e.g. above about 600 Da.

For example:

Mass: C20H30 C40H60 C60H90

Nominal 270 540 810 Monoisotopic 270.235 540.470 810.705

Whole Number 270 540 811

Page 6: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Common isotopes of the lighter elements

Isotope Mass Relative

Abundance

Isotope Mass Relative

Abundance

1H 1.00782 100 2H 2.01410 0.016

12C 12.000 100 13C 13.0033 1.12

14N 14.0031 100 15N 15.0001 0.36

16O 15.9949 100 18O 17.9992 0.2

32S 31.9721 100 33S 32.9715 0.78

34S 33.9679 4.39

35Cl 34.9689 100 37Cl 36.9659 32.7

79Br 78.9183 100 81Br 80.9163 97.5

Page 7: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Relative Molecular MassesNaturally occurring compounds contain isotopes in their natural abundances. The inclusion of the less common isotopes leads to a higher average RMM.Hence we can define three different RMMs:

Nominal RMMMonoisotopic RMMAverage RMM

Which do we measure by mass spectrometry?

Page 8: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Measurement of RMMFor RMMs of < 500, normally the nominal RMM is obtained at low resolving power

The monoisotopic RMM can be measured if the resolving power is high enough to resolve isotope peaks and any other interfering peaks.

If the resolving power is insufficient to resolve isotope peaks, the average RMM is measured. If interfering peaks are not resolved, an accurate RMM cannot be measured.

Page 9: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Isotope Peaks - 1Ions containing elements that have more than one isotope exhibit isotope peaks on the high mass side of the peak due to the lightest ion and may indicate the elements present in an ion.

The probability that any one carbon atom is a 13C isotope is 1.1%

If there are n carbon atoms in an ion, the probability that at least one carbon atom is a 13C isotope is n x 1.1% so that I[(M+1)+]/I[(M+)] = (n x 1.1)/100

Page 10: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Isotope Peaks - 2For relatively low RMM samples, this ratio indicates the number of carbon atoms in the ion providing that there is no contribution from MH+ ions, e.g. for alcohols and amines at high sample pressures.For example, if M+. is at m/z 112, this could be due to C8H16

+., C7H12O+. or C6H8O2+. for which the ratio

I[(M+1)+]/I[(M+)] would ideally be 0.09, 0.08 and 0.07 respectively. This ratio exceeds unity once the number of C atoms in the molecule exceeds 85-90.

Page 11: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Molecular Ion Regions

C80H160 C80H160S3 C100H200

1121 1217 1402

Page 12: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Calculation of Isotope Patterns

Exemplified for Cl and Br atoms in C6H4OClBr

Write out isotopic abundances of Cl and Br isotopes (including zero abundances) across the top and down the side of a square.

Construct a matrix by multiplying each member of the row with each member of the column.

Sum the diagonals (top right to bottom left) to produce relative abundances.

Page 13: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Isotopic Peaks due to Cl + Br

3 0 1 35Cl, 36Cl, 37Cl abundances 1 | 3 0 1

0 | 0 0 0 79Br, 80Br, 81Br abundances 1 1 | 3 0 1 (vertical)

Relative Abundance. 3 : 0 : 4 : 0 : 1

RMM 114 116 118

Comp. 35Cl79Br 35Cl81Br 37Cl81Br 37Cl79Br

Page 14: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

1-Bromo-4-Chlorobenzene Mass Spectrum

The 3:4:1 pattern in M+ region (with 13C peaks between). The fragment ion at m/z 111, 113 shows the 3:1 pattern indicating the presence of a Cl atom and that the Br atom has been lost.

(m a in lib ) Be n ze n e , 1-b ro m o -4-c h lo ro -10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

0

50

100

12 2438

44

50

55 61

75

85 96104

111

128 141 155

192

Br

C l

Page 15: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

(m a in lib ) Be n ze n e , 1-b ro m o -4-c h lo ro -10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

0

50

100

12 2438

44

50

55 61

75

85 96 104

111

128 141 155

192

Br

C l

(m a in lib ) Be n ze n e , 1-b ro m o -2-c h lo ro -20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

0

50

100

2537

50

55 61

75

8591

96104

111

128 140 155

192

C l

Br

Page 16: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

The Molecular Ion RegionFor compounds containing only C, H, O and N atoms, 13C isotopes control the isotope pattern. In large molecules, such as proteins, few ions contain less than 2 or 3 heavy isotopes; the peak due to ions containing only light isotopes is of very low intensity.

The presence of S, Cl and Br atoms leads to characteristic peaks at M+2, M+4, etc.

