Intramolecular energy transfer and vibrational redistribution in chiral molecules: experiment and theory

Abstract

The quantum vibrational dynamics of the CH-chromophore in a chiral environment are studied with the examples CHDTMu, CHDTF and CHFClBr. For the chiral methane isotopomer we use a recently established nine-dimensional potential hypersurface to extract the three-dimensional short-time quantum dynamics and the related CH-overtone spectra. We have carried out ab initio(MP2) calculations in the appropriate normal coordinate subspace for CHDTF, a chiral isotopomer of methylfluoride for which we have previously carried out extensive related calculations, and also experimental investigations on other isotopomers. For CHFClBr we report the first experimental and theoretical study of the CH-chromophore overtone spectra. The results are systematically analysed in terms of anharmonic coupling constants of the effective hamiltonian as well as the potential hypersurfaces in the appropriate three-dimensional subspaces. We show that the chiral, symmetry-breaking coupling constant k_sab is of appreciable absolute magnitude for all three cases (ca. 25 cm^–1 for CHFClBr). The resulting fast intramolecular vibrational redistribution in the highly excited CH-chromophore, on the femtosecond timescale, leads to appreciable population transfer between states of dynamical a′ and a″ symmetry for the electronically, ‘chemically’ chiral CHFClBr and for CHDTF, which is chiral only by isotope substitution. The symmetry-breaking intra-molecular redistribution processes in chiral molecules are briefly discussed in relation to dynamical chirality, time-dependent optical activity and fundamental symmetry violations of parity, and even of charge conjugation, parity and time-reversal symmetry and their combinations.