Unimolecular Dissociation Research Articles

Mass spectrometry evidence for self-rigidification of π-conjugated oligomers containing 3,4-ethylenedioxythiophene groups using RRKM theory and internal energy calibration.

The self-rigidification of ionized π-conjugated systems based on two combinations of thiophene (T) and 3,4-Ethylenedioxythiophene (E) is investigated using mass-analyzed ion kinetic energy spectrometry (MIKES) of ions produced from electron impact ionization at 70 eV. The m/z 446 radical cations of the two isomers ETTE and TEET lead to detect m/z 418 and 390 daughter ions. The MIKE spectra differ only by the intensities of these fragment ions. As the m/z 418 daughter ion is produced through a same retro-Diels Alder reaction whatever the fragmenting isomer, the difference in daughter ion intensities is interpreted in term of unimolecular dissociation rate constants ( k( Eint)) ratios. Considering that the transition state (TS) of such reaction is attributed to a quinoid form, equivalent vibration modes are assumed for the TS of both dissociating ETTE and TEET radical cations. As a result, by using the Rice-Ramsperger-Kassel-Marcus (RRKM) theory, the difference in daughter ion intensities is interpreted by considering that the fragmenting ion is more or less ordered in its ground state than at the transition state, resulting from the influence of the number of the S…O interactions in the planarization of the TEET ion toward the ETTE charged species. The comparison of this behavior in MIKES experiments is supported by the modeling of ion behavior in mass spectrometer and the calibration in internal energy of the radical cations produced in an EI source.

A Better Understanding of the Unimolecular Dissociation Dynamics of Weakly Bound Aromatic Compounds at High Temperature: A Study on C6H6-C6F6 and Comparison with C6H6 Dimer.

Chemical dynamics simulations are performed to study the unimolecular dissociation of the benzene (Bz)-hexafluorobenzene (HFB) complex at five different temperatures ranging from 1000 to 2000 K, and the results are compared with that of the Bz dimer at common simulation temperatures. Bz-HFB, in comparison with Bz dimer, possesses a much attractive intermolecular interaction, a very different equilibrium geometry, and a lower average quantum vibrational excitation energy at a given temperature. Six low-frequency modes of Bz-HFB are formed by Bz + HFB association which are weakly coupled with the vibrational modes of Bz and HFB. However, this coupling is found much stronger in Bz-HFB compared to the same in the Bz dimer. The simulations are done with very good potential energy parameters taken from the literature. Considering the canonical (TST) model, the unimolecular dissociation rate constant at each temperature is calculated and fitted to the Arrhenius equation. An activation energy of 5.0 kcal/mol and a pre-exponential factor of 2.39 × 1012 s-1 are obtained, which are of expected magnitudes. The responsible vibrational mode for dissociation is identified by performing normal-mode analysis. Simulations with random excitations of high-frequency Bz and HFB modes and low-frequency inter-Bz-HFB vibrational modes of the Bz-HFB complex are also performed. The intramolecular vibrational energy redistribution (IVR) time and the unimolecular dissociation rate constants are calculated from these simulations. The latter shows good agreement with the same obtained from simulation with random excitation of all vibrational modes.

The K2(9-ethylguanine)122+ quadruplex is more stable to unimolecular dissociation than the K(9-ethylguanine)8+ quadruplex in the gas phase: a BIRD, energy resolved SORI-CID, IRMPD spectroscopic, and computational study.

