Inter- and intramolecular electronic transfer on formation of H3P center dot center dot center dot ICl as determined by rotational spectroscopy

Ground-state rotational spectra of the three isotopomers H3P ... (ICl)-Cl-3 5, H3P ... (ICl)-Cl-37 and D3P ... (ICl)-Cl-35 of a symmetric-top complex f ormed between phosphine and iodine monochloride were observed by pulsed-noz zle, Fourier transform microwave spectroscopy. The spectroscopic constants B-0, D-J, D-JK, chi(aa)(I), chi(aa)(Cl), M-aa(I) and M-bb(I) were determine d in each case. Changes in B-0 on isotopic substitution established that th e nuclei lie in the order H3P ... ICl, with the distance r(P ... I)=2.963(1 ) Angstrom. The changes in the halogen nuclear quadrupole coupling constant s chi(aa)(I) and chi(aa)(Cl) from their counterparts in the free ICl molecu le led to the conclusion that a fraction delta(i) = 0.144(7) of an electron is transferred from P to I on complex formation and that the polarization of ICl by PH3 is equivalent to the intramolecular transfer of a fraction de lta(p)(Cl) = 0.157(2) from I to Cl. The complex is about as strongly bound as H3N ... HCl, when the intermolecular stretching force constant k(sigma) is used as a criterion. It is found that the distance r(P ... I) contracts by 0.017 Angstrom between H3P ... (ICl)-Cl-35 and D3P ... (ICl)-Cl-35. The systematic variation of delta(i) within a series of complexes B ... ICl is discussed.