Probing Hyperconjugation Experimentally with the Conformational Deuterium Isotope Effect

Hyperconjugation underlies many chemical phenomena of fundamental and practical importance. Owing to a great deal of interest in the anomeric effect, anomeric-like hyperconjugative effects have been thoroughly investigated in oxygen-containing systems. However, such interactions in the second- and third-row chalcogens are less well-understood and have generated some controversy. Here, we show that the conformational deuterium isotope effect, in combination with Saunders' isotopic perturbation method, permits sensitive and direct experimental probing of the conformational equilibria in dioxane, dithiane, and diselenane analogues by variable-temperature, dynamic NMR spectroscopy. We find that the magnitude of the conformational deuterium isotope effect is 252.1, 28.3, and 7.1 J/mol (±10%) for the oxygen, sulfur, and selenium analogues, respectively. These results reveal the periodic trend for hyperconjugation in the chalcogens, which reflect a decreasing n(x)→σ(C-H(D)) interaction throughout the period, as supported by IR spectroscopy and in agreement with DFT calculations and a natural bond order analysis.