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Donald G Truhlar - One of the best experts on this subject based on the ideXlab platform.
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hydrogen abstraction reactions from phenolic compounds by peroxyl radicals multireference character and density functional Theory Rate constants
Journal of Physical Chemistry A, 2016Co-Authors: Annia Galano, Leonardo Munozrugeles, Juan Raul Alvarezidaboy, Donald G TruhlarAbstract:An assessment of multireference character in transition states is considered to be an important component in establishing the expected reliability of various electronic structure methods. In the present work, the multireference characters of the transition states and the forming and breaking of bonds for a large set of hydrogen abstraction reactions from phenolic compounds by peroxyl radicals have been analyzed using the T1, M, B1, and GB1 diagnostics. The extent of multireference character depends on the system and on the conditions under which the reaction takes place, and some systematic trends are observed. In particular, the multireference character is found to be reduced by solvation, the size of the phenolic compound, and deprotonation in aqueous solution. However, the deviations of calculated Rate constants from experimental ones are not correlated with the extent of multireference character. The performance of single-determinant density functional Theory was investigated for the kinetics of these...
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path dependent variational effects and multidimensional tunneling in multi path variational transition state Theory Rate constants calculated for the reactions of ho2 with tert butanol by including all 46 paths for abstraction at c and all six paths
Physical Chemistry Chemical Physics, 2016Co-Authors: Junwei Lucas Bao, Pattrawan Sripa, Donald G TruhlarAbstract:Multi-path variational transition state Theory (MP-VTST) provides a conformationally complete framework for calculating gas-phase Rate constants. For reactions in which the transition state has distinguishable torsional minima (which include most reactions), there are multiple possible reaction paths. In principle MP-VTST includes the contributions from all the reaction paths, and it should explicitly treat the variational and tunneling effects of each path, but in practice one may need to truncate the number of paths included in MP-VTST calculations in order to achieve a balance between computational cost and accuracy. In this work, we present calculations including all paths for two prototype combustion reactions, namely the two hydrogen abstraction reactions from tert-butanol by HO2 radical. For both reactions we included all the reaction paths. Since abstraction at C has 46 paths, it provided a good opportunity to carry out a case study in which we investigated the errors introduced by truncating the number of paths. For the reaction studied, we found that the variational and multidimensional tunneling transmission coefficients are very different for different reaction paths, which provides new evidence that MP-VTST is necessary for treating path-dependent variational effects and multidimensional tunneling. We found that tunneling transmission coefficients can be much larger for higher-energy paths than for lower-energy ones. Interestingly, the simple hypothesis that higher barriers are narrower does not explain this finding in the present case; we found instead that the effect is due to higher-energy barriers having the possibility of tunneling at energies farther below the barrier top. We also show that a previously applied criterion for judging convergence with respect to the number of paths may not be reliable at low temperature.
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a product branching ratio controlled by vibrational adiabaticity and variational effects kinetics of the h trans n2h2 reactions
Journal of Chemical Physics, 2012Co-Authors: Jingjing Zheng, Francisco B C Machado, Orlando Robertoneto, Edson F. V. Carvalho, Roberta J Rocha, Marina Pelegrini, Luiz F A Ferrao, Donald G TruhlarAbstract:The abstraction and addition reactions of H with trans-N(2)H(2) are studied by high-level ab initio methods and density functional Theory. Rate constants were calculated for these two reactions by multistructural variational transition state Theory with multidimensional tunneling and including torsional anharmonicity by the multistructural torsion method. Rate constants of the abstraction reaction show large variational effects, that is, the variational transition state yields a smaller Rate constant than the conventional transition state; this results from the fact that the variational transition state has a higher zero-point vibrational energy than the conventional transition state. The addition reaction has a classical barrier height that is about 1 kcal∕mol lower than that of the abstraction reaction, but the addition Rates are lower than the abstraction Rates due to vibrational adiabaticity. The calculated branching ratio of abstraction to addition is 3.5 at 200 K and decreases to 1.2 at 1000 K and 1.06 at 1500 K.
