Intermolecular hydrogen tunneling in solids. Comparison between diabatic and adiabatic rate expressions

1988 ◽  
Vol 66 (4) ◽  
pp. 875-880 ◽  
Author(s):  
Philip D. Pacey ◽  
Willem Siebrand
Keyword(s):  
Rate Constants ◽  
Potential Energy Surfaces ◽  
Degrees Of Freedom ◽  
Hydrogen Transfer ◽  
Hydrogen Abstraction ◽  
Golden Rule ◽  
Slow Motion ◽  
Similar Rate ◽  
State Theory ◽  
Rate Expression

Rate expressions are derived for hydrogen transfer between two molecules in a solid, typical examples being hydrogen abstraction by methyl radicals in solid methane and in glassy methanol. These expressions are based on two-dimensional potential-energy surfaces describing the motion of the hydrogen atom along with that of the atoms between which it is transferred. A diabatic rate expression, based on the Golden Rule, is compared with an adiabatic rate expression, based on transition-state theory with a tunneling correction. In both cases, the two degrees of freedom must be treated quantum-mechanically rather than classically. For the adiabatic case, this leads to a new expression in which the barrier permeability is averaged over the wavefunction of the slow motion. The result differs from the Golden-Rule expression but yields similar rate constants. Numerical estimates are presented to illustrate the temperature and isotope dependence of these rate constants. The concept of a tunneling path is shown to break down at low temperature, so that the conventional one-dimensional tunneling approach becomes invalid.

scholarly journals Chemical insights into the atmospheric oxidation of thiophene by hydroperoxyl radical

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Maryam Seyed Sharifi ◽  
Hamed Douroudgari ◽  
Morteza Vahedpour
Keyword(s):  
Rate Constants ◽  
Potential Energy Surfaces ◽  
Hydrogen Abstraction ◽  
State Theory ◽  
Stationary Points ◽  
Positive Temperature ◽  
Oxidation Reactions ◽  
Hydroperoxyl Radical ◽  
Title Reaction ◽  
High Level

AbstractThe reaction mechanisms and kinetics of thiophene oxidation reactions initiated by hydroperoxyl radical, and decomposition of the related intermediates and complexes, have been considered herein by using high-level DFT and ab initio calculations. The main energetic parameters of all stationary points of the suggested potential energy surfaces have been computed at the BD(T) and CCSD(T) methods, based on the geometries optimized at the B3LYP/6-311 + g(d,p) level of theory. Rate constants of bimolecular reactions (high-pressure limit rate constants) at temperatures from 300 to 3000 K for the first steps of the title reaction have been obtained through the conventional transition state theory (TST), while the pressure dependent rate constants and the rate constants of the second and other steps have been calculated employing the Rice–Ramsperger–Kassel–Marcus/Master equation (RRKM/ME). The results show that the rate constants of addition to α and β carbons have positive temperature dependence and negative pressure dependence. It is found that the additions of HO2 to the α and β carbons of thiophene in the initial steps of the title reaction are the most favored pathways. Also, the addition to the sulfur atom has a minor contribution. But, all efforts for simulating hydrogen abstraction reactions have been unsuccessful. In this complex oxidation reaction, about 12 different products are obtained, including important isomers such as thiophene-epoxide, thiophene-ol, thiophene-oxide, oxathiane, and thiophenone. The calculated total rate constants for generation of all minimum stationary points show that the addition reactions to the α and β carbons are the fastest among all at temperatures below 1000 K, while the proposed multi-step parallel reactions are more competitive at temperatures above 1200 K. Furthermore, important inter-and intra-molecular interactions for some species have been investigated by two well-known quantum chemistry method, the NBO and AIM analyses. Thermochemical properties such as free energy, enthalpy, internal energy, and entropy for thiophene and hydroperoxyl radical and related species in the simulated reactions have been predicted using a combination of the B3LYP and BD(T) methods.

