A Study of C5H10 radical cations by collisionally activated dissociation,charge stripping,and low energy ion-molecule reactions

C₅H₁₀ radical cations generated from a variety of olefins and cycloalkanes were investigated by collisionally activated dissociation, charge stripping, and low energy ion-molecule reactions. It has been determined that all of the isomers studied can be distinguished by charge stripping, whereas collisionally activated dissociation and the ion-molecule reactions are less informative. The radical cations from cyclopentane, methylcyclobutane and substituted cyclopropanes retain their cyclic structures for at least a fraction of the population having lifetimes in the microsecond range.     

Electronic absorption spectroscopy of matrix-isolated polycyclic aromatic hydrocarbon cations. II. The phenanthrene cation (C14H10+) and its 1-methyl derivative

The ultraviolet, visible, and near infrared absorption spectra of phenanthrene (C14H10), 1-methylphenanthrene [(CH3)C14H9], and their radical ions [C14H10+; (CH3)C14H9+], formed by vacuum-ultraviolet irradiation, were measured in neon matrices at 4.2 K. The associated vibronic band systems and their spectroscopic assignments are discussed. The oscillator strengths were calculated for the phenanthrene ion and found lower than the theoretical predictions. This study presents the first spectroscopic data for phenanthrene and its methyl derivative trapped in a neon matrix where the perturbation of the isolated species by its environment is minimum; a condition crucial to astrophysical applications.     

Collisionally activated dissociation of 2,4,6-triphenylpyridinium cations

Thresholds for the appearance of fragment ions allowed the estimation of threshold fragmentation energies (TFE) for the collisionally activated dissociation (CAD) in the gas phase of laser-desorbed pyridine-ring substituted N-benzylpyridinium cations to form pyridine and a carbocation. p-Methylbenzylpyridinium cation underwent an alternative CAD into pyridinium cation and the p-quinodimethane. The TFE are discussed in comparison with the energy differences (ΔΔH_f = ΔH_f(Py) + ΔH_f(R⁺) ? ΔH_f(Py ⁺R) calculated by the AM1 method to provide strong evidence for benzyl to tropylium cation rearrangement in an ion-molecule pair.     

Isomerization and dissociation of n-butylbenzene radical cation

Fragmentation mechanisms of ionized butylbenzene to give m/z 91 and m/z 92 fragment ions have been examined at the G3B3 and G3MP2B3 levels of theory. It is shown that the energetically favored pathways lead to tropylium, Tr(+), and methylene-2,4-cyclohexadiene, MCD(?+), ions. Formation of m/z 91 benzyl ions, Bz(+), by a simple bond fission (SBF) process, needs about 30 kJ/mol more energy than Tr(+). Possible formation of C(7)H(8)(?+) ions of structures different from the retro-ene rearrangement (RER) product, MCD(?+), has been also considered. Comparison with experimental data of this "thermometer" system is done through a kinetic modeling using Rice-Ramsperger-Kassel-Marcus (RRKM) and orbiting transition state (OTS) rate constant calculations on the G3MP2B3 0 K energy surface. The results agree with previous experimental observation if (i) the competitive formation of Tr(+) and Bz(+) is taken into account in the m/z 91 pathway, and (ii) the stepwise character of the RER fragmentation is introduced in the m/z 92 fragmentation route.     

An ab initio calculation of the dissociation C2H2(1Σ+) → C2H(2Σ+) + H(2S)

The collinear dissociation of acetylene to C₂H and H is studied by a generalized self-consistent procedure. The dissociation energy, the C-H force constant and stretching frequency are computed.     

