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The primary and secondary fragmentation dynamics of iodobenzene following its ionization at 120 eV were determined using three-dimensional velocity map imaging and covariance analysis. Site-selective iodine 4d ionization was used to populate a range of excited polycationic parent states, which primarily broke apart at the carbon-iodine bond to produce I+ with phenyl or phenyl-like cations (C n H x + or C n H x 2 + , with n = 1 – 6 and x = 1 – 5). The molecular products were produced with varying degrees of internal excitation and dehydrogenation, leading to stable and unstable outcomes. This further allowed the secondary dynamics of C 6 H x 2 + intermediates to be distinguished using native-frame covariance analysis, which isolated these processes in their own centre-of-mass reference frames. The mass resolution of the imaging mass spectrometer used for these measurements enabled the primary and secondary reaction channels to be specified at the level of individual hydrogen atoms, demonstrating the ability of covariance analysis to comprehensively measure the competing fragmentation channels of aryl cations, including those involving intermediate steps. [ABSTRACT FROM AUTHOR]

Structural imaging of transient excited-state species is a key goal of molecular physics, promising to unveil rich information about the dynamics underpinning photochemical transformations. However, separating the electronic and nuclear contributions to the spectroscopic observables is challenging, and typically requires the application of high-level theory. Here, we employ site-selective ionisation via ultrashort soft X-ray pulses and time-resolved Coulomb explosion imaging to interrogate structural dynamics of the ultraviolet photochemistry of carbon disulfide. This prototypical system exhibits the complex motifs of polyatomic photochemistry, including strong non-adiabatic couplings, vibrational mode couplings, and intersystem crossing. Immediately following photoexcitation, we observe Coulomb explosion signatures of highly bent and stretched excited-state geometries involved in the photodissociation. Aided by a model to interpret such changes, we build a comprehensive picture of the photoinduced nuclear dynamics that follows initial bending and stretching motions, as the reaction proceeds towards photodissociation. Coulomb Explosion imaging is a promising technique to study the ultrafast nuclear dynamics which underpin molecular photochemistry. By initiating Coulomb explosion through soft X-ray ionization, the authors are able to image ultrafast nuclear dynamics of a prototypical photoreaction. [ABSTRACT FROM AUTHOR]

By employing the vacuum ultraviolet (VUV) laser velocity-map imaging (VMI) photoelectron scheme to discriminate energetic photoelectrons, we have measured the VUV-VMI-threshold photoelectrons (VUV-VMI-TPE) spectra of propargyl radical [C3H3(X2B1)] near its ionization threshold at photoelectron energy bandwidths of 3 and 7 cm-1 (full-width at half-maximum, FWHM). The simulation of the VUV-VMI-TPE spectra thus obtained, along with the Stark shift correction, has allowed the determination of a precise value 70 156 ± 4 cm-1 (8.6982 ± 0.0005 eV) for the ionization energy (IE) of C3H3. In the present VMI-TPE experiment, the Stark shift correction is determined by comparing the VUV-VMI-TPE and VUV laser pulsed field ionization-photoelectron (VUV-PFI-PE) spectra for the origin band of the photoelectron spectrum of the X+-X transition of chlorobenzene. The fact that the FWHMs for this origin band observed using the VUV-VMI-TPE and VUV-PFI-PE methods are nearly the same indicates that the energy resolutions achieved in the VUV-VMI-TPE and VUV-PFI-PE measurements are comparable. The IE(C3H3) value obtained based on the VUV-VMI-TPE measurement is consistent with the value determined by the VUV laser PIE spectrum of supersonically cooled C3H3(X2B1) radicals, which is also reported in this article. [ABSTRACT FROM AUTHOR]

