Geometry changes accompanying electronic excitation of nitrosomethane in the 650 nm region

The vapour phase absorption spectrum of nitrosomethane in the 600-720 nm region has been re-examined. The overall complexity of the spectrum is shown to be due to a conformational change from the eclipsed ground state to a staggered excited state. The prominent torsional features have been reassigned and are found to be consistent with a barrier of 500 ± 100 cm^-1 to internal rotation in the excited state. Progressions of 390 and 1420 cm ^-1 have been assigned to the CNO bending and NO stretching modes in the excited state. Only one electronic absorption system with a Franck-Condon forbidden origin at 694.1 nm corresponding to the ¹A? ? ¹ A' (n_-, p*) transition has been observed. The geometry changes accompanying electronic excitation have been estimated using ab initio calculations for the ¹ A? state. A substantial lengthening of the CN and NO bonds, a widening of their bond angle and a change in the phase of the torsional barrier are predicted.