On the model dependence of kinetic shifts in unimolecular reactions: the dissociation of the cations of benzene and n-butylbenzene

Statistical adiabatic channel model/classical trajectory (SACM/CT) calculations have been performed for transitional mode dynamics in the simple bond fission reactions of C(6)H(6)(+) --> C(6)H(5)(+) + H and n-C(6)H(5)C(4)H(9)(+) --> C(7)H(7)(+) + n-C(3)H(7). Reduced-dimensionality model potentials have been designed that take advantage of ab initio results as far as available. Average anisotropy amplitudes of the potentials were fitted by comparison of calculated specific rate constants k(E,J) with measured values. The kinetic shifts of the calculated k(E) curves and the corresponding bond energies E(0)(J=0), derived as 3.90 +/- 0.05 eV for C(6)H(6)(+) and 1.78 +/- 0.05 eV for n-C(6)H(5)C(4)H(9)(+), were in good agreement with literature values from thermochemical studies. Kinetic shifts from fixed tight activated complex Rice-Ramsperger-Kassel-Marcus (RRKM) theory, which also reproduces the measured k(E), were larger than the present SACM/CT results as well as earlier results from variational transition state theory (for C(6)H(6)(+)). The approach using RRKM theory was found to underestimate E(0)(J=0) by about 0.2-0.3 eV. A simplified SACM/CT-based method is also proposed which circumvents the trajectory calculations and allows derivation of E(0)(J=0) on the basis of measured k(E) and which provides similar accuracy as the full SACM/CT treatment.     

Unsaturated dodecahedranes--in quest of the C20H14 1,4,16-triene and C20H12 1,4,10(14),16-tetraene, and their cations and anions

The highly pyramidal, highly strained 1,4,16-dodecahedratriene (4) and C20H12 1,4,10(14),16-dodecahedratetraene (5) are cage olefins with an intriguing "inner life". For 5 DFT calculations give information about the energetic and geometrical consequences of one-/two-electron oxidation and reduction. Attempts to prepare 4 and 5 through thermal retro[2+2]/[4+2]cycloaddition strategies proved unsuccessful. Still, the C20H14/C20H12 cage cations and anions are liberated upon electron impact or gas-discharge ionization of their thermally extremely stable tris-/tetrakisanthraceno-anellated derivatives. Mass-selection (photoelectron (PE) characterization) of the anions failed, however, due to the very small anion intensity, the preferential formation of hydrogen-poor ions, and minor cage disruption.     

Tri- and tetraprotonated ethane (C2H9(3+) and C2H10(4+)) containing five- and six-coordinate carbons

Triprotonated ethane (C(2)H(9)(3+)) 4 and tetraprotonated ethane (C(2)H(10)(4+)) 6 were found by ab initio MP2/cc-pVTZ calculations as viable energy minima. Their structure has three and four two-electron three-center (2e-3c) bonds, respectively. In contrast, calculations showed no minimum-energy structure on the potential energy surface of pentaprotonated ethane (C(2)H(11)(5+)). Charge-charge repulsion may approach its limit in this case. Sufficient stabilization of polycations by Schmidbaur-type auration with (C(6)H(5))(3)PAu, an isolobal analogue of H(+), should be possible for the preparation of the polyaurate derivatives of poly protonated ethane allowing their structural study.     

Electronic structure and photochemical rearrangements of matrix-isolated C8H10 radical cations

The C₈H₁₀ isomers 1,3,5,7-octatetraene (OTE), 1,3,5-cyclooctatriene (COT) and bicyclo[4.2.0]octa-1,4-diene (BCO) were subjected to ionization by X-irradiation in argon matrices at 20 K. The electronic structure of the parent radical cations is discussed on the basis of their spectral properties and qualitative theoretical considerations. Photolysis of the cyclic cations leads to the formation of OTE¹ in at least six different conformation which can be distinguished by selective bleaching experiments. The complex band structure of the all-trans-OTE¹absorptions is demonstrated to arise from the presence of this species in at least five different matrix sites. By very narrow-bandwidth irradiation, single sites can be bleached or populated and the resulting difference spectra allow a detailed vibronic analysis of all-trans-OTE¹     

Collisionally activated dissociation of some pyridinium cations: Novel fragmentation pathways

Three novel collisionally induced dissociation pathways, additional to the usual formation of pyridine or pyridinium cation, are described for laser-desorbed N-substituted pyridinium cations. Particularly prevalent is the formation of an ion of m/z 94, corresponding to [PyCH₃]⁺. Doubly charged pyridiniums tend to lose H⁺, and one example of the apparent formation of the neutral radical C₅H₆N˙ is reported.     

