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1.
Density functional theory has been used to investigate the structure, stability, and aromaticity of a series of nitrogen-containing
thiocarbons, which are related to C
n
S
n
2− (n = 3–5). We have identified a large number of minimum-energy species which might be synthesized and calculated their aromaticity
using the nucleus-independent chemical shift (NICS) method of Schleyer and magnetic susceptibility. Successive substitution
of carbon by nitrogen reduces their stability, as reflected in the calculated bond orders. In general, there is no close correlation
between the stability and π-aromaticity of the species studied. 相似文献
2.
Correlations were studied between the properties of Bader's ring and bond critical points calculated for variously substituted
malonaldehyde (containing intramolecular H-bond) and its π-electron delocalisation expressed by aromaticity indices HOMA and NICSs. It was observed that π-electron delocalisation of the system strongly depends on the substituent and its position. Replacement of the H+ with Li+ in malonaldehyde allowed us to study the role of unoccupied 2p orbital in π-electron delocalisation. In the case of lithium system the aromaticity is increased as compared with the malonaldehyde itself
and moreover with the malonaldehyde anion. This proves that the unoccupied 2p orbital of Li+ may play a significant role in delocalisation of π-electrons due to its low orbital energy. In the case of lithium system the substituent effect is negligible, which resembles
the situation in benzene derivatives.
Dedicated to Prof. Karl Jug on the occasion of his 65th birthday anniversary. 相似文献
3.
The boron rings containing planar octacoordinate transition metals, D
8h
FeB8
2−, CoB8
− and CoB8
3+, C
2v
FeB8, D
2h
CoB8
+ and CoB8, are optimized with all real vibrational frequencies at the B3LYP/6–311+G* level of the theory. The D
8h
FeB8
2− and CoB8
− isomers are global minima, while D
8h
CoB8
3+ is only local minimum. The electronic structure character of these systems is revealed by natural bond orbital (NBO) analysis,
showing that the boron rings containing planar octacoordinate transition metals have stability and aromaticity with six π
electrons. The aromaticity is confirmed by nucleus independent chemical shifts (NICS) calculations.
Supported by the specialized research fund for the doctoral program of higher education (20060007030) 相似文献
4.
The antioxidant activity of heterocyclic thioamides based on imidazole, triazole, tetrazole, thiazole, thiazoline, and thiadiazole
is estimated by spectrophotometry using the rate constant of reaction with the chromogenic radical 2,2′-diphenyl-1-picrylhydrazyl.
The rate constant of direct transfer of a hydrogen atom to the radical in carbon tetrachloride is maximal for 1-methylimidazoline-2-thione.
The protective antioxidant effect of the preparations in ethanol falls down abruptly from 4-phenylthiazoline-2-thione (k = 1.06 × 104 M−1min−1) to the thioamides based on triazole and thiazoline (k ∼ 20 M−1min−1). In acetonitrile, thiazole derivatives show the most substantial antioxidant activity, k = n ×104 M−1min−1, which goes down to zero if the aromaticity of the heterocycle is broken. As established, for the pseudo first order reaction
between thioamides and the chromogenic radical, the reaction rate linearly depends on the concentration of thioamides. A spectrophotometry
kinetic method for the quantification of heteroaromatic thioamides is elaborated. 相似文献
5.
Theoretical observation of hexaatomic molecules containing pentacoordinate planar carbon 总被引:1,自引:1,他引:0
Qiong Luo 《中国科学B辑(英文版)》2008,51(11):1030-1035
The smallest molecules up to date containing a D5h pentacoordinate planar carbon (PPC) atom, CBe5 and CBe54-, are presented by means of ab initio calculations. To gain a better understanding about which electronic factors contribute to their stabilization, natural bond orbital (NBO) analysis and the nucleus independent chemical shifts (NICS) were calculated. The data reported here suggest that D5h CBe5 is σ aromaticity in nature, while in D5h CBe54- π aromaticity is dominating. The classical octet rule is well satisfied in both molecules, and is one of the fundamental reasons to understand the stability of the pentagon structures. The Be5 ring serves as σ donor in D5h CBe5, and π-acceptor in D5h CBe54-. The D5h CBe54- possessing 18 valence electrons with a closed-shell electron configuration is the most plau-sible candidate for experimental detection. 相似文献
6.
