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1.
L. I. Belen’kii I. D. Nesterov N. D. Chuvylkin 《Chemistry of Heterocyclic Compounds》2007,43(1):28-33
The relative stability of the isomeric σ-complexes formed in the sulfonation of pyrrole at the α-or β-position (the α-isomer
is energetically more favorable) does not agree with the experimentally established positional selectivity of substitution
(the formation of β-pyrrolesulfonic acid). However, quantum-chemical calculations of the energy parameters for the reaction
of pyrrole and SO3 with due regard to the solvation effect in the model solvent methylene chloride (ε = 8.93) lead to the conclusion that the
calculated activation energy of the rearrangement to the more favorable β-pyrrolesulfonic acid for the less favorable β-isomer
of the σ-complex is lower than on the path to the formation of the α-pyrrolesulfonic acid. It was shown that the significant
increase in the polarity of the model medium in the transition to DMSO (ε = 46.7) does not lead to substantial change in the
energy parameters of the reaction. The explanation for the positional selectivity during the sulfonation of pyrrole using
Py·SO3, according to previous data, involves the participation of the pyridine in the transformation of the σ-complexes into the
products. The calculations were made by the B3LYP/6-31G(d) and HF/3-21+G methods using the model of overlapping spheres to
take account of solvation.
Dedicated to E. J. Lukevics on his 70th birthday
__________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 34–40, January, 2007. 相似文献
2.
L. I. Belen’kii N. D. Chuvylkin I. D. Nesterov 《Chemistry of Heterocyclic Compounds》2012,48(2):241-257
Existing experimental data on positional selectivity in electrophilic substitution reactions of π-excessive heterocycles are classified. These data are discussed basing on the results of the authors' quantum-chemical calculations [RHF/6-31G(d), MP2/6-31G(d), and B3LYP/6-31G(d)] of the σ-complexes formed during attack of electrophiles such as H+, Me+, Me3Si+, Br+, NO2 +, MeCO+, and SO3 at the α- and β-positions of furan, thiophene, selenophene, pyrrole and its N-substituted derivatives, N-R-pyrroles (R = Me, t-Bu, SiMe3, Si(i-Pr)3, C6H4(p-NO2), SO2Ph, CHO, CO2Me), and the corresponding α- and β-substituted electrophilic substitution products. The differences in energies of the α-and β-isomers of the σ-complexes characterize the preferred direction of electrophilic attack, while the differences in the energies of the isomeric products make it possible to assess the energy preference of one of them. Analysis of the obtained data demonstrates the effects of the studied heterocycles' structure, the nature of the electrophile, and the thermal and steric factors on the positional selectivity (α/β ratio) in electrophilic substitution reactions of π-excessive five-membered heteroaromatic compounds. 相似文献
3.
E. Cengiz V. Aylıkcı N. Kaya G. Apaydın E. Tıraşoğlu 《Journal of Radioanalytical and Nuclear Chemistry》2008,278(1):89-96
In this study, the chemical effects on σKi
(i = α, β), σLα cross sections, Kβ/Kα X-ray intensity ratios and vacancy transfer probabilities from K to L (η
KL) for pure Nb and Nb compounds were investigated. The samples were excited by 59.5 keV γ-rays from 241Am and 5.96 keV photon energy from a 55Fe annular radioactive sources. K and L X-rays emitted by samples were counted by an Ultra-LEGe detector with a resolution
of 150 eV at 5.9 keV. While it was observed that the chemical bonding had an effect on the σKβ, σLα cross sections and Kβ/Kα X-ray intensity ratios for compounds, it was almost negligible for σKα cross section because Kα transitions (2P3/2,1/2→1S1/2) occurred in inner shells. It is well known that interactions between central element atom and ligands come into existence
in valence state, so outer energy levels are sensitive to the chemical environment. The experimental values of σKα cross section and η
KL are in good agreement with theoretically calculated and other experimental values of pure niobium, but the experimental values
of the σKβ, σLα cross sections and Kβ/Kα X-ray intensity ratios have differences for some compounds because valence electrons have different bond distances and binding
energies in different compounds. 相似文献
4.