Vacuum ultraviolet (VUV) laser pulsed field ionization-photoelectron (PFI-PE) spectroscopy has been applied to the study of the sulfur monoxide radical (SO) prepared by using a supersonically cooled radical beam source based on the 193 nm excimer laser photodissociation of SO2. The vibronic VUV-PFI-PE bands for the photoionization transitions SO+(X2Π1/2; v+ = 0) ← SO(X3Σ-; v = 0); and SO+(2Π3/2; v+ = 0) ← SO(X3Σ-; v = 0) have been recorded. On the basis of the semiempirical simulation of rotational branch contours observed in these PFI-PE bands, we have obtained highly precise ionization energies (IEs) of 83 034.2 ± 1.7 cm-1 (10.2949 ± 0.0002 eV) and 83 400.4 ± 1.7 cm-1 (10.3403 ± 0.0002 eV) for the formation of SO+(X2Π1/2; v+ = 0) and SO+(2Π3/2; v+ = 0), respectively. The present VUV-PFI-PE measurement has enabled the direct determination of the spin-orbit coupling constant (A0) for SO+(X2Π1/2,3/2) to be 365.36 ± 0.12 cm-1. We have also performed high-level ab initio quantum chemical calculations at the coupled-cluster level up to full quadruple excitations and complete basis set (CBS) extrapolation. The zero-point vibrational energy correction, the core-valence electronic correction, the spin-orbit coupling, and the high-level correction are included in the calculation. The IE[SO+(X2Π1/2,3/2)] and A0 predictions thus obtained are found to be in remarkable agreement with the experimental determinations. [ABSTRACT FROM AUTHOR]

The ionisation energy (IE) of ZrO, the bond dissociation energies (D 0s) and the heats of formation at 0 K (ΔH ○ 0K f ) and 298 K (ΔH ○ 298K f ) for ZrO/ZrO+ are predicted by the coupled cluster methods utilizing upto single, double, triple and quadruple excitations and complete basis set limit approximation (CCSDTQ/CBS). The CCSDTQ/CBS approach also includes the zero-point vibrational energy, high-order correlation, core-valence (CV) electronic and spin-orbit coupling corrections while the scalar relativistic contribution is handled by employing pseudopotential basis sets. The present calculations yield IE(ZrO) = 6.794 eV and D 0(Zr+-O) − D 0(Zr-O) = −0.193 eV which are in good agreement with the respective experimental values of 6.81272(10) eV and −0.1788(1) eV determined in a two-colour laser-pulsed field ionisation-photoelectron study. The CCSD(T) and multireference configuration interaction (MRCI) methods with CV correlations included give equally remarkable predictions of the harmonic frequencies and the bond lengths for ZrO/ZrO+. This study together with the previous investigations has demonstrated that, with pseudopotential basis sets, the CCSDTQ/CBS protocol can be readily extended to investigate 4d-transition metal-containing diatomic molecules to yield comparable accuracy (±20 meV) to that achieved in the IE and D 0 predictions of 3d-transition metal-containing species. [ABSTRACT FROM AUTHOR]

The ionisation energy (IE) of ZrO, the bond dissociation energies (D 0s) and the heats of formation at 0 K (ΔH ○ 0K f ) and 298 K (ΔH ○ 298K f ) for ZrO/ZrO+ are predicted by the coupled cluster methods utilizing upto single, double, triple and quadruple excitations and complete basis set limit approximation (CCSDTQ/CBS). The CCSDTQ/CBS approach also includes the zero-point vibrational energy, high-order correlation, core-valence (CV) electronic and spin-orbit coupling corrections while the scalar relativistic contribution is handled by employing pseudopotential basis sets. The present calculations yield IE(ZrO) = 6.794 eV and D 0(Zr+-O) − D 0(Zr-O) = −0.193 eV which are in good agreement with the respective experimental values of 6.81272(10) eV and −0.1788(1) eV determined in a two-colour laser-pulsed field ionisation-photoelectron study. The CCSD(T) and multireference configuration interaction (MRCI) methods with CV correlations included give equally remarkable predictions of the harmonic frequencies and the bond lengths for ZrO/ZrO+. This study together with the previous investigations has demonstrated that, with pseudopotential basis sets, the CCSDTQ/CBS protocol can be readily extended to investigate 4d-transition metal-containing diatomic molecules to yield comparable accuracy (±20 meV) to that achieved in the IE and D 0 predictions of 3d-transition metal-containing species. [ABSTRACT FROM AUTHOR]

The ferrate(vi) ion, [FeO4]2−, has attracted much interest in recent years because of its potential use as a green oxidant in organic synthesis and water treatment. Although there have been several reports on the use of ferrate(vi) for the oxidation of alcohols to the corresponding carbonyl compounds, the mechanism remains unclear. In this work, the kinetics of the oxidation of a series of alcohols with α-C–H bond dissociation energies ranging from 81 to 95 kcal mol−1 have been studied by UV/Vis spectrophotometry. The reactions are first-order in both [FeO4]2− and [alcohol]. The deuterium isotope effects for the oxidation of methanol/d4-methanol, ethanol/d6-ethanol and benzyl alcohol/d7-benzyl alcohol are 18.0 ± 0.1, 4.1 ± 0.1 and 11.2 ± 0.1, respectively. A linear correlation is found between the second-order rate constants and the α-C–H bond dissociation energies (BDEs) of the alcohols, consistent with a hydrogen atom transfer (HAT) mechanism. The proposed HAT mechanism is supported by DFT calculations. [ABSTRACT FROM AUTHOR]

