The effects of nonzero total electron spin in the (X)over-tilde(3)B(1) state of methylene CH2

We report here how we have incorporated the effects of a nonzero total electron spin in the MORBID Hamiltonian and computer program [P. Jensen, J. Mol. Spectrosc. 125, 478-501 (1988); J. Chem. Sec. Faraday Trans. 2 84, 1315-1340 (1988); in ''Methods in Computational Molecular Physics'' (S. Wilson and G. H. F. Diercksen, Eds.), Plenum Press, New York, 1992] for calculating the rovibronic energies of a triatomic molecule directly from the potential energy function, The spin-spin and spin-rotation Hamiltonian terms. given in a form depending on the vibrational coordinates, have been expressed in terms of isotope-independent functions and added to the MORBID rotation-vibration Hamiltonian. The eigenvalues of the resulting Hamiltonian are obtained in a variational procedure. This method is tested on the methylene radical CH2 in the (X) over tilde(3)B(1) electronic ground state for which we describe simultaneously the splittings due to electron spin for the isotopomers (CH2)-C-12, (CD2)-C-12, and (CH2)-C-13. For these molecules, experimental data are available, and we compare the results of least-squares fits to these data with predictions from ab initio theory. (C) 1997 Academic Press.