The concerted mechanism of photo-induced biprotonic transfer in 7-azaindole dimers: A model for the secondary evolution of the classic C_2h dimer and comparison of four mechanisms

Abstract

A mechanism is proposed for the formation in gas phase, during a short time, of the delicately symmetrical coplanar C _2h classic 7-azaindole (7AI) doubly hydrogen-bonded dimer. Of the five card-pack or otherwise random geometry structures most likely to be formed in the supersonic jet expansion molecular beam, none would be an obvious precursor to the C _2h dimer. One unstable dimer with dipole–dipole, van der Waals, and plane-to-plane hydrogen bonding is shown to be capable of unhinging about the hydrogen-bond pair as an axis, from 0° to 90° to 180°, yielding a deep minimum for the C _2h structure with its delicate geometry and symmetry. This relaxation mechanism is feasible in the 3-μs interval between the nozzle escape and the first laser pulse interception of the molecular beam. In the second part of the paper four published mechanisms are compared for concerted vs. two-step biprotonic phototransfer for the 7AI dimers. The dependence of the latter two models on H-atom instead of proton-transfer as an intermediate step negates the mechanism in a singlet (π,π*) electronic state by the valency repulsion, in the 3-electron orbital that would be generated. The concerted mechanism for biprotonic phototransfer is reaffirmed by the analysis of the quantum mechanical conditions set on the biprotonic transfer in the photo-excited molecular 7AI pair.