Direct Ab Initio Dynamics Research Articles

Theoretical studies on dynamics and thermochemistry of the reactions CHClFCHO, CHF 2CHO and CClF 2CHO with the Cl atom

The theoretical investigations are performed on the reaction mechanisms, including hydrogen abstraction and addition reaction channels for the three reactions CHClFCHO + Cl → products (R1), CHF 2CHO + Cl → products (R2), and CClF 2CHO + Cl → products (R3) by ab initio direct dynamics approach. The electronic structure information for the stationary points is obtained at the MP2 level of theory using the cc-pVDZ and aug-cc-pVDZ basis sets. The classical energy profile is refined by multi-coefficient correlation method based on quadratic configuration interaction with single and double excitation (MC-QCISD) using the MP2 optimized geometries. The enthalpies of formation for the reactants and product radicals involved in the three reactions are estimated at the MC-QCISD//MP2 level via isodesmic reactions. We find that each addition reaction pathway has a much higher potential energy barrier, and therefore its contribution to the total rate constants can be neglected. The rate constants for the H-abstraction reactions, which are evaluated by canonical variational transition state theory with the small-curvature tunneling correction over a range of temperatures from 220 to 2000 K, are in good agreement with the available experimental values. The Arrhenius expressions are fitted to be (in cm 3 molecule −1 s −1) k 1 = 5.08 × 10 −160 T 1.60 exp (244.6/ T), k 2 = 4.80 × 10 −17 T 1.86 exp (274.9/ T), and k 3 = 2.34 × 10 −16 T 1.67 exp (37.1/ T), respectively. Our conclusions show that for reaction CHClFCHO + Cl → products (R1), the channel of hydrogen abstraction from the formyl (–CHO) position is the primary pathway at low temperature, but as the temperature increases the hydrogen abstraction from the –CHClF group is more probable. While for reaction CHF 2CHO + Cl → products (R2), the pathway of hydrogen abstraction from the formyl position is always the primary channel over the whole temperature range. We also find that the halogen substitute (F or Cl-substitution) reduces the reactivity of the corresponding species.

Direct Ab Initio Dynamics Study for the Hydrogen Abstraction Reaction: CH $$_{2} ({^{3}}B_{1})$$ + H2CO $$\rightarrow $$ CH3 + CHO

We present a direct ab initio dynamics study of thermal rate constants of the hydrogen abstraction reaction of CH \(_{2}(^{3}B_{1})\) + H2CO \(\rightarrow \) CH3 + CHO. The MP2/cc-pVDZ method is employed to optimize the geometries of stationary points as well as the points on the minimum energy path. The energies of all the points were further refined at the CCSD(T)/cc-pVTZ level of theory based on the Moller– Plesset perturbation theory (MP2) optimized geometries. The rate constants were evaluated using the conventional transition state theory, the canonical variational TST, and the improved canonical variational TST, also both including small-curvature tunneling correction in the temperature range of 300–2,500 K. The calculated results show that the rate constants have positive temperature dependence in the calculated temperature range. The calculated results show that the tunneling effect is important at low temperature region.

Quantum Molecular Dynamics Study of the Reaction of O2 with HOCO

The reaction of O2 with HOCO has been studied by using an ab initio direct dynamics method based on the UB3PW91 density functional theory. Results show that the reaction can occur via two mechanisms: direct hydrogen abstraction and an addition reaction through a short-lived HOC(O)O2 intermediate. The lifetime of the intermediate is predicted to be 660 +/- 30 fs. Although it is an activated reaction, the activation energy is only 0.71 kcal/mol. At room temperature, the obtained thermal rate coefficient is 2.1 x 10(-12) cm3 molecule(-1) s(-1), which is in good agreement with the experimental results.

