WHY IS FORMALDEHYDE NONPLANAR IN ITS $S_{1}(\pi^{*},n)$ ELECTRONIC STATE?

Author Institution: Department of Chemistry, State University of New YorkAbnormally low energy levels observed for the out-of-plane vibration ${\nu_{4}}(b_{1})$ of the $\bar{A}^{1}A_{2}(S_{1})$ electronic states of $H_{2}CO, HDCO$ and $D_{2}CO$ are modeled in turn by means of two-state calculations of vibronic coupling between $S_{1}$ and a higher-lying singlet state $B^{1}B_{2}(S_{2})$. The active vibration is $\nu_{4}$. In each case the vibronic calculations reproduce also the large positive anharmonicity of $\nu_{4}$ in the $S_{1}$ states; the calculated vibrational spacing alternates as observed and consistent with the known pyramidal geometry. Accordingly, the known double-minimum potential for $\nu_{4}$ in the $S_{1}$ electronic state is attributed to vibronic interaction of $S_{1}$ and $S_{2}$ with consequent ""pseudo Jahn-Teller distortions."" Vibronic coupling of $S_{1}$ and $S_{2}$ by $\nu_{4}$ was earlier proposed to account for most of the intensity of the (electric-dipole-forbidden) $S_{1}-S_{0}$ transition. The coupling constant estimated from the intensity data is close to that obtained in the present modeling of energy levels. This agreement is taken as confirmation that the coupling is a major cause af the change af shape of formaldehyde in its $S_{1}-S_{0}$ transition. The more extreme distortion of the potential of the analogous triplet state $^{3}A_{2} (\pi*, n)$ of$D_{2}CO$ also is modeled successfully