共查询到20条相似文献,搜索用时 31 毫秒
1.
A. E. Kuramshina S. A. Bochkor V. V. Kuznetsov 《Russian Journal of Organic Chemistry》2009,45(4):496-499
Quantum-chemical study on the potential energy surface of 5-methyl-1,3-dioxane at the nonempirical RHF//STO-3G, RHF//3-21G, RHF//6-31G(d), RHF//6-31G(d,p), and MP2//6-31G(d,p) levels of theory revealed two energy-equivalent paths of conformational transformation of the equatorial and axial chair conformers. Potential barriers to these processes were estimated. The δG° value for the methyl substituent on C5 in 1,3-dioxane ring, determined on the basis of the experimental (NMR) and calculated vicinal 1H-1H coupling constants, was very consistent with published data 相似文献
2.
A. E. Kuramshina S. A. Bochkor V. V. Kuznetsov 《Russian Journal of General Chemistry》2009,79(4):787-790
The conformational isomerization of 2-oxo-1,3-dioxane and its methyl analogs was investigated by means of ab initio RHF//6-31G(d,p) and MP2//6-31G(d,p) quantum-chemical methods. It is shown that in comparison with 1,3-dioxanes the potential energy surface of the mentioned compounds has a fewer number of stationary points and includes two minima corresponding to conformers of sofa or distorted sofa configuration and one maximum corresponding to 2,5-twist form. The value of potential barrier of interconversion of cyclic carbonates is significantly lower than that of the analogously substituted 1,3-dioxanes. 相似文献
3.
A study of the potential energy surface of 4-phenyl-1,3-dioxane by non-empirical quantum chemical RHF/STO-3G and 6-31G(d) approximations reveals six energetically inequivalent pathways of conformational isomerization of equatorial and axial chair forms. 相似文献
4.
Using nonempirical quantum-chemical approximations RHF//STO-3G, 3-21G, 6-31G(d) and MP2//6-31G(d,p) a conformation isomerism of 2,4-dimethyl-1,3,2-dioxaborinane and its oxonium ions was studied. It was shown that the potential energy surface of the studied molecules has minima corresponding to equatorial (main minimum) and axial sofa forms and maxima corresponding to equatorial and axial conformations of 2,5-twist forms. Calculated values of the barriers of internal rotation of methyl group at the ring C4 atom were found. It was also established that the heat of protonation of the cyclic boric ester was smaller than of the non-boric analog, cis-2,4-dimethyl-1,3-dioxane, owing to decrease in basicity of the oxygen atoms in the cyclic boric ester due to partial double bond character of B-O bond. 相似文献
5.
V. V. Kuznetsov 《Russian Journal of Organic Chemistry》2010,46(11):1667-1669
It was shown by ab initio quantum-chemical approximations HF/6-31G(d) and MP2/6-31G(d)//HF/6-31G(d) that the conformational
isomerization of 1,3-dioxane and 1,3-dithiane proceeded along common routes. The potential energy surface of both compounds
contains six minima including the chair invertomers and enantiomeric flexible forms. They are separated by several potential
barriers. It was established by molecular dynamics method that the flexible conformers at heating and keeping at 295–300 K
transformed into each other and in the chair conformer. 相似文献
6.
Quantum-chemical study on the potential energy surface of 5-alkyl- and 5-phenyl-1,3-dioxanes at the RHF/6-31G(d) level of theory revealed two pathways for conformational isomerizations of the equatorial and axial chair conformers. Potential barriers to this process were estimated. The Gibbs conformational energies ΔG° of substituents at C5 in the 1,3-dioxane ring were determined on the basis of experimental (1H NMR) and theoretical vicinal coupling constants, which turned out to be consistent with published data. 相似文献
7.
A. E. Kuramshina S. A. Bochkor V. V. Kuznetsov 《Chemistry of Heterocyclic Compounds》2009,45(5):536-538
The whole possible course of conformational isomerization of 4-methyl-1,3-dioxane has been established using empirical (MM+)
and nonempirical [STO-3G, 3-21G, 6-31G(d) and 6-31G(d,p)] approximations within the limits of the Hartree-Fock method. It
was shown that the potential energy surface of this compound contains a principal (equatorial chair conformer) and local minima
corresponding to the axial chair conformer and series flexible forms. 相似文献
8.
9.
