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

A simple proton-selective α/β-HSQMBC experiment is proposed for the accurate measurement of long-range proton-carbon coupling constants (nJCH) 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 JHH evolution. Accurate nJCH 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 nJCH 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.