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Case Western Reserve University

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Presentation on theme: "Case Western Reserve University"— Presentation transcript:

1 Case Western Reserve University
Chapter 14 NMR Spectroscopy Organic Chemistry 4th Edition Paula Yurkanis Bruice Irene Lee Case Western Reserve University Cleveland, OH ©2004, Prentice Hall

2 Nuclear Magnetic Resonance (NMR) Spectroscopy
Identify the carbon–hydrogen framework of an organic compound Certain nuclei such as 1H, 13C, 19F, and 31P have allowed spin states of +1/2 and –1/2; this property allows them to be studied by NMR

3 The spin state of a nucleus is affected by an applied
magnetic field

4 The energy difference between the two spin states
depends on the strength of the magnetic field

5 a-spin states b-spin states absorb DE release DE Signals detected by NMR

6 An NMR Spectrometer

7 The electrons surrounding a nucleus affect the effective
magnetic field sensed by the nucleus

8 Chemically equivalent protons: protons in the same chemical environment
Each set of chemically equivalent protons in a compound gives rise to a signal in an 1H NMR spectrum of that compound

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11 The Chemical Shift The reference point of an NMR spectrum is defined by the position of TMS (zero ppm) The chemical shift is a measure of how far the signal is from the reference signal The common scale for chemical shifts = d d = distance downfield from TMS (Hz) operating frequency of the spectrometer (MHz)

12 1-bromo-2,2-dimethylpropane
1H NMR spectrum of 1-bromo-2,2-dimethylpropane

13 The chemical shift is independent of the operating
frequency of the spectrometer

14 Electron withdrawal causes NMR signals to appear at
higher frequency (at larger d values)

15 Characteristic Values of
Chemical Shifts

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17 1-bromo-2,2-dimethylpropane
1H NMR spectrum of 1-bromo-2,2-dimethylpropane

18 Integration Line The area under each signal is proportional to the number of protons that give rise to that signal The height of each integration step is proportional to the area under a specific signal The integration tells us the relative number of protons that give rise to each signal, not absolute number

19 Diamagnetic Anisotropy
The p electrons are less tightly held by the nuclei than are s electrons; they are more free to move in response to a magnetic field Causes unusual chemical shifts for hydrogen bonded to carbons that form p bonds

20 Splitting of the Signals
An 1H NMR signal is split into N + 1 peaks, where N is the number of equivalent protons bonded to adjacent carbons Coupled protons split each other’s signal The number of peaks in a signal is called the multiplicity of the signal The splitting of signals, caused by spin–spin coupling, occurs when different kinds of protons are close to one another

21 1H NMR Spectrum of 1,1-Dichloroethane

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24 The ways in which the magnetic fields of three protons
can be aligned

25 Splitting is observed if the protons are separated by more
than three s bonds Long-range coupling occurs when the protons are separated by more than three bonds and one of the bonds is a double or a triple bond

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27 More Examples of 1H NMR Spectra

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29 The three vinylic protons are at relatively high frequency
because of diamagnetic anisotropy

30 The Difference between a Quartet and a Doublet of Doublets

31 The signals for the Hc, Hd, and He protons overlap

32 The signals for the Ha, Hb, and Hc protons do not overlap

33 Coupling Constants The coupling constant (J) is the distance between two adjacent peaks of a split NMR signal in hertz Coupled protons have the same coupling constant

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35 The trans coupling constant is greater than the cis

36 A Splitting Diagram for
a Doublet of Doublets

37 A Splitting Diagram for
a Quartet of Triplets

38 When two different sets of protons split a signal, the
multiplicity of the signal is determined by using the N + 1 rule separately for each set of the hydrogens when the coupling constants for the two sets are different When the coupling constants are similar, the N + 1 rule can be applied to both sets simultaneously

39 The three methyl protons are chemically equivalent due
to rotation about the C–C bond We see one signal for the methyl group in the 1H NMR spectrum

40 1H NMR spectra of cyclohexane-d11 at various temperatures
axial equatorial equatorial axial the rate of chair–chair conversion is temperature dependent

41 Protons Bonded to Oxygen and Nitrogen
These protons can undergo proton exchange The greater the extent of the hydrogen bond, the greater the chemical shift They always appear as broad signals

42 dry ethanol ethanol with acid

43 To observe well-defined splitting patterns, the difference
in the chemical shifts (in Hz) must be 10 times the coupling constant values

44 13C NMR Spectroscopy The number of signals reflects the number of different kinds of carbons in a compound The overall intensity of a 13C signal is about 6400 times less than the intensity of an 1H signal The chemical shift ranges over 220 ppm The reference compound is TMS

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46 Proton-Decoupled 13C NMR of
2-Butanol

47 Proton-Coupled 13C NMR of 2-Butanol

48 DEPT 13C NMR distinguish among CH3, CH2, and CH
groups

49 The COSY spectrum identifies protons that are coupled
Cross peaks indicate pairs of protons that are coupled

50 COSY Spectrum of 1-Nitropropane

51 The HETCOR spectrum of 2-methyl-3-pentanone
indicates coupling between protons and the carbon to which they are attached


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