A Conical Intersection Influences the Ground State Rearrangement of Fulvene to Benzene

Abstract

The rearrangement of fulvene to benzene is believed to play an important role in the formation of soot during hydrocarbon combustion. Previous work has identified two possible mechanisms for the rearrangementa unimolecular path and a hydrogen-atomassisted, bimolecular path. Computational results to date have suggested that the unimolecular mechanism faces a barrier of about 74 kcal/mol, which makes it unable to compete with the bimolecular mechanism under typical combustion conditions. This computed barrier is about 10 kcal/mol higher than the experimental value, which is an unusually large discrepancy for modern electronic structure theory. In the present work, we have reinvestigated the unimolecular mechanism computationally, and we have found a second transition state that is approximately 10 kcal/mol lower in energy than the previously identified one and, therefore, in excellent agreement with the experimental value. The existence of two transition states for the same rearrangement arises because there is a conical intersection between the two lowest singlet states which occurs in the vicinity of the reaction coordinates. The two possible paths around the cone on the lower adiabatic surface give rise to the two distinct saddle points. The lower barrier for the unimolecular mechanism now makes it competitive with the bimolecular one, according to our calculations. In support of this conclusion, we have reanalyzed some previous experimental results on anisole pyrolysis, which leads to benzene as a significant product and have shown that the unimolecular and bimolecular mechanisms for fulvene → benzene must be occurring competitively in that system. Finally, we have identified that similar conical intersections arise during the isomerizations of benzofulvene and isobenzofulvene to naphthalene.

Automated Reaction Kinetics of Gas-Phase Organic Species over Multiwell Potential Energy Surfaces

“…Without a detailed calculation, finding one or the other saddle point depends strongly on the details of the saddle-point guess, and there is no simple systematic way to perceive the presence of both possibilities. Relative to fulvene ( IV ), the energy of the higher saddle point from B to IX is ∼74.3 kcal/mol, whereas that of the lower one ranges from 61.3 to 65.4 kcal/mol using various multireference methods . The barrier height we found is 66.2 kcal/mol at the CCSD­(T)-F12a/cc-pVTZ-F12/ωB97X-D/6-311++G­(d,p) level of theory, and, by confirming the similarity of the geometries as well, we conclude that KinBot converged to the lower, kinetically more important saddle point.…”

“…91,94,95 Interest in this system has not ceased, marked by, for instance, a recent study on its photochemistry using a nonadiabatic nanoreactor, 48 an experimental reinvestigation of 1,5-hexadiyne pyrolysis using PEPICO, 96 and an elegant study that uncovered an alternative, lower-energy structure for a key saddle point. 43 Our motivation to revisit this system is the appearance of new theories about molecular weight growth 92 The reaction pathways are color-coded. Black: identical to the MK PES; cyan: new channels at L3//L2; blue: new channels at L3//L1; brown: channels that are different or missing in our calculations.…”

“…The recombination reaction of propargyl radicals is thought to play a central role in the formation of soot, as it is a key reaction that leads to the formation of the first aromatic ring during combustion. , The reaction takes place on the ground C 6 H 6 PES, which is an extraordinarily complex surface with at least 215 stable isomers . The kinetically relevant, low-energy portion of the PES, when starting from the propargyl + propargyl reactants, is described in the landmark paper of Miller and Klippenstein (MK) that builds on the PESs developed in prior work. ,, Interest in this system has not ceased, marked by, for instance, a recent study on its photochemistry using a nonadiabatic nanoreactor, an experimental reinvestigation of 1,5-hexadiyne pyrolysis using PEPICO, and an elegant study that uncovered an alternative, lower-energy structure for a key saddle point . Our motivation to revisit this system is the appearance of new theories about molecular weight growth and particle formation, such as the one put forward by Johansson et al, and because formerly undiscovered intermediates and pathways can be of great importance in predicting products of complex chemical systems.…”

“…The saddle-point energies can be significantly different for the alternative pathways, such as in an antarafacial and suprafacial sigmatropic H atom transfer 10 or a conical intersection around which the reaction coordinate can traverse on either side. 43 While in our experience most reactions on most PESs do not suffer from such complications, there is no search algorithm that can ensure that the lowest-energy pathway is found between any two species that are larger than just a few atoms; edge cases will always present themselves and likely require expert insight (often motivated by experimental observations) to resolve discrepancies.…”

Mechanism, thermochemistry, and kinetics of the reversible reactions: C2H3 + H2 ⇌ C2H4 + H ⇌ C2H5

“…This has been called the H-assisted pathway, 128,129 and starts with H-atom addition to the double bond, followed by rearrangement. Recently, the rst experimental evidence 130 for the H-assisted pathway has been found in the radical/radical reaction: CH 3 with C 5 H 5 / C 6 H 6 (benzene) (Scheme 1).…”