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multipath variational transition state Theory Rate constant of the 1 4 hydrogen shift isomerization of the 2 cyclohexylethyl radical
Journal of Physical Chemistry A, 2012Co-Authors: Jingjing Zheng, Donald G TruhlarAbstract:We propose a new formulation of variational transition state Theory called multipath variational transition state Theory (MP-VTST). We employ this new formulation to calculate the forward and reverse thermal Rate constant of the 1,4-hydrogen shift isomerization of the 2-cyclohexylethyl radical in the gas phase. First, we find and optimize all the local-minimum-energy structures of the reaction, product, and transition state. Then, for the lowest-energy transition state structures, we calculate the reaction path by using multiconfiguration Shepard interpolation (MSCI) method to represent the potential energy surface, and, from this representation, we also calculate the ground-state vibrationally adiabatic potential energy curve, the reaction-path curvature vector, and the generalized free energy of activation profile. With this information, the path-averaged generalized transmission coefficients ⟨γ⟩ are evaluated. Then, thermal Rate constant containing the multiple-structure anharmonicity and torsional anh...
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Kinetics of the Hydrogen Abstraction from Carbon-3 of 1-Butanol by Hydroperoxyl Radical: Multi-Structural Variational Transition-State Calculations of a Reaction with 262 Conformations of the Transition State
The journal of physical chemistry letters, 2012Co-Authors: Prasenjit Seal, Ewa Papajak, Donald G TruhlarAbstract:We estimated Rate constants for the hydrogen abstraction from carbon-3 of 1-butanol by hydroperoxyl radical, a critically important reaction in the combustion of biofuel. We employed the recently developed multi-structural variational transition-state Theory (MS-VTST), which utilizes a multifaceted dividing surface that allows us to include the contributions of multiple structures for reacting species and transition states. First, multiconfigurational Shepard interpolation—based on molecular-mechanics-guided interpolation of electronic-structure Hessian data obtained by the M08 HX/jun-cc-pVTZ electronic model chemistry—was used to obtain the portion of the potential energy surface needed for single-structure variational transition-state Theory Rate constants including multidimensional tunneling; then, the M08-HX/MG3S electronic model chemistry was used to calculate multi-structural torsional anharmonicity factors to complete the MS-VTST Rate constant calculations. The lowest-energy structures of the trans...
William L. Hase - One of the best experts on this subject based on the ideXlab platform.
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anharmonic semiclassical variational transition state Theory Rate constant model for h atom association with different sites on the diamond 111 surface
Journal of Physical Chemistry A, 2001Co-Authors: Kihyung Song, William L. HaseAbstract: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...
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Linear Free Energy of Activation Relationship for Barrierless Association Reactions
Journal of the American Chemical Society, 1997Co-Authors: Pascal De Sainte Claire, Gilles H. Peslherbe, And Haobin Wang, William L. HaseAbstract:An analysis of variational transition state Theory Rate constants, for the association reactions Cl- + CH3Cl, Cl- + CH3Br, H + CH3, H + diamond{111}, CH3 + CH3, and Al + Al2 shows that the free energy of activation ΔG⧧ varies nearly linearly with temperature, over a broad temperature range, for each reaction. This suggests that association Rate constants can be parametrized by the free energy of activation at 300 K, Δ , and the change in ΔG⧧ with temperature, ΔΔG⧧/ΔT. The near linear dependence of ΔG⧧ with temperature is supported by a semiempirical model for association kinetics. The Rate constants for Cl- + CH3Cl, Cl- + CH3Br, and CH3 + CH3 decrease with increase in temperature, while those for H + CH3, H + diamond{111}, and Al + Al2 slightly increase. For the six association reactions considered here, the linear free energy relationship gives semiquantitative Rate constants over a rather broad temperature range of 200−2000 K. To calculate accuRate Rate constants over a broader temperature range, partic...
