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.

Deoxygenation of nitroaromatics is a classic synthetic method for the construction of nitrogen heterocycles. The generally accepted mechanism involves exhaustive deoxygenation to a singlet nitrene. We present theoretical evidence for an alternative, 6pi-electron 5-atom electrocyclization of nitroso-styrenes, -stilbenes, and -biphenyls to nitronates. A downstream 1,5-H shift and tautomerization leads to N-hydroxy heterocycles. [reaction: see text]

The activation enthalpies for the reactions of cyclopentadiene with ethylene, acrylonitrile, fumaronitrile, maleonitrile, vinylidene cyanide, tricyanoethylene, and tetracyanoethylene have been computed with the Hartree-Fock quantum mechanical method and a variety of density functionals (B3LYP, BPW91, and MPW1K) and compared to classic experimental data obtained by Sauer, Wiest, and Mielert (Chem. Ber. 1964, 97, 3183). Some significant errors in B3LYP activation enthalpies involving cyano groups, especially 3 or 4 cyano groups, have been identified, whereas HF and MPW1K reproduce substituent effects quite accurately.

The performance of two local exchange functionals, OLYP and O3LYP, developed by Handy and Cohen (Mol. Phys. 2001, 99, 403), has been assessed for predicting activation barriers and reaction energies for a set of eleven pericyclic reactions for which experimental data are well established. The results are compared to B3LYP and CBS-QB3 results previously reported (Guner et al. J. Phys. Chem. A 2003, 107, 11445).