Browsing by Author "Pham, Tien, V"

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A model study on the mechanism and kinetics for reactions of the hydrated electron with H3O+ and NH4+ ions
(2019-09-16)
Le Huyen, Trinh
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Pham, Tien, V
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Minh Tho Nguyen
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Lin, M. C.
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Mechanism and kinetics of the reactions of the hydrated electron (e(aq)(-)) with H3O+ and NH4+ cations were determined using quantum chemical computations with both density functional theory (B3LYP) and wavefunction (MP2 and CCSD(T)) methods and the 6-311 + + G(3df,2p) basis set, in conjunction with a PCM method for treating structures in solution. Although both reactions occur with a hydrogen atom transfer, their mechanisms differ from each other by kinetic results. While the reaction of e(aq)(-) and H3O+ in solution is diffusion-controlled, NH4+ reacts with e(aq)(-) via a transition structure with H-tunneling. The predicted rate constants agree well with available experimental results.
Computational Study on the Mechanisms and Rate Constants for the O(P-3,D-1) + OCS Reactions
(2019-10-03)
Chen, Hsin-Tsung
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Pham, Tien, V
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Lin, M. C.
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The mechanisms and kinetics of O(P-3,D-1) + OCS(X-1 Sigma(+)) reactions have been studied by the high-level G2M(CC2) and CCSD(T)/6-311+G(3df)//B3LYP/6-311+G(3df) methods in conjunction with the transition-state theory and variational Rice-Ramsperger-Kassel-Marcus theory calculations. The result shows that the triplet surface proceeds directly by abstraction and substitution channels to produce SO(P-3) + CO(X-1 Sigma(+)) and S(P-3) + CO2(X-1 Sigma(+)(g)) bypassing the barriers of 7.6 and 9.1 kcal.mol(-1) at the G2M(CC2)//B3LYP/6-311+G(3df) level, respectively, while two stable intermediates, LM1 (OSCO1) and LM2 (SC(O)O-1), are formed barrierlessly from O(D-1 ) + OCS(X-1 Sigma(+)) in the singlet surface, which lie at -40.5 and -50.1 kcal.mol(-1) relative to O(P-3) + OCS(X-1 Sigma(+)) reactants and decompose to CO (X-1 Sigma(+)) + SO (a(1)Delta) and S(D-1) + CO2(X-1 Sigma g(+)). LM1 and LM2 may also be produced by singlet-triplet surface crossings via MSX1 and MSX2; the predicted total rate constant for the O(P-3) + OCS(X-1 Sigma(+)) reaction including the crossings, 9.2 X 10(-11) exp(-5.18 kcal-mol(-1)/RT) cm(3) molecule(-1) s(-1), is in good agreement with available experimental data. The branching ratio of the CO2 product channel, 0.22-0.32, between 1200 and 1600 K, is also in excellent agreement with the value of 0.2-0.3 measured by Isshiki et al. (J. Phys. Chem. A. 2003, 107, 2464).
On the hypergolicity of trimethyl aluminum in air
(2020-03-01)
Pham, Tien, V
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Nguyen, Hue M. T.
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Lin, M. C.
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A theoretical study on the mechanisms and kinetics of the reaction of (CH3)(3)Al (TMA), an important industrial compound, with O-2 has been carried out at the CCSD(T)/6-311 + +G(3df,2p)//B3LYP/6-311 + + G(3df,2p) level in conjunction with the conventional transition state theory (TST) calculations. The potential energy surface (PES) of the reaction indicates that the TMA + O-2 system has two pathways leading to different product pairs: the first one passes through a tight transition state with a high energy barrier, 16.9 kcal/mol, producing (CH3)(2)AlO2 + CH3 and the other one goes via a loose roaming-like transition state with a much lower energy barrier, 6.8 kcal/mol, yielding CH3Al(O)OCH3 + CH3. The barrier predicted for the former was found to be too high for combustion initiation under the ambient condition. The latter, however, may play a key role in initiating the hypergolic reaction of TMA in the air. Rate constants for both channels have been calculated for the temperature range of 300-2000 K. The pressure-independent rate constant for the TMA + O-2 reaction via the loose transition state is predicted to be k(T) = 3.05 x 10(-21)T(3.003) exp( - 3226.1/T) cm(3) molecule(-1) s(-1); this result gives the half-life of TMA in air under the ambient condition to be as short as 1.7 x 10(-2) s, which is sufficiently short for the hypergolic combustion initiation without even considering the ensuing rapid radical chain reactions.
Theoretical Study of the Kinetics of the Gas-Phase Reaction between Phenyl and Amino Radicals
(2020-01-21)
Pham, Tien, V
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Tue Trang, Hoang T.