Yuri B. Vysotsky Valery A. Kuzmitsky Konstantin N. Solovyov 《Theoretical chemistry accounts》1981,59(5):467-485
The coupled variant of double-parameter perturbation theory in the MO LCAO SCF method in the London approximation has been
used for the calculation of π-electron current distributions in the molecules of porphin and its derivatives. The chemical
shifts of1H-NMR have been computed on the basis of calculations of ring currents and charge distributions. It is shown that π-electron
ring currents are responsible for the dominant contribution to the shielding of protons. The theoretical and experimental
values of proton chemical shifts are in a good agreement. Chemical shifts of the13C and15N nuclei have also been estimated. Two aromaticity scales are proposed for the compounds under study based on the calculations
of the π-electron contribution to the diamagnetic susceptibility and of π-electron currents, respectively. 相似文献
7.
Fernando Blanco Daniel H. O’ Donovan Ibon Alkorta José Elguero 《Structural chemistry》2008,19(2):339-352
A theoretical study of the monosubstitution effects of all atoms of the second and third rows of the periodic table on the
α, β and γ positions of neutral and protonated pyridine has been carried out by means of B3LYP/6-31 + G(d,p) DFT calculations. The geometric
and electronic properties, calculated using the Atoms in Molecules methodology, and the electrostatic potential have been
analysed. Concurrently, three separate aromaticity indexes (NICS(0), NICS(1) and HOMA) have been evaluated and compared to
the above results. Furthermore, the effect of protonation on these parameters has been investigated. A comparison with analogous
results for benzene derivatives has also been carried out. 相似文献
8.
p-Polyphenyl-α,ω-quinododimethides were predicted to be moderately aromatic and diatropic, although they exist only in a single
classical resonance structure with no aromatic conjugated circuits. Such a dichotomy was resolved using our graph theory of
aromaticity and ring-current diamagnetism. Six-site non-conjugated circuits were found to contribute appreciably to aromaticity
and ring-current diamagnetism. Within each quinododimethide molecule, local aromaticity increases on going from the outermost
to inner phenylene rings. 相似文献
9.
Clusters Al2P2
n− (n = 1–4) were theoretically investigated using density functional theory (DFT) methods at the B3LYP/6-311+G* and B3PW91/6-311+G*
levels of theory. The calculated results showed that the planar structure (D
2h
symmetry) of Al2P2
n− (n = 1–4) species was the global minimum. And the negative nucleus-independent chemical shift (NICS) value of Al2P2
n− (n = 1–4) species indicated the existence of a ring current in the planar structure (D
2h
symmetry). A detailed molecular orbital (MO) analysis revealed that the planar structures (D
2h
symmetry) had π aromaticity, which further exhibited the strongly aromatic character for Al2P2
n− (n = 1–4) species. 相似文献
10.
Boron rings containing planar octa-and enneacoordinate cobalt,iron and nickel metal elements 总被引:2,自引:2,他引:0
Qiong Luo 《中国科学B辑(英文版)》2008,51(7):607-613
For a series of boron rings with planar hyper-coordinate 8th group transition metal atoms, singlet 1FeB8-2, multiplet kFeB9n (n = -1, k = 1; n = 0, k = 2), singlet 1CoB8n(n = -1, 1, 3), multiplet kCoB9n (n = 1, k = 2; n = 0, k = 1) and singlet 1NiB9 , the geometry structures have been optimized to be local minima on corresponding potential hyper-surfaces. The electron structures are discussed by orbital analysis and the aromaticity is predicted by nucleus-independent chemical shifts calculation at both the B3LYP/6-311 G* and BP86/6-311 G* levels of theory, respectively. The results suggest that all these structures with high symmetry planar geometries are stable and have aromatic properties with six π valence electrons. 相似文献
11.