A combination of experimental trapped-ion mass spectrometric studies and computational chemistry has been used in the present work to assess the intrinsic properties of the potassiated 9-ethylguanine (9eG) self-assembled quadruplex, K2(9eG)122+, in the gas phase. Infrared multiple photon dissociation (IRMPD) spectroscopy in the N-H/C-H stretching region (2700-3800 cm-1) revealed that this G-quadruplex is a sandwich-type structure with two G-tetrads sandwiching each of the two K+, very similar to the structure determined previously for the K(9eG)8+ complexes. The stability of K2(9eG)122+ toward unimolecular dissociation and its binding energy were examined using energy-resolved sustained off-resonance collision induced dissociation (SORI-CID) and blackbody infrared radiative dissociation (BIRD) kinetics experiments. SORI-CID experiments showed that the self-assembled K2(9eG)122+ complex undergoes charge separation forming K(9eG)8+ and K(9eG)4+ compared to K(9eG)8+ which loses neutral 9eG. More interestingly, K2(9eG)122+ is more stable toward unimolecular dissociation activated by SORI-CID than the K(9eG)8+ complex. Temperature dependent BIRD kinetics for K2(9eG)122+ were consistent with energy-resolved SORI-CID results showing K2(9eG)122+ to have an activation energy of 225 ± 15 kJ mol-1, approximately 50 kJ mol-1 greater than that determined for K(9eG)8+. The extra stability of K2(9eG)122+ is apparently not thermodynamic stability, but most likely due to an energy barrier for dissociation.

Falloff curves and mechanism of thermal decomposition of CF3I in shock waves.

The falloff curves of the unimolecular dissociation CF3I (+Ar) → CF3 + I (+Ar) are modelled by combining quantum-chemical characterizations of the potential energy surface for the reaction, standard unimolecular rate theory, and experimental information on the average energy transferred per collision between excited CF3I and Ar. The (essentially) parameter-free theoretical modelling gives results in satisfactory agreement with data deduced from earlier shock wave experiments employing a variety of reactant concentrations (between a few ppm and a few percent in the bath gas Ar). New experiments recording absorption-time signals of CF3I, I2, CF2 and (possibly) IF at 450-500 and 200-300 nm are reported. By analysing the decomposition mechanism, besides the unimolecular dissociation of CF3I, these provide insight into the influence of secondary reactions on the experimental observations.

Experimental and kinetic modeling investigation on anisole pyrolysis: Implications on phenoxy and cyclopentadienyl chemistry

In this work, the flow reactor pyrolysis of anisole was studied at pressures of 0.04 and 1 atm and temperatures from 850 to 1160 K. Comprehensive speciation was achieved using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). A detailed kinetic model for anisole combustion was developed and validated against experimental results in the present work. Fuel decomposition and aromatics formation processes were investigated based on modeling analyses. The results show that the dominant decomposition pathway of anisole is the unimolecular OCH3 bond dissociation reaction at both pressures, while the role of bimolecular reactions becomes significant at 1 atm. At lower temperatures, phenoxy radical is mainly consumed via the reactions with methyl radical, producing methylcyclohexadienone. At higher temperatures, it is mainly consumed via the unimolecular decomposition reaction producing cyclopentadienyl. Cyclopentadienyl is responsible for the abundant production of aromatic products such as benzene, toluene, styrene and naphthalene. Furthermore, the bimolecular reactions of anisole also contribute to the formation of aromatic products at lower temperatures. Possible formation pathways of oxygenated aromatics such as benzofuran and dibenzofuran were also analyzed in this work. The present model was also validated against literature experimental data of anisole combustion, including global combustion parameters like ignition delay times and speciation profiles in flow reactor pyrolysis and jet stirred reactor pyrolysis and oxidation.

The gas‐phase ozonolysis reaction of methylbutenol: A mechanistic study

AbstractThe gas‐phase ozonolysis reaction of methylbutenol through the Criegee mechanism is investigated. The initial reaction leads to a primary ozonide (POZ) formation with barriers in the range of 10–28 kJ mol−1. The formation of 2‐hydroxy‐2‐methyl‐propanal (HMP) and formaldehyde‐oxide is more favorable, by 10 kJ mol−1, than the syn‐CI and formaldehyde. The unimolecular dissociation of the more stable syn‐CI via 1,5‐H transfer into an epoxide is more favored than the epoxide and 3O2 formation. The ester channel led to the formation of the acetone and formic acid favorably from the anti‐CI. The hydration of the anti‐CI with H2O and (H2O)2 is significantly barrierless with a higher plausibility to the latter, and thus they may lead to the formation of peroxides and ultimately OH radicals, as well as airborne particulate matter. Reaction of anti‐CI with water dimers enhances its atmospheric reactivity by a factor of 28 in reference to water monomers.

