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1.
Langmuir ; 37(33): 10233-10240, 2021 Aug 24.
Artigo em Inglês | MEDLINE | ID: mdl-34387499

RESUMO

An improved procedure for the preparation of aerogel granules of polytetrafluoroethylene-graphene oxide (PTFE-GO) with a composition of 50:50 (in wt %) and a specific density of 35 ± 2 mg/cm3 is described. The technique practically excludes the granule cracking. The specific density of the pellets after reduction using hydrazine vapor and annealing at 370 °C decreased to 29 ± 2 mg/cm3. The PTFE-reduced GO (rGO) pellets obtained were tested as a recyclable sorbent for isopropyl alcohol (IPA) in sorption/combustion cycles. It has been shown that the aerogel sorption capacity for IPA increases from 35.6 to 39.3 g/g as a result of alcohol burning off. During the combustion of IPA, the temperature of an individual pellet can exceed 300 °C. When several contingent pellets are burned, the temperature of their heating increases. The fine-pored structure of the near-surface layer of the granule is destroyed during the alcohol burning, the internal structure with larger pores is exposed, and the relative proportion of PTFE on the surface of the granules decreases. It was also shown that the specific surface area of PTFE-rGO increases from 26 to 49 m2/g during cycling.

2.
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.

3.
Nanomaterials (Basel) ; 11(5)2021 May 17.
Artigo em Inglês | MEDLINE | ID: mdl-34067894

RESUMO

The alkaline activation of a carbonized graphene oxide/dextrin mixture yielded a carbon-based nanoscale material (AC-TR) with a unique highly porous structure. The BET-estimated specific surface area of the material is 3167 m2/g, which is higher than the specific surface area of a graphene layer. The material has a density of 0.34 g/cm3 and electrical resistivity of 0.25 Ω·cm and its properties were studied using the elemental analysis, transmission electron microscopy (TEM), electron diffraction (ED), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray induced Auger electron spectroscopy (XAES), and electron energy loss spectroscopy (EELS) in the plasmon excitation range. From these data, we derive an integral understanding of the structure of this material. The concentration of sp3 carbon atoms was found to be relatively low with an absolute value that depends on the measurement method. It was shown that there is no graphite-like (002) peak in the electron and X-ray diffraction pattern. The characteristic size of a sp2-domain in the basal plane estimated from the Raman spectra was 7 nm. It was also found that plasmon peaks in the EELS spectrum of AC-TR are downshifted compared to those of graphite.

4.
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.

5.
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.

6.
Materials (Basel) ; 14(2)2021 Jan 09.
Artigo em Inglês | MEDLINE | ID: mdl-33435447

RESUMO

A graphene oxide aerogel (GOA) was formed inside a melamine sponge (MS) framework. After reduction with hydrazine at 60 °C, the electrical conductive nitrogen-enriched rGOA-MS composite material with a specific density of 20.1 mg/cm3 was used to fabricate an electrode, which proved to be a promising electrocatalyst for the oxygen reduction reaction. The rGOA-MS composite material was characterized by elemental analysis, scanning electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. It was found that nitrogen in the material is presented by different types with the maximum concentration of pyrrole-like nitrogen. By using Raman scattering it was established that the rGOA component of the material is graphene-like carbon with an average size of the sp2-domains of 5.7 nm. This explains a quite high conductivity of the composite obtained.

7.
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.

8.
Phys Chem Chem Phys ; 21(39): 21746-21752, 2019 Oct 09.
Artigo em Inglês | MEDLINE | ID: mdl-31552405

RESUMO

A theoretical study of geometrical structures and electronic properties of niobium-doped boron clusters is performed using the CALYPSO approach for the global minimum search followed by density functional theory calculations. It is found that the global minima obtained for the neutral clusters correspond to the half-sandwich structures at n = 10-17 and the tubular-type structures at n = 18-20. The geometrical patterns in the anion series are more complex. The geometries undergo a transformation from the wheel-like structure of NbB10- to the half-sandwich ones beginning at n = 11 and finally to the drum-shaped structures at n ≥ 18. A fascinating NbB12- cluster is uncovered by our structural search, which shows robust stability and can be considered as a new member of the half-sandwich transition metal doped boron clusters. The chemical bond analysis indicates that the high stability is due to the strong interactions between the Nb atom and the B12 host as well as to the strong B-B covalent bonds. Our study will enrich the database of geometrical structures of transition metal doped boron clusters and will stimulate future synthesis of boron-based nanomaterials.