D. V. Steglenko M. E. Kletsky S. V. Kurbatov V. I. Minkin R. Goumont F. Terrier 《Russian Chemical Bulletin》2009,58(8):1602-1608
The reaction mechanisms of cycloaddition of nitrobenzofuroxan to cyclopentadiene are studied using DFT calculations at the
B3LYP/6-31G* level in the gas phase and with account of the solvation effects. The reaction follows a three-step mechanism
and includes the formation of a prereaction donor-acceptor complex, endo [4+2] cycloaddition with inverse electron demands (IED), and [3,3]-sigmatropic rearrangement to the final endo [2+4] adduct. No formation of stable bipolar σ-complexes on the minimum-energy reaction pathways occurs. 相似文献
5.
The influence of the solvent effect, zero-point energy correction, and electron correlation on conclusions about positional
orientation of pyrrole sulfonation was studied by the B3LYP, HF, and MP2 methods with the 6-31G(d) and 6-31+G(d) basis sets.
The most important role is played by the solvation effects. Basis set extension effects and the inclusion of zero-point energy
correction contribute insignificantly. According to calculations using all the methods listed above, the formation of β-pyrrolesulfonic
acid in polar solvent is thermodynamically more favorable and less kinetically hindered. B3LYP calculations seem to be more
appropriate, because the method allows experimental data on positional orientation of the sulfonation of pyrroles to be interpreted
in a simpler and more reliable fashion.
Dedicated to Academician V. A. Tartakovsky on the occasion of his 75th birthday.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1425–1430, August, 2007. 相似文献
6.
The interactions of nicotinic acid with α-D-glucose and maltose, and with α-, β-, hydroxypropyl-α- and hydroxypropyl-β-cyclodextrins were studied by using solution calorimetry at T = 298.15 K and pH = 3.4. The thermodynamic parameters (log10 K, Δ G∘c, Δ H∘c and Δ S∘c) were calculated for the systems in which complex formation was observed. Systems with weak interparticle interactions lacking
complex formation were characterized by enthalpic virial coefficients calculated on the basis of the McMillan–Mayer theory.
It was found that the complexation affinity of α-cyclodextrin to nicotinic acid is stronger in comparison to β-cyclodextrin and the mono- and disaccharides. The influence of different factors, such as the availability of the macrocyclic
hydrophobic cavity, the relationship of the sizes of guest molecule to the host cavity, the presence of bulky hydroxypropyl
substitutes and their structure, and the solvation of guest molecules on the stability of complexes and their thermodynamic
parameters of interaction is discussed. 相似文献
7.
The aim of the present work was to provide arguments to the almost ‘hystorical’ problem of what β-tungsten is.
WO3was reduced in dry H2gas atmosphere in order to examine, whether β-tungsten formed in such a way contains oxygen as part of the lattice described
as WxO (e.g. W20O) or is a pure metallic phase of tungsten.
As a result of thermoanalytical measurements and of chemical analysis for oxygen, the assumption is supported that in the
600-800°C temperature range of metal formation not the WxO (β-W)→W(α-W) transformation but the β-W→α-W structural rearrangement of materials with identical chemical composition is
the most probable process.
The earlier opinion that the formation of the β-W structure requires the presence of oxygen atoms was not verified by our
results.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
8.
O. E. Ayala D. Lardizábal A. Reyes M. I. Rosales J. A. Matutes A. González Arias 《Journal of Thermal Analysis and Calorimetry》2000,59(3):943-949
Expressions for calculating the cation vacancy contents of MnZn ferrites from thermogravimetric curves are presented together
with some experimental data. In a single-phase MnZn ferrite synthesized by conventional ceramic procedures, the O2 evolution accompanying ferrite formation follows the formal equation.
Mn2+
σα Znσβ Fe3+
2σ(1–γ) [V ]σ/4(1–2γ) O4 =σ'/σ Mn2+
σ(α–2ϕ) Znσβ Fe2+
2σθ Mn3+
2σϕ Fe3+
2σ(1–γ–θ) [V ]σ/4(1–2γ–3ϕ) O4 +σ'φ/2O2 (g)
where α and β denote the MnO and ZnO mole fractions in the primary mixture γ=α+β, θ and ϕ depend on the quantities of Fe2+ and Mn3+ formed, respectively, φ=θ–ϕ and σ'/σ is a function of the former parameters. Even though the relative amounts of Fe2+ /Fe3+ and Mn2+ /Mn3+ remain uncertain, the vacancy content [V ] of the ferrite can be determined because it depends on φ alone, which is related to the change in mass of the sample as
the synthesis takes place through the equation
φ=(1.5–γ) μβ /μO2 (1–m
f /m
i )
Here, m
i and m
f are the masses of the sample before and after O2 evolution, μB is the formula mass of the ferrite and μO2 is the O2 molar mass. Practically vacancy-free single-phase MnZn ferrite samples were obtained by sintering in air at 1250°C and cooling
in pure N2 .