Acridin-9(10H)-one-based blue thermally activated delayed fluorescence materials: improvement of color purity and efficiency stability

A series of donor-acceptor-donor (D-A-D) type blue thermally activated delayed fluorescence (TADF) emitters, namely, 2,7-DtBuCz-AD, 3,6-DtBuCz-AD, 3,6-DMAC-AD, and 3,6-DMAC-AD-CF₃, were developed with highly rigid acridin-9(10H)-one (i.e. acridone [AD]) as acceptor. The regioisomeric effect study revealed that the attachment of donors at 3,6-sites of AD ring dramatically enhanced TADF ratio in comparison with the 2,7-site isomer. On the one hand, by varying donors from dimethylacridine (DMAC) to tert-butylcarbazole (tBuCz) at 3,6-sites of AD ring, the emission color purity of blue TADF emitters was improved from sky blue to deep blue. On the other hand, by introducing trifluoromethyl (CF₃) onto 9-site phenyl ring of 3,6-DtBuCz-AD, the efficiency stability of the sky blue emission for 3,6-DMAC-AD-CF₃ was remarkably improved. The deep blue organic light-emitting diode (OLED) of 3,6-DtBuCz-AD exhibited a maximum external quantum efficiency (EQE_max) of 17.88% with CIE coordinates of (0.15, 0.08), which is among the best performances ever reported for deep blue TADF-OLEDs. The sky-blue OLED of 3,6-DMAC-AD realized an EQE_max of 23.16%. And with the incorporation of CF₃, the sky blue device of 3,6-DMAC-AD-CF₃ exhibited extremely low efficiency loss of only 5.1% at the high brightness of 1,000 cd/m².     

Release of 3H and 14C during sampling and speciation in activated concrete

Journal of Radioanalytical and Nuclear Chemistry - Characterisation of contaminated and activated decommissioning waste require sampling of the studied material for the analysis of different...     

Quantum-chemical study of silacyclohexanes C5H10SiHCN, C5H10SiH(t-Bu), C5H10Si(t-Bu)CN, and C5H10SiHF

By quantum-chemical calculations at the M06-2X/aug-cc-pVTZ level of theory geometrical parameters, dipole moments, polarizabilities, first hyperpolarizabilities and relative energies of the axial and equatorial conformers in gaseous phase were determined for 1-cyano-1-silacyclohexane, 1-tert-butyl-1-silacyclohexane, 1-tert-butyl-1-cyano-1-silacyclohexane, and 1-fluoro-1-silacyclohexane. For the cyano group and fluorine atom the axial position is more preferable whereas for tert-butyl group, equatorial one. Polarizabilities of conformers are similar but optical anisotropy of equatorial conformers of C₅H₁₀SiHCN and C₅H₁₀SiH(t-Bu) molecules is much larger than that of axial conformers. Upon substitution in nitriles of C¹ atom by Si atom the hyperpolarizability is many times increased.     

Ab initio/GIAO-CCSD(T) study of propenoyl (H2C=CH-CO+) and isopentenoyl ((CH3)2C=CH-CO+) cations and their superelectrophilic protonated dications

Structures of superelectrophilic protonated propenoyl (H2C=CH-COH2+) and isopentenoyl ((CH3)2C=CH-COH2+) dications and their parent cations were calculated using ab initio methods at the MP2/6-311+G and MP2/cc-pVTZ levels. Energies were calculated using Gaussian-2 (G2) theory. The alpha-carbon (Calpha) protonated 3 and 7 were found to be the global minima for protonated propenoyl and isopentenoyl dications, respectively. 13C NMR chemical shifts of the cations were also calculated using the GIAO-CCSD(T), GIAO-MP2 and GIAO-SCF methods. 13C NMR chemical shifts of the related tert-butyl cation ((CH3)3C+) and protonated tert-butyl dication ((CH3)2CCH4(2+)) were also computed at the same level to compare and explore the effect of the additional charge in dications.     

