Accurate measurement of small heteronuclear coupling constants from pure-phase α/ β HSQMBC cross-peaks

Abstract

A simple proton-selective α/ β-HSQMBC experiment is proposed for the accurate measurement of long-range proton–carbon coupling constants ( n J CH) in small molecules without need for an individualized and time-consuming post-processing fitting procedure. The method acquires two pure-phase In-phase (IP) and Anti-phase (AP) multiplets completely free of any phase distortion due to the absence of J HH evolution. Accurate n J CH values can be directly measured analyzing the relative displacement of the resulting IPAP cross-peaks. Discussion about signal intensity dependence and cross-talk is made for a range of experimental conditions. The robustness of the method is evaluated by comparing the n J CH value measured from the analysis of the three available IP, AP and IPAP multiplet patterns. Multiple-frequency and region-selective versions of the method can also be efficiently recorded provided that excited protons are not mutually coupled.