Observation and rovibrational analysis of the ν2 band of HCN-H35Cl

The high-resolution infrared absorption spectrum of an equilibrium mixture of HCN and HCl in a static gas long-path absorption cell is recorded in the 2500-2900 cm−1 spectral region at 205 K. The spectrum shows rovibrational structure which has the typical appearance of a parallel band of a linear molecule and is assigned to the intramolecular H-Cl stretching vibration band ν2 of the linear HCN-H35Cl heterodimer. The rovibrational analysis of the band yield a band origin ν0 of 2779.0968(12) cm−1 together with a value for the upper-state rotational constant B′ of 0.067722(2) cm−1. The observed red shift of 107 cm−1 for the ν2 band of HCN-H35Cl relative to the H-Cl stretching vibration band of monomer H35Cl is in excellent agreement with results from the MP2/6−311++G** level of theory. The value of the upper-state rotational constant shows that the intermolecular hydrogen bond shortens by 0.022 Å upon intramolecular vibrational excitation of the ν2 mode.