The parity-violating energy difference between enantiomeric molecules

Abstract

The parity-violating weak neutral current perturbation of the groundstate electronic energy has been calculated, by ab initio methods, for the (S)-isomer of hydrogen peroxide, over a range of standard Gaussian basis sets. The STO-N-31G, but not the STO-NG, sets give energy shifts E _pv which converge with increasing N towards the corresponding E _pv value given by an extended basis. Calculations for α-amino acids, peptides, model helical systems, with the STO-6-31G basis, or with an analogous set for corre-sponding sulphur analogues, show that the E _pv shift is sensitive in sign and magnitude to the molecular conformation generally. It is found that L-alanine and the L-peptides in the α-helix and the β-sheet conformation have a lower ground state energy than the corresponding D-enantiomers due to the WNC interaction. The energy shift E _pv only approximates to an expected Z ⁵ proportionality, and it does not correlate consistently in sign with the handedness of a helical system.