Dissociative photoionization of chromium hexacarbonyl: A round-trip ticket to non-statisticality and a detective story in thermochemistry

The fragmentation processes of internal energy selected chromium hexacarbonyl cations, Cr(CO)6+, were investigated by imaging photoelectron photoion coincidence (iPEPICO) spectroscopy at the VUV beamline of the Swiss Light Source. In the 9.3–21.5 eV photon energy range, Cr(CO)6+ dissociates by six sequential carbonyl ligand losses. The fragment ion fractional abundances, plotted in the breakdown diagram, along with the time-of-flight mass spectra for the first three metastable CO-loss channels were modeled using a statistical approach. Between 12 and 16 eV, the statistical model overestimates the degree of fragmentation, which is explained by enhanced kinetic energy release in impulsive CO loss on repulsive electronic states of the parent ion Cr(CO)6+, as confirmed by TD-DFT calculations. This is the first reported example for an embedded non-statistical unimolecular dissociation regime, bracketed by statistical regimes at low and at high energies. The statistical model was employed to derive 0 K appearance energies for Cr(CO)n+ (n = 0–5). The Cr(CO)5+ appearance energy and the literature COCr(CO)5+ bond dissociation energy yield an adiabatic Cr(CO)6+ ionization energy of 8.195 ± 0.120 eV, independent of photoelectron spectroscopy measurements. The 0 K appearance energy of Cr+ is 61 kJ mol–1 higher than predicted based on literature enthalpies of formation, which we suggest is most likely due to an error in the enthalpy of formation of Cr(CO)6, and propose a revised ΔfH[Cr(CO)6, g] = –972.1 ± 4.1 and –968.9 ± 4.1 kJ mol–1 at 0 and 298 K, respectively, and ΔfH298K[Cr(CO)6, c] = –1040.6 ± 4.2 kJ mol–1 for the crystalline state, based on the known enthalpy of sublimation. The measured CrCO bond dissociation energies in [(CO)nCrCO]+ (n = 0–5), and the enthalpies of formation of the chromium carbonyl ion series are also reported.

Gas phase fragmentation of adducts between dioxygen and closo-borate radical anions

The icosahedral closo-borate anions, [closo-B12X12]2− (B12) and [1-H-closo-1-CB11X11]− (CB11) with X=H or halogen, are exceptionally stable weakly coordinating anions. Recent studies, which focused on the gas phase ion chemistry of this class of anions, revealed several unusual ion-molecule and unimolecular dissociation pathways. Herein, we demonstrate that collisional activation of highly iodinated closo-borate anions results in a loss of an iodine atom followed by a spontaneous addition of O2 present in a mass spectrometer background producing B12-O2 and CB11-O2 species. We examined the electronic structure of a representative O2 adduct using photoelectron spectroscopy and explored collision-induced dissociation (CID) pathways of a series of iodinated B12-O2 and CB11-O2 anions with different substituents. A systematic study of the gas phase dissociation pathways of the B12-O2 and CB11-O2 anions revealed several unusual dissociation pathways characteristic of this class of species. Abundant fragment ions are formed by neutral loss of BOX2 (X = boron bound substituent), Y2H (Y = boron bound halogen, if any H is present in the molecular ion) and BO2. Loss of BO2 was observed for CB11-O2 but not for B12-O2 anions. Substantial differences in stability of the corresponding fragments are responsible for the observed differences in dissociation of B12-O2 and CB11-O2 species. Furthermore, we show that in addition to O2 other molecules with a radical character (e.g. SCH3) can form adducts with closo-borate anions after iodine abstraction. The boron cage fragmentation observed for the O2 adducts is atypical for highly iodinated closo-borates and is attributed to the strength of BO bonds which dominate the thermochemistry of B12-O2 and CB11-O2 fragmentation reactions.

Evaluation of Advanced Precursor Determination for Tandem Mass Tag (TMT)-Based Quantitative Proteomics across Instrument Platforms.

