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Direct dynamics simulations with the M06/6-311++G(d,p) level of theory were performed to study the 3CH2 + 3O2 reaction at 1000 K temperature on the ground state singlet surface. The reaction is complex with formation of many different product channels in highly exothermic reactions. CO, CO2, H2O, OH, H2, O, H, and HCO are the products formed from the reaction. The total simulation rate constant for the reaction at 1000 K is (1.2 ± 0.3) × 10-12 cm3 molecule-1 s-1, while the simulation rate constant at 300 K is (0.96 ± 0.28) × 10-12 cm3 molecule-1 s-1. The simulated product yields show that CO is the dominant product and the CO:CO2 ratio is 5.3:1, in good comparison with the experimental ratio of 4.3:1 at 1000 K. On comparing the product yields for the 300 and 1000 K simulations, we observed that, except for CO and H2O, the yields of the other products at 1000 K are lower at 300 K, showing a negative temperature dependence.

The dynamics of the HBr+ + CO2 → HOCO+ + Br reaction was recently investigated with guided ion beam experiments under various excitations (collision energy of the reactants, rotational and spin-orbital states of HBr+, etc.), and their impacts were probed through the change of the cross section of the reaction. The potential energy profile of this reaction has also been accurately characterized by high-level ab initio methods such as CCSD(T)/CBS, and the UMP2/cc-pVDZ/lanl08d has been identified as an ideal method to study its dynamics. This manuscript reports the first ab initio molecular dynamics simulations of this reaction at two different collision energies, 8.1 kcal/mol and 19.6 kcal/mol. The cross sections measured from the simulations agree very well with the experiments measured with HBr+ in the 2∏1/2 state. The simulations reveal three distinct mechanisms at both collision energies: direct rebound (DR), direct stripping (DS), and indirect (Ind) mechanisms. DS and Ind make up 97% of the total reaction. The dynamics of this reaction is also compared with nucleophilic substitution (SN2) reactions of X- + CH3Y → CH3X + Y- type. In summary, this research has revealed interesting dynamics of the HBr+ + CO2 → HOCO+ + Br reaction at different collision energies and has laid a solid foundation for using this reaction to probe the impact of rotational excitation of ion-molecule reactions in general.

The 3CH2 + 3O2 reaction has a quite complex ground state singlet potential energy surface (PES). There are multiple minima and transition states before forming the 10 possible reaction products. A ...

The potential energy curves (PECs) for the interaction of 3CH2 with 3O2 in singlet and triplet potential energy surfaces (PESs) leading to singlet and triplet Criegee intermediates (CH2OO) are studied using electronic structure calculations. The bonding mechanism is interpreted by analyzing the ground state multireference configuration interaction (MRCI) wave function of the reacting species and at all points along the PES. The interaction of 3CH2 with 3O2 on the singlet surface leads to a flat long-range attractive PEC lacking any maxima or minima along the curve. The triplet surface stems into a maximum along the curve resulting in a transition state with an energy barrier of 5.3 kcal/mol at CASSCF(4,4)/cc-pVTZ level. The resulting 3CH2OO is less stable than the 1CH2OO. In this study, the biradical character (β) is used as a measure to understand the difference in the topology of the singlet and triplet PECs and the relation of the biradical nature of the species with their structures. The 3CH2OO has a larger biradical character than 1CH2OO, and because of the larger bond order of 1CH2OO, the C-O covalent bond becomes harder to break, thereby stabilizing 1CH2OO. Thus, this study provides insights into the shape of the PEC obtained from the reaction between 3CH2 and 3O2 in terms of their bonding nature and from the shape of the curves, the temperature dependence or independence of the rate of the reaction is discussed.

