Parity violation leads to energy differences delta(pv)H(o)(0)=N(A)delta(pv)E of enantiomers in the femtojoule to picojoule per mole range. Recently introduced methods of electroweak quantum chemistry predict such energy differences to be one to two orders of magnitude larger than previously accepted-but still very small. How can such small energies be measured and what are the consequences for our understanding of molecular chirality, biomolecular homochirality, and perhaps fundamental physics? The review gives some tentative answers to these questions. We discuss the current status of theory and some of the current experimental approaches.

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We review the high-resolution spectroscopic approach toward the study of intramolecular dynamics, emphasizing molecular parity violation. Theoretical work in the past decade has shown that parity-violating potentials in chiral molecules are much larger (typically one to two orders of magnitude) than anticipated on the basis of older theories. This makes experimental approaches toward small molecular parity-violating effects promising. The concepts and results of intramolecular dynamics derived from spectroscopy are analyzed as a sequence of symmetry breakings. We summarize the concepts of symmetry breakings (de facto and de lege) in view of parity violation in chiral molecules. The experimental schemes and the current status of spectroscopic experiments on molecular parity violation are established. We discuss the promises of detecting and accurately measuring parity-violating energy differences Delta(pv) E on the order of 10(11) J mol(1) (approximately 100 aeV) in enantiomers of chiral molecules with regard to their contribution to fundamental physics in the framework of the standard model of particle physics and more speculative future fundamental symmetry tests such as for the combined charge conjugation, parity, and time-reversal (CPT) symmetry violation.

Ten stationary points on the water dimer potential energy surface have been characterized with the coupled-cluster technique which includes all single and double excitations as well as a perturbative approximation of triple excitations [CCSD(T)]. Using a triple-ζ basis set with two sets of polarization functions augmented with higher angular momentum and diffuse functions [TZ2P(f,d)+dif], the fully optimized geometries and harmonic vibrational frequencies of these ten stationary points were determined at the CCSD(T) theoretical level. In agreement with other ab initio investigations, only one of these ten stationary points is a true minimum. Of the other nine structures, three are transition structures, and the remaining are higher order saddle points. These high-level ab initio results indicate that the lowest lying transition state involved in hydrogen interchange is chiral, of C1 symmetry rather than Cs as suggested by recently developed 6D potential energy surfaces. The one- and n-particle limits of the electronic energies of these ten stationary points were probed by systematic variation of the atomic orbital basis sets and the treatment of electron correlation within the framework of the focal-point analysis of Allen and co-workers. The one-particle limit was approached via extrapolation of electronic energies computed with the augmented correlation consistent basis sets (aug-cc-pVXZ, X=D−6), and, independently, by estimating the basis set incompleteness effect with the explicitly-correlated second-order Møller-Plesset method (MP2-R12). Electron correlation was evaluated at levels as high as the Brueckner coupled cluster method with double excitations and perturbatively treated triple and quadruple excitations [BD(TQ)]. Core correlation and relativistic effects were also assessed. Consideration of the aforementioned electronic effects as well as basis set superposition error leads to an estimate of 21.0 kJ mol−1 for the electronic dissociation energy of (H2O)2.

The rate of unimolecular processes is calculated by means of a statistical adiabatic channel model. The rates are mainly determined by the maxima of the channel energies. The channels are constructed by correlating reactant and product states; the channel energies are computed by a simple interpolation procedure. The coupling between the various vibrational‐rotational motions is taken into account. The influence of parameters of the potential surface on channel energies and on rate constants is studied. Numerical results for the NO2, CH4, CD4, C2H6 and C2D6 dissociation – recombination kinetics are compared to experimental data. The statistical adiabatic channel model gives similar results as minimum local entropy models. Both these approaches lead to stronger curvatures of k(E) and smaller activation energies for the thermal rate constant k∞ than the corresponding RRKM calculations.