- Mass-Spectrometric Study on Ion-Molecule Reactions of CF3+ with Nitrogen-Containing Benzene Derivatives, Pyridine, Pyrrole, and Acetonitrile at Near-Thermal Energy
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The gas-phase ion-molecule reactions of CF3+ with nitrogen-containing benzene derivatives (C6H5Y : Y = NH2, NO2, and CN), pyridine, pyrrole, and acetonitrile have been studied at near-thermal energy using an ion-beam apparatus.The major product channels are charge transfer for aniline (71.7 +/- 0.5), O- abstraction for nitrobenzene (91.7 +/- 0.5percent), electrophilic addition leading to initial adducts ions for benzonitrile (97.5 +/- 0.8percent), acetonitrile (100percent), and pyridine (94.8 +/- 0.4percent), and electrophilic addition followed by HF elimination for pyrrole (80.0 +/- 1.4percent).The reaction mechanism is discussed based on product ion distributions and theoretical calculations of the energies of reaction pathways.
- Tsuji, Masaharu,Aizawa, Masato,Ujita, Hiroki,Nishimura, Yukio
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- Hydrogen atom adducts to nitrobenzene: Formation of the phenylnitronic radical in the gas phase and energetics of wheland intermediates
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The phenylnitronic radical (1) was prepared in the gas phase by collisional electron transfer to stable C6H5NO2H+ cation (1+) and found to be stable on the microsecond time scale. The major unimolecular dissociation of 1 was loss of OH radical to form nitrosobenzene as determined by variable-time neutralization-reionization mass spectrometry. Ab initio calculations at the effective QCISD(T)/6-311+G(3df,2p) level and combined Moller-Plesset and density functional theory calculations identified loss of OH as the lowest-energy dissociation of 1 that proceeded at the thermochemical threshold with no reverse activation barrier. Dissociations of 1 by loss of syn- and anti-HONO and a hydrogen atom were more endothermic than loss of OH and had activation barriers above the thermochemical thresholds. The internal energy of 1 formed by electron transfer in the ground electronic state (X) was insufficient to cause the observed dissociations. The dissociations are postulated to take place from the metastable excited electronic B state formed by vertical electron transfer. Wheland intermediates for hydrogen atom additions to the ortho (2), meta (3), para (4), and ipso (5) positions in nitrobenzene were calculated to be 75, 98, 78, and 101 kJ mol-1 less stable than 1. Radicals 2-4 existed in substantially deep potential energy wells to allow their generation as transient intermediates. Radical 5 resided in a shallow potential energy minimum and was predicted to dissociate exothermically to benzene and NO2. Relative thermal rate constants for hydrogen atom additions to nitrobenzene were calculated and found to correlate with the regioselectivities for additions of other radicals.
- Polasek, Miroslav,Turecek, Frantisek
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- Unimolecular Dissociation Rate Constants: Chlorobenzene Cations Revisited by Using a New Method
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Unimolecular dissociation rates of metastable molecular ions formed at the laser focus of a time-of-flight mass spectrometer (TOFMS) have been determined by measuring the relative concentrations of parent metastable ions and the daugther fragment ions as a function of their retained kinetic energies upon dissociation.This is accomplished by the use of a simple one-stage ion mirror/energy analyzer.For a given reflection potential on the ion mirror, only those ions with a kinetic energy equal to or less than the reflection potential are detected.In this way, the relative concentrations of the parent and daughter ions as a function of their position of dissociation in the time-of-flight (TOF) lens can be measured, and consequently the dissociation lifetime can be derived.This technique has been applied to measuring the unimolecular dissociation rate of chlorobenzene ion (C6H5Cl+) in the three-photon energy range of 13.64-13.92 eV through resonance-enhanced two-photon state-selected ionization of chlorobenzene in a supersonic molecular jet.The method demonstrates a minimum dynamic range of 105 to 2*107 s-1 and is in good agreement with previous work.The data suggest that rotational temperature plays a role in the unimolecular dissociation lifetime of the metastable ions.
- Stanley, R. J.,Cook, M.,Castleman, A. W.
