Exploring the Origin of the Generalized Anomeric Effects in the Acyclic Nonplanar Systems

Contrary to the published conclusions in the literature concerning the origin of the generalized <i>anomeric</i> relationships in open-chain nonplanar systems, its origin has remained an open question. In order to explore the origin of the generalized <i>anomeric</i> relationships in open-chain nonplanar systems, we assessed the roles and contributions of the effective factors on the conformational properties of methyl propargyl ether (<b>1</b>), methyl propargyl sulfide (<b>2</b>), and methyl propargyl selenide (<b>3</b>) by means of the G3MP2, CCSD­(T), MP2, LC-ωPBE, and B3LYP methods and natural bond orbital (NBO) interpretations. We examined the contributions of the hyperconjugative interactions on the conformational preferences of compounds <b>1</b>–<b>3</b> by the deletions of the orbitals overlapping from the Fock matrices of the <i>gauche</i>- and <i>anti</i>-conformations. The trend observed for energy changes in the Fock matrices justify the variations of the <i>gauche</i>-conformations preferences going from compound <b>1</b> to compound <b>3</b>, revealing that the hyperconjugative interactions are solely responsible for the generalized <i>anomeric</i> relationships in compounds <b>1</b>–<b>3</b>. Accordingly, the conclusions published in the literature concerning the origin of the generalized <i>anomeric</i> effect in the acyclic nonplanar compounds should be revised by these findings. The Pauli exchange type repulsions (PETR) are in favors of the <i>gauche</i>-conformations and the variations of the PETR differences between the <i>gauche</i>- and <i>anti</i>-conformations of compounds <b>1</b>–<b>3</b> correlate well with their <i>gauche</i>-conformations preferences, revealing that the generalized <i>anomeric</i> relationships in compounds <b>1</b>–<b>3</b> have also the Pauli exchange-type repulsions origin. The resemblance between the preorthogonal natural bond orbitals (that are involved in the hyperconjugative interactions) and their corresponding molecular orbitals have been investigated