RESUMEN

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 C3H4 and 1,3-difluoroallene C3H2F2, 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 Epv 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 tau with maximum values of about 0.5 pJ mol(-1) for C3H4 and 2 pJ mol(-1) for C3H2F2. However, in the latter case they are asymmetrically shifted around tau=90 degrees , with a nonzero value at the chiral equilibrium geometry resulting in a parity-violating energy difference between enantiomers DeltapvE=Epv(P)-Epv(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 DeltapvE and thus parity violation dominates over tunneling for the stereomutation dynamics in 1,3-difluoroallene. Therefore, DeltapvE is predicted to be a spectroscopically measurable energy difference. Two of the lower excited electronic states of C3H2F2 (1A and 3A) 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.

Asunto(s)

Alcadienos/química , Alelos , Química Física/métodos , Electrónica , Electrones , Fluoruros/química , Modelos Químicos , Modelos Estadísticos , Conformación Molecular , Teoría Cuántica , Espectrofotometría , Estereoisomerismo , Termodinámica

RESUMEN

Meaninglessly small would be the effects from parity violation according to the traditional point of view on the structure and dynamics of chiral molecules; enantiomers would thus exist as symmetry-related structures, which are de facto stable because of very long tunneling times. With ClSSCl as the first example, electroweak and tunneling dynamics calculations demonstrate that the de lege asymmetry arising from the parity-violating energy difference ΔEpv between the two enantiomers ((P)- and (M)-ClSSCl, the figure shows the torsional potentials) dominates by far over the tunneling splitting in the symmetrical case. These results are of fundamental interest for our concept of molecular chirality.