Unimolecular Dissociation Dynamics Research Articles

Isomer-resolved unimolecular dynamics of the hydroperoxyalkyl intermediate (•QOOH) in cyclohexane oxidation

The oxidation of cycloalkanes is important in the combustion of transportation fuels and in atmospheric secondary organic aerosol formation. A transient carbon-centered radical intermediate (•QOOH) in the oxidation of cyclohexane is identified through its infrared fingerprint and time- and energy-resolved unimolecular dissociation dynamics to hydroxyl (OH) radical and bicyclic ether products. Although the cyclohexyl ring structure leads to three nearly degenerate •QOOH isomers (β-, γ-, and δ-QOOH), their transition state (TS) barriers to OH products are predicted to differ considerably. Selective characterization of the β-QOOH isomer is achieved at excitation energies associated with the lowest TS barrier, resulting in rapid unimolecular decay to OH products that are detected. A benchmarking approach is employed for the calculation of high-accuracy stationary point energies, in particular TS barriers, for cyclohexane oxidation (C6H11O2), building on higher-level reference calculations for the smaller ethane oxidation (C2H5O2) system. The isomer-specific characterization of β-QOOH is validated by comparison of experimental OH product appearance rates with computed statistical microcanonical rates, including significant heavy-atom tunneling, at energies in the vicinity of the TS barrier. Master-equation modeling is utilized to extend the results to thermal unimolecular decay rate constants at temperatures and pressures relevant to cyclohexane combustion.

Unimolecular dissociation dynamics of electronically excited HCO(Ã2A''): rotational control of nonadiabatic decay.

The photoinduced unimolecular decay of the electronically excited HCO(Ã2A'') is investigated in a combined experimental-theoretical study. The molecule is excited to the (1, n2, 0) combination bands, which decay via Renner-Teller coupling to the ground electronic state. The rovibrational state distribution of the CO fragment was measured via the high-n Rydberg H-atom time-of-flight method and calculated using a wave packet method on an accurate set of potential energy surfaces. It is shown that the non-adiabatic decay rate is strongly modulated by the HCO rotational angular momentum, which leaves unique signatures in the product state distribution. The experimentally observed bimodal rotational distribution of the dominant CO(v = 0) fragment is likely due to decay of different vibronic states populated by the excitation and modulated by the excited state lifetime, which is in turn controlled by the parent rotational quantum number.

Unimolecular Dissociation of C6H6-C6Cl6 Complex and Effect of Mode-Mode Coupling.

The unimolecular dissociation dynamics of the C6H6-C6Cl6 (Bz-HCB) complex is studied with initial excitation of all vibrational modes for a temperature range of 1000-2000 K and with mode-specific excitations at 1500 K. The results are compared with those of the C6H6-C6F6 [Bz- HFB] complex. When all modes of Bz-HCB are initially excited, the rate of dissociation is slower with respect to Bz-HFB. However, the rate of dissociation is faster when simulations with nonrandom excitation of the specific vibrational modes are performed. The rate of dissociation of Bz-HCB is found to become slower when a few intramolecular modes are excited along with all inter-fragment modes compared to the simulation when only inter-fragment modes of the same complex are excited. Such an energy-transfer dynamics is absent if both intramolecular and inter-fragment modes are not initially excited. Thus, a "stimulated" resonance energy-transfer dynamics is observed in Bz-HCB dissociation dynamics.

Unimolecular dissociation dynamics of C6H6–C6Cl6 complex and the effect of anharmonicity

Chemical dynamics simulations are performed to study the unimolecular dissociation of C6H6–C6Cl6 (Bz-HCB) complex at a temperature range of 1000–2000 K. The intramolecular and intermolecular potential energy parameters are optimized to reproduce the experimental frequencies, equilibrium energy and the minimum energy geometry of the complex. The microcanonical and canonical rate constants are fit to the harmonic classical Rice–Ramsperger–Kassel–Marcus (RRKM) and transition state theory (TST) equations, respectively. The role of anharmonicity on the dissociation of the complex is examined by considering the modified TST equation with the inclusion of an exponential anharmonic correction. The requirement of such a correction in analyzing the dissociation dynamics of Bz-HCB complex is validated by a Monte Carlo based calculation.