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- Kinetic energy release in thermal ion-molecule reactions: The Nb2+ (benzene) single charge-transfer reaction
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We have adapted the techniques originally developed to measure ion kinetic energies in ion cyclotron resonance (ICR) spectrometry to study the single charge-transfer reaction of Nb2+ with benzene under thermal conditions in a Fourier transform ion cyclotron resonance mass spectrometer (FTICRMS).The partitioning of reaction exothermicity among the internal and translational modes available is consistent with a long-distance electron-transfer mechanism, in which the reactants approach on an ion-induced dipole attractive potential and cross to a repulsive potential at a critical separation of ca. 7.5 Angstroem when electron transfer occurs.The reaction exothermicity, 5.08 eV, is partitioned to translation of Nb+, 0.81 +/- 0.25 eV, translation of C6H6+, 1.22 +/- 0.25 eV, and internal excitation of C6H6+ to produce the la2u electronic state, which is ca. 3 eV above to ground state of the ion.We have also studied the kinetics of the reaction of Nb2+ with benzene and determined the rate constant, k = 1.4*10-9 cm3 molecule-1 s-1, and the efficiency, 0.60, of the process.These also support the proposed charge-transfer mechanism.In addition to the charge-transfer pathway, which accounts for 95percent of the reaction products, Nb2+ is observed to dehydrogenate benzene to form Nb2+ (benzyne).This process implies D(Nb2+ -benzyne) above 79 kcal/mol.
- Gord, James R.,Freiser, Ben S.,Buckner, Steven W.
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- Infrared multiple-photon dissociation of the nitrobenzene radical cation. A paradigm for competitive reactions
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The dissociation of nitrobenzene cation displays a variety of surprising and even apparently nonstatistical reaction behaviors. We have used infrared multiple-photon dissociation experiments to further study the reactions of this system. These experiments along with a previous photoelectron photoion coincidence study indicate that, for some products, the nitrobenzene cation dissociates to form an ion-neutral complex and then reassociates to give the phenyl nitrite cation. A reaction mechanism involving statistical dissociation is shown to account for the experimental data.
- Osterheld, Thomas H.,Baer, Tomas,Brauman, John I.
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- Unimolecular reaction dynamics from Kinetic Energy Release Distributions. III. A comparative study of the halogenobenzene cations
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The translational kinetic energy release distribution (KERD) in the halogen loss reaction of the chloro-, bromo-, and iodobenzene cations has been experimentally determined in the microsecond time scale and theoretically analyzed by the maximum entropy method. The KERD is constrained by the square root of the translational energy, i.e., by the momentum gap law. This can be understood in terms of quantum-mechanical resonances controlled by a matrix element involving a localized bound state and a rapidly oscillating continuum wave function, as in the case of a vibrational predissociation process. The energy partitioning between the reaction coordinate and the set of the remaining coordinates is nearly statistical, but not quite: less translational energy is channeled into the reaction coordinate than the statistical estimate. The measured entropy deficiency leads to values of the order of 80% for the fraction of phase space sampled by the pair of fragments with respect to the statistical value. In the case of the dissociation of the chlorobenzene ion, it is necessary to take into account a second process which corresponds to the formation of the chlorine atom in the excited electronic state 2P1/2 in addition to the ground state 2P3/2. The observations are compatible with the presence of a small barrier (of the order of 0.12 eV) along the reaction path connecting the D2A1 state of C6H5Cl+ to the Cl(2P1/2) + C6H5+(X1A1) asymptote.
- Urbain,Leyh,Remacle,Lorquet,Flammang,Lorquet
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- A combined theoretical and experimental study of the dissociation of benzene cation
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Variational Rice-Ramsperger-Kassel-Marcus (RRKM) theory calculations of the energy and angular momentum dependence of the rate constant for the dissociation of C6H6+ into C6H5+ and an H atom are reported.In these variational calculations both the definition of the reaction coordinate and its value are independently optimized.A model potential-energy surface which interpolates between a Morse potential at short range and an ion-induced dipole potential at long range is employed in these variational calculations.The fully optimized variational results indicate that the transition state for this dissociation occurs at separation distances of about 3-4 Angstroem and that the available phase space in the transition state is typically a factor of 5 lower than the predicted by phase space theory.Experimental measurements were made of the time-resolved product ion intensity resulting from the laser-induced dissociation of a thermal (ca. 375 K) distribution of benzene cations.An ion cyclotron resonance trap was used over a range of photolysis wavelengths from 266 to 285 nm.The observed time dependences in the product ion signals are a result of both dissociative and radiative relaxation processes with a deconvolution procedure yielding estimated dissociation rate constants. Satisfactory agreement between the theoretical and experimental results, including the previous experimental results of Neusser and co-workers is obtained for an assumed dissociation energy of 3.88 eV to the lowest triplet state of C6H5+.
