Unimolecular 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.

Diabatic Valence-Hole States in the C2 Molecule: “Putting Humpty Dumpty Together Again”

Despite the long history of spectroscopic studies of the C$_2$ molecule, fundamental questions about its chemical bonding are still being hotly debated. The complex electronic structure of C$_2$ is a consequence of its dense manifold of near-degenerate, low-lying electronic states. A global multi-state diabatic model is proposed here to disentangle the numerous configuration interactions within four symmetry manifolds of C$_2$ ($^{1}\Pi_g$, $^{3}\Pi_g$, $^{1}\Sigma_u^+$, and $^{3}\Sigma_u^+$). The key concept of our model is the existence of two "valence-hole" configurations, $2\sigma_g^22\sigma_u^11\pi_{u}^33\sigma_g^2$ for $^{1,3}\Pi_g$ states and $2\sigma_g^22\sigma_u^11\pi_{u}^43\sigma_g^1$ for $^{1,3}\Sigma_u^+$ states that derive from $3\sigma_g\leftarrow2\sigma_u$ electron promotion. The lowest-energy state from each of the four C$_2$ symmetry species is dominated by this type of valence-hole configuration at its equilibrium internuclear separation. As a result of their large binding energy (nominal bond order of 3) and correlation with the 2s$^2$2p$^2$+2s2p$^3$ separated-atom configurations, the presence of these valence-hole configurations has a profound impact on the $global$ electronic structure and unimolecular dynamics of C$_2$.

The dynamics of CO production from the photolysis of acetone across the whole S1 ← S0 absorption spectrum: Roaming and triple fragmentation pathways.

The unimolecular photodissociation dynamics of acetone spanning the entire S1 ← S0 absorption spectrum have been reinvestigated, with a focus on mechanisms that produce CO. At excitation wavelengths of λ > 305.8 nm, all photoproducts are formed on the S0 state after internal conversion. A roaming mechanism forming C2H6 + CO is active in the window λ = 311.2-305.8 nm. From λ = 305.8to262 nm, little or no CO is produced with the photochemistry dominated by the Norrish-type I C-C bond cleavage on the lowest excited triplet state, T1. At higher energy (λ < 262 nm), an increasing fraction of CH3CO radicals from the primary reaction have sufficient internal energy to spontaneously decompose to CH3 + CO. A new model is presented to account for the kinetic energy distribution of the secondary CH3 radical, allowing us to determine the height of the energetic barrier to CH3CO decomposition as 68 ± 4 kJ mol-1, which lies midway between previous measurements. The fraction of CO from triple fragmentation rises smoothly from 260 to 248nm. We see no evidence of the return of roaming, or any other S0 reaction, in this higher energy region of the first electronic absorption band.

Fragmentation dynamics of CH3Clq+ (q = 2,3): theory and experiment.

A combined theoretical and experimental study of the dissociation of the di- and trication of the CH3Cl molecule has been performed. Experimentally, these multi-charged ions were produced after interactions of a CH3Cl effusive jet with a mono-energetic beam of H+ or Ar9+ projectile ions. Theoretically, we mapped the multi-dimensional potential energy surfaces of CH3Cl2+, H2CClH2+ and CH3Cl3+ species in their electronic ground and electronically excited states using post-Hartree-Fock configuration interaction methods. In addition to the obvious bond-breaking ionic fragments (i.e. H+ + CH2Cl+, H+ + CH2Cl2+ and CH3+ + Cl+), the formation of H2+ (+CHCl+ or CHCl2+), H3+ (+CCl+) and HCl+ (+CH2+) was observed upon bond rearrangement after ion impact of CH3Cl. The interaction strength of the incident projectiles is found to affect the relative yields on the observed dissociation channels, however, it has no effect on the kinetic energy releases of the fragmentation pathways. For the observed dissociation channels, plausible formation mechanisms were proposed. These reaction pathways take place on the ground and/or electronic excited potential energy surfaces of the doubly and triply charged CH3Cl ions, where spin-orbit and vibronic couplings are in action. Moreover, this work suggests that the mechanisms undertaken may depend on the multiply charged ion preparation by valence or inner-shell single photon photoionization, fast ion beam impact or ultrafast intense laser ionization.

Artificial Neural Networks for Noise Removal in Data-Sparse Charged Particle Imaging Experiments.