The completely optimized structure and harmonic force field of s-trans-buta-1,3-diene are reported at the MP2/6-31G and MP2/6-31G* levels of computation. Sets of empirical scale factors for the calculated force fields are derived and compared with the corresponding values computed at the RHF/4-31G and RHF/6-31G levels. Changes in the scale factors for this series of force fields are discussed. The vibrational frequencies are also reported for thirteen isotopomers of s-trans-buta-1,3-diene using the MP2/6-31G* force field. Some characteristics of the gauche and cis forms of buta-1,3-diene are also given. 相似文献
10.
É. R. Latypova R. Ya. Kharisov I. V. Vakulin R. F. Talipov G. Yu. Ishmuratov 《Journal of Structural Chemistry》2007,48(1):46-50
Using RHF/3-21G, RHF/6-31G(d, p), MP2/6-31G(d, p), B3LYP/6-31G(fd, p) approximations the structure and 13C NMR spectra of 2-alkylsubstituted cyclohexene-2-ones and 2-alkylacroleins are studied and calculated. In the series of 2-alkylcyclohexene-2-ones the effect of the substituent on a deviation from coplanarity of the C=C-C=O fragment is more expressed in comparison with 2-alkylacroleins. This deviation (5°) is not enough to explain the observed properties of 2-alkylcyclohexene-2-ones due to disturbed conjugation. The particular behavior of (R)-4-mentenone in reactions of 1,4-addition and ozonolysis is explained by a more expressed +I-effect of the alkyl substituent in α-position. 相似文献
11.
O. Yu. Valiakhmetova S. A. Bochkor V. V. Kuznetsov 《Journal of Structural Chemistry》2010,51(3):573-576
With the use of RHF/6-31G(d) and MP2/6-31G(d)//RHF/6-31G(d) semi-empirical (AMI) and non-empirical quantum chemical approximations conformational isomerization of methylboronic acid and its methyl, isopropyl, tert-butyl, and phenyl esters is studied. With the exception of the tert-butyl analogue, the potential energy surface of molecules in these compounds, is shown to contain three minima that correspond to planar and near-planar conformers: cis-cis, trans-trans, and cis-trans, with the latter being the main one. The minima are separated by two conformational isomerization barriers corresponding to the orthogonal arrangement of one of the OH(OR) groups. 相似文献
12.
G. S. Khaibullina S. A. Bochkor V. V. Kuznetsov 《Russian Journal of Organic Chemistry》2014,50(5):725-728
Study of conformational transformations of 5,5-dinitro-1,3-dioxane by the method HF/6-31G(d), and also hybrid DFT PBE/3z and RI-MP2/λ2 indicated the only inversion route chair-chair through an intermediate minimum corresponding to the 2,5-twist-form. The potential barrier to this process was estimated and also the features of the intramolecular rotation of the axial and equatorial nitro groups in the chair conformer were investigated. 相似文献
13.
O. Yu. Valiakhmetova S. A. Bochkor V. V. Kuznetsov 《Chemistry of Heterocyclic Compounds》2010,46(8):1006-1010
Computer modeling using the quantum-chemical empirical MM+ method and nonempirical RHF//6-31G(d), MP2//6-31G(d), and, in individual
cases, RHF//3-21G methods was employed to study the potential energy surface of 2,4-dialkyl-1,3,2-dioxaborinanes. The optimal
geometry and 1H NMR spectral data gave values for ∆G
0 of the methyl and hexyl substituents at C-4 of the heterocyclic ring equal to 0.6 and 1.6 kcal/mol, respectively. 相似文献
14.
V. P. Feshin E. V. Feshina L. I. Zhizhina 《Russian Journal of General Chemistry》2007,77(8):1359-1362
Calculations of two forms of 3,4-dimethylanisole molecule with full geometry optimization are performed by AM1, RHF/6-31G(d), and MP2/6-31G(d) methods. The form with cis location of H3C-O and C1–C6 bonds is somewhat more preferable energetically than that with the trans location. By the RHF/6-31G(d) method, this molecule is also calculated with fixed rotation angles of methoxy group around the C-O bond. It is shown that, in the first form, the negative charge on the C6 atom is considerably smaller than on C2, while in the second form the charges are practically equal, which does not confirm the concerted effects of steric and electronic factors in reactions of 3,4-dimethylanisole with methyl isocyanate and isobutyraldehyde. The charge difference is due to the effect of methyl group in the 3-position of the aromatic ring and to the orientation of the methoxy group toward geminal C-C bonds of the ring. 相似文献
15.
