The C₄H₆^•+ Potential Energy Surface. 1. The Ring-Opening Reaction of Cyclobutene Radical Cation and Related Rearrangements

Abstract

Ab initio MO and density functional calculations indicate that the ring opening of the cyclobutene radical cation (CB •+) follows two competitive pathways, whose energy barriers differ by less than 1 kcal/mol at the highest level of theory employed, RCCSD(T)/cc-pVTZ//UQCISD/6-31G*. The first corresponds to a conrotatory rearrangement to the cis-butadiene radical cation (cis-BD •+). The second one leads to trans-BD •+ via a very flat potential energy plateau which comprises cyclopropylcarbinyl-type structures of the type proposed some time ago by Bauld, but the controtatory stereochemistry is preserved also along this process. State correlation diagrams indicate that the rearrangement leading to trans-BD •+ may occur adiabatically along a C 2 reaction coordinate. Despite this, the transition state has no symmetry. This seemingly “unnecessary” loss of symmetry is traced back to the proximity of the 2A and 2B surfaces in the vicinity of the C 2 stationary points, where the two states encounter a strong vibronic interaction which leads to breaking of the C 2 symmetry. These vibronic interactions are also responsible for the general flattening of the potential energy surface in this area.

Branching ratios for the dissociative recombination of hydrocarbon ions

“…9) is the most stable form of the ion [40][41][42][43] and of the neutral [20]. The predominant dissociation pathway is seen to be C 2 + C 2 with C 4 being twice as likely.…”

Unusual Rearrangements of Radical Cations: The Role of Vibronic Coupling

“…11. [14] On the right hand side of this Figure one can see that, if the radical cation of cyclobutene (CB •+ ) undergoes electrocyclic ring-opening to cis-BD •+ in the con-or disrotatory fashion, while an axis or a plane of symmetry is retained, respectively, the ground states of the two compounds invariably correlate with excited states of the products. Thus the systems evolve towards conical intersections which are avoided by vibronic coupling which breaks the remaining symmetry.…”

“…The Ring Opening of the Cyclobutene Radical Cation (CB •+ ) [14] During the first years of chemistry we learn that the ring-opening of cyclobutene is a textbook example for the operation of the Woodward-Hoffman rules for electrocyclic reactions. We wanted to investigate whether similar rules can also be applied to radical cations, but we found that this is not the case, because ionization of CB in Ar matrices led directly to the trans isomer of the butadiene radical cation (BD •+ ), while no trace of the cis-isomer was ever seen.…”

Selectivity in Radical Cation Cycloadditions

“…Furthermore, the symmetry preserving pathway is subject to Jahn-Teller distortions that increase the activation energy even more. 28,36 The overall result is an acceleration of slow or symmetry forbidden cycloadditions, [37][38][39] electrocyclic reactions, [40][41][42][43][44][45] sigmatropic shifts, [46][47][48] An explanation for the increased reaction rates observed for radical cation Diels-Alder reactions has been proposed by Bauld and is shown in Scheme 4.1. 51 The same principle is applicable for other radical cation pericyclic reactions.…”