Nuclear Magnetic Resonance Spectroscopy
Nuclear Magnetic Resonance Spectroscopy
Chapter 13 Part 2
13.10 Integration of 1H NMR Absorptions: Proton Counting Each of the following compounds has a single 1H NMR peak. Approximately where would you expect each compound to absorb?
The relative intensity of a signal (integrated area) is proportional
to the number of protons causing the signal
This information is used to deduce the structure For example in ethanol (CH3CH2OH), the signals have the
integrated ratio 3:2:1
For narrow peaks, the heights are the same as the areas and
Cyclohexane
can be measured with a ruler
(CH3)3N CH3COCH3
13.10 Integration of 1H NMR Absorptions: Proton Counting
13.11 Spin-Spin Splitting in 1H NMR Spectra
This is proportional to the relative number of
Peaks are often split into multiple peaks due to
protons causing each signal. An integration ratio of 1.5:1 is consistent with a 6:4 ratio of protons as with a 3:2 ratio of protons. How many signals would you expect from the 1H NMR spectrum of chloromethyl methyl ether, ClCH2OCH3, and what would you expect the signal area ratios to be?
interactions between nonequivalent protons on adjacent carbons, called spin-spin splitting The splitting is into one more peak than the number of H’s on the adjacent carbon (“n+1 rule”) The relative intensities are in proportion of a binomial distribution and are due to interactions between nuclear spins that can have two possible alignments with respect to the magnetic field The set of peaks is a multiplet (2 = doublet, 3 = triplet, 4 = quartet)
1
Simple Spin-Spin Splitting An adjacent CH3 group can have four
different spin alignments as 1:3:3:1 This gives peaks in ratio of the adjacent H signal An adjacent CH2 gives a ratio of 1:2:1 The separation of peaks in a multiplet is measured is a constant, in Hz
J (coupling constant)
Coupling constant (J) Measured in Hz. Usually ranges from 0
18 Hz
Note: the same coupling constant is shared by both groups of protons whose spins are coupled. They are independent of spectrometer field strength
The intensity of the peaks are dependent upon n
n=1: yields a doublet with a peak ratio of 1:1 n=2: yields a triplet with a peak ratio of 1:2:1 n=3: yields a quartet with a peak ratio of 1:3:3:1
Two groups of protons coupled to each other have
the same coupling constant, J
Rules for Spin-Spin Splitting Equivalent protons do not split each other The signal of a proton with n equivalent
neighboring H’s is split into n + 1 peaks Protons that are farther than two carbon
atoms apart do not split each other
2
13.12 More Complex Spin-Spin Splitting Patterns Spectra can be more complex due to
overlapping signals, multiple nonequivalence Example: trans-cinnamaldehyde
H
O H H
3
Predict the splitting patterns
Draw structures for compounds that meet the following descriptions: C2H6O; one singlet C3H7Cl; one doublet, one septet C4H8Cl2O; two triplets C4H8O2
Analysis of NMR Spectra The NMR spectra provides the following
information that can assist in the determination of chemical structure The number of signals The chemical shift The intensity of the signal (area under each peak) The splitting of each signal
13.4 13C NMR Spectroscopy: Signal Averaging and FT-NMR Carbon-13: only carbon isotope with a nuclear spin Natural abundance 1.1% of C’s in molecules Sample is thus very dilute in this isotope Sample is measured using repeated accumulation of
data and averaging of signals, incorporating pulse and the operation of Fourier transform (FT-NMR) All signals are obtained simultaneously using a broad pulse of energy and resonance recorded Frequent repeated pulses give many sets of data that are averaged to eliminate noise Fourier-transform of averaged pulsed data gives spectrum (see Figure 13-6)
4
13.5 Characteristics of 13C NMR Spectroscopy Provides a count of the different types of
environments of carbon atoms in a molecule 13C resonances are 0 to 220 ppm downfield from TMS (Figure 13-7) Chemical shift affected by electronegativity of nearby atoms
O, N, halogen decrease electron density and shielding (“deshield”), moving signal downfield.
signal is at δ 0 to 9; sp2 C: δ 110 to 220 C(=O) at the low field, δ 160 to 220 Spectrum of 2-butanone is illustrative- signal for C=O
sp3 C
carbons on left edge
Read about para-bromoacetophenone (Figure 13-8 b).
13.6 DEPT 13C NMR Spectroscopy Improved pulsing and computational methods
give additional information DEPT-NMR (distortionless enhancement by
polarization transfer) Normal spectrum shows all C’s then:
Obtain spectrum of all C’s except quaternary (broad band decoupled) Change pulses to obtain separate information for CH2, CH Subtraction reveals each type (See Figure 13-10)
5
Predict the number of carbon resonance lines in the 13C NMR spectrum of: Methylcyclopentane 1 - Methylcyclohexene 1,2 2 -
Dimethylbenzene Methyl- 2 - butene
Last Slide Chap 13 part 2
6
Chapter 13 Part 2
13.10 Integration of 1H NMR Absorptions: Proton Counting Each of the following compounds has a single 1H NMR peak. Approximately where would you expect each compound to absorb?
