The hydrogen-bonded complex H3P···HCl investigated by a combination of rotational spectroscopy and ab initio SCF calculations

Abstract

Ground-state rotational spectra were observed for the four symmetric-top isotopomers H₃P···H³⁵Cl, H₃P···H³⁷Cl, H₃P···D³⁵Cl and D₃P···H³⁵Cl and the four asymmetric-rotor isotopomers H₂DP···H³⁵Cl, H₂DP···H³⁷Cl, HD₂P···D³⁵Cl and H₂DP···D³⁵Cl of a complex formed in mixtures of phosphine and hydrogen chloride. Rotational constants, centrifugal distortion constants, Cl nuclear quadrupole coupling constants and spin–rotation coupling constants were determined through spectral analyses. Interpretation of various of the spectroscopic constants with the aid of models of the complex provided information about its symmetry and geometry, about the strength of the intermolecular binding and about the subunit dynamics. The complex has C_3v symmetry and the order of the nuclei is unambiguously established as H₃P···HCl. Ab initio calculations carried out with three different basis sets [6-311++G(d,p), aug-cc-pVDZ, and aug-cc-pVTZ] and with electron correlation taken into account at the MP2 level were used to optimise the geometry under the assumption of C_3v symmetry and indicated that only small geometrical distortions of the PH₃ and HCl subunits accompany formation of the complex. By assuming that similar distortions are also appropriate to experimental geometries of the subunits, the rotational constants of the symmetric-top isotopomers of the complex were used to establish the distances r(P···H) and r(P···Cl). Although r(P···H) was constant at 2.5802(2) Å for the three species based on PH₃, this distance shortened to 2.5736(5) Å in D₃P···H³⁵Cl. An alternative analysis indicated that r(P···Cl) shortened by 0.0014 Å per D atom substituted for phosphine H atoms in H₃P···H³⁵Cl, an effect attributed to an increased magnitude of the electric dipole moment of PH₃ as it is deuteriated. The centrifugal distortion constants D_J and D = 2D_J+D_JK were used to determine values of the quadratic intermolecular stretching and bending force constants k_σ and k_θθ, respectively. The latter also yielded an average value 〈θ²〉^1/2 of the PH₃ oscillation angle θ, which decreased significantly for D₃P···H³⁵Cl relative to the (constant) value for species based on PH₃. After correction of the Cl nuclear quadrupole coupling constant of H₃P···H³⁵Cl for the effects of the electric field gradient due to PH₃ by using estimates from the MP2/aug-cc-pVTZ calculation, the corrected value was used to give the average β_av = cos⁻¹〈cos²β〉^1/2 = 15° for the oscillation angle of the HCl subunit.