The absorption spectrum of the free NCO radical

Abstract
Two systems of absorption bands have been observed in the visible and ultra-violet regions of the spectrum during the flash photolysis of several organic cyanates, and have been photographed under high resolution with long absorbing paths. Extensive vibrational and rotational analyses have been carried out for the bands of one system and show that the spectrum is due to an electronic transition A ( 2 Z + ) <-- X ( 2 II < i ) of the free NCO radical, which is linear in both states. All three vibrational frequencies and the first-order anharmonic constants have been obtained for the upper state, A ( 2 { + ), and give a close fit to the term values of 21 observed vibrational levels. A Fermi resonance has been observed between v ' 1 and 2v' 2 . In addition, the rotational constants B' and D' and their variations with all three fundamental vibrations have been obtained for this state. Transitions have been observed from four excited levels of the bending vibration in the lower state, X ( 2 II i ), and the rotational constants have been determined for some of these levels. Interaction between the electronic and vibrational motions (Renner effect) complicates the vibrational structure of this state. The state belongs to Hund’s coupling case ( a ), and the spin-orbit coupling gives a splitting A" = —95.6 cm-1. In a 2 { + vibronic level of this state (arising from l = 1 and A = 1) the spin sp litting is proportional to N +1/2, but the spin-splitting constant y is unusually large, and amounts to 30 % of the B value. The electronic states of NCO are correlated with those of its dissociation products. This shows that the bond dissociation energy of the CO bond is slightly greater than that of the CN bond in the three known states.

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