Large Nonstatistical Branching Ratio in the Dissociation of Pentane-2,4-dione Radical Cation: An Ab Initio Direct Classical Trajectory Study

Abstract

The dissociation of pentane-2,4-dione radical cation has been studied by ab initio direct classical trajectory calculations at the MP2/6-31G(d) level of theory. A bond additivity correction has been used to improve the MP2 potential energy surface (BAC-MP2). A microcanonical ensemble was constructed using quasiclassical normal-mode sampling by distributing 10 kcal/mol of excess energy above ZPE for the transition state for the tautomerization of the enol with a terminal double bond, 4-hydroxypent-4-en-2-one radical cation, to the diketo form. A total of 244 trajectories were run starting from this transition state, yielding pentane-2,4-dione radical cation and depositing energy in the terminal CC bond. As a result, the branching ratio for dissociation of the terminal CC bond versus the interior CC bonds is significantly larger than expected from RRKM theory. The branching ratio for the dissociation of the two interior CC bonds is ∼20:1, with the one closest to the activated methyl breaking more often. Since the two interior bonds are equivalent and should dissociate with equal probability, this branching ratio represents a very large deviation from statistical behavior. A simple kinetic scheme has been constructed to model the dissociation rates. The nonstatistical behavior is seen because the rate of energy flow within the molecule is comparable to or less than the rates of dissociation for the activated system. In addition to the expected dissociation products, some of the trajectories also lead to the formation of an ester-like product, prop-1-en-2-yl acetate radical cation.

QM/MM Protocol for Direct Molecular Dynamics of Chemical Reactions in Solution: The Water-Accelerated Diels–Alder Reaction

“…An increasing number of studies point to the importance of nonstatistical dynamics in organic reactions. − Experimental rate or product ratios in these reactions are poorly described by statistical theories (transition state theory or RRKM theory) but can be understood in terms of nonstatistical dynamical effects revealed by classical trajectory calculations. One area of concern is that most reactions are carried out in solution, but most trajectory calculations either have been applied to gas-phase reactions or have used a dynamical model without explicit solvent for reactions that occur in solution.…”

Nonstatistical Dynamics in Unlikely Places: [1,5] Hydrogen Migration in Chemically Activated Cyclopentadiene

“…Accumulating evidence from laboratories around the world, studying an ever-widening range of reaction types, points to an important role for nonstatistical dynamical effects in the thermal reactions of polyatomic molecules. − When such effects occur, the results can be disruptive to the conventional models of reaction mechanisms. For example, reactive intermediates can behave in ways that depend on how they were prepared, they can form products in ratios that are not predictable from the barriers on the standard free energy surface (in extreme cases even favoring the pathway that has the highest barrier), and they can choose exit channels in a manner that shows oscillatory time dependence .…”

Ab initio and semi-empirical Molecular Dynamics simulations of chemical reactions in isolated molecules and in clusters

“…The method was used to study the formation 12 and protonation of nitric acid, 13 as well as related systems; 14 photodetachment of an anion complex, (F À )(H 2 O) 4 , 15 S N 2 reactions; 16 unimolecular dissociation of H 2 CO and related molecules; 17,18 abstraction reactions of H atoms 19 and a few others. As the merits of MP2 in MD calculations became increasingly recognized, many more applications of the method were reported, including additional studies of nitric acid formation; [20][21][22][23][24] ionization dynamics (and the reverse process of electron capture); [25][26][27][28][29][30][31] S N 2 reactions; [32][33][34] dynamics in the transition state region, also in the context of unimolecular reactions; [35][36][37][38] hydrogen atom abstraction and elimination reactions [39][40][41][42][43][44] and a host of other processes. In the last several years, the use of MD-MP2 has become quite extensive, especially for polyatomic systems of small to moderate sizes.…”