Tandem mass tag (TMT)-based quantitation is a strong modality for quantitative proteomics, as samples can be multiplexed, creating large-scale data sets with high precision and minimal missing values. However, coisolation/cofragmentation of near isobaric, coeluting precursor peptide analytes has been well-documented to show ratio compression, compromising the accuracy of peptide/protein quantitation. Advanced peak determination (APD) is a new peak-picking algorithm that shows improved identification of peak detection in survey scans (MS1) to increase the number of precursors selected for unimolecular dissociation (MS2). To increase the number of these "features" selected for MS2 APD purposefully selects multiple peptide precursors of very similar m/ z that often derive from different proteins-a major source of ratio compression in TMT quantification. Here, we evaluate the effects of various data acquisition parameters combined with APD on ratio compression. We find that data acquisition with APD enabled results in more coisolated precursors, more mixed spectra, and in turn, fewer peptide spectral matches, especially at standard on-column loads. We conclude that APD should not be utilized for isobaric tagging, MS2-based experiments.

What do we expect from the dissociation of ionized nitro-substituted polycyclic aromatic hydrocarbons in the interstellar medium?

The unimolecular dissociations of four nitro-substituted polycyclic aromatic hydrocarbon (PAH) ions were explored using tandem mass spectrometry, imaging photoelectron photoion coincidence (iPEPICO) spectroscopy and computational chemistry. Ionized 1-nitronaphthalene and 9-nitroanthracene both exhibit the loss of C2O2 (likely sequential loss of two CO molecules), NO and CO2 as primary fragmentation channels, while for the larger 3-nitrofluoranthene and 1-nitropyrene ions CO2 loss is replaced by NO2 loss. The reaction pathways for 1-nitropyrene ions were calculated at the CCSD/6-31G(d)//B3-LYP/6-31G(d) level of theory. NO loss is preceded by a nitro-to-nitrite isomerization requiring 2.09 eV, while NO2 loss was found to form the triplet state [pyrene – H]+ ion (requiring 2.36 eV), consistent with the behavior of this class of PAH ions. iPEPICO-derived energies for these two reactions are in excellent agreement with theory, 2.05 ± 0.09 eV and 2.38 ± 0.12 eV, respectively. The iPEPICO-derived reaction energy for C2O2 loss is 2.76 ± 0.08 eV, but we were unable to calculate this reaction pathway. The most energetically-favorable products of sequential CO loss lie 3.2 eV and 2.05 eV, respectively, lower in energy than the precursor ion, indicating that what was measured was an effective barrier height for this reaction.

Exploration of the pyrolysis chemistry of 1,1-diethoxybutane: A flow reactor and kinetic modeling study

1,1-Diethoxybutane is a promising next-generation biofuel and this work reports the first study for its pyrolysis chemistry. Pyrolysis of 1,1-diethoxybutane was investigated in a flow reactor at 0.04 and 1 atm using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). A series of oxygenated and hydrocarbon products, including several radicals, were detected and their mole fractions were evaluated. A detailed kinetic model of 1,1-diethoxybutane, including 253 species and 1577 reactions, was developed and validated against the present experimental data. Modeling analyses including the rate of production analysis and sensitivity analysis were performed to reveal the crucial consumption pathways of 1,1-diethoxybutane, as well as the formation and consumption pathways of intermediates and products. The H-abstraction reactions of 1,1-diethoxybutane play a key role in fuel consumption. The intra-molecular elimination reactions and unimolecular CC or CO dissociation reactions of 1,1-diethoxybutane make minor contributions to fuel consumption. The sensitivity analysis also shows that the fuel consumption reactions have large sensitivities to 1,1-diethoxybutane. Further β-CC and β-CO scission reactions closely connect the consumption of six fuel radicals and the formation of most oxygenated products. The observation of high mole fractions of C1–C4 acids and aldehydes demonstrates the structural features of 1,1-diethoxybutane. Besides, several hydrocarbon products, such as C2–C4 alkenes, benzene and its precursors, were detected, some of which, such as propene, are also produced from the consumption reactions of fuel radicals. Besides, hydrocarbon products can also be produced from the combination reactions of small species, such as the combination reactions of C3 species to produce benzene and the combination reaction of allyl and methyl to produce 1-butene. The low benzene production ability of 1,1-diethoxybutane implies its low sooting tendency compared with fossil-derived transportation fuels.

