The topic of my talk will concern molecules containing large amplitude motions and in particular molecules with one or two methyl (CH₃) internal rotors. Internal rotors are present everywhere in our environment, and they are important indicators of the physico-chemical conditions which exist in it. They are also excellent “sensors” for molecular structure determinations.

The high resolution microwave, millimeter and infrared spectra of those molecules cannot be treated by traditional Hamiltonian methods [1]. One of the goals of our work is to develop theoretical methods and codes to calculate the energy levels, and then to fit the observed line positions for this type of molecules. Following this strategy, we can provide reliable predictions of line positions and intensities for astrophysical molecules containing one internal rotor CH₃, such as methanol CH₃OH, methyl formate HCOOCH₃, acetic acid CH₃COOH, acetaldehyde CH₃CHO, acetamide CH₃CONH₂ or two-top molecules such as the methyl acetate molecule CH₃-O-C(=O)-CH₃ [2]. The major new facilities, i.e. the Atacama Large Millimeter Array (ALMA), in Chili, have opened the sub-millimeter region for astronomical observations. Molecules which undergo internal rotation of a methyl group present thousands of lines in this spectral range and therefore their spectra are particularly important to model. Several internal rotors of astrophysical interest will be presented as well as the latest results obtained with a newly written code to deal with one Large-Amplitude Rotatory Motion and one Large-Amplitude Oscillatory Motion such as in the excited states of methyl amine [3].

Another goal of our work is to acquire knowledge on structural properties of small organic molecules or biomimetic molecules. Fourier transform or “chirped pulse” microwave spectroscopy in the gas phase coupled with high level quantum chemical calculations has led to precise and detailed determinations of molecular structures for lowest energy conformers of a number of molecules. In this talk, I will show results for molecules with internal rotors, which are prototype for odorant molecules like linalool, an acyclic mono-terpene present in many plants and emitted in the earth atmosphere, important phytohormones produced by plants such as methyl jasmonate or bee pheromones like heptan-2-one. Recent results obtained on methyl and dimethyl derivatives of five or six-membered nitrogen aromatic rings of biological interest will be also presented.

This work has been supported by French National programs PCMI (Programme National de Physique Chimie du Milieu Interstellaire) and LEFE (Les Enveloppes Fluides et l'Environnement) of CNRS.

[1] C. Lin and J. D. Swalen, Rev. Mod. Phys., 1959, 31, 841-892.
[2] I. Kleiner, ACS Earth and Space Chemistry, 2019, 3, 1812-1842.
[3] I. Kleiner and J. T. Hougen, J. Mol. Spectrosc., 2020, 368, 111255.