In a previous work [ Lakshmanan , S. ; J. Phys. Chem. A 2018 , 122 , 4808 - 4818 ], direct dynamics simulations at the M06/6-311++G(d,p) level of theory were reported for 3CH2 (X3B1) + 3O2 (X3∑g-) reaction on its ground-state singlet potential energy surface (PES) at 300 K. However, further analyses revealed the simulations are unstable for the 3CH2 (X3B1) + 3O2 (X3∑g-) reactants on the ground-state singlet surface and the trajectories reverted to an excited-state singlet surface for the 1CH2 (a1A1) + 1O2 (b1∑g+) reactants. Thus, the dynamics reported previously are for this excited-state singlet PES. The PESs for the 3CH2 (X3B1) + 3O2 (X3∑g-) and 1CH2 (a1A1) + 1O2 (b1∑g+) reactants are quite similar, and this provided a means to perform simulations for the 3CH2 (X3B1) + 3O2 (X3∑g-) reactants on the ground-state singlet PES at 300 K, which are reported here. The reaction dynamics are quite complex with seven different reaction pathways and nine different products. A consistent set of product yields have not been determined experimentally, but the simulation yields for the H atom, CO, and CO2 are somewhat lower, higher, and lower respectively, than the recommended values. The yields for the remaining six products agree with experimental values. Product decomposition was included in determining the product yields. The simulation 3CH2 + 3O2 rate constant at 300 K is only 3.4 times smaller than the recommended value, which may be accommodated if the 3CH2 + 3O2 → 1CH2O2 potential energy curve is only 0.75 kcal/mol more attractive at the variational transition state for 3CH2 + 3O2 → 1CH2O2 association. The simulation kinetics and dynamics for the 3CH2 + 3O2 and 1CH2 + 1O2 reactions are quite similar. Their rate constants are statistically the same, an expected result, since their transition states leading to products have energies lower than that of the reactants and the attractive potential energy curves for 3CH2 + 3O2 → 1CH2O2 and 1CH2 + 1O2 → 1CH2O2 are nearly identical. The product yields for the 3CH2 + 3O2 and 1CH2 + 1O2 reactions are also nearly identical, only differing for the CO2 yield. The reaction dynamics on both surfaces are predominantly direct, with negligible trapping in potential energy minima, which may be an important contributor to their nearly identical product yields.

Collisional energy transfer of highly vibrationally excited propylbenzene cation in a N2 bath has been studied with chemical dynamics simulations. In this work, an intermolecular potential of propylbenzene cation interacting with N2 was developed from SCS-MP2/6-311++G** ab initio calculations. Using a particle swarm optimization algorithm, the ab initio results were simultaneously fit to a sum of three two-body potentials, consisting of Ca–N, Cb–N, and H–N, where Ca is carbon on the benzene ring and Cb is carbon on the propyl side chain. Using the developed intermolecular potential, classical trajectory calculations were performed with a 100.1 kcal/mol excitation energy at 473 K to compare with experiment. Varying the density of the N2 bath, the single collision limit of propylbenzene cation with the N2 bath was obtained at a density of 20 kg/m3 (28 atm). For the experimental excitation energy and in the single collision limit, the average energy transferred per collision, ⟨ΔEc⟩, is 1.04 ± 0.04 kcal/mol a...

Following work by Slater and Bunker, the unimolecular rate constant versus collision frequency, kuni(ω,E), is expressed as a convolution of unimolecular lifetime and collisional deactivation probabilities. This allows incorporation of nonexponential, intrinsically non-RRKM, populations of dissociating molecules versus time, N(t)/N(0), in the expression for kuni(ω,E). Previous work using this approach is reviewed. In the work presented here, the biexponential f1 exp(−k1t) + f2 exp(−k2t) is used to represent N(t)/N(0), where f1k1 + f2k2 equals the RRKM rate constant k(E) and f1 + f2 = 1. With these two constraints, there are two adjustable parameters in the biexponential N(t)/N(0) to represent intrinsic non-RRKM dynamics. The rate constant k1 is larger than k(E) and k2 is smaller. This biexponential gives kuni(ω,E) rate constants that are lower than the RRKM prediction, except at the high and low pressure limits. The deviation from the RRKM prediction increases as f1 is made smaller and k1 made larger. Of c...

The reaction of 3CH2 with 3O2 is of fundamental importance in combustion, and the reaction is complex as a result of multiple extremely exothermic product channels. In the present study, direct dynamics simulations were performed to study the reaction on both the singlet and triplet potential energy surfaces (PESs). The simulations were performed at the UM06/6-311++G(d,p) level of theory. Trajectories were calculated at a temperature of 300 K, and all reactive trajectories proceeded through the carbonyl oxide Criegee intermediate, CH2OO, on both the singlet and triplet PESs. The triplet surface leads to only one product channel, H2CO + O(3P), while the singlet surface leads to eight product channels with their relative importance as CO + H2O > CO + OH + H ∼ H2CO + O(1D) > HCO + OH ∼ CO2 + H2 ∼ CO + H2 + O(1D) > CO2 + H + H > HCO + O(1D) + H. The reaction on the singlet PES is barrierless, consistent with experiment, and the total rate constant on the singlet surface is (0.93 ± 0.22) × 10–12 cm3 molecule–1...