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Comparison of zero‐point energy constrained and quantum anharmonic Rice–Ramsperger–Kassel–Marcus and phase space Theory Rate constants for Al3 dissociation
The Journal of Chemical Physics, 1996Co-Authors: Gilles H. Peslherbe, William L. HaseAbstract:The ZPE constrained trajectory model is found to retain the ergodicity and intrinsic Rice–Ramsperger–Kassel–Marcus (RRKM) behavior observed previously [J. Chem. Phys. 101, 8535 (1994)] in unconstrained trajectories of Al3 decomposition. Microcanonical unimolecular Rate constants for Al3 decomposition are calculated from the ZPE constrained trajectories and compared with the predictions of the vibrator and flexible transition state models of RRKM Theory, phase space Theory, and the orbiting transition state model of phase space Theory (OTS/PST). Quantum anharmonic Al3 vibrational densities of state, determined by a semiclassical approach, are used to calculate these statistical Rate constants. Anharmonicity increases the density of states threefold for total energies 1–2 kcal/mol above the classical product asymptotic limit, but has a negligible effect on the Al2‐‐‐Al transition state sum of states. The ZPE constrained trajectory unimolecular Rate constants are in poor agreement with the quantum anharmonic OTS/PST and flexible RRKM Rate constants. This is because the ZPE constraint is too restrictive and some of the ZPE constrained trajectories are temporarily trapped in the ZPE forbidden region of phase space. The ZPE constrained trajectory Rate constants are smaller than their purely classical counterparts, since Al2 is not formed without its ZPE and thus the effective dissociation threshold is larger for the ZPE constrained trajectories. ZPE constrained sums and densities are calculated by including the ZPE constraint when solving the classical phase integral. RRKM Rate constants calculated from these ZPE constrained sums and densities are in much better agreement with the quantum anharmonic OTS/PST and flexible RRKM Rate constants, than are those calculated from the ZPE constrained trajectories. The difference between the ZPE constrained RRKM and quantum flexible RRKM Rate constants becomes small and much less than the anharmonic correction, for energies slightly in excess of the Al2+Al classical asymptotic limit. This is because the number of real frequencies in the instantaneous normal mode analysis decreases as the total energy is increased, which makes the ZPE constrained RRKM Rate constant more accuRate. Product energy partitioning from the ZPE constrained trajectories is in good agreement with the predictions of quantum phase space theories, except that the product diatom is formed too rotationally excited. The ZPE constraint scheme retains a spurious frequency and zero‐point energy for the Al2‐‐‐Al bending motion at large separations, which increases the Al2 product rotational energy. The work reported here supports the proposal that a ZPE constraint model, based on an instantaneous normal mode analysis, may be a valid approach for including zero‐point energy effects in trajectory simulations of ergodic anharmonic coupled systems. However, additional work needs to be done to remove some of the numerical problems with the current ZPE constraint model and to make the model less restrictive.
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ab initio potential and variational transition state Theory Rate constant for h atom association with the diamond 111 surface
Journal of Chemical Physics, 1994Co-Authors: Pascal De Sainte Claire, P. Barbarat, William L. HaseAbstract:High‐level ab initio calculations were performed to determine accuRate analytic potential energy functions for interactions a gas‐phase H‐atom has with H‐atoms attached to the diamond (111) surface and with C‐atom radical sites on this surface. The nonbonded potential between the gas‐phase H‐atom and H‐atoms attached to the surface was determined from coupled‐cluster ab initio calculations, including single, double, and perturbatively applied triple excitations [CCSD(T)], with the 6‐311++G(2df,p) basis set. The resulting nonbonded potential is nearly identical to that found previously from both Theory and experiment for interactions between H‐atoms on different hydrocarbon molecules. In the ab initio calculations, a C‐atom radical site on the diamond surface is represented by a constrained tert‐butyl radical. Radial and small‐displacement angular potentials for a H‐atom interacting with this radical were determined from unrestricted quadratic configuration interaction calculations, with single, double and...
