Broadband-decoupled proton spectroscopy

Abstract

Proton NMR spectra are presented in a form that incorporates chemical-shift effects but excludes spin-spin splittings, as if homonuclear broadband decoupling had been employed. By Fourier transformation of spin echoes modulated by proton-proton coupling, a two-dimensional J spectrum is obtained. A 50 ms purging pulse at the end of the evolution period suppresses antiphase product-operator terms; this changes the character of the two-dimensional spin-multiplet patterns and ensures that the signals are in the pure absorption mode. In the limit of weak coupling, each multiplet pattern possesses C 4 symmetry and a software “symmetry filter” separates overlapping multiplets and rejects all other signal components. By the appropriate projection of the processed J spectrum, a one-dimensional spectrum is obtained with narrow singlet responses at the chemical-shift frequencies and no spin-spin splittings. The peak heights are proportional to the numbers of equivalent protons at each site. A separate spin-multiplet pattern from each site is available in the other frequency dimension ( F 1). The technique can be incorporated into more complicated experiments such as spin-lattice relaxation studies.