Interaction of benzene and halogens in the gas-phase: Rotational spectrum of C₆H₆⋯ClF

Ground-state rotational spectra of three isotopomers, C₆H₆⋯³⁵ClF, C₆H₆⋯³⁷ClF and C₆D₆⋯³⁵ClF, of a complex formed by benzene with chlorine monofluoride have been observed by pulsed-nozzle, Fourier-transform microwave spectroscopy. Chemical reaction between the two components was precluded by use of a fast-mixing nozzle. A vibrational satellite associated with a low lying state (υ = 1) was also observed for the C₆H₆⋯³⁵ClF and C₆D₆⋯³⁵ClF isotopomers. The spectra were established to be of the symmetric-top type, but with large centrifugal distortion effects associated both with the frequencies of the unperturbed line centres and with the Cl nuclear quadrupole hyperfine structure. In the ground state, these effects were reflected in a large centrifugal distortion constant D_JK, in higher order distortion constants of significant magnitude, and in a strong dependence of the Cl nuclear quadrupole coupling constant on the quantum number K. In the υ = 1 state, the constant D_JK and the centrifugal distortion terms _χK and _χD that affect the nuclear quadrupole hyperfine structure were a similar magnitude to those of the υ = 0 state, but of opposite sign. The nuclear quadrupole coupling constants, _χR, appropriate to the J = 0, K = 0 states, and the changes in the rotational constants B₀ with isotopic substitution, were interpreted in terms of a model for the complex in which, at equilibrium, the ClF axis is inclined at an angle θ ≈ 14° to the C₆ axis of benzene, with the two axes intersecting at the ClF centre of mass. The Cl atom lies closer to the benzene ring than does the F atom. The extrapolation of the ClF internuclear axis intersects the plane of the benzene ring at a point (*) that lies at a distance of ca. 0.24 Å inside the ring from the centre of a C-C bond. The conformation with θ = 0° (ClF axis and benzene C₆ axis coincident) corresponds to a potential energy maximum, concentric with which is an approximately circular valley corresponding to θ ≈ 14°. This type of potential energy surface is consistent with the symmetric-top nature of the observed spectra, with the relatively small energy separation of the υ ≈ 0 and 1 states, and with large, but oppositely signed, centrifugal distortion effects in the two states originating in a Coriolis interaction between them. The distance r(*⋯-Cl) = 3.313 Å was also determined.