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

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Abstract
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″â†�1A′(n–, π*) transition has been observed. The geometry changes accompanying electronic excitation have been estimated using ab initio calculations for the 1A″ 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.

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