We present the first demonstration of artificial neural networks (ANNs) for the removal of Poissonian noise in charged particle imaging measurements with very low overall counts. The approach is successfully applied to both simulated and real experimental image data relating to the detection of photoions/photoelectrons in unimolecular photochemical dynamics studies. Specific examples consider the multiphoton ionization of pyrrole and (S)-camphor. Our results reveal an extremely high level of performance, with the ANNs transforming images that are unusable for any form of quantitative analysis into statistically reliable data with an impressive similarity to benchmark references. Given the widespread use of charged particle imaging methods within the chemical dynamics community, we anticipate that the use of ANNs has significant potential impact - particularly, for example, when working in the limit of very low absorption/photoionization cross-sections, or when attempting to reliably extract subtle image features originating from phenomena such as photofragment vector correlations or photoelectron circular dichroism.

Unraveling Conformer-Specific Sources of Hydroxyl Radical Production from an Isoprene-Derived Criegee Intermediate by Deuteration.

Ozonolysis of isoprene, the most abundant volatile organic compounds emitted into the Earth's troposphere after methane, yields three distinct Criegee intermediates. Among these, methyl vinyl ketone oxide (MVK-oxide) is predicted to be the major source of atmospheric hydroxyl radicals (OH) from isoprene ozonolysis. Previously, Barber et al. [ J. Am. Chem. Soc., 2018, 140, pp 10866-10880] demonstrated that syn-MVK-oxide conformers undergo unimolecular decay via 1,4-hydrogen (H) transfer from the methyl group to the adjacent terminal oxygen atom, followed by the prompt release of OH radical products. Here, we selectively deuterate the methyl group of MVK-oxide (d3-MVK-oxide) and record its IR action spectrum in the vinyl CH stretch overtone (2νCH) region. The resultant time-dependent appearance of OD radical products, detected by laser-induced fluorescence, demonstrates that a unimolecular decay of d3-MVK-oxide proceeds by an analogous 1,4-deuterium (D) atom transfer mechanism anticipated for syn conformers. The experimental spectral and temporal results are compared with the calculated IR absorption spectrum and unimolecular decay rates predicted by the Rice-Ramsperger-Kassel-Marcus (RRKM) theory for syn-d3-MVK-oxide, as well as the prior study on syn-MVK-oxide. The d3-MVK-oxide IR action spectrum is similar to that for MVK-oxide, yet exhibits notable changes as the overtone and combination transitions involving CD stretch shift to a lower frequency. The unimolecular decay rate for d3-MVK-oxide is predicted to be a factor of 40 times slower than that for MVK-oxide in the 2νCH region. Experimentally, the temporal profile of the OD products reflects the slower unimolecular decay of d3-MVK-oxide compared to that for MVK-oxide to OH products as well as experimental factors. Both experiment and theory demonstrate that quantum mechanical tunneling plays a very important role in the 1,4-H/D-transfer processes at energies in the vicinity of the transition-state barrier. The similarities of the IR action spectra and changes in the unimolecular decay dynamics upon deuteration indicate that syn conformers make the main contribution to the IR action spectra of MVK-oxide and d3-MVK-oxide.

Nonstatistical Reaction Dynamics.

Nonstatistical dynamics is important for many chemical reactions. The Rice-Ramsperger-Kassel-Marcus (RRKM) theory of unimolecular kinetics assumes a reactant molecule maintains a statistical microcanonical ensemble of vibrational states during its dissociation so that its unimolecular dynamics are time independent. Such dynamics results when the reactant's atomic motion is chaotic or irregular. Intrinsic non-RRKM dynamics occurs when part of the reactant's phase space consists of quasiperiodic/regular motion and a bottleneck exists, so that the unimolecular rate constant is time dependent. Nonrandom excitation of a molecule may result in short-time apparent non-RRKM dynamics. For rotational activation, the 2J + 1 K levels for a particular J may be highly mixed, making K an active degree of freedom, or K may be a good quantum number and an adiabatic degree of freedom. Nonstatistical dynamics is often important for bimolecular reactions and their intermediates and for product-energy partitioning of bimolecular and unimolecular reactions. Post-transition state dynamics is often highly complex and nonstatistical.

Is CH3NC isomerization an intrinsic non-RRKM unimolecular reaction?

