A gas phase complex of acetylene and bromine: geometry, binding strength and charge transfer from rotational spectroscopy

Ground-state rotational spectra of a 1:1 complex of acetylene and bromine were observed with a Fourier-transform microwave spectrometer. Rotational, centrifugal distortion and Br nuclear hyperfine coupling constants were determined for the isotopomers C2H2⋯79Br79Br, C2H2⋯79Br81Br, C2H2⋯81Br79Br and C2H2⋯81Br81Br. The complex possesses a T-shaped, C2v geometry at equilibrium with a zero-point distance 3.144 Å between the centre (∗) of the CC bond and the inner bromine atom. The inter- and intra-molecular electron transfers on complex formation are δ(∗→Bri)e=0.010e and δ(Bri→Bro)e=0.037e, respectively, where i = inner and o = outer. The inter-molecular stretching force constant is kσ=7.80(6) N m−1.