Wereport a study of the unimolecular dissociation of BrCH2CH2O radicals produced from the photodissociation of BrCH2CH2ONO at 351/355 nm. Using both a crossed laser-molecularbeam scattering apparatus with electron bombardment detection anda velocity map imaging apparatus with tunable VUV photoionizationdetection, we investigate the initial photodissociation channels ofthe BrCH2CH2ONO precursor and the subsequentdissociation of the vibrationally excited BrCH2CH2O radicals. The only photodissociation channel of the precursor wedetected upon photodissociation at 351 nm was O–NO bond fission.C–Br photofission and HBr photoelimination do not compete significantlywith O–NO photofission at this excitation wavelength. The measuredO–NO photofission recoil kinetic energy distribution peaksnear 14 kcal/mol and extends from 5 to 24 kcal/mol. There is alsoa small signal from lower kinetic energy NO product (it would be 6%of the total if it were also from O–NO photofission). We usethe O–NO photofission P(ET) peaking near 14 kcal/mol to help characterize the internalenergy distribution in the nascent ground electronic state BrCH2CH2O radicals. At 351 nm, some but not all of theBrCH2CH2O radicals are formed with enough internalenergy to unimolecularly dissociate to CH2Br + H2CO. Although the signal at m/e=93 (CH2Br+) obtained with electron bombardmentdetection includes signal both from the CH2Br productand from dissociative ionization of the energetically stable BrCH2CH2O radicals, we were able to isolate the signalfrom CH2Br product alone using tunable VUV photoionizationdetection at 8.78 eV. We also sought to investigate the source ofvinoxy radicals detected in spectroscopic experiments by Miller andco-workers (J. Phys. Chem.A2012, 116, 12032) from the photodissociation of BrCH2CH2ONO at 351 nm. Using velocity map imaging and photodissociatingthe precursor at 355 nm, we detected a tiny signal at m/e= 43 and a larger signal at m/e= 15 that we tentatively assign to vinoxy. Anunderlying signal in the time-of-flight spectra at m/e= 29 and m/e= 42, the two strongest peaks in the literature electron bombardmentmass spectrum of vinoxy, is also apparent. Comparison of those signalstrengths with the signal at HBr+, however, shows thatthe vinoxy product does not have HBr as a cofragment, so the priorsuggestion by Miller and co-workers that the vinoxy might result froma roaming mechanism is contraindicated. [ABSTRACT FROM AUTHOR]

A new mechanochemical method was developed to convert polymer wastes, polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC), to fuel gases (H2, CH4, and CO) under ball-milling with KMnO4 at room temperature. By using various solid-state characterizations (XPS, SEM, EDS, FTIR, and NMR), and density functional theory calculations, it was found that the activation followed the hydrogen atom transfer (HAT) mechanism. Two metal oxidant molecules were found to abstract two separate hydrogen atoms from the α–CH and β–CH units of substrates, [–βCH2–αCH(R)–]n, where R = H in PE, R = γCH3 in PP, and R = Cl in PVC, resulting in a di-radical, [–βCH•–αC•(R)–]. Subsequently, the two unpaired electrons of the di-radical were recombined into an alkene intermediate, [–βCH = αC(R)–], which underwent further oxidation to produce H2, CH4, and CO gases. [ABSTRACT FROM AUTHOR]

A correction to the article "High-resolution threshold photoelectron study of the propargyl radical by the vacuum ultraviolet laser velocity-map imaging method" which was published online on August 19, 2013 is presented.