- Klippenstein, Stephen J.,Faulk, James D.,Dunbar, Robert C.
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- Time-Dependent Mass Spectra and Breakdown Graphs. 6. Slow Unimolecular Dissociation of Bromobenzene ions at Near Threshold Energies
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Time-resolved photoionization mass spectrometry (TPIMS) in the millisecond range has been employed to study the reaction C6H5Br(1+) radical -> C6H5(1+) + Br radical in bromobenzene.Experimental photoionization efficiency curves were fitted with a QET model calculation by assuming a critical energy Eo = 2.76 eV and an activation entropy ΔS(excit.) = 8.07 eu.The activation entropy coresponds to the totally loose (orbiting) transititon state.Ultraslow unimolecular dissociations having microcanonical rate coefficients k(E) /= 10 /sec, at near threshold energies, can be sampled by TPIMS, in spite of competing IR-raditive decay of the parent ion.
- Malinovich, Y.,Arakawa, R.,Haase, G.,Lifshitz, C.
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- C6H5Br+? → C6H5+ + Br? occurs via orbiting transition state
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Photodissociation of the bromobenzene molecular ion has been investigated on a nanosecond time scale by photodissociation mass-analyzed ion kinetic energy spectrometry. The rate constant and kinetic energy release distribution have been determined. The present experimental data together with the previous milli- to microsecond data have been compared with theoretical calculations. The rate-energy data available over 6 orders of magnitude in time scale could be fit with a nontotally loose transition state model (RRKM) reported by Rosenstock and co-workers. However, the model has been found to predict rate constant values larger than theoretically acceptable at high internal energy. The completely loose transition state model, namely the reaction occurring via orbiting transition state, seems to be a better description of the reaction.
- Lim, Sang-Hyun,Choe, Joong Chul,Kim, Myung Soo
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- Mass-Spectrometric Study on Ion-Molecule Reactions of CF3+ with PhX at Near-Thermal Energy
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The gas-phase ion-molecule reactions of CF3+ with benzene, toluene, ethylbenzene, styrene, and ethynylbenzene have been studied at near-thermal energy using an ion-beam apparatus.The major product channels are electrophilic addition followed by HF elimination for benzene (93.4+/-2.2percent), toluene (84.3+/-2.4percent) and ethynylbenzene (76.9+/-0.9percent).The dominant product channels for ethylbenzene are electrophilic addition followed by C2H4 and C2H4+HF eliminations (78.9+/-4.7percent), while those for styrene are electrophilic addition followed by one or two HF eliminations and C2H2F2 elimination (91.7+/-0.4percent).Only ethynylbenzene gives an initial adduct ion with a small branching ratio of 6.2+/-0.4percent.As minor product channels, hydride transfer occurs for benzene (6.6+/-2.2percent) and toluene (7.8+/-1.5percent), and charge transfer takes place for toluene (7.9+/-2.0percent), styrene (8.3+/-1.4percent), and ethynylbenzene (6.2+/-0.4percent).The reaction mechanisms are discussed on the basis of product ion distributions and semi-empirical calculations of potential energies of reaction pathways.
- Tsuji, Masaharu,Aizawa, Masato,Nishimura, Yukio
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- On The Structure and Thermochemistry of the van der Waals Molecule C6H6*HCl and Its Photoion (C6H6*HCl)(1+)
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The dissociation energy of the van der Waals complex C6H6*HCl was determined to be 4.79 +/- 0.12 kcal mol-1 by measurement of the difference between the threshold for the dissociative photoionization of the complex and the ionization potential of C6H6.This value is compared to potential well depths obtained by analysis of centrifugal distortion constants for the same complex.The photoionization efficiency function for the production of (C6H6*HCl)(1+) from C6H6*HCl was obtained between 1280 and 1380 Angstroem.A weak threshold for direct ionization at 1357 +/- 7 Angstroem (9.14 +/- 0.05 eV) leads to a dissociation energy for (C6H6*HCl)(1+) of 7.3 +/- 1.2 kcal mol-1.The shape of the photoionization efficiency function is interpreted in terms of the involvement of the structure of the complex in the dissociation dynamics.Standard heats of formation are reported for C6H6*HCl (-10.4 kcal mol-1) and (C6H6*HCl)(1+) (204 kcal mol-1).