MECHANISM AND KINETICS OF THE CH3CH2C(O)OCH2CH3 + OH REACTION: A THEORETICAL STUDY

2011 ◽  
Vol 10 (05) ◽  
pp. 691-709 ◽  
Author(s):  
CONG HOU ◽  
CHENG-GANG CI ◽  
TONG-YIN JIN ◽  
YONG-XIA WANG ◽  
JING-YAO LIUM
Keyword(s):  
Rate Constants ◽  
Hydrogen Abstraction ◽  
Negative Temperature ◽  
State Theory ◽  
Measured Temperature ◽  
Temperature Range ◽  
Global Rate ◽  
Abstraction Reaction ◽  
Lower Temperature ◽  
Kinetic Calculations

The hydrogen abstraction reaction of CH 3 CH 2 C(O)OCH 2 CH 3 + OH has been studied theoretically by dual-level direct dynamics method. Six H-abstraction channels were found for this reaction. The required potential energy surface information for the kinetic calculations was obtained at the MCG3-MPWB//M06-2X/aug-cc-pVDZ level. The rate constants were calculated by the improved canonical variational transition-state theory with small-curvature tunneling correction (ICVT/SCT) approach in the temperature range of 200–2000 K. It is shown that the "methylene H-abstraction" from the alkoxy end of the ester CH 3 CH 2 C(O)OCH 2 CH 3 is the dominant channel at lower temperature (< 400 K), while the other channels from the acetyl end should be taken into account as the temperature increases and become the competitive ones at higher temperature. The calculated global rate constants are in good agreement with the experimental ones in the measured temperature range and exhibit a negative temperature dependence below 500 K. A four-parameter rate constant expression was fitted from the calculated kinetic data between 200–2000 K.

Theoretical study on the chemical formation mechanism of a β-myrcene ozonolysis reaction under atmospheric conditions

2012 ◽  
Vol 90 (8) ◽  
pp. 708-715 ◽  
Author(s):  
Yuyang Zhao ◽  
Jing Bai ◽  
Chenxi Zhang ◽  
Chen Gong ◽  
Xiaomin Sun
Keyword(s):  
Rate Constants ◽  
Potential Energy Surfaces ◽  
Density Functional ◽  
Single Point ◽  
State Theory ◽  
Tunneling Effect ◽  
Atmospheric Conditions ◽  
Functional Theory ◽  
Real Atmosphere ◽  
Energy Surfaces

Density functional theory (DFT) was used to study the β-myrcene ozonolysis reaction. The reactants, intermediates, transition states, and products were optimized at the MPWB1K/6–31G(d,p) level. The single-point energies were performed at the MPWB1K/6–311+G(3df,2p) level. The profiles of the potential energy surfaces were constructed and the rate constants of the reaction steps were analyzed. The possible reaction mechanisms for the ozonolysis intermediates in real atmosphere are also discussed. Based on quantum chemistry information, the rate constants were calculated using Rice–Ramsperger–Kassel–Marcus (RRKM) theory and the canonical variational transition-state theory (CVT) with small curvature tunneling effect (SCT). Arrhenius equations of rate constants over the temperature range of 200–800 K are provided, and the lifetimes of the reaction species in the troposphere were estimated according to rate constants.

Étude théorique des réactions d'abstraction d'hydrogène , avec R, X≡H, CH3, NH2,OH et F

1985 ◽  
Vol 63 (7) ◽  
pp. 1447-1456 ◽  
Author(s):  
Georges Leroy ◽  
Michel Sana ◽  
Anne Tinant
Keyword(s):  
Transition State ◽  
Rate Constants ◽  
Transition State Theory ◽  
Hydrogen Abstraction ◽  
State Theory ◽  
Arrhenius Behavior ◽  
Activation Barriers ◽  
Empirical Procedure ◽  
Semi Empirical ◽  
Theory And Experiment