近十几年来,硅烯(:SN小'一到作为活性中间体的研究引起化学界的广泛兴趣,形成了内容丰富的硅烯化学.1975年Ilass*等研究了硅烯和乙炔的加成反应,指出该反应的中间体为硅杂环丙烯,硅杂环丙烯异构化为硅甲基乙炔.Boatz问等利用:j-ZIG(d)基组对金属杂环丙烯小[*xZC 相似文献
16.
David H. Setiadi Gregory A. Chass Ladislaus L. Torday Andras Varro Julius G. Papp 《Journal of Molecular Structure》2002,594(3):7080-172
Tetralin, chroman as well as its' S and Se containing congeners were subjected to ab initio (RHF/3-21G and RHF/6-31G(d)) and DFT (B3LYP/6-31G(d)) computation. Molecular geometries and the activation energies for ring inversions were determined with full geometry optimizations. 相似文献
17.
L. I. Belen'kii N. D. Chuvylkin A. I. Serykh I. A. Suslov 《Russian Journal of Organic Chemistry》2005,41(9):1337-1340
Positional selectivity (α:β ratio) of electrophilic substitution in pyrrole, N-methylpyrrole, and N-tert-butylpyrrole was analyzed by ab initio [RHF/6-31G(d), MP2/6-31G(d)//RHF/6-31G(d)] and DFT [B3LYP/6-31G(d)] calculations of some σ-complexes derived from the substrates. The results of calculations with the use as model electrophilic species of trimethylsilyl cation [MP2/6-31G(d)//RHF/6-31G(d) and B3LYP/6-31G(d)] and SO3 molecule [B3LYP/6-31G(d)] instead of proton are fairly consistent with the experimental data, according to which trimethylsilylation of pyrrole and its N-substituted derivatives with trimethylsilyl trifluoromethanesulfonate, as well as sulfonation with pyridine-sulfur trioxide complex, gives the corresponding β-substituted products. 相似文献
18.
《Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy》1994,50(3):609-619
The conformational stability, barriers to internal rotation and vibrational frequencies of trivinylborane have been determined from the vibrational spectra and ab initio calculations. The ab initio calculations have been carried out utilizing the RHF/3-21G, RHF/6-31G*, and MP2/6-31G* basis sets and support the vibrational data that there are two stable conformations in the fluid phases separated by a relatively small energy difference. One of the conformations is a near-planar form which has the three vinyl groups twisted out of the BC3 plane and belongs to the C3 point group. The other conformer has a non-planar structure and belongs to the C1 point group. These and other calculated results are compared to the corresponding quantities obtained from the experiment. 相似文献
19.
V. V. Kuznetsov 《Russian Journal of General Chemistry》2012,82(5):869-873
Computer simulation of pathways of conformational isomerization of 1,3-oxathiane molecule carried out with the help of HF/6-31G(d), MP2.6-31G(d)/HF/6-31G(d), and PNE/3z quantum-chemical approximations showed that interconversion between the degenerate in energy chair conformers proceeds through seven independent pathways: directly and via six flexible forms. Potential energy surface contains eight minimum points including chair conformers and enantiomeric pairs of twist forms, and also five transition states, among them different modification of semi-chair, symmetric and unmmetrical boat. Molecular dynamics methods show that flexible forms at room temperature convert into one another and into the chair conformers. 相似文献
20.
Bombasaro JA Zamora MA Baldoni HA Enriz RD 《The journal of physical chemistry. A》2005,109(5):874-884
The full conformational space of N-acetyl-l-cysteine-N-methylamide was explored by ab initio (RHF/ 6-31G(d)) and DFT (B3LYP/6-31G(d)) computations. Multidimensional conformational analysis predicts 81 structures in N-acetyl-l-cysteine-N-methylamide, but only 47 relaxed structures were previously determined at the RHF/3-21G level of theory. These structures were now optimized using RHF/6-31G(d) and B3LYP/6-31G(d) approaches. Seven conformational migrations were observed when recalculated at higher level of theory. Besides these major changes, only smaller conformational shifts were operative for the remaining stationary points. The exploration of the whole conformational space of N-acetyl-l-cysteine-N-methylamide, including the transition-state structures allowing the conformational interconversion among the low-energy forms, was analyzed in this study. Our results offer new insights into the influence of polar side chains on the conformational preferences of peptide structures. 相似文献