We have conducted a two-color visible-ultraviolet (VIS-UV) resonance-enhanced laser photoionization efficiency and pulsed field ionization-photoelectron (PFI-PE) study of gaseous cobalt carbide (CoC) near its ionization onset in the total energy range of 61 200-64 510 cm-1. The cold gaseous CoC sample was prepared by a laser ablation supersonically cooled beam source. By exciting CoC molecules thus generated to single N′ rotational levels of the intermediate CoC*(2Σ+; v′) state using a VIS dye laser prior to UV laser photoionization, we have obtained N+ rotationally resolved PFI-PE spectra for the CoC+(X1Σ+; v+ = 0 and 1) ion vibrational bands free from interference by impurity species except Co atoms produced in the ablation source. The rotationally selected and resolved PFI-PE spectra have made possible unambiguous rotational assignments, yielding accurate values for the adiabatic ionization energy of CoC(X2Σ+), IE(CoC) = 62 384.3 ± 0.6 cm-1 (7.73467 ± 0.00007 eV), the vibrational frequency ωe+ = 985.6 ± 0.6 cm-1, the anharmonicity constant ωe+χe+ = 6.3 ± 0.6 cm-1, the rotational constants (Be+ = 0.7196 ± 0.0005 cm-1, αe+ = 0.0056 ± 0.0008 cm-1), and the equilibrium bond length re+ = 1.534 Å for CoC+(X1Σ+). The observation of the N+ = 0 level in the PFI-PE measurement indicates that the CoC+ ground state is of 1Σ+ symmetry. Large ΔN+ = N+ - N′ changes up to 6 are observed for the photoionization transitions CoC+(X1Σ+; v+ = 0-2; N+) ← CoC*(2Σ+; v′; N′ = 6, 7, 8, and 9). The highly precise energetic and spectroscopic data obtained in the present study have served as a benchmark for testing theoretical predictions based on state-of-the-art ab initio quantum calculations at the CCSDTQ/CBS level of theory as presented in the companion article. [ABSTRACT FROM AUTHOR]

The ionization energy (IE) of CoC and the 0 K bond dissociation energies (D0) and the heats of formation at 0 K (ΔH°f0) and 298 K (ΔH°f298) for CoC and CoC+ are predicted by the wavefunction based coupled-cluster theory with single, double, triple and quadruple excitations (CCSDTQ) and complete basis set (CBS) approach. The CCSDTQ/CBS calculations presented here involve the approximation to the CBS limit at the coupled cluster level up to full quadruple excitations along with the zero-point vibrational energy, high-order correlation, core-valence (CV) electronic, spin-orbit coupling, and scalar relativistic effect corrections. The present calculations provide the correct symmetry, 1Σ+, for the ground state of CoC+. The CCSDTQ/CBS IE(CoC) = 7.740 eV is found in good agreement with the experimental IE value of 7.73467 ± 0.00007 eV, determined in a two-color laser photoion and pulsed field ionization-photoelectron study. This work together with the previous experimental and theoretical investigations support the conclusion that the CCSDTQ/CBS method is capable of providing reliable IE predictions for 3d-transition metal carbides, such as FeC, CoC, and NiC. Among the single-reference based coupled-cluster methods and multi-reference configuration interaction (MRCI) approach, the CCSDTQ and MRCI methods give the best predictions to the harmonic frequencies ωe (ωe+) = 956 (992) and 976 (1004) cm-1 and the bond lengths re (re+) = 1.560 (1.528) and 1.550 (1.522) Å, respectively, for CoC (CoC+) in comparison with the experimental values. The CCSDTQ/CBS calculations give the prediction of D0(Co+-C) - D0(Co-C) = 0.175 eV, which is also consistent with the experimental determination of 0.14630 ± 0.00014 eV. The theoretical results show that the CV and valence-valence electronic correlations beyond CCSD(T) wavefunction and the relativistic effect make significant contributions to the calculated thermochemical properties of CoC/CoC+. For the experimental D0 and ΔHof0 values of CoC/CoC+, which are not known experimentally, we recommend the following CCSDTQ/CBS predictions: ΔHof0(CoC) = 775.7 kJ/mol and ΔHof0(CoC+) = 1522.5 kJ/mol, ΔHof298(CoC) = 779.2 kJ/mol and ΔHo298(CoC+) = 1526.0 kJ/mol. [ABSTRACT FROM AUTHOR]