Chemical dynamics simulations were performed to study collisional intermolecular energy transfer from a thermalized N2 bath at 300 K to vibrationally “cold” C6F6. The vibrational temperature of C6F6 is taken as 50 K, which corresponds to a classical vibrational energy of 2.98 kcal/mol. The temperature ratio between C6F6 and the bath is 1/6, the reciprocal of the same ratio for previous “hot” C6F6 simulations (J. Chem. Phys. 2014, 140, 194103). Simulations were also done for a C6F6 vibrational temperature of 0 K. The average energy of C6F6 versus time is well fit by a biexponential function which gives a slightly larger short time rate component, k1, but a four times smaller long time rate component, k2, compared to those obtained from the “hot” C6F6 simulations. The average energy transferred per collision depends on the difference between the average energy of C6F6 and the final C6F6 energy after equilibration with the bath, but not on the temperature ratio of C6F6 and the bath. The translational and rot...

Anti-E2, syn-E2, inv-, and ret-SN2 reaction channels for the gas-phase reaction of F– + CH3CH2I were characterized with a variety of electronic structure calculations. Geometrical analysis confirmed synchronous E2-type transition states for the elimination of the current reaction, instead of nonconcerted processes through E1cb-like and E1-like mechanisms. Importantly, the controversy concerning the reactant complex for anti-E2 and inv-SN2 paths has been clarified in the present work. A positive barrier of +19.2 kcal/mol for ret-SN2 shows the least feasibility to occur at room temperature. Negative activation energies (−16.9, −16.0, and −4.9 kcal/mol, respectively) for inv-SN2, anti-E2, and syn-E2 indicate that inv-SN2 and anti-E2 mechanisms significantly prevail over the eclipsed elimination. Varying the leaving group for a series of reactions F– + CH3CH2Y (Y = F, Cl, Br, and I) leads to monotonically decreasing barriers, which relates to the gradually looser TS structures following the order F > Cl > Br ...

A zero-point energy (ZPE) constraint model is proposed for classical trajectory simulations of unimolecular decomposition and applied to CH4* → H + CH3 decomposition. With this model trajectories are not allowed to dissociate unless they have ZPE in the CH3 product. If not, they are returned to the CH4* region of phase space and, if necessary, given additional opportunities to dissociate with ZPE. The lifetime for dissociation of an individual trajectory is the time it takes to dissociate with ZPE in CH3, including multiple possible returns to CH4*. With this ZPE constraint the dissociation of CH4* is exponential in time as expected for intrinsic RRKM dynamics and the resulting rate constant is in good agreement with the harmonic quantum value of RRKM theory. In contrast, a model that discards trajectories without ZPE in the reaction products gives a CH4* → H + CH3 rate constant that agrees with the classical and not quantum RRKM value. The rate constant for the purely classical simulation indicates that anharmonicity may be important and the rate constant from the ZPE constrained classical trajectory simulation may not represent the complete anharmonicity of the RRKM quantum dynamics. The ZPE constraint model proposed here is compared with previous models for restricting ZPE flow in intramolecular dynamics, and connecting product and reactant/product quantum energy levels in chemical dynamics simulations.

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.

