Interaction of benzene and halogens in the gas-phase: rotational spectrum of C6H6···ClF

Abstract

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 (v=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 DJK, 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 v=1 state, the constant DJK and the centrifugal distortion terms χK and χD that affect the nuclear quadrupole hyperfine structure were a similar magnitude to those of the v=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 Ainside 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 v=0 and v=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 Awas also determined.