Direct Dynamics Study on the Hydrogen Abstraction Reaction CH2O + HO2 → CHO + H2O2

We present a direct ab initio dynamics study on the hydrogen abstraction reaction CH2O + HO2 --> CHO + H2O2, which is predicted to have four possible reaction channels caused by different attacking orientations of HO2 radical to CH2O. The structures and frequencies at the stationary points and the points along the minimum energy paths (MEPs) of the four reaction channels are calculated at the B3LYP/cc-pVTZ level of theory. Energetic information of stationary points and the points along the MEPs is further refined by means of some single-point multilevel energy calculations (HL). The rate constants of these channels are calculated using the improved canonical variational transition-state theory with the small-curvature tunneling correction (ICVT/SCT) method. The calculated results show that, in the whole temperature range, the more favorable reaction channels are Channels 1 and 3. The total ICVT/SCT rate constants of the four channels at the HL//B3LYP/cc-pVTZ level of theory are in good agreement with the available experiment data over the measured temperature ranges, and the corresponding three-parameter expression is k(ICVT/SCT) = 3.13 x 10(-20) T(2.70) exp(-11.52/RT) cm3 mole(-1) s(-1) in the temperature range of 250-3000 K. Additionally, the flexibility of the dihedral angle of H2O2 is also discussed to explain the different experimental values.

Dynamics of the Concerted Triple Proton Transfer in Cyclic Water Trimer Using the Multiconfiguration Molecular Mechanics Algorithm

Cyclic water clusters are important molecular species to understand the nature of hydrogen bonded networks. Theoretical studies for the dynamics of triple proton transfer in the cyclic water trimer were performed. The potential energy surface (PES) of triple proton transfer is generated by the multiconfiguration molecular mechanics (MCMM) algorithm. We have used the MP2/6-31G(d,p) level for high-level ab initio data (energies, gradients, and Hessians), which are used in the Shepard interpolation. Eight high-level reference points were added step by step, including two points for the critical configurations of the large curvature tunneling paths. The more high-level points are used, the better the potential energy surfaces become. The rate constant and kinetic isotope effect (KIE) for the triple proton transfer at 300 K, which have been calculated by the canonical variational transition-state theory with microcanonical optimized multidimensional semiclassical tunneling approximation, are 1.6 x 10(-3) s(-1) and 230, respectively. Tunneling is very important not only for the triple proton transfer but also for the triple deuterium transfer. The MCMM results show good agreement with those from the direct ab initio dynamics calculations.

Direct dynamics studies on the isomerization and H 2 elimination reactions of H 3PS

Direct ab initio dynamics methods are using to study the H 3 PS → 1 trans - H 2 PSH → 2 cis - H 2 PSH , and H 3 PS → 3 HPS + H 2 → 4 PSH + H 2 reactions. The minimum energy paths (MEPs) of the two reaction channels, geometries, and harmonic vibrational frequencies for all stationary points and selected points along the MEPs are calculated using the QCISD/6-311++G(d,p) method. The single-point calculations are carried out by QCISD(T)/6-311++G(2df,2pd). The fitted three-parameter expressions of the calculated CVT/SCT and TST/Eckart rate coefficients in 200–2000 K for reactions (1) and (3) are k 1 CVT / SCT = 2.69 × 10 - 42 T 16.52 exp ( - 9811.9 / T ) s - 1 , k 1 TST / Eckart = 1.27 × 10 - 31 T 13.39 exp ( - 12180.6 / T ) s - 1 , k 3 CVT / SCT = 8.93 × 10 - 20 T 9.82 exp ( - 20612.1 / T ) s - 1 , and k 3 TST / Eckart = 1.31 × 10 - 26 T 11.86 exp ( - 18896.4 / T ) s - 1 , respectively.

Ab Initio and Variational Transition State Approach to β-C3N4Formation: Kinetics for the Reaction of CH3NH2with H

The CH3NH2 molecule has been considered as either an important intermediate in methane and ammonia mixtures or a precursor in methylamine and hydrogen mixtures in the synthesis of carbon nitride thin films. The fast Hydrogen (H) abstraction from CH3NH2 is an important process involved in the formation of HCN or CNH in the chemical vapor deposition (CVD) of carbon nitride thin films. The energetic and kinetic characteristics of the H abstraction reaction from CH3NH2 by atomic H used in CVD of beta-C3N4 were studied using ab initio direct dynamics methods for the first time. Two primary processes were identified for this reaction: H abstraction from the CH3 group and H abstraction from the NH2 group. On the basis of ab initio data, the rate constants of each channel have been deduced by canonical variational transition state theory with small-curvature tunneling correction over a wide temperature range of 200 to approximately 3000 K. The theoretical results were compared with available experimental data.