Chemical Dynamics Simulations on Association and Ensuing Dissociation of a Benzene-Hexafluorobenzene Molecular System.

Chemical dynamics simulations are performed to study the association of benzene (Bz) and hexafluorobenzene (HFB) followed by the ensuing dissociation of the Bz-HFB complex. The calculations are done for 1000, 1500, and 2000 K with an impact parameter ( b) range of 0-10 Å at each temperature. Almost no complexes are observed to form at b = 8 and 10 Å. Following three different methods of calculation of the temperature-dependent association rate constant kasso( T), the values obtained are 1.67 × 10-10, 1.86 × 10-10, and 2.05 × 10-10 cm3/molecule·s with a standard deviation of approximately 0.1 × 10-10 cm3/molecule·s for T = 1500 K. Among those values of kasso( T), the middle one is obtained by considering a relative translational energy of 3 RT/2 at T = 1500 K, and the same is followed to calculate kasso( T) at 1000 and 2000 K. The Arrhenius parameters, using the kasso( T) values at three temperatures, are 0.203 × 10-10 cm3/molecule·s for the pre-exponential factor and -5.79 kcal/mol for the activation energy. The absolute value of the latter is similar to the Bz + HFB association energy of 5.93 kcal/mol. The ensuing dissociation dynamics of the complex is significantly different from the unimolecular dissociation dynamics, and an exponential function fits the N( t - t0)/ N( t0) curves comparatively well. The ensuing dissociation is also observed to be independent of time for a statistically large sample size.

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.

Exploring unimolecular dissociation kinetics of ethyl dibromide through electronic structure calculations

Pathway for dissociation of multihalogenated alkyls is observed to be competitive between molecular and atomic elimination products. Factors such as molecular structure, temperature and pressure are known to influence the same. Hence present work is focussed to explore mechanism and kinetics of atomic (Br) and molecular (HBr and Br2) elimination upon pyrolysis of 1,1- and 1,2-ethyl dibromide (EDB). For this purpose, electronic structure calculations were performed at DFT and CCSD(T) level of theory. In addition to concerted mechanism, an alternate energetically efficient isomerisation pathway has been exploited for molecular elimination. Energy calculations are further complimented by detailed kinetic investigation, over wide range of temperature and pressure, using suitable models like Canonical Transition State Theory, Statistical Adiabatic Channel Model and Troe’s formalism. Our calculations suggest high branching ratio for dehydrohalogentation reaction, from both isomers of EDB. Fall off curve depicts good agreement between theoretically estimated and experimentally reported values.

Velocity map imaging of OH radical products from IR activated (CH3)2COO Criegee intermediates.

The unimolecular dissociation dynamics of the dimethyl-substituted Criegee intermediate (CH3)2COO is examined experimentally using velocity map imaging to ascertain the translational and internal energy distributions of the OH and H2CC(CH3)O radical products. The energy profile of key features along the reaction coordinate is also evaluated theoretically. Unimolecular decay of (CH3)2COO is initiated by vibrational activation in the CH stretch overtone region and the resultant OH X(2)Π3/2 (v = 0) products are state-selectively ionized and imaged. Analysis reveals an isotropic spatial distribution, indicative of a 3 ps lower limit for the timescale of dissociation, and a broad and unstructured total kinetic energy release distribution. The energy released to products is partitioned principally as internal excitation of the H2CC(CH3)O fragments with modest translational excitation of the fragments and a small degree of OH rotational excitation. The total kinetic energy release distribution observed for (CH3)2COO is compared with that predicted for statistical partitioning over product quantum states, and contrasted with recent experimental and quasi-classical trajectory results for syn-CH3CHOO [N. M. Kidwell et al., Nat. Chem. 8, 509 (2016)].

