The topic of my talk will concern 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 methodsC. Lin and J. D. Swalen, Rev. Mod. Phys., 1959, 31, 841-892. Dedicated theoretical methods and codes have been developed to calculate the energy levels, and then to fit the observed line positions for internal rotors. First I will briefly review those approaches. Following this strategy reliable predictions of line positions and intensities for astrophysical molecules containing one internal rotor CH₃ or two-top molecules can be providedI. Kleiner, ACS Earth and Space Chemistry, 2019, 3, 1812-1842. I will present several internal rotors of interstellar interest as well as the latest results obtained with a code dealing with one Large-Amplitude Rotatory Motion and one Large-Amplitude Oscillatory MotionI. Kleiner and J. T. Hougen, J. Mol. Spectrosc., 2020, 368, 111255. Internal rotation can be also used to acquire knowledge on structural properties for small organic molecules or biomimetic molecules, which can serve as benchmark, and be compared to quantum chemical calculations. In this talk, I will show results for internal rotors, which are prototype for odorant molecules, phytohormones or bee pheromones. Recent results obtained on methyl and dimethyl derivatives of five or six-membered nitrogen aromatic rings of biological interest will be also presentedThis 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 C. Lin and J. D. Swalen, Rev. Mod. Phys., 1959, 31, 841-892.. I. Kleiner, ACS Earth and Space Chemistry, 2019, 3, 1812-1842.. I. Kleiner and J. T. Hougen, J. Mol. Spectrosc., 2020, 368, 111255.. 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.

The coupling between ambient ionization sources, developed for mass spectrometric analysis of biomolecules, and cryogenic ion processing, originally designed to study interstellar chemistry, creates a new and general way to capture transient chemical species and elucidate their structures with optical spectroscopies. Advances in non-linear optics over the past decade allow single-investigator, table top lasers to access radiation from 550 cm-1 in the infrared to the vacuum ultraviolet. When spectra are acquired using predissociation of weakly bound rare gas “tags,” the resulting patterns are directly equivalent to absorption spectra of target ions at temperatures below 10 K, and quenched close to their global minimum energy geometries. Taken together, what emerges is a new and powerful structural capability that augments the traditional tools available in high resolution mass spectrometry. Currently, these methods are being exploited to monitor chemical and physical processes in assemblies with well-defined temperatures and compositions. Recent applications, ranging from the mechanisms of small molecule activation by homogeneous catalysts to the microscopic mechanics underlying the ultrafast spectral diffusion in water, emphasize the generality and utility of the methods in contemporary chemistry.

Each of the six C,N,O diatomic molecules has a unique role in shaping our intuitive understanding of electronic structure theory. In this work, the pathologically pervasive configuration interactions that occur in four electronic symmetry manifolds (¹Π_g, ³Π_g, ¹Σ_u⁺, and ³Σ_u⁺) of the C₂ molecule are disentangled by a global multi-state diabatization scheme. The key concept of our model is the existence of two “valence-hole” configurations, 2σ_g²2σ_u¹2π_u³3σ_g² (^1,3Π_g) and 2σ_g²2σ_u¹2π_u⁴3σ_g¹ (^1,3Σ_u⁺) that derive from a 3σ_g←2σ_u electron promotion. The lowest energy state from each of the four C₂ symmetry species is dominated by this type of valence-hole configuration at its equilibrium internuclear separation. These valence-hole configurations have a nominal bond order of 3 and correlate with the 2s²2p²+2s2p³ separated-atom configurations. Facilitated by chemical intuition, the diabatic picture uncovers the disruptive impact of the valence-hole configurations on the global electronic structure and unimolecular dynamics of C₂. In each of the four symmetry manifolds studied in this work, the strongly-bound diabatic valence-hole state, the energy of which starts low and ends high, crosses multiple weakly-bound and repulsive states that are composed of electron configurations with a 2σ_g²2σ_u² valence-core. These diabatic crossings result in an extensive, interconnected network of avoided-crossings among the low-lying electronic states of C₂. The C₂ molecule behaves “badly”, yet its secrets are revealed by diabatic modeling of their lumpy adiabatic potentials and broken spectroscopic patterns. Based on our demonstration of the importance of valence-hole configurations in C₂, we propose a diabatic model re-analysis of similar interactions in the other second-row diatomic molecules, for which the valence-hole states are expected to have a similar impact on their global electronic structure.

I will give an overview about our work on development of novel computational methods for spectroscopy with emphasis on dynamic methods and approaches for the condensed phase. I will describe the efficient calculation of Infrared spectra for periodic systems using subsystem density functional theory (DFT) as well as Raman and sum frequency generation spectra by means of DFT-based molecular dynamics. This has allowed a realistic description of (large) compounds including finite temperature and environmental effects. Moreover, pioneering Raman optical activity spectra for the investigation of chiral compounds using DFT-based molecular dynamics have been presented and a novel approach for vibrational circular dichroism. In addition, I will show our developments for excited state dynamics and using real time propagation for the study of absorption and vibrational spectra for (chiral) compounds in the gas and condensed phase.