Page 17: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Electron Ionization Spectra

Page 18: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Production of EI Spectra70 eV electrons bombard the sample vapour and deposit typically 0 - 20 eV in a molecule.Typically 8 - 10 eV is needed to produce M+. in its ground state so M+. ions are produced with internal energies of approximately 0 - 10 eVThese undergo one or more fragmentations in the ion source and products ejected after about 1 ms are collected to produce the mass spectrum. Discrimination effects differ between instruments so that the spectrum of a given sample varies somewhat from instrument to instrument.

Page 19: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Initial Inspection of the Spectrum - 1

Look at the overall appearance of the spectrum: try to identify the molecular ion, M+. and obtain information from any isotope peaks present.

If the major peaks are at low m/z and M+. is under 20% of the most intense peaks, the sample is probably aliphatic.

The more intense M+. is, the greater the degree of unsaturation is present (alkene, carbonyl compound).

Page 20: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Hexane

(m a in lib ) He xa n e0 10 20 30 40 50 60 70 80 90 100

0

50

100

2 1215

27

28

29

30

39

41

42

43

55

57

58 7184

86

M+.

[M-C2H5]+

Page 21: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

2-Hexene

(m a in lib ) 2-He xe n e10 20 30 40 50 60 70 80 90 100

0

50

100

15 26

27

28

29

30 38

39

40

41

42

43

44 5153

55

56

57 61 63 65 67

69

70 74 77 79 81 83

84

85

M+.

[M-C2H4]+.

[M-CH3]+

Page 22: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Cyclohexane

(m a in lib ) C yc lo h e xa n e10 20 30 40 50 60 70 80 90 100

0

50

100

1315 26

27

28

30 38

39

40

41

42

43

44 5154

55

56

5761 63 65 67

69

70 74 77 79 8183

84

85

M+.

[M-CH3]+

[M-C2H4]+.

Page 23: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Cyclohexene

(m a in lib ) C yc lo h e xe n e10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

0

50

100

14 1626

27

29 3238

39

40

41

42 49

51 53

54

55 6365

67

6874

77 7980

81

82

83

M+.

[M-CH3]+

[M-C2H4]+.

Page 24: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

1,3-Cyclohexadiene

(m a in lib ) 1,3-C yc lo h e xa d ie n e10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

0

50

100

15

27

2838

39

40 49

5152

54 63 65 74 76

77

78

79

80

81

M+.

Page 25: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Benzene

(m a in lib ) Be n ze n e10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

0

50

100

15 26 28

39

4049

51

53 6163 74 76

77

78

79

M+.

Page 26: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

1,5-hexadien-3-yne

(m a in lib ) 1,5-He xa d ie n -3-yn e10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

0

50

100

12 14 2427

29 31 3438

39

40 42 44 4749

50

51

52

53 55 6163

64 67 6973

7476

77

78

79

M+.

[M-C2H2]+.

Page 27: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Initial Inspection of the Spectrum - 2

If peaks due to M+. and other high mass ions dominate the spectrum, the sample is probably aromatic.

A large number of peaks often indicates a large number of H atoms are present.

The lack of any dominant peaks suggests the absence of a hetero-atom.

The simpler the spectrum, the more symmetry is likely to be present in the sample molecule.

Page 28: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

1-Napthalenol

(m a in lib ) 1-Na p h th a le n o l10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

0

50

100

18 26 39 43 51 58 63 72 8993 97 101

115

125

144

O H

M+.

[M-CO]+.

Page 29: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

(m a in lib ) 2-Na p h th a le n o l10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

0

50

100

15 2739

4351 57

6374 79 87

89

92 97 102

115

126

144

HO

(m a in lib ) 1-Na p h th a le n o l10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

0

50

100

18 2639

43 5058 63 72

7789

97 102

115

125

144

O H

Page 30: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Nonanal

(m a in lib ) No n a n a l10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

0

50

100

15 19

27

29

31

41

57

60

70

79

82

8591

95

98

114124

141

O

Page 31: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

M+.

[M-C2H5]+

C6H13+

[M-C5H10]+

[M-C6H13]+

C3H7+

C2H5+

Page 32: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

M+.[M-C3H6]+.

[C4H9CO]+

[M-C3H6-C3H6]+.

Page 33: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

M+.

[M-C2H5O]+

C5H7+

Page 34: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

CH2NH2+

Base peak of primary amines

Found in all amine spectra and in spectra of amides

M+.

Page 35: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Identifying the Molecular Ion - 1

The common isotopes of elements C, O, S have even relative atomic masses and even valencies whereas the common isotopes of H, F, Cl, Br, P and Na have odd relative atomic masses and odd valencies. Hence organic compounds that contain only these elements (i.e. no nitrogen atoms) have an even relative molecular mass (RMM) so that M+. occurs at an even value of m/z or the MH+ ion appears at an odd value of m/z .

