The spectroscopy of formaldehyde .3. Out-of-plane potentials and geometry optimizations for singlet states

Out-of-plane potentials and optimized geometries for excited singlet states of formaldehyde, including n,pi*, a,pi*, pi,pi*, n,3s, and n,3p, were obtained by multireference CI methods. The (1)(n,pi*) and (1)(sigma,pi*) states were found to be nonplanar, with out-of-plane angles of 34.5 degrees and 42.1 degrees, respectively. Agreement with the experimental geometry and energy of (1)(n,pi*) is good, and even the inversion barrier is well reproduced. The (1)(n,3s), (1)(sigma,pi*), and 1(pi,pi*) (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 --> 3s Rydberg transition to be perturbed by (1)(sigma,pi*) and (1)(pi,pi*). At nonplanar geometries, (1)(sigma,pi*) mixes heavily with (1)(pi,pi*); therefore, the pi --> pi* band system, which may be observable from the higher vibrational levels of the ground state, is expected to be significantly perturbed. The (C) over tilde <-- (X) over tilde system, recently reassigned to n --> 3p(z), should be perturbed not only by (1)(n,3p(y), as previously suggested, but also by (1)(pi,pi*) through the nu(6) (CH2 rocking) mode. Furthermore, the Q(6) potential of (1)(n,3p(z) is expected to interact with (1)(n,sigma(CH)*). The (D) over tilde <-- (X) over tilde system, assigned to n --> 3p(y), is predicted to be perturbed by (1)(pi,pi*), as well as by (1)(sigma,pi*) through the out-of-plane bending mode. (C) 1996 Academic Press, Inc.