- Walters, E. A.,Grover, J. R.,White, M. G.,Hui, E. T.
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- Unimolecular Reaction Dynamics from Kinetic Energy Release Distributions. 1 - Dissociation of Bromobenzene Ions
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Preliminary data on a new method for extracting dynamic data from the kinetic energy release distribution of metastable fragment ions are presented.This method can give information concerning the energy threshold of the reaction and the energy dependence of the rate constant (i.e. its logarithmic derivative).It was applied to bromine loss from bromobenzene cations.
- Lorquet, J. C.,Leyh, B.
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- Molecular Beam Chemistry. Formation of Phenyl Cations from C6H5X Molecules
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Ractive scattering experiments between Cs(+) and C6H5X molecules with X=F, Br, I, and NO2 are reported and the results compared to previous experiments with X=Cl.The formation of the phenyl cation is discussed in terms of the intersections of two potentia
- Safron, S.A.,King, G.A.,Horvat, R.C.
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- Time-Dependent Mass Spectra and Breakdown Graphs. 13. Time-Resolved Photoionization Mass Spectrometry of 1,5-Hexadiyne
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Time-resolved photoionization in the millisecond range has been employed to study parallel dissociation reactions of the 1,5-hexadiyne radical cation.Experimental results were observed to compare successfully with RRKM/QET calculations, which assume isome
- Lifshitz, C.,Ohmichi, N.
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- One-photon infrared photodissociation of polyatomic ions in a fast beam
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Photodissociation of 22 vibrationally excited polyatomic ions, ranging in size from four to thirteen atoms, has been observed following absorption of a single CO2-laser photon. In a number of cases the infrared wavelength dependence of the process shows well-defined peaking, which can be interpreted as absorption at the normal-mode frequencies of the ion. Kinetic energy release and order-of-magnitude fragmentation rate information have also been obtained for both photon-induced and metastable decomposition of a number of the ions. RRKM theory modeling indicates that these data are compatible with a quasi-equilibrium theory description of the unimolecular decomposition of highly vibrationally excited molecular ions. Considered as resulting from the last photon absorption of a multiphoton dissociation process, these results are relevant to understanding infrared multiphoton photochemistry of polyatomic molecules.
- Coggiola,Cosby,Helm,Peterson,Dunbar, Robert C.
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- Time-Resolved Unimolecular Dissociation of Styrene Ion. Rates and Activation Parameters
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The rate of unimolecular dissociation of styrene ion into benzene ion plus acetylene was measured by time-resolved photodissociation at 308 nm in the ICR ion trap.A rate constant of 1.10E5 s-1 was obtained at a total ion internal energy of 4.20 eV.Using accurate heat of formation data for the reactant and products, a 0 K reaction enthalpy of 2.42 eV was assigned.RRKM rate-energy curves were calculated for comparison with the present measurement and with previous photoionization coincidence (PEPICO) data.The shape of the RRKM curve matches experiment, andquantitative rate agreement is obtained assuming an activation energy of 2.32 eV and a tight transition state.The activation parameters derived from the activated complex, ΔS(excit.)(1000 K) = -6.4 eu and Ainfinite(1000 K) = 2.3E12 s-1, are compared with values for other ion dissociations and with neutral-molecule rearrangements and rearrangement dissociations.
- Dunbar, Robert C.