Violeta Yeguas Gloria Inés Cárdenas-Jirón María Isabel Menéndez Ramón López 《Journal of mathematical chemistry》2010,48(1):137-144
The B3LYP density functional theory methodology in conjunction with the 6-31G(d,p) basis set has been used to characterize
triply N-confused meso-tetraphenylporphyrins. According to our computations, there is no a direct correlation between stability and aromaticity
as already found for non-substituted confused porphyrins. The inclusion of these substituents in the calculations provokes
a decrease of the planarity and aromaticity of these macrocycles along with a notable rise of their relative stability with
respect to the non-substituted case. Steric repulsions, both among phenyl rings and β atoms in the pyrrolic rings, and among H atoms in the core of the macrocycles, dominate over aromaticity in the establishment
of the most stable conformation of each isomer. 相似文献
12.
Sandeep Nigam Chiranjib Majumder S. K. Kulshreshtha 《Journal of Chemical Sciences》2006,118(6):575-578
Ground state geometry and electronic structure of M
4
2-
cluster (M = B, Al, Ga) have been investigated to evaluate their aromatic properties. The calculations are performed by employing
the Density Functional Theory (DFT) method. It is found that all these three clusters adopt square planar configuration. Results
reveal that square planar M
4
2-
dianion exhibits characteristics of multifold aromaticity with two delocalised π-electrons. In spite of the unstable nature
of these dianionic clusters in the gas phase, their interaction with the sodium atoms forms very stable dipyramidal M4Na2 complexes while maintaining their square planar structure and aromaticity. 相似文献
13.
DFT (B3LYP/6-31G*) and G3//B3LYP (usually referred as G3B3 in the literature) calculations have been carried out on annular tautomers of C- and/or N-functionalized imidazoles (both the aromatic 1H- and nonaromatic 2H-) and pyrazoles (the aromatic 1H- and 2H-, and nonaromatic 3H- and 4H-). The aromaticity of 1H-imidazole and 1H- and 2H-pyrazole results in these species being more stable than their nonaromatic tautomers. However, this stability is reversed when the hydrogen on the azole nitrogen or methylene carbon is substituted by OH or by F, e.g., in increasing order of stability we find 1-fluoro-1H-imidazole <2-fluoro-2H-imidazole (<2-fluoro-1H-imidazole). These results are related to a recent report of a highly substituted imidazole that exists in the “nonaromatic” 2H-tautomeric form, and discussed subsequently in a purely thermochemical context. 相似文献
14.
The structure and aromatic properties of Rees hydrocarbons 7bH-cyclopent[cd]indene and its benzo-annelated derivative 1a and 2a, respectively, are examined by the B3LYP/6−31+G(d) calculations employing HOMA criterion of Krygowski and coworkers. It is
shown that 1a possesses strong π-electron delocalization over the perimeter of the CC bonds, thus forming a quasi-[10]annulene pattern.
Its aromatic character is determined to be 83%. In contrast, 2a is less convenient model system for [14]annulene. The reason behind is that the perimeter network of the potentially aromatic
14π-electrons is supplemented by two additional more local aromatic patterns involving 10π and 6π electrons. Consequently,
the π-electron delocalization over the molecular rim is incomplete being thus diminished. The aromatic character of the peripheral
bonds in both 1a
−
and 2a
−
anions formed upon deprotonation of the central C–H bond is decreased, since the role of the smaller rings in forming aromatic
subsystems is increasing. Finally, polycyano substitution of 1a and 2a decreases aromaticity due to the price paid for the resonance effect taking place between the carbocyclic π-network and the
double bonds of the CN groups. The resonance effect is particularly strong in anions derived by heterolytic cleavage of the
C–H bond emanating from the central sp
3 carbon atom.
Dedicated to Professor T. M. Krygowski for his outstanding scientific achievements on the occasion of his 70th birthday. 相似文献
15.