Anharmonic effect of the unimolecular dissociation of Glycerol to Glycidol

The unimolecular dissociation rate constants of the dehydration of Glycerol to Glycidol were calculated at the MP2/6–311G(d,p) level using the Rice–Ramsperger–Kassel–Marcus (RRKM) theory. The anharmonic effect of the reactions was examined by comparing the rate constants at temperatures (700–3000[Formula: see text]K) of the canonical case and total energies (25654–53089[Formula: see text]cm[Formula: see text]) of the microcanonical system. The calculations showed that high temperatures are required for the reaction to proceed. As the temperatures and total energies increased, the rate of reactions increased. However, the growth rate of the unimolecular dissociation rate constants was high and slower both in the canonical and microcanonical systems. Comparative analysis showed that the anharmonic effect was most significant for the reaction [Formula: see text] and least significant for the reaction [Formula: see text]. The anharmonic effect became more significant as the temperatures and total energies increased. Compared with the microcanonical situation, the anharmonic effect of the canonical system was more pronounced.

Direct versus Indirect Photodissociation of Isoxazole from Product Branching: A Chirped-Pulse Fourier Transform mm-Wave Spectroscopy/Pulsed Uniform Flow Investigation.

The UV photodissociation of isoxazole (c-C3H3NO) is studied in this work by chirped-pulse Fourier transform mm-wave spectroscopy in a pulsed uniform Laval flow. This approach offers a number of advantages over traditional spectroscopic detection methods due to its broadband, sub-MHz resolution, and fast-acquisition capabilities. In coupling this technique with a quasi-uniform Laval flow, we are able to obtain product branching fractions in the 193 nm photodissociation of isoxazole. Five dissociation channels are explored through direct detection of seven different photoproducts. These species and their respective branching fractions (%) include the following: HCN (53.8 ± 1.7), CH3CN (23.4 ± 6.8), HCO (9.5 ± 2.3), CH2CN (7.8 ± 2.9), CH2CO (3.8 ± 0.9), HCCCN (0.9 ± 0.2), and HNC (0.8 ± 0.2). Guided by previous electronic structure and dynamics simulations, we are able to elucidate the dissociation dynamics that govern the final product branching fractions observed in this work, which differ significantly from previous reports on the thermal decomposition of isoxazole. Interestingly, both direct and indirect dynamics contribute to its dissociation, and clear signatures of both are manifested in the relative branching ratios obtained. Consistent with previous studies on the unimolecular dissociation of isoxazole, our findings also suggest the importance of the open-shell singlet diradicaloid species vinylnitrene in the dissociation dynamics, regardless of the initially populated excited state. This work, taken together with previous investigations, provides a global picture of the complex dissociation pathways involved in the photodissociation of isoxazole.

Ultraviolet photodissociation dynamics of 1-pentyl radical

The ultraviolet (UV) photodissociation of jet-cooled 1-pentyl radical is investigated in the wavelength region of 236−254 nm using the high-n Rydberg-atom time-of-flight (HRTOF) technique. The H-atom photofragment yield spectrum of the 1-pentyl radical shows a broad UV absorption feature peaking near 245 nm, similar to the 2pz →3s absorption bands of ethyl and n-propyl. The center-of-mass translational energy distribution, P (ET), of the H+C5H10 product channel is bimodal, with a slow peak at ~5 kcal/mol and a fast peak at ~50 kcal/mol. The fraction of the average translational energy release in the total available energy, ⟨fT⟩, is 0.30, with those of the slow and fast components being 0.13 and 0.58, respectively. The slow component has an isotropic product angular distribution, while the fast component is anisotropic with an anisotropy parameter ~0.4. The bimodal translational energy and angular distributions of the H+C5H10 products indicate two H-atom elimination channels in the photodissociation of 1-pentyl: (i) a direct, prompt dissociation from the electronic excited state and/or the repulsive part of the ground electronic state potential energy surface; and (ii) a unimolecular dissociation of internally hot radical in the ground electronic state after internal conversion from the electronic excited state.

