Excited potential energy hypersurfaces for hydrogen (H4) at trapezoidal geometries. Relation to photochemical 2s + 2s processes

Abstract

Ab initio VB calculations, complete within minimum basis set (STO-4G), were performed on the Born-Oppenheimer potential surfaces of the three lowest singlet electronic states of the H4 molecule for the three-dimensional subspace of all trapezoids. Perspective drawings of equipotential surfaces aid the visualization of the results. The hypersurface of the ground state is in excellent agreement with previous results and suggests that the transition state of the isotopic exchange reaction Hz + Dz e 2HD lies outside the trapezoidal subspace. An exothermic reaction of singlet excited H2 (BIZ,+) with ground state Hz (XIZ;,+) without activation energy and adiabatic formation of an H4* excimer is predicted. The bonding in the excimer is due to exciton resonance and charge transfer. The state is purely "zwitterionic". However, at all energetically favorable excimer geometries, a purely dissociative state originating in overall singlet coupling of two parallel triplet Hz molecules is almost degenerate with the excimer state, and this may limit the lifetime of the excimer The dissociative doubly excited state is approximately half zwitterionic and half covalent and can also be viewed as originating in two crossed ground state Hz molecules.Paths of steepest descent on this surface lead to 4H, H2 + 2H, or 2H2 (diagonal bonding). The case of HZ + H2 can be viewed as the simplest possible model for organic 2s + 2s processes. Our results support the picture outlined by van der Lugt and Oosterhoff for photochemical pericyclic processes (return to So through a "pericyclic" minimum in a doubly excited state) and suggest answers to questions concerning photocycloadditions and other pericyclic processes: (i) the relation of the pericyclic minimum to the excimer minimum; (ii) the origin of activation energy; (iii) the physical factors affecting the ordering of excited states at the geometry of the pericyclic minimum; (iv) occurrence of diagonal bonding; (v) occurrence of excited product formation; (vi) relation of the photochemical process to triplet-triplet annihilation; (vii) its relation to radical ion recombination; and (viii) interrelation of the "supermolecule" correlation diagrams to interaction and correlation diagrams based on MO's and states of partners in the photocycloaddition.Born-Oppenheimer hypersurfaces for electronically excited singlet states in the six-dimensional nuclear configuration space of H4 are of interest for several reasons. First, their availability might form a basis for eventual better understanding of processes such as collisional energy transfer, one of the simplest conceivable bimolecular photochemical reactions (H2 + H2* -products), and one of the simplest molecular ion recombination reactions (H3+ + H--products).Although the last named reaction is one between two ground state species, it starts on an electronically excited surfaces of the total H4 system and therefore has potential for production of H**, particularly since excess energy can be carried away by Hz so that no stabilization by a third b...

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