Conformational dependence of deuterium-induced isotope effects on the olefinic one-bond 13C–1H and three-bond 1H–2H coupling constants in cis- and trans-stilbene

Abstract

Secondary deuterium isotope effects on the olefinic one-bond ¹³C, ¹H and three-bond ¹H, ²H spin–spin coupling constant were determined in a series of specifically deuteriated cis- and trans-stilbene isotopomers. Inverse (¹H detected) heteronuclear multiple quantum correlation (HMQC) NMR pulse sequence was used to measure the coupling constants in these π-conjugated systems of a moderate size. Complete spectral analyses were performed in cases where higher order perturbations took place. All secondary isotope effects on ¹J(¹³C_α, ¹H)/Hz are negative, i.e. the reduced values from –0.06 to –0.02 were observed. In contrast, the effects on ³J(¹H_α, ²H_α′)/HZ are all positive from 0.04 to 0.13, thus being on average relatively higher than those on ¹J(¹³C_α, ¹H). Only two primary effects on ¹J(¹³C_α, ¹H) were determined to see whether the same trend holds as for secondary effects. The real values of olefinic coupling constants are given with a high accuracy. In order to relate isotope effects with structural parameters, AM1 semiempirical calculations were performed. Subtle conformational changes due to the deuteriation are discussed in terms of rovibrational averaging and overall geometry.