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Comparison of canonical variational transition state Theory Rate constants for hydrogen atom association with alkyl radicals and with the (111) surface of diamond
The Journal of Physical Chemistry, 1993Co-Authors: P. Barbarat, Cedric Accary, William L. HaseAbstract:A model potential energy function developed previously for H + CH[sub 3] [yields] CH[sub 4] association is extended, with transfer of parameters, to H atom association with other alkyl radicals and with the diamond (111) surface. Reaction path following calculations are performed to determine canonical variational transition state Theory (CVTST) Rate constants for these association reactions. The CVTST Rate constants for H atom association with C[sub 2]H[sub 5], i-C[sub 3]H[sub 7], and t-C[sub 4]H[sub 9] agree with experimental and/or estimated Rate constants to within a factor of 2. This finding indicates it is not a severe approximation to assume transferability of potential energy parameters for different H atom and alkyl radical association reactions. Differences between the CVTST Rate constants for these associations are discussed in terms of moment of inertia ratios between the transition state and reactants and frequencies for the transitional bending modes. The CVTST Rate constant for H atom association with the diamond (111) surface is approximately 2 times smaller than that for H + t-C[sub 4]H[sub 9] association, which results from a factor of 2 difference in reaction path degeneracies for these two associations and agrees with a kinetic model proposed previously. CVTST may overestimate the Hmore » + diamond (111) surface association Rate constant, since it does not treat this energy-transfer process. 53 refs., 5 figs., 2 tabs.« less
Pascal De Sainte Claire - One of the best experts on this subject based on the ideXlab platform.
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The Pitzer Free Rotor Model for NondegeneRate Modes: Application to the Long-Range Behavior of Halogen Radical Reactions with Substituted Olefins
Journal of Physical Chemistry A, 2003Co-Authors: Mohamed Kharroubi, Pascal De Sainte ClaireAbstract:While the Pitzer model had been used in the past with great success for degeneRate internal motions, e.g., transitional bending motions in the recombination of hydrogen and methyl radicals, simpler models are often used in the treatment of nondegeneRate modes, such as the transitional bending motions in the recombination reaction of hydrogen and choloromethyl radicals. In this work, highly detailed analysis of the Pitzer free rotor treatment for nondegeneRate internal motions is presented. The addition reaction of a chlorine radical with 1,1,-dichloroethylene is used here for illustrative purposes. The reaction proceeds in two steps: the reactants reach a bridged intermediate that can dissociate back to reactants or yield the *C2H2Cl3 adducts. Only the long-range part of the reaction was investigated in this paper. Electronic structure calculations were performed at the UQCISD(T)/6-31G**//UMP2/6-31G** level of Theory, and canonical variational transition state Theory Rate constants were computed. In conclusion, it is shown that two different pathways lead to the bridged intermediate. Consideration of the forward and backward Rate constants demonstRated that the more direct route to the bridged intermediate is favored kinetically.
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Linear Free Energy of Activation Relationship for Barrierless Association Reactions
Journal of the American Chemical Society, 1997Co-Authors: Pascal De Sainte Claire, Gilles H. Peslherbe, And Haobin Wang, William L. HaseAbstract:An analysis of variational transition state Theory Rate constants, for the association reactions Cl- + CH3Cl, Cl- + CH3Br, H + CH3, H + diamond{111}, CH3 + CH3, and Al + Al2 shows that the free energy of activation ΔG⧧ varies nearly linearly with temperature, over a broad temperature range, for each reaction. This suggests that association Rate constants can be parametrized by the free energy of activation at 300 K, Δ , and the change in ΔG⧧ with temperature, ΔΔG⧧/ΔT. The near linear dependence of ΔG⧧ with temperature is supported by a semiempirical model for association kinetics. The Rate constants for Cl- + CH3Cl, Cl- + CH3Br, and CH3 + CH3 decrease with increase in temperature, while those for H + CH3, H + diamond{111}, and Al + Al2 slightly increase. For the six association reactions considered here, the linear free energy relationship gives semiquantitative Rate constants over a rather broad temperature range of 200−2000 K. To calculate accuRate Rate constants over a broader temperature range, partic...
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Linear Free Energy of Activation Relationship for Barrierless Association Reactions
Journal of the American Chemical Society, 1997Co-Authors: Pascal De Sainte Claire, Gilles H. Peslherbe, William Hase, H. WangAbstract:An analysis of variational transition state Theory Rate constants, for the association reactions Cl- + CH3Cl, Cl- + CH3Br, H + CH3, H + diamond{111}, CH3 + CH3, and Al + Al2 shows that the free energy of activation ΔG varies nearly linearly with temperature, over a broad temperature range, for each reaction. This suggests that association Rate constants can be parametrized by the free energy of activation at 300 K, Δ , and the change in ΔG with temperature, ΔΔG/ΔT. The near linear dependence of ΔG with temperature is supported by a semiempirical model for association kinetics. The Rate constants for Cl- + CH3Cl, Cl- + CH3Br, and CH3 + CH3 decrease with increase in temperature, while those for H + CH3, H + diamond{111}, and Al + Al2 slightly increase. For the six association reactions considered here, the linear free energy relationship gives semiquantitative Rate constants over a rather broad temperature range of 200−2000 K. To calculate accuRate Rate constants over a broader temperature range, particularly at low temperatures, nonlinear terms must be included in the free energy expansion.
