AB-INITIO POTENTIAL-ENERGY SURFACE FOR H- EXTENDED BASIS-SETS AND CORRELATION TREATMENT(OCS REACTIONS )

Ab initio calculations are presented for the potential energy surface (PES) of H + OCS. The two triple-zeta AO basis sets used are the 6-311 +G(2df,2p) and aug-cc-pVTZ. The highest levels of correlation employed are MP4 and QCISD(T) starting with UHF zeroth-order wave functions. T here are two major reaction channels on the PES: reaction I is H(S-2) + OCS((1) Sigma) --> OH((II)-I-2) + CS((1) Sigma), and reaction II is H(S-2) + OCS((1) Sigma) --> SH((II)-I-2) + CO((1) Sigma). Multiple pat hways leading from reactants to (I) and (II) were determined. Results of this study substantiate findings from an earlier quantum chemical s tudy using a lower level of theory, including (1) the existence of 12 transition states and six stable four-body intermediates; (2) the qual itative description of the PES; i.e., geometries, relative barriers, a nd well depths are similar to those in the earlier study; and (3) the entrance channel transition states leading to (II) are tight, as sugge sted by experiment. The results presented here also support the explan ations of observed product energy distributions for (I) and (II) based on the earlier ab initio study. An additional transition state connec ting the cis-HOCS and cis-HSCO minima was located, confirming a previo us suggestion that reaction II could result from hydrogen migration af ter HOCS formation. The size of the barrier to hydrogen migration on t his PES, however, indicates that it is a high-energy reaction. The low -energy pathway leading to (II) is through direct formation of HSCO. T he current results show a substantial improvement in the quantitative agreement with experiment over the previously calculated values. Our b est calculations predict reaction enthalpies for (I) and (II) to be 57 .3 and -11.5 kcal/mol, respectively, well within the uncertainty of th e experimental data. The measured activation energy for (II), which in cludes all dynamic and quantum effects such as tunneling, is 3.9 kcal/ mol, Our best theoretical value for the lowest energy barrier leading to (II) is 5.7 kcal/mol (including zero-point corrections). We emphasi ze that corrections due to tunneling are not included in this value.