Van der Waals versus hydrogen-bonding in complexes of indole with argon, water, and benzene by mass-analyzed pulsed field threshold ionization

By using the technique of mass-analyzed threshold ionization spectroscopy, we were able to measure ionization energies of indole-Ar (62505 cm(-1)), indole-H2O (59433 cm(-1)), and the indole-benzene (59833 cm(-1)) complexes, as well as their dissociation energies in the cationic ground state. The dissociation energies in the neutral ground state are calculated from the experimental data. The ionic E-0 = 537 +/- 10 cm(-1) and neutral dissociation energy D-0 = 451 +/- 15 cm(-1) of the indole-Ar complex are much smaller than those of the indole-H2O complex; E-0 = 4790 +/- 10 cm(-1), D-0 = 1632 +/- 15 cm(-1) and of the indole-C6H6 complex: E-0 = 4581 +/- 10 cm(-1), D-0 = 1823 +/- 15 cm(-1). This demonstrates the van der Waals character of the indole-Ar complex and the hydrogen bonding type in indole-water. Furthermore, we conclude that the indole-benzene complex is hydrogen-bonded with the benzene pi-cloud serving as electron donator and indole serving as hydrogen donator.