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
9.
The conformational stabilities of the α- and β-substituted enamines and vinyl ethers were predicted by orbital phase theory
and confirmed by ab initio molecular orbital calculations. Cyclic interaction significantly occurs among the nonbonding orbital
n
Y for the lone pair on the hetero atom Y (N in the enamines or O in the ethers), the π and π* orbitals of the CC bond, and the
σC-H or σ*C-X orbitals on the substituent CH2X. The cyclic -n
Y-π-σC-H-π*- interaction is favored by the orbital phase continuity in the α-substituted molecules, while the cyclic -n
Y-π-σ*C-X-π*- interaction is favored in the β-substituted molecules. The most stable conformation was then predicted to be synperiplanar
or (pseudo)equatorial in the α-substituted molecules and anticlinical or (pseudo)axial in the β-substituted molecules.
Received: 8 May 1998 / Accepted: 30 July 1998 / Published online: 16 November 1998 相似文献
10.
Carbon-carbon bond deformation curves for fluorinated ethylene molecules and the corresponding carbocations were calculated
by the ab initio self-consistent field method in the 5-31G basis set. The maximum force required for bond cleavage was taken
as a criterion for bond strength. It has been found that for ethylene, replacement of hydrogen by fluorine insignificantly
strengthens the C=C bonds in symmetric molecules. However, in molecules with an asymmetric arrangement of fluorine atoms,
the bond is slightly weakened due to different charges on the carbon atoms. The configuration of the corresponding carbocations
also depends on the bond polarity: an assymmetric distribution of electron density in the C=C bond region leads to the formation
of σ-complexes, while a symmetric distribution of electron density (pure covalent bonding) gives π-complexes. Since the carbon-carbon
bond in the σ-complexes is essentially weaker, one should expect significant weakening of the bond in high-acidity media if
the bond exhibits any kind of asymmetry (chain branching, defects, etc.). For the considered molecules, an antibatic correlation
has been established between the strength criterion Fmax (unlike the dissociation energy) and the bond length.
Institute of Physical Chemistry, Russian Academy of Sciences, Moscow. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 1, pp. 34–41, January–February, 1995.
Translated by I. Izvekova 相似文献
11.
The formation constants of mixed-ligand complexes with hetero donors of the type CuAB where A is a carboxylic acid such as
lactic acid or oxalic acid and B is an α−or β-amino acid are studied by polarography. The mixed-ligand stabilisation constant
clearly indicates the preferred formation of copper(II) mixedligand complexes with hetero donors over homo donors. The other
driving forces leading to the favoured formation of mixed-ligand complexes are also discussed. 相似文献
12.
L. I. Belen'kii G. P. Gromova B. V. Lichitskii M. M. Krayushkin 《Chemistry of Heterocyclic Compounds》1997,33(11):1276-1283
The interaction of 2,4-disubstituted thiophenes with AlCl3 and CCl4 or tert-BuCl proceeds through cationic σ-complexes corresponding to electrophilic substitution; the deprotonation of these
complexes to form 2,3,5-trisubstituted products is sterically hindered, since this stage (rearomatization) involves a change
in hybridization of the C(2) atom from sp3 to sp2, which leads to an increase of repulsion between the substituent R in position 3 and the bulky trichloromethyl or tert-butyl
group that has entered into position 2 and is coplanar with R. It has been concluded tentatively that in the case of 2,4-alkylthiophenes
that have even one hydrogen atom on the α-C atom of the substituent, deprotonation of the σ-complexes that are formed leads
to the corresponding 3-alkylidene-5-R-2-dichloromethylene-2,3-dihydrothiophenes or 2-alkylidene-4-R-5-dichloromethylene-2,
5-dihydrothiophenes, which then readily undergo oligomerization, as is also observed for such compounds under conditions of
trichloromethylation. The hydrogen chloride that is evolved forms a stable 2H-thiophenium ion with the unreacted 2,4-dialkylthiophene.