Direct dynamics simulations and selected ion flow tube (SIFT) experiments were performed to study the kinetics and dynamics of the OH(-) + CH3I reaction versus temperature. This work complements previous direct dynamics simulation and molecular beam ion imaging experiments of this reaction versus reaction collision energy (Xie et al. J. Phys. Chem. A 2013, 117, 7162). The simulations and experiments are in quite good agreement. Both identify the SN2, OH(-) + CH3I → CH3OH + I(-), and proton transfer, OH(-) + CH3I → CH2I(-) + H2O, reactions as having nearly equal importance. In the experiments, the SN2 pathway constitutes 0.64 ± 0.05, 0.56 ± 0.05, 0.51 ± 0.05, and 0.46 ± 0.05 of the total reaction at 210, 300, 400, and 500 K, respectively. For the simulations this fraction is 0.56 ± 0.06, 0.55 ± 0.04, and 0.50 ± 0.05 at 300, 400, and 500 K, respectively. The experimental total reaction rate constant is (2.3 ± 0.6) × 10(-9), (1.7 ± 0.4) × 10(-9), (1.9 ± 0.5) × 10(-9), and (1.8 ± 0.5) × 10(-9) cm(3) s(-1) at 210, 300, 400, and 500 K, respectively, which is approximately 25% smaller than the collision capture value. The simulation values for this rate constant are (1.7 ± 0.2) × 10(-9), (1.8 ± 0.1) × 10(-9), and (1.6 ± 0.1) × 10(-9) cm(3)s(-1) at 300, 400, and 500 K. From the simulations, direct rebound and stripping mechanisms as well as multiple indirect mechanisms are identified as the atomic-level reaction mechanisms for both the SN2 and proton-transfer pathways. For the SN2 reaction the direct and indirect mechanisms have nearly equal probabilities; the direct mechanisms are slightly more probable, and direct rebound is more important than direct stripping. For the proton-transfer pathway the indirect mechanisms are more important than the direct mechanisms, and stripping is significantly more important than rebound for the latter. Calculations were performed with the OH(-) quantum number J equal to 0, 3, and 6 to investigate the effect of OH(-) rotational excitation on the OH(-) + CH3I reaction dynamics. The overall reaction probability and the probabilities for the SN2 and proton-transfer pathways have little dependence on J. Possible effects on the atomistic mechanisms were investigated for the SN2 pathway and the probability of the direct rebound mechanism increased with J. However, the other atomistic mechanisms were not appreciably affected by J.

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.

MP2 and DFT electronic structure theories, with the functionals OPBE, OLYP, HCTH407, BhandH, and B97-1 for the latter, were used to investigate stationary point properties on the F(-) + CH(3)I → FCH(3) + I(-) potential energy surface (PES). The aug-cc-pVDZ and aug-cc-pVTZ basis sets for C, H, and F, with Wadt and Hay's 3s3p valence basis and an effective core potential (ECP) for iodine, were employed for both MP2 and DFT. Single-point CCSD(T) calculations were also performed to obtain the complete basis set (CBS) limit for the stationary point energies. The CCSD(T)/CBS reaction exothermicity is only 5.0 kJ/mol different than the experimental value. MP2 and DFT do not predict the same stationary points on the PES. MP2 predicts the C(3v) F(-)-CH(3)I and FCH(3)-I(-) ion-dipole complexes and traditional [F-CH(3)-I](-) central barrier as stationary points, as well as a C(s) hydrogen-bonded F(-)-HCH(2)I complex and a [F-HCH(2)-I](-) transition state connecting this latter complex to the F(-)-CH(3)I complex. A CCSD(T)/CBS relaxed potential energy curve, calculated for the MP2 structures, shows that going from the F(-)-CH(3)I complex to the [F-CH(3)-I](-) TS is a barrierless process, indicating these two structures are not stationary points. This is also suggested by the DFT calculations. The structures and frequencies for CH(3)I and CH(3)Cl given by MP2 and DFT are in overall good agreement with experiment. The calculations reported here indicate that the DFT/B97-1 functional gives the overall best agreement with the CCSD(T) energies, with a largest difference of only 7.5 kJ/mol for the FCH(3)-I(-) complex.

International audience; Quantum mechanical plus molecular mechanical direct chemical dynamics were used, with electrospray tandem mass spectrometry experiments, potential energy surface calculations, and RRKM analyses, to study the gas-phase collision-induced dissociation (CID) of protonated urea. The direct dynamics were able to reproduce some of the experimental observations, in particular the presence of two fragmentation pathways, and, thus, to explain the dynamical origin of the two fragmentation ions observed in the CID spectra. A shattering dissociation mechanism takes place during the collision, and it becomes more important as the collision energy increases, thus explaining the linear increase of the high-energy reaction path (loss of ammonia) versus collision energy. By combining the different theoretical and experimental findings, a complete dynamical picture leading to the fragmentation was identified: (i) Oxygen-protonated urea, the most stable structure in the gas phase, must first isomerize to the nitrogen-protonated form. This can happen by multiple CID collisions or in the electrospray ionization process. (ii) Once the nitrogen-protonated isomer is formed, it can dissociate via two mechanisms: i.e, a slow, almost statistical, process forming a NH4+--NHCO intermediate that rapidly dissociates or a fast nonstatistical process which may lead to the high-energy products.

