Browsing by Author "Minh Tho Nguyen"

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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
;
Pham, Tien, V
;
Minh Tho Nguyen
;
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.
A model study on the mechanism and kinetics for the dissociation of water anion
(2019-08-01)
Trinh Le Huyen
;
Long Van Duong
;
Minh Tho Nguyen
;
Ming Chang Lin
;
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Quantum chemical computations using both density functional theory (B3LYP functional) and wavefunction (MP2 and CCSD(T)) methods, with the 6-311++G(3df,2p) and aug-cc-pVnZ (n = D,T,Q) basis sets, in conjunction with a polarizable continuum model (PCM) method for treating structures in solution, were carried out to look again at a series of small negatively charged water species [(H2O)(n)](center dot-). For each size n of [(H2O)(n)](center dot-) in aqueous solution with n = 2, 3, and 4, two distinct structural motifs can be identified: a classical water radical anion formed by hydrogen bonds and a molecular pincer in which the excess electron is directly interacting with H atoms. In aqueous solution, both motifs have comparable energy content and likely coexist and compete for the ground state. Some water anion isomers can dissociate when interaction with a water molecule, [(H2O)(n)](center dot-) + H2O -> H-center dot(H2O)(m) + OH-(H2O)(n)(-)(m), through successive hydrogen transfers with moderate energy barriers. This reaction can also be regarded as a water-splitting process in which the H transfers involved take place mainly within a water trimer, whereas other water molecules tend to stabilize transition structures through microsolvation rather than direct participation. Calculated absolute rate constants for the reversed reaction H-center dot(H2O)(2) + OH-(H2O)(2) -> [(H2O)(4)](center dot) + H2O with both H and D isotopes agree well with the experimentally evaluated counterpart and lend a kinetic support for the involvement of a tetramer unit.