We present an on-the-fly ab initio semiclassical study of vibrational energy levels of glycine, calculated by Fourier transform of the wavepacket correlation function. It is based on a multiple coherent states approach integrated with monodromy matrix regularization for chaotic dynamics. All four lowest-energy glycine conformers are investigated by means of single-trajectory semiclassical spectra obtained upon classical evolution of on-the-fly trajectories with harmonic zero-point energy. For the most stable conformer I, direct dynamics trajectories are also run for each vibrational mode with energy equal to the first harmonic excitation. An analysis of trajectories evolved up to 50 000 atomic time units demonstrates that, in this time span, conformers II and III can be considered as isolated species, while conformers I and IV show a pretty facile interconversion. Therefore, previous perturbative studies based on the assumption of isolated conformers are often reliable but might be not completely appropriate in the case of conformer IV and conformer I for which interconversion occurs promptly.

Halogen bonding is a noncovalent interaction between a halogen atom and a nucleophilic site. Interactions involving the π electrons of aromatic rings have received, up to now, little attention, despite the large number of systems in which they are present. We report binding energies of the interaction between either NCX or PhX (X = F, Cl, Br, I) and the aromatic benzene system as determined with the coupled cluster with perturbative triple excitations method [CCSD(T)] extrapolated at the complete basis set limit. Results are compared with those obtained by Møller-Plesset perturbation theory to second order (MP2) and density functional theory (DFT) calculations by using some of the most common functionals. Results show the important role of DFT in studying this interaction.

Quantum mechanical simulations unequivocally explain experimental IR spectra of protonated supramolecular systems.