BIRD-J-resolved HMBC and BIRD-high-resolution HMBC pulse sequences for measuring heteronuclear long-range coupling constants and proton-proton spin coupling constants in complicated spin systems

摘要

Efficient pulse sequences for measuring long-range C-H coupling constants (J_(C-H)) and proton-proton spin coupling constants (J_(H-H)), named BIRD-J-resolved HMBC and BIRD-high-resolution HMBC, respectively, have been developed. In spin systems possessing a secondary methyl group positioned between protonated carbons (e.g.-CH_2-CH(CH_3)-CH _(2-)), the methine proton splits in a complicated fashion, resulting in difficulty in the determination of its spin coupling constants. For easy and accurate measurements of the long-range J_(C-H) and J_(H-H) in such a spin system, the BIRD pulse [90°x(H)- 180°x (H/C)- 90° (-x)(H)] or [90°x(H)- 180°x(H/C)- 90° (-x)(H)180°x(C)] is incorporated into the J-resolved portion of the pulse sequence. As a result, the above secondary methyl group can be selectively decoupled, providing simplified cross-peak patterns, which are suitable for the accurate measurements of the long-range J_(C-H) and J_(H-H).