Exploring the origin of the anomeric relationships in 2-cyanooxane, 2-cyanothiane, 2-cyanoselenane and their corresponding isocyano isomers. Correlations between hyper-conjugative anomeric effect, hardness and electrostatic interactions

Abstract

LC-wPBE, LC-BLYP, B3LYP, M06-2X and MP2 methods with the 6-311+G** basis set and natural bond orbital (NBO) interpretation were performed to explore the origin of the anomeric relationships in 2-cyanooxane (1), 2-cyanothiane (2), 2-cyanoselenane (3) and their isocyano isomers [i.e. 2-iso-cyanooxane (4), 2-iso-cyanothiane (5), 2-isocyanoselenane (6)]. All levels of theory used in this work showed that the axial conformations of compounds 1–6 are more stable than their corresponding equatorial forms. By deletion of the electron delocalization between the non-bonded lone pairs of the six-membered rings and the σ^*_C–CN and σ^*_C–NC anti-bonding orbitals from the Fock matrixes of the axial and equatorial conformations and with re-diagonalization and comparison of the current Fock matrixes with their original forms, we found that these electron delocalizations are responsible for the axial conformation preferences in compounds 1–3 and 4–6. Importantly, the approach mentioned above revealed that the through-space electronic interactions between the non-bonded lone pairs of the heteroatoms of the six-membered rings and the π* anti-bonding orbitals of the cyano and isocyano groups (π^*_CN and π^*_NC) do not play a significant role here. Also, the variations of the differences in the total energies among the axial and equatorial conformations of compounds 1–3 correlate with the variations of differences in their corresponding dipole moments. Although the hyper-conjugative anomeric effect (HCAE) and Pauli exchange-type repulsions (PETR) are in favor of the axial conformations of compounds 1–3, seemingly, the electrostatic model associated with the dipole–dipole interactions is a determinant here. Interestingly, the attractive electrostatic interactions between two adjacent atoms (AEI) explain reasonably the variations of the structural parameters of compounds 1–3 and 4–6 which are in line with the resultant justification from the HCAE. The axial conformations of compounds 1–3 and 4–6 are harder than their corresponding equatorial conformations but their conformational behaviors could not be interpreted with the Maximum Hardness Principle.