Radical Addition to Alkenes:  Further Assessment of Theoretical Procedures

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 .