Structural imaging of transient excited-state species is a key goal of molecular physics, promising to unveil rich information about the dynamics underpinning photochemical transformations. However, separating the electronic and nuclear contributions to the spectroscopic observables is challenging, and typically requires the application of high-level theory. Here, we employ site-selective ionisation via ultrashort soft X-ray pulses and time-resolved Coulomb explosion imaging to interrogate structural dynamics of the ultraviolet photochemistry of carbon disulfide. This prototypical system exhibits the complex motifs of polyatomic photochemistry, including strong non-adiabatic couplings, vibrational mode couplings, and intersystem crossing. Immediately following photoexcitation, we observe Coulomb explosion signatures of highly bent and stretched excited-state geometries involved in the photodissociation. Aided by a model to interpret such changes, we build a comprehensive picture of the photoinduced nuclear dynamics that follows initial bending and stretching motions, as the reaction proceeds towards photodissociation. Coulomb Explosion imaging is a promising technique to study the ultrafast nuclear dynamics which underpin molecular photochemistry. By initiating Coulomb explosion through soft X-ray ionization, the authors are able to image ultrafast nuclear dynamics of a prototypical photoreaction. [ABSTRACT FROM AUTHOR]

Hydrogen- or proton-migration-induced isomerization has recently been of concern for its critical role in the dissociation of organic molecules of astrophysical or biological relevance. Herein we present a combined experimental and theoretical study of the two-body C-C bond breakdown dissociation of ethane dication. For the asymmetric fragmentation channel CH 2 + + CH 4 + , the kinetic energy release measurements and ab initio quantum chemical calculations demonstrate that the reaction pathway involving hydrogen-migration-induced isomerization of [CH 3 -CH 3 ] 2+ to [CH 2 -CH 4 ] 2+ can be accessed via the lowest triplet state rather than the ground singlet state of ethane dication. Interestingly, it is found that a considerable proportion of the yield of symmetric fragmentation CH 3 + + CH 3 + , which is usually considered from a direct Coulomb explosion and seemingly independent of isomerization, could come from the dissociation of ethane dication in the ground singlet state with the involvement of [CH 3 -CH 3 ] 2+ isomerization to intermediate [H 2 C(H 2 )CH 2 ] 2+ of the diborane-like double-bridged structure.

We present a composite procedure based on explicitly correlated CCSD(T)-F12 calculations for accurate energetic predictions for carbon chain molecules HC n H encompassing both the even (HC 2 k H) and odd series (HC 2 k -1 H), with the shorter members playing a key role in the evolution of cosmic carbon compounds in both circumstellar envelopes and interstellar medium. This approach considers the contributions of core-valence correlation, scalar relativistic effect, spin-orbit coupling, and zero-point vibrational energy in an additive manner. The computed ionization energies demonstrate outstanding agreement (±0.07 eV) up to a chain size of k = 6 and the literature heats of formation for k ≤ 2 are reproduced with "chemical accuracy" of 1 kcal mol -1 . Among the various corrections included, the importance of core-valence correlation effect has been highlighted in the thermochemical calculations for carbon chain growth. The thermochemical trend toward infinite length is also highlighted by extrapolation of ionization energy and triplet-singlet splitting at the CCSD(T) level for k up to 15. The correlation between the end-group effect and the even-odd parity effect observed for HC n H chains has been established with the aid of intrinsic bond orbital localization.

The ionization energies of VCH 2 and VCH 3 , the various 0 K bond dissociation energies ( D 0 s) in their neutrals and cations, and their respective heats of formation at 0 and 298 K are computed by the single-reference, wave function-based CCSDTQ/CBS procedure. The core of the composite method is the approximation to the complete basis set (CBS) limit at the coupled cluster (CC) level which includes up to full quadruple excitations. The zero-point vibrational energy, core-valence correlation, spin-orbit coupling, and scalar relativistic effects have their contributions incorporated in an additive manner. For the species in the current study, this protocol requires geometry optimizations and harmonic frequency calculations practically no higher than the CCSD(T)/aug-cc-pwCVTZ and CCSD(T)/aug-cc-pVTZ levels, respectively. The present calculations successfully predict D 0 (V + -CH 3 ) = 2.126 eV and D 0 (V + -CH 2 ) = 3.298 eV in remarkable agreement with the data recently measured by a spin-orbit state selected V + + CH 4 collision experiment ( Phys. Chem. Chem. Phys . 2021 , 23 , 273-286). The good accord encourages the use of CCSDTQ/CBS protocol in thermochemical predictions of various feasible product channels identified in methane activation by transition metal species.

