Rotational Spectra of the Less Common Isotopomers, Electric Dipole Moment and the Double Minimum Inversion Potential of H₂O···HCl

Abstract

Rotational spectra of 14 different isotopomers of the hydrogen-bonded dimer H2O···HCl have been measured. On application of a method of analysis designed to minimize the effect of vibration−rotation contributions to the ground state geometry, it was found that the experimental rotational constants imply a nonplanar geometry with R0(OCl) = 3.2273(3) Å, and an out-of-plane bend angle of the water subunit φ0 = 34.7(4)°. This angle is consistent with results of ab initio calculations which, at the BSSE corrected, aug-cc-pVDZ/MP2 level, give φ = 46° for the equilibrium configuration and φ = 35.2° for the ground state. The electric dipole moment of H2O···HCl, μ = 3.437(4) D, was determined from Stark effect measurements and, after allowance for the zero-point inversion motion of the water subunit, leads to dipole moment enhancement on complexation Δμ = 0.81 D. Theoretical analysis of contributions to the chlorine nuclear quadrupole splitting constant χaa for H2O···HCl shows that most of the reduction relative to its value in free HCl is accounted for by the response of the complexed HCl to the nonuniform field of the nearby H2O subunit. The remaining difference, ascribed to motional averaging, implies an operationally defined angle of oscillation for the HCl moiety of 10°−12°.