Nascent product excitations in unimolecular reactions: The separate statistical ensembles method

Photodissociation Dynamics of CO-Forming Channels on the Ground-State Surface of Methyl Formate at 248 nm: Direct Dynamics Study and Assessment of Generalized Multicenter Impulsive Models

The photodissociation dynamics of methyl formate in the electronic ground state S₀, initiated by a 248 nm-wavelength laser, is studied by direct dynamics simulations. We analyze five channels, where four of them have as products CH₃OH + CO, one leading to the formation of three fragments, H₂CO + H₂ + CO, and a channel characterized by a roaming transition state. The analysis of energy distribution among the degrees of freedom of the product and the comparison with experimental results previously published by other groups provide the ingredients to distinguish the examined dissociation pathways. The interpretation of the results proves that the characterization of dissociation mechanisms must rely on a dynamics approach involving multiple electronic states, including considerations on the features of the S₁/S₀ conical intersection. Here, we also assess the generalized multicenter impulsive model, GMCIM, that has been designed for dissociation processes with exit barriers, and the energy distribution in the products is predicted on the basis of information from the saddle points and the intrinsic reaction coordinates. Main features, advantages, limits, and future perspectives of the method are reported and discussed.

Competing pathways in the near-UV photochemistry of acetaldehyde

Time-resolved ion imaging measurements have been performed to explore the photochemistry of acetaldehyde at photolysis wavelengths spanning the range 265–328 nm.

Regulation of nonadiabatic processes in the photolysis of some carbonyl compounds

An energy scheme involving S_o → S₁ excitation, followed by dissociation channels via diabatic coupling, internal conversion, transition state and roaming mechanisms.

Direct comparison of 3-centre and 4-centre HBr elimination pathways in methyl-substituted vinyl bromides

HBr elimination from energized methyl vinyl bromide molecules can occur through competing 3- and 4-centre transition states.

Note: Photodissociation of CH3COCN at 308 nm by time-resolved Fourier-transform infrared emission spectroscopy: is CO a primary or secondary product?

This Note aims to clarify the source of CO in photodissociation of acetyl cyanide (CH3COCN) at 308 nm. From the theoretical aspects, a new pathway via isomerization transition state (TS) at 391 ± 8 kJ∕mol is found leading to the CO + CH3NC products. An amount of 60% reactant molecules at 300 K is estimated to successfully surpass the average TS barrier lying above the excitation energy by 3.5 kJ∕mol. Further, a prior distribution method is conducted to characterize the vibrational energy distribution of CO on a statistical basis. The pathway to CH3NC + CO yields a vibrational branching ratio (v = 0:v = 1:v = 2:v = 3∼0.63:0.25:0.093:0.032) in excellent agreement with the observation (0.62:0.25:0.09:0.05).

Simultaneous velocity and temperature measurements in gaseous flowfields using the vibrationally excited nitric oxide monitoring technique: a comprehensive study

The performance of the vibrationally excited nitric oxide monitoring (VENOM) technique for simultaneous velocity and temperature measurements in gaseous flowfields is presented. Two different schemes were investigated, employing different methods to "write" a transient NO grid in the flow using the 355 nm photolysis of NO(2), which was subsequently probed by planar laser induced fluorescence imaging to extract velocity maps. We find that only one scheme provides full-frame temperature maps. The most accurate velocity measurement was attained by writing an NO pattern in the flow using a microlens array and then comparing the line displacement with respect to a reference image. The demonstrated uncertainty of this approach was 1.0%, corresponding to 7 m/s in a 705 m/s uniform flow. We found that the uncertainty associated with the instantaneous temperature measurements using the NO two-line thermometry technique was largely determined by the shot-to-shot power fluctuations of the probe lasers and, for the flows employed, were determined to range from 6% to 7% of the mean freestream temperature. Finally, simultaneous and local velocity/temperature measurements were performed in the wake of a cylinder in a uniform Mach 4.6 flowfield. The mean and fluctuation velocity and temperature maps were computed from 5000 single-shot measurements. The wake temperature and velocity fluctuations, with respect to the freestream values, were 15% to 30% and 5% to 20%, respectively. The spatial distributions agree with the results of computational fluid dynamics (CFD) simulations. Our results suggest that the VENOM technique holds promise for interrogating high-speed unsteady flowfields.