Direct dynamics simulations, using B3LYP/6-311++G(2d,2p) theory, were used to study the unimolecular and intramolecular dynamics of vibrationally excited CH3NC. Microcanonical ensembles of CH3NC, excited with 150, 120, and 100 kcal/mol of vibrational energy, isomerized to CH3CN nonexponentially, indicative of intrinsic non-Rice-Ramsperger-Kassel-Marcus (RRKM) dynamics. The distribution of surviving CH3NC molecules vs time, i.e., N(t)/N(0), was described by two separate functions, valid above and below a time limit, a single exponential for the former and a biexponential for the latter. The dynamics for the short-time component are consistent with a separable phase space model. The importance of this component decreases with vibrational energy and may be unimportant for energies relevant to experimental studies of CH3NC isomerization. Classical power spectra calculated for vibrationally excited CH3NC, at the experimental average energy of isomerizing molecules, show that the intramolecular dynamics of CH3NC are not chaotic and the C-N≡C and CH3 units are weakly coupled. The biexponential N(t)/N(0) at 100 kcal/mol is used as a model to study CH3NC → CH3CN isomerization with biexponential dynamics. The Hinshelwood-Lindemann rate constant kuni(ω,E) found from the biexponential N(t)/N(0) agrees with the Hinshelwood-Lindemann-RRKM kuni(ω,E) at the high and low pressure limits, but is lower at intermediate pressures. As found from previous work [S. Malpathak and W. L. Hase, J. Phys. Chem. A 123, 1923 (2019)], the two kuni(ω,E) curves may be brought into agreement by scaling ω in the Hinshelwood-Lindemann-RRKM kuni(ω,E) by a collisional energy transfer efficiency factor βc. The interplay between the value of βc, for the actual intermolecular energy transfer, and the ways the treatment of the rotational quantum number K and nonexponential unimolecular dynamics affect βc suggests that the ability to fit an experimental kuni(ω,T) with Hinshelwood-Lindemann-RRKM theory does not identify a unimolecular reactant as an intrinsic RRKM molecule.

Direct dynamics simulations of the unimolecular dissociation of dioxetane: Probing the non-RRKM dynamics.

In a previous UB3LYP/6-31G* direct dynamics simulation, non-Rice-Ramsperger-Kassel-Marcus (RRKM) unimolecular dynamics was found for vibrationally excited 1,2-dioxetane (DO); [R. Sun et al., J. Chem. Phys. 137, 044305 (2012)]. In the work reported here, these dynamics are studied in more detail using the same direct dynamics method. Vibrational modes of DO were divided into 4 groups, based on their characteristic motions, and each group excited with the same energy. To compare with the dynamics of these groups, an additional group of trajectories comprising a microcanonical ensemble was also simulated. The results of these simulations are consistent with the previous study. The dissociation probability, N(t)/N(0), for these excitation groups were all different. Groups A, B, and C, without initial excitation in the O-O stretch reaction coordinate, had a time lag to of 0.25-1.0 ps for the first dissociation to occur. Somewhat surprisingly, the C-H stretch Group A and out-of-plane motion Group C excitations had exponential dissociation probabilities after to, with a rate constant ∼2 times smaller than the anharmonic RRKM value. Groups B and D, with excitation of the H-C-H bend and wag, and ring bend and stretch modes, respectively, had bi-exponential dissociation probabilities. For Group D, with excitation localized in the reaction coordinate, the initial rate constant is ∼7 times larger than the anharmonic RRKM value, substantial apparent non-RRKM dynamics. N(t)/N(0) for the random excitation trajectories was non-exponential, indicating intrinsic non-RRKM dynamics. For the trajectory integration time of 13.5 ps, 9% of these trajectories did not dissociate in comparison to the RRKM prediction of 0.3%. Classical power spectra for these trajectories indicate they have regular intramolecular dynamics. The N(t)/N(0) for the excitation groups are well described by a two-state coupled phase space model. From the intercept of N(t)/N(0) with random excitation, the anharmonic correction to the RRKM rate constant is approximately a factor of 1.5.

Sub-500 fs electronically nonadiabatic chemical dynamics of energetic molecules from the S1 excited state: Ab initio multiple spawning study.