Unimolecular dissociation dynamics of vibrationally activated CH3CHOO Criegee intermediates to OH radical products.

The hydroxyl radical is an important atmospheric oxidant, and a significant source of its production occurs through alkene ozonolysis. This takes place via a cycloaddition reaction and subsequent fragmentation to form an energized carbonyl oxide (for example, CH3CHOO), known as a Criegee intermediate, which can then either react with another atmospheric species or decay and, in doing so, produce the hydroxyl radical. Here, we examine the dissociation dynamics of a prototypical Criegee intermediate by characterizing the translational and internal energy distributions of the OH radical products, which reflect critical configurations along the reaction pathway. Experimentally, the kinetic energy release to OH products is ascertained through velocity map imaging. Theoretically, quasi-classical trajectories are performed on a new full-dimensional, ab initio potential energy surface. Both experiment and theory show that most of the available energy flows into internal excitation of the vinoxy products. The isotropic angular distribution of OH fragments indicates that dissociation occurs in ≥2 ps, in agreement with theory.

Chemical Dynamics Simulations of Benzene Dimer Dissociation.

Classical chemical dynamics simulations were performed to study the intramolecular and unimolecular dissociation dynamics of the benzene dimer, Bz2 → 2 Bz. The dissociation of microcanonical ensembles of Bz2 vibrational states, at energies E corresponding to temperatures T of 700-1500 K, were simulated. For the large Bz2 energies and large number of Bz2 vibrational degrees of freedom, s, the classical microcanonical (RRKM) and canonical (TST) rate constant expressions become identical. The dissociation rate constant for each T is determined from the initial rate dN(t)/dt of Bz2 dissociation, and the k(T) are well-represented by the Arrhenius eq k(T) = A exp(-E(a)/RT). The E(a) of 2.02 kcal/mol agrees well with the Bz2 dissociation energy of 2.32 kcal/mol, and the A-factor of 2.43 × 10(12) s(-1) is of the expected order-of-magnitude. The form of N(t) is nonexponential, resulting from weak coupling between the Bz2 intramolecular and intermolecular modes. With this weak coupling, large Bz2 vibrational excitation, and low Bz2 dissociation energy, most of the trajectories dissociate directly. Simulations, with only the Bz2 intramolecular modes excited at 1000 K, were also performed to study intramolecular vibrational energy redistribution (IVR) between the intramolecular and intermolecular modes. Because of restricted IVR, the initial dissociation is quite slow, but N(t) ultimately becomes exponential, suggesting an IVR time of 20.7 ps.

Ab initio molecular dynamics of high-temperature unimolecular dissociation of gas-phase RDX and its dissociation products.

Unimolecular dynamics of gas-phase hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and its dissociation products were simulated using density functional theory (DFT) at the M06-L level. The simulations of RDX at 2000 K showed that dissociation proceeds from multiple conformers, mostly via homolytic fission of an N-N bond with a minor contribution from elimination of HONO, in agreement with previous transition state theory calculations. However, the simulations of the fission and elimination products revealed that secondary N-N fission is facile and, at the simulated temperature of 1750 K, dominant over other mechanisms. The simulations of the resulting intermediates revealed a number of new unimolecular pathways that have not been previously considered. The transition structures and minimal energy paths were calculated for all reactions to confirm these observations. Based on these findings, a revised set of the unimolecular reactions contributing to gas-phase RDX decomposition is proposed.

Breaking of a bond: when is it statistical?

Unimolecular dissociation dynamics of a model three degree of freedom triatomic molecule is studied in order to understand the mechanisms for deviations from statisticality. Performing a wavelet based time-frequency analysis of the dynamics allows for the dynamics to be followed on the network of nonlinear resonances, also called as the Arnold web. The results indicate that the long lifetime trajectories spend a considerable amount of time trapped near junctions in the web. It is argued that characterizing the dynamics near such junctions might lead to deeper insights into the origins of nonstatistical dynamics.