Ab initioEhrenfest dynamics

We present an ab initio direct Ehrenfest dynamics scheme using a three time-step integrator. The three different time steps are implemented with nuclear velocity Verlet, nuclear-position-coupled midpoint Fock integrator, and time-dependent Hartree-Fock with a modified midpoint and unitary transformation algorithm. The computational cost of the ab initio direct Ehrenfest dynamics presented here is found to be only a factor of 2-4 larger than that of Born-Oppenheimer (BO) dynamics. As an example, we compute the vibration of the NaCl molecule and the intramolecular torsional motion of H2C=NH2+ by Ehrenfest dynamics compared with BO dynamics. For the vibration of NaCl with an initial kinetic energy of 1.16 eV, Ehrenfest dynamics converges to BO dynamics with the same vibrational frequency. The intramolecular rotation of H2C=NH2+ produces significant electronic excitation in the Ehrenfest trajectory. The amount of nonadiabaticity, suggested by the amplitude of the coherent progression of the excited and ground electronic states, is observed to be directly related to the strength of the electron-nuclear coupling. Such nonadiabaticity is seen to have a significant effect on the dynamics compared with the adiabatic approximation.

The reaction of the aminoboranylidene-iminoborane isomerization: a CASSCF direct dynamics study

The barrier and the potential-energy surface of the isomerization from aminoboranylidene (BNH2) to iminoborane (HBNH) have been studied using complete active space self-consistent field (CASSCF) with the 6-31 + G(d, p) basis set and higher-level energy methods. The rate constants of the isomerization reaction are reported by employing the direct ab initio dynamics method. The geometries of all the stationary points were optimized using the B3LYP and CCSD methods with the cc-pVTZ and cc-pVQZ basis sets. The information along the intrinsic reaction coordinate (IRC) was also calculated at the CASSCF/6-31 + G (d,p) level of theory. The energies were refined at the G3, G3MP2, G3MP2B3, CBS-Q, CBS-QB3, and two high-level (HL) methods based on the geometries optimized using CASSCF/6-31 + G(d,p). The rate constants were evaluated using conventional transition-state theory (TST), canonical variational transition-state theory (CVT), and canonical variational transition-state theory with small curvature tunneling correction (CVT/SCT) and conventional transition-state theory with Eckart tunneling correction (TST/Eckart). According to the calculated results, we conclude that the tunneling effect is very important to this isomerization reaction.

Ab initio direct dynamics studies on the reactions of chlorine atom with CH 3 − nCl nCHO ( n = 1–3)

The H-abstraction reactions CH 3 − n Cl n CHO + Cl ( n = 1–3) ((R1)–(R3)) have been studied by the ab initio direct dynamics method. The potential energy surface (PES) information required for the rate constant calculation for each reaction is obtained at the MP2/cc-pVDZ and CCSD(T)/6-311+G(d, p) (single-point) levels of theory. The hydrogen atom can be abstracted from both the formyl (–CHO) positions and the chlorinated methyl (–CH 3 − n Cl n ) positions for CH 2ClCHO, CHCl 2CHO, i.e., there are two probable channels for reactions (R1) and (R2). The enthalpies of formation for the species CH 2ClCHO, CHCl 2CHO, CCl 3CHO, and the radicals CH 2ClCO, CHClCHO, CHCl 2CO, CCl 2CHO, CCl 3CO are computed at the CCSD(T)/6-311+G(d, p)//MP2/cc-pVDZ level. The rate constants for each reaction channel are evaluated by using improved canonical variational transition state theory (ICVT) with a small-curvature tunneling correction (SCT) over a wide range of temperatures from 220 to 2000 K. The calculated ICVT/SCT rate constants are found to be in good agreement with the available experimental values. It is shown that in the low temperature range, the reactions proceed predominantly via H-abstraction from formyl position, while the other H-abstraction channel from methyl should be taken into account with the increase of the temperature. Also, the reactivity decreases substantially with chloride substitution at the methyl position of acetaldehyde.

