Chiral Torsional Double Wells: Tunneling and Polarizability

Abstract

Quantum tunneling between mirror-related wells of a torsional potential sets the timescale for coherent interconversion of chiral conformers. We ask whether slower tunneling also weakens the transition polarizability in hydrogen peroxide, hydrogen disulfide, and hydrogen diselenide. We use relaxed CCSD(T)//MP2 potentials and a kinetic model that removes overall molecular rotation to obtain the torsional states, then use static polarizabilities to calculate transition matrix elements between them. In the molecular frame used here, symmetry makes the matrix element of the even dipole component vanish between opposite-parity torsional states, while the odd polarizability component has a nonzero matrix element. The two quantities probe different parts of the torsional landscape: tunneling depends on penetration through the barriers, while the polarizability matrix element forms mainly within the wells. Consequently, tunneling slows sharply across the molecular series even as the matrix element grows. Higher torsional doublets and deuterium substitution further show that large changes in tunneling time can occur with little change in the transition polarizability matrix element. Calculated torsional energies for H$_2$O$_2$ and D$_2$O$_2$ agree with experimental values. Thus, slowing coherent interconversion need not weaken the intrinsic transition polarizability.