Molecular supersonic jet studies of aniline solvation by helium and methane

Abstract

The technique of two color resonant two photon ionization coupled with time of flight mass spectroscopy has been employed to study aniline–He (AnHex) and aniline–CH4 [An(CH4)x] van der Waals clusters generated in a supersonic molecular jet. This technique allows identification of spectroscopic transitions with clusters of known mass because no ion fragmentation is observed. Specific features in the optical fluorescence excitation and dispersed emission spectra can thereby be uniquely identified with a particular cluster. Cluster vibrations can be analyzed by a Morse potential to yield the An–He bond dissociation energy D0∼100±50 cm−1. Careful analysis of the dispersed emission from AnHex suggests 145<D0<155 cm−1. It is found that the van der Waals bond stretching frequency is nearly the same in the ground and excited states and that there is a strong propensity rule for ΔV=0(V=vdW bond mode) as expected in this case, although ΔV=±1 transitions can be observed. The AnHe1 and AnHe2 origins are slightly red shifted with respect to the An origins, while the AnHex (x≥3) origin is broad and nearly unshifted. This pattern is followed for An(CH4)x clusters; AnCH4 transitions are red shifted 80 cm−1 from the comparable An features and An(CH4)2 transitions are 160 cm−1 below their comparable An feature. The An(CH4)x (x≥3) transitions appear at ∼200–300 cm−1 below their comparable An mode. The binding energy for the An–CH4 bond is found to be 500<D0<700 cm−1 in the 1B2 state of aniline. Aniline has a strong preference for binding the solvent above and below the aromatic ring. Since the D0 is large for An–CH4 and the stretching mode is only ∼25 cm−1 the An(CH4)x system builds up a large density of states in the van der Waals degrees of freedom. This density of states allows intramolecular vibrational redistribution (IVR) to take place, if the An mode excited is lower in energy than the D0 value. The rate of IVR from 6a1 (O00+500 cm−1) is somewhat faster than the 5 ns fluorescence rate but much slower than the rate of vibrational predissociation (VP) from higher levels. Both the IVR process, due to the van der Waals vibrational density of states, and the limiting solvent red shift, at a value similar to that found for cryogenic solutions, are discussed in terms of these clusters as model solute/solvent systems.