共查询到17条相似文献,搜索用时 9 毫秒
1.
N. D. Chuvylkin A. M. Tokmachev A. V. Fionov E. V. Lunina 《Russian Chemical Bulletin》1997,46(10):1649-1662
The results of ESR-spectroscopic and quantum-chemical investigations of the coordination of 2,2,6,6-tetramethylpiperidineN-oxyl, 2,2,3,4,5,5-hexamethyl-3-imidazolidineN-oxyl, 2,2,4,5,5-pentamethyl-3-imidazolineN-oxyl, 2,4,5,5-tetramethyl-2-phenyl-3-imidazolineN-oxyl, and 2,4,5,5-tetramethyl-2-octyl-3-imidazolineN-oxyl to Lewis acid sites (LAS) on alumina surface are described systematically and analyzed. The cluster models of LAS accepted
in radiospectroscopy and based on experimental data on g-factors and hyperfine coupling constants with N and Al nuclei in
the corresponding donor-acceptor complexes are discussed. Within the framework of the unrestricted Hartree—Fock (UHF) method
using the STO-3G, STO-6G, 3–21G, and 6–31G basis sets and also in terms of semiempirical MNDO, AM1, and PM3 procedures, comparative
quantum-chemical analysis of the structural, spin, electrostatic, energy, and radiospectroscopic characteristics of the coordination
of the model cluster LAS to the simplest representative of nitroxides is performed. Three illustrative types of structures
of the resulting surface complex are considered. A semiquantitative interpretation of the whole set of features found experimentally
for the coordination of nitroxide probes to the surface LAS on alumina is given.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1743–1756, October, 1997. 相似文献
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The conformations of 2-cyclopenten-1-ol (2CPOL) have been investigated by high-level theoretical computations and infrared spectroscopy. The six conformational minima correspond to specific values of the ring-puckering and OH internal rotation coordinates. The conformation with the lowest energy possesses intramolecular π-type hydrogen bonding. A second conformer with weaker hydrogen bonding has somewhat higher energy. Ab initio coupled-cluster theory with single and double excitations (CCSD) was used with the cc-pVTZ (triple-ζ) basis set to calculate the two-dimensional potential energy surface (PES) governing the conformational dynamics along the ring-puckering and internal rotation coordinates. The two conformers with the hydrogen bonding lie about 300 cm−1 (0.8 kcal/mole) lower in energy than the other four conformers. The lowest energy conformation has a calculated distance of 2.68 Å from the hydrogen atom on the OH group to the middle of the C=C double bond. For the other conformers, this distance is at least 0.3 Å longer. The infrared spectrum in the O-H stretching region agrees well with the predicted frequency differences between the conformers and shows the conformers with the hydrogen bonding to have the lowest values. The infrared spectra in other regions arise mostly from the two hydrogen-bonded species. 相似文献
5.
The complex doublet potential energy surface of the CH(2)NO(2) system is investigated at the B3LYP/6-31G(d,p) and QCISD(T)/6-311G(d,p) (single-point) levels to explore the possible reaction mechanism of the triplet CH(2) radical with NO(2). Forty minimum isomers and 92 transition states are located. For the most relevant reaction pathways, the high-level QCISD(T)/6-311 + G(2df,2p) calculations are performed at the B3LYP/6-31G(d,p) geometries to accurately determine the energetics. It is found that the top attack of the (3)CH(2) radical at the N-atom of NO(2) first forms the branched open-chain H(2)CNO(2) a with no barrier followed by ring closure to give the three-membered ring isomer cC(H(2))ON-O b that will almost barrierlessly dissociate to product P(1) H(2)CO + NO. The lesser followed competitive channel is the 1,3-H-shift of a to isomer HCN(O)OH c, which will take subsequent cis-trans conversion and dissociation to P(2) OH + HCNO. The direct O-extrusion of a to product P(3) (3)O + H(2)CNO is even much less feasible. Because the intermediates and transition states involved in the above three channels are all lower than the reactants in energy, the title reaction is expected to be rapid, as is consistent with the measured large rate constant at room temperature. Formation of the other very low-lying dissociation products such as NH(2) + CO(2), OH + HNCO and H(2)O + NCO seems unlikely due to kinetic hindrance. Moreover, the (3)CH(2) attack at the end-O of NO(2) is a barrier-consumed process, and thus may only be of significance at very high temperatures. The reaction of the singlet CH(2) with NO(2) is also briefly discussed. Our calculated results may assist in future laboratory identification of the products of the title reaction. 相似文献
6.