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- Dissociative proton transfer reactions of H3+, N2H+, and H3O+ with acyclic, cyclic, and aromatic hydrocarbons and nitrogen compounds, and astrochemical implications
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A flowing afterglow-selected ion flow drift tube has been used to measure the rate coefficients and product ion distributions for reactions of H3O+, N2H+, and H3+ with a series of 16 alkanes, alkenes, alkynes, and aromatic hydrocarbons as well as acrylonitrile, pyrrole, and pyridine. Exothermic proton transfer generally occurs close to the collision rate. The reactions of H3O+ are mostly nondissociative and those of H3+ are mostly dissociative, but many reactions, especially those of N2H+, have both dissociative and nondissociative channels. The dissociative channels result mostly in H2 and/or CH4 loss in the small hydrocarbons and in toluene, loss of C2H2 from acrylonitrile, and loss of HCN from pyrrole. Only nondissociative proton transfer is observed with benzene, pyridine, and larger aromatics. Drift tube studies of N2H+ reactions with propene and propyne showed that increased energy in the reactant ion enhances fragmentation. Some D3+ reactions were also investigated and the results suggest that reactions of H3+ with unsaturated hydrocarbons B proceed through proton transfer that forms excited (BH+)* intermediates. Pressure effects suggest that a fraction of the (BH+)* intermediates decomposes too rapidly to allow collisional stabilization in the flow tube (t -8 s). The other low-energy (BH+)* intermediates are formed by the removal of up to 40% of the reaction exothermicity as translational energy, and these intermediates result in stable BH+ products. The results suggest that, in hydrogen-dominated planetary and interstellar environments, the reactions of H3+ can convert C2-C6 hydrocarbons to smaller and less saturated molecules, but polycyclic aromatics are stable against decomposition by this mechanism. The dissociative reactions of H3+ can therefore favor the accumulation of small unsaturated hydrocarbons and aromatics in astrochemical environments.
- Milligan, Daniel B.,Wilson, Paul F.,Freeman, Colin G.,Meot-Ner (Mautner), Michael,McEwan, Murray J.
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p. 9745 - 9755
(2007/10/03)
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- Photodissociation Dynamics of Nitrobenzene Molecular Ion on a Nanosecond Time Scale
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Photodissociation dynamics of nitrobenzene molecular ion has been investigated on a nanosecond time scale by mass-analyzed ion kinetic energy spectrometry.Photodissociation has been found to occur through various reaction channels.Three dissociation channels to C6H5O(1+), C6H5(1+), and NO(1+) have been observed together with a consecutive dissociation to C5H5(1+) via C6H5O(1+).The photodissociation rate constant of the molecular ion and the kinetic energy release distributions in each channel have been determined.The rate constant of the second step of the consecutive reaction has been determined in real time also.It has been found that the direct bond cleavage to C6H5(1+) occurs in competition with the rearrangement processes to C6H5O(1+) and NO(1+) on a nanosecond time scale.Statistical theories have been used to gain insight into the dynamics of the reactions.
- Hwang, Wan Goo,Kim, Myung Soo,Choe, Joong Chul
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p. 9227 - 9234
(2007/10/03)
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- Structures of Product Ions C6H7(+) and C6H9(+) of Ion-Molecule Reactions with Allyl Bromide
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The ion-molecule reactions of allyl bromide with the molecular ion of allyl bromide and with its major fragment, the allyl ion, yield the C6H7(+) and C6H9(+) ions.The structure of these product ions was explored by means of photofragmentation with laser light in the 10 μm region and by proton transfer reaction to selected reagents.These product ions were also formed by other reactions and their reactivities compared.In both cases the presence of at least two populations is demonstrated.For C6H9(+) these two populations are initially present, whereas for C6H7(+) an isomer is formed by the infrared light before the loss of H2.When this ion is produced by photofragmentation of C6H9(+), at least one third, stable isomer is formed.Two isomers of C6H5(+) are formed in the photofragmentation of C6H7(+), but only one form photofragments further by loss of C2H2.The use of non-linear least-squares fitting does not allow definite conclusions to be drawn concerning the kinetics of the consecutive photofragmentations.
- Zhu, Zhiqing,Gaeumann, Tino
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p. 1111 - 1118
(2007/10/02)
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- Thermal kinetics in small systems. II. Generalized Arrhenius plots with applications to the dissociation of benzene and substituted-benzene cations
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Rate constans for the dissociation of isolated benzene and substituted-benzene cations are analyzed.In the first section Arrhenius activation energies are deduced.A primitive version of the adiabatic channel model is then used to reduce these activation energies to their values at zero Kelvin.A value for the heat of formation at that temperature of the C6H5+ ion equal to 1130 kJ/mol is reported.Attention is drawn to a remarkable constancy (and also two anomalies) in the frequency factors for these reactions.The predictive capability of the adiabatic channel model is then examined.It is found to yield kinetic isotope effects in good agreement with experiment.