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ab initio potential and variational transition state Theory Rate constant for h atom association with the diamond 111 surface
Journal of Chemical Physics, 1994Co-Authors: Pascal De Sainte Claire, P. Barbarat, William L. HaseAbstract:High‐level ab initio calculations were performed to determine accuRate analytic potential energy functions for interactions a gas‐phase H‐atom has with H‐atoms attached to the diamond (111) surface and with C‐atom radical sites on this surface. The nonbonded potential between the gas‐phase H‐atom and H‐atoms attached to the surface was determined from coupled‐cluster ab initio calculations, including single, double, and perturbatively applied triple excitations [CCSD(T)], with the 6‐311++G(2df,p) basis set. The resulting nonbonded potential is nearly identical to that found previously from both Theory and experiment for interactions between H‐atoms on different hydrocarbon molecules. In the ab initio calculations, a C‐atom radical site on the diamond surface is represented by a constrained tert‐butyl radical. Radial and small‐displacement angular potentials for a H‐atom interacting with this radical were determined from unrestricted quadratic configuration interaction calculations, with single, double and...
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Ab initio potential and variational transition state Theory Rate constant for H‐atom association with the diamond (111) surface
Journal of Chemical Physics, 1994Co-Authors: Pascal De Sainte Claire, P. Barbarat, William HaseAbstract:High‐level ab initio calculations were performed to determine accuRate analytic potential energy functions for interactions a gas‐phase H‐atom has with H‐atoms attached to the diamond (111) surface and with C‐atom radical sites on this surface. The nonbonded potential between the gas‐phase H‐atom and H‐atoms attached to the surface was determined from coupled‐cluster ab initio calculations, including single, double, and perturbatively applied triple excitations [CCSD(T)], with the 6‐311++G(2df,p) basis set. The resulting nonbonded potential is nearly identical to that found previously from both Theory and experiment for interactions between H‐atoms on different hydrocarbon molecules. In the ab initio calculations, a C‐atom radical site on the diamond surface is represented by a constrained tert‐butyl radical. Radial and small‐displacement angular potentials for a H‐atom interacting with this radical were determined from unrestricted quadratic configuration interaction calculations, with single, double and perturbatively applied triple excitations [UQCISD(T)], with the 6‐31G∗∗ basis set. UQCISD(T) calculations were performed on the H+CH3→CH4 reaction system with both the 6‐31G∗∗ and 6‐311++G(3df,3pd) basis sets to calibRate the accuracy of the 6‐31G∗∗ basis set results for the H‐atom plus constrained tert‐butyl radical. The above information was used to construct an analytic potential energy function for H‐atom association with a radical site on the (111) surface of diamond, which was then employed in a canonical variational transition state Theory (CVTST) calculation of the association Rate constant. The resulting Rate constant is 1.8-2.1×1013 cm3 mol−1 s−1 for the 1000-2000 K temperature range. It is insensitive to the gas‐phase H‐atom/surface H‐atom nonbonded potential and the potential for the diamond lattice. The H+diamond (111) CVTST Rate constant is used to estimate a Rate constant of 4×1013 cm3 mol−1 s−1 for H+tert‐butyl association at 298 K. The UQCISD(T)/6‐31G∗ calculations give a H--C(CH3)3 bond dissociation energy which is only 1 kcal/mol lower than the experimental value.
José M. Lluch - One of the best experts on this subject based on the ideXlab platform.