Energetic materials store a large amount of chemical energy. Different ignition processes, including laser ignition and shock or compression wave, initiate the energy release process by first promoting energetic molecules to the electronically excited states. This is why a full understanding of initial steps of the chemical dynamics of energetic molecules from the excited electronic states is highly desirable. In general, conical intersection (CI), which is the crossing point of multidimensional electronic potential energy surfaces, is well established as a controlling factor in the initial steps of chemical dynamics of energetic molecules following their electronic excitations. In this article, we have presented different aspects of the ultrafast unimolecular relaxation dynamics of energetic molecules through CIs. For this task, we have employed ab initio multiple spawning (AIMS) simulation using the complete active space self-consistent field (CASSCF) electronic wavefunction and frozen Gaussian-based nuclear wavefunction. The AIMS simulation results collectively reveal that the ultrafast relaxation step of the best energetic molecules (which are known to exhibit very good detonation properties) is completed in less than 500 fs. Many, however, exhibit sub-50 fs dynamics. For example, nitro-containing molecules (including C-NO2, N-NO2, and O-NO2 active moieties) relax back to the ground state in approximately 40 fs through similar (S1/S0)CI conical intersections. The N3-based energetic molecule undergoes the N2 elimination process in 40 fs through the (S1/S0)CI conical intersection. Nitramine-Fe complexes exhibit sub-50 fs Fe-O and N-O bond dissociation through the respective (S1/S0)CI conical intersection. On the other hand, tetrazine-N-oxides, which are known to exhibit better detonation properties than tetrazines, undergo internal conversion in a 400-fs time scale, while the relaxation time of tetrazine is very long (about 100 ns). Many other characteristics of sub-500 fs nonadiabatic decay of energetic molecules are discussed. In the end, many unresolved issues associated with the ultrafast nonadiabatic chemical dynamics of energetic molecules are presented.

Effects of vibrational and rotational energies on the lifetime of the pre-reaction complex for the F− + CH3I SN2 reaction

Direct dynamics simulations were performed to investigate unimolecular dynamics of the F−⋯HCH2I pre-reaction complex for the F−+CH3I SN2 reaction. The simulations were performed for a total energy commensurate with the 0.32eV collision energy considered in previous experiments and simulations of this reaction. Two excitation patterns of the complex were considered for the simulations: one with the excitation energy primarily in vibration and the other with the energy primarily in rotation. For the latter equal amounts of energy were added about the three external rotation axes of the complex, giving rise to J and K rotation quantum numbers of 357 and 66 for the initial excitation. For the vibrational excitation the unimolecular dynamics agrees with RRKM theory and dissociation of the complex to the F−+CH3I reactants is negligible, since the barrier for this reaction is 20.2kcal/mol in contrast to the 2.4kcal/mol barrier for accessing the SN2 transition state (TS). The unimolecular dynamics are much different for the rotational excitation simulation. They are non-RRKM and the instantaneous unimolecular rate constant for F−⋯HCH2I decomposition increases with time. Apparently, this arises from K-mixing and vibration/rotation energy redistribution. At the end of the 10ps simulation, the instantaneous simulation rate constant for accessing the SN2 transition state is an order of magnitude smaller than the harmonic RRKM rate constant with the K quantum number treated as an active degree of freedom, indicating that K-mixing and vibration/rotation energy redistribution are not complete on this time scale. For rotational excitation, dissociation to the F−+CH3I reactants is the dominant unimolecular pathway, instead of forming the SN2 products. This is a result of a much higher rotational barrier at the SN2 TS than for the TS leading to F−+CH3I.

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.

Energy and temperature dependent dissociation of the Na(+)(benzene)1,2 clusters: importance of anharmonicity.

Chemical dynamics simulations were performed to study the unimolecular dissociation of randomly excited Na(+)(Bz) and Na(+)(Bz)2 clusters; Bz = benzene. The simulations were performed at constant energy, and temperatures in the range of 1200-2200 K relevant to combustion, using an analytic potential energy surface (PES) derived in part from MP2/6-311+G* calculations. The clusters decompose with exponential probabilities, consistent with RRKM unimolecular rate theory. Analyses show that intramolecular vibrational energy redistribution is sufficiently rapid within the clusters that their unimolecular dynamics is intrinsically RRKM. Arrhenius parameters, determined from the simulations of the clusters, are unusual in that Ea is ∼10 kcal/mol lower the Na(+)(Bz) → Na(+) + Bz dissociation energy and the A-factor is approximately two orders-of-magnitude too small. Analyses indicate that temperature dependent anharmonicity is important for the Na(+)(Bz) cluster's unimolecular rate constants k(T). This is consistent with the temperature dependent anharmonicity found for the Na(+)(Bz) cluster from a Monte Carlo calculation based on the analytic PES used for the simulations. Apparently temperature dependent anharmonicity is quite important for unimolecular dissociation of the Na(+)(Bz)1,2 clusters.

