Your browser doesn't support javascript.
loading
Mostrar: 20 | 50 | 100
Resultados 1 - 9 de 9
Filtrar
Mais filtros










Base de dados
Intervalo de ano de publicação
1.
J Phys Chem A ; 125(36): 7891-7899, 2021 Sep 16.
Artigo em Inglês | MEDLINE | ID: mdl-34464135

RESUMO

First-principles density functional theory calculations on neutral and singly negatively and positively charged iron clusters Fen and iron nitride clusters FenN and FenN2 (n = 1-10) in the range of 1 ≤ n ≤ 10 revealed that there is a strong competition between ferromagnetic and antiferromagnetic states especially in the FenN20,±1 cluster series. This phenomenon was related to superexchange via a bridging N atom between two iron atoms in the FenN20,±1 cluster series and to a double superexchange effect via a Fe atom shared by two N atoms in the FenN20,±1 series. A thorough examination of the structure-energy-spin state relationships in these clusters is conducted, leading to new insights and confirmation of available experimental results on structural parameters and dissociation energetics. The bond energies of both nitrogen atoms in the FenN2 series are approximately the same. They weakly depend on the charge of the host cluster and fluctuate around 5.5 eV when moving along the series. The energy of N2 desorption is relatively small; it varies by about 1.0 eV and depends on the charge of the cluster. The experimental finding that N2 dissociates on the Fen+ clusters beginning with n = 4 was supported by the results of our computations. Our computed values of the Fen+-N bonding energies agree with the experimental data within the experimental uncertainty bars. It was found that the attachment of one or two N atoms does not seriously affect the polarizability, electron affinity, or ionization energy of the host iron clusters independent of the charge.

2.
J Phys Chem A ; 125(20): 4409-4419, 2021 May 27.
Artigo em Inglês | MEDLINE | ID: mdl-33979167

RESUMO

The ground states of the neutral and anionic tetrafluoride and hexafluoride series of 3d-metal atoms from Sc to Zn were assigned by using a double-check approach in which the pure and hybrid density functional methods were interchangeably used. It was confirmed that all these neutral fluorides are superhalogens except for TiF4. The electron affinities of the hexafluorides were shown to be consistently higher than those of the tetrafluorides in accordance with the superhalogen conception of the extra electron delocalization over a larger number of the electronegative ligands. In the search for mononuclear fluorides possessing higher electron affinities, we considered the M(F2)6- and M(F3)6- series where M = Sc-Zn. We found that the optimized geometrical structures in both series may be described as MF6-- k(F2), k = 3 and 6, of which the geometry of the MF6- core mimics that of the corresponding hexafluoride anion and the F2 dimers are kept in a bound state by polarizing forces. In these cases, the electron affinity is decreased by tenths of eV with respect to the electron affinity of the core hexafluorides due to a confinement of the extra electron by the F2 environment.

3.
Phys Chem Chem Phys ; 23(3): 2166-2178, 2021 Jan 28.
Artigo em Inglês | MEDLINE | ID: mdl-33438692

RESUMO

The coalescence of two Fe8N as well as the structure of the Fe16N2 cluster were studied using density functional theory with the generalized gradient approximation and a basis set of triple-zeta quality. It was found that the coalescence may proceed without an energy barrier and that the geometrical structures of the resulting clusters depend strongly on the mutual orientations of the initial moieties. The dissociation of N2 is energetically favorable on Fe16, and the nitrogen atoms share the same Fe atom in the lowest energy state of the Fe16N2 species. The attachment of two nitrogen atoms leads to a decrease in the total spin magnetic moment of the ground-state Fe16 host by 6 µB due to the peculiarities of chemical bonding in the magnetic clusters. In order to gain insight into the dependence of properties on charge and to estimate the bonding energies of both N atoms, we performed optimizations of Fe16N and the singly charged ions of both Fe16N2 and Fe16N. It was found that the electronic properties of the Fe16N2 cluster, such as electron affinity and ionization energy, do not appreciably depend on the attachment of nitrogen atoms but that the average binding energy per atom changes significantly. The lowering in total energy due to the attachment of two N atoms was found to be nearly independent of charge. The IR and Raman spectra were simulated for Fe16N2 and its ions, and it was found that the positions of the most intense peaks in the IR spectra strongly depend on charge and therefore present fingerprints of the charged states. The chemical bonding in the ground-state Fe16N20,±1 species was described in terms of the localized molecular orbitals.

