A pseudo-π analogue of a Mulliken bπ.aσ type complex: The rotational spectrum of cyclopropane–chlorine monofluoride

Abstract

The rotational spectrum of an asymmetric-rotor complex formed by cyclopropane with chlorine monofluoride has been observed by using a pulsed-nozzle, Fourier-transform microwave spectrometer. A fast-mixing nozzle was employed to preclude any chemical reaction between the components. Ground-state spectroscopic constants A ₀ , B ₀ , C ₀ , Δ _J , Δ _JK , δ _J , χ _aa (Cl), [χ _bb (Cl)-χ _cc (Cl)] and 12[M _bb (Cl)+M _cc (Cl)] were obtained by fitting observed transition frequencies for each of the two isotopomers (CH ₂ ) ₃ ··· ³⁵ ClF and (CH ₂ ) ₃ ··· ³⁷ ClF. A detailed interpretation of these constants leads to the conclusion that the three C nuclei, the Cl nucleus and the F nucleus are coplanar and that in the equilibrium conformation the axis of ClF coincides or nearly coincides with the a-axis of the complex, with Cl closer than F to the cyclopropane ring. The distance from the cyclopropane centre of mass to the Cl nucleus is 3.395(1) Å. Although the principal moments of inertia of the observed isotopomers of the complex are independent of the angle of rotation ϕ of the cyclopropane subunit about its local C ₃ axis, it is most likely that the a-axis coincides with a median of the C ₃ equilaterial triangle. In that case, the electrophilic end of ^δ+ ClF ^δ- interacts with the pseudo-π electron density of the C–C single bond, with r(*···Cl)=2.958(1) Å, where * defines the mid-point of the C–C bond. Comparison of the series B···HCl and B···ClF, where B is ethyne, ethene or cyclopropane, reveals that cyclopropane···HCl has an enhanced binding strength (as measured by the force constant k _σ ) relative to other members of the B···HCl series that results from a secondary interaction between the nucleophilic Cl atom of HCl and the four equivalent H atoms at the binding edge of the cyclopropane subunit. The corresponding H···F secondary interactions are negligible in cyclopropane···ClF.