The ionization energy (IE) of VCH, the 0 K V-CH/VC-H bond dissociation energies ( D 0 s), and the heats of formation at 0 K (Δ H f0 ° ) and 298 K (Δ H f298 ° ) for VCH/VCH + are predicted by the wave function-based CCSDTQ/CBS approach. This composite-coupled cluster method includes full quadruple excitations in conjunction with the approximation to the complete basis set (CBS) limit. The contributions of zero-point vibrational energy, core-valence (CV) correlation, spin-orbit coupling, and scalar relativistic corrections are taken into account. The present calculations show that adiabatic IE(VCH) = 6.785 eV and demonstrate excellent agreement with an IE value of 6.774 7 ± 0.000 1 eV measured with two-color laser-pulsed field ionization-photoelectron spectroscopy. The CCSDT and MRCI+Q methods which include CV correlations give the best predictions of harmonic frequencies: ω 2 (ω 2 + ) (bending) = 534 (650) and 564 (641) cm -1 and the V-CH stretching ω 3 (ω 3 + ) = 835 (827) and 856 (857) cm -1 compared with the experimental values. In this work, we offer a streamlined CCSDTQ/CBS approach which shows an error limit (≤20 meV) matching with previous benchmarking efforts for reliable IE and D 0 predictions for VCH/VCH + . The CCSDTQ/CBS D 0 (V + -CH) - D 0 (V-CH) = -0.012 eV and D 0 (VC + -H) - D 0 (VC-H) = 0.345 eV are in good accord with the experimentally derived values of -0.028 4 ± 0.000 1 and 0.355 9 ± 0.000 1 eV, respectively. The present study has demonstrated that the CCSDTQ/CBS protocol can be readily extended to investigate triatomic molecules containing 3d-metals.

The ferrate(vi) ion, [FeO 4 ] 2- , has attracted much interest in recent years because of its potential use as a green oxidant in organic synthesis and water treatment. Although there have been several reports on the use of ferrate(vi) for the oxidation of alcohols to the corresponding carbonyl compounds, the mechanism remains unclear. In this work, the kinetics of the oxidation of a series of alcohols with α-C-H bond dissociation energies ranging from 81 to 95 kcal mol -1 have been studied by UV/Vis spectrophotometry. The reactions are first-order in both [FeO 4 ] 2- and [alcohol]. The deuterium isotope effects for the oxidation of methanol/d 4 -methanol, ethanol/d 6 -ethanol and benzyl alcohol/d 7 -benzyl alcohol are 18.0 ± 0.1, 4.1 ± 0.1 and 11.2 ± 0.1, respectively. A linear correlation is found between the second-order rate constants and the α-C-H bond dissociation energies (BDEs) of the alcohols, consistent with a hydrogen atom transfer (HAT) mechanism. The proposed HAT mechanism is supported by DFT calculations.

We investigate the unimolecular dissociation of the vinoxy radical (CH2CHO) prepared with high internal energy imparted from the photodissociation of chloroacetaldehyde (CH2ClCHO) at 157 nm. Using a velocity map imaging apparatus, we measured the speed distribution of the recoiling chlorine atoms, Cl((2)P3/2) and Cl((2)P1/2), and derived from this the resulting distribution of kinetic energy, P(ET), imparted to the Cl + vinoxy fragments upon dissociation. Using conservation of energy, the distribution of kinetic energy was used to determine the total internal energy distribution in the radical. The P(ET) derived for the C-Cl bond fission presented in this work suggests the vinoxy radicals are mostly formed in the à state. We also took ion images at m/z = 42 and m/z = 15 to characterize the branching between the unimolecular dissociation channels of the vinoxy radical to H + ketene and methyl + CO products. Our results show a marked change in the branching ratio between the two channels from the previous study on the photodissociation of chloroacetaldehyde at 193 nm by Miller et al. (J. Chem. Phys., 2004, 121, 1830) in that the production of ketene is now favored over the production of methyl. To help analyze the data, we developed a model for the branching between the two channels that takes into account how the change in rotational energy en route to the products affects the vibrational energy available to surmount the barriers to the channels. The model predicts the portion of the C-Cl bond fission P(ET) that produces dissociative vinoxy radicals, then predicts the branching ratio between the H + ketene and CH3 + CO product channels at each ET. The model uses Rice-Ramsperger-Kassel-Marcus rate constants at the correct sums and densities of vibrational states while accounting for angular momentum conservation. We find that the predicted portion of the P(ET) that produces H + ketene products best fits the experimental portion (that we derive by taking advantage of conservation of momentum) if we use a barrier height for the H + ketene channel that is 4.0 ± 0.5 kcal/mol higher than the isomerization barrier en route to CH3 + CO products. Using the G4 computed isomerization barrier of 40.6 kcal/mol, this gives an experimentally determined barrier to the H + ketene channel of 44.6 kcal/mol. From these calculations, we also predict the branching ratio between the H + ketene and methyl + CO channels to be ∼2.1:1.