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Canonical Variational Transition-State Theory Study of the CF3CHFCH2F + OH Reaction
The journal of physical chemistry. A, 2010Co-Authors: Àngels González-lafont, José M. Lluch, Adrián Varela-Álvarez, José SordoAbstract:Variational transition-state Theory Rate constants with multidimensional tunneling contributions using the small curvature method have been calculated for the CF3CHFCH2F (HFC-245eb) + OH reaction over a temperature range from 200 to 800 K. The mPW1B95−41.0 hybrid functional, parametrized by Albu and Swaminathan to geneRate theoretical Rate constants nearly identical to the experimental values for the CH3F + OH reaction, has been used in conjunction with the 6-31+G(d,p) basis set to explore the potential energy surface of the title reaction. The functional provides results within the limits of chemical accuracy, supporting the conclusions about transferability of a previous study on the CF3CH2CH3 + OH reaction. Fourteen different reaction channels have been explored, all of them with significant contributions to the global Rate constants.
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Variational Transition-State Theory Rate Constant Calculations of the OH + CH3SH Reaction and Several Isotopic Variants
The Journal of Physical Chemistry A, 2003Co-Authors: Laura Masgrau, Àngels González-lafont, José M. LluchAbstract:In this paper the first variational transition-state Theory Rate constant calculation for the OH + CH 3 SH reaction and several isotopic variants involving OD, CH3SD, and CD 3 SH is presented. Multidimensional tunneling corrections have been included when necessary. The potential energy surface has been described by low-level calculations at the MP2(full)/cc-pVDZ level combined with higher level calculations using the multilevel MCCM-CCSD(T)-CO-2m method. We have found that the reaction takes place by forming first a weakly bound complex that presents two hydrogen bonds between the hydroxyl radical and methanethiol. From this complex two different reaction pathways have been found: abstraction of the hydrogen atom attached to the sulfur atom and abstraction of the hydrogen atom of the methyl group. To take into account the different kinetic channels, the canonical and the competitive canonical unified statistical theories have been used to calculate the global Rate constant of the perprotio reaction. Due to the slower nature of the H-abstraction of the methyl group, the global Rate constant turns out to be equivalent to the overall Rate constant for the abstraction of the hydrogen atom attached to the sulfur atom. It is only at the higher temperature range when a significant percentage of CH 2 SH is predicted. An activation energy of -0.54 kcal/mol in the range 225-430 K is obtained for the global reaction, in very good agreement with the experimental results. Several kinetic isotope effects of the global reactions have also been calculated and analyzed in view of previously published experimental results.
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Variational Transition-State Theory Rate Constant Calculations with Multidimensional Tunneling Corrections of the Reaction of Acetone with OH
The Journal of Physical Chemistry A, 2002Co-Authors: Laura Masgrau, Àngels González-lafont, José M. LluchAbstract:In this paper the first variational transition-state Rate constant calculation for the OH + CH3COCH3 → P reaction is presented. The potential energy surface has been described by low level calculations at the B3LYP/6-31G* level combined with higher level calculations using the multilevel CBS−RAD technique. Three different reaction pathways have been found: abstraction of an H atom eclipsed to the carbonyl group of acetone, abstraction of an H atom alternated to the carbonyl group of acetone, and addition of an OH molecule to the carbon atom of the carbonyl group of acetone. To take into account the three different kinetic channels, the competitive canonical unified statistical Theory has been used to calculate the global Rate constant. However, in practice the global Rate constant of the acetone + OH reaction turns out finally to be the sum of the eclipsed and alternated abstraction Rate constants, leading to a clearly curved Arrhenius plot. The addition−elimination mechanism has an almost negligible con...
Radu A. Miron - One of the best experts on this subject based on the ideXlab platform.
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Thermal desorption of large molecules from solid surfaces.
Physical Review Letters, 2002Co-Authors: Kristen A. Fichthorn, Radu A. MironAbstract:We use molecular-dynamics simulations and importance sampling to obtain transition-state-Theory Rate constants for thermal desorption of an n-alkane series from Au(111). We find that the binding of a large molecule to a solid surface involves different types of local minima. The preexponential factors increase with increasing chain length and can be substantially larger than typical estimates for small molecules. Our results match recent experimental studies and indicate that a proper treatment of conformational isomerism and entropy, heretofore not found in coarse-grained models, is essential to quantitatively describe the thermal desorption of large molecules from solid surfaces.