Structural and electronic properties of the charge-transfer complex H3P center dot center dot center dot Br-2 determined by Fourier transform microwave spectroscopy

The ground-state rotational spectra of six isotopomers of the symmetric-top complex H3P ... Br-2 have been measured by the technique of pulsed-nozzle. Fourier transform microwave spectroscopy. The spectroscopic constants B-0, D-J, D-JK, chi (aa)(Br-x) and M-bb(Br-x), x=i (inner) or o (outer) bromine atom, were obtained from analysis of the spectra. Interpretation of these constants with the aid of models revealed that the pre-reactive complex has an intermolecular bond of length r(P ... Br) = 3.0440(4) Angstrom between the P atom of PH3 and one Br atom of Br, and that this bond is a relatively strong one, as measured by the intermolecular stretching force constant k( o) = 9.8 Nm(-1) The complex was discovered to have a significant contributi on from charge transfer in the ground state by establishing the fraction of intermolecular charge transferred from P to Br-i [delta (i) = 0.077(23)] a nd the fraction of intramolecular charge transferred from Br, to Br, [delta (Br)(P) = 0.11(1)].