The 4 550 å Band System of Glyoxal: I. Rotational Analyses of the (0–0) Bands for C₂h₂o₂, C₂hdo₂, and C₂d₂o₂

Rotational analyses of the (0–0) bands of the 4 550 Å band system of C 2 H 2 O 2 (glyoxal), C 2 HDO 2 , and C 2 D 2 O 2 have been performed. The analyses show unambiguously that the bands have type C structures, in agreement with the assignment to a 1 A u – 1 A g , (or 1 A″– 1 A′), π*–n transition. Rotational constants, A, B, C, and D K have been determined for each isotope and probable structures for the molecule in the ground and excited states have been determined. The CO bond is longer and the CC bond shorter in the excited state, in agreement with the predictions of molecular orbital theory.