Results are reported for a direct dynamics simulation of NH(4)(+) + CH(4) gas phase collisions. We interpret the results with protonated peptide/hydrogenated alkanethiolate self-assembled monolayer (H-SAM) surface collisions in mind. Previous theoretical studies of such systems have made use of nonreactive surfaces, and therefore, our goal is to investigate the types and likelihood of peptide/H-SAM reactions. In that vein, the NH(4)(+) + CH(4) reaction represents a simple gas phase system which includes many of the important interactions present in protonated peptide/H-SAM surfaces. Thirty-seven open pathways are seen in the 5-35 eV collision energy range. An energy dependence on the likelihood of forming CN bonds is found. This type of bonding could deposit both the peptide and its molecular fragments on the H-SAM surface. For our gas phase collision system, around 50% of the trajectories result in the formation of CN bonds. For all collision energies in which reactive scattering occurs, CN bond formation is an important reaction pathway.

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.

Dynamics of Ar atom collisions with a perfluorinated alkanethiol self-assembled monolayer (F-SAM) surface on gold were investigated by classical trajectory simulations and atomic beam scattering techniques. Both explicit-atom (EA) and united-atom (UA) models were used to represent the F-SAM surface; in the UA model, the CF3 and CF2 units are represented as single pseudoatoms. Additionally the nonbonded interactions in both models are different. The simulations show the three limiting mechanisms expected for collisions of rare gas atoms (or small molecules) with SAMs, that is, direct scattering, physisorption, and penetration. Surface penetration results in a translational energy distribution, P(Ef), that can be approximately fit to the Boltzmann for thermal desorption, suggesting that surface accommodation is attained to a large extent. Fluorination of the alkanethiol monolayer leads to less energy transfer in Ar collisions. This results from a denser and stiffer surface structure in comparison with that of the alkanethiol SAM, which introduces constraints for conformational changes which play a significant role in the energy-transfer process. The trajectory simulations predict P(Ef) distributions in quite good agreement with those observed in the experiments. The results obtained with the EA and UA models are in reasonably good agreement, although there are some differences.

Linear molecules with degenerate bending modes have states, which may be represented by the quantum numbers N and L. The former gives the total energy for these modes and the latter identifies their vibrational angular momentum jz. In this work, the classical mechanical analog of the N,L-quantum states is reviewed, and an algorithm is presented for selecting initial conditions for these states in quasiclassical trajectory chemical dynamics simulations. The algorithm is illustrated by choosing initial conditions for the N = 3 and L = 3 and 1 states of CO2. Applications of this algorithm are considered for initial conditions without and with zero-point energy (zpe) included in the vibrational angular momentum states and the C-O stretching modes. The O-atom motions in the x,y-plane are determined for these states from classical trajectories in Cartesian coordinates and are compared with the motion predicted by the normal-mode model. They are only in agreement for the N = L = 3 state without vibrational angular momentum zpe. For the remaining states, the Cartesian O-atom motions are considerably different from the elliptical motion predicted by the normal-mode model. This arises from bend-stretch coupling, including centrifugal distortion, in the Cartesian trajectories, which results in tubular instead of elliptical motion. Including zpe in the C-O stretch modes introduces considerable complexity into the O-atom motions for the vibrational angular momentum states. The short-time O-atom motions for these trajectories are highly irregular and do not appear to have any identifiable characteristics. However, the O-atom motions for trajectories integrated for substantially longer period of times acquire unique properties. With C-O stretch zpe included, the long-time O-atom motion becomes tubular for trajectories integrated to approximately 14 ps for the L = 3 states and to approximately 44 ps for the L = 1 states.