Hydrogen abstraction reactions [Formula: see text] with R, X≡H, CH3, NH2,OH, and F have been studied at the abinitio 6-31G – UHF level. However, energetic properties were computed at the CI level. Rate constants and Arrhenius parameters have been obtained using the transition state theory formalism with Eckart's tunneling correction. The discrepancy between theoretical and experimental results led us to elaborate a semi-empirical procedure to calculate activation barriers, in which the bonds R—H and X—H are represented by Morse curves. Thus, the agreement between theory and experiment is much better. Moreover, the results obtained by this procedure demonstrate the non-Arrhenius behavior of all the reactions under consideration and allow us to rationalize a large number of experimental facts.

scholarly journals Theoretical Kinetics Study Of The HO2 + C2H5OH Hydrogen Abstraction Reaction

2020 ◽  
Vol 36 (1) ◽  
Author(s):  
Nguyen Trong Nghia
Keyword(s):  
Transition State ◽  
Rate Constants ◽  
Transition State Theory ◽  
Hydrogen Abstraction ◽  
Branching Ratios ◽  
State Theory ◽  
Kinetics Study ◽  
Abstraction Reaction ◽  
Hydrogen Abstraction Reaction ◽  
Ho2 Radical

C2H5OH has been using as an alternative fuel for decades; HO2 also plays a pivotal role in the combustion. The kinetics and mechanism for the reaction between C2H5OH and HO2 radical has been investigated using the molecular parameters for the reactants, transition states and products predicted at the CCSD(T)//B3LYP/6-311++G(3df,2p) level of theory. There are ten pair products have been found including C2H5O + H2O2 (PR1), CH3CHOH + H2O2 (PR2), CH2CH2OH + H2O2 (PR3), CH3CH2OOOH + H (PR4), C2H5 + HOOOH (PR5), CH3CH2OOH + OH (PR6), CH3CH(OH)OOH + H (PR7), HOCH2CH2OH + H (PR8), HOOCH3 + CH2OH (PR9), and CH3 + HOOCH2OH (PR10) in which the second and third ones are the major channels. The rate constants and branching ratios for all H-abstraction reactions have been calculated using the conventional transition state theory with asymmetric Eckart tunneling corrections for the temperature ranging from 298 to 2000 K.

scholarly journals Reaction Mechanisms and Kinetics of the Hydrogen Abstraction Reactions of C4–C6 Alkenes with Hydroxyl Radical: A Theoretical Exploration

2019 ◽  
Vol 20 (6) ◽  
pp. 1275 ◽  
Author(s):  
Quan-De Wang ◽  
Mao-Mao Sun ◽  
Jin-Hu Liang
Keyword(s):  
Rate Constants ◽  
Reaction Mechanisms ◽  
Hydrogen Abstraction ◽  
Geometry Optimization ◽  
State Theory ◽  
Basis Set ◽  
Oh Radical ◽  
Combustion Chemistry ◽  
Reaction Barriers ◽  
High Level

The reaction of alkenes with hydroxyl (OH) radical is of great importance to atmospheric and combustion chemistry. This work used a combined ab initio/transition state theory (TST) method to study the reaction mechanisms and kinetics for hydrogen abstraction reactions by OH radical on C4–C6 alkenes. The elementary abstraction reactions involved were divided into 10 reaction classes depending upon the type of carbon atoms in the reaction center. Geometry optimization was performed by using DFT M06-2X functional with the 6-311+G(d,p) basis set. The energies were computed at the high-level CCSD(T)/CBS level of theory. Linear correlation for the computed reaction barriers and enthalpies between M06-2X/6-311+G(d,p) and CCSD(T)/CBS methods were found. It was shown that the C=C double bond in long alkenes not only affected the related allylic reaction site, but also exhibited a large influence on the reaction sites nearby the allylic site due to steric effects. TST in conjunction with tunneling effects were employed to determine high-pressure limit rate constants of these abstraction reactions and the computed overall rate constants were compared with the available literature data.

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