Analysis of Complex Nuclear Magnetic Resonance Spectra

Abstract

In previous chapters we examined the connection between the structure of organic molecules and the parameters of the NMR spectra, namely, the chemical shifts and the spin—spin coupling constants. The determination of these parameters is quite simple when the substance investigated gives a simple spectrum of the first order. Spectra of the first order (Ch. I) arise when the differences in the chemical shifts of groups of equivalent nuclei considerably exceed the constants for spin-spin coupling between these nuclei. Since an increase in the strength of the magnetic field of the spectrometer (with a corresponding increase in frequency) leads to a proportionate increase in the chemical shifts, but leaves the spin-spin coupling constants unchanged (if they are both expressed in frequency units, Hz), plotting the NMR spectra on spectrometers with a high magnetic field strength leads as a rule to simplification of the spectra and makes them closer to spectra of the first order. Figure IV-1 gives spectra of acrylonitrile plotted at frequencies of 40, 60, and 100 MHz.