Steps towards molecular parity violation in axially chiral molecules. I. Theory for allene and 1,3-difluoroallene

Abstract

In view of exploring possibilities for an experimental investigation of molecular parity violation we report quantum-chemical calculations of the parity-conserving and parity-violating potentials in the framework of electroweak quantum chemistry in allene C 3 H 4 and 1,3-difluoroallene C 3 H 2 F 2 , which is nonplanar and axially chiral in the electronic ground state but expected to be nearly planar and achiral in several electronically excited states. The parity-violating potentials E pv for allene and 1,3-difluoroallene calculated with the multiconfiguration linear-response (MC-LR) approach of Berger and Quack [J. Chem. Phys.112, 3148 (2000)] show qualitatively similar behavior as a function of torsional angle τ with maximum values of about 0.5 pJ mol − 1 for C 3 H 4 and 2 pJ mol − 1 for C 3 H 2 F 2 . However, in the latter case they are asymmetrically shifted around τ = 90 ° , with a nonzero value at the chiral equilibrium geometry resulting in a parity-violating energy difference between enantiomers Δ pv E = E pv ( P ) − E pv ( M ) = 1.2 pJ mol − 1 (equivalent to about 10 − 13 cm − 1 ). The calculated barrier heights corresponding to the nonrigid (multiple, and in part chiral) transition states in 1,3-difluoroallene fall in the range of 180 – 200 kJ mol − 1 . These high barriers result in hypothetical tunneling splittings much smaller than Δ pv E and thus parity violation dominates over tunneling for the stereomutation dynamics in 1,3-difluoroallene. Therefore, Δ pv E is predicted to be a spectroscopically measurable energy difference. Two of the lower excited electronic states of C 3 H 2 F 2 ( A 1 and A 3 ) are calculated to be planar or quasiplanar, allowing, in principle, for spectroscopic state selection of states of well-defined parity. The results are discussed in relation to possible schemes of measuring parity violation in chiral molecules.