Identification and characterization of a gas-phase complex of methylenecyclopropane and chlorine monofluoride by rotational spectroscopy

Abstract

The rotational spectrum of a complex formed between methylenecyclopropane (mecp) and chlorine monofluoride was observed by using a fast-mixing nozzle in combination with a Balle–Flygare microwave spectrometer. Rotational constants A₀, B₀ and C₀, quartic centrifugal distortion constants Δ_J, Δ_JK, δ_J and δ_K, and the components χ_aa, χ_bb − χ_cc and χ_ab of the Cl nuclear quadrupole coupling tensor were evaluated for each of the isotopomers mecp···³⁵ClF and mecp···³⁷ClF. Interpretations of the various spectroscopic constants demonstrated that the observed species is a Mulliken outer complex which may be described formally as of the bπ.aσ type, i.e. with the electrophilic end ^δ+Cl of ClF interacting weakly with the π bond of mecp and with only minor electric charge redistribution within the ClF subunit. The geometry of the complex was established to be of C_s symmetry. The ClF molecule lies in the principal inertial plane ab, which is the molecular symmetry plane and is perpendicular to the plane of the four carbon atoms of mecp. The angle ϕ, subtended at the centre (*) of the π bond by the unique carbon atom of the mecp ring and the Cl atom, is 92.5(5)° and the distance r(*···Cl) = 2.675(10) Å. The system *···Cl–F deviates from collinearity by 4.9(1)°. These properties of mecp···ClF are discussed in the context of two different series of B···ClF complexes. In the first, the molecules B are the prototype π or pseudo-π electron donors ethene, cyclopropane, allene, mecp and buta-1,3-diene. The molecules B in the second series are the three-membered rings (CH₂)₂Y, where Y is either one of the n-pair donors O or S or the π donor C=CH₂.