A generalized unimolecular impulsive model for curved reaction path.

This work aims to introduce a generalized impulsive model for unimolecular dissociation processes. This model allows us to take into account the curvature of the reaction path explicitly. It is a generalization of the previously developed multi-center impulsive model [P.-Y. Tsai and K.-C. Lin, J. Phys. Chem. A 119, 29 (2015)]. Several limitations of conventional impulsive models are eliminated by this study: (1) Unlike conventional impulsive models, in which a single molecular geometry is responsible for the impulse determination, the gradients on the whole dissociation path are taken into account. The model can treat dissociation pathways with large curvatures and loose saddle points. (2) The method can describe the vibrational excitation of polyatomic fragments due to the bond formation by multi-center impulse. (3) The available energy in conventional impulsive models is separated into uncoupled statistical and impulsive energy reservoirs, while the interplay between these reservoirs is allowed in the new model. (4) The quantum state correlation between fragments can be preserved in analysis. Dissociations of several molecular systems including the roaming pathways of formaldehyde, nitrate radical, acetaldehyde, and glyoxal are chosen as benchmarks. The predicted photofragment energy and vector distributions are consistent with the experimental results reported previously. In these examples, the capability of the new model to treat the curved dissociation path, loose saddle points, polyatomic fragments, and multiple-body dissociation is verified. As a cheaper computational tool with respect to ab initio on-the-fly direct dynamic simulations, this model can provide detailed information on the energy disposal, quantum state correlation, and stereodynamics in unimolecular dissociation processes.

Experimental and kinetic modeling investigation on methyl decanoate pyrolysis at low and atmospheric pressures

The pyrolysis of methyl decanoate (MD), an ideal surrogate of biodiesels, was investigated in a flow reactor at the pressures of 30 and 760 Torr and the temperature ranging from 773 to 1198 K. A great variety of pyrolysis products including free radicals, n-alkanes, 1-alkenes, alkynes, unsaturated esters and aromatics were comprehensively observed and identified by employing synchrotron vacuum ultraviolet photoionization mass spectrometry. A new kinetic model for MD pyrolysis was constructed and applied to validate the experimental data. Modeling analyses involving rate of production analysis and sensitivity analysis were performed to help explore the pyrolysis kinetics of MD and the formation mechanisms of key species. The analysis results show that the decomposition of MD is determined by the H-abstraction and unimolecular dissociation reactions during the whole pyrolysis process, whereas the contributions of H-abstraction reactions are enhanced as the pressure elevates. C4-C9 unsaturated esters are principally yielded from the β-scission of ester radicals; while the β-scission reactions of MD radicals are responsible for the formation of C5-C9 1-alkenes. In addition, 1-alkenes can be further decomposed to form small radicals and molecules. Through the combination reactions such as the reaction routes of C3 + C3, C4 + C2 and C5 + C2, these radicals and molecules can be transformed into benzene and benzyl radical, which are demonstrated as the crucial precursors of polycyclic aromatic hydrocarbons. In conclusion, the pyrolysis of MD would not only significantly enhance the cognitions of various pollutant formation mechanisms but also have a guiding significance for the combustion in the fuel-cooled engine.

High-energy collision-induced dissociation of radiosensitizer anions: Nimorazole and metronidazole

The formation of radical anions of nitroimidazolic radiosensitizers, nimorazole and metronidazole, and their unimolecular dissociation reactions upon high-energy collisions with He gas were investigated with a double focusing mass spectrometer equipped with an electrospray ionization (ESI) source. Radical anions of both, nimorazole and metronidazole, are readily formed in the ESI process, while for the metronidazole also the deprotonated anion is formed. A variety of dissociation channels is observed and the associated kinetic energy released in the dissociation for most of these channels is reported. The marker anion NO2− is observed in the dissociation of all studied anions. The release of NO radical from the radical anions is associated with surprisingly small kinetic energy release in comparison to simple nitroimidazoles. On the other hand, the highest kinetic energy release is noted for the release of the neutral side group from N1 position of the imidazole ring, which suggests that the substitution at the N1 plays a crucial role in the decomposition of anions of radiosensitizers.