Direct ab Initio Dynamics Calculation of the Reaction Rates of CH3OCl with OH

A direct ab initio dynamics method was carried out for the reaction CH3OCl + OH --> products. Three abstraction channels from chlorine atom, in-plane hydrogen, and out-of-plane hydrogen atoms at the CH3 group have been found. The optimized geometries and frequencies of the stationary points and the minimum-energy paths (MEPs) were calculated at the MP2/6-311G(d,p) level. To improve the reaction enthalpy and potential barrier, single-point calculations were made at three higher levels of theory, the approximate QCISD(T)/6-311++G(3df,2pd), G3, and G3(MP2) levels. Furthermore, the rate constants for three abstraction channels were evaluated using canonical variational transition state theory (CVT) with the small-curvature tunneling correction (SCT) over a wide temperature range of 220-2000 K at above three higher theory levels, respectively. The calculated rate constants as well as branching rates are in reasonable agreement with the experimental values in the temperature region 250-341 K. The present results indicate H-abstraction especially from out-of-plane hydrogen is the main reaction pathway, while Cl-abstraction is much less competitive.

A DFT and ab initio direct dynamics study on the hydrogen abstract reaction of H 3BNH 3 → H 2 + H 2BNH 2

A direct ab initio dynamics study is presented on the hydrogen abstraction reaction of H 3BNH 3 → H 2 + H2BNH 2. The geometries of all the stationary points are optimized at the B3LYP and MP2 levels of theory with a series of basis sets up to aug-cc-pVTZ. The energies are refined using the G3, G3MP2, G3MP2B3, CBS-Q, CBS-Q//B3, and a combined high-level (HL) method based on the geometries optimized using the B3LYP/aug-cc-pVTZ level of theory. The rate constants are evaluated using the conventional transition-state theory and canonical variational transition-state theory (CVT). The fitted Arrhenius expression calculated from the CVT/SCT method is k ( T ) = 6.86 × 10 6 × T 1.69 × e ( - 1.37 × 10 4 / T ) s - 1 . The estimated apparent activation energy is in accordance with experimental results.

DFT and ab initio direct dynamics study on the reaction of H 2 loss reaction from H 2BNH 2

DFT and ab initio molecular orbital calculations are carried out for the H 2 loss reaction from aminoborane (H 2BNH 2). The geometries of all the stationary points are optimized at the B3LYP and MP2 methods with a series of basis sets up to aug-cc-pVTZ. The harmonic vibrational frequencies are calculated at the same level of theory. One transition state is found at the B3LYP/(aug-)cc-pVTZ and MP2/(aug-)cc-pVTZ level of theory. The energies and enthalpies are refined at the G3, G3MP2, G3MP2B3, CBS-Q, CBS-QB3, and HL methods based on the geometries optimized using B3LYP/aug-cc-pVTZ method. The rate constants are evaluated using the conventional transition-state theory (TST), canonical variational transition-state theory (CVT), and canonical variational transition-state theory with small curvature tunneling correction (CVT/SCT) and conventional transition-state theory with Eckart tunneling correction (TST/Eckart).

Direct dynamics study on the reaction of O( 3P)+PH 3→PH 2+OH

A direct ab initio dynamics study of thermal rate constants is presented for the hydrogen abstraction reaction of O( 3P)+PH 3→PH 2+OH. The UQCISD/6-311++G(d,p) method was employed to optimize geometry structures and to calculate the minimum energy path (MEP). Frequencies for all stationary points were obtained at the UQCISD/6-311++G(d,p) and UQCISD(T)/6-311++G(2df,2pd) levels of theory, respectively. The energies of the stationary points and 22 points selected along MEP were refined at the UQCISD(T)/6-311++G(2df,2pd)//UQCISD/6-311++G(d,p) level of theory. Reaction rate constants were evaluated by using the conventional transition state theory (TST), the canonical variational transition state theory (CVT) and the CVT with the small curvature tunneling correction method (CVT/SCT) over the temperature range of 200–2000 K, respectively. The results show that the classical potential barrier Δ E ≠ and the reaction enthalpy Δ H 298 0 calculated are 3.35 and −19.44 kcal/mol, respectively, for title reaction. The variational effect is significant in the whole temperature range, while the SCT tunneling correction is important within the temperature range of T≤500 K for the calculation of rate constant.