The effects of temperature on energy transfer during collisions of protonated diglycine ions, Gly(2)-H(+), with a diamond {111} surface were investigated by chemical dynamics simulations. The simulations were performed for a collision energy of 70 eV and angle of 0 degrees with respect to the surface normal. In one set of simulations the initial surface temperature, T(surf), was varied from 300 to 2000 K, while the Gly(2)-H(+) vibrational and rotational temperatures were maintained at 300 K. For the second set of simulations the Gly(2)-H(+) vibrational temperature, T(vib), was varied from 300 to 2000 K, keeping both the Gly(2)-H(+) rotational and surface temperatures at 300 K. Increasing either the surface temperature or Gly(2)-H(+) vibrational temperature to values as high as 2000 K has, at most, only a negligible effect on the partitioning of the incident collision energy to the surface and to the vibrational and rotational modes of Gly(2)-H(+). To a good approximation, the initial surface and peptide ion energies are nearly adiabatic during the collisional energy transfer. This adiabaticity of the initial peptide ion energy agrees with experiments (J. Phys. Chem. A 2004, 108, 1). A more quantitative analysis of the effects of T(vib) and T(surf) shows there are small, but noticeable, effects on the energy transfer efficiencies. Namely, increasing the vibrational or surface temperature results in a near-linear decrease in the energy transfer to the degrees of freedom associated with this temperature.

A molecular dynamics simulation was performed to study the effect of an applied force on heat transfer at the interface of model diamond [111] nanosurfaces. The force was applied to a small, hot nanosurface at 800, 1000, or 1200 K brought into contact with a larger, colder nanosurface at 300 K. The relaxation of the initial nonequilibrium interfacial force occurs on a subpicosecond time scale, much shorter than that required for heat transfer. Heat transfer occurs with exponential kinetics and a rate constant that increases linearly with the interfacial force according to 7 x 10(-4) ps(-1)/nN. This rate constant only increases by at most 10% as the temperature of the hot surface is increased from 800 to 1200 K. Replacing the interfacial H-atoms on both surfaces by D atoms also has a very small effect on the heat transfer. However, if one nanosurface has H atoms on its interface and the other nanosurface's interface has D atoms, then there is a marked 25% decrease in the rate constant for heat transfer. Increasing the size of the hot surface, and, thus, the interfacial contact area, increases the rate of heat transfer but not the rate constant. For the same interfacial force, different anharmonic models for the nanosurfaces' potential energy function give the same heat transfer rate constant. The possibility of quantum effects for heat transfer across the diamond interface is considered.

Quasiclassical, direct dynamics trajectories have been used to study the reaction of formaldehyde cation with molecular hydrogen, simulating the conditions in an experimental study of H2CO+ vibrational effects on this reaction. Effects of five different H2CO+ modes were probed, and we also examined different approaches to treating zero-point energy in quasiclassical trajectories. The calculated absolute cross-sections are in excellent agreement with experiments, and the results provide insight into the reaction mechanism, product scattering behavior, and energy disposal, and how they vary with impact parameter and reactant state. The reaction is sharply orientation-dependent, even at high collision energies, and both trajectories and experiment find that H2CO+ vibration inhibits reaction. On the other hand, the trajectories do not reproduce the anomalously strong effect of nu2(+) (the CO stretch). The origin of the discrepancy and approaches for minimizing such problems in quasiclassical trajectories are discussed.

The PM3 semiempirical electronic structure theory is reparametrized with specific reaction parameters (SRPs) to develop a potential energy surface (PES) for O(3P) processing of alkanes. The results of high-level ab initio calculations for the O(3P) + C2H6 primary reactions, yielding OH + C2H5, C2H5O + H, and CH3O + CH3, 11 ensuing secondary and unimolecular dissociation reactions involving products of these primary reactions, and additional reactions were used to develop two PM3-SRP models for the PES. The ab initio results used for this fitting were taken from previous multiconfiguration calculations and additional PMP2/cc-pVTZ calculations reported here. Even though these two PM3-SRP models are unable to quantitatively represent the many reactions that occur in high-energy collisions of O(3P) with alkanes, they are vast improvements over the PES of PM3 theory. These models are used in direct dynamics classical trajectory simulations of the O(3P) + C2H6 reaction at a 5 eV collision energy. The results of...

