Potential surface distortion and orbital reorganization upon change of electronic state. Formaldehyde

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Abstract
Near Hartree–Fock calculations on the 1A1 ground, 2B1, 2B2 cationic, 2B1 anionic, and 1,3A2 optical excited states of formaldehyde are reported. Reorganization effects are analyzed at the group symmetry and orbital levels. The results are as follows: Energy—large reorganization effects are found on excitation to all states. Comparison of equivalent excitations to the isoelectronic molecule ethylene shows that the reorganization effects are significantly more severe in the polar H2CO. Orbitals—the b2 and π orbitals are strongly sensitive to the electronic state, leading to poor frozen orbital predictions of excited state properties (e.g., dipole moments). The frontier 2b2 orbital contains strongly bonding and antibonding regions depending on the electronic state; consequently it is better to label it as frontier sigma (σf) rather than as nonbonding (n). Dipole moments—the large dipole moments in the σfπ* states can be largely understood from the centroids of σf and 1b1(π) shifting towards the oxygen in σfπ*. Potential surfaces—only the B1 anionic and A2 optical states are predicted to have nonplanar potential minima. Fermi correlation alone cannot explain either the smaller out‐of‐plane angle or larger inversion barrier height of the 1A2 vs 3A2 states.