A spin-vibronic model Hamiltonian accounting for the linear, quadratic, cubic, and quartic Jahn-Teller interactions with the inclusion of spin-orbit coupling and all important anharmonic effects was developed to study electronic and nuclear dynamics in twofold degenerate electron systems and was applied to the (XE)-E-2 ground electronic state of the CH3O methoxy radical (C3(v)). The problem of spin-vibronic eigenvalues and eigenfunctions was solved in a basis set of products of electronic, electron spin, and vibrational functions. The results of calculations show the importance of inclusion of cubic vibronic terms into the model Hamiltonian and the insignificance of quartic terms. (c) 2005 Elsevier B.V. All rights reserved.