Page 36: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Identifying the Molecular Ion - 2

The one common element that is an exception to this rule is nitrogen, the most common isotope of which has a relative atomic mass of 14 and a valency of 3.Hence an odd relative molecular mass results when the molecule contains an odd number of N atoms. Thus if M+. has an odd m/z, it suggests a possible amide, amine, nitrile or N-heterocyclic compound.

Page 37: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Uses of Isotope PeaksCommon elements that give M+2 isotope peaks:

35Cl:37Cl rel. ab. ~ 3 : 1 79Br:81Br rel. ab. ~ 1 : 1 32S:34S rel. ab. ~ 100 : 4 28Si:30Si rel. ab. ~ 100 : 3.4

Hence peaks at M+2, M+4, etc. indicate the presence of Cl, Br, S, Si; the absence of these peaks indicates the absence of these elements.Common elements that give rise to M+1 isotope peaks are C and N but only C isotope peaks need be considered:

12C:13C rel. ab. ~ 100 : 1.1

So that I([M+1]+)/I([M+]) = n x 1.1/100 for an ion containing n C atoms.

Page 38: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Approximate ratio

27:27:9:1

Approximate ratio

9:6:1

Approximate

ratio 3:1

Page 39: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

81:108:54:12:1

27:27:9:1

9:6:1

Page 40: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

1:4:6:4:1

1:3:3:1

Page 41: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

C12H22S2+.

m/z 230 231 232 Intensities 100 13 9

[M-(C6H10)]+

m/z 148 149 150 Intensities 100 7 9

Page 42: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Is it Really the Molecular Ion?

Check that the supposed M+. loses neutral species that are sensible, e.g. radicals such as alkyl radicals or OH. or molecules such as alkenes, CO, HCl, H2O, etc.

If there are losses that cannot be explained, e.g. 3 - 13, 21 - 25 Da, the assignment should be re-examined.

If it appears that M+. loses 3 Da, this could arise from losses of CH3 and H2O giving peaks due to the ions [M-15]+ and [M-18]+..

Page 43: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Is it Really the Molecular Ion?

Try to identify the main species lost by M+.. These often indicate the type of compound to which the sample belongs.

Rearrangement ions formed by loss of a molecule are often particularly informative. If no nitrogen is present, these appear at an even value of m/z.

Identify ions characteristic of a compound type: m/z 105, 77, 51 for benzoyl compounds, m/z 91, 65, 39 for alkylbenzenes, m/z 30 for amines, etc.

Page 44: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

M+.

absent

[M-CH3]+

[M-H2O]+.

[M-C3H7]+

[M-C4H9]+

Page 45: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

M+.

C3H7CO+

C2H5CO+

Page 46: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

[M-CH3]+ M+.

[M-C3H6]+.

[CH3CO]+

Page 47: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

M+.

C7H7+

C5H5+

C3H3+

Page 48: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

How to Work Out the Molecular Formula - 1

Start with the RMM and the value of I([M+1])/I([M]) which gives an indication of the number of C atoms present.

Suppose the RMM is 136. The maximum possible number of C atoms is found by dividing this by 12. This gives 11 but C11H4 is very unlikely to be correct. Try 10 C atoms, converting the other 12 Da to H atoms, giving C10H16 (e.g. pinene or limonene, etc).

Page 49: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

How to Work Out the Molecular Formula - 2

Repeating the process for 9 C atoms gives C9H28 which is unacceptable. Convert 16 H atoms to one O atom giving C9H12O as a possibility, e.g. benzyl ethyl ether. Repeating this for 8 C atoms gives C8H8O2 which could be an aromatic acid or ester.Knowing the number of C atoms present, one can suggest a molecular formula. In this case, suppose C8H8O2 is suggested.

Page 50: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Rings + Double Bonds: 1For a molecule of molecular formula CxHyNzOa the number of double bond equivalents (DBEs) is given by the expression

DBEs = x - (y/2) + (z/2) + 1

A DBE is a unit of unsaturation, e.g. an alkene or carbonyl group is 1 DBE, a saturated ring is 1 DBE, an aromatic ring is 4 DBEs, a triple bond is 2 DBEs. The number of DBEs is independent of the number of O atoms present. Halogen atoms are counted as H atoms, S and P atoms are counted as O and N atoms respectively.

Page 51: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Rings + Double Bonds: 2If the formula C8H8O2 is suggested, the number of RDBs is

8 - (8/2) + (0/2) + 1 = 5.

