Density functional theory study of (HCN) n clusters up to n = 10

Abstract

Hydrogen cyanide is an extraterrestrial material which is also recognized today as a plausible prebiotic molecule of major interest about the origins of life. This theoretical study is devoted to the quantum molecular modelling of HCN and (HCN) n clusters up to n = 10, using the density functional theory method BPW91 with the large basis set 6-311G++3D3P. We have obtained the optimized geometries of linear clusters up to decamer and of cyclic trimer and tetramer, with the help of the vibrational frequencies method. Electrostatic properties (dipole moments, tensor of polarizabilities, molecular electrostatic potential), thermochemistry and vibrational spectra were computed. For the linear clusters (HCN) n , the dipole moment and the polarizability show a linear variation versus n; this behaviour implies a linear variation of the infrared intensity versus the Raman activity for the CH and the CN stretchings. The thermochemistry at T = 298.15 K and P = 1 atm of the linear polymerization (addition of HCN molecules each to other through hydrogen bonds) indicates that the reaction is exothermic (Δ H < 0) but not spontaneous (Δ G > 0 and Δ S < 0). The pure rotational lines of HCN in the far infrared band, detected by the recent Cassini-Huygens mission to Titan are interpreted through our obtained rotational constant for the HCN monomer.