Potential energy surface and infrared spectrum of the Ar-H2Cl+ ionic complex

The infrared photodissociation spectrum of the Ar-H2Cl+ dimer has been reco rded in the vicinity of the Cl-H stretch fundamentals of bare H2Cl+. Eleven Q branches of a strong perpendicular transition of a (near) prolate symmet ric top are observed. The position and rotational structure of the band are consistent with an assignment to the free Cl-H stretch fundamental of a pr oton-bound Ar-HClH+ dimer. The global minimum on the intermolecular potenti al energy surface of Ar-H2Cl+, calculated at the MP2/aug-cc-pVTZ(#) level o f theory, corresponds to the proton-bound structure with an intermolecular separation of R-e=1.97 Angstrom and a well depth of D-e=1860 cm(-1). The sl ightly nonlinear ionic hydrogen bond is directional with large barriers (V- b) for internal rotation of H2Cl+ via planar transition states with C-2v sy mmetry: V(b)similar to 750 and 1330 cm(-1) for the bridged (R-e=3.45 Angstr om, D-e=1107 cm(-1)) and chlorine-bound (R-e=3.38 Angstrom, D-e=531 cm(-1)) structures. The molecular constants of the observed transition, nu(0)=2663 .1 cm(-1) and A=10.35 cm(-1), are in good agreement with the values calcula ted for the proton-bound equilibrium geometry, nu(0)=2665.4 cm(-1) and A(e) =10.28 cm(-1). (C) 2000 American Institute of Physics. [S0021-9606(00)00425 -6].