In the example of 2-tert-butyl-4-methylthiophene, formation of the stable 2,5-di-tert-butyl-3-methylthiophenium ion is observed;
upon treatment with water, this ion does not lose a proton, but rather a tert-butyl group, thus being converted to the original
2,4-dialkylthiophene.
Devoted to the bright memory of I. N. Goncharova.
N. D. Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, Moscow 117913. Translated from Khimiya Geterotsiklicheskikh
Soedinenii, No. 11, pp. 1477–1485, November, 1997. 相似文献
13.
I. V. Terekhova N. A. Obukhova A. V. Agafonov G. I. Kurochkina A. N. Syrtsev M. K. Gratchev 《Russian Chemical Bulletin》2005,54(8):1883-1886
The interaction of ascorbic acid with hydroxypropyl-α- and hydroxypropyl-β-cyclodextrins of different degree of substitution
was studied at 298.15 K and different pH using solution calorimetry. In an aqueous solution, only hydroxypropyl-β-cyclodextrins
form weak molecular complexes with the nonionized form of ascorbic acid. The thermodynamic functions of complex formation
and stability constants of the complexes were calculated. The systems with weak intermolecular interaction without complex
formation were characterized by enthalpic virial coefficients. On the basis of the obtained thermodynamic characteristics
it was shown that the selectivity of complex formation of hydroxypropyl-α- and hydroxypropyl-β-cyclodextrins with ascorbic
acid is determined by the size of the macrocyclic cavity, the presence of the hydroxypropyl substituent, and the medium acidity.
The degree of substitution of hydroxypropyl-β-cyclodextrins exerts no substantial effect on the thermodynamic parameters of
interaction with ascorbic acid.
__________
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1828–1831, August, 2005. 相似文献
14.
Seto Y. Umemoto K. Arii T. Masuda Y. 《Journal of Thermal Analysis and Calorimetry》2004,76(1):165-177
New solid complex of nitrilotriacetic acid and bismuth trichloride was synthesized by a solid phase reaction of nitrilotriacetic
acid and bismuth trichloride at room temperature. The composition of the sample is BiCl3[N(CH2COOH)3]2.5. The crystal structure of the complex belongs to triclinic system with the lattice parameters: α=0.7849 nm, β=0.9821 nm,
χ=2.0021 nm, α=96.50°, β=98.76° and γ=90.49°. The far-infrared spectra show the bonding between the Bi ion and N atom of nitrilotriacetic
acid. The thermal analysis also demonstrates the complex formation between the bismuth ion and nitrilotriacetic acid. The
gaseous pyrolysis product and the final residue in the thermal decomposition process are determined to check the thermal decomposition
reaction.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
15.
The effect of solvent on the efficiency of exchange-resonant energy transfer from d0-organometallic π-complexes to nonphosphorescent cyclopentadiene in glassy organic solvents at 77 K was studied. The radius
of nonradiative energy transfer depends on the solvation of metal complexes of this series. 相似文献
16.
L. I. Belen'kii T. G. Kim I. A. Suslov N. D. Chuvylkin 《Russian Chemical Bulletin》2005,54(4):853-863
Data on the relative reactivities (substrate selectivity) of five-membered heterocycles in electrophilic substitution reactions
and positional selectivity (α : β ratio) in these reactions were analyzed. Unlike the substrate selectivity (pyrrole ≫ furan
> selenophene > thiophene) determined by the position of heteroatoms in the Periodic Table, the positional selectivity decreases
in the order corresponding to the change in the relative stability of the onium states of the elements (O+ < Se+ ≤ S+ < N+) and reflects the predominant role of heteroatoms in the stabilization of σ complexes formed upon β-substitution. These differences
in the positional selectivity of the parent heterocycles have a substantial effect on the orientation in electrophilic substitution
reactions in their derivatives and the corresponding benzoannelated systems. This interpretation was confirmed by ab initio quantum chemical calculations (RHF/6–31G(d) and MP2/6– 31G(d)//RHF/6–31G(d)) and density functional theory calculations (B3LYP/6–31G(d)).