Erich Kleinpeter Andreas Koch Bagrat A. Shainyan 《Journal of Molecular Structure》2008,863(1-3):117-122
The spatial magnetic properties (Through Space NMR Shieldings – TSNMRS) of two cyclobutadiene derivatives (2 and 5) and of a number of cyclobutadiene dianion derivatives (3, 4 and 6–8) have been calculated by the GIAO perturbation method employing the Nucleus-Independent Chemical Shift (NICS) concept of P. v. Ragué Schleyer, and visualized as Iso-Chemical-Shielding Surfaces (ICSS) of various size and direction. TSNMRS values can be successfully employed to quantify and visualize the (anti)aromaticity of the compounds studied and to discuss the influence of Li+ complexation to cyclobutadiene dianion (4a, 7 and 8) on planar 4c,6e or three-dimensional 6c,6e aromaticity. 相似文献
16.
Hoda Pasdar Reza Ghiasi Shahram Ajoudani 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2012,86(3):402-407
The structure and properties of AlN-polycycles were studied by DFT (density functional theory) method. The results of calculations
were obtained at B3LYP/6-311G(d, p) level on model species. Topological parameters such as electron density, its Laplacian, kinetic electron energy density,
potential electron energy density, and total electron energy density at the ring critical points (RCP) from Bader’s ‘Atoms
in molecules’ (AIM) theory were analyzed in detail. These results indicate a good correlation between ρ(3, +1), G(r), H(r), and V(r) averaged values and hardness of AlN-polycycles. The aromaticity of all molecules has been studied by nucleus-independent
chemical shift. There is a linear correlation between ΣNICS(0.0)molecule values and polarizability. 相似文献
17.
Lixin Zhou 《Theoretical chemistry accounts》2000,105(1):86-92
Results of ab initio self-consistent-field (SCF) and density functional theory (DFT) calculations of the gas-phase structure,
acidity (free energy of deprotonation, ΔGo), and aromaticity of 1,2-diseleno-3,4-dithiosquaric acid (3,4-dithiohydroxy-3-cyclobutene-1,2-diselenone, H2C4Se2S2) are reported. The global minimum found on the potential energy surface of 1,2-diseleno-3,4-dithiosquaric acid presents a
planar conformation. The ZZ isomer was found to have the lowest energy among the three planar conformers and the ZZ and ZE
isomers are very close in energy. The optimized geometric parameters exhibit a bond length equalization relative to reference
compounds, cyclobutanediselenone, and cyclobutenedithiol. The computed aromatic stabilization energy (ASE) by homodesmotic
reaction (Eq 1) is −20.1 kcal/mol (MP2(fu)/6-311+G** //RHF/6-311+G**) and −14.9 kcal/mol (B3LYP//6-311+G**//B3LYP/6-311+G**).
The aromaticity of 1,2-diseleno-3,4-dithiosquaric acid is indicated by the calculated diamagnetic susceptibility exaltation
(Λ) −17.91 (CSGT(IGAIM)-RHF/6-311+G**//RHF/6-311+G**) and −31.01 (CSGT(IGAIM)-B3LYP/6-311+G**//B3LYP/6-311+G**). Thus, 1,2-diseleno-3,4-dithiosquaric
acid fulfils the geometric, energetic and magnetic criteria of aromaticity. The calculated theoretical gas-phase acidity is
ΔGo
1(298K)=302.7 kcal/mol and ΔGo
2(298K)=388.4 kcal/mol. Hence, 1,2-diseleno-3,4-dithiosquaric acid is a stronger acid than squaric acid(3,4-dihydroxy-3-cyclobutene-1,2-dione,
H2C4O4).
Received: 11 April 2000 / Accepted: 7 July 2000 / Published online: 27 September 2000 相似文献
18.
