Hugh Chaffey-Millar

Improving Upon String Methods for Transition State Discovery

Transition state discovery via application of string methods has been researched on two fronts. The first front involves development of a new string method, named the Searching String method, while the second one aims at estimating transition states from a discretized reaction path. The Searching String method has been benchmarked against a number of previously existing string methods and the Nudged Elastic Band method. The developed methods have led to a reduction in the number of gradient calls required to optimize a transition state, as compared to existing methods. The Searching String method reported here places new beads on a reaction pathway at the midpoint between existing beads, such that the resolution of the path discretization in the region containing the transition state grows exponentially with the number of beads. This approach leads to favorable convergence behavior and generates more accurate estimates of transition states from which convergence to the final transition states occurs more readily. Several techniques for generating improved estimates of transition states from a converged string or nudged elastic band have been developed and benchmarked on 13 chemical test cases. Optimization approaches for string methods, and pitfalls therein, are discussed.

Radical Addition to Thioketones: Computer-Aided Design of Spin Traps for Controlling Free-Radical Polymerization

An extensive study has been undertaken of the radical affinity of a number of thioketones (S [Formula: see text] C(X)(Y)) with the aim of selecting combinations of X and Y that render the substrate suitable for the mediation of free radical polymerizations. Using high level ab initio molecular orbital calculations, enthalpies at 0 K were determined for the reactions R(•) + S [Formula: see text] C(X)(Y) → R-S-C(•)(X)(Y) for R(•) = CH3, CH2OH, CH2CN, and benzyl, in reactions with a variety of thioketones, including various combinations of X and Y taken from H, CH3, Ph, CN, OCH3, C(CH3)3 and para-CN-Ph as well as several compounds in which the X and Y are bonded, namely xanthene-9-thione, fluorine-9-thione, and cyclopenta[def]phenanthrene-4-thione. The radical affinities of the various thioketones has been discussed in terms of the radical stabilization energies (RSEs) of the adduct radicals and stabilities of the S [Formula: see text] C bonds. From these studies, the two thioketones S [Formula: see text] C(CN)(Ph) and fluorene-9-thione were selected as being potentially suitable candidates for use in controlling free radical polymerizations due to their high radical affinities. However, based on transition state theory calculations of the rate coefficients for homo/copolymerization of S [Formula: see text] C(CN)(Ph) with itself and styrene at 333.15 K, this substrate was deemed to be unsuitable, as it was likely to undergo side reactions. Instead, the more-hindered fluorine-9-thione was identified as the ideal thioketone, and the equilibrium constants at 333.15 K for the reactions of the styryl and vinyl acetate dimer radicals with fluorine-9-thione were made. These two reactions, at 333.15 K, displayed equilibrium constants in the vicinity of 10(14) L mol(-)(1) and 10(16) L mol(-)(1), respectively, indicating that there is significant scope within the thioketone class of compounds to mediate free radical polymerizations via radical stability alone.