9.
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.

10.
J Phys Condens Matter ; 31(48): 485302, 2019 Dec 04.
Artigo em Inglês | MEDLINE | ID: mdl-31412328

RESUMO

Doping of boron-based materials with transition metal atoms allows one to tune or modify the properties and structure of the materials. In this work, an extensive search for the global minima on potential energy surfaces of ScB n and ScB[Formula: see text] clusters has been performed using the CALYPSO method. The structural evolution of scandium doped boron clusters of this range is found to proceed in three steps; namely, the formation of half-sandwich type structures is followed by the formation of drum-like structures with the Sc atom located at the center and terminates with the cage-like structures. It is also found that highly symmetrical geometric structures are more common for the smaller size range of [Formula: see text]. The neutral ScB13 cluster is identified as magic on the basis of an analysis of relative stabilities in the ScB n series. Our analysis of chemical bonding has shown that the stability of this cluster is mainly due to the formation of several delocalized [Formula: see text]-bonding molecular orbitals composed of Sc 3d and B 2s atomic orbitals. These bonds appear to be responsible for the enhanced stability of ScB13 with respect to other Sc-doped boron clusters.

11.
Phys Chem Chem Phys ; 20(48): 30376-30383, 2018 Dec 12.
Artigo em Inglês | MEDLINE | ID: mdl-30489588

RESUMO

Modification of properties of boron nanoparticles by doping with transition metals presents a challenging problem because the number of isomers of both doped and un-doped nanoparticles rapidly increases with the nanoparticle size. Here, we perform a study of neutral and anionic Ru-doped boron clusters RuBn (n = 9-20) using the unbiased CALYPSO structural search method in combination with density functional theory calculations. Our results show that the neutral RuB9 cluster possesses a perfect planar wheel-like geometrical structure, whereas the RuBn clusters prefer structures of the half-sandwich type in the range of 10 ≤ n ≤ 14, drum-like type in the range of 15 ≤ n ≤ 18 and cage-like structures for larger n values. The geometrical structures of the lowest total energy states of the RuBn- anions are similar to those of the corresponding neutrals, except for RuB10-, RuB11-, RuB14-, RuB15- and RuB20-. The neutral RuB12 and RuB14 clusters are found to exhibit enhanced stability with respect to the rest of the RuBn clusters due to the delocalized bonding between the Ru atom and the boron host.

13.
J Chem Phys ; 148(13): 134305, 2018 Apr 07.
Artigo em Inglês | MEDLINE | ID: mdl-29626885

RESUMO

Monosubstituted nitrotoluenes serve as important model compounds for nitroaromatic energetic molecules such as trinitrotoluene. This work investigates the ultrafast nuclear dynamics of 3- and 4-nitrotoluene radical cations using femtosecond pump-probe measurements and the results of density functional theory calculations. Strong-field adiabatic ionization of 3- and 4-nitrotoluene using 1500 nm, 18 fs pulses produces radical cations in the ground electronic state with distinct coherent vibrational excitations. In both nitrotoluene isomers, a one-photon excitation with the probe pulse results in NO2 loss to form C7H7+, which exhibits out-of-phase oscillations in yield with the parent molecular ion. The oscillations in 4-nitrotoluene with a period of 470 fs are attributed to the torsional motion of the NO2 group based on theoretical results showing that the dominant relaxation pathway in 4-nitrotoluene radical cations involves the rotation of the NO2 group away from the planar geometry. The distinctly faster oscillation period of 216 fs in 3-nitrotoluene is attributed to an in-plane bending motion of the NO2 and CH3 moieties based on analysis of the normal modes. These results demonstrate that coherent nuclear motions determine the probability of C-NO2 homolysis in the nitrotoluene radical cations upon optical excitation within several hundred femtoseconds of the initial ionization event.

