Showing papers by "Thomas Bally published in 2000"

Electronic Absorption Spectra of Neutral Pentacene (C22H14) and Its Positive and Negative Ions in Ne, Ar, and Kr Matrices

Abstract: The absorption spectra of neutral pentacene (C22H14) and its radical cation (C22H14+) and anion (C22H14-) isolated in solid Ne, Ar, and Kr have been measured from the ultraviolet to the near-infrared. The associated vibronic band systems and their spectroscopic assignments are discussed together with the physical and chemical conditions governing molecular anion production in matrices doped with alkali metal (Na or K). TD-DFT calculations were carried out to assist in the assignments for the measured spectral features of the pentacene ions.

Hydrogen-Transferred Radical Cations of NADH Model Compounds. 1. Spontaneous Tautomerization

Abstract: The radical cations generated from three NADH model compounds were investigated with particular emphasis on the possible formation of tautomeric enol radical cations. To a small extent, hydrogen transfer occurs upon ionization of all three compounds in argon matrixes. In the bicyclic seven-membered ring derivative enolization continues in a thermal reaction at 12 K (with highly dispersive kinetics), or upon visible irradiation. These processes are not observed in methylcyclohexane glasses at 77 K, which indicates that a cage effect is preventing the attainment of a reactive conformation in this medium. The assignment of electronic spectra of the primary (keto) cations and of their tautomeric (enol) forms is supported by CASSCF/CASPT2 and TD-B3LYP calculations.

Spectroscopic Characterization of Matrix-Isolated Phenylcarbene and Cycloheptatetraene†

Abstract: The full UV/VIS and IR spectra are presented for phenylmethylene (1) and its rearrangement product, cycloheptatetraene (2). The spectra are assigned and discussed on the basis of CASSCF/CASPT2 excited-state and of scaled B3LYP force-field calculations, respectively. The optical spectrum of 1 is very similar in appearance to that of the iso-π-electronic benzyl radical, which demonstrates that the unpaired σ-electron does not interfere noticeably with the π-electronic structure. The IR spectrum of 1 is not very well reproduced by the unscaled B3LYP calculations because of pronounced anharmonicity of the Ph−C−H bending mode, and even a differentiated scaling of the harmonic force field cannot fully remedy this. In contrast, the IR spectrum of 2 is very well reproduced and the corresponding valence force field shows the expected properties for this cyclic allene.

Attempts to model neutral solitons in polyacetylene by ab initio and density functional methods. The nature of the spin distribution in polyenyl radicals

Abstract: Different quantum chemical methods have been examined, in order to test their abilities to model long polyenyl radicals and, in the limit, a neutral soliton in polyacetylene. For polyenyl radicals up to C9H11, it was possible to use a range of methods up to CCSD(T). A number of interesting features of the geometries and spin distributions were found in these radicals. Coupled cluster methods reproduced rather well the experimental ratios of the spins at each of the carbons in allyl, pentadienyl, and heptatrienyl radicals. In contrast, both restricted and unrestricted Hartree–Fock methods gave very poor results. CASSCF gave good geometries and π spin distributions, but only if all the π MOs were included in the active space. Both pure (BLYP) and hybrid (B3LYP) DFT methods were found to over-estimate the widths of solitons in long polyenyl chains.

Hydrogen-Transferred Radical Cations of NADH Model Compounds. 2. Sequential Electron−Proton Addition to NAD+

Abstract: Sequential electron−proton addition to different NAD+ model compounds was studied by pulse radiolysis. Under mildly acidic conditions, the radicals obtained by reduction of the cations are protonat...