This immediately suggests the presence of an aromatic ring (4 RDBs) and in view of the presence of two O atoms, the other is almost certainly a carbonyl group.

A formula such as C9H12O would have

9 - (12/2) + (0/2) + 1 = 4

This suggests, for example, an aromatic ring (4 RDBs) and a saturated substituent.

Page 52: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Suggesting Possible Structures

C6H5COOCH3 A benzoyl compound so m/z 105, 77 and 51 should be prominent features.

C6H5CH2COOH A benzyl compound so m/z 91, 65 and 39 should be prominent features.

CH3C6H4COOH The o-, m- and p-isomers all give a prominent peak at m/z 119 (loss of OH), least intense for the m-isomer. The o-isomer gives an m/z 118 ion (loss of H2O formed by the OH of the -COOH group and H from the methyl group. Comparison of the spectra with those of authentic samples would confirm the identification.

Page 53: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

M+.

[M-OH]+

[M-COOH]+

Page 54: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

M+.

[M-H2O]+.

C7H7+

[M-H2O-CO]+.

[M-OH]+

Page 55: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Fragmentation of M+.

Ions

Page 56: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Unknown mass Spectrum

Odd m/z suggests a N atom present

m/z 105, 77, 51 suggests benzoyl

Since N accounts for 14 of the other 16 Da, 2 H atoms are present

Hence the sample is benzamide, C6H5CONH2.

Page 57: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Common Neutral Losses of Diagnostic Value - 1

15 CH3 Alkyl branching if intense peak, otherwise neglect

16 O Nitroaromatic, oxime, sulfoxide

16 NH2 RCONH2

18 H2O Alcohol, (ketone, aldehyde, less common)

20 HF Alkyl fluoride

26 C2H2 Aromatic hydrocarbon

27 HCN ArCN, N-heterocylic compounds, ArNH2 rarely

27 C2H3 Ethyl ester (low abundance)

28 CO Quinones, some phenols

28 C2H4 n-Propyl ketones, ethyl esters, ArOC2H5

29 C2H5 Ethyl ketones, Ar - n-C3H7 compounds

30 CH2O Aromatic methyl esters

31,32 CH3O,CH3OH Methyl esters of carboxylic acids

33,34 SH, H2S RSH

Page 58: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Common Neutral Losses of Diagnostic Value - 2

41 C3H5 Propyl ester

42 C3H6 n-butyl ketone

CH2CO RCOCH3, ArOCOCH3, ArNHCOCH3

43 C3H7 RCOC3H7, Ar-n-C4H9 compounds

44 CO2 Anhydrides, esters

45 COOH RCOOH

OC2H5 Ethyl esters of carboxylic acids

46 NO2 Aromatic nitrocompounds

48 SO Aromatic sulfoxide

55 C4H7 Butyl ester of carboxylic acid

56 C4H8 RCOC5H11, ArOC4H9, Ar-C5H11 (n- or i-)

57 C4H9 RCOC4H9

C2H5CO RCOC2H5

60 CH3COOH Acetate

Page 59: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

M+.

[M-O]+.

[M-NO]+

[M-NO2]+

Page 60: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

“McLafferty+1” peak given by ethyl esters

[M-C2H3]+

[M-C6H12]+. [CH2C(OH)OC2H5]+.

Characteristic of ethyl ester of long-chain carboxylic acid

M+.

Page 61: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Common Characteristic Ions

m/z 105 + 77 + 51 Benzoyl compounds

m/z 91 + 65 + 39 Alkyl benzenes, benzyl compounds

m/z 30 Base peak RNH2 otherwise other amines

m/z 44, 58, 72, . . . Amines, amides

m/z 31 Primary alcohol; low intensity, other alcohols, ethers

m/z 31, 45, 59, . . . Ethers

m/z 74 Methyl esters of carboxylic acids

m/z 60 Straight chain carboxylic acids

m/z 77 or 76 Mono- or di-substituted benzene (low intensity)

Page 62: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

[CH2NH2]+

Suggests amine or amide

[CH3CO]+

[C2H4NH2]+ [M-C3H6]+.

[M-CH3]+ M+.

Page 63: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

[M-C8H16]+. [CH2C(OH)OCH3]+.

Characteristic of methyl ester of a long-chain carboxylic acid

M+.

CH3(CH2)8CO.OCH3

[M-OCH3]+

Characteristic H-rearrangement ions of straight chain methyl esters

143

Page 64: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

The Odd-Even Electron Rule - 1

For molecules that do not contain an odd number of N atoms, M+. is an even mass, odd electron ion. If it loses a molecule in a rearrangment process, the resulting fragment ion is again an even mass, odd electron ion. If it loses a radical, which is an odd mass, odd electron species, this produces an odd mass, even electron fragment ion.