SACM/CT Study of Product Energy Distributions in the Dissociation of n-Propylbenzene Cations

The distribution of translational, rotational, and vibrational energies in the fragments (benzylium ions and ethyl radicals) of the dissociation of n-propylbenzene cations has been determined by statistical adiabatic channel model/classical trajectory (SACM/CT) calculations. The reaction was treated by CT calculations of capture processes for transitional modes, starting with specified fragment energies. A short-range valence/long-range ion-induced dipole potential model for the transitional modes was employed. The derived distributions approach the results from phase space theory (PST) at small energies and angular momenta. At larger energies and angular momenta, the shapes of the distribution functions remain similar to those from PST; however, the average translational, rotational, and vibrational energies of the fragments increasingly differ from PST predictions. The present results are consistent with separate SACM/CT calculations on the same potential of specific rate constants k(E,J) and thermally averaged rate constants k ∞(T) of the dissociation/recombination reaction.

Second OH overtone excitation and statistical dissociation dynamics of peroxynitrous acid

The second OH overtone transition of the trans-perp conformer of peroxynitrous acid (tp-HOONO) is identified using infrared action spectroscopy. HOONO is produced by the recombination of photolytically generated OH and NO(2) radicals, and then cooled in a pulsed supersonic expansion. The second overtone transition is assigned to tp-HOONO based on its vibrational frequency (10 195.3 cm(-1)) and rotational band contour, which are in accord with theoretical predictions and previous observations of the first overtone transition. The transition dipole moment associated with the overtone transition is rotated considerably from the OH bond axis, as evident from its hybrid band composition, indicating substantial charge redistribution upon OH stretch excitation. The overtone band exhibits homogeneous line broadening that is attributed to intramolecular vibrational redistribution, arising from the coupling of the initially excited OH stretch to other modes that ultimately lead to dissociation. The quantum state distributions of the OH X (2)Pi (nu=0) products following first and second OH overtone excitation of tp-HOONO are found to be statistical by comparison with three commonly used statistical models. The product state distributions are principally determined by the tp-HOONO binding energy of 16.2(1) kcal mol(-1). Only a small fraction of the OH products are produced in nu=1 following the second overtone excitation, consistent with statistical predictions.

Master equation simulations of competing unimolecular and bimolecular reactions: application to OH production in the reaction of acetyl radical with O2

Master equation calculations were carried out to simulate the production of hydroxyl free radicals initiated by the reaction of acetyl free radicals (CH3(C=O).) with molecular oxygen. In particular, the competition between the unimolecular reactions and bimolecular reactions of vibrationally excited intermediates was modeled by using a single master equation. The vibrationally excited intermediates (isomers of acetylperoxyl radicals) result from the initial reaction of acetyl free radical with O2. The bimolecular reactions were modeled using a novel pseudo-first-order microcanonical rate constant approach. Stationary points on the multi-well, multi-channel potential energy surface (PES) were calculated at the DFT(B3LYP)/6-311G(2df,p) level of theory. Some additional calculations were carried out at the CASPT2(7,5)/6-31G(d) level of theory to investigate barrierless reactions and other features of the PES. The master equation simulations are in excellent agreement with the experimental OH yields measured in N2 or He buffer gas near 300 K, but they do not explain a recent report that the OH yields are independent of pressure in nearly pure O2 buffer gas.

A combined experimental and theoretical study of the reaction between methylglyoxal and OH/OD radical: OH regeneration

Experimental studies have been conducted to determine the rate coefficient and mechanism of the reaction between methylglyoxal (CH(3)COCHO, MGLY) and the OH radical over a wide range of temperatures (233-500 K) and pressures (5-300 Torr). The rate coefficient is pressure independent with the following temperature dependence: k(3)(T) = (1.83 +/- 0.48) x 10(-12) exp((560 +/- 70)/T) cm(3) molecule(-1) s(-1) (95% uncertainties). Addition of O(2) to the system leads to recycling of OH. The mechanism was investigated by varying the experimental conditions ([O(2)], [MGLY], temperature and pressure), and by modelling based on a G3X potential energy surface, rovibrational prior distribution calculations and master equation RRKM calculations. The mechanism can be described as follows: Addition of oxygen to the system shows that process (4) is fast and that CH(3)COCO completely dissociates. The acetyl radical formed from reaction (4) reacts with oxygen to regenerate OH radicals (5a). However, a significant fraction of acetyl radical formed by reaction (R4) is sufficiently energised to dissociate further to CH(3) + CO (R4b). Little or no pressure quenching of reaction (R4b) was observed. The rate coefficient for OD + MGLY was measured as k(9)(T) = (9.4 +/- 2.4) x 10(-13) exp((780 +/- 70)/T) cm(3) molecule(-1) s(-1) over the temperature range 233-500 K. The reaction shows a noticeable inverse (k(H)/k(D) < 1) kinetic isotope effect below room temperature and a slight normal kinetic isotope effect (k(H)/k(D) > 1) at high temperature. The potential atmospheric implications of this work are discussed.

