Zahra Homayoon

Quasiclassical Trajectory Study of CH3NO2 Decomposition via Roaming Mediated Isomerization Using a Global Potential Energy Surface

We report a global potential energy surface (PES) for CH3NO2 based on fitting roughly 114,000 density functional theory (UB3LYP/6-311+g(d,p)) electronic energies. The PES is a precise, permutationally invariant fit to these energies. Properties of the PES are described, as are some preliminary quasiclassical trajectory calculations. An isomerization-roaming pathway to the CH3ONO isomer and then to the CH3O + NO products is found. Although the pathway occurs at larger distances than a related loose saddle-point on the PES, the pathway supports the supposition of such a pathway based on locating a loose first-order saddle point and associated IRC, reported previously by Zhu and Lin [Zhu, R. S. and Lin, M. C. Chem. Phys. Lett. 2009, 478, 11].

Direct Chemical Dynamics Simulations

In a direct dynamics simulation, the technologies of chemical dynamics and electronic structure theory are coupled so that the potential energy, gradient, and Hessian required from the simulation are obtained directly from the electronic structure theory. These simulations are extensively used to (1) interpret experimental results and understand the atomic-level dynamics of chemical reactions; (2) illustrate the ability of classical simulations to correctly interpret and predict chemical dynamics when quantum effects are expected to be unimportant; (3) obtain the correct classical dynamics predicted by an electronic structure theory; (4) determine a deeper understanding of when statistical theories are valid for predicting the mechanisms and rates of chemical reactions; and (5) discover new reaction pathways and chemical dynamics. Direct dynamics simulation studies are described for bimolecular SN2 nucleophilic substitution, unimolecular decomposition, post-transition-state dynamics, mass spectrometry experiments, and semiclassical vibrational spectra. Also included are discussions of quantum effects, the accuracy of classical chemical dynamics simulation, and the methodology of direct dynamics.

Ab Initio and RRKM Study of the HCN/HNC Elimination Channels from Vinyl Cyanide

Ab initio CCSD and CCSD(T) calculations with the 6-311+G(2d,2p) and the 6-311++G(3df,3pd) basis sets were carried out to characterize the vinyl cyanide (C(3)H(3)N) dissociation channels leading to hydrogen cyanide (HCN) and its isomer hydrogen isocyanide (HNC). Our computations predict three elimination channels giving rise to HCN and another four channels leading to HNC formation. The relative HCN/HNC branching ratios as a function of internal energy of vinyl cyanide were computed using RRKM theory and the kinetic Monte Carlo method. At low internal energies (120 kcal/mol), the total HCN/HNC ratio is about 14, but at 148 kcal/mol (193 nm) this ratio becomes 1.9, in contrast with the value 124 obtained in a previous ab initio/RRKM study at 193 nm (Derecskei-Kovacs, A.; North, S. W. J. Chem. Phys.1999, 110, 2862). Moreover, our theoretical results predict a ratio of rovibrationally excited acetylene over total acetylene of 3.3, in perfect agreement with very recent experimental measurements (Wilhelm, M. J.; Nikow, M.; Letendre, L.; Dai, H.-L. J. Chem. Phys.2009, 130, 044307).

Experimental and Theoretical Studies of Roaming Dynamics in the Unimolecular Dissociation of CH3NO2 to CH3O + NO

Abstract Characteristic signatures of roaming are seen in the translational energy distribution and rotational energy distributions in the products CH3O + NO from the unimolecular dissociation of nitromethane in both theory and experiment. Calculations on a new global potential energy surface reveal the detailed roaming-isomerization dynamics in this process, and these are supported by the experimental observations. Calculations find a characteristic of roaming, namely production of vibrationally excited CH3O, which is consistent with experiment. This continues to challenge the conventional idea in transition state theory of a localized transition state bottleneck that is a cornerstone of chemical reaction theory.

Visible/Infrared Dissociation of NO3: Roaming in the Dark or Roaming on the Ground?

We present a DC slice imaging study of roaming dynamics in the photodissociation of the nitrate radical, NO3, contrasting pure visible excitation with a combination of visible and CO2 laser excitation at 10.6 μm. Images of specific rotational levels of NO are seen to reflect dissociation on the ground and first excited electronic states, as reported in previous work. The branching is obtained for specific rotational levels by comparison to quasiclassical trajectory calculations of the dynamics on these two surfaces. The results for the visible dissociation are found to be very similar to the combination of visible and infrared, raising questions about the nature of the coupling of these surfaces, the extent to which roaming takes place on both, and how the final product branching is determined.

Model Simulations of the Thermal Dissociation of the TIK(H+)2 Tripeptide: Mechanisms and Kinetic Parameters

