AB-INITIO INVESTIGATIONS OF THE ELECTRONIC-STRUCTURE OF HECH+ AND HECH2+

The (1)A' and (3)A '' electronic states of HeCH+ and the (2)A' ground electronic state of HeCH2+ have been investigated using an all electro n coupled cluster single, double and triple excitation [CCSD(T)] metho d coupled with an augmented correlation-consistent polarised core vale nce triple zeta basis set (aug-cc-pCVTZ). For the (1)A' and (3)A '' el ectronic states of HeCH+, the CCSD(T)/aug-cc-pCVTZ model yielded an op timised geometry of C-s symmetry with a {r(C-He), r(C-H), theta(H-C-He )} structural parameters of {2.172 Angstrom, 1.129 Angstrom, 81.4 degr ees} and {1.675 Angstrom, 1.121 Angstrom, 101.4 degrees}, respectively . Moreover, this electronic model yielded the singlet state to be 26.3 kcal mol(-1) lower in energy when compared with the triplet state. Fo r the (2)A' ground electronic state of HeCH2+ C the optimised structur al parameters were {1.388 Angstrom, 1.253 Angstrom, 110.5 degrees}, re spectively. The calculated harmonic frequencies for all these electron ic states were found to be real. (C) 1997 Published by Elsevier Scienc e B.V.