Direct ab initio dynamics studies of the hydrogen abstraction reactions of hydrogen atom with n-propyl radical and isopropyl radical

The kinetics of the hydrogen abstraction reactions of hydrogen atom with n-propyl radical and isopropyl radical were studied using the direct ab initio dynamics approach. BHandHLYP/cc-pVDZ method was employed to optimize the geometries of stationary points as well as the points on the minimum energy path (MEP). The energies of all the points for the two reactions were further refined at the QCISD(T)/cc-pVTZ level of theory. No barrier was found at the QCISD(T)/cc-pVTZ//BHandHLYP/cc-pVDZ level of theory for both reactions. The forward and reverse rate constants were evaluated with both canonical variational transition state theory (CVT) and microcanonical variational transition state theory (mu VT) in the temperature range of 300-2,500 K. The fitted three-parameter Arrhenius expression of the calculated CVT rate constants at the QCISD(T)/cc-pVTZ//BHandHLYP/cc-pVDZ level of theory are k(CVT) (n-C3H7) = 1.68 x 10(-14) T(0.84) e((319.5/T)) cm3 molecule(-1) s(-1) and k(CVT) (iso-C3H7)=4.99 x 10(-14) T(0.90) e((159.5/T)) cm3 molecule(-1) s(-1) for reactions of n-C3H7 + H and iso-C3H7 + H, respectively, which are in good agreement with available literature data. The variational effects were analysed.

Direct ab initio dynamics study on the rate constants and kinetics isotope effects of CH3O+H→CH2O+H2 reaction

We present a direct ab initio dynamics study of thermal rate constants of the hydrogen abstraction reaction of CH(3)O+H-->CH(2)O+H(2). The unrestricted Becke's half-and-half hybrid functional using the Lee-Yang-Parr correlation functional with Dunning's correlation consistent polarized valence double-zeta basis set, the unrestricted quadratic configuration interaction calculation including single and double substitutions with Dunning's correlation consistent polarized valence double-zeta basis set, and the unrestricted quadratic configuration interaction calculation including single and double substitutions with a triples contribution with Dunning's correlation consistent polarized valence triple-zeta basis set methods were employed to optimize the structures and to calculate frequencies for all stationary points. Minimum energy paths were obtained by the unrestricted Becke's half-and-half hybrid functional using the Lee-Yang-Parr correlation functional and the unrestricted quadratic configuration interaction calculation including single and double substitutions with the same Dunning's correlation consistent polarized valence double-zeta basis set levels of theory. No barrier is found at the unrestricted Becke's half-and-half hybrid functional using the Lee-Yang-Parr correlation functional with Dunning's correlation consistent polarized valence double-zeta basis set level of theory in contrast to a small barrier of 1.43 kcal mol(-1) at the unrestricted quadratic configuration interaction calculation including single and double substitutions with Dunning's correlation consistent polarized valence double-zeta basis set level of theory. In particular, the barrier vanishes as the energies along the minimum energy path MEP are refined at the unrestricted quadratic configuration interaction calculation including single and double substitutions with a triples contribution with Dunning's correlation consistent polarized valence triple-zeta basis set level of theory. Smaller barriers of 0.47 and 0.17 kcal mol(-1) were obtained at the unrestricted quadratic configuration interaction calculation including single and double substitutions with a triples contribution with Dunning's correlation consistent polarized valence triple-zeta basis set and the unrestricted quadratic configuration interaction calculation including single and double substitutions with a triples contribution with Dunning's correlation consistent polarized valence triple-zeta basis set based on the geometries at the unrestricted quadratic configuration interaction calculation including single and double substitutions with Dunning's correlation consistent polarized valence triple-zeta basis set levels of theory, respectively. The forward rate constants are evaluated with the canonical variational transition state theory in the temperature range of 300-2500 K. The calculated forward rate constants at the unrestricted quadratic configuration interaction calculation including single and double substitutions with a triples contribution with Dunning's correlation consistent polarized valence triple-zeta basis set based on the geometries at the unrestricted quadratic configuration interaction calculation including single and double substitutions with Dunning's correlation consistent polarized valence double-zeta basis set level of theory are in good agreement with the available experimental data. The kinetic isotope effects are estimated.