Direct dynamics simulations, utilizing the RM1 semiempirical electronic structure theory, were performed to study the thermal dissociation of the doubly protonated tripeptide threonine-isoleucine-lysine ion, TIK(H+)2, for temperatures of 1250-2500 K, corresponding to classical energies of 1778-3556 kJ/mol. The number of different fragmentation pathways increases with increase in temperature. At 1250 K there are only three fragmentation pathways, with one contributing 85% of the fragmentation. In contrast, at 2500 K, there are 61 pathways, and not one dominates. The same ion is often formed via different pathways, and at 2500 K there are only 14 m/z values for the product ions. The backbone and side-chain fragmentations occur by concerted reactions, with simultaneous proton transfer and bond rupture, and also by homolytic bond ruptures without proton transfer. For each temperature the TIK(H+)2 fragmentation probability versus time is exponential, in accord with the Rice-Ramsperger-Kassel-Marcus and transition state theories. Rate constants versus temperature were determined for two proton transfer and two bond rupture pathways. From Arrhenius plots activation energies Ea and A-factors were determined for these pathways. They are 62-78 kJ/mol and (2-3) × 1012 s-1 for the proton transfer pathways and 153-168 kJ/mol and (2-4) × 1014 s-1 for the bond rupture pathways. For the bond rupture pathways, the product cation radicals undergo significant structural changes during the bond rupture as a result of hydrogen bonding, which lowers their entropies and also their Ea and A parameters relative to those for C-C bond rupture pathways in hydrocarbon molecules. The Ea values determined from the simulation Arrhenius plots are in very good agreement with the reaction barriers for the RM1 method used in the simulations. A preliminary simulation of TIK(H+)2 collision-induced dissociation (CID), at a collision energy of 13 eV (1255 kJ/mol), was also performed to compare with the thermal dissociation simulations. Though the energy transferred to TIK(H+)2 in the collisions is substantially less than the energy for the thermal excitations, there is substantial fragmentation as a result of the localized, nonrandom excitation by the collisions. CID results in different fragmentation pathways with a significant amount of short time nonstatistical fragmentation. Backbone fragmentation is less important, and side-chain fragmentation is more important for the CID simulations as compared to the thermal simulations. The thermal simulations provide information regarding the long-time statistical fragmentation.

Quasiclassical Trajectory Calculations of the N((2)D) + H2O Reaction Elucidating the Formation Mechanism of HNO and HON Seen in Molecular Beam Experiments.

The N((2)D) + H2O is a reaction with competitive product channels, passing through several intermediates. Dynamics of this reaction had been investigated by two of the present authors at two collision energies, Ec, using the crossed molecular beams mass spectrometric method ( Faraday Discuss. 2001 , 119 , 27 - 49 ). The complicated mechanism of this reaction and puzzling results encouraged us to investigate the reaction in a joint experimental/theoretical study. Quasiclassical trajectory (QCT) calculations on an ab initio potential energy surface describing all channels of the title reaction are done with a focus on the N/H exchange channels. Interesting results of QCT calculations, in very good agreement with experimental data, reveal subtle details of the reaction dynamics of the title reaction to HNO/HON + H exit channels by disentangling the different routes to formation of the two possible HNO/HON isomers and therefore assisting in a critical manner the derivation of the reaction mechanism. Results of the present study show that the nonstatistical HNOH intermediate governs exit channels; therefore, the HON channel is as important as that of HNO. The study also confirms that the H2 + NO molecular channel is negligible even though the barrier to its formation is calculated to be well below the reactant asymptote.

Multichannel RRKM-TST and CVT rate constant calculations for reactions of CH2OH or CH3O with HO2.

The kinetics and mechanism of the gas-phase reactions between hydroxy methyl radical (CH(2)OH) or methoxy radical (CH(3)O) with hydroproxy radical (HO(2)) have been theoretically investigated on their lowest singlet and triplet surfaces. Our investigations indicate the presence of one deep potential well on the singlet surface of each of these systems that play crucial roles on their kinetics. We have shown that the major products of CH(2)OH + HO(2) system are HCOOH, H(2)O, H(2)O(2), and CH(2)O and for CH(3)O + HO(2) system are CH(3)OH and O(2). Multichannel RRKM-TST calculations have been carried out to calculate the individual rate constants for those channels proceed through the formation of activated adducts on the singlet surfaces. The rate constants for direct hydrogen abstraction reactions on the singlet and triplet surfaces were calculated by means of direct-dynamics canonical variational transition-state theory with small curvature approximation for the tunneling.

Communication: Rate coefficients from quasiclassical trajectory calculations from the reverse reaction: The Mu + H2 reaction re-visited

In a previous paper [P. G. Jambrina et al., J. Chem. Phys. 135, 034310 (2011)] various calculations of the rate coefficient for the Mu + H(2) → MuH + H reaction were presented and compared to experiment. The widely used standard quasiclassical trajectory (QCT) method was shown to overestimate the rate coefficients by several orders of magnitude over the temperature range 200-1000 K. This was attributed to a major failure of that method to describe the correct threshold for the reaction owing to the large difference in zero-point energies (ZPE) of the reactant H(2) and product MuH (∼0.32 eV). In this Communication we show that by performing standard QCT calculations for the reverse reaction and then applying detailed balance, the resulting rate coefficient is in very good agreement with the other computational results that respect the ZPE, (as well as with the experiment) but which are more demanding computationally.

A Global Potential Energy Surface Describing the N(2D) + H2O Reaction and a Quasiclassical Trajectory Study of the Reaction to NH + OH

We report a global potential energy surface (PES) for the N((2)D) + H2O reaction based on fitting roughly 312 000 UCCSD(T)-F12/aug-cc-pVTZ electronic energies. The surface is a linear least-squares fit using a permutationally invariant basis with Morse-type variables. Quasiclassical trajectory calculations of the N((2)D) + H2O(D2O) reaction with focus on the NH(D) + OH(D) exit channel are performed. An analysis of the internal-state distributions shows that the NH(D) fragment has more internal energy, both rotational and vibrational than the OH(D) fragment, in good agreement with experiment. This difference is traced to nonstatistical dynamics.