Figure 8.3 gives the basic layout of a continuous wave NMR spectrometer. These intruments were the original type of instrument and have largely been replaced by Fourier transform instruments. However, the principles of operation are broadly

The sample is placed in a narrow glass NMR tube and is spun in the fixed magnetic field at ca 30 revolutions/s by means of an air turbine thus ensuring uniformity of the magnetic field across the sample in a horizontal direction. The sample is analysed in solution in a deuterated solvent to interference from protons in the relatively much larger amount of solvent with the signal from the sample protons.

)The reference point of 0 parts per million (ppm) is determined by the frequency at which the protons in tetramethylsilane (TMS) absorb. Sometimes residual protons in the solvent are used to lock the protons in a spectrum, e.g. the residual proton in deuterated chloroform is at 7.25 ppm relative to TMS.

)In order to obtain a proton spectrum the radiofrequency radiation is swept across a range of ca 10 ppm, e.g Hz when the magnetic field is recorded on a 100 mHz instrument or 6000 Hz when the spectrum is recorded on a 600 mHz instrument. The receiver coil measures the absorption of radiation as the frequency is swept over the range being examined.

)As well as determining the frequency at which protons in the molecule absorb, the instrument determines the area of each signal which is proportional to the number of protons absorbing radiation, e.g. three protons give an area three times as large as a signal due to one proton in the same molecule.

)Modern instruments, rather than being based on a continuous wave, are based on a pulsed wave. In brief, the short powerful pulse used in this type of spectroscopy behaves as a spread of frequencies covering the Hz range of interest, e.g. the range in which protons resonate. Most of the principles of the continuous wave instrument still hold but rather than the absorption of radiation by the sample being observed emission is observed as the excited protons relax back to the ground state following the short high energy pulse of radiation. Thus spectra are accumulated using a high intensity pulse followed by a time delay of a few seconds while the relaxation data of different protons in the molecule are collected. This type of procedure enables a spectrum to be acquired every few seconds as opposed to a few minutes required to collect the data using a frequency sweep on a continuous wave instrument. The data from a number of pulses are accumulated using a computer, undergo mathematical manipulation (Fourier transformation) and are combined to produce a spectrum in which the signal to noise characteristics are much improved compared to a spectrum obtained on a single scan continuous wave instrument.

Proton NMR

Chemical shifts Proton ('H) NMR is the most commonly used form of NMR because of its sensitivity and the large amount of structural information it yields. The exact absorption or resonance frequency of a proton depends on its environment. For example, a proton attached to carbon atom is affected predominantly by the groups which are separated from the carbon atom to which it is attached by one bond or to a lesser extent two bonds.

As discussed earlier, the chemical shift of a proton is determined in relation to the protons of tetramethylsilane, which are arbitrarily assigned a shift of 0 ppm. Shift values for individual protons in a molecule are expressed in ppm and the value of 1 ppm in Hertz depends on the strength of the applied magnetic field which

magnetic field, such as occurs in the case of protons attached to an aromatic ring, its a value will increase, i.e. it will resonate at lower field (lower frequency).

Alkyl protons such as those in CH 3 and CH, groups not attached to adjacent electronegative groups resonate between a ppm. Protons on CH 3, CH2 and CH groups attached to electronegative atoms or groups such as 0, N, F, Cl, CN, C=C and C=0 resonate between a 2-5.

Protons attached directly to C=C resonate between a 4-7. Protons attached to aromatic rings resonate between a 6-9. Tables 8.1 and 8.2 show a values in ppm for protons attached to some common organic groups.

If the NMR spectrum of methylacetate is examined, (Figure 8.4) it can be seen to yield two signals of the same size at a 2.06 and a 3.67 more or less as predicted

Aniline: In aniline the 1 and 5 and 2 and 4 protons are equivalent: H-1 and-H-5 shift = = 6.51 ppm. H-2 and H-4 shift = = 7.02 ppm. H-3 shift = = 6.64 ppm. Thus the spectrum of aniline would contain: 2H 6.51 ppm; 2H 7.02 ppm and 1H 6.64 ppm.

Nitrobenzene: In nitrobenzene the 1 and 5 and 2 and 4 protons are equivalent. H-1 and H-5 shift = = 8.21 ppm. H-2 and H-4 shift = = 7.45 ppm. H- 3 shift = = 7.66 ppm. Thus the spectrum of nitrobenzene would contain: 2H 8.21 ppm; 2H 7.45 ppm and 1H 7.66 ppm.

Nitroaniline: In nitroaniline the 1 and 4 and 2 and 3 protons are equivalent H-1 and H-4 shift = = 6.69 ppm. H-2 and H-3 shift = = 7.96 ppm. Thus the spectrum of nitroaniline would contain: