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

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.