Selective COSY‐J‐resolved‐HMBC, a new method for improving sensitivity of cross peaks of methine proton signals attached to a methyl group

Abstract

Improved pulse sequences for measuring long-range C-H coupling constants ((n)J(C-H)), named selective COSY-J-resolved HMBC-1 and -2, have been developed. In the spin systems, such as -CH(C)-CH(A)(CH(3))-CH(B)-, a methine proton H(A) splits into a multiplet owing to several vicinal couplings with protons, resulting in attenuation of its cross-peak intensity. Therefore, the measurements of (n)J(C-H) with H(A) are generally difficult in the J-resolved HMBC or selective J-resolved HMBC spectrum. With the aim of accurate measurements of (n)J(C-H) in such a spin system, we have developed new pulse sequences, which transfer the magnetization of a methyl group to its adjacent methine proton. The proposed pulse sequences successfully enable to enhance the sensitivity of H(A) cross peak in comparison with the selective J-resolved HMBC pulse sequence.