Thermodynamic parameters for acid dissociation of the conjugate acids of poly(N-vinylimidazole) and polyallylamine have been determined in the presence of sodium chloride and sodium nitrate. Even though
the plots of ΔG
0 against the degree of dissociation, α, are highly dependent on the added salt concentration levels, the concentration effect
has never been observed for the corresponding ΔH
0 versus α plots. The effect on the ΔG
0 versus α plots has been attributed to the entropy change of the counterions between a polyelectrolyte phase and a bulk solution
phase. The α dependency of ΔH
0 is affected remarkably by the kinds of cationic polymers and counter-anions. Each ΔH
0 value at completely neutralized conditions is quite close to the corresponding ΔH
0 value of the monomer analog. The difference in the ΔH
0 values at fully charged conditions has been explained by the heats due to
The ion-pair formation of chloride anion to the conjugate acids of poly(N-vinylimidazole) and polyallylamine has been supported by 35Cl NMR measurement. It has also been suggested that chloride anions bind the basic polymer molecules even at fully neutralized
conditions.
Received: 2 June 1999/Accepted in revised form: 19 July 1999 相似文献
19.
Boris N. Afanasyev Yulia P. Akulova Maria M. Kotlyar 《Journal of Solid State Electrochemistry》1997,1(1):68-76
The function Δ(ΔG
A
0), which is the difference of Gibbs energies characterizing surface-active substance (surfactant, SAS) adsorption at metal/solution
and air/solution surfaces, has been introduced. The equation connecting the function Δ(ΔG
A
0) with SAS ionization potential has been obtained using the elementary theory of donor-acceptor interactions. Published experimental
data on SAS adsorption at mercury, bismuth and gold have been used for Δ(ΔG
A
0) calculation. The dependence of Δ(ΔG
A
0) on ionization potentials can be described by an equation derived in this work. It has been demonstrated that the value of
the hydrophilicity of gold is much higher than the values for mercury and bismuth. The lifetime of SAS molecules at a metal
surface has been estimated. The question of the possibility of theoretica
l estimation of standard energies ΔG
A
0 characterizing SAS adsorption at a metal/solution surface has been discussed.
Received: 9 December 1996 / Accepted: 13 January 1997 相似文献
20.
E. J. Padma Malar 《Theoretical chemistry accounts》2005,114(1-3):213-221
Structure and bonding in triple-decker cationic complexes [(η5-Cp)Fe(μ,η:η5-E5) Fe(η5-Cp)]+ (1: E = CH, 2: E = P, 3: E = As) and [(η5-Cp)Fe(μ,η:η5-Cp)Fe(η5-E5)]+ (E = P, As) are examined by density functional theory (DFT) calculations at the B3LYP/6-31+G* level. These species exhibit
the lowest energy when all the three ligands are eclipsed. In the complexes with bifacially coordinated cyclo-E5, the perfectly eclipsed D5h sandwich structure a is found to be a potential minimum. The energy difference between the fully eclipsed and the staggered conformations b and c are within 1.0, 2.1, and 6.3 kcal/mol, respectively, for E = CH, P, and As. The isomeric species with monofacially coordinated
cyclo-E5 (E =P, As), [(η5 -Cp)Fe(μ,η :η5-Cp)Fe(η5-E5)]+ are predicted to be about 30 and 60 kcal/mol higher in energy , respectively, for E = P and As. The calculations predict
that the bifacially coordinated cyclo-E5 (E =P, As) undergoes significant ring expansion leading to ``loosening of bonds' as observed experimentally. The consequent
loss of aromaticity in the central cyclo-E5 indicates that significant π-electron density from the ring can be directed towards bonding with the iron centers on both sides. The diffuse nature of
the π-orbitals of cyclo-P5 and cyclo-As5 can lead to better overlap with the iron d-orbitals and result in stronger bonding. This is reflected in the bond order values
of 0.377 and 0.372 for the Fe-P and Fe-As bonds in 2a and 3a, respectively. The natural population analysis reveals that the Fe atom that is coordinated to a cyclo-E5 (E = P, As) possesses a negative charge of −0.23 to −0.38 units due to transfer of electron density from the inorganic ring
to the metal center. 相似文献