Laboratoire de Physique des Lasers, Atomes et Molécules

The accurate determination of the equilibrium structure of molecules by combining quantum chemistry calculations and spectroscopy is an historical activity of the team. Many results have been gathered in the book "Equilibrium Molecular Structures : From Spectroscopy to Quantum Chemistry”.

Experimentally, molecules or molecular complexes can be isolated and cooled down in an adiabatic expansion, then observed in the microwave and/or infrared regions. Such analyses applied to semi-rigid molecular systems allow for example :

• The unambiguous identification of the energetically most stable structures. Despite the increase in the number of possible structures with the size of the molecule, the nature does only retains a few of them, the most stable ones energetically.

Folded (left) and unfolded (right) structure of a small protected peptide : acylalanine

• The determination of the structure of the PAHs (polycyclic aromatic hydrocarbons) and their derivatives (nitrogen substituted, ions, protonated, etc.). The geometry is the basis element of the physicochemical properties of this important class of molecules (representing about 2/3 of the known chemical species). It is also essential to be able to model their spectra for eventual identification during environmental observation campaigns (atmospheric or interstellar).

Analysis of rotationally resolved infrared bands of PAHs

• The torsional dynamics of a molecular bond. The treatment of large amplitude motions remains a challenging issue in spectroscopy. However, as with the PAHs, it is essential to model their spectra in order to identify them during environmental observation campaigns.

Coupled large amplitude motions (methyl torsion) from structure calculations and microwave spectroscopy.

• Understanding the peculiar structure and dynamics of the hydrogen bond, whether intra- or inter-molecular. This kind of very floppy interaction cannot be treated as a standard covalent bond. It is necessary to understand these properties in order to take them into account in theoretical models.

Structure and vibrational dynamics of CH₃Cl-HCl hydrogen bond complex

• The analysis of prototypes molecules as part of the implementation of parity violation experimental evidence.

Recent publications :

• Equilibrium Molecular Structures : From Spectroscopy to Quantum Chemistry, Jean Demaison (Editor), James E. Boggs (Editor), Attila G. Csaszar (Editor), CRC Press 2010

• Coupled large amplitude motions : a case study of the dimethylbenzaldehyde isomers, M. Tudorie, I. Kleiner, M. Jahn, J.-U. Grabow, M. Goubet, O. Pirali, J. Phys. Chem. A 117 13636 (2013)

• Structural and Dynamic Properties of a Hydrogen Bond from the Study of the CH₃Cl-HCl Complex and Isotopic Species, M. Goubet, P. Asselin, P. Soulard and B. Madebène, J. Phys. Chem. A 117 12569 (2013)

• The far-infrared spectrum of azulene and isoquinoline and supporting anharmonic density functional theory calculations to high resolution spectroscopy of polycyclic aromatic hydrocarbons and derivatives, M. Goubet, O. Pirali, J. Chem. Phys. 140 044322 (2014)

Collaborations