The Spectroscopy of Formaldehyde

Out-of-plane potentials and optimized geometries for excited singlet states of formaldehyde, including n ,π*, σ,π*, π,π*, n ,3 s , and n ,3 p , were obtained by multireference CI methods. The 1 ( n ,π*) and 1 (σ,π*) states were found to be nonplanar, with out-of-plane angles of 34.5° and 42.1°, respectively. Agreement with the experimental geometry and energy of 1 ( n ,π*) is good, and even the inversion barrier is well reproduced. The 1 ( n ,3 s ), 1 (σ,π*), and 1 (π,π*) (mixed with n 2 ) states were found to have minima on different sections of the S 2 surface, separated from one another by low barriers of 0.2 to 0.7 eV. This causes the n → 3 s Rydberg transition to be perturbed by 1 (σ,π*) and 1 (π,π*). At nonplanar geometries, 1 (σ,π*) mixes heavily with 1 (π,π*); therefore, the π → π* band system, which may be observable from the higher vibrational levels of the ground state, is expected to be significantly perturbed. The C ← X system, recently reassigned to n → 3 p z , should be perturbed not only by 1 ( n ,3 p y ), as previously suggested, but also by 1 (π,π*) through the ν 6 (CH 2 rocking) mode. Furthermore, the Q 6 potential of 1 ( n ,3 p z ) is expected to interact with 1 ( n ,σ * CH ). The D ← X system, assigned to n → 3 p y , is predicted to be perturbed by 1 (π,π*), as well as by 1 (σ,π*) through the out-of-plane bending mode.