AN ANALYTICAL REPRESENTATION OF THE GROUND POTENTIAL-ENERGY SURFACE ((2)A') OF THE H-2-HCL+CL AND CL+HCL-]HCL+CL REACTIONS, BASED ON AB-INITIO CALCULATIONS(CL)

In this work we have studied at an ab initio level the lowest (2)A' po tential energy surface (PES) of the HCl2 system. This PES is involved in the H(S-2)+Cl-2(X (1)Sigma(g)(+))--> HCl(X (1)Sigma(g)(+))+Cl(P-2) and Cl(P-2) + HCl(X (1)Sigma(+)) --> HCl(X (1)Sigma(+)) + Cl(P-2) gas phase elementary chemical reactions. The former reaction is an importa nt chemical laser while the second one is the most frequently used pro totype of heavy-light-heavy reaction. A large number of points on the 2A' PES have been calculated at the PUMP2/6-311G(3d2 f,3p2d) ab initio level. The ab initio calculations show the existence of two angular t ransition states with negligible or very small barriers to collinearit y. This and other properties of the PES are in agreement with previous studies. An analytical expression based on a many-body expansion has been used to obtain a satisfactory fit of the 740 ab initio points cal culated, with a root-mean-square deviation within the range of the est imated ab initio method error margin. This analytical representation o f the (2)A' PES has been used to evaluate the variational transition s tate theory thermal rate constants of the above-mentioned reactions, i ncluding also the Cl + DCl reaction, and quite good agreement has been obtained when comparing with experimental results. The analytical PES obtained in this work is suitable for use in studies on the kinetics and dynamics of the HCl2 system. (C) 1998 American Institute of Physic s.