Low-J rotational spectra, internal rotation, and structures of several benzene–water dimers

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Abstract
Low J (0–4) rotational transitions have been observed for the benzene–water dimer of which high J (≥4) transitions were reported recently by Blake [Science 257, 942 (1992)]. Our experiments used a modified Balle/Flygare Fourier transform microwave spectrometer, with a pulsed supersonic nozzle as the sample source, and examined a variety of isotopic species in the ground and first excited internal rotor states (m=0 and 1). The dimers of the parent C6H6 benzene with H2O, HDO, D2O, and H218O have symmetric top spectra characteristic of two coaxial rotors with a symmetric top frame and a very low effective V6 barrier. The dimers of H2O and D2O with the 13C and D monosubstituted benzenes have asymmetric top spectra of which only the m=0 state was assigned. However, doublets in the m=1, J=0→1 transitions show that there is a V2 term of ∼0.5 MHz in their barriers. A substitution analysis was made of the rotational constants found for the m=0 state of the dimers with H218O, D2O, and the 13C and D monosubstituted benzenes. It shows that the oxygen is at the a axis of the dimer, well outside (0.48 Å) the hydrogens. However, the C2 axis of the H2O is not coincident with the a axis but is at an angle β of 37° to it, rotated so that the two hydrogens are equivalent. The sixfold axis of the benzene corresponds to the a axis, there is little or no tilt (γ) of the benzene. The c.m. (C6H6) to c.m. (H2O) distance R is 3.329 Å. The closely spaced hyperfine structure from the proton–proton magnetic dipole interaction and the deuterium quadrupole interaction was resolved for several dimers and transitions, principally J=0→1 and 1→2. The results demonstrate effective nuclear equivalence in dimers with H2O and D2O. Also, the symmetries found for their nuclear spin functions correlate with the lowest rotational levels of free water, the m=0 state with 000 and m=1 with 101 and 111. For the m=1, K=0 transitions of C6H6–H2O the correlation is with 111 and for the K=±1, with 101. These assignments are reversed for C6H6–D2O.