These experiments photolytically generate two key intermediates in the decomposition mechanisms of energetic materials with nitro substituents, 2-nitropropene, and 2-nitro-2-propyl radicals. These intermediates are produced at high internal energies and access a number of competing unimolecular dissociation channels investigated herein. We use a combination of crossed laser-molecular beam scattering and velocity map imaging to study the photodissociation of 2-bromo-2-nitropropane at 193 nm and the subsequent unimolecular dissociation of the intermediates above. Our results demonstrate that 2-bromo-2-nitropropane has four primary photodissociation pathways: C-Br bond fission yielding the 2-nitro-2-propyl radical, HBr elimination yielding 2-nitropropene, C-N bond fission yielding the 2-bromo-2-propyl radical, and HONO elimination yielding 2-bromopropene. The photofragments are formed with significant internal energy and undergo many secondary dissociation events, including the exothermic dissociation of 2-nitro-2-propyl radicals to NO + acetone. Calculations at the G4//B3LYP/6-311++g(3df,2p) level show that the presence of a radical at a nitroalkyl center changes the mechanism for and substantially lowers the barrier to NO loss. This mechanism involves an intermediate with a three-center ring rather than the intermediate formed during the traditional nitro-nitrite isomerization. The observed dissociation pathways of the 2-nitro-2-propyl radical and 2-nitropropene help elucidate the decomposition mechanism of larger energetic materials with geminal dinitro groups.

In this study, we present a novel mechanism for NO loss from nitroalkyl radicals that circumvents the traditional higher-energy nitro-nitrite isomerization. We characterize the intrinsic reaction coordinate at the B3LYP/6-311++g(3df,2p) level of theory and calculate the transition-state energies using the G4 composite method; the subsequent dynamics en route to the highly exothermic NO + acetone product channel proceeds through a three-membered ring intermediate. Crossed laser-molecular beam scattering experiments on the 2-nitro-2-propyl radical confirm the importance of this new mechanism in determining the product branching.

The photoionization and dissociative photoionizations of ethylenediamine have been studied both experimentally and theoretically. In experiments, photoionization efficiency spectra for ions NH(2)CHCH(3)(+), NH(2)CH=CH(2)(+), CH(2)NH(2)(+), NH(3)(+), NH(2)CH(2)CHNH(2)(+) and NH(2)CH(2)CH(2)NH(2)(+) have been obtained. In addition, the energetics of the dissociative photoionization is investigated with ab initio Gaussian-3 (G3) calculations. The computational results are useful in analyzing the dissociation channels near the ionization thresholds. With the help of the G3 results, the dissociation channels for the formation of the aforementioned fragment ions have been established.

The fragmentation process of ionized 1,4-dioxane and the reactions between the C3H6O+* ions, one of the major fragments, and various reactants (including acetonitrile, formaldehyde, ethylene, and propene) have been studied experimentally with mass spectrometry. In the present work, G3(MP2) calculations were carried out to investigate these processes theoretically. In agreement with experiment, isomers CH3OCHCH2+* (1) and *CH2CH2OCH2+ (2) were found to be the C3H6O+* ions fragmented from ionized 1,4-dioxane, with 2 being the major product. The mechanisms of the formation of 1 and 2 were successfully established. In addition, the characteristic reactivities, as well as the corresponding reaction mechanisms, of both isomers were rationalized with the aid of calculations. Finally, a minor reaction between isomer 2 and propene was identified, and the presence of the product of this reaction was found to be useful in explaining the aforementioned mass spectrometric data.

The photoionization and dissociative photoionization of acetone have been studied at the photon energy range of 8-20 eV. Photoionization efficiency spectra for ions CH3COCH3+, CH3+, C2H3+, C3H3+, C3H5+, CH(2-)CO+, CH3CO+, C3H4O+, and CH3COCH2+ have been measured. In addition, the energetics of the dissociative photoionization has been examined by ab initio Gaussian-3 (G3) calculations. The computational results are useful in establishing the dissociation channels near the ionization thresholds. With the help of G3 results, the dissociation channels for the formation of the fragment ions CH3CO+, CH2CO+, CH3+, C3H3+, and CH3COCH2+ have been established. The G3 results are in fair to excellent agreement with the experimental data.