Nonlinear hydrogen bonds of the type (CH2)2Z···HY: The rotational spectrum of a complex of methylenecyclopropane and hydrogen bromide

Abstract

The rotational spectra of the isotopomers mecp···H⁷⁹Br, mecp···H⁸¹Br and mecp···D⁷⁹Br (mecp=methylenecyclopropane) of a complex formed between mecp and hydrogen bromide were observed by using a pulsed-nozzle, Fourier-transform microwave spectrometer. Accurate values of the rotational constants A₀, B₀ and C₀, the centrifugal distortion constants Δ_J, Δ_JK and δ_J, and the components χ_aa, (χ_bb-χ_cc) and χ_ab of the Br nuclear quadrupole coupling tensor were determined in each case. These spectroscopic constants were interpreted to show that mecp···HBr has C_s symmetry, with the HBr subunit lying in the principal inertial plane ab, which coincides with the molecular symmetry plane. The H atom of HBr forms a hydrogen bond to the centre (*) of the mecp π bond. The angle φ included by the unique C atom of the cyclopropyl ring, the centre of the π bond and the H atom of HBr is 88.8(10)° and the distance r(*···H)=2.353(18) Å. The hydrogen bond *···H–Br deviates from linearity by ϑ=18.03(23)° in a direction that suggests a secondary interaction of ^δ-Br with two of the protons of the cyclopropyl ring. The properties φ, ϑ, r(Z···X) and r(H_ring···Y) of the complexes (CH₂)₂Z···XY, where Z=O, S or C2CH₂ and XY=HCl, HBr or ClF are compared and the reason why the hydrogen bonds Z···H–Y are significantly nonlinear, while the ‘chlorine’ bonds Z···Cl–F remain close to linearity, is considered.