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

Ground-state rotational spectra were observed for the four symmetric-top isotopomers H3P . . .(HCl)-Cl-35, H3P . . .(HCl)-Cl-37, H3P . . .(DCl)-Cl-35 and D3P . . .(HCl)-Cl-35 and the four asymmetric-rotor isotopomers H2DP . . .(HCl)-Cl-35, H2DP . . .(HCl)-Cl-37, HD2P . . .(DCl)-Cl-35 and H2DP . . .(DCl)-Cl-35 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) Angstrom for the three species based on PH3, this distance shortened to 2.5736(5) Angstrom in D3P . . .(HCl)-Cl-35. An alternative analysis indicated that r(P . . . Cl) shortened by 0.0014 Angstrom per D atom substituted for phosphine H atoms in H3P . . .(HCl)-Cl-35, 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(sigma) and k(theta theta), respectively. The latter also yielded an average value [theta (2)](1/2) of the PH3 oscillation angle theta, which decreased significantly for D3P . . .(HCl)-Cl-35 relative to the (constant) value for species based on PH3. After correction of the Cl nuclear quadrupole coupling constant of H3P . . .(HCl)-Cl-35 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 beta (av)=cos(-1)[co beta](1/2)=15 degrees for the oscillation angle of the HCl subunit.