Geometry and binding strength of a pi-type hydrogen-bonded complex of ethene and hydrogen bromide determined by rotational spectroscopy

The ground-state rotational spectra of the four isotopomers C2H4... (HBr)-B r-79, C2H4... (HBr)-Br-81, C2H4... (DBr)-Br-79 and C2H4... (DBr)-Br-81 of a n asymmetric-top complex formed by ethene with hydrogen bromide were observ ed by pulsed-nozzle, Fourier-transform, microwave spectroscopy. A fast-mixi ng nozzle was used to preclude any reaction induced by formation of Br atom s on vessel walls. Large Br nuclear quadrupole hyperfine splittings combine d with HBr subunit angular oscillations of wide amplitude allowed the detec tion of centrifugal distortion effects on the hyperfine patterns, These eff ects were manifest in a dependence of chi(aa) on K-1 through a term chi(K)K (-1)(2) and in an additional term chi(d) K--1(2) (4 K--1(2)-1)/{J(J+ 1)} in the quadrupole energy expression. The set of spectroscopic constants A(0), B-0, C-0, Delta(J), Delta(JK), delta(J), chi(aa), %bb-chi(cc), chi(K), chi (d), M-aa and M-bb = M-cc was determined for each isotopomer investigated. Detailed analyses of various spectroscopic constants showed that, in the C2 H4... HBr complex, the HBr subunit forms a weak hydrogen bond to the centre of the pi-bond of ethene, as characterised by an intermolecular stretching force constant k(sigma) = 5.2 Nm(-1). In the equilibrium form of the compl ex, the HBr subunit therefore lies along the C-2 axis of ethene that is per pendicular to the nuclear plane. In the zero-point state, the distance from the centre (*) of the pi bond to Br is r(*... Br) = 3.916 (4) Angstrom. Sy stematic behaviour of distances r(Z ... X), where Z is the electron donor a tom or centre of B, and of k(sigma) values in the series B ... HX/B ... XY, where X = Br or Cl and Y = Cl, is discussed. (C) 2000 Elsevier Science S.A . All rights reserved.