Page 65: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

The Odd-Even Electron Rule - 2

Once a radical has been lost to produce an even electron, closed shell ion, further fragmentations can occur only by the loss of molecules to produce further odd mass, even electron ions.Successive loss of two radicals NEVER occurs.Do not assume that an ion is always formed from the next highest mass fragment ion. Ions may fragment by several routes so that adjacent peaks may not belong to ions of the same fragmentation sequence.

Page 66: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

(CH3)2CH-C6H4COOH

All fragment ions are odd mass, even electron ions M+.

Page 67: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Charge Localisation - 1Although the charge on a molecular ion may be delocalised, it is useful to consider it formally as localised. Where on the molecular ion is the charge located?Which is the easiest (lowest energy) electron to remove?These are usually

(a) lone pair electrons on heteroatoms

(b) -electrons in unsaturated systems

Page 68: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Charge Localisation - 2If there is a choice of electrons that could be removed, the formal charge may be placed on one of several atoms.

Hence, formally, one can think of M+. ions as consisting of a mixture of ions with the formal charge being on one of several possible sites.

Each type of molecular ion can give rise to a different type of fragmentation and the spectrum observed will be the weighted sum of the products of these.

Page 69: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Examples of Charge Localisation

Carbonyl compounds are assumed to lose a lone pair electron from the carbonyl oxygen

Ionized toluene is assumed to have lost a ring

-electron

Page 70: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Charge Localisation at Several Sites

NH2+.

O

R

NH2

O+.

R

NH2

O

R

+.

Here there are three possible sites for charge localisation. Fragmentation may be rationalised in terms of the decomposition of three different types of molecular ion

Page 71: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

[M-CH3]+ -cleavage

[M-CH3CO]+

inductive cleavage M+.

Page 72: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Factors Influencing Ion Abundance - 1

Eint required for decomposition: in general, low energy processes will predominate but different ionization methods yield different internal energy distributions and hence different mass spectra from a particular sample.

Stability of the product ion

Page 73: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Factors Influencing Ion Abundance - 2

Stability of the neutral productDelocalization of electron e.g. in allyl radical

Placing of electron on electronegative atom e.g. .OH

Loss of small stable molecule containing multiple bonds, e.g. CO, C2H2, HCN

Stevenson’s Rule

AB+. A+ + B. or A. + B+

Preference for formation of ion from fragment having lower IE (except largest R. is lost preferentially)

Page 74: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Radical Site Initiation -Cleavage) - 1

This is particularly important for ions that contain N or O atoms. Electron pairing occurs by the transfer of an odd electron from the bond alpha to the atom carrying the charge and the transfer of the odd lone pair electron to form a new bond. The remaining electron from the alpha bond is lost on the radical that is eliminated as a result of the bond breaking.

Page 75: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Radical Site Initiation -Cleavage) - 2

In this type of fragmentation, the charge site does not move but the radical site moves as a result of the alpha bond breaking. In the example below, C - X is the alpha bond broken. An example is the fragmentation of carbonyl compounds:

R - C - X RCO+ + X.

|| O+.

(X = H, R, OH, OR, Cl, NH2) aldehydes, ketones, acids, esters, acid halides, amides

Page 76: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Mechanisms of Alpha Cleavage

X = H, R, OH, OR, Cl, NH2 for aldehydes, ketones, acids, esters, acid halides and amides.

m/z 30 is the base peak in primary amine spectra

Primary alcohols give the m/z 31, ethers often give m/z 45, 59 . . by this reaction

The ease of loss of alkyl groups is R1>R2>R3 where this is also the order of decreasing size.

Page 77: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

M+.

[M-H]+ C6H5CO+

C6H5+

C4H3+

Page 78: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

[M-R.]+ -cleavage, 57+, 71+

[R]+ inductive cleavage 29+, 43+

M+.

Page 79: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

[M-16]+.

characteristic of amides

M+.

Page 80: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

M+. absent

[M-H2O]+.

[M-H2O-C3H6]+.[CH2OH]+

Page 81: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

M+.

[C4H9OCH2]+

3 examples of [CH2OR]+ ions at m/z 31, 59 and 87

[C2H5OCH2]+

[CH2OH]+

Page 82: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

[CH2NH2]+ base peak of primary amines

M+.

Page 83: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Loss of the Largest Alkyl Radical -1

Where there is a choice of alkyl radicals that can be lost in an alpha-cleavage, the largest radical is lost preferentially followed by the next largest, etc.

