The suitability of the self-consistent reaction field (SCRF) strategy for the study of solutes in chloroform solution has been examined. The SCRF method developed by Miertus, Scrocco and Tomasi (MST) has been parametrized at both ab initio 6-31G* and semiempirical AM1, MNDO, and PM3 levels. The reliability of the MST/SCRF model has been assessed from the comparison with results derived from classical Monte Carlo-free energy perturbation simulations and from mixed Monte Carlo-quantum mechanical/molecular mechanical computations, as well as with experimental data. The parametrized MST/SCRF method estimates the free energy of solvation with a root-mean-square deviation of 0.4 kcal/mol from the experimental value for the molecules studied. Further confidence in the optimized method stems from its ability to reproduce the tautomeric change of 2-and 4-pyridone upon transfer from gas phase to chloroform and the partition coefficient for compounds not considered in the parametrization. The results indicate the suitability of the MST/SCRF model for the study of solvent effects in dilute chloroform solutions.

The toluene radical ion C6H5CH3 •+, generated by resonance two-photon ionization, does not react with a single isobutene molecule (i-C4H8) which has a significantly higher ionization potential (ΔIP = 0.42 eV). However, a reaction is observed involving two i-C4H8 molecules, to form the dimer ion C8H16 •+. A coupled reaction of dimer formation and charge transfer to the dimer is exothermic if the product is an ionized hexene with a low IP. Correspondingly, the observed nominal second-order rate coefficients, (5−25) × 10-12 cm3 s-1, are enhanced by a factor of >105 over the expected value for direct endothermic charge transfer. Pressure and concentration effects suggest a sequential mechanism that proceeds through a C6H5CH3 •+(i-C4H8) reactive π complex. The complex can isomerize to a nonreactive CH3C6H4-t-C4H9 •+ adduct, or react with a second i-C4H8 molecule to form a C6H5CH3 •+(i-C4H8)2 complex, in which the olefin molecules are activated by the aromatic ion. Similar reactions are observed in the benzene/propene system with a somewhat larger ΔIP of 0.48 eV, suggesting that the charge density on the olefin in the complex is still sufficient to activate it for nucleophilic attack. However, aromatic/olefin systems with ΔIP > 0.87 eV show no olefin dimer formation. At low [i-C4H8] and [Ar] number densities, the rate of formation of C8H16 •+ is proportional to [i-C4H8]2[Ar]. The corresponding fourth-order rate coefficient shows a strong negative temperature coefficient with k = 11 × 10-42 cm9 s-1 at 300 K and 2 × 10-42 cm9 s-1 at 346 K, suggesting that the mechanism can be efficient in low-temperature industrial and interstellar environments. The direct formation of the dimer bypasses the monomer olefin cation and its consequent side-reactions, and directs the products selectively into radical ion polymerization. The products and energy relationships that apply in the gas phase are observed also in clusters.