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1.
Dalton Trans ; 41(37): 11361-8, 2012 Oct 07.
Artigo em Inglês | MEDLINE | ID: mdl-22886001

RESUMO

The formation mechanism is always a fundamental and confused issue for polyoxometalate chemistry. Two formation mechanisms (M1 and M2) of the Lindqvist anion [W(6)O(19)](2-) have been adopted to investigate it's self-assembly reaction pathways at a density functional theory (DFT) level. The potential energy surfaces reveal that both the mechanisms are thermodynamically favorable and overall barrierless at room temperature, but M2 is slightly dominant to M1. The formation of the pentanuclear species [W(5)O(16)](2-) and [W(5)O(15)(OH)](-) are recognized as the rate-determining steps in the whole assembly polymerization processes. These two steps involve the highest energy barriers with 30.48 kcal mol(-1) and 28.90 kcal mol(-1), respectively, for M1 and M2. [W(4)O(13)](2-) and [W(4)O(12)(OH)](-) are proved to be the most stable building blocks. In addition, DFT results reveal that the formation of [W(3)O(10)](2-) experiences a lower barrier along the chain channel.


Assuntos
Teoria Quântica , Compostos de Tungstênio/química , Água/química , Ânions , Modelos Moleculares , Soluções
2.
J Mol Model ; 18(7): 3219-25, 2012 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-22246287

RESUMO

In view of their intriguing structural and electrical properties, the linear and nonlinear optical (NLO) responses of six carbon nanotube (CNT) molecules substituted by nitrogen atoms at one end have been explored by using the CAM-B3LYP method. Molecules 1, 2 and 3 were obtained by increasing the lengths of the CNTs, and 1-Li, 2-Li and 3-Li were constructed by doping one Li atom into the N-substituted end of 1, 2 and 3 (mentioned above), respectively. Two effective approaches have been proposed to increase nonlinear optical properties(NLO): increasing the length of the CNT as well as doping one Li atom into the N-substituted end. The results show that both the linear polarizabilities (α(0)) and nonlinear first hyperpolarizabilities (ß(tot)) values increase with increasing the lengths of the CNTs: 188 of 1 < 307 of 2 < 453 of 3 for α(0) and 477 of 1 < 2654 of 2 < 3906 au of 3 for ß(tot). Significantly, compared with the non-doped CNTs, the ß(tot) values are remarkably enhanced by doping one Li atom into the N-substituted end: 477 of 1 < 23258 of 1-Li, 2654 of 2 < 37244 of 2-Li, and 3906 of 3 < 72004 au of 3-Li. Moreover, the ß(vec) values show a similar trend to the ß(tot) values. Our results may be beneficial to experimentalists in exploring high-performance nonlinear optical materials based on CNT.


Assuntos
Nanotubos de Carbono/química , Nitrogênio/química , Óptica e Fotônica , Modelos Moleculares
3.
J Phys Chem A ; 114(10): 3516-22, 2010 Mar 18.
Artigo em Inglês | MEDLINE | ID: mdl-20175584

RESUMO

Both singlet and triplet potential energy surfaces for the reaction of ground-state formaldehyde (CH(2)O) and ozone (O(3)) are theoretically investigated at the BMC-CCSD//BHandHLYP/6-311+G(d,p) level. Various possible isomerization and dissociation pathways are probed. Hydrogen abstraction, oxygen abstraction, and C-addition/elimination are found on both the singlet and the triplet surfaces. The major products for the total reaction are HCO and HOOO, which are generated via hydrogen abstraction. The transition state theory (TST) and multichannel RRKM calculations have been carried out for the total and individual rate constants for determinant channels over a wide range of temperatures and pressures.

4.
J Chem Phys ; 132(6): 064301, 2010 Feb 14.
Artigo em Inglês | MEDLINE | ID: mdl-20151737

RESUMO

The low-lying triplet and singlet potential energy surfaces of the O((3)P)+CH(3)CN reaction have been studied at the G3(MP2)//B3LYP/6-311+G(d,p) level. On the triplet surface, six kinds of pathways are revealed, namely, direct hydrogen abstraction, C-addition/elimination, N-addition/elimination, substitution, insertion, and H-migration. Multichannel Rice-Ramsperger-Kassel-Marcus theory and transition-state theory are employed to calculate the overall and individual rate constants over a wide range of temperatures and pressures. It is predicted that the direct hydrogen abstraction and C-addition/elimination on triplet potential energy surface are dominant pathways. Major predicted end products include CH(3)+NCO and CH(2)CN+OH. At atmospheric pressure with Ar and N(2) as bath gases, CH(3)C(O)N (IM1) formed by collisional stabilization is dominated at T<700 K, whereas CH(3) and NCO produced by C-addition/elimination pathway are the major products at the temperatures between 800 and 1500 K; the direct hydrogen abstraction leading to CH(2)CN+OH plays an important role at higher temperatures in hydrocarbon combustion chemistry and flames, with estimated contribution of 64% at 2000 K. Furthermore, the calculated rate constants are in good agreement with available experimental data over the temperature range 300-600 K. The kinetic isotope effect has also been calculated for the triplet O((3)P)+CH(3)CN reaction. On the singlet surface, the atomic oxygen can easily insert into C-H or C-C bonds of CH(3)CN, forming the insertion intermediates s-IM8(HOCH(2)CN) and s-IM5(CH(3)OCN) or add to the carbon atom of CN group in CH(3)CN, forming the addition intermediate s-IM1(CH(3)C(O)N); both approaches were found to be barrierless. It is indicated that the singlet reaction exhibits a marked difference from the triplet reaction. This calculation is useful to simulate experimental investigations of the O((3)P)+CH(3)CN reaction in the singlet state surface.

