Multiphoton ionization studies of clusters of immiscible liquids. I. C6H6–(H2O)n, n=1,2

Abstract

Resonant two-photon ionization (R2PI) time-of-flight mass spectroscopy is used to record S0–S1 spectra of the neutral complexes C6H6–H2O, C6H6 –HDO, C6H6–D2O, C6H6–(H2O)2, and C6H6–(D2O)2. In C6H6–H2O, the lack of an S0–S1 origin transition and the presence of a splitting at 610 (which is absent in C6H6 –HDO) provide vibronic level evidence that the water molecule is on the sixfold axis undergoing internal rotation about that axis. Rotational band contour analysis of the 610 transitions of the isotopomers confirms this picture and also determines a ground state center-of-mass separation between C6H6 and D2O of 3.32±0.07 Å, very close to that predicted by ab initio calculations. R2PI scans of the van der Waals structure in the isotopic series C6H6–H2O, C6H6 –HDO, and C6H6–D2O provide tentative assignments for three of the six van der Waals modes in the complex. In C6H6–(H2O)2, rotational band contour analysis of the origin transition provides a best-fit structure in which the two water molecules reside on the same side of the benzene ring at a H2O–H2O separation close to that in the free water dimer. Qualitatively, the structure of the 1:2 cluster is thus one which maximizes the strength of the water–water hydrogen bond at the expense of a somewhat poorer interaction of the second water molecule with the benzene ring in an off-axis geometry. Several intriguing features of the structure are suggested by our analysis, but are near the limit of our ability to distinguish from band contour fitting. Among these features are (i) the on-axis water molecule is pulled slightly in toward the ring from that in the 1:1 complex; (ii) the water dimer prefers an orientation bisecting a C–C bond in the benzene ring; (iii) the water–water separation is ∼0.2 Å less than that in the free water dimer; and (iv) the water dimer axis is tilted by about 10° relative to the plane of the benzene ring. Finally, the van der Waals structure in C6H6–(H2O)2 and C6H6–(D2O)2 suggests the possibility of large amplitude motion in these complexes as well. We postulate that this motion involves a hindered rotation of the on-axis water molecule.