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Trinh Le Huyen
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Tue Ngoc Nguyen
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The potential energy surface (PES) of the C6H5 + NH2 reaction has been investigated by using ab initio CCSD(T)//B3LYP/ 6-311++G(3df,2p) calculations. The conventional transition-state theory (TST) and the variable reaction coordinate-TST (VRC-TST) have been used to predict the rate constants for the channels possessing tight and barrierless transition states, respectively. The Rice-Ramsperger-Kassel-Marcus/Master equation (RRKM/ME) theory has been utilized to determine the pressure-dependent rate constants for these reactions. The PES shows that the reaction begins with an exothermic barrierless addition of NH2 to C6H5 producing the vital intermediate state, namely, aniline (C6H5NH2, IS1). Once IS1 is generated, it can further isomerize to various intermediate states, which can give rise to different products, including PR4 (4,5,6-trihydro-1-amino phenyl + H-2), PR5 (3,4,5,6-tetrahydro phenyl + NH3), PR6 (2,3,5,6-tetrahydro-1-imidogen phenyl + H-2), PR9 (3,4,5,6-tetrahydro-1-imidogen phenyl + H-2), and PR10 (2,5,6-trihydro-1-amino phenyl + H-2), of which the most stable product, PR5, was formed by the most favorable channel going through the two advantageous transition states T1/11 (-28.9 kcal/mol) and T11P5 (-21.5 kcal/mol). The calculated rate constants for the low-energy channel, 1.37 x 10(-9) and 2.16 x 10(-11) cm(3) molecule(-1) s(-1) at T = 300, P = 1 Torr and T = 2000 K, P = 760 Torr, respectively, show that the title reaction is almost pressure-and temperature-dependent. The negative temperature-dependent rate coefficients can be expressed in the modified Arrhenius form of k(1) = 8.54 x 10(13) T-7.20 exp (-7.07 kcal.mol(-1)/RT) and k(2) = 2.42 x 10(15) T-7.61 exp (-7.75 kcal.mol(-1)/RT) at 1 and 10 Torr, respectively, and in the temperature range of 300-2000 K. The forward and reverse rate coefficients as well as the high-pressure equilibrium constants of the C6H5 + NH2 <-> IS1 process were also predicted; their values revealed that its kinetics do not depend on pressure at low temperature but strongly depend on pressure at high temperature. Moreover, the predicted formation enthalpies of reactants and the enthalpy changes of some channels are in good agreement with the experimental results.
Thermal decomposition of N2O near 900 K studied by FTIR spectrometry: Comparison of experimental and theoretical O(P-3) formation kinetics
(1970-01-01)
Pham, Tien, V
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Tsay, T. J.
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Lin, M. C.
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The spin-forbidden dissociation reaction of the N2O(X-1 sigma(+)) ground state has been investigated by both quantum calculations and experiments. Ab initio prediction at the CCSD(T)/CBS(TQ5)//CCSD(T)/aug-cc-pVTZ+d level of theory gave the crossing point (MSX) energy at 60.1 kcal/mol for the N2O(X-1 sigma(+)) -> N-2(X1 sigma g+) + O(P-3) transition, in good agreement with published data. The T- and P-dependent rate coefficients, k(1)(T,P), for the nonadiabatic thermal dissociation predicted by nonadiabatic Rice-Ramsperger-Kassel-Marcus (RRKM) calculations agree very well with literature data. The rate constants at the high- and low-pressure limits, k(1)(infinity) = 10(11.90) exp (-61.54 kcal mol(-1)/RT) s(-1) and k(1)(o) = 10(14.97) exp(-60.05 kcal mol(-1)/RT) cm(3) mol(-1) s(-1), for example, agree closely with the extrapolated results of Rohrig et al. at both pressure limits. The second-order rate constant (k(1)(o)) is also in excellent agreement with our result measured by FTIR spectrometry in the present study for the temperature range of 860-1023 K as well as with many existing high-temperature data obtained primarily by shock-wave heating up to 3340 K. Kinetic modeling of the NO product yields measured at long reaction times in the present work also allowed us to reliably estimate the rate constant for reaction (3), O + N2O -> N-2 + O-2, based on its strong competition with the NO formation from reaction (2) which has been better established. The modeled values of k(3) confirmed the previous finding by Davidson et al. with significantly smaller values of A-factor and activation energy than the accepted ones. A least-squares analysis of both sets of data gave k(3) = 10(12.22 +/- 0.04) exp[- (11.65 +/- 0.24 kcal mol(-1)/RT)] cm(3) mol(-1) s(-1), covering the wide temperature range of 988-3340 K.