A hydrogen bonded complex C2H2…HBr isolated and characterized in the gas phase using pulsed-jet, Fourier transform microwave spectroscopy

Abstract

A complex formed from ethyne and hydrogen bromide has been isolated and characterized by using a fast-mixing nozzle in conjunction with a pulsed-jet, Fourier transform microwave spectrometer. Any possible chemical reaction between the two components when mixed in the usual way was thereby precluded. The rotational constants A, B and C, the quartic centrifugal distortion constants δJ, δJk and δJ and the Br nuclear hyperfine coupling constants Xaa and Xbb—Xcc (nuclear quadrupole) and Mbb (spin-rotation) were determined for each of the five isotopomers C2H2…H79Br, C2H2…H81 Br, C2H2…D79Br, C2H2…D81Br and C2D2…H79Br. Interpretations of the spectroscopic constants show that the complex is planar and T shaped in the equilibrium conformation, with HCCH internuclear axis forming the cross of the T and the HBr internuclear axis lying along the C2 axis of C2H2. The H of HBr is closer to the centre (*) of the π bond of ethyne, and therefore HBr is involved in a hydrogen bond to the π system, in which the distance of H from * is r (*…H) = 2.469(1) A. The intermolecular stretching force constant is estimated as kσ = 5.38(2) Nm−1 for the species involving a hydrogen bond (C2H2…H79Br, C2H2…H81Br and C2D2…H79Br) while this quantity increases to kσ = 5.68(2)Nm−1 for those complexes bound through a deuterium bond (C2H2…D79Br and C2H2…D79Br). The opportunity is taken to consider similarities in the properties of complexes within the two series B…HBr and B…HC1 for a range of Lewis bases B, including B = C2H2. Some family relationships are identified in the two series.