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

Ground-state rotational spectra were observed for the four symmetric-top is otopomers 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 chlori de. Rotational constants, centrifugal distortion constants, Cl nuclear quad rupole coupling constants and spin-rotation coupling constants were determi ned through spectral analyses. Interpretation of various of the spectroscop ic constants with the aid of models of the complex provided information abo ut its symmetry and geometry, about the strength of the intermolecular bind ing and about the subunit dynamics. The complex has C-3v symmetry and the o rder of the nuclei is unambiguously established as H3P . . . HCl. Ab initio calculations carried out with three different basis sets [6-311++G(d,p), a ug-cc-pVDZ, and aug-cc-pVTZ] and with electron correlation taken into accou nt 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 th at similar distortions are also appropriate to experimental geometries of t he subunits, the rotational constants of the symmetric-top isotopomers of t he complex were used to establish the distances r(P . . .H) and r(P . . . C l). Although r(P . . .H) was constant at 2.5802(2) Angstrom for the three s pecies based on PH3, this distance shortened to 2.5736(5) Angstrom in D3P . . .(HCl)-Cl-35. An alternative analysis indicated that r(P . . . Cl) short ened 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 el ectric dipole moment of PH3 as it is deuteriated. The centrifugal distortio n constants D-J and D=2D(J+)D(JK) were used to determine values of the quad ratic 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 significantl y for D3P . . .(HCl)-Cl-35 relative to the (constant) value for species bas ed 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 correct ed value was used to give the average beta (av)=cos(-1)[co beta](1/2)=15 de grees for the oscillation angle of the HCl subunit.