Anomeric Effect on Geminal and Vicinal J_HH NMR Coupling Constants

Abstract

Trends for geminal ( 2 J HH ) and vicinal ( 3 J 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 2 J HH with the coupling pathway occupation numbers taken from the literature. The present analysis suggests that both 2 J HH and 3 J HH coupling constants are sensitive probes to gauge the anomeric effect, as well as other hyperconjugative interactions.

The quantum-chemical calculation of NMR indirect spin–spin coupling constants

“…For another NBO study of the anomeric effect on coupling constants, see Ref. [218]. We note, however, that there is an element of arbitrariness in all methods based on an analysis of orbital contributions.…”

Optimal transfer of spin-order between a singlet nuclear pair and a heteronucleus

“…As a practical matter, J 12 in saturated analogs of fig. 1 is typically ~4–8 Hz [8], implying efficient spin-order transfer when | J 1C − J 2C | > 0.5–1 Hz. This is usually the case in biological molecules of interest.…”

A computational study of the nuclear magnetic resonance parameters for double proton exchange pathways in the formamide–formic acid and formamide–formamidine complexes

“…A strong negative hyperconjugation into the three-bond coupling pathway has been proposed as a factor responsible for this effect. 21,54 b Intermolecular coupling constants. In contrast to the intramolecular spin-spin coupling constants, changes in the intermolecular coupling constants may be much more substantial, since they are closely connected to the breaking and creation of hydrogen bonding.…”