Observation and assignment of the hot band nu(2)+nu(1)(9)-nu(1)(9) in (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 nu(2) of (HCN)(2) and several hot bands associated to nu(2). We manage to assign 63 new rovibrational transitions [P(97) to R(98)] associated with the nu(2) band leading to an extended analysis of the nu(2) band. This enables the assignment of 60 rovibrational transitions associated with the hot band nu(2) + nu(9)(1) - nu(9)(1) in (HCN)(2) together with the first rovibrational analysis of this hot band. The determined positive value of the anharmonicity constant X-2,X-9 of 0.2569(13) cm(-1) indicates that the intermolecular low-frequency bending vibration nu(9)(1) increases in frequency upon simultaneous excitation of the nu(2) mode. This result confirms that the HCN dimer is stiffer and more tightly bound upon vibrational excitation of the bound C-H stretching mode.