Three-dimensional N.M.R. correlation spectroscopy with selective radiofrequency pulses

Abstract

Three-dimensional N.M.R. correlation spectroscopy makes it possible to identify three chemical species, I, S and R that have resolved scalar couplings. This experiment can be implemented by applying two simultaneous selective radiofrequency pulses that are scanned independently through narrow frequency ranges (F ₁ and F ₂) encompassing the I and S-spin chemical shifts. Coherence transfer signals are detected in the form of a three-dimensional cross-peak at the R-spin frequency. This soft-pulse experiment avoids the problem of very large three-dimensional data matrices implicit in the three-dimensional Fourier transform method. A product operator treatment is presented which indicates that coherence is transferred by way of an intermediate stage of pure zero-quantum or pure double-quantum coherence. Consequently, relative signs of coupling constants may be obtained by inspection of the fine structure of the cross-peaks. The predictions of the theory are borne out by experiments carried out on proton three-spin systems in acrylic acid and 2,3-dibromopropanoic acid. Density matrix theory has been used to predict the lineshape in the F ₁ and F ₂ dimensions. For regressive I and S transitions it resembles the ‘phase-twist’ lineshape familiar to two-dimensional spectroscopy. For progressive I and S transitions it has a novel two-dimensional lineshape with two positive and two negative lobes.