- Klots, Cornelius E.
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p. 2513 - 2520
(2007/10/02)
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- Rotationally Resolved Intramolecular Processes in Benzene
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The influence of the molecular rotation on intramolecular processes is investigated for both a non-reactive and a reactive large polyatomic system.The non-reactive system chosen is the S1 state of benzene, C6H6.Doppler-free spectra of the 1401 band show isolated rotational perturbations due to a highly selective coupling of the light zero-order states to dark rovibrational states within the S1 state.These perturbations are caused by J, K dependent Coriolis coupling, which will induce energy randomization in the excited molecule.The reactive process investigated is the unimolecular dissociation of rotationally selected excited benzene cations.It is found that energy is completely randomized and the benzene cations behave like a statistical system.Dissociation rates are found to decrease with rotational quantum number, J.It is concluded that rotational selection has to be included in precise experiments and theoretical models of intramolecular processes. - Keywords: Chemical Kinetics / Elementary Reactions / Gases / Multi-photon Processes / Spectroscopy, Ultraviolet
- Neusser, H. J.,Schubert, U.,Riedle, E.,Kiermeier, A.,Kuehlewind, H.,Schlag, E. W.
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p. 322 - 329
(2007/10/02)
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- Thermochemistry of Silaethylene and Methylsilylene from Experiment and Theory
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Fourier transform ion cyclotron resonance spectroscopy has been used to examine the deprotonation energetics of the methylsilyl cation, CH3SiD2+, to yield silaethylene and methylsilylene proton affinities of 205 +/- 3 and 215 +/- 4 kcal/mol, respectively.These values combined with the known heat of formation of methylsilyl cation, yield ΔHof 298(CH2SiH2) = 43 +/- 3 kcal/mol and ΔHof 298(CH3SiH) = 53 +/- 4 kcal/mol.These results are corroborated by ab initio generalized valence bond-configuration interaction calculations which indicate that silaethylene is more stable than methylsilylene by 11.6 kcal/mol, in excellent agreement with the experimental difference (10 +/- 3 kcal/mol).The adiabatic ionization potential of methylsilylene is calculated to be 8.22 eV, which is lower than the value of 8.85 eV determined for silaethyene using photoelectron spectroscopy.
- Shin, Seung Koo,Irikura, Karl K.,Beauchamp, J. L.,Goddard, William A.
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- Time-Resolved Photodissociation of Chlorobenzene Ion in the ICR Spectrometer
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Following photoexcitation by 308-nm light, the dissociation of chlorobenzene ions was followed over times from 7 to 1000 μs after the laser pulse, using the ICR spectrometer configured for rapid detection.Time resolution was of the order of 10 μs and it is pointed out that the ICR technique is well suited to observing ion dissociation processes on time scales ranging from 10 μs to seconds.By use of literature rate-energy relations, the time resolved photodissociation measurement was calibrated to give the internal energy of the dissociating parent ion.Collisional thermalization and collisionless thermalization of the parent ion were characterized by observing time-resolved dissociation at varying delay times after electron-impact ion formation, giving rate constants of 0.4 s-1 for collisionless energy dissipation and 1.3x10-10 cm3/(molecule s) for collisional energy dissipation in chlorobenzene bath gas.
- Dunbar, Robert C.
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p. 2801 - 2804
(2007/10/02)
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- Reaction of Ammonia with Accelerated Benzoyl Ions under Multiple-collision Conditions in a Triple Quadrupole Instrument
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The reaction of benzoyl ion with ammonia in multiple-collision conditions in the second quadrupole assembly of a triple quadrupole mass spectrometer at (laboratory) ion kinetic energies from 0 to 20 eV produced the even-electron ions +, m>+ (m=0,1) and + (n=0,1,2,3) and the odd-electron ions p>+. (p=0,1).Thermochemical information could not be obtained under multiple-collision conditions: both exotermic and endotermic reactions were observed, with no translational-energy onset measurable for the endothermic pr/cesses, nor decrease in the yield of the exothermic processes at high energies.The behaviour of cluster-ions intensities as pressure varied was qualitatively as expected.There are pressure and energy regions where spectra change little; if this feature were to be general, it would point to some utility for these conditions in qualitative analysis.