Potential Energy Surface for Large Barrierless Reaction Systems: Application to the Kinetic Calculations of the Dissociation of Alkanes and the Reverse Recombination Reactions.

The isodesmic reaction method is applied to calculate the potential energy surface (PES) along the reaction coordinates and the rate constants of the barrierless reactions for unimolecular dissociation reactions of alkanes to form two alkyl radicals and their reverse recombination reactions. The reaction class is divided into 10 subclasses depending upon the type of carbon atoms in the reaction centers. A correction scheme based on isodesmic reaction theory is proposed to correct the PESs at UB3LYP/6-31+G(d,p) level. To validate the accuracy of this scheme, a comparison of the PESs at B3LYP level and the corrected PESs with the PESs at CASPT2/aug-cc-pVTZ level is performed for 13 representative reactions, and it is found that the deviations of the PESs at B3LYP level are up to 35.18 kcal/mol and are reduced to within 2 kcal/mol after correction, indicating that the PESs for barrierless reactions in a subclass can be calculated meaningfully accurately at a low level of ab initio method using our correction scheme. High-pressure limit rate constants and pressure dependent rate constants of these reactions are calculated based on their corrected PESs and the results show the pressure dependence of the rate constants cannot be ignored, especially at high temperatures. Furthermore, the impact of molecular size on the pressure-dependent rate constants of decomposition reactions of alkanes and their reverse reactions has been studied. The present work provides an effective method to generate meaningfully accurate PESs for large molecular system.

Unimolecular Dissociation of 1-Methylpyrene Cations: Why Are 1-Methylenepyrene Cations Formed and Not a Tropylium-Containing Ion?

1-Methylpyrene radical cations undergo the loss of a hydrogen atom at internal energies above the first dissociation threshold. Imaging photoelectron photoion coincidence spectroscopy was employed in combination with RRKM modeling to determine a 0 K activation energy of 2.78 ± 0.25 eV and an entropy of activation of 6 ± 19 J K-1 mol-1 for this H-loss reaction. The ionization energy of 1-methylpyrene was measured by mass-selected threshold photoelectron spectroscopy to be 7.27 ± 0.01 eV. These values were found to be consistent with calculations at the CCSD/6-31G(d)//B3-LYP/6-31G(d) level of theory showing that the formation of the 1-methylenepyrene cation (resulting from H loss from the methyl group) is kinetically more favorable than the formation of a tropylium-containing product ion that is structurally analogous to the formation of the tropylium cation in H loss from ionized toluene. The shift away from a tropylium-containing structure was found to be due to the increased ring strain imposed on the C7 moiety when it is bound to three fused benzene rings. The RRKM results allow for the derivation of the Δf H0o (1-methylenepyrene cation) of 945 ± 31 kJ mol-1.

Free radical-initiated peptide sequencing (FRIPS)-based cross-linkers for improved peptide and protein structure analysis.

Free radical-initiated peptide sequencing (FRIPS) has recently been introduced as an analytical strategy to create peptide radical ions in a predictable and effective way by collisional activation of specifically modified peptides ions. FRIPS is based on the unimolecular dissociation of open-shell ions and yields fragments that resemble those obtained by electron capture dissociation (ECD) or electron transfer dissociation (ETD). In this review article, we describe the fundamentals of FRIPS and highlight its fruitful combination with chemical cross-linking/mass spectrometry (MS) as a highly promising option to derive complementary structural information of peptides and proteins. FRIPS does not only yield exhaustive sequence information of cross-linked peptides, but also defines the exact cross-linking sites of the connected peptides. The development of more advanced FRIPS cross-linkers that extend the FRIPS-based cross-linking/MS approach to the study of large protein assemblies and protein interaction networks can be eagerly anticipated.