Experimental and theoretical data are provided for a set of 11 pericyclic reactions of unsaturated hydrocarbons. Literature experimental data are evaluated and standardized to ∆H q 0K for comparison to theory. Hartree-Fock, MP2, CASSCF, CASPT2, density functional theory (B3LYP, BPW91, MPW1K, and KMLYP functionals), and CBS-QB3 transition-structure geometries, activation enthalpies and entropies, and reaction enthalpies and entropies for these reactions are reported and are compared to experimental results. For activation enthalpies, several density functionals rival CASPT2 and CBS-QB3 for closest agreement with experiment, while CASPT2 and CBS-QB3 provide the most accurate heats of reaction. Transition-structure geometries are reproduced well by all methods with the exception of the Cope rearrangement and cyclopentadiene dimerization transition structures.

The Mayo and Flory mechanisms for the self-initiation of styrene polymerization were explored with B3LYP and BPW91 density functional calculations. The Diels-Alder dimer (AH) is the key intermediate, and the lowest energy pathway for AH formation is a stepwise mechanism via a gauche/sickle (*M2*Gs) or gauche/U-shaped (*M2*Gu) diradical. Ring closure of the 1,4-diradical to diphenylcyclobutane (DCB) is predicted to have a lower barrier than ring closure to AH. Dynamic effects are likely to play an important role in determining the rate of AH versus DCB formation. Hydrogen transfer from AH to styrene to generate two monoradical species is predicted to be a reasonable process that initiates monoradical polymerization.

Highly functionalized, cyclobutene-containing adducts are afforded through intramolecular cycloadditions between cyclobutadiene and tethered dienes. The cycloaddition displays the following reactivity trend: cyclobutadiene serves as a dienophile in intramolecular reactions when it is connected to the diene through a four-atom tether. In cases where a three-atom linker separates the two reaction partners, the cyclobutadiene can function as both a diene and dienophile, affording a mixture of vinylcyclobutane (2 + 2) and cyclohexene-containing cycloadducts (4 + 2). Theoretical studies provide insight into the factors influencing the various pericyclic pathways operative in this system. In cases where cyclobutadiene functions as a diene to generate vinylcyclobutanes, these (2 + 2) adducts can be converted into the corresponding (4 + 2) cyclohexenyl products through a [3,3]-sigmatropic rearrangement.