Anomeric effect on geminal and vicinal J_HH NMR coupling constants

Trends for geminal (²J_HH) and vicinal ( ³J_HH) nuclear magnetic resonance indirect spin-spin coupling constants, SSCCs, for 2-methylthiirane (5) and 2-methyloxirane (6) are studied both from experimental and theoretical points of view to determine the influence of hyperconjugative interactions on these couplings. These two analogous compounds were chosen because it was expected that they exhibit quite different anomeric effects. Hyperconjugative interactions are investigated using the "natural bond orbital" method. Coupling constants are calculated within the density functional theory including all four scalar contributions, that is, the Fermi contact, the spin-dipolar, and the paramagnetic and diamagnetic spin-orbital contributions. Solvent dielectric effects are taken into account using Tomasi's polarizable continuum model. Results for geminal couplings are consistent with linear correlations connecting ²J _HH with the coupling pathway occupation numbers taken from the literature. The present analysis suggests that both ²J_HH and ³J_HH coupling constants are sensitive probes to gauge the anomeric effect, as well as other hyperconjugative interactions.