The radical-molecule reaction mechanism of (1)CHCl with NO(2) has been explored theoretically at the B3LYP/6-311G(d, p) and CCSD(T)/6-311G(d, p) (single-point) levels of theory. Thirteen minimum isomers and 29 transition states are located. The initial association between (1)CHCl and NO(2) proceeds most likely through the carbon-to-middle-nitrogen attack leading to an energy-rich adduct a (HClCNO(2)), which is found to be a barrierless process. Staring from a, the most feasible channel is to undergo a concerted O-shift and C--N bond rupture leading to product P(2) (NO + HClCO). The minor product pathways are the direct O-extrusion of a to P(3) (O + HClCNO-cis) as well as the 1,3-H-shift of a to isomer b (ClCNOOH) followed by a concerted OH-shift leading to d (HOClCNO), which will dissociate to product P(8) (NO + ClCOH) via C--N cleavage. Because the transition states and isomers involved in the most feasible channel all lie below the reactants, the title reaction is expected to be rapid, as is consistent with the measured rate constant at 296 K. The comparison with the analogous reactions (3)CH(2) + NO(2) are discussed. The present study may be useful for further experimental investigation of the title reaction. 相似文献
7.
N. D. Chuvylkin A. M. Tokmachev A. V. Fionov E. V. Lunina 《Russian Chemical Bulletin》1998,47(6):1048-1053
The results of ESR-spectroscopic and quantum-chemical studies of the coordination of 2,2,4,5,5-pentamethyl-3-imidazoline-N-oxyl and 2-phenyl-2,4,5,5-tetramethyl-3-imidazoline-N-oxyl with the surface acid sites (AS) of silica are analyzed. Cluster models of AS, corresponding to one- and two-point coordination
and accepted in radiospectroscopy on the basis ofg-factors and constants of hyperfine splitting at the N nuclei in the resulting donor-acceptor complexes, are discussed. Within
the framework of the unrestricted Hartree-Fock method using the MNDO approach, a comparative quantum-chemical analysis of
the structural, spin, electrostatic, energy, and radiospectroscopic characteristics of the coordination of the model cluster
AS to the imidazoline nitroxide probes is performed. For the two-center adsorption, three types of AS structures are considered,
together with the paramagnetic surface complexes they form. A consistent semi-quantitative interpretation of the whole set
of features found experimentally for the coordination of imidazoline nitroxides to the surface AS on silica is given.
Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 6, pp. 1081–1086, June, 1998. 相似文献
8.
A. L. Chistyakov I. V. Stankevich N. P. Gambaryan I. S. Akhrem 《Russian Chemical Bulletin》2000,49(5):799-805
Fragments of the potential energy surfaces (PES) of the AlBr5 (I) and [AlBr5+CH4] (II) systems were studied by the MNDO/PM3 method. Five local minima corresponding to Br2·AlBr3 donor-acceptor complexes were found on the PES of system I. Two of these complexes have a pronounced ionic character. In
system II, among the products of barrierless addition of Br2·AlBr3 complexes to CH4, the methane molecule is activated only in two complexes. These are products of the attack of the most electrophilic AlBr5 complexes on a H atom of the methane molecule. The potential barriers to conversion of these products into complexes with
structures formally corresponding to the products of the attack of electrophiles on a C−H bond (the Olah scheme) or the C
atom of methane molecule (the Schreiner scheme) were calculated.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 802–808, May, 2000. 相似文献
9.
The structure of the conformationally flexible 2-fluoroethanal molecule (CH2FCHO, FE) in the ground (S0) and lowest excited triplet (T1) and singlet (S1) electronic states was investigated by ab initio quantum-chemical methods. The FE molecule in the S0 state was found to exist as two conformers, viz., as cis (the F—C—C—O angle is 0°) and trans (the F—C—C—O angle is 180°) conformers. On going both to the T1 and S1 states, the FE molecule undergoes substantial structural changes, in particular, the CH2F top is rotated with respect to the core and the carbonyl CCHO fragment becomes nonplanar. The potential energy surfaces for the T1 and S1 states are qualitatively similar, viz., six minima in each of the excited states of FE correspond to three pairs of mirror-symmetrical conformers. Based on the potential energy surfaces calculated for the FE molecule in the T1 and S1 states, the one-dimensional problems on the torsion and inversion nuclear motions as well as the two-dimensional torsion-inversion problems were solved. 相似文献
10.
