Mass spectroscopy & nmr spectroscopy

Mass Spectroscopy &

NMR Spectroscopy

Big Picture

What information is contained in each type of spectroscopy?

O Mass SpectroscopyO Molecular weight from molecular ion

peakO Molecular formula from molecular

weight, M+1, M+2 peaksO Functional group and hydrocarbon

branches from fragmentation patters

What information is contained in each type of spectroscopy?

O 13C NMRO Number of signals = types of

chemically nonequivalent carbonsO Chemical shift

O Alkane, alkene, aromatic carbonsO Presence (or absence) of bonded

electron withdrawing group (electronegative atom)

What information is contained in each type of spectroscopy?

O 1H (proton) NMRO Number of signals = types of chemically

nonequivalent hydrogensO Chemical shift

O Alkane, alkene, aromatic hydrogensO Presence (or absence) of nearby electron

withdrawing group (electronegative atom)O Integration – number of hydrogens which

give rise to each signalO Splitting patterns – number of hydrogens

on adjacent carbons

Mass Spectra: Information from Molecular Ions

O Check M+ O Odd number – contains odd number N O Even number – contains combination

of C, H, and possibly OO Check M, M+2 peaks

O Two peaks of equal intensity Br present

O Two peaks, M+2 about 1/3 of M Cl present

Mass Spectra: Molecular Formula Determination

O Rule of 13 can be used for hydrocarbons (see class notes). Before applying rule of 13 subtract outO If N, Br or Cl present (previous slide),

subtract mass of lower molecular weight isotope (halogens) from M peak value

O If IR is available, check for presence of O functional groups and subtract O or O2 (carboxylic acid)

O Given relative abundance of M, M+1 peaks, use 13C abundance of 1.10% to determine number of C’s in molecule (see class notes)

Mass Spectra: Fragmentation PatternsO Hydrocarbons will fragment at branches and

favor more stable fragment (radical) patterns

CH3-CH2-CH2-CH2-CH2-CH3 CH3CH2

CH3CH2CH2

CH3CH2CH2CH2

Mass Spectra: Fragmentation Patterns

CH3CH2CH2

And (CH3)2CH

M-15 (CH3)

CH3CH

CH3

CH2CH2

CH3

Mass Spectra: Fragmentation Patterns

M-15 (CH3)

CH3

C

CH3

CH2CH3

CH3 CH3CH2

(CH3)3C

13C NMRO How many different signals?

O Peak at 77 ppm is CDCl3 solvent

O # signals = how many chemically different carbons

O Chemical shiftO High C=O (150-220 ppm)O Mid alkene, aromatic, nitrileO Low

O 15-60 ppm sp3 CO 20-90 ppm sp3 C bonded to electronegative

atom

1 H (Proton) NMRO How many different signals? Gives

how many types of chemically nonequivalent H’s

1 H (Proton) NMR Chemical Shift

O High (9-12 ppm) aldehyde or carboxylic acid H

O Mid O Aromatic H 6-8.5 ppmO Phenol H 4.5-7.5 ppmO Alkene H 4-6.5 ppm

O Low O H on sp3 C 0.9 – 1.8 ppmO H on sp3 C next to C=O or C=C (alkene or

aromatic), alkyne H 2-3 ppmO H on sp3 C bonded to halogen, N or O 3.0-4.2 ppmO NH or OH 1-6 ppm

1 H (Proton) NMRO Integration (area under peaks)

O Gives relative number of H giving rise to each signal

O When done manually, results may be a bit off

O Splitting patternsO Split into n+1 peaks, n= number of H

on adjacent carbonsO More than 4 peaks may be hard to

interpret correctly