Elucidation of the Forces Governing the Stereochemistry of Biphenyl

Abstract

AbstractBiphenyl prefers a twisted conformation rather than a planar or perpendicular structure. Although conjugation (stabilization) and steric repulsion (destabilization) coexist in the planar structure, both would be relieved in the perpendicular structure in which an alternative stabilizing interaction, namely hyperconjugation, dominates. It has been assumed that the twisted conformer results from a balance of the attractive and repulsive forces. By using the block‐localized wavefunction (BLW) method, which is an ab initio valence bond (VB) approach, we have quantified the stabilization (conjugation and hyperconjugation) and destabilization (steric repulsion) energy changes as a function of the twist angle, and demonstrated that the minimum equilibrium structure indeed is a result of the coupling of these two conflicting forces. At the B3LYP/TZVPP level of theory, the BLW computations showed that the π‐conjugation effect contributes to the stability of the planar biphenyl by 46.4 kJ/mol, whereas the σ–π* hyperconjugation stabilization energy in the perpendicular structure is 36.4 kJ/mol. However, we note that there is also a weak σ–σ* hyperconjugative interaction between the two phenyl rings in the planar structure, which is estimated to stabilize the planar structure by an additional 6.6 kJ/mol. The overall steric effect was determined to be 17.1 kJ/mol, which is higher than the ethane rotational barrier (12 kJ/mol) and highlights the repulsion between the adjacent C–C bonds and the ortho‐hydrogen atoms in the planar biphenyl.