Parity Non-Conservation in Molecular Systems – NCPCHEM

The project, involving both chemists and physicists, theoreticians and experimentalists, aims at providing the first experimental observation of the energy difference between a chiral molecule and its mirror image due to the weak force. The energy difference is predicted by theory to show up as a minute frequency shift, on the order of less than a Hertz, in the vibrational spectra of chiral molecule. The total energy differences measured in a vibrational spectrum are typically at least thirteen order of magnitude larger, and so an experiment of extreme sensitivity is needed to detect such small frequency shifts. The laser experiment being developed is based on an interferometry technique called two-photon Ramsey fringes and depends crucially on intial spectroscopic characterization of candidate molecules by two teams specializing in infrared and microwave spectroscopy, respectively. The experiment was recenty listed by the «Nature« magazine as one of five experiments “as hard as finding the Higgs”.

The development of such a difficult experiment is a long process, but progress is now steady, and the first tests on an achiral training molecule are expected to take place within a year. A key challenge of the project is to find a suitable chiral molecule for the experiment. The chiral effect of the weak force increases significantly with nuclear charge. A team of synthetic chemists has therefore prepared chiral organometallic complexes of the heavy atom rhenium for which relativistic quantum chemical calculations indicate a sizeable effect. The present major concern is to find molecules of sufficient stability to be put into a molecule beam for the laser experiment.

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. If successful, the experiment would provide a low-energy route to explore the standard model of our universe, contrasting with high-energy particle accelerators.

The collaboration has so far led to nine publications in international peer-reviewd journals.

Main project partners: Trond Saue (UMR 7177 CNRS Strasbourg), Jeanne Crassous (UMR 6226 CNRS Rennes 1), Benoit Darquié (UMR7538 CNRS Paris 13), Pierre Asselin (UMR 7075 CNRS Paris 6) and Thérèse Huet (UMR7538 CNRS Lille 1).

Chirality is a fundamental concept in physics, chemistry and biology. For chemistry chirality is a challenge, notably in the development and synthesis of molecules for pharmaceuticals, agrochemicals, flavors and, more recently, nanotechnology. From physics it is known that the weak force does not conserve parity and thus induces an energy difference between enantiomers of opposite handedness. Such an energy difference, albeit tiny, could possibly be connected with the observation in biology of the overwhelming dominance of L-amino acids and D-sugars over their mirror images.
The main objective of the present project is to achieve the first experimental observation of parity violation in molecular systems, by detecting a frequency difference in the vibrational spectrum of left- and right-handed molecules induced by parity violation (PV). The project is a follow-up of the previous ANR project NCPMOL which led to the creation of a consortium of physicists and chemists, theoreticians and experimentalists, unique in France. We proposed a high-resolution laser experiment based on the Doppler-free two photon Ramsey fringes which we expect can attain a resolution of a few hundredths of a Hz in differential frequency measurements. A computational protocol for the screening of candidate molecules were developed based on 2-component relativistic density functional theory (DFT) calculations. Two families of chiral oxorhenium complexes were synthesized and characterized. Although we predict PV frequency shifts within experimental resolution for some of them, they had to be rejected, being either too bulky or not sublimating properly for injection into the molecular beam.
In the present project NCPCHEM emphasis is on chemistry. We will develop a new class of simple and compact chiral rhenium complexes starting from achiral methylrhenium trioxide (MTO), a molecule known from catalysis, and furthermore associate two experienced synthetic chemists, profs. Rémi Chauvin and Antoine Baceiredo (Toulouse), with the project for further impetus and ideas about candidate molecules and synthetic pathways. The computational protocol will be further developed by inclusion of conformational averaging and we will develop an alternative, much more expensive protocol, based on relativistic Coupled Cluster theory, for benchmark studies. This theoretical work will be carried out in collaboration with profs. Peter Schwerdtfeger (Auckland) and Lucas Visscher (Amsterdam). The candidate molecules will be characterized stereochemically by vibrational circular dichroism (VCD) spectroscopy. In collaboration with prof. Kenneth Ruud and dr. Radovan Bast (Tromsø) we will develop software for the simulation of VCD spectra at the relativistic level, thus allowing unambiguous determination of the absolute configuration of candidate molecules. In order to find transitions suitable for the high-resolution laser experiment the candidate molecules will be further characterized by microwave and infrared spectroscopy. In the present project the relevant project partners will improve the sensitivity of their spectrometers or the ability to work in different pressure conditions. Finally, for the high-resolution experiment we will use MTO as a test molecule for developing procedures for bringing heavy atom molecules into the supersonic beam from the solid state and for improving the sensitivity of the experiment.
A successful experiment would have a major impact in physics, chemistry and biology, in that it would enable low-energy tests of the weak interaction, shed light in the stability and dynamics of chiral molecules provide the condition sine qua non for establishing a link between the weak inteaction and biochirality.