Experimental and theoretical study of the photochemical properties of 4-azidopyridine and its derivatives

The quantum yield of the photodissociation of 4-azidopyridine (in acetonitrile) decreases from 0.8 to 0.2-0.3 upon protonation or methylation at the endocyclic nitrogen atom. The semiempirical (PM3) and ab initio (HF, MP2, and DFT(B3LYP)) methods were used to perform quantum-chemical calculations of the structure of azides in the ground So and lowest excited singlet S-1 states. For all the azides studied, it was found that, in the S-0 state, the N-2 terminal group carries a significant positive charge and that, in the S-1 state sigma(*)(NN), an antibonding orbital with respect to the N-N-2 bond is occupied, a factor that determines the dissociation of this bond. The results obtained conform to the known correlation between the type of the orbital occupied in the excited state and the photochemical activity of the azide molecule. Calculations of the potential energy surface and the activation energy of dissociation in the S-1 state demonstrated that a decrease in the quantum yield in going from an azide to its cation may be caused by an increase in the activation barrier of the reaction.