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

Abstract Calculated energy levels and vibronic oscillator strengths for the CO-stretch vibration ν 2 show that the 1 (π, π ∗ ) state of H 2 CO participates in all strong and most medium intensity absorption bands above 8.2 eV, which previously were given Rydberg assignments. The 2 1 0 band of n,3p y at 8.32 eV appears to have a large contribution from the 2 3 0 band of π, π ∗ . The 0 0 0 and 3 1 0 (HCH bend) bands of n,3d, at 8.88 eV and 9.04 eV, respectively, are reassigned as 0 0 0 and 2 1 0 of the avoided crossing state, π, π ∗ / n ,3 p y . The 3 2 0 band of n,3d at 9.18 eV can be related to both 2 2 0 of π, π ∗ / n ,3 p y , and 0 0 0 and of n,3d yz . Some previously reported ‘anomalies’ have been resolved.