Easy measurement of one-bond proton-carbon and geminal proton-proton coupling constants in diastereotopic methylene groups

“Straightforward measurement of individual ¹J(CH) and ²J(HH) in diastereotopic CH₂ groups” by Josep Saurí, Laura Castañar, Pau Nolis, Albert Virgili and Teodor Parella. Journal of Magnetic Resonance 242 (2014) 33–40. DOI: 10.1016/j.jmr.2014.02.003

The C-H_A cross-peak corresponding to a diastereotopic CH_AH_B 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 ¹³C Boltzmann polarization instead of the regular INEPT-based ¹H Boltzmann polarization makes visible the central lines of this multiplet pattern. A spin-state-selective method is proposed for the efficient measurement of both ¹J(CH_A) and ¹J(CH_B) along the indirect dimension of a 2D spectrum as well as to the magnitude and the sign of the geminal ²J(H_AH_B) coupling constant from the straightforward analysis of a single four-component E.COSY cross-peak. Additionally, the extraction of¹J(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 ¹D(CH) and ²D(HH) coupling constants in a small molecule weakly aligned in a PMMA swollen gel.