Showing papers by "Ffrancon Williams published in 1995"
TL;DR: In this article, the electronic and molecular structure of BOD{sup+} and its photoprecursor, the radical cation of cyclooctatetraene (COT{sup +}), are discussed on the basis of their EA spectra and ab initio calculations.
Abstract: The recently discovered access paths to the radical cation of bicyclo[3,3,0]octa-2,6-diene-4,8-diyl (BOD{sup +}) are explored by electronic absorption (EA) spectroscopy whereby previous ESR results are confirmed. The electronic and molecular structure of BOD{sup +} and of its photoprecursor, the radical cation of cyclooctatetraene (COT{sup +}), are discussed on the basis of their EA spectra and ab initio calculations. The ground and excited state potential surfaces common to the title cations are explored, and it is shown that the COT{sup +} {yields} BOD{sup +} photorearrangement proceeds mainly by virtue of a pronounced Jahn-Teller distortion of the second excited state of COT{sup +} ({sup 2}E). This distortion competes effectively with internal conversion to the first excited state, leads to an inversion of the ground state symmetry, and covers a substantial part of the reaction path leading to the bisallylic cation. 37 refs., 8 figs., 4 tabs.
27 citations
TL;DR: In this article, the electronic structure of 1,2-, 1,4-, and 1,5-DHP{sup +} is discussed on the basis of their photoelectron and optical spectra and INDO/S calculations.
Abstract: Starting from the recently characterized radical cation of bicyclo[3,3,0]octa-2,6-diene-4,8-diyl, four additional dihydropentalene radical cations (DHP{sup -4}) can be formed by phototautomerization in Freon glasses and argon matrices where they can be characterized by optical (Freon, argon) and ESR spectroscopy (Freon). Two of these DHP isomers can be prepared independently, while the cations of the other two are identified by analogy of their spectra with those of related compounds. The electronic structure of 1,2-, 1,4-, and 1,5-DHP{sup +}, which have linear and cross-conjugated triene {pi}-systems is discussed on the basis of their photoelectron and optical spectra and INDO/S calculations. The part of the C{sub 8}H{sub 8}{sup +} potential surface comprising all ten possible DHP{sup +} tautomers and some related valence isomers is explored by high-level ab initio calculations. An FMO-based set of rules for sigmatropic rearrangements in radical cations is presented and serves to rationalize the observed H-shifts. 40 refs., 12 figs., 4 tabs.
8 citations
TL;DR: In this article, the authors demonstrate the occurrence of isomerization in the gas phase by use of tandem (MS/MS) sector and Fourier transform (FT) mass spectrometries.
Abstract: Investigation by matrix-isolation ESR has shown that 4-vinylcyclohexene, 1, surprisingly undergoes isomerization to the bicyclo[3.2.1]oct-2-ene ion, 3. Here we demonstrate the occurrence of this isomerization in the gas phase by use of tandem (MS/MS) sector and Fourier transform (FT) mass spectrometries. The radical cations of 4-vinylcyclohexene (IE = 8.93 eV) or bicyclo[3.2.1]oct-2-ene (approximately 14 kcal/mol more stable than that of 4-vinylcyclohexene) were formed, in separate trials, in a chemical ionization (CI) source by electron ionization (EI). The radical cations were then studied by obtaining their collisionally activated decomposition (CAD) spectra. The CAD spectra are similar, indicating that the isomerization has occurred. Both the sector and the FT mass spectrometer results reflect those obtained in the matrix-isolation ESR investigation. That is isomerizes to 3 at high internal energy, but is stable at low internal energy. Two mechanisms explain this rearrangement. The second mechanism is questionable because the most stable olefin radical cation formed from 5 is that of bicyclo[2.2.2]-2-octene, which gives different ESR and CAD spectra than those of 1 or 3. The CAD spectrum of bicyclo[2.2.2]-2-octene radical cation indicates that the retro-Diels-Alder loss of ethylene is more facile than that from 1 or 3. 18 refs., 3 figs.
7 citations