Spectroscopic and computational studies on the rearrangement of ionized [1.1.1]propellane and some of its valence isomers: the key role of vibronic coupling.

The [1.1.1]propellane radical cation 1(•+), generated by radiolytic oxidation of the parent compound in argon and Freon matrices at low temperatures, undergoes a spontaneous rearrangement to form the distonic 1,1-dimethyleneallene (or 2-vinylideneallyl) radical cation 3(•+) consisting of an allyl radical substituted at the 2-position by a vinyl cation. In similar matrix studies, it is found that the isomeric dimethylenecyclopropane radical cation 2(•+) also rearranges to 3(•+). The unusual molecular and electronic structure of 3(•+) has been established by the results of ESR, UV-vis, and IR spectroscopic measurements in conjunction with detailed theoretical calculations. Also of particular interest is an NIR photoinduced reaction by which 3(•+) is cleanly converted to the vinylidenecyclopropane radical cation 4(•+), a process that can be represented in terms of a single electron transfer from the allyl radical to the vinyl cation followed by allyl cation cyclization. The specificity of this photochemical reaction provides additional strong chemical evidence for the structure of 3(•+). Theoretical calculations reveal the decisive role of vibronic coupling in shaping the potential energy surfaces on which the observed ring-opening reactions take place. Thus vibronic interaction in 1(•+) mixes the (2)A(1)' ground state, characterized by its "non-bonding" 3a(1)' SOMO, with the (2)E'' first excited state resulting in the destabilization of a lateral C-C bond and the initial formation of the methylenebicyclobutyl radical cation 5(•+). The further rearrangement of 5(•+) to 3(•+) occurs via 2(•+) and proceeds through two additional lateral C-C bond cleavages characterized by transition states of extremely low energy, thereby explaining the absence of identifiable intermediates along the reaction pathway. In these consecutive ring-opening rearrangements, the "non-bonding" bridgehead C-C bond in 1(•+) is conserved and ultimately transformed into a normal bond characterized by a shorter C-C bond length. This work provides strong support for the Heilbronner-Wiberg interpretation of the vibrational structure in the photoelectron spectrum of 1 in terms of vibronic coupling.

Field evaluation of essential oils for reducing attraction by the Japanese beetle (Coleoptera: Scarabaeidae).

Forty-one plant essential oils were tested under field conditions for the ability to reduce the attraction of adult Japanese beetles, Popillia japonica Newman (Coleoptera: Scarabaeidae), to attractant-baited or nonbaited traps. Treatments applied to a yellow and green Japanese beetle trap included a nonbaited trap, essential oil alone, a Japanese beetle commercial attractant (phenethyl proprionate:eugenol:geraniol, 3:7:3 by volume) (PEG), and an essential oil plus PEG attractant. Eight of the 41 oils reduced attractiveness of the PEG attractant to the Japanese beetle. When tested singly, wintergreen and peppermint oils were the two most effective essential oils at reducing attractiveness of the PEG attractant by 4.2x and 3.5x, respectively. Anise, bergamont mint, cedarleaf, dalmation sage, tarragon, and wormwood oils also reduced attraction of the Japanese beetle to the PEG attractant. The combination of wintergreen oil with ginger, peppermint, or ginger and citronella oils reduced attractiveness of the PEG attractant by 4.7x to 3.1x. Seventeen of the 41 essential oils also reduced attraction to the nonbaited yellow and green traps, resulting in 2.0x to 11.0x reductions in trap counts relative to nonbaited traps. Camphor, coffee, geranium, grapefruit, elemi, and citronella oils increased attractiveness of nonbaited traps by 2.1x to 7.9x when tested singly, but none were more attractive than the PEG attractant. Results from this study identified several plant essential oils that act as semiochemical disruptants against the Japanese beetle.

Ionized bicyclo[2.2.2]oct-2-ene: a twisted olefinic radical cation showing vibronic coupling.

The bicyclo[2.2.2]oct-2-ene radical cation (1(.+)) exhibits matrix ESR spectra that have two very different types of gamma-exo hydrogens (those hydrogens formally in a W-plan with the alkene pi bond), a(2H) about 16.9 G and a(2H) about 1.9 G, instead of the four equivalent hydrogens as would be the case in an untwisted C(2v) structure. Moreover, deuterium substitution showed that the vinyl ESR splitting is not resolved (and under about 3.5 G); this is also a result of the twist. Enantiomerization of the C(2) structures is rapid on the ESR timescale above 110 K (barrier estimated at 2.0 kcalmol(-1)). Density functional theory calculations estimate the twist angle at the double bond to be 11-12 degrees and the barrier as 1.2-2.0 kcalmol(-1). Single-configuration restricted Hartree-Fock (RHF) calculations at all levels that were tried give untwisted C(2v) structures for 1(.+), while RHF calculations that include configuration interactions (CI) demonstrate that this system undergoes twisting because of a pseudo Jahn-Teller effect (PJTE). Significantly, twisting does not occur until the sigma-orbital of the predicted symmetry is included in the CI active space. UHF calculations at all levels that include electron correlation (even semiempirical) predict twisting at the alkene pi bond because they allow the filled alpha and the beta hole of the SOMO to have different geometries. The 2,3-dimethylbicyclo[2.2.2]oct-2-ene radical cation (2(.+)) is twisted significantly less than 1(.+), but has a similar temperature for maximum line broadening. Neither the 2,3-dioxabicyclo[2.2.2]octane radical cation (3(.+)) nor its 2,3-dimethyl-2,3-diaza analogue (5(.+)) shows any evidence of twisting. Calculations show that the orbital energy gap between the SOMO and PJTE-active orbitals for 3(.+) is too large for significant PJTE stabilization to occur.