TOC

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  1. Introduction and Retrospective
  2. Structure of TSs and Intrinsic Reaction Coordinate of Radical Addition
  3. Objects of Investigation: Basic Set of Reactions
  4. Thermochemical Aspect
  5. Energy Partitioning
  6. Results of the PMO Theory Approach
  7. Alternative Form of the PMO Equation: Effective Electronegativity of Reagent
  8. Comparison of Direct TS Calculations with Indices of Reactivity
  9. Empirical Schemes of Vinyl Monomers' Relative Reactivity in Radical Addition Reactions
  10. Conclusions
  11. References

Radical addition to the vinyl C=C bond: Quantum chemistry model of the reaction

Objects of Investigation: Basic Set of Reactions

To make a background of discussions of reactivity, the following set of reagents was chosen: radicals CH3•, CH3O•, CCl3•, CF3•, and HOO• and vinyl monomers CH2CHX: ethene (Eth, H), propene (Prop, CH3), styrene (St, C6H5), vinylacetate (VA, OCOCH3), methylacrylate (MA, COOCH3), acrylic nitryl (AN, CN3), vinylchloride (VCl, Cl), and vinylidenchloride (VCl2). For these monomers, both positions of attack were considered. In addition, reactivity of monomers with conjugated bonds toward CH3• and HOO• were considered *. The total number of calculated TSs was somewhere around 100.

All TSs had the same topology as shown above. Among possible conformations of products those ones were chosen that had a former radical group perpendicular to the former plane of the monomer. Table I presents the main geometric characteristics of TSs of β-attack (attack on the unsubstituted position of the C=C bond). One may judge from the table that TS geometries within a series remain practically constant. Comparison of rprod/r≠ values shows that the CH3• addition reaction has the earliest TS and the HOO• addition the latest, in due agreement with experimental facts.

* Butadiene (Bu2ene), hexatriene (Hex3ene), vinylnaphthalene (ViNaph), 2- and 4-vinylpyridines (ViPy), and 2- and 4-vinylpyrimidines (ViPm).

Table I. Geometries of TSs of β-addition of radicals to the main set of monomers.
ParameterHOO•CH3•CH3O•CCl3•CF3•
r≠, distance between reactive centers in TS (rprod – in product); φ, angle of attack; θ, torsion of CH bonds relative to the horizontal plain.
r≠ (Å)1.945 ± 0.012.24 ± 0.022.05 ± 0.022.21 ± 0.012.22 ± 0.01
φ (°)101105100107106
θ (°)1510111311
RCC (Å)1.405 ± 0.011.385 ± 0.011.395 ± 0.011.395 ± 0.011.395 ± 0.01
rprod/r≠0.735 ±.0050.685 ±.0050.689 ±.0010.702 ±.0050.721 ±.003