Mechanism of the Primary Photodissociation Processes of Organic Molecules

Abstract

Two rules have been derived experimentally from investigations of the continuous absorption spectra of inorganic molecules¹. First, the action of light may produce the splitting of a single bond, the two products of dissociation, namely, atoms or radicals, possessing free valencies. This process occurs as long as the central atom of the original molecule is in a lower state of valency, molecule formation being brought about by its p electrons only. The first long wave limit then agrees with the single bond energy concerned. The second mechanism refers to molecules the central atom of which is in its maximal state of valency, its s electrons being responsible for chemical linkage in addition to its p electrons. In this case, one of the dissociation products does not possess free valencies, but is a saturated molecule of lower valency state in a ¹Σ term. Here, the first long wave limit is not correlated to a single bond energy, but agrees with the energy difference of the total energy of formation of the original molecule and that of the produced molecule of lower valency. In other words, the action of light produces a transition to that repulsive electronic state, which is due to the repulsive character of the s² group of electrons and predicted by the pair bond theory of valency, originally advanced by Heitler and London.