Observation and assignment of the hot band ν2 + ν91 − ν91 in (HCN)2

Abstract

The high-resolution infrared absorption spectrum for (HCN)₂ in the spectral region 3100–3300 cm⁻¹ is recorded in a static multipass cell at 240 K under equilibrium conditions. The spectrum shows extensive rovibrational structures which are due to the bound C–H stretching vibration band ν₂ of (HCN)₂ and several hot bands associated to ν₂. We manage to assign 63 new rovibrational transitions [P(97) to R(98)] associated with the ν₂ band leading to an extended analysis of the ν₂ band. This enables the assignment of 60 rovibrational transitions associated with the hot band ν₂ + ν₉¹ − ν₉¹ in (HCN)₂ together with the first rovibrational analysis of this hot band. The determined positive value of the anharmonicity constant X_2,9 of 0.2569(13) cm⁻¹ indicates that the intermolecular low-frequency bending vibration ν₉¹ increases in frequency upon simultaneous excitation of the ν₂ mode. This result confirms that the HCN dimer is stiffer and more tightly bound upon vibrational excitation of the bound C–H stretching mode.