Straightforward measurement of individual (1)J(CH) and (2)J(HH) in diastereotopic CH(2) groups

Abstract

The C-H(A) cross-peak corresponding to a diastereotopic CHAHB methylene spin system exhibits a characteristic 1:0:1 multiplet pattern along the indirect dimension of a ω1-coupled HSQC spectrum. It is shown here that the use of the initial (13)C Boltzmann polarization instead of the regular INEPT-based (1)H Boltzmann polarization makes visible the central lines of this multiplet pattern. A spin-state-selective method is proposed for the efficient measurement of both (1)J(CHA) and (1)J(CHB) along the indirect dimension of a 2D spectrum as well as to the magnitude and the sign of the geminal (2)J(HAHB) coupling constant from the straightforward analysis of a single four-component E.COSY cross-peak. Additionally, the extraction of (1)J(CH) values for CH and CH3 multiplicities can be also performed from the same spectrum. The success of the method is also illustrated for the determination of residual dipolar (1)D(CH) and (2)D(HH) coupling constants in a small molecule weakly aligned in a PMMA swollen gel.

Keywords: E.COSY; Inverse INEPT; Methylene spin systems; One-bond proton–carbon coupling constants; Residual dipolar couplings; Two-bond proton–proton coupling constants.