Ab initio calculations and mechanism of two proton migration reactions of ethoxy radical

Abstract We present a direct ab initio and density functional theory dynamics study of the thermal rate constants of the two H-migration reactions of C 2 H 5 O radical. MPW1K/6-31+G(d,p) methods were employed to optimize the geometries of all stationary points and to calculate the minimum energy path (MEP). The energies of all the stationary points were refined at the QCISD(T)/aug-cc-pVTZ level of theory. The thermal gas phase rate constants were evaluated based on the energetics from the QCISD(T)/aug-cc-pVTZ//MPW1K/6-31+G(d,p) level of theory using both microcanonical variational transition state theory (μVT) and canonical variational transition state theory (CVT) with the Eckart tunneling correction in the temperature range of 200–2500 K. The extended Arrhenius expression fitted from the μVT/Eckart rate constants of 1,2 H-shift and 1,3 H-shift reactions of C 2 H 5 O radical in the temperature range of 200–2500 K are k = 3.90 × 10 −31 T 12.4 e (−2.13 × 10 3 / T ) and k = 2.83 × 10 −29 T 11.9 e (−2.24 × 10 3 / T ) s −1 , respectively. The two isomerization rate constants exhibited positive temperature dependence in the calculated temperature region. The variational effects for the two isomerizations of ethoxy radical are small and the tunneling effects are important in the low temperature range. The titled reactions are minor and not essential compared to the decomposition pathways of ethoxy radical.

Mechanism of the inner hydrogen atom transfer in free base porphyrazine: a direct ab initio dynamics study

A theoretical study of the mechanism and kinetics of the inner double hydrogen atom-transfer process in free base porphyrazine is presented. The structures and energies of reactant, product, intermediate, transition-state, and second-order saddle-point in the transfer reaction of inner hydrogen atoms in porphyrazine are calculated by using B3LYP/6-31G** method under certain symmetry restriction. The results show that the probability of asynchronous mechanism is larger than that of synchronous mechanism via a second-order saddle-point. Changes from porphyrin to porphyrazine have a less dramatic effect on the mechanism of NH tautomerism. Our analysis reveals that the stepwise mechanism first requires the porphyrazine ring to compress followed by transfer of a H atom with an additional energy requirement of 9.9 kcal mol −1. Meanwhile, the difference of theoretical calculated results between porphyrazine and porphyrin are discussed.

Direct ab initio molecular dynamics study of [formula omitted

An ab initio direct molecular dynamics study of the reaction of CH 3 + with benzene has been performed and the mechanism of reaction examined. Ab initio energy and gradient evaluations were done at the QCISD/6-31+G** and MP2/6-31+G** levels. The primary products of reaction were several isomeric forms of arenium ion (σ complex). The reaction proceeds through a single channel, a direct barrierless insertion of a methyl cation into benzene to form a σ complex. The direct insertion mechanism supports a recent quantum chemical study but differs from that posited in earlier experimental and theoretical studies of the reaction, which have assumed formation of a stable π complex. The molecular dynamics simulation shows that product arenium ions are internally energetic enough for a hydrogen to shift about the benzenium ring to form several isomers.

Ab initio and direct dynamics study on the hydrogen abstraction reaction C2H3+HCHO

Abstract We present an ab initio direct dynamics study of thermal rate constants of the hydrogen abstraction reaction of C 2 H 3 + HCHO → C 2 H 4 + CHO. The MP2/cc-pVDZ method is employed to optimize the geometries of stationary points as well as the points on the minimum energy path (MEP). The energies of all the points were further refined at the QCISD(T)/cc-pVTZ level of theory. The rate constants were evaluated using canonical variational transition state theory (CVT) and canonical variational transition state theory with small-curvature tunneling contributions (CVT/SCT) in the temperature range of 300–2500 K. The calculated results show that the rate constants have positive temperature dependence in the calculated temperature range. The calculated rate constants at the QCISD(T)/cc-pVTZ//MP2/cc-pVDZ level of theory are in good agreement with available literature data.