A classical trajectory study of the photodissociation of T1 acetaldehyde: The transition from impulsive to statistical dynamics

Previous experimental and theoretical studies of the radical dissociation channel of T(1) acetaldehyde show conflicting behavior in the HCO and CH(3) product distributions. To resolve these conflicts, a full-dimensional potential-energy surface for the dissociation of CH(3)CHO into HCO and CH(3) fragments over the barrier on the T(1) surface is developed based on RO-CCSD(T)/cc-pVTZ(DZ) ab initio calculations. 20,000 classical trajectories are calculated on this surface at each of five initial excess energies, spanning the excitation energies used in previous experimental studies, and translational, vibrational, and rotational distributions of the radical products are determined. For excess energies near the dissociation threshold, both the HCO and CH(3) products are vibrationally cold; there is a small amount of HCO rotational excitation and little CH(3) rotational excitation, and the reaction energy is partitioned dominantly (>90% at threshold) into relative translational motion. Close to threshold the HCO and CH(3) rotational distributions are symmetrically shaped, resembling a Gaussian function, in agreement with observed experimental HCO rotational distributions. As the excess energy increases the calculated HCO and CH(3) rotational distributions are observed to change from a Gaussian shape at threshold to one more resembling a Boltzmann distribution, a behavior also seen by various experimental groups. Thus the distribution of energy in these rotational degrees of freedom is observed to change from nonstatistical to apparently statistical, as excess energy increases. As the energy above threshold increases all the internal and external degrees of freedom are observed to gain population at a similar rate, broadly consistent with equipartitioning of the available energy at the transition state. These observations generally support the practice of separating the reaction dynamics into two reservoirs: an impulsive reservoir, fed by the exit channel dynamics, and a statistical reservoir, supported by the random distribution of excess energy above the barrier. The HCO rotation, however, is favored by approximately a factor of 3 over the statistical prediction. Thus, at sufficiently high excess energies, although the HCO rotational distribution may be considered statistical, the partitioning of energy into HCO rotation is not.

Correlated product distributions from ketene dissociation measured by dc sliced ion imaging

Speed distributions of spectroscopically selected CO photoproducts of 308 nm ketene photodissociation have been measured by dc sliced ion imaging. Structured speed distributions are observed that match the clumps and gaps in the singlet CH2 rotational density of states. The effects of finite time gates in sliced ion imaging are important for the accurate treatment of quasicontinuous velocity distributions extending into the thickly sliced and fully projected regime, and an inversion algorithm has been implemented for the special case of isotropic fragmentation. With accurate velocity calibration and careful treatment of the velocity resolution, the new method allows us to characterize the coincident rotational state distribution of CH2 states as a smoothly varying deviation from an unbiased phase space theory (PST) limit, similar to a linear-surprisal analysis. High-energy rotational states of CH2 are underrepresented compared to PST in coincidence with all detected CO rotational states. There is no evidence for suppression of the fastest channels, as had been reported in two previous studies of this system by other techniques. The relative contributions of ground and first vibrationally excited singlet CH2 states in coincidence with selected rotational states of CO (upsilon=0) are well resolved and in remarkably good agreement with PST, despite large deviations from the PST rotational distributions in the CH2 fragments. At 308 nm, the singlet CH2 (upsilon2=0) and (upsilon2=1) channels are 2350 and 1000 cm(-1) above their respective thresholds. The observed vibrational branching is consistent with saturation at increasing energies of the energy-dependent suppression of rates with respect to the PST limit, attributed to a tightening variational transition state.

Molecular elimination in photolysis of fluorobenzene at 193 nm: Internal energy of HF determined with time-resolved Fourier-transform spectroscopy

Following photodissociation of fluorobenzene (C(6)H(5)F) at 193 nm, rotationally resolved emission spectra of HF(1<or= v <or= 4) in the spectral region 2800-4000 cm(-1) are detected with a step-scan Fourier-transform spectrometer. In the period 0.1-1.1 mus after photolysis, HF(v <or= 4) shows similar Boltzmann-type rotational distributions corresponding to a temperature approximately 1830 K; a short extrapolation from data in the period 0.1-4.1 mus leads to a nascent rotational temperature of 1920+/-140 K with an average rotational energy of 15+/-3 kJ mol(-1). The observed vibrational distribution of (v = 1):(v = 2):(v = 3):(v = 4) = (60+/-7):(24+/-3):(10.5+/-1.2):(5.3+/-0.5) corresponds to a vibrational temperature of 6400 +/- 180 K. An average vibrational energy of 33 +/- 9/3 kJ mol(-1) is derived based on the observed population of HF(1 <or= v <or= 4) and an estimate of the population of HF(v = 0) by extrapolation. The observed internal energy distribution of HF is consistent with that expected for the four-center (alpha,beta) elimination channel. A modified impulse model taking into account geometries and displacement vectors of transition states during bond breaking predicts satisfactorily the rotational excitation of HF. We also compare internal energies of HF observed in this work with those from photolysis of vinyl fluoride (CH(2)CHF) and 2-chloro-1,1-difluoroethene (CF(2)CHCl) at 193 nm.

