described as first orderwhen the numbers of components and their relative
intensities are as predicted by the n+ 1 rule and Pascal’s triangle. This generally
requires that the ratio of the chemical shift difference between the resonance
signals of the two groups of protons to their coupling constant, J, is at least
seven. As this ratio diminishes, the relative intensities within each multiplet
become distorted, and additional splitting is observed making interpretation
more difficult. Such patterns are then described as second order. Capital letters
at opposite ends of the alphabet, with subscripted numbers to indicate the
numbers of protons in each set of nuclei, are used to designate first order
patterns, and these are included in Figure 8.
It should be noted that:(i) protons within the same group are normally chemicallyand magnetically
equivalent, and do not show any splitting, although they are coupled;
(ii) coupling constants, J, are independent of the magnitude of the applied
magnetic field, whereas chemical shift differences (measured in Hz)
increase in proportion to it. Hence, NMR spectra can be simplified by using
more powerful spectrometers, as complex spectra become closer to first
order. Proton coupling constants range from 0 to over 20 Hz.Spin–spin splitting patterns for three coupled groups are observed in unsatu-
rated structures because the coupling extends through more than three bonds.
Each of the groups interacts with the other two, giving three coupling constants
and, depending on their relative magnitudes, the spectra may be complex due
to second order effects.
Examples of spectra are given in Topic E13.Spectrometers were originally designed to scan and record an NMR spectrum
by progressively changing (sweeping) the applied magnetic field at a fixed
radiofrequency (RF), or sweeping the frequency at a fixed field. Sample
resonances were recorded as a series of sharp absorption peaks along the
frequency/field axis, which is calibrated in ppm. These continuous wave(CW)
instruments have been largely superseded by pulsed Fourier transform (FT)
spectrometers. Samples are subjected to a series of rapid, high-energy RF pulses
of wide frequency range, between which a decaying emission signal from nuclei
excited by the pulse and then relaxing to the ground state is monitored by the
receiver circuit. The detector signal, or free induction decay(FID), contains all
of the spectral information from the sample, but in the form of a time-dependent
interferogram. This can be digitized and converted into a conventionalNMR
spectrometers
E12 – Nuclear magnetic resonance spectrometry: principles and instrumentation 257
Table 5. Multiplicity and relative intensities of resonance signals from coupled groups of
nuclei in saturated structures (I =^1 ⁄ 2 )
Number of Multiplicity of observed Relative intensities of
adjacent nuclei resonance components of multiplets
0 Singlet 1
1 Doublet 1 1
2 Triplet 1 2 1
3 Quartet 1331
4 Quintet 14641
5 Sextet 1 5 10 10 5 1
6 Septet 1 6 15 20 15 6 1