The evidence of the mode selectivity of the infrared predissociation reaction of the hydrogen bonds in the aniline–water–pyrrole cluster cation

The infrared spectrum of aniline–water–pyrrole cluster cation has been measured in the 3 μm region. Four strong vibrational bands were observed at 3313, 3515, 3637, and 3721 cm−1, and the cluster dissociated into fragments in two reaction paths, AWP+→AW++P and AWP+→AP++W (A: aniline, W: water, P: pyrrole) when it absorbed an infrared photon. The branching ratio (AW+/AP+) has been determined to be 0.20±0.02 for the NH stretching vibration of aniline (3313 cm−1, hydrogen-bonded to pyrrole) and 0.14±0.03 for the other three bands. This vibrational mode dependence should be due to the fast energy transfer from the NH to the intermolecular vibration through the hydrogen bond.