1. Nuclear Magnetic Resonance Spectroscopy
Renee Y. Becker
Valencia Community College
CHM 2011C

2. The Use of NMR Spectroscopy
Used to determine relative location of atoms within a molecule
Most helpful spectroscopic technique in organic chemistry
Related to MRI in medicine (Magnetic Resonance Imaging)
Maps carbon-hydrogen framework of molecules
Depends on very strong magnetic fields

4. Nuclear Magnetic Resonance Spectroscopy
1H or 13C nucleus spins and the internal magnetic field aligns parallel to or against an aligned external magnetic field (See Figure 13.1)
Applying an external magnetic field, Bo, the proton or nucleus will orient parallel or anti-parallel to the orientation of the external field.
The parallel orientation of the proton or nucleus is lower in energy than the anti-parallel orientation.
Radio energy of exactly correct frequency (resonance) causes nuclei to flip into anti-parallel state
Energy needed is related to molecular environment (proportional to field strength, Bo ) – see Figure 13.2

7. Nuclear Magnetic Resonance Spectroscopy (1H)
The energy of the radiation required is within the radio frequency range.
The energy required is dependent upon the nucleus and the strength of the magnetic field.
A proton in a magnetic field of 1.41 telsa requires a E.M. radiation of 60 MHz to resonate.
E = 2.4 x 10-5 kJ/mol
I.R. energies  48 kJ/mol

8. The Nature of NMR Absorptions
Electrons in bonds shield nuclei from magnetic field
Different signals appear for nuclei in different environments

9. 1H
13C

10. The NMR Measurement
The sample is dissolved in a solvent that does not have a signal itself and placed in a long thin tube
The tube is placed within the gap of a magnet and spun
Radiofrequency energy is transmitted and absorption is detected
Species that interconvert give an averaged signal that can be analyzed to find the rate of conversion

12. Chemical Shifts
The relative energy of resonance of a particular nucleus resulting from its local environment is called chemical shift
NMR spectra show applied field strength increasing from left to right
Left part is downfield right part is upfield
Nuclei that absorb on upfield side are strongly shielded.
Chart calibrated versus a reference point, set as 0, tetramethylsilane [TMS]

13. Chemical Shifts Let’s consider the just the proton (1H) NMR.
60 MHz NMR experiments are carried out with a constant RF of 60 MHz and the magnetic field is varied. When a spin-flip occurs (resonance), it is detected by an R.F. receiver.

14. Measuring Chemical Shift
Numeric value of chemical shift: difference between strength of magnetic field at which the observed nucleus resonates and field strength for resonance of a reference
Difference is very small but can be accurately measured
Taken as a ratio to the total field and multiplied by 106 so the shift is in parts per million (ppm)
Absorptions normally occur downfield of TMS, to the left on the chart

16. Measuring Chemical Shift
Remember: the chemical shift is in ppm.

17. 1H NMR Spectroscopy and Proton Equivalence
Proton NMR is much more sensitive than 13C and the active nucleus (1H) is nearly 100 % of the natural abundance
Shows how many kinds of nonequivalent hydrogens are in a compound
Equivalent H’s have the same signal while nonequivalent are different
There are degrees of nonequivalence

20. Chemical Shifts in 1H NMR Spectroscopy
Lower field signals are H’s attached to sp2 C
Higher field signals are H’s attached to sp3 C
Electronegative atoms attached to adjacent C cause downfield shift
See Tables 13-2 and 13-3 for a complete list

23. Integration of 1H NMR Absorptions: Proton Counting
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 can be measured with a ruler

24. Integration of 1H NMR Absorptions: Proton Counting
This is proportional to the relative number of 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?

25. Spin-Spin Splitting in 1H NMR Spectra
Peaks are often split into multiple peaks due to 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 set of peaks is a multiplet (2 = doublet, 3 = triplet, 4 = quartet)

27. 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

33. 13.12 More Complex Spin-Spin Splitting Patterns
Spectra can be more complex due to overlapping signals, multiple nonequivalence
Example: trans-cinnamaldehyde

35. p-bromotoluene

36. 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