Page 84: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Loss of the Largest Alkyl Radical - 2

The loss of H or an alkyl radical gives rise to the following series of ions for different classes of compoundsAldehydes and ketones m/z 29, 43, 57, 71 . . . Aldehydes usually give a fairly prominent m/z 29 peak (CHO+) and a weaker [M - 1]+ peak.

Methyl ketones often give m/z 43 (CH3CO+) as the base peak.

Page 85: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Loss of the Largest Alkyl Radical - 3Alcohols and ethers m/z 31, 45, 59, 73 . . . Primary alcohols typically give a m/z 31 peak (CH2OH+), often of fairly low intensity.Methyl ethers give m/z 45 peaks, again often of low intensity (charge induced fragmentation usually predominates).Amines m/z 30, 44, 58, 72 . . . n-alkyl amines often give m/z 30 as the base peak whereas secondary and tertiary amines often give m/z 44 and 58 as base peaks together with m/z 30

Page 86: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Loss of the Largest Alkyl Radical – 4

(amines)

[A] C3H7 loss predominates to give an intense m/z 30 peak; little H loss evident, m/z 72 negligible; M+. m/z 73 (10%)

[B] CH3 loss is preferred to H loss giving m/z 58 as the base peak; some H loss giving m/z 72 (20%); M+. m/z 73 (30%)

[C] CH3 loss is only possible alpha-cleavage so that m/z 58 is again the base peak but there is no peak at either m/z 72 (H loss) or 73, M+.

C3H7HC

H

NH2 H3CHC

H

HN

HC CH3

H

H3C C

CH3

CH3

NH2

A B C

Page 87: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

[CH2NHC2H5]+

M+.

-C2H4

Page 88: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

M+.

[M-CH3]+

[M-C3H7]+

[M-C5H11]+

Page 89: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

M+. absent

CH3 loss

C2H5 loss

C3H7 loss

Page 90: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Allylic Cleavage

This is a major type of fragmentation for alkenes leading to alkyl radical loss. Unfortunately, migration of the double bond occurs before fragmentation so that the observation of ions of this type is of little structural value. The mass spectra of many alkenes, especially polyenes, tend to be independent of the position of the double bond so that isomers cannot be distinguished.The m/z 41 ion is the most common ion observed in the mass spectra of aliphatic compounds, together with homologues of m/z 55, 69, 83 . . .

Page 91: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Allylic ions at m/z 41, 55, 69, 83

M+.

Page 92: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify
Page 93: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Charge Site Initiation (Inductive Cleavage)

Page 94: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

[C2H5OCH2]+

M+.

[HOCH2]+

C3H7+

C2H5+

Page 95: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

M+.

[CH2SC3H7]+

[CH2SC2H5]+

[C3H7]+

[C2H3]+

Page 96: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Rearrangement Reactions

Page 97: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

McLafferty Rearrangement - 1

Page 98: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

McLafferty Rearrangement - 2

Deuterium labelling studies show that a -H atom is transferred quite specifically through a six-membered transition state. If there are no -H atoms, the rearrangement does not occur.

Note that substituents on the -carbon atom are retained in the ion but substituents on the - and -carbon atoms are lost as part of the alkene - useful in locating site of branching in an alkyl chain.

Page 99: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

No alkyl chain 3 or more C atoms long so no McLafferty Rearrangement leading to alkene loss

M+.

[C2H5CO]+

[C2H5]+

Page 100: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

C2H4 loss from C3H7 alkyl chain

CH3 loss

M+.

[C3H7]+, [CH3CO]+

Page 101: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Loss of C4H8 from C5H11 alkyl chain (McLafferty Rearrangement)

[M-OCH3]+

M+.

[M-C5H11]+

Page 102: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Loss of C4H8 from

C5H11 alkyl chain

[M-NH2]+

M+.

Page 103: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

McLafferty Rearrangement - 3

If an even mass fragment ion is found which could be formed from the molecular ion by the loss of 28, 42, 56, 70, . . . Da., always suspect that it results from a McLafferty rearrangement or of a related process.The driving force for the rearrangement is the formation of a strong bond between the H atom and the unsaturated heteroatom carrying the charge. Similar reactions occur with H-transfer to other heteroatoms.

Page 104: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

McLafferty Rearrangement - 4

Page 105: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Peaks due to alkene losses at m/z 112, 98, 84, 70, 56, 42

M+.

Page 106: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

M+.

[M-C3H6]+.

Page 107: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

M+.

[M-C2H4]+.

Page 108: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

H-Atom Rearrangement to a Saturated Heteroatom -

1

Page 109: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

M+.