Internal state distributions of fragment HCO via S0 and T1 pathways of glyoxal after photolysis in the ultraviolet region

The dynamics of photodissociation of glyoxal (HOC-COH) near the dissociation threshold on the triplet manifold are studied through measurement of distributions of nascent fragment HCO in various internal states. Three rotational levels 1(01) (*), 4(13) (*), and 3(21) (*)+3(22) (*) of vibrational state U (excitation wavelength approximately 394.4 nm, origin at 25,331.865 cm(-1)) of glyoxal in state A (1)A(u) and two other vibrational states at excitation wavelengths 390.33 and 382.65 nm are selected to produce fragment HCO. By means of fluorescence in the transition B (2)A(')-X (2)A(') of HCO, we determined the relative populations of internal states of that fragment. Rotational states of product HCO up to N=26 and K=2 are populated, and bimodal distributions of these rotational states are observed for the photolysis wavelengths used in this work. The high rotational part of the distribution with average energy near values calculated on the basis of the statistical model-phase-space theory is assigned to arise from glyoxal on its S(0) surface, and the low rotational part from the T(1) surface with an exit barrier. After photolysis near the threshold region on the triplet surface, HCO arising from the T(1) state appears to be a major component of products because these rotational levels 1(01) (*), 4(13) (*), and 3(2) (*) of U state selected are gateway states with an enhanced rate of intersystem crossing.

Statisticodynamical approach of state distributions in the products of four-atom planar unimolecular reactions. I. Formal developments for conserved vibrations

Traditional statistical approaches, entirely based on transition state theory (TST), do not allow the description of rotational state distributions in the products of indirect reactions governed by short-range forces. Owing to the interpretative power of TST, this limitation has long been acting as a brake upon a deep understanding of determining attributes of indirect reaction dynamics. Recently, however, we developed a statisticodynamical approach (SDA) of final state distributions for triatomic unimolecular reactions [P. Larregaray, L. Bonnet, and J. C. Rayez, J. Chem. Phys. 114, 3349 (2001); Phys. Chem. Chem. Phys. 4, 1577 (2002); 4, 1781 (2002)]. The approach combines TST for the description of state distributions at the transition state (TS) and the linear transformation model for the description of their alteration on the way from the TS to the products. The whole description is mostly analytical, thus keeping the rationalizing spirit of TST. The goal of the present series is to extend SDA to the case of four-atom planar unimolecular reactions, assuming that internal vibrations of the nascent products are conserved from the TS on. This first part is concerned with formal developments while the remaining parts deal with their validation and application, in particular to the fragmentation of isocyanic acid.

Photodissociation Dynamics of Vinyl Chloride Investigated with a Pulsed Slit-Jet and Time-Resolved Fourier-Transform Spectroscopy

Following photodissociation of vinyl chloride seeded in a He supersonic jet at 193 nm, rotationally resolved infrared emission of HCl (v) are recorded to yield nascent rotational and vibrational distributions. Preliminary results show that the rotational distribution of HCl free from rotational quenching deviates slightly from Boltzmann-type distribution and agrees well with trajectory calculations; a portion of the low-J component observed previously in a flow system is attributed to quenching. The implications for photodissociation dynamics are discussed.

Treatment of the K-quantum number in unimolecular reaction theory: insights from product correlations

The connection between the K-quantum number and product correlations in the barrierless unimolecular dissociation of symmetric-top molecules is explored to establish a qualitative diagnostic for the treatment of the K-rotor dynamics in unimolecular reaction theory. We find that fragment scalar and vector correlations can provide guidance in this matter, and the photodissociation dynamics of thermal NCNO to form CN and NO at several dissociation wavelengths are presented to demonstrate the utility of this approach. The "goodness" of the K-quantum number can be related to the amount of energy in the conserved vibrational modes at the inner transition state. On the basis of measured correlated vibrational distributions, the K-quantum number is found to be approximately conserved at the inner transition state for the photodissociation of NCNO at 514, 520, and 526 nm. The methodology, involving a comparison of product distributions from the photodissociation of jet and thermal ensembles at identical wavelengths, is general and may be applied to previously studied systems that dissociate along barrierless potential energy surfaces, CF(3)NO and CH(2)CO. In addition, vector correlations serve as a means to probe the K-mixing at the outer transition state, and measured v-j correlations in the photodissociation of thermal NCNO are presented.