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

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 (ΔH < 0) but not spontaneous (ΔG > 0 and ΔS < 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.