14.
Phys Chem Chem Phys ; 20(7): 4636-4640, 2018 Feb 14.
Artigo em Inglês | MEDLINE | ID: mdl-29239428

RESUMO

Femtosecond pump-probe measurements of the nerve agent simulant dimethyl methylphosphonate (DMMP) demonstrate the preparation of a robust coherent vibrational state in the corresponding radical cation. The oscillations in the transient ion yields have a period of 45 fs (750 cm-1), which is at least 3 times faster than any previously observed oscillations in polyatomic radical cations. Use of 1200-1600 nm, as opposed to 800 nm, wavelengths for ionization increases the oscillation amplitude by a factor of 5 and doubles the number of visible oscillation periods from 6 to 12, indicating that an adiabatic ionization mechanism significantly enhances preparation of the coherent state. The coherent motion is assigned to a bending mode in DMMP+ with frequency in the range of 742.2-754.7 cm-1 based on the results of DFT calculations. The observation of coherent nuclear dynamics in the dissociation of DMMP+ suggests the potential utility of coherent control schemes for controlling the dissociation of DMMP and related molecules, which has important implications for developing detection and decontamination technologies for organophosphorus chemical warfare agents.

15.
J Phys Chem A ; 121(34): 6510-6516, 2017 Aug 31.
Artigo em Inglês | MEDLINE | ID: mdl-28787163

RESUMO

The geometrical structure and electronic properties of the neutral RhBn and singly negatively charged RhBn- clusters are obtained in the range of 3 ≤ n ≤ 10 using the unbiased CALYPSO structure search method and density functional theory (DFT). A combination of the PBE0 functional and the def2-TZVP basis set is used for determining global minima on potential energy surfaces of the Rh-doped Bn clusters. The photoelectron spectra of the anions are simulated using the time-dependent density functional theory (TD-DFT) method. Good agreement between our simulated and experimentally obtained photoelectron spectra for RhB9- provides support to the validity of our theoretical method. The relative stabilities of the ground-state RhBn and RhBn- clusters are estimated using the calculated binding energies, second-order total energy differences, and HOMO-LUMO gaps. It is found that RhB7 and RhB8- are the most stable species in the neutral and anionic series, respectively. The chemical bonding analysis reveals that the RhB8-cluster possesses two sets of delocalized σ and π bonds. In both cases, the Hückel 4N + 2 rule is fulfilled and this cluster possesses both σ and π aromaticities.

16.
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.

17.
J Comput Chem ; 33(4): 416-24, 2012 Feb 05.
Artigo em Inglês | MEDLINE | ID: mdl-22121015

RESUMO

The geometrical and electronic structures of Al(BO(2))(n) and Al(BO(2))(n)(-) (n = 1-4) clusters are computed at different levels of theory including density functional theory (DFT), hybrid DFT, double-hybrid DFT, and second-order perturbation theory. All aluminum borates are found to be quite stable toward the BO(2) and BO(2)(-) loss in the neutral and anion series, respectively. Al(BO(2))(4) belongs to the class of hyperhalogens composed of smaller superhalogens, and should possess a large adiabatic electron affinity (EA(ad)) larger than that of its superhalogen building block BO(2). Indeed, the aluminum tetraborate possesses the EA(ad) of 5.6 eV, which, however, is smaller than the EA(ad) of 7.8 eV of the AlF(4) supehalogen despite BO(2) is more electronegative than F. The EA(ad) decrease in Al(BO(2))(4) is due to the higher thermodynamic stability of Al(BO(2))(4) compared to that of AlF(4). Because of its high EA and thermodynamic stability, Al(BO(2))(4) should be capable of forming salts with electropositive counter ions. We optimized KAl(BO(2))(4) as corresponding to a unit cell of a hypothetical KAl(BO(2))(4) salt and found that specific energy and energy density of such a salt are competitive with those of trinitrotoluol (TNT).

