Radical Addition to Alkenes:  Further Assessment of Theoretical Procedures

Abstract

Ab initio molecular orbital calculations at a variety of levels of theory have been carried out for a number of prototypical radical addition reactions with a view to determining a level of theory suitable for predicting reliable barriers. Closest agreement with experimental barriers is achieved with a variant of the recently introduced CBS-RAD procedure. At this level, the mean absolute deviation from experimental barriers for methyl radical additions in solution is just 1.4 kJ mol-1. A second high-level theoretical procedure examined is a variant of G2(MP2,SVP), corresponding effectively to QCISD(T)6-311+G(3df,2p) energy calculations on QCISD/6-31G(d) optimized geometries and incorporating scaled B3-LYP/6-31G(d) zero-point vibrational energy corrections. At this level, the mean absolute deviations from the experimental barriers is significantly larger at 7.7 kJ mol-1, the calculated barriers being consistently too high. The effect of quadruple excitations is found to be small. The considerably less expensive B3-LYP/6-311+G(3df,2p)//B3-LYP/6-31G(d) procedure performs quite well, with a mean absolute deviation of about 5.6 kJ mol-1. Solvent effects were estimated using the SCIPCM model. For a dielectric constant of 2 (nonpolar medium), the effect on barrier ranges from −1.1 to +1.1 kJ mol-1, while for a dielectric constant of 40 (polar medium), the effects range from −3.0 to +2.8 kJ mol-1.

On the Use of Quantum Chemistry for the Determination of Propagation, Copolymerization, and Secondary Reaction Kinetics in Free Radical Polymerization

“…In the mid-1990s, great efforts were made to clarify which computational methods were best suited to the estimation of the Arrhenius parameters of radical addition to alkenes [65,77,78]. Different levels of theory were applied on various propagation reactions and a suitable approach for the prediction of energy barrier was proposed.…”

Ab initio group contribution method for activation energies for radical additions

“…The ever-increasing computational power and the development of better algorithms bring the accurate calculation of kinetic and thermodynamic parameters for industrially relevant gas-phase reactions from first principles within reach. Hydrocarbon free-radical reactivity and thermochemistry have been studied with a variety of computational methods, ranging from semiempirical to density functional theory (DFT) and postHartree-Fock methods (Wong et al, 1994;Barone and Orlandini, 1995;Heuts et al, 1996;Xiao et al, 1997;Wong and Radom, 1998;Blowers and Masel, 2000;Chuang et al, 2000;Stahl et al, 2001;Sumathi et al, 2001aSumathi et al, ,b, 2002. It is found that in order to obtain results with so-called chemical accuracy for standard enthalpies of formation, that is, better than 4 kJ/mol, calculations on radical species and on radical reactions require large basis set post-Hartree-Fock methods (Mayer et al, 1998;Chuang et al, 2000;Saeys et al, 2003).…”

Kinetic Solvent Effects in Organic Reactions

“…Integrating such methods with computational methods to calculate reaction rates in liquids, such as the continuum methods discussed in Section 3.2 and used in several of the discussed studies 24,52,54,73,78,79 , could provide a systematic approach to calculating many reaction rates in different solvents. This large volume of reaction rates could then be used to train a machine learning model to predict solvent effect from molecular properties.…”

“…and Radom [54][55][56][57] . In one such study, when various radicals were added to methyl acrylate using DFT calculations with continuum solvation, the rate of reaction in various solvents correlated well with the dipole moment of the solvents; however, it was argued that a multipole approach was still needed 52 .…”

“…Polar solvents still had a small effect on the rate if the charge transfer from the reactants to transition states, calculated using a Mulliken charge analysis 58 , was low. Wong and Radom performed calculations using the self-consistent isodensity polarizable continuum model (SCIPCM) 54,59 . In the case of radicals with saturated substituents adding to alkenes with saturated substituents, addition of any solvent increases the reaction barrier.…”