The important role of 1(π, π∗) in the UV absorption spectrum of formaldehyde, as shown by ab initio MR CI studies

Calculated energy levels and vibronic oscillator strengths for the CO-stretch vibration ν2 show that the 1(π, π∗) state of H2CO participates in all strong and most medium intensity absorption bands above 8.2 eV, which previously were given Rydberg assignments. The 210 band of n,3py at 8.32 eV appears to have a large contribution from the 230 band of π, π∗. The 000 and 310 (HCH bend) bands of n,3d, at 8.88 eV and 9.04 eV, respectively, are reassigned as 000 and 210 of the avoided crossing state, π, π∗/n,3py. The 320 band of n,3d at 9.18 eV can be related to both 220 of π, π∗/n,3py, and 000 and of n,3dyz. Some previously reported ‘anomalies’ have been resolved.