4.
J Comput Chem ; 41(30): 2583-2590, 2020 Nov 15.
Artigo em Inglês | MEDLINE | ID: mdl-32964509

RESUMO

The structure and properties of ZnO quantum dots is a very popular and rapidly growing field of research for which accurate quantum calculations are challenging to perform. Since the dependence between system size and wall time scales nonlinearly, certain compromises have to be made. A particularly important limiting factor is the size of the basis used, this is especially the case if accurate large calculations are to be carried out. In our work, we discovered that an important O(2p)->Zn(4p) back donation, which greatly influences the strength of the ZnO bond, can be reproduced only if diffuse functions are added to the basis set. We further tested the basis dependence for the magic-sized wurtzite nanophase ZnO clusters which were previously shown to be able to accurately reproduce the magnetically doped II-IV Q-dots. In this work, we outline the minimal basis sets required to properly describe ZnO bonds in a nanocluster. It was demonstrated that the rock salt nanophase is incorrectly stabilized if a basis set does not contain sufficiently diffuse functions while the correct wurtzite phase is stabilized when diffuse functions are added. This tendency, similar to that in the ZnO dimer case, was shown to stem from the incorrect lack of Zn(4p) electron density in calculations when using the diffuse-free basis set.

5.
J Phys Chem A ; 124(37): 7427-7438, 2020 Sep 17.
Artigo em Inglês | MEDLINE | ID: mdl-32841027

RESUMO

Dissociation pathways of singly- and multiply charged gas-phase nitromethane cations were investigated with strong-field laser photoionization mass spectrometry and density functional theory computations. There are multiple isomers of the singly charged nitromethane radical cation, several of which can be accessed by rearrangement of the parent CH3-NO2 structure with low energy barriers. While direct cleavage of the C-N bond from the parent nitromethane cation produces NO2+ and CH3+, rearrangement prior to dissociation accounts for fragmentation products including NO+, CH2OH+, and CH2NO+. Extensive Coulomb explosion in fragment ions observed at high laser intensity indicates that rapid dissociation of multiply charged nitromethane cations produces additional species such as CH2+, H+, and NO22+.  On the basis of analysis of Coulomb explosion in the mass spectral signals and pathway calculations, sufficiently intense laser fields can remove four or more electrons from nitromethane.

6.
J Phys Chem Lett ; 11(7): 2630-2636, 2020 Apr 02.
Artigo em Inglês | MEDLINE | ID: mdl-32178515

RESUMO

In this work, we report a comparative study of the gamma ray stability of perovskite solar cells based on a series of perovskite absorbers including MAPbI3 (MA = methylammonium), MAPbBr3, Cs0.15FA0.85PbI3 (FA = formamidinim), Cs0.1MA0.15FA0.75PbI3, CsPbI3, and CsPbBr3. We reveal that the composition of the perovskite material strongly affects the radiation stability of the solar cells. In particular, solar cells based on the MAPbI3 were found to be the most resistant to gamma rays since this perovskite undergoes rapid self-healing due to the special gas-phase chemistry analyzed with ab initio calculations. The fact that the solar cells based on MAPbI3 can withstand a 1000 kRad gamma ray dose without any noticeable degradation of the photovoltaic properties is particularly exciting and shifts the paradigm of research in this field toward designing more dynamic rather than intrinsically robust (e.g., inorganic) materials.

