Quantum-chemical interpretation of photo-induced electrocyclic reactions

Predistortion amplified in the excited state

“…For the corresponding states, this implies a correlation of the ground state of the reactant with a twoelectron excited state of the product, and the other way around. Where the two lines would intersect, the crossing is avoided, resulting in a barrier in S 0 and a minimum in the "dark state", as shown for the butadiene/cyclobutene ring closure/opening by van der Lugt and Oosterhoff [106,107] (see also [108]). If the reaction is WH-allowed, this "pericyclic minimum" is low, due to its origin from the correlation, typically lower than the lowest one-electron excited ("spectroscopic") state.…”

“…Van der Lugt and Oosterhoff suggested that a molecule that happens to arrive there finds its way back and then to the deeper minimum, from where the WH-allowed reaction takes place [106][107][108] (in a more modern view, the forbidden path meets a barrier before the last CI, whereas the allowed path is barrierless [113]). In other words, in this suggestion the WH selectivity is based on dark-state minima of different energies and lifetimes, which are reversibly populated, so that the reactions from there can compete with each other.…”

“…Since the times of van der Lugt and Oosterhoff [106][107][108], cyclobutene was the prototype of photochemical electrocyclic (or even pericyclic) reactions governed by the WH rules. It was therefore very disconcerting, when the work of Leigh and coworkers demonstrated the various violations of these rules by just this group of compounds (although ring cleavage of cyclobutenes follow the stereochemical WH predictions, as also does the cyclobutene formation from dienes).…”

Forward and Backward Pericyclic Photochemical Reactions Have Intermediates in Common, Yet Cyclobutenes Break the Rules

“…The crossing of the two pairs of correlated states is avoided and creates 1) a barrier in S 0 that prevents the reaction from occurring in the ground state, and 2) a "pericyclic minimum" (p*) on the lowest excited surface S 1 that is common to both the forward and backward reactions; this collects population from any initially excited state and funnels it through a conical intersection (CI) to the S 0 minima of product and reactant. An early version of this scheme was developed by van der Lugt and Oosterhoff; [4] modern quantum chemistry and ultrafast spectroscopy have added the CI as the funnel outlet from p* and introduced considerations of dynamics instead of equilibration (see refs. [2], [3], [5] and [6], for example).…”

Visual valence bond rules for chemical reactions

“…Based on generalized electron-pair wave functions and the primitive symmetry classi®cation of the prinicpal structure of a molecular system, the VB approach to conservation of symmetry has been suggested [5]. The idea of permutational symmetry control was introduced to pericyclic reactions by van der Lugt and Oosterho [6,7] and by Mulder and coworkers [8±10], and photoinduced electrocyclic reactions were discussed in terms of the VB orbitals of a cyclic system. The method is rather mathematical and complex.…”