5.
J Phys Chem B ; 113(52): 16472-8, 2009 Dec 31.
Artigo em Inglês | MEDLINE | ID: mdl-19968318

RESUMO

Tris(o-phenylenedioxy)cyclotriphosphazene (TPP) became the compound of choice to investigate the structural features of organic zeolites and their potential applications as soft materials. A van der Waals crystal of the TPP analogue (host) with the thiophene side fragment tris(3,4-thiophenedioxy)cyclotriphosphazene (TTP) was designed to investigate the selective adsorption among some common gases (guest): methane (CH(4)), carbon dioxide (CO(2)), nitrogen (N(2)), or hydrogen (H(2)). The crystal structure of TTP was modeled by applying minimization methods using the COMPASS (condensed-phase optimized molecular potentials for atomic simulation studies) force field. Interaction energies and structural properties of van der Waals complexes of the crystal of TTP and gas molecules were studied using the dispersion corrected density functional theory (DFT-D). The proper functional and basis set were selected after comparing with benchmark data of the coupled-cluster calculations with singles, doubles, and perturbative triple excitations [CCSD(T)] estimated at the complete basis set (CBS) limit. On the basis of our results, the interaction energy between the host and the guest molecules was predicted in the increasing order of host-H(2) << host-N(2) < host-CH(4) < host-CO(2), suggesting the designed TTP is a good candidate as an organic zeolite for potential fuel storage, hydrogen purification, carbon dioxide removal from the air, as well as safety care in a coal mine.

6.
J Phys Chem A ; 113(20): 5951-7, 2009 May 21.
Artigo em Inglês | MEDLINE | ID: mdl-19397309

RESUMO

A direct ab initio dynamics method was used to study the mechanism and kinetics of the reaction CF(3)CHFOCH(3) + OH. Two reaction channels, R1 and R2, were found, corresponding to H-abstraction from a CH(3) group and a CHF group, respectively. The potential energy surface (PES) information was obtained at the G3(MP2)//MP2/6-311G(d,p) level. The standard enthalpies of formation for the reactant (CF(3)CHFOCH(3)) and products (CF(3)CHFOCH(2) and CF(3)CFOCH(3)) were evaluated via isodesmic reactions at the same level. Furthermore, the rate constants of two channels were calculated using the canonical variational transition state theory (CVT) with small-curvature tunneling (SCT) contributions over a wide temperature range of 200-3000 K. The dynamic calculations demonstrate that reaction R1 dominates the overall reaction when the temperature is lower than 800 K whereas reaction R2 becomes more competitive in the higher temperature range. The calculated rate constants and branching ratios are both in good agreement with the available experimental values.

7.
J Chem Phys ; 130(12): 124705, 2009 Mar 28.
Artigo em Inglês | MEDLINE | ID: mdl-19334870

RESUMO

Stimulated by the mass spectroscopic observation of the metallofullerene Ca@C(44), we have performed a systematic investigation to search for the most stable isomer using HF/3-21G approximately LanL2DZ, HF/6-31+G(d), B3LYP/6-31+G(d), and MP2/6-31+G(d)//B3LYP/6-31+G(d) methods. The Ca@C(44) (D(2):53) isomer with eight adjacent pentagons in the fullerene framework is predicted to possess the lowest energy. The thermodynamics stability explorations of Ca@C(44) isomers at different temperatures show that Ca@C(44) (D(2):53) is the most thermodynamically stable in the temperature range of absolute zero to 4000 K. The encapsulation of Ca atom in C(44) fullerene is exothermic, and the electronic structure of Ca@C(44) (D(2):53) can be described formally as Ca(2+)@C(44) (2-). Further analysis on the frontier molecular orbitals and density of states of Ca@C(44) (D(2):53) suggests that both highest occupied molecular orbital and lowest unoccupied molecular orbital are carbonlike with low Ca character, and the carbon cage possesses high chemical activity. In addition, the vibrational spectrum of Ca@C(44) (D(2):53) has been simulated and analyzed to gain an insight into the metal-cage vibrations.

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