Review lecture: Nuclear magnetic resonance spectroscopy in two frequency dimensions

Abstract

A spectrum is normally thought of in terms of a graph of absorption intensity as a function of frequency, and in this form it has served us well for many years. A recent development extends this idea into a second frequency dimension, the spectrum now being represented by a surface in three-dimensional space. The examples are in the field of high resolution nuclear magnetic resonance (n.m.r.) spectroscopy, although the principle is rather more general, being based on the two-dimensional Fourier transformation of a transient response that is a function of two independent time variables $t_1$ and $t_2$. By arranging for different experimental conditions to prevail during $t_1$ and $t_2$, it is possible to separate different n.m.r. parameters, for example chemical shifts and spin coupling constants, into the two frequency dimensions. There is also an important element of correlation involved, since during t$_2$ the nuclei `remember' their past history during $t_1$, and this has been used to correlate proton and carbon-13 chemical shifts.