The calculation of the 13C and 15N NMR chemical shifts by a combined molecular mechanics (Pcmodel 9.1/MMFF94) and ab initio (GIAO (B3LYP/DFT, 6-31 + G(d)) procedure is used to investigate the conformations of a variety of alkyl substituted anilines. The 13C shifts are obtained from the GIAO isotropic shielding (Ciso) with separate references for sp3 and sp2 carbons (δc = δref − Ciso). The 15N shifts are obtained similarly from the GIAO isotropic shielding (Niso) with reference to the 15N chemical shift of aniline. Comparison of the observed and calculated shifts provides information on the molecular conformations. Aniline and the 2,6-dialkylanilines exist with a rapidly inverting symmetric pyramidal nitrogen atom. The 2-alkylanilines have similar conformations with the NH2 group tilted away from the 2-alkyl substituent. The N,N-dialkylanilines show more varied conformations. N,N-dimethylaniline has a similar structure to aniline, but N-ethyl, N-methylaniline, N,N-diethylaniline, and N,N-diisopropylaniline are conformationally mobile with two rapidly interconverting conformers. In contrast, the anilines substituted at C2 and the nitrogen atom exist as one conformer where the steric interaction between the C2 substituent and the N substituent determines the conformation. In 2-methyl-N-methylaniline, the nitrogen atom is pyramidal as usual with the N-methyl opposite to the 2-methyl, but in 2-methyl-N,N-dimethyl aniline, the NMe2 group is now almost orthogonal to the phenyl plane. This is also the case with 2-methyl-N,N-diethylaniline and 2,6-diisopropyl-N,N-dimethylaniline. The comparison of the observed and calculated 15N chemical shifts confirms the above findings, in particular the pyramidal conformation of aniline and the above observations with respect to the conformations of the N,N-dialkylanilines. 相似文献
11.
Harrison JJ Chan CY Onopchenko A Pradhan AR Petersen M 《Magnetic resonance in chemistry : MRC》2008,46(2):115-124
A full line-shape analysis of the VT 31P NMR spectra was carried out for the monomer-dimer equilibrium of neutral ZDDP. The energy surface and the energetics of the monomer-dimer equilibrium (DeltaH degrees , DeltaG degrees , Ea, DeltaH(not equal), and DeltaG(not equal)) are reported for three variants wherein the alkyl groups in the ZDDP are 2-ethylhexyl, isopropyl, and isobutyl. We explored a reaction pathway between the monomer and dimer form by means of density functional theory (DFT). The linear combination of atomic orbitals (LCAO) code DMol3 was used together with a synchronous transient method to effectively locate transition states. Vibrational eigenmodes of all intermediates were computed to capture finite temperature effects. Methyl and ethyl were considered as alkyl groups. Two novel intermediates were located-a four-membered ring and a six-membered ring intermediate along the reaction coordinate. Comparison of the experimentally derived and computed energy surfaces was carried out. 相似文献
12.
The radical-molecule reaction mechanism of CHCl(2) and CCl(3) with NO(2) have been explored theoretically at the B3LYP/6-311G(d,p) and MC-QCISD (single-point) levels. For the singlet potential energy surface (PES) of CHCl(2) + NO(2) reaction, the association of CHCl(2) with NO(2) was found to be a barrierless carbon-to-nitrogen approach forming an energy-rich adduct a (HCl(2)CNO(2)) followed by isomerization to b(1) (trans-cis-HCl(2)CONO), which can easily interconvert to b(2), b(3), and b(4). Subsequently, the most feasible pathway is the 1,3-chlorine migration associated with N-O1 bond cleavage of b(1) leading to P(1) (CHClO + ClNO). The second competitive pathway is the 1,4-chlorine migration along with N-O1 bond rupture of b(4) giving rise to P(2) (CHClO + ClON). Moreover, some of P(1) and P(2) can further dissociate to give P(6) (CHClO + Cl + NO). The lesser followed competitive channel is the 1,3-H-shift from C to N atom along with N-O1 bond rupture of b(1) to form P(3) (CCl(2)O + HNO). The concerted 1,4-H-shift accompanied by N-O1 bond fission of b(3) to product P(4) (CCl(2)O + HON) is even much less feasible. For the singlet PES of CCl(3) + NO(2) reaction, the only primary product is found to be P(1) (CCl(2)O + ClNO), which can lead to P(2) (CCl(2)O + Cl + NO) via dissociation of ClNO. The obtained major products CHClO and CCl(2)O for CHCl(2) + NO(2) and CCl(3) + NO(2) reactions, respectively, are in good agreement with kinetic detection in experiment. Compared with the singlet pathways, the triplet pathways may have less contributions to both reactions. Because the rate-determining transition state involved in the feasible pathways lie above the reactