Spectroscopy of Formaldehyde. 1. Ab Initio Studies on Singlet Valence and Rydberg States of Planar H2CO, with Emphasis on 1(.pi.,.pi.) and 1(.sigma.,.pi.)

Abstract

CO stretching potentials were obtained by ab initio multireference CI calculations for singlet valence and Rydberg states of H2CO in CzV symmetry. Within the Franck-Condon region, the valence states 'Bl(o,n*) and 'Al(n,n*) cross all members of the n -Ryd series as well as 'Bl(n,3s). The structures of the absorption bands are therefore extensively altered by vibronic mixing, intensity borrowing, and predissociative interactions.This explains why the n -Ryd bands are observed to be vibrationally more complex and of higher intensity than expected. Since lA~(n,n*) becomes the ground state at large R(CO), n,n* is capable of coupling all 'AI Rydberg states with the ground state, which dissociates to CH2 + 0 fragments at 7.64 eV. Thus, the observed absorption continuum above 7.5 eV is attributable to predissociating interactions. In the vertical region, the 'Al(n,n*) valence state lies at 9.6 eV and exhibits a heavy mixing with the no,n*2 configuration. The reported "n,3d" band at 8.88 eV is reassigned to the 0-0 band of the 3'A1 -%'Al transition, where the minimum of 3'Al results from an avoided crossing between n,n* and n,3p,. This explains the apparent large quantum defect and other anomalies reported for the so-called "n,3d" band.

Quasi‐degenerate perturbation theory with general multiconfiguration self‐consistent field reference functions

“…Harchey et al 27 These values are all close to the GMC-QDPT values, supporting the present results. Before these studies, many works were conducted, three of which are listed in Table 4.…”

“…The calculations with CAS-SCF and CAS-QDPT are far too large to be done. We, therefore, compare the results with available experimental results and some recent theoretical results, i.e., MR-CI results by Hachey et al, 27 the second-order complete active space perturbation theory (CASPT2) calculations by Merchán and Roos, 28 and the equation of motion coupled cluster (EOM-CC) calculations by Gwaltney et al 29 As can be computed from Table 3, the maximum differences in excitation energy for the largest three (two) numbers of active orbitals is 0.09 (0.05) eV. We can therefore consider that the excitation energies at the MC-SCF level are almost converged values for the change of the active orbital numbers.…”

Simultaneous calculation of Rydberg and valence excited states of formaldehyde

“…The spectroscopic identi®cation of the r±p à state is still also uncertain. Similar to the p±p à state it was shown that this state undergoes avoided crossings with Rydberg states as well [6].…”

“…The aforementioned puzzling experimental ®ndings concerning irregularities in certain Rydberg series and the missing p±p à state have been explained by Hachey et al [6] in their MR double excitation CI (MRD-CI) calculations by interactions between Rydberg and valence states, associated in particular with the stretching coordinate of the CO bond. For vertical excitations, they found the p±p à state, with 9.60 eV excitation energy, to be well above the n±p à and r±p à valence and several Rydberg states.…”

“…Apart from the need for¯exible and extended wave functions, questions concerning eciency and economy of the calculation cannot be ignored either. In the MRD-CI calculations by Hachey et al [6] one set of self-consistent-®eld (SCF) molecular orbitals (MOs) computed for the n±p à state was used for the CI expansion of both the Rydberg and the valence states. The bias connected with this particular choice of MOs had to be counterbalanced by means of extrapolations to full CI results.…”