ncpchem

A project funded by The French National Research Agency for the period 20/12/2010 - 19/12/2014.

The main objective of the present proposal is to observe the effect of parity violation in the vibrational spectra of chiral molecules by high resolution laser spectroscopy.

Chirality is a fundamental concept in physics, chemistry and biology. In physics objects of opposite chirality are connected by the parity operation (P), which combines with charge conjugation (C) and time reversal (T) in the CPT theorem: a mirror universe, where all particle positions are reflected about some plane (parity inversion), where all particles are replaced by their anti-particles (charge conjugation) and where all their momenta reversed (time reversal), will evolve according to the same physical laws as the present universe. Although the theorem states that the combined CPT symmetry is conserved by all physical processes and by the four fundamental forces (gravitation, the electromagnetic force, the strong and the weak force), individual symmetries may be broken. Of interest here is that parity may not be conserved in processes involving the weak force, as suggested by Lee and Yang in 1956 and soon after confirmed by experiment.

The weak interaction is also at play in molecular systems, notably in the interaction between electrons and nuclei, and the standard model therefore predicts a tiny energy difference between enantiomers of chiral molecules. In biology chirality is a hallmark of life in that Nature shows, with very few exceptions, a distinct preference for L-amino acids and D-sugars over their mirror images. A chiral host molecule displays typically a different response to the two enantiomers of a chiral guest molecule. For instance, the (S)- and (L) -forms of limonene smell of lemons and oranges, respectively. More tragically, this was demonstrated by the administration of thalidomide to pregnant women in the 1960s. The (R)-form has the desired sedative properties, whereas the (S)-form led to birth defects and one of the biggest medical tragedies of modern times. The origin of biochirality remains a mystery and was listed by the special 2006 125th anniversary issue of Science as one of the great unanswered questions in science. For chemistry chirality is a challenge, notably in the development and synthesis of molecules for pharmaceuticals, agrochemicals, flavors and, more recently, nanotechnology. The development of catalysts for stereoselective synthesis is one of the most important tasks of modern chemistry, as demonstrated by the award of the Nobel Prize in Chemistry for 2001 to William S. Knowles, Ryoji Noyori and K. Barry Sharpless for the development of catalytic asymmetric synthesis.

The present project addresses a more fundamental aspect of chirality, namely it aims at providing the first experimental observation of the energy difference between enantiomers of chiral molecules due to parity violation (PV). A successful experiment will have a significant impact on our understanding of the stability and dynamics of chiral molecules as well as the origin of biochirality. Such a project is inherently multidisciplinary and is expected to lead to important advances in spectroscopy, chemical synthesis and quantum chemical methods.

A number of experimental techniques have been proposed for the observation of parity non-conservation (PNC) in molecular systems, including vibrational-rotational, electronic , Mössbauer and NMR spectroscopy, as well as crystallization and solubility experiments, but no unambiguous observation has so far been made. Theory plays a very important role in this field since it can provide predictions of PNC effects and thus guide research. In 1974 Bouchiat and Bouchiat predicted that PNC effects should scale as Z^{3} with respect to nuclear charge Z in atomic systems and suggested to search for such effects in highly forbidden transitions of atomic spectra. Such effects were indeed observed soon thereafter [Sapirstein:Parity-violation]. In molecular systems the situation is even more favorable with a Z^{5} scaling law, although additional complications are introduced by the nuclear degrees of freedom. There has therefore been a quite significant activity in the quantum chemical community in the past years with important contributions from the groups of M. Quack, R. Berger, P. Lazzeretti, P. Schwerdtfeger, P. Manninen and T. Saue that have been summarized in recent reviews.