Characterization of ground and excited electronic state deprotonation energies of systems containing double bonds using natural bond orbital analysis

Natural bond orbital analysis is applied to the ground and excited states of a set of neutral, cationic, and anionic doubly bonded species HnC=XH(n) (X=C, N, O) isoelectronic with ethylene. The character of the excitation is correlated with calculated charge shifts and geometry changes upon relaxation. For these planar molecules, depopulation of the pi bond or population of the pi* antibond causes an out-of-plane twist or pyramidalization upon relaxation correlated to the amount of charge shift. These nonplanar distortions generally lower the energy more than changes in bond lengths and angles. Population of a sigma(XH)* antibond by more than similar to 0.4 e often leads to dissociation of that proton. The character and symmetry of the transition are used to match excited states in the protonated and deprotonated species so as to extract an excited state deprotonation energy. The vertical deprotonation energy of the pi-->pi* state tends to be higher than that of the ground state due to greater electronic destabilization of the deprotonated species, while Rydberg excited states take less energy to deprotonate. Adiabatic deprotonation energies can be greater or less than that of the ground state depending on whether the protonated or deprotonated species is more stabilized by geometry relaxation. (C) 1996 American Institute of Physics.