The energy-transfer dynamics associated with Ne atom collisions with a n-hexylthiolate self-assembled monolayer (SAM) surface at 20 kcal/mol collision energy are studied in chemical dynamics simulations by using both harmonic/separable and anharmonic/coupled surface models and by considering a SAM in both its classical potential energy minimum and containing a 293 K energy distribution. For the anharmonic surface, excitation of higher-energy potential energy minima arising from different intermolecular conformations of the alkyl chains and intramolecular vibrational energy redistribution (IVR) between the surface modes during the collision enhance the energy transfer from Ne atom translation to the surface. IVR is efficient for the alkyl chain's torsional modes and intramolecular chain-bending modes and occurs on the time scale of the Ne atom collisions. It does not occur during the collisions for other modes, such as the higher-frequency intrachain CCC bends. This IVR between surface modes, during the co...

Direct dynamics classical trajectory simulations are used to study energy transfer and unimolecular dissociation in collisions of N-protonated glycine, (gly-H)+, with an argon atom and a hydrogenated diamond {111} surface. The (gly-H)+ potential is represented by the AM1 semiempirical electronic structure theory and analytic potentials developed previously are used for the diamond surface and the (gly-H)+/Ar and (gly-H)+/diamond intermolecular potentials. The AM1 potential for (gly-H)+ gives the same collisional energy transfer distributions as does the AMBER empirical force field. For (gly-H)+ + diamond {111} at a collision energy and angle of 70 eV and 45°, the average percent energy transfer to (gly-H)+ vibration/rotation, to the surface, and to final ion translation are 12, 38, and 50, respectively. A distribution of (gly-H)+ dissociation products are observed in these collisions, with ∼55% of the dissociations occurring while (gly-H)+ collides with the surface, i.e., shattering fragmentation. Shatter...

Recent quantum dynamical calculations [Hauschildt, J.; et al. Chem. Phys. Lett. 1999, 300, 569] have shown that HOCl → OH + Cl dissociation on the ground-state potential energy surface, and for J = 0, occurs predominantly via isolated compound-state resonances, whose rates are highly mode-specific. In this work, these resonance rates are averaged to calculate the HOCl → OH + Cl unimolecular rate constant as a function of temperature and pressure. The result is compared with the standard pressure and temperature dependent RRKM unimolecular rate constant. It is found that the state-specificity makes the pressure-dependent rates significantly lower than the RRKM rates in the intermediate pressure regime.

Canonical variational transition-state theory (CVTST) rate constants, for H atom association to terrace and ledge sites on the diamond {111} surface, are calculated using anharmonic energy levels f...

A Hamiltonian is described in which some degrees of freedom are represented by normal modes and the remainder retain their complete couplings and anharmonicities. The classical equations of motion for this Hamiltonian may be efficiently integrated in Cartesian coordinates. This Hamiltonian is used to study the mode specificity of energy transfer in Ne-atom collisions with alkanethiolate chains and a monolayer of n-hexyl thiolate chains self-assembled on Au{111}. The intermolecular and intramolecular degrees of freedom for these chain and self-assembled monolayer (SAM) systems are represented by normal modes. Collinear collisions with n-hexyl and n-octadecyl thiolate chains show that only one mode is excited at low collision energies. Mode specificity is also observed in Ne-atom collisions with the SAM. As expected from the adiabatic/impulsive model of T → V energy transfer, higher frequency modes of the chains and monolayer are excited as the Ne-atom translational energy is increased. A comparison, betwee...

A previous classical trajectory simulation showed that the unimolecular decompositions of Al 6 and Al13 are intrinsically RRKM. In the work presented here, this study is further analyzed to determine the Al 5 + Al and Al12 + Al product energy distributions, which are compared with the predictions of statistical theories. Orbiting transition state/phase space theory (OTS/PST) gives distributions in excellent agreement with the trajectory results. Assuming a loose, product-like transition state gives a lower average product translational energy, 〈Et〉, than what is found with the orbiting transition state. Including anharmonicity, in the calculation of the product vibrational density of states, increases the energy partitioned to product vibration. The Engelking model for cluster decomposition overestimates 〈Et〉. One Klots model gives an inaccurate 〈Et〉, but a second model more firmly rooted in phase space theory performs quite well. The Engelking model, for deducing the cluster dissociation energy from the measured 〈Et〉, does not give accurate results for Al 6 and Al13 dissociation.

Ab initio calculations were used to investigate properties of complexes formed from the association of CH4 with Al2O3, Al4O6, and Al8O12 alumina clusters. Methane attaches to a surface Al atom of t...

