The Effects of Nonzero Total Electron Spin in the X ; 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. 128, 478-501 (1988); J. Chem. Soc. 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 ; 3 B 1 electronic ground state for which we describe simultaneously the splittings due to electron spin for the isotopomers 12 CH2 , 12 CD2 , and 13 CH2 . 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.