Direct Dynamics Simulation of the Activation and Dissociation of 1,5-Dinitrobiuret (HDNB)

Certain room-temperature ionic liquids exhibit hypergolic activity as liquid bipropellants. Understanding the chemical pathways and reaction mechanisms associated with hypergolic ignition is important for designing new fuels. It has been proposed (J. Phys. Chem. A 2008, 112, 7816) that an important ignition step for the hypergolic ionic liquid bipropellant system of dicyanamide/nitric acid is the activation and dissociation of the 1,5-dinitrobiuret anion DNB(-). For the work reported here, a quasiclassical direct dynamics simulation, at the DFT/M05-2X level of theory, was performed to model H(+) + DNB(-) association and the ensuing unimolecular decomposition of HDNB. This association step is 324 kcal/mol exothermic, and the most probable collision event is for H(+) to directly scatter off of DNB(-), without sufficient energy transfer to DNB(-) for H(+) to associate and form a highly vibrationally excited HDNB molecule. Approximately 1/3 of the trajectories do form HDNB, which decomposes by eight different reaction paths and whose unimolecular dynamics is highly nonstatistical. Some of these paths are the same as those found in a direct dynamics simulation of the high-temperature thermal decomposition of HDNB (J. Phys. Chem. A 2011, 115, 8064), for a similar total energy.

Comparisons of classical chemical dynamics simulations of the unimolecular decomposition of classical and quantum microcanonical ensembles

Previous studies have shown that classical trajectory simulations often give accurate results for short-time intramolecular and unimolecular dynamics, particularly for initial non-random energy distributions. To obtain such agreement between experiment and simulation, the appropriate distributions must be sampled to choose initial coordinates and momenta for the ensemble of trajectories. If a molecule's classical phase space is sampled randomly, its initial decomposition will give the classical anharmonic microcanonical (RRKM) unimolecular rate constant for its decomposition. For the work presented here, classical trajectory simulations of the unimolecular decomposition of quantum and classical microcanonical ensembles, at the same fixed total energy, are compared. In contrast to the classical microcanonical ensemble, the quantum microcanonical ensemble does not sample the phase space randomly. The simulations were performed for CH(4), C(2)H(5), and Cl(-)---CH(3)Br using both analytic potential energy surfaces and direct dynamics methods. Previous studies identified intrinsic RRKM dynamics for CH(4) and C(2)H(5), but intrinsic non-RRKM dynamics for Cl(-)---CH(3)Br. Rate constants calculated from trajectories obtained by the time propagation of the classical and quantum microcanonical ensembles are compared with the corresponding harmonic RRKM estimates to obtain anharmonic corrections to the RRKM rate constants. The relevance and accuracy of the classical trajectory simulation of the quantum microcanonical ensemble, for obtaining the quantum anharmonic RRKM rate constant, is discussed.

Algorithms for Sampling a Quantum Microcanonical Ensemble of Harmonic Oscillators at Potential Minima and Conical Intersections

Algorithms are presented for sampling quantum microcanonical ensembles for a potential energy minimum and for the conical intersection at the minimum energy crossing point of two coupled electronic states. These ensembles may be used to initialize trajectories for chemical dynamics simulations. The unimolecular dynamics of a microcanonical ensemble about a potential energy minimum may be compared with the dynamics predicted by quantum Rice-Ramsperger-Kassel-Marcus (RRKM) theory. If the dynamics is non-RRKM, it will be of particular interest to determine which states have particularly long lifetimes. Initializing a microcanonical ensemble for the electronically excited state at a conical intersection is a model for electronic nonadiabatic dynamics. The trajectory surface-hopping approach may be used to study the ensuing chemical dynamics. A strength of the model is that zero-point energy conditions are included for the initial nonadiabatic dynamics at the conical intersection.

