Direct Ab Initio Dynamics Methods for Calculating Thermal Rates of Polyatomic Reactions

Truong, Thanh N.; Duncan, Wendell T.; Bell, Robert L.

doi:10.1021/bk-1996-0629.ch006

Cited by 63 publications

(15 citation statements)

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“…As an alternative we explored the use of the BH&HLYP functional, which has been reported to yield transition-state energies comparable to those attained from second-order MollerPlesset perturbation theory. 63,64 Indeed this functional predicts activation energies 10-15 kJ/mol lower than those determined using the B3LYP functional. We also explored the consequences of keeping the distances between pairs of support atoms fixed.…”

Section: Oxidation Of Methoxy Speciesmentioning

confidence: 86%

A Density Functional Theory Study of the Oxidation of Methanol to Formaldehyde over Vanadia Supported on Silica, Titania, and Zirconia

2002

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Density functional theory was used to investigate the mechanism and kinetics of methanol oxidation to formaldehyde over vanadia supported on silica, titania, and zirconia. The catalytically active site was modeled as an isolated VO 4 unit attached to the support. The calculated geometry and vibrational frequencies of the active site are in good agreement with experimental measurements both for model compounds and oxidesupported vanadia. Methanol adsorption is found to occur preferentially with the rupture of a V-O-M bond (M = Si, Ti, Zr) and with preferential attachment of a methoxy group to V. The vibrational frequencies of the methoxy group are in good agreement with those observed experimentally as are the calculated isobars. The formation of formaldehyde is assumed to occur via the transfer of an H atom of a methoxy group to the O atom of the V=O group. The activation energy for this process is found to be in the range of 199-214 kJ/mol and apparent activation energies for the overall oxidation of methanol to formaldehyde are predicted to lie in the range of 112-123 kJ/mol, which is significantly higher than that found experimentally. Moreover, the predicted turnover frequency (TOF) for methanol oxidation is found to be essentially independent of support composition, whereas experiments show that the TOF is 10 3 greater for titania-and zirconia-supported vanadia than for silica-supported vanadia. Based on these findings, it is proposed that the formation of formaldehyde from methoxy groups may require pairs of adjacent VO 4 groups or V 2 O 7 dimer structures.3

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Section: Oxidation Of Methoxy Speciesmentioning

confidence: 86%

A Density Functional Theory Study of the Oxidation of Methanol to Formaldehyde over Vanadia Supported on Silica, Titania, and Zirconia

2002

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“…9 However, the BH&HLYP functional has been shown to give more accurate determination of transition-state energies comparable to those attained from second-order Møller-Plesset perturbation theory (MP2). 42 Since we are interested in transition states in this study, we rely primarily on the latter hybrid method.…”

Section: Theoretical Methodsmentioning

confidence: 99%

Density Functional Theory Study of Proton Mobility in Zeolites: Proton Migration and Hydrogen Exchange in ZSM-5

2000

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Acidic protons in zeolites are known to be mobile at elevated temperatures. In this study, density functional theory was used to identify the reaction pathways for proton migration in a model that represents the zeolite ZSM-5. In the absence of water, the acidic proton "hops" or migrates between two of the four O atoms surrounding an aluminum center with an activation barrier of 28 kcal/mol. During proton transfer, the O atoms stretch closer together in order to stabilize the transition state. This is revealed by a 13.4°decrease in the O-Al-O bond angle. Adsorbed water bridges the proton donor and acceptor sites, reducing the barrier height by 24 kcal/mol. Hence proton migration depends heavily on the local geometry and conditions of the zeolite. We show that experimentally undetectable amounts of water can greatly influence the measured rates and apparent activation barriers. We broaden the scope of our study to consider hydrogen exchange with other gas-phase species of the form RO-H (RO ) CH 3 O, CH 3 CH 2 O) and R-H (R ) H, CH 3 , C 2 H 5 , C 3 H 7 , C 6 H 5 ). It is evident that to a first approximation the activation barrier increases with an increase in the polarizability of the species RO-H. For the chemical series R-H, the activation energy increases with the deprotonation energy of the interacting species R-H. We also calculate the overall reaction rate constants for proton hopping and hydrogen exchange.

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“…Throughout this paper we will refer to these structures using the sequence of alpha-beta substituents. Out of 20 structures used, 17 as closed-shell singlet monocations using B3LYP [50][51][52][53] hybrid functional with VTZ basis set for cobalt [54] and 6-31G(d) basis set [55,56] for the remaining atoms [10][11][12]14], and six using B3LYP functional and lacv3p** basis set [1][2][3][4][5][6][7][8]15,[57][58][59]. All of them were treated with RHF formalism.…”

Section: Computational Detailsmentioning

confidence: 99%

Validation of semiempirical methods for modeling of corrinoid systems

Kwiecień¹,

Dybała-Defratyka²,

Paneth³

2004

Journal of Inorganic Biochemistry

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Direct Ab Initio Dynamics Methods for Calculating Thermal Rates of Polyatomic Reactions

Cited by 63 publications

References 0 publications

A Density Functional Theory Study of the Oxidation of Methanol to Formaldehyde over Vanadia Supported on Silica, Titania, and Zirconia

A Density Functional Theory Study of the Oxidation of Methanol to Formaldehyde over Vanadia Supported on Silica, Titania, and Zirconia

Density Functional Theory Study of Proton Mobility in Zeolites: Proton Migration and Hydrogen Exchange in ZSM-5

Validation of semiempirical methods for modeling of corrinoid systems

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