Cyclopropane center dot center dot center dot sulfur dioxide and ethylene center dot center dot center dot sulfur dioxide van der Waals complexes: A theoretical study

Ab initio calculations at different levels of theory [HF, MP2, MP4SDTQ, and QCISD(T)] and using different basis sets (Pople's 6-31G**, Dunning-Huzinag a's D95**, and Dunning's aug-cc-pVDZ) were tarried out to analyze the poten tial energy hypersurfaces of the cyclopropane sulfur dioxide and ethylene.. .sulfur dioxide van der Waals complexes. Both the theoretical methods emplo yed and the basis set chosen exert a great influence on the geometry and na ture of the stationary points located: The MP2/aug-cc-pVDZ structures are c onsistent with experimental data from microwave spectroscopy. Binding energ ies, dipole moments, and rotational constants are in reasonable agreement w ith the corresponding experimental values. Calculations predict the existen ce of tunneling pathways between equivalent structures in accord with the e xperimentally observed splittings in the rotational spectra: The QCISD(T)/a ug-cc-pVDZ//MP2/aug-cc-pVDZ energy barrier in the case of ethylene...sulfur dioxide is 48 cm(-1), in good agreement with the experimental estimate (30 cm(-1)). For the cyclopropane...sulfur dioxide complex where an experiment al estimate cannot be done, the computed barrier is 69 cm(-1). Although cov alent forces make appreciable contributions to the stabilization of both va n der Waals complexes, the electrostatic component of the intermolecular in teraction seems to play an especially relevant role in determining the rela tive orientation of the two subunits in each complex. (C) 1999 American Ins titute of Physics. [S0021-9606(98)30648-0].