Benchmark, DFT assessments, cooperativity, and energy decomposition analysis of the hydrogen bonds in HCN/HNC oligomeric complexes.

Hydrogen cyanide (HCN) and its tautomer hydrogen isocyanide (HNC) are relevant for extraterrestrial chemistry and possible relation to the origin of biomolecules. Several processes and reactions involving these molecules depend on their intermolecular interactions that can lead to aggregates and liquids especially due to the hydrogen bonds. It is thus important to comprehend, to describe, and to quantify their hydrogen bonds, mainly their nature and the cooperativity effects. A systematic study of all linear complexes up to pentamers of HCN and HNC is presented. CCSD(T)/CBS energy calculations, with and without basis set superposition error (BSSE) corrections for energies and geometries, provided a suitable set of benchmarks. Approximated methods based on the density functional theory (DFT) such as BP86, PBE, TPSS, B3LYP, CAM-B3LYP with and without dispersion corrections and long-range corrections, were assessed to describe the interaction energies and cooperativity effects. These assessments are relevant to select DFT functionals for liquid simulations. Energy decomposition analysis was performed at the PBE/STO-TZ2P level and provided insights into the nature of the hydrogen bonds, which are dominated by the electrostatic component. In addition, several linear relationships between the various energy components and the interaction energy were obtained. The cooperativity energy was also found to be practically linear with respect to the interaction energy, which may be relevant for designing and/or correcting empirical force fields. Graphical Abstract Hydrogen bonds in HCN/HNC oligomeric complexesᅟ.