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Overview of information that can be obtained from mass spectra, C-13 and proton nmr spectra
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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