ChemInform Abstract: Accurate Phase Space Theory and Molecular Dynamics Calculations of Aluminum Cluster Dissociation

Calculations of phase space theory and molecular dynamics rate constants and product energy and angular momentum distributions for Al6 and Al13 dissociation are reviewed. The molecular dynamics simulations indicate the unimolecular dynamics of these clusters are ergodic and agree with the random lifetime assumption of Rice-Ramsperger-Kassel-Marcus (RRKM) theory. With anharmonicity effects properly included, orbiting transition state theory (OTS/PST) gives rate constants and product energy and angular momentum distributions in very good agreement with those obtained from the molecular dynamics simulations.

Fragmentation Channels in Dissociative Electron Recombination with Hydronium and Other Astrophysically Important Species

We report on our recent studies of dissociative recombination (DR) employing two different fragment imaging detection techniques at the TSR storage ring in Heidelberg, Germany. Principles of an upgraded 3D optical system and the new energy-sensitive multistrip detector (EMU) are explained together with possible applications in reaction dynamics studies. With the EMU imaging detector we succeeded to observe the branching ratios after DR of deuterated hydronium ions D(3)O(+) at energies of 0-0.5 and 4-21 eV. The branching ratios are almost constant at low energies while above 6 eV both oxygen-producing channels O + D + D + D and O + D(2) + D strongly increase and dominate by about 85% at 11 eV. To demonstrate further capabilities of our fragment imaging detectors, we also summarize some of our additional recent studies on DR of molecular ions important for astrophysics as well as for fundamental unimolecular dynamics.

Vibrational specificity of proton-transfer dynamics in ground-state tropolone

The vibrational dependence of large-amplitude proton transfer taking place in the ground electronic state (X1A1) of tropolone has been explored by implementing a coherent variant of the stimulated emission pumping (SEP) technique within the framework of two-color resonant four-wave mixing (TC-RFWM) spectroscopy. The lowest 1700 cm(-1) portion of this potential surface has been interrogated under ambient bulk-gas conditions, enabling rotationless term energies (Tv+) and tunneling-induced bifurcations Delta(v)X to be extracted for 43 assigned vibrational features of a1 and b2 symmetry. The resulting values of Delta(v)X reflect the state-specificity long attributed to the hydron-migration pathways of tropolone and range in magnitude from 0.0 cm(-1) to 17.8 cm(-1), where the former implies essentially complete quenching of unimolecular dynamics whilst the latter represents nearly a twenty-fold increase in reaction rate over that of the zero-point level. This vibrational mediation of tunneling behavior is discussed in terms of attendant atomic displacements and permutation-inversion symmetries, with choreographed motion of the five-member reaction site (C-O-H...O=C) found to exert the most significant influence on the efficacy of proton transfer.

Theoretical and Computational Studies of Non-RRKM Unimolecular Dynamics

A survey is presented of theoretical models and computational studies for unimolecular reaction dynamics. Intrinsic RRKM and non-RRKM dynamics are described, and properties of the unimolecular reactant's classical phase space giving rise to these dynamics are discussed. Quantum dynamical calculations of isolated resonances and state-specific decomposition are reviewed, and the resulting possible mode-specific or statistical state-specific decomposition is delineated. The relationship between the latter and RRKM theory is described. Computational studies give the probability that a molecule dissociates in a time interval of t --> t + dt, that is, the lifetime distribution P(t), and determining unimolecular rate constants versus pressure, energy, and temperature from P(t) is outlined. Non-RRKM behavior evident in P(t) is not always present in the rate constants. The need to include anharmonicity and the proper treatment of the K quantum number, in calculating the RRKM unimolecular rate constant, is explained. The possibility of observing "steps" in unimolecular rate constants is considered. The extensive experimental non-RRKM dynamics found for several classes of chemical reactions are surveyed. The direct coupling of chemical dynamics with electronic structure theory, that is, direct dynamics, has allowed one to study the atomic-level dynamics for numerous unimolecular reactions, and extensive non-RRKM and nonintrinsic reaction coordinate (IRC) dynamics have been discovered. These dynamics for OH(-) + CH(3)F and F(-) + CH(3)OOH are reviewed.