Probing Hyperconjugation Experimentally with the Conformational Deuterium Isotope Effect

Abstract

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 variabletemperature, 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.

Pyrolytic reactivities of deuterated β-ether-type lignin model dimers

“…In contrast, no such systematic shifts were observed for the corresponding signals of Ph-␣-d dimer 4. These unexpected results may originate from conformational changes, which may alter the chemical Table 1 Deuterium isotope effects ( ) on 13 Conformational isotope effects are secondary effects of deuteration [17][18][19][20][21]. Hyperconjugation is suggested to be ineffective after deuteration of C H [22][23][24][25].…”

The effect of N-methylation of amino acids (Ac-X-OMe) on solubility and conformation: a DFT study

“…In second-order perturbation analysis of NBOs, larger E(2) values represent stronger interactions between electron-donors and electron-acceptors in a molecule (an increased hyperconjugative interaction of the system). [119][120][121][122] The largest contribution to stabilization of the amide bond through hyperconjugation is indeed due to [n(N) → σ*(CO)]. Interestingly, after N-methylation, this stabilization effect for the amide bond is smaller in all cases, which is indicative of reduced double bond character (N-methyl peptides have slightly longer C-N bondssee Table 2S, ESI †).…”

“…The obtained data in Table 4 implies that for most cases (except the cis-form in the gas phase) the around the amide bond. [119][120][121][122] The largest contribution to stabilization of the amide bond through hyperconjugation is indeed due to [n(N)→σ*(CO)]. Table 5.…”

Synthesis and Screening of α-Xylosides in Human Glioblastoma Cells

“…This phenomenon reduces the electron density at the anomeric proton H-1 resulting downfield shift in the NMR spectrum (resonance structure III, Supporting Figure S1A). , Karplus equation predicts coupling constant of 4 Hz for compound 3A (the dihedral angle of 60°) and 13 Hz in compound 3B (the dihedral angle of 180°) (Supporting Figure S1B). − …”