7.
J Phys Chem A ; 123(45): 9693-9700, 2019 Nov 14.
Artigo em Inglês | MEDLINE | ID: mdl-31557029

RESUMO

Perfluorobenzenes are reactive species with the lowest magnesium metalation barriers among all hexahalobenzenes. This fact makes them good candidates for the study of heterocoupling reactions of the Grignard type. In this work, we investigated a number of pathways for both heterocoupling and homocoupling reactions and estimated the solvated energy barrier heights. According to the results of our density functional theory (DFT)-based computations, the heterocoupling reaction (C6F5)MgF + C6F6 is a single-step process. We have also studied the (C6F5)MgF + (C6F5)MgF homocoupling reaction with an aryne intermediate. In this particular reaction, a carbon-carbon bond is formed between two nucleophilic carbon centers in a chemically predictable way. The final product, (C12F9)Mg2F3, retains even stronger nucleophilic activity than that of the starting (C6F5)MgF reagent. A very surprising result of our calculations is that this homocoupling of two nucleophilic centers is spontaneous in THF solvent.

8.
J Phys Chem A ; 123(6): 1140-1152, 2019 Feb 14.
Artigo em Inglês | MEDLINE | ID: mdl-30668102

RESUMO

2-Nitrotoluene (2-NT) is a good model for both photolabile protecting groups for organic synthesis and the military explosive 2,4,6-trinitrotoluene (TNT). In addition to the direct C-NO2 bond-cleavage reaction that initiates detonation in TNT, 2-NT undergoes an H atom attack reaction common to the photolabile 2-nitrobenzyl group, which forms the aci-nitro tautomer. In this work, femtosecond pump-probe measurements with mass spectrometric detection and density functional theory (DFT) calculations demonstrate that the initially prepared vibrational coherence in the 2-NT radical cation (2-NT+) is preserved following H atom attack. Strong-field adiabatic ionization is used to prepare 2-NT+, which can overcome a modest 0.76 eV energy barrier to H atom attack to form the aci-nitro tautomer as soon as ∼20-60 fs after ionization. Once formed, the aci-nitro tautomer spontaneously loses -OH to form C7H6NO+, which exhibits distinctly faster oscillations in its ion yield (290 fs period) as compared to the 2-NT+ ion (380 fs period). The fast oscillations are attributed to the coherent torsional motion of the aci-nitro tautomer, which has a significantly faster computed torsional frequency (86.9 cm-1) than the 2-NT+ ion (47.9 cm-1). Additional DFT calculations identify reaction pathways leading to the formation of the dissociation products C7H6NO+, C7H7+, and C6H6N+. Collectively, these results reveal a rich picture of coherently and incoherently driven dissociation pathways in 2-NT+.

9.
J Comput Chem ; 37(28): 2527-36, 2016 10 30.
Artigo em Inglês | MEDLINE | ID: mdl-27557091

RESUMO

Geometrical and electronic structures of the neutral and singly negatively charged Fe6 On and Fe7 Om clusters in the range of 1 ≤ n ≤ 20 and 1 ≤ m ≤ 24, respectively, are computed using density functional theory with the generalized gradient approximation. The largest clusters in the two series, Fe6 O20 and Fe7 O24 , can be described as Fe(FeO4 )5 and Fe(FeO4 )6 or alternatively as [FeO5 ](FeO3 )5 and [FeO6 ](FeO3 )6 , respectively. The Fe6 O20 and Fe7 O24 clusters possess adiabatic electron affinities (EAad ) of 5.64 eV and 5.80 eV and can be attributed to the class of hyperhalogens since FeO4 is an unique closed-shell superhalogen with the EAad of 3.9 eV. The spin character of the lowest total energy states in both series changes from ferromagnetic to ferrimagnetic or antiferromagnetic when the first FeOFe bridge is formed. Oxidation decreases substantially the polarizability per atom of the initial bare clusters; namely, from 5.98 Å(3) of Fe6 to 2.47 Å(3) of Fe6 O20 and from 5.67 Å(3) of Fe7 to 2.38 Å(3) of Fe7 O24 . The results of our computations pertaining to the binding energies of O, Fe, O2 , and FeO in the Fe7 Om series provide an explanation for the experimentally observed abundance of the iron oxide nanoparticles with stoichiometric compositions. © 2016 Wiley Periodicals, Inc.

SELEÇÃO DE REFERÊNCIAS
DETALHE DA PESQUISA
...