AttoPhotoChemistry. Probing ultrafast electron dynamics by the induced nuclear motion: The prompt and delayed predissociation of N2

Quantum mechanical wavepacket dynamics simulation that includes the nuclear motion exhibit a prompt, few fs, dissociation of electronically attosecond excited N2 in addition to the slow dissociation evident from spectral line broadening in well resolved spectra. The simulations show that nuclear motion can probe early times electron dynamics. The separation of time scales is mimicked by a model study fashioned like chemical kinetics of unimolecular dissociation. The physical origin of the separation into prompt and delayed decay is argued to be the same in the vibrational and the present case, namely that there are more bound than dissociative channels.

Coupled Unimolecular Dissociation Kinetics of Bromotoluene Radical Cations

The unimolecular dissociations of o-, m-, and p-bromotoluene radical cations to C7H7(+) (benzylium and tropylium) are examined by considering the coupling of the three isomers in the dissociation pathways. The potential energy surface obtained from ab initio calculations suggests the interconversion of isomers through methylene and hydrogen migrations on the ring. The rate equations for each isomer are combined together to form a rate matrix for coupled reactions. The rate matrix contains the microcanonical rate constants for all elementary steps, which are calculated using Rice-Ramsperger-Kassel-Marcus theory based on the molecular parameters obtained from density functional theory. The unimolecular dissociation rates for coupled reactions are determined by numerically solving the matrix equation. As a result of reaction coupling, the product branching ratio becomes time-dependent and the reaction rates of three isomers become parallel to one another as the energy increases, although their initial rates differently vary with energy. The calculated rate-energy curves fall below the time-resolved photodissociation data in the energy range 2.2-2.7 eV but are in line with the photoelectron photoion coincidence data in the energy range 2.7-3.5 eV. The discrepancy between experiment and theory in the low-energy region is ascribed to the uncertainties of the potential energy surface as well as the contribution of the radiative relaxation rate that has not been taken into account in the theoretical calculations. The rate-energy curves are then used to calculate the thermal reaction rate constants, and the Arrhenius parameters are determined in the temperature range 700-1300 K. Comparison of the activation energy and entropy obtained from the Arrhenius plot with the calculated enthalpy and entropy changes between the reactant and the highest-lying transition state suggests that a series of [1,2] H-atom migrations occurring near the entrance comprise the rate-determining steps and the subsequent [1,2] H-atom migrations play an important role in increasing the activation energy and decreasing the entropy by reducing the net flux to the exit.

N-Acylated Dipeptide Tags Enable Precise Measurement of Ion Temperature in Peptide Fragmentation

Peptide fragmentations into b- and y-type ions are useful for the identification of proteins. The b ion, having the structure of a N-protonated oxazolone, dissociates to the a-type ion with loss of CO. This CO-loss process affords the possibility of characterizing the temperature of the b ion. Herein, we used N-acylated dipeptide tags, isobaric tags originally developed for protein quantification, as internal standards for the measurement of the ion temperature in peptide fragmentation. Amine-reactive dipeptide tags were attached to the N-termini of sample peptides. Collision-induced dissociation (CID) of the tagged peptides yielded a b-type quantitation signal (b(S)) from the tag, which subsequently dissociated into the a(S) ion with CO-loss. As the length of alkyl side chain on the dipeptide tag was extended from C(1) to C(8), the yield of a(S) ion gradually increased for the 4-alkyl-substituted oxazolone ion but decreased for the 2-alkyl-substituted one. To gain insights into the unimolecular dissociation kinetics, we obtained the potential energy surface from ab initio calculations. Theoretical study suggested that the 4-alkyl substitution on N-protonated oxazolone decreased the enthalpy of activation by stabilizing the productlike transition state, whereas the 2-alkyl substitution increased it by stabilizing the reactant. Resulting potential energy surfaces were used to calculate the microcanonical and canonical rate constants as well as the a(S)-ion yield. Arrhenius plots of canonical rate constants provided activation energies and pre-exponential factors for the CO-loss processes in the 600-800 K range. Comparison of experimental a(S)-ion yields with theoretical values led to precise determination of the temperature of b(S) ion. Thus, the b(S)-ion temperature of tagged peptide can be measured simply by combining kinetic parameters provided here and a(S)-ion yields obtained experimentally. Although the b-type fragment patterns varied with the chain length and position of alkyl substituent on the N-protonated oxazolone, the y-type fragment patterns were almost identical under these conditions. Furthermore, b(S)-ion temperatures were nearly the same with only a few degrees K difference. Our results demonstrate a novel use of N-acylated dipeptide tags as internal temperature standards, which enables the reproducible acquisition of peptide fragment spectra.