Theoretical study on the HACA chemistry of naphthalenyl radicals and acetylene: The formation of C12H8, C14H8, and C14H10 species

The hydrogen-abstraction-C₂H₂-addition (HACA) chemistry of naphthalenyl radicals has been studied extensively, but there is a significant discrepancy in product distributions reported or predicted in literature regarding appearance of C₁₄H₈ and C₁₄H₁₀ species. Starting from ab initio calculations, a comprehensive theoretical model describing the HACA chemistry of both 1- and 2-naphthalenyl radicals is generated. Pressure-dependent kinetics are considered in the C₁₂H₉, C₁₄H₉, and C₁₄H₁₁ potential energy surfaces including formally direct well-skipping pathways. On the C₁₂H₉ PES, reaction pathways were found connecting two entry points: 1-naphthalenyl (1-C₁₀H₇) + acetylene (C₂H₂) and 2-C₁₀H₇ + C₂H₂. A significant amount of acenaphthylene is predicted to be formed from 2-C₁₀H₇ + C₂H₂, and the appearance of C₁₄H₈ isomers is predicted in the model simulation, consistent with high-temperature experimental results from Parker et al. At 1500 K, 1-C₁₀H₇ + C₂H₂ mostly generates acenaphthylene through a formally direct pathway, which predicted selectivity of 66% at 30 Torr and 56% at 300 Torr. The reaction of 2-C₁₀H₇ with C₂H₂ at 1500 K yields 2-ethynylnaphthalene as the most dominant product, followed by acenaphthylene mainly generated via isomerization of 2-C₁₀H₇ to 1-C₁₀H₇. Both the 1-C₁₀H₇ and 2-C₁₀H₇ reactions with C₂H₂ form some C₁₄H₈ products, but negligible phenanthrene and anthracene formation is predicted at 1500 K. A rate-of-production analysis reveals that C₁₄H₈ formation is strongly affected by the rates of H-abstraction from acenaphthylene, 1-ethynylnaphthalene, and 2-ethynylnaphthalene, so the kinetics of these reactions are accurately calculated at the high level G3(MP2,CC)//B3LYP/6-311G^** level of theory. At intermediate temperatures like 800 K, acenaphthylene + H are the leading bimolecular products of 1-C₁₀H₇ + C₂H₂, and 1-acenaphthenyl radical is the most abundant C₁₂H₉ isomer due to its stability. The predicted product distribution of 2-C₁₀H₇ + C₂H₂ at 800 K, in contrast to the results of Parker et al is predicted to consist primarily of species containing three fused benzene rings—for example, phenanthrene and anthracene—as the leading products, indicating HACA chemistry is valid from two to three ring polycyclic aromatic hydrocarbons under some conditions. Further experiments are needed for validation.     

Isomerization of chemically activated bicyclo[3.1.0]hex-2-ene and related C6H8 isomers

Singlet methylene was reacted with cyclopentadiene to give chemically activated bicyclo[3.1.0]hex-2-ene (BCH). The rate of isomerization of BCH to 1,4-cyclohexadiene, 1,3-cyclohexadiene, cis-1,3,5-hexatriene, and l-methylcyclopentadiene is compared with calculated rate constants using the RRKM theory and measured or estimated thermal Arrhenius parameters. Subsequent isomerizations of the C₆H₈ products are also measured and calculated. These include 1,4-cyclohexadiene to benzene and the reversible reactions between 1,3-cyclohexadiene, cis-1,3,5-hexatriene, and trans-1,3,5-hexatriene. The results provide new data for several of these reactions which have not been observed in thermal studies. Agreement between the observed and calculated rates using the strong collision assumption is satisfactory except for the trans-1,3,5-hexatriene to cis-1,3,5-hexatriene reaction.