We have calculated the potential surface with respect to the torsional angles of the methyl groups, the torsional energy level structure, and the infrared and overtone Raman intensities by ab initio methods using extended basis sets. The conclusions that were drawn from a correlation to the observed spectra are the following, (i) The modest basis set MP2/6-31G(d, p) yields a value of 238.32 cm⁻¹ for the v₁₆ gearing mode in reasonable agreement with the observed value of 241.0 cm⁻¹. Extension of the basis set to MP2/6-311G(df, p) marginally improves this value to 239.36 cm⁻¹. A somewhat larger basis set, RHF/6-311+G(3df, 3p) was needed for a satisfactory description of the components of the polarizability and the dipole moment. (ii) The technique of full relaxation of the coordinates of the top and the frame was unable to account for the strong resonance between the 2v₁₂ overtone gearing level and the v₇ COC in-plane bending mode. (iii) The asymmetry along the diagonals of the potential surface that determines the sign and magnitude of the sine.sine gearing term was found to display complex behaviour and was positive in the region of the eclipsed-eclipsed equilibrium conformation and negative for the staggered–staggered conformation.

The density functional theory (DFT) is applied to theoretically study the capture and storage of three different nitro polycyclic aromatic hydrocarbons, 4-nitrophenol, 2-nitrophenol, and 9-nitroanthracene by activated carbon, with and without the presence of water. These species are pollutants derived from vehicle and industry emissions. The modeling of adsorption is carried out at the molecular level using a high-level density functional theory with the B3LYP-GD(BJ)/6-31+G(d,p) level of theory. The adsorption energies of polluting gases considered isolated and in a humid environment are compared to better understand the role of water. The calculations reveal different possible pathways involving the formation of chemical bonds between adsorbent and adsorbate on the formation of intermolecular van der Waals interactions. The negative adsorption energy on AC for the three species is obtained when they are treated individually and in mixture with H₂O. The basis-set superposition error, estimated using the counterpoise correction, varies the adsorption energies by 2-13%. Dispersion effects were also taken into account. The adsorption energy ranges from -10 to -414 kJ/mol suggesting a diversity of pathways. The resulting analysis suggests three preferred pathways for capture. The main pathway is physical interaction due to π-π stacking. Other means are capture due to the formation of hydrogen bonds resulting from water adsorbed on the surface and the simultaneous adsorption of pollutant and water where water can act as a link that promotes adsorption. The thermodynamic properties give a clue to the most eco-friendly approaches for molecular adsorption.

The photochemistry of the ethyl radical following excitation to the 3p Rydberg state is investigated in a joint experimental and theoretical study. Velocity map images for hydrogen atoms detected from photoexcited isotopologues CH₃CH₂, CH₃CD₂ and CD₃CH₂ at ∼201 nm, are discussed along with high-level ab initio electronic structure calculations of potential energy curves and non-adiabatic coupling matrix elements (NACME). A novel mechanism governed by a conical intersection allowing prompt site-specific hydrogen-atom elimination is presented and discussed. For this mechanism to occur, an initial ro-vibrational excitation is allocated to the radical permitting to access this reaction pathway and thus to control the ethyl photochemistry. While hydrogen-atom elimination from cold ethyl radicals occurs through internal conversion into lower electronic states followed by slow statistical dissociation, prompt site-specific C_α elimination into CH₃CH + H, occurring through a fast non-adiabatic crossing to a valence bound state followed by dissociation through a conical intersection, is accessed by means of an initial ro-vibrational energy content into the radical. The role of a particularly effective vibrational promoting mode in this prompt photochemical reaction pathway is discussed.

The far infrared spectra of three deuterated isotopologues of ethylene glycol, CH₂OD–CH₂OD, CH₂OD–CH₂OH and CH₂OH–CH₂OD, where the latter two species differ in their intramolecular hydrogen-bonding arrangement, are studied using highly correlated ab initio methods, vibrational second order perturbation theory and a variational procedure of reduced dimensionality. New subroutines suitable for the study of large systems with more than two interacting large amplitude motions were implemented and applied. The molecular symmetry of ethylene glycol decays by the formation of weak intramolecular bonds producing very asymmetrical stable structures. Three internal rotations contribute to the formation of a very anisotropic potential energy surface and to the puzzling distribution of the rovibrational energy levels. The ground vibrational state rotational constants and the centrifugal distortion constants (S-reduction, I^r representation) corresponding to the aGg′ (G1) and gGg′ (G2) forms are provided for the studied isotopologues. The low-lying vibrational levels up to 550 cm⁻¹ are obtained variationally for J = 0. Two series of sublevels of the ground vibrational state are obtained: eight components localized in G1 lying between 0.0 and 0.3 cm⁻¹ and eight sublevels localized in G2 lying between 138.1 and 138.4 cm⁻¹. The gap between both sets is lower in CH₂OD–CH₂OD and more dispersed in the monodeuterated variety.