Comparing the ultraviolet photostability of azole chromophores

Ultrafast time-resolved velocity map imaging methods are used to interrogate the timescales for H-atom elimination in the azole isomers imidazole and pyrazole, the former of which is a prevalent moiety in biomolecules that exhibit a high degree of photostability following the absorption of ultraviolet (UV) radiation (e.g. DNA bases and aromatic amino acids). The results presented here, for the first time, draw focus on the statistical H-atom elimination dynamics in these two heteroaromatics, which result from vibrationally hot ground state (S0) molecules that are formed following ultrafast internal conversion from an initially populated excited electronic state (1ππ* or 1πσ*) at 200 nm. Measurements on imidazole suggest that statistical H-atom elimination is minimal over the temporal window of these experiments (which extends to 600 ps) and occurs on a timescale of >270 ps. Conversely, pyrazole shows a significant statistical H-atom yield by 600 ps with a time constant of 165 ± 30 ps. This highlights statistical unimolecular dissociation dynamics which, on these timescales, cannot be interpreted with traditional RRKM theory. Additional experiments on deuterated isotopomers of the two species also reveal that in imidazole statistical H-atom generation is localized to N–H bond fission, while in pyrazole there is approximately a 1 : 1 ratio between statistical C–H and N–H cleavage, and the two processes have associated time constants of 151 ± 20 ps and 193 ± 35 ps, respectively. We postulate that the observed high fraction of rapid irreversible C–H fission in pyrazole, relative to imidazole, may lead to the formation of toxic free radicals within specific biological environments, whereas statistical dissociation, restricted to only the N–H coordinate, may hypothetically quench UV photodamage yields via H-atom ‘caging’ and ‘recombination’ dynamics in hydrogen bonded networks (e.g. secondary protein structures).

Determining the CH3SO2→CH3+SO2 barrier from methylsulfonyl chloride photodissociation at 193 nm using velocity map imaging

These imaging experiments study the formation of the methylsulfonyl radical, CH(3)SO(2), from the photodissociation of CH(3)SO(2)Cl at 193 nm and determine the energetic barrier for the radical's subsequent dissociation to CH(3) + SO(2). We first state-selectively detect the angular and recoil velocity distributions of the Cl((2)P(3/2)) and Cl((2)P(1/2)) atoms to further refine the distribution of internal energy partitioned to the momentum-matched CH(3)SO(2) radicals. The internal energy distribution of the radicals is bimodal, indicating that CH(3)SO(2) is formed in both the ground state and low-lying excited electronic states. All electronically excited CH(3)SO(2) radicals dissociate, while those formed in the ground electronic state have an internal energy distribution which spans the dissociation barrier to CH(3) + SO(2). We detect the recoil velocities of the energetically stable methylsulfonyl radicals with 118 nm photoionization. Comparison of the total recoil translational energy distribution for all radicals to the distribution obtained from the detection of stable radicals yields an onset for dissociation at a translational energy of 70+/-2 kcal/mol. This onset allows us to derive a CH(3)SO(2) --> CH(3) + SO(2) barrier height of 14+/-2 kcal/mol; this determination relies on the S-Cl bond dissociation energy, taken here as the CCSD(T) predicted energy of 65.6 kcal/mol. With 118 nm photoionization, we also detect the velocity distribution of the CH(3) radicals produced in this experiment. Using the velocity distributions of the SO(2) products from the dissociation of CH(3)SO(2) to CH(3) + SO(2) presented in the following paper, we show that our fastest detected methyl radicals are not from these radical dissociation channels, but rather from a primary S-CH(3) bond photofission channel in CH(3)SO(2)Cl. We also present critical points on the ground state potential energy surface of CH(3)SO(2) at the //CCSD(T)/aug-cc-pV(Q + d)ZCCSD(T)/6-311++G(2df,p) level. We include harmonic zero-point vibrational corrections as well as core-valence and scalar-relativistic corrections. The CCSD(T) predicted barrier of 14.6 kcal/mol for CH(3)SO(2) --> CH(3) + SO(2) agrees well with our experimental measurement. These results allow us to predict the unimolecular dissociation kinetics of CH(3)SO(2) radicals and critique the analysis of prior time-resolved photoionization studies on this system.