The relative intensity of a signal (integrated area) is proportional
to the number of protons causing the signal
This information is used to deduce the structure For example in ethanol (CH3CH2OH), the signals have the
integrated ratio 3:2:1
For narrow peaks, the heights are the same as the areas and
Cyclohexane
can be measured with a ruler
(CH3)3N CH3COCH3
13.10 Integration of 1H NMR Absorptions: Proton Counting
13.11 Spin-Spin Splitting in 1H NMR Spectra
This is proportional to the relative number of
Peaks are often split into multiple peaks due to
protons causing each signal. An integration ratio of 1.5:1 is consistent with a 6:4 ratio of protons as with a 3:2 ratio of protons. How many signals would you expect from the 1H NMR spectrum of chloromethyl methyl ether, ClCH2OCH3, and what would you expect the signal area ratios to be?
interactions between nonequivalent protons on adjacent carbons, called spin-spin splitting The splitting is into one more peak than the number of H’s on the adjacent carbon (“n+1 rule”) The relative intensities are in proportion of a binomial distribution and are due to interactions between nuclear spins that can have two possible alignments with respect to the magnetic field The set of peaks is a multiplet (2 = doublet, 3 = triplet, 4 = quartet)
1
Simple Spin-Spin Splitting An adjacent CH3 group can have four
different spin alignments as 1:3:3:1 This gives peaks in ratio of the adjacent H signal An adjacent CH2 gives a ratio of 1:2:1 The separation of peaks in a multiplet is measured is a constant, in Hz
J (coupling constant)
Coupling constant (J) Measured in Hz. Usually ranges from 0
18 Hz
Note: the same coupling constant is shared by both groups of protons whose spins are coupled. They are independent of spectrometer field strength
The intensity of the peaks are dependent upon n
n=1: yields a doublet with a peak ratio of 1:1 n=2: yields a triplet with a peak ratio of 1:2:1 n=3: yields a quartet with a peak ratio of 1:3:3:1
Two groups of protons coupled to each other have
the same coupling constant, J
Rules for Spin-Spin Splitting Equivalent protons do not split each other The signal of a proton with n equivalent
neighboring H’s is split into n + 1 peaks Protons that are farther than two carbon
atoms apart do not split each other
2
13.12 More Complex Spin-Spin Splitting Patterns Spectra can be more complex due to
overlapping signals, multiple nonequivalence Example: trans-cinnamaldehyde
H
O H H
3
Predict the splitting patterns
Draw structures for compounds that meet the following descriptions: C2H6O; one singlet C3H7Cl; one doublet, one septet C4H8Cl2O; two triplets C4H8O2
Analysis of NMR Spectra The NMR spectra provides the following
information that can assist in the determination of chemical structure The number of signals The chemical shift The intensity of the signal (area under each peak) The splitting of each signal
13.4 13C NMR Spectroscopy: Signal Averaging and FT-NMR Carbon-13: only carbon isotope with a nuclear spin Natural abundance 1.1% of C’s in molecules Sample is thus very dilute in this isotope Sample is measured using repeated accumulation of
data and averaging of signals, incorporating pulse and the operation of Fourier transform (FT-NMR) All signals are obtained simultaneously using a broad pulse of energy and resonance recorded Frequent repeated pulses give many sets of data that are averaged to eliminate noise Fourier-transform of averaged pulsed data gives spectrum (see Figure 13-6)
4
13.5 Characteristics of 13C NMR Spectroscopy Provides a count of the different types of
environments of carbon atoms in a molecule 13C resonances are 0 to 220 ppm downfield from TMS (Figure 13-7) Chemical shift affected by electronegativity of nearby atoms
O, N, halogen decrease electron density and shielding (“deshield”), moving signal downfield.
signal is at δ 0 to 9; sp2 C: δ 110 to 220 C(=O) at the low field, δ 160 to 220 Spectrum of 2-butanone is illustrative- signal for C=O
sp3 C
carbons on left edge
Read about para-bromoacetophenone (Figure 13-8 b).
13.6 DEPT 13C NMR Spectroscopy Improved pulsing and computational methods
give additional information DEPT-NMR (distortionless enhancement by
polarization transfer) Normal spectrum shows all C’s then:
Obtain spectrum of all C’s except quaternary (broad band decoupled) Change pulses to obtain separate information for CH2, CH Subtraction reveals each type (See Figure 13-10)
5
Predict the number of carbon resonance lines in the 13C NMR spectrum of: Methylcyclopentane 1 - Methylcyclohexene 1,2 2 -
Dimethylbenzene Methyl- 2 - butene
Last Slide Chap 13 part 2
6