Classical trajectory simulations are performed to study the efficiency of energy transfer in the collisional activation of polyglycine and polyalanine peptide ions with β-sheet and α-helix structures. Energy-transfer efficiencies for collisions with Ar are determined versus impact parameter, peptide size and structure, mass of the collider, the collision energy, and the form of the intermolecular potential between the peptide and argon. High-level ab initio calculations, for Ar interacting with small molecules representing the peptides' functional groups, are performed to determine an accurate Ar + peptide intermolecular potential. Energy transfer may be efficient and in some cases as high as 80%. There is a low collision energy regime in which the percent energy transfer increases as the peptide size increases. However, at higher energies, an apparent impulsive energy-transfer regime is reached where the peptide size has a negligible effect on the energy-transfer efficiency. For a certain peptide size, s...

The PES(F,Cl) analytic potential energy surface, developed previously, is used in a trajectory study of the F- + CH3Cl SN2 reaction. The trajectory SN2 rate constants, as a function of reactant relative translational energy Erel and CH3Cl temperature, are in good overall agreement with the experimental rate constants and those calculated using an ion−molecule capture/RRKM statistical model applied to PES(F,Cl). The latter agreement exists even though the reaction dynamics is decidedly nonstatistical. For high Erel the reaction is direct. At lower Erel there is evidence for formation of an ion−molecule complex; however, its lifetime is too short for complete energy randomization to occur. The velocity scattering angle is isotropic at low Erel but becomes anisotropic with forward scattering as Erel is increased. The reaction exothermicity is primarily partitioned to product vibration, in disagreement with a previous experimental study. Energy transfer from the reactants to products is very selective. Excess...

Classical trajectory simulations are used to study the translational activation of the Cl- + CH3Cl SN2 reaction at energies in the range 20−80 kcal/mol. The trajectories are calculated on the PES3 analytic potential energy surface. The shape of the reactive cross section versus relative translational energy Erel and the translational threshold of 18 kcal/mol are both similar to recent experimental results [J. Phys. Chem. A 1997, 101, 5969]. The reactive trajectories are direct, with negligible trapping in the ion−dipole complexes. The product energy is primarily partitioned to relative translation with small and similar amounts of energy partitioned to vibration and rotation. The velocity scattering angle distribution suggests backward scattering and a rebound mechanism for translational activation at low Erel, with increasing importance of forward scattering and a stripping mechanism as Erel is increased. An analysis of angular momenta terms and their correlations shows that the total angular momentum is...

Classical trajectories are computed for the stereomutations of cyclopropane-1,2-d2 at 695 K using direct dynamics on an AM1-SRP potential energy surface (AM1 with specific reaction parameters). The trajectories were initiated on a dividing surface, separating the cyclopropane reactants from the stereomutation products, with momenta and coordinates sampled from a 695 K Boltzmann distribution. The quasiclassical procedure was used, with two different sampling schemes, to ensure zero-point energy and vibrational quantization conditions for the energy levels on the dividing surface. From the product distribution, the ratio of rate constants for double and single methylene rotation, k12/k1, is 2.3 and 3.5 for the two sampling methods. These are between the values of 1.0 and 5−42 deduced from two separate experimental studies. A harmonic-oscillator/rigid-rotor transition-state theory calculation of k12/k1, assuming trimethylene as an intermediate, gives k12/k1 = 1.35 on the AM1-SRP potential energy surface. The...

Different levels of ab initio theory are used to calculate geometries, vibrational frequencies, and energies for stationary points on the H + C2H4 ⇌ C2H5 potential energy surface. Frequencies and geometries calculated for C2H4 at the QCISD/6-311G** and MRCI/cc-pVDZ levels of theory are in very good agreement with experiment. The ab initio normal modes for the H- - -C2H4 transition state are similar to those for C2H4, and ratios between the C2H4 experimental anharmonic frequencies and ab initio harmonic frequencies are used to estimate anharmonic frequencies for the transition state. Nine of the 10 frequencies assigned for C2H5 from experiment are consistent with the ab initio calculations. The CC stretch was apparently misassigned and reassigned here. The ab initio calculations do not give definitive values for the H + C2H4 → C2H5 barrier height and heat of reaction. Using these two energy terms as adjustable parameters in concert with the above stationary point geometries and frequencies, transition stat...