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

Abstract

Ground-state rotational spectra of the three isotopomers H₃P···I³⁵Cl, H₃P···I ³⁷Cl and D₃P···I³⁵Cl of a symmetric-top complex formed between phosphine and iodine monochloride were observed by pulsed-nozzle, Fourier transform microwave spectroscopy. The spectroscopic constants B₀, D_J, D_JK, χ_aa(I), χ_aa(Cl), M_aa(I) and M_bb(I) were determined in each case. Changes in B₀ on isotopic substitution established that the nuclei lie in the order H₃P···ICl, with the distance r(P···I)=2.963(1) Å. The changes in the halogen nuclear quadrupole coupling constants χ_aa(I) and χ_aa(Cl) from their counterparts in the free ICl molecule led to the conclusion that a fraction δ_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 δ_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_σ is used as a criterion. It is found that the distance r(P···I) contracts by 0.017 Å between H₃P···I³⁵Cl and D₃P···I³⁵Cl. The systematic variation of δ_i within a series of complexes B···ICl is discussed.