On the Statistical Nature of Collision and Surface-Induced Dissociation: A Theoretical Investigation of Aluminum Clusters

The unimolecular dissociation dynamics of aluminum clusters following collision with either a rare gas atom or a surface is investigated by classical trajectory simulations with model potentials. Two conformers of Al(6) with very distinct shapes, i.e., the spherical O(h) and planar C(2)(h) clusters, are considered in this work. The initial vibrational energy and angular momentum distributions resulting from collision, as well as the energy and angular momentum resolved lifetime distributions, of excited clusters were determined for both collision-induced dissociation (CID) and surface-induced dissociation (SID) processes. The partitioning of excitation energy acquired upon collision was found to depend on the excitation mechanism (CID or SID), as well as on the cluster molecular shape, especially in the case of CID. For both types of processes, the energy and angular momentum resolved excited cluster lifetime distributions were found to decay exponentially, in agreement with statistical theories of chemical reactions, suggesting intrinsic Rice-Ramsperger-Kassel-Marcus (RRKM) behavior. Moreover, the simulated microcanonical rate constants determined from the cluster lifetime distributions are in good agreement with the predictions of the orbiting transition state model of phase space theory (OTS/PST), which further supports the statistical character of cluster CID and SID. Thus, in the CID and SID of highly fluxional systems such as aluminum clusters, the rate of intramolecular vibrational energy redistribution (IVR) is much faster than the dissociation rate, which validates one of the key assumptions, i.e., post-collision statistical behavior, underlying the models that are routinely used to determine cluster binding energies from experimental CID/SID cross sections.

Spectroscopy and dynamics of A [2B1] allyl radical

The A [(2)B1] <--X [(2)A2] band system between 380 and 420 nm was observed in a supersonic jet expansion. The allyl radical was found to dissociate following electronic excitation, releasing a hydrogen atom. Monitoring the appearance of the hydrogen atom photoproduct as a function of the excitation laser wavelength, similar spectral features are observed as in earlier absorption experiments. Time- and frequency-resolved photoionization of the hydrogen atom product provides information on the unimolecular dissociation dynamics. The measured dissociation rates and kinetic energy releases of both allyl radical, C3H5, and partially deuterated allyl radical, C3DH4, suggest direct loss of the central hydrogen atom, leading to allene as the major product.

Probing Centrifugal Barriers in Unimolecular Dissociation of the Allyl Radical

Time-resolved photoionization of the hydrogen atom product from the allyl radical, C3H5, dissociation with 115 kcal/mol total energy provides information on the unimolecular dissociation dynamics. Vibrationally hot ground-state allyl radicals in both low and high J-states are prepared by electronic excitation to selected rovibrational states of C-state allyl followed by internal conversion. The measured dissociation rates and kinetic energy release are independent of the allyl parent rotational energy and suggest that centrifugal effects are unimportant in allyl radical dissociation at 115 kcal/mol.