Quantum chemical calculations were performed by the above-mentioned methods for model N-R-pyrroles (R = Me, Et, Pri, But, CH=CH2, C≡CH, Ph, PhSO2, and 4-O2NC6H4) and their α- and β-protonated σ complexes. The results of these calculations demonstrated that it is the steric factors
and charges on the β-C, α-C, and N atoms and the substituents at the N atom (the kinetic control), as well as the nature of
the electrophile, rather than the difference in the relative stabilities of the onium states of N+ (which depends on the nature of the substituent at the N atom and reflects the role of the heteroatom in stabilization of
σ complexes formed via β-substitution; the thermodynamic control) that are responsible for the type of orientation (α or β) that prevails.
Dedicated to Academician V. I. Minkin on the occasion of his 70th birthday.
__________
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 837–846, April, 2005. 相似文献
17.
Sargaeva NP Lin C O'Connor PB 《Journal of the American Society for Mass Spectrometry》2011,22(3):480-491
Ion-electron reaction based fragmentation methods (ExD) in tandem mass spectrometry (MS), such as electron capture dissociation
(ECD) and electron transfer dissociation (ETD) represent a powerful tool for biological analysis. ExD methods have been used
to differentiate the presence of the isoaspartate (isoAsp) from the aspartate (Asp) in peptides and proteins. IsoAsp is a
β3-type amino acid that has an additional methylene group in the backbone, forming a Cα–Cβ bond within the polypeptide chain. Cleavage of this bond provides specific fragments that allow differentiation of the isomers.
The presence of a Cα–Cβ bond within the backbone is unique to β-amino acids, suggesting a similar application of ExD toward the analysis of peptides
containing other β-type amino acids. In the current study, ECD and ETD analysis of several β-amino acid containing peptides
was performed. It was found that N–Cβ and Cα–Cβ bond cleavages were rare, providing few c and z type fragments, which was attributed to the instability of the Cβ radical. Instead, the electron capture resulted primarily in the formation of a and y fragments, representing an alternative fragmentation pathway, likely initiated by the electron capture at a backbone amide
nitrogen protonation site within the β amino acid residues. 相似文献
18.
A synthesis of 15-benzoyloxybutyl-20-hydroxymethylpregn-16-enes, the intermediates in the synthesis of brassino- and ecdysteroids
modified in the D ring was performed starting with 2α,3α-isopropylidenedioxy-6,6-ethylenedyoxy-5α-androst-15-ene-17-one and
its 2β,3β-isomer through a sequence of reactions involving Michael addition, Wittig reaction and ene reaction. Structures
of the compounds were proved by the methods of two-dimensional NMR spectroscopy. 相似文献
19.
Varga J. Mudra I. Ehrenstein G. W. 《Journal of Thermal Analysis and Calorimetry》1999,56(3):1047-1057
Ca salts of suberic (Ca-Sub) and pimelic acid (Ca-Pim) were synthesized and used as β-nucleating agents in different grades
of isotactic polypropylene (IPP). Propylene homo-, random- and block-copolymers containing these additives crystallize principally
in pure β-modification as demonstrated in isothermal and non-isothermal crystallization experiments. Ca-Sub proved the most
effective β-nucleating agent known, so far. It broadens the upper crystallization temperature range of pure β-IPP formation
up to 140°C. The effect of the additives on the crystallization and melting characteristics of the polymers was studied. The
degree of crystallinity of the β-modification was found to be markedly higher than that of α-IPP. High temperature melting
peak broadening was first observed and discussed in literary results regarding the same phenomenon for α-IPP.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
20.
Alfréd Menyhárd Gábor Dora Zsuzsanna Horváth Gábor Faludi József Varga 《Journal of Thermal Analysis and Calorimetry》2012,108(2):613-620
Crystallization kinetics of β-nucleated isotactic polypropylene (β-iPP) under isothermal conditions were investigated by differential
scanning calorimetry. iPP was nucleated by a trisamide derivative, namely tris-2,3-dimethyl-hexylamide of trimesic acid (TATA).
In the presence of TATA possessing dual nucleating ability, the formation of the α- and β-form occurs simultaneously. An isothermal
stepwise crystallization method is suggested in this study, which can separate the crystallization process of β- and α-iPP
and consequently their crystallization kinetics can be evaluated separately. The results indicated that the mechanism of crystallization
changes in temperature especially in the vicinity of the upper critical temperature of the formation of the β-phase. In addition,
it was found that the ratio of the growth rates of β- and α-modification determines the characteristics of crystallization
and influences the apparent rate constant of crystallization of both polymorphs. 相似文献