[M-HCl]+

Page 110: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

H-Atom Rearrangement to a Saturated Heteroatom - 2In these reactions, an unpaired electron on the heteroatom is donated to form a new bond with a H atom with cleavage of the original bond to that H atom.

A second radical site reaction leading to H2O+. formation is not favoured since this is an energetic process that does not lead to particularly stable products.

Page 111: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

H-Atom Rearrangement to a Saturated Heteroatom - 3

Instead, a charge site reaction occurs leading to the loss of H2O (very common for primary alcohols, much less so for secondary and tertiary alcohols because of α-cleavage competition) followed by a further charge site reaction in which C2H4 is lost.

In general, the loss of a small neutral species is more energetically favorable than formation of a small ionic species.

Page 112: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Fragmentation of Aromatic Molecular Ions

Many simple aromatic molecular ions fragment by elimination of a small, unsaturated molecule by breaking the aromatic ring but giving a further, stable cyclic ion as a product.

Examples of small molecules lost include: -Benzene, C2H2, Pyridine, HCN, Thiophene, HCS, Furan, HCO, Phenols, CO, Anilines, HCN

Page 113: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

M+.

[M-C2H2]+.

Page 114: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

M+.

[M-HCN]+.

Page 115: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

M+.

[M-HCO]+

Page 116: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

M+.[M-C2H2]+.

[M-C3H3]+

[M-CHS]+

Page 117: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

H-Atom Rearrangement in Phenols and Anilines

Phenols expel CO to give an [M-28]+. peak often accompanied by an [M-29]+ peak due to the loss of a H atom.Replacing the O atom by NH to give aniline, the loss of HCN to give an [M-27]+. ion can be similarly rationalized.Deuterium labeling shows considerable H/D scrambling indicates that the mechanisms are more complicated than indicated above.

Page 118: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Phenol

Aniline

M+.

M+.

[M-CO]+.

[M-HCN]+.

Page 119: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

H-Atom Rearrangement to a Saturated Heteroatom - 4

THE ORTHO EFFECT requires a labile H atom (OH, NH2, CHO) and formation of a stable ionic product by charge migration, eliminating a small saturated molecule e.g. H2O, CH3OH, HCl. These products are more energetically favourable than those given by charge retention reactions.

Page 120: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

2-Hydroxybenzaldehyde

(m a in lib ) Be n za ld e h yd e , 2-h yd ro xy-10 20 30 40 50 60 70 80 90 100 110 120 130

0

50

100

13 1729

39

42 4750 53

63

65

71 74

76

87

93

104

122

O H

O

M+.[M-H]+

[M-H2O]+.

[M-CO]+.

[M-CO-H2O]+.

Page 121: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

2-Hydroxybenzoic acid ethyl ester

(m a in lib ) Be n zo ic a c id , 2-h yd ro xy-, e th yl e ste r20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180

0

50

100

27

39

4353

65

76 81

92

104 109

120

138

166

O H

O

O

Loss of C2H5OHLoss of CO from

m/z 120

M+.

Page 122: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Even-Electron Ions, CI,

Page 123: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Even-Electron Ions - 1Under EI conditions, M+. ions are formed and a major fragmentation process is the loss of a radical, R., producing an even-electron ion.

Once a radical has been lost, all subsequent fragmentations involve the loss of a molecule to form further even-electron ions.

Under CI conditions, an even-electron ion, such as MH+, is formed; subsequent fragmentations involve the loss of a molecule to form further even-electron ions.

Page 124: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Sites of ProtonationIn order to rationalize the fragmentation of MH+ ions, one must consider at which sites in the sample molecule the proton is attached. The spectrum may then be rationalized in terms of the fragmentation of the different types of MH+ ions.In general, protonation occurs on heteroatoms having lone pairs of electrons, such as O, N and Cl. This frequently followed by charge-induced elimination of a molecule containing the hetero-atom. Other possible protonation sites are aromatic rings and regions of unsaturation.

Page 125: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Even Electron Ions

Ephedrine ionized by methane CI may protonate on the O atom of the OH group:

Protonation on the N atom leads to the loss of CH3NH2 by a similar mechanism, yielding an ion of m/z 135. Both m/z 148 and 135 are observed in the CI spectrum, indicating the presence of OH and HNCH3 groups in the molecule.

