Description: This article discusses carbon tunneling. Abstract: Calculations on the role of tunneling in the degenerate Cope rearrangements of semibullvalene (1) and barbaralane (3) predict that, at temperatures below 40 K, tunneling from the lowest vibrational level should make the temperature-independent rate constants k = 1.43 x 10(-3) s(-1) and k = 7.28 x 10(-9) s(-1), respectively. An experiment, using semibullvalene-2(4)-d1, is proposed to test the prediction of rapid tunneling by 1 at cryogenic temperatures.

Description: This article discusses cyclooctatetraenes. Abstract: (U)B3LYP calculations with the 6-31+G(d) and 6-311+G(2df) basis sets have been carried out on cyclooctatetraenes 6 and 7, in which the COT ring is tetrakis-annelated with α-dithio- or α-diselenocarbonyl groups. Transferring two electrons from the high-lying b1g and eu σ MOs in 6 and 7 into the unoccupied, nonbonding, COT π orbital is computed to be energetically favorable. The lowest D4h electronic state is calculated to be 3Eu, which formally contains 10 π electrons in the eight-membered ring and has two unpaired electrons in σ MOs. The 3Eu state undergoes a first-order Jahn-Teller distortion to form 6d and 7d, in which the pair of one-electron holes in the σ MOs is stabilized by the formation of two, two-center, three-electron bonds between pairs of chalcogen atoms that are diagonally across the eight-membered ring from each other. The corresponding open-shell singlets are computed to be about 1 kcal/mol lower in energy than the Jahn-Teller distorted triplets. Molecules 6i and 7i, in which the C-C bond in one four-membered ring is cleaved, are computed to be lower in energy than 6d and 7d. However, a substantial barrier is predicted to separate each of the two pairs of isomers ...

Description: This article discusses cooperative effects in the annelation of benzene by multiple etheno groups. Abstract: The results of B3LYP/6-31G(d) calculations show that there is a strong cooperative effect on the energies that result from annelating benzene with 1, 2, and 3 etheno groups, to form 3, 4, and 5 respectively. The etheno annelation energies have been decomposed into two major contributors - etheno annelation, with all the bonds in the six-membered ring constrained to have the same C-C bond length as in D6h benzene, and optimization of the C-C bond lengths in the annelated benzene ring. The energies computed for each of these two steps show that the etheno groups behave cooperatively in both of them. The origin of the cooperativity, calculated for each step, is described and discussed.

Description: This article discusses the effects of geminal methyl groups on the tunnelling rates in the ring opening of cyclopropylcarbinyl radical at cryogenic temperature. Abstract: CVT + SCT calculations on the rate of tunnelling at 20 K in the ring opening of cyclopropylcarbinyl radical, substituted with geminal methyl groups at a ring carbon (1b), have been performed. The calculations predict that, contrary to expectations based on the effect of mass on the rate of tunnelling, the geminal methyl substituents in 1b should make the rate of ring opening to 1,1-dimethyl-3-butenyl radical (2b) 10(4) times faster than the rate of ring opening of unsubstituted cyclopropylcarbinyl radical (1a) to 3-butenyl radical (2a) and almost 10(6) times faster than the rate of ring opening of 1b to 2,2-dimethyl1-3-butenyl radical (2c). The reasons for these unexpected findings are discussed.

Description: This article discusses photoelectron spectroscopic study of the oxyallyl diradical. The photoelectron spectrum of the oxyallyl (OXA) radical anion has been measured. The radical anion has been generated in the reaction of the atomic oxygen radical anion (O߭߫) with acetone. Three low-lying electronic states of OXA have been observed in the spectrum. Electronic structure calculations have been performed for the triplet states (3B2 and 3B1) of OXA and the ground doublet state (2A2) of the radical anion using density functional theory (DFT). Spectral simulations have been carried out for the triplet states based on the results of the DFT calculations. The simulations identifies a vibrational progression of the CCC bending mode of the 3B2 state of OXA in the lower electron binding energy (eBE) portion of the spectrum. On top of the 3B2 feature, however, the experimental spectrum exhibits additional photo-electron peaks whose angular distribution is distinct from that for the vibronic peaks of the 3B2 state. Complete active space self-consistent field (CASSCF) method and second-order perturbation theory based on the CASSCF wave function (CASPT2) have been employed to study the lowest singlet state (1A1) of OXA. The simulation based on the results of these electronic structure calculations establishes that ...