The Radical Cation of syn‐Tricyclooctadiene and Its Rearrangement Products

Abstract: The syn dimer of cyclobuta- diene (tricyclo(4.2.0.0 2,5 )octa-3,7-diene, TOD) is subjected to ionization under different conditions and the resulting species are probed by optical and ESR spectroscopy. By means of quantum chemical modelling of the potential energy surfaces and the optical spectra, it is possible to assign the different products that arise spontaneously after ionization or after subsequent warming or illumination of the samples. Based on these findings, we propose a mechanistic scheme which involves a partitioning of the incipient radical cation of TOD between two electronic states. These two states engage in (near) activation- less decay to the more stable valence isomers, cyclooctatetraene (COT .a ) and a bis-cyclobutenylium radical cation BCB .a . The latter product undergoes further rearrangement, first to tetracy- clo(4.2.0.0 2,4 .0 3,5 )oct-7-ene (TCO .a ) and eventually to bicyclo(4.2.0)octa-2,4,7-tri- ene (BOT .a ) which can also be gener- ated photochemically from BCB .a or TCO .a . The surprising departure of syn- TOD .a from the least-motion reaction path leading to BOT .a can be traced to

Ion chemistry of anti-o,o dibenzene.

Abstract: The ion chemistry of anti-o,o′-dibenzene (1) was examined in the gaseous and the condensed phase. From a series of comparative ion cyclotron resonance (ICR) mass spectrometry experiments which involved the interaction of Cu+ with 1, benzene, or mixtures of both, it was demonstrated that 1 can be brought into the gas phase as an intact molecule under the experimental conditions employed. The molecular ions, formally 1.+ and 1.−, were investigated with a four-sector mass spectrometer in metastable-ion decay, collisional activation, charge reversal, and neutralization–reionization experiments. Surprisingly, the expected retrocyclization to yield two benzene molecules was not dominant for the long-lived molecular ions; however, other fragmentations, such as methyl and hydrogen losses, prevailed. In contrast, matrix ionization of 1 in freon (77 K) by γ-radiation or in argon (12 K) by X-irradiation leads to quantitative retrocyclization to the cationic dimer of benzene, 2.+. Theoretical modeling of the potential-energy surface for the retrocyclization shows that only a small, if any, activation barrier is to be expected for this process. In another series of experiments, metal complexes of 1 were investigated. 1/Cr+ was formed in the ion source and examined by metastable ion decay and collisional activation experiments, which revealed predominant losses of neutral benzene. Nevertheless, comparison with the bis-ligated [(C6H6)2Cr]+ complex provided evidence for the existence of an intact 1/Cr+ under these experimental conditions. No evidence for the existence of 1/Fe+ was obtained, which suggests that iron mediates the rapid retrocyclization of 1/Fe+ into the bis-ligated benzene complex [(C6H6)2Fe]+.

Radical cations of perinaphthocyclopropanes. Conditions for the observation of 1,3-perinaphthadiyl radical cations

Abstract: γ-Irradiation of 7,7a-dihydro-6bH-cycloprop[a]acenaphthylene (1) and its 7,7-dimethyl derivative (2) in haloalkane glasses at 77 K yields persistent parent radical cations of these compounds. On visible irradiation, 1˙++++ is transformed into the radical cation of phenalene, 3˙+++, which was also generated independently from neutral 3. The transformation 1˙++++→3˙+++ presumably proceeds by opening of the central cyclopropane bond and subsequent H-shift in the resulting perinaphthadiyl radical cation which is, however, not observed. Surprisingly, the dimethyl derivative, 2˙++, is also transformed into a phenalene radical cation by visible photolysis. In this case the mechanism is probably a different one, involving a distonic diyl cation obtained by cleavage of a lateral cyclopropane bond. In contrast to the above, the ring opening of the dicyclopropa[a,g]pyracyclene 4 radical cation proceeds stepwise, and the perinaphthadiyl cation intermediate can be observed. A second photochemical ring opening yields the radical cation of 2,7-dihydro-2,2,7,7-tetramethylpyrene that was generated independently from the neutral precursor. The electronic absorption spectra of all observed radical cations are discussed in conjunction with the corresponding photoelectron spectra (where available) and with quantum chemical calculations. p