Page 126: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Ephedrine EI and CI Spectra

Ephedrine, RMM 165, gives an EI spectrum dominated by the m/z 58 fragment ion and no M+. ion. Methane CI gives an MH+ ion at m/z 166 and fragments at m/z 148, 135 and 58 due to protonation on the OH and NHCH3 groups or on the aromatic ring respectively

Page 127: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Field’s Rule - 1In the decomposition of even electron ions , if more than one neutral can be eliminated, the lower the proton affinity of the neutral, the greater the tendency to leave.Hence under CI conditions, a protonated chloroalkane has a high tendency to lose HCl (PA 5.8 eV), a protonated alcohol has a moderate tendency to lose H2O (PA 7.2 eV) and a protonated amine has a low tendency to lose NH3 (PA 9.0 eV), consistent with the [M-18]+ and [M-31]+ relative abundances in the CI mass spectrum of ephedrine.

Page 128: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Field’s Rule - 2In the case of unsaturated ions which could lose

unsaturated neutrals, the higher proton affinities of these species often result in other more complex H-rearrangement modes of fragmentation being more favourable. For example,

C2H5O+=CH2 CH2O + C2H5+ (9% CID) (inductive cleavage)

C2H5O+=CH2 C2H4 + HO+=CH2 (58% CID)

(H rearrangement) predominates since the PAs of C2H4 and CH2O are respectively 7.0 and 7.4 eV.

Page 129: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Charge Site Rearrangements - 1

Even-electron ions undergo H-transfer to the charge site; all electrons remain paired (low energy process), the charge site does not move and an unsaturated or cyclic neutral species is lost

Page 130: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Charge Site Rearrangements - 2

Very important in the fragmentation of amines. After an initial α-cleavage, this fragmentation requires the presence of at least one ethyl group or larger so that an alkene may be lost.

Page 131: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

[M-CH3]+ -cleavage

M+.

-C2H4

[CH2NH2]+

Page 132: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Violations of Even Electron Rule - 1

Usually R - Y+ undergoes heterolytic cleavage to give R+ + Y, all species being even-electron species.

This is a much lower energy process than homolytic cleavage with charge retention giving R. + Y+. so producing two odd-electron species.

Page 133: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Violations of Even Electron Rule - 2

The most common exceptions are in the spectra of aromatic molecules containing electronegative substituents, Cl, CN, NO2, etc.

In the spectra of 1,3- and 1,4-dinitrobenzenes, the ions C6H4

+. and C6H3+ are of almost equal

abundance (losses of 2 NO2 and NO2 + HNO2 respectively) not only because IE(benzyne) < IE(NO2) but also because other reactions giving even-electron products are not energetically more competitive. Other examples of this behaviour are common.

Page 134: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

M+.

[M-NO2]+

-NO

-O

-C2H2

NO+

Page 135: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Practical Problems - 1

Beware of spurious peaks such as the following:Background peaks from previous samples, pump oil or from an air leak, e.g. m/z 40, 32, 28, 18 etc.Peaks arising from incomplete removal of common solvents such as m/z 83, 85, 87 from CHCl3, m/z 58, 43 from acetonePeaks present due to incomplete reaction leaving traces of starting materials in the samplePeaks due to homologues, e.g. at 14 m/z units above or below the true molecular ion peak

Page 136: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

Practical Problems - 2

• Compare your spectrum with that of an authentic sample obtained by use of the same ionisation technique but remember that an exact match of relative intensities is unlikely to be found because of varying mass discrimination effects.

Page 137: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

General Hints - 1Aromatic or aliphatic? Provisionally identify M+.

Check that proposed neutral losses are sensible

Is N present? Is assignment of M+. incorrect?

Check for isotope peaks for Cl, Br, S, heavy metals

Use I([M+1]+)/I([M]+.) to estimate number of C atoms present

Postulate a molecular formula and estimate the double bond equivalents

Page 138: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

General Hints - 2Inspect higher mass ions, possibly formed from M+. in one step, e.g. even mass fragments formed in a rearrangment processLook for characteristic neutral losses such as 16 Da, O from ArNO2 or NH2 from an amide, 30 Da, CH2O from ArOCH3 and characteristic ions, m/z 30, amines, m/z 74 methyl esters, 105/77/51, 91/65/39 for benzoyl and alkyl benzene compounds Do not assume adjacent peaks are due to sequential losses of neutrals; two or more charge sites lead to competing fragmentation routes.

Page 139: Interpretation of Mass Spectra Part 4. Objectives To describe the main features of EI, CI, ESI spectra of organic compounds To indicate how to identify

General Hints - 3Do not try to interpret every small peak, especially those at low m/z which result from sequential fragmentation

Never postulate the loss of a radical from an even electron ion without very good reason

Use negative evidence as well as positive evidence: e.g. if there is no peak at m/z 91, the sample is unlikely to be an alkyl benzene.