THE DISSOCIATION MECHANISM OF TRIPLET FORMALDEHYDE

Ab initio molecular electronic structure theory has been used in conjunction with flexible basis sets to predict the barrier height to radical dissociation for the lowest triplet state (T1) of formaldehyde (3A"H2CO --> H. + HCO.). Self-consistent-field (SCF), complete active space SCF (CASSCF), and configuration interaction with single and double excitations (CISD) levels of theory were employed with basis sets ranging from double zeta plus polarization (DZP) to quadruple zeta plus triple polarization (QZ3P). Complete geometry optimizations of the equilibrium structure of X-approximately A1 H2CO, a-approximately 3A "H2CO, the transition state, and the dissociated radical on the potential energy surface were carried out. Improved basis set, triple zeta plus double polarization with higher angular momentum polarization functions [TZ(2df,2pd)], single point methods were used to further refine relative energies. Higher correlated level, multireference CISD (MR-CISD), was employed to verify the calculations involving higher excitations. At the highest level of theory [CISD(Q) with the TZ(2df,2pd) basis set], the exit barrier height at 0 K for the T1 state is predicted to be 7.8 kcal mol-1 with the zero point vibrational energy (ZPVE) correction with an estimated error bar of 3.0 kcal mol-1, favorably comparing with the most recent and accurate experimental estimate of 2.9-6.0 kcal mol-1 by Chuang, Foltz, and Moore [J. Chem. Phys. 87, 3855 (1987)]. This study also presents the most sophisticated theoretical predictions to date on the equilibrium structure and physical properties of the lowest triplet state, a-approximately 3A", of formaldehyde.