Rotational spectroscopy of H3P center dot center dot center dot BrCl and the systematics of intermolecular electron transfer in the series B center dot center dot center dot BrCl, where B=CO, HCN, H2O, C2H2, C2H4, H2S, NH3, and PH3

Ground-state rotational spectra of the isotopomers H3P ...(BrCl)-Br-79-Cl-3 5, H3P ...(BrCl)-Br-81-Cl-35, H3P ...(BrCl)-Br-79-Cl-37, H3P ...(BrCl)-Br-8 1-Cl-37, D3P ...(BrCl)-Br-79-Cl-35, and D3P ...(BrCl)-Br-81-Cl-35, of the p hosphine-bromine monochloride complex were observed by the pulsed-jet, Four ier-transform method, incorporating a mixing nozzle to preclude reaction am ong the component gases. Each isotopomer exhibited a symmetric-top-type spe ctrum which yielded accurate values of the spectroscopic constants B-0, D-J , D-JK, chi(aa) (Br), chi(aa) (Cl), M-aa (Br), and M-bb (Br) on analysis. I nterpretations of the changes in the B-0 values with isotopomer showed that the intermolecular bond involves P and Br, with r(P ... Br)=2.869(1) Angst rom and that the BrCl bond increases in length by similar to 0.04 Angstrom on complex formation. Changes in the halogen nuclear quadrupole coupling co nstants when H3P ... BrCl is formed lead, with the aid of the Townes-Dailey model, to the conclusion that a fraction delta(i)=0.100(5) of an electron is transferred from P to Br on complex formation, while the polarization of BrCl by PH3 can be viewed as the transfer of 0.128(2)e from Br to Cl, lead ing to a net change of -0.028(5)e in the population of the 4p(z) orbital of Br. The complex is only of moderate strength, with an intermolecular stret ching force constant k(sigma)=11.5 Nm(-1). Values of delta(i), similarly de termined, for the series B ... BrCl, where B=CO, HCN, H2O, C2H2, C2H4, H2S, NH3, or PH3, are presented. It is shown that the variation of delta(i) wit h the ionization energy I-B of the Lewis base B can be described by an expr ession delta(i)=A exp(-bI(B)). This behavior is compared with that for the corresponding series B ... ICl. (C) 2000 American Institute of Physics. [S0 021-9606(00)01037-0].