Observation and assignment of the hot band ν2 + ν91 − ν91in (HCN)2

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 X2,9 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.