18.
J Chem Phys ; 134(23): 234311, 2011 Jun 21.
Artigo em Inglês | MEDLINE | ID: mdl-21702560

RESUMO

Using density functional theory with hybrid exchange-correlation potential, we have calculated the geometrical and electronic structure, relative stability, and electron affinities of MnX(n) compounds (n = 1-6) formed by a Mn atom and halogen atoms X = F, Cl, and Br. Our objective is to examine the extent to which the Mn-X interactions are similar and to elucidate if/how the half-filled 3d-shell of a Mn atom participates in chemical bonding as the number of halogen atoms increases. While the highest oxidation number of the Mn atom in fluorides is considered to be +4, the maximum number of halogen atoms that can be chemically attached in the MnX(n)(-) anions is 6 for X = F, 5 for X = Cl, and 4 for X = Br. The MnCl(n) and MnBr(n) neutrals are superhalogens for n ≥ 3, while the superhalogen behavior of MnF(n) begins with n = 4. These results are explained to be due to the way different halogen atoms interact with the 3d electrons of Mn atom.

19.
J Chem Phys ; 134(14): 144305, 2011 Apr 14.
Artigo em Inglês | MEDLINE | ID: mdl-21495753

RESUMO

Using density functional theory with generalized gradient approximation, we have performed a systematic study of the structure and properties of neutral and charged trioxides (MO(3)) and tetraoxides (MO(4)) of the 3d-metal atoms. The results of our calculations revealed a number of interesting features when moving along the 3d-metal series. (1) Geometrical configurations of the lowest total energy states of neutral and charged trioxides and tetraoxides are composed of oxo and∕or peroxo groups, except for CuO(3)(-) and ZnO(3)(-) which possess a superoxo group, CuO(4)(+) and ZnO(4)(+) which possess two superoxo groups, and CuO(3)(+), ZnO(3)(+), and ZnO(4)(-) which possess an ozonide group. While peroxo groups are found in the early and late transition metals, all oxygen atoms bind chemically to the metal atom in the middle of the series. (2) Attachment or detachment of an electron to∕from an oxide often leads to a change in the geometry. In some cases, two dissociatively attached oxygen atoms combine and form a peroxo group or a peroxo group transforms into a superoxo group and vice versa. (3) The adiabatic electron affinity of as many as two trioxides (VO(3) and CoO(3)) and four tetraoxides (TiO(4), CrO(4), MnO(4), and FeO(4)) are larger than the electron affinity of halogen atoms. All these oxides are hence superhalogens although only VO(3) and MnO(4) satisfy the general superhalogen formula.

20.
J Chem Phys ; 133(14): 144301, 2010 Oct 14.
Artigo em Inglês | MEDLINE | ID: mdl-20949994

RESUMO

A systematic density functional theory based study of the structure and spectroscopic properties of neutral and negatively charged MX(n) clusters formed by a transition metal atom M (M=Sc,Ti,V) and up to seven halogen atoms X (X=F,Cl,Br) has revealed a number of interesting features: (1) Halogen atoms are bound chemically to Sc, Ti, and V for n≤n(max), where the maximal valence n(max) equals to 3, 4, and 5 for Sc, Ti, and V, respectively. For n>n(max), two halogen atoms became dimerized in the neutral species, while dimerization begins at n=5, 6, and 7 for negatively charged clusters containing Sc, Ti, and V. (2) Magnetic moments of the transition metal atoms depend strongly on the number of halogen atoms in a cluster and the cluster charge. (3) The number of halogen atoms that can be attached to a metal atom exceeds the maximal formal valence of the metal atom. (4) The electron affinities of the neutral clusters abruptly rise at n=n(max), reaching values as high as 7 eV. The corresponding anions could be used in the synthesis of new salts, once appropriate counterions are identified.

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