- White, Earl L.,Tabet, Jean-Claude,Bursey, Maurice M.
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p. 132 - 139
(2007/10/02)
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- Time-Dependent Mass Spectra and Breakdown Graphs. 7. Time Resolved Photoionization Mass Specrometry of Iodobenzene. The Heat of Formation of C6H5+
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Time-resolved photoionization mass spectrometry (TPIMS) in the millisecond range has been employed to study the reaction C6H5I+. -> C6H5+ + I. in iodobenzene.Experimental photoionization efficiency curves were fitted with a QET model calculation assuming a critical enregy E0 = 2.38 eV and an activation entropy ΔS* = 7.44 eu.The activation entropy corresponds to the totally loose (orbiting) transition state.Discrepancies with previous data for bromobenzene are traced to the neutral thermochemistry and resolved, yielding a 0 K heat of formation of the phenyl cation ΔHf00(C6H5+) = 1141 +/- 10 kJ mol-1.
- Malinovich, Y.,Lifshitz, C.
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p. 2200 - 2203
(2007/10/02)
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- GAS-PHASE ION-MOLECULE NITRATION CHEMISTRY: THE NITRATION OF AROMATIC RADICAL CATIONS BY NITROGEN DIOXIDE.
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The authors have observed in gas-phase work at 10 torr that aromatic cations (ArH** plus multiplied by (times) )readily react with NO//2 to give the sigma -bonded ArH-NO//2** plus . Under the same conditions the reactions of nitronium ion (NO//2** plus ) with neutral ArH yield only ArH** plus multiplied by (times) and ArH-O** plus multiplied by (times) , products of electron or oxygen transfer. The radical cations giving the sigma intermediate were those from benzene, methylbenzenes up to mesitylene and 1,2,4-trimethylbenzene, phenol, and several fluorobenzenes up to 1,2,4-trifluorobenzene. In contrast tetrafluorobenzenes, furan, and pyridine did not yield the sigma intermediate. Naphthalene reacted to give the sigma -bonded C//1//0H//8-NO//2** plus , but surprisingly slowly. The parallels to condensed-phase nitration are yet to be fully demonstrated, but it is clear that the odd-electron system NO//2 plus ArH** plus multiplied by (times) yields the sigma -bonded intermediate leading to nitration.
- Schmitt,Buttrill Jr.,Ross
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p. 926 - 930
(2007/10/02)
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- Production and Unimolecular Decay Rate of Internal Energy Selected Molecular Ions in a Laser Reflection Time-of-Flight Mass Spectrometer
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Monoenergetic, internal energy selected polyatomic ions are produced in a new two-laser pump-pump experiment.Low energetic parent ions produced with the first laser are further excited with the tunable second laser to a specified internal energy after extraction from the ion source.The metastable decay of energy-selected ions is monitored in a reflectron time-of-flight mass spectrometer, and as a first example, dissociation rates have been measured for two decay channels of C6H6+.The result clearly demonstrates directly that C-loss as well as H-loss channels are competing and originate from the same electronic state of the benzene cation.
- Kuehlewind, H.,Neusser, H. J.,Schlag, E. W.
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p. 6104 - 6106
(2007/10/02)
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- Diazonium salts in photochemistry. I. Quenching of triplet photosensitizers
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Aromatic diazonium tetrafluoroborates have been shown to be excellent triplet quenchers.Typical rate constants obtained using laser flash photolysis are in the 10*109 M-1 s-1 range in acetonitrile at 300 K.With most senzitizers the interaction leads to the dediazoniation reaction, via either energy transfer or electron transfer mechanisms.In the case of anthracene, where the rate constant for benzenediazonium tetrafluoroborate is (5.4+/-0.3)*109 M-1s-1, the reaction involves the transfer of an electron, leading to the formation of anthracene radical-cation, which has been characterized by absorption spectroscopy.High energy senzitizers such as benzophenone, are believed to lead to aryl cations and nitrogen; the process may involve some degree of charge transfer, since it is somewhat faster when electron donating substituents are present in the diazonium salt.
- Scaiano, J. C.,Kim-Thuan, Nguyen
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p. 2286 - 2291
(2007/10/02)
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