1H-and 13C-NMR

-6-

Spin Systems: Analysis of the ^H-NiVIR

Spectra

6.1 SECOND-ORDER SPECTRA

In the previous chapters we discussed only first-order spectra, from which the coupHng

constants and chemical shifts can be directly obtained. All general rules concerning the

splitting, intensity distribution and integration of the peaks can be applied to first-order

NMR spectra. To obtain a first-order NMR spectrum, the chemical shift difference (AS)

between the coupled protons in hertz has to be much larger than the coupling constant

(see page 32)

*^ab 10 (25)

If this ratio is greater than 10, the spectra are considered to he first-order spectra. If this

condition is not met, the term second-order spectrum is used.

The differences between first-order and second-order NMR spectra are as follows:

First-order spectra Second-order spectra

The multiplicity of the signal splitting

can be determined by the number of

neighboring protons (the n + 1 rule)

The relative intensity distribution can be

determined by Pascal's triangle only

Spectra are simple

Spectra can be analyzed directly The multiplicity of the signal splitting

cannot be determined by the number of

neighboring protons. The (n + 1) is not

valid. There is increased multiplicity

There are no definitive rules to determine

the peak intensities. Intensity distribution

is altered

Spectra are complex

The analysis of spectra is complicated. It

can be performed only by a computer

As one can see from the above table, second-order NMR spectra are more complex and

their interpretation is much more complicated. Before going in detail, let us analyze the

H-NMR spectrum of acrylonitrile obtained on spectrometers operating at three different

frequencies (Figure 79) [62].

159

160 6. SPIN SYSTEMS: ANALYSIS OF THE ^H-NMR SPECTRA

HBV^ /HA

He CN

Figure 79 ^H-NMR spectra of acrylonitril recorded on (a) 220 , (b) 100 , and (c) 60 MHz NMR instruments. (Reprinted with permission of American Chemical Society from Anal. Chem., 1971, 42, 29A.)

Acrylonitrile has three different olefinic protons. All of these protons have different

chemical shifts and different coupling constants due to their configuration (trans, cis and

geminal). They will resonate as a doublet of doublets. The 220 MHz NMR spectrum

(Figure 79a) shows the resonances of three protons, which are sufficiently resolved, so

that all coupling constants and proton chemical shifts can be extracted. In the 100 MHz

NMR spectrum of the same compound, the signal groups of protons HB and He approach

each other and some signals overlap, whereas HA proton signal intensities are changed.

However, quite a different situation is found in the 60 MHz NMR spectrum. It has a

completely different appearance (instead of the expected doublet of doublets), where

neither the number of lines nor their intensities are in accordance with the rules of the

first-order NMR spectra that we have seen so far. This kind of NMR signal belongs to the

class of second-order NMR spectra and their analysis can be done with the help of

suitable computer programs. Comparison of these NMR spectra recorded at three

different magnetic fields leads to the conclusion that the number of signals and their

intensities of a given compound depend on the strength of the applied magnetic field. One

point that always has to be kept in mind is that changes in magnetic field strength do not

affect coupling constants or chemical shifts. The chemical shift difference in hertz

between two protons changes upon changing the strength of the magnetic field. The

chemical shift difference in ppm is not affected by the magnetic field. Since the coupling

6.2 TWO-SPIN SYSTEMS 161

constant is not affected by the magnetic field, consequently, any increase in magnetic

field strength must increase the ratio A6// and therefore result in first-order splitting for

spectra that are second order at lower field strengths.

To interpret NMR spectra more easily, first-order spectra have to be recorded by

increasing the chemical shift difference between the coupled protons by using a

spectrometer with a higher magnetic field strength. Therefore, NMR machines operating

at higher magnetic fields are always required. In the next chapter we will classify second-

order NMR spectra and study them in detail. For now, we will limit ourselves to the

consideration of the more frequently encountered spin systems.

6.2 TWO-SPIN SYSTEMS

In this section, we shall begin our discussion of simple spin systems. Since the simplest

spin system is formed between two different protons, we will examine these systems first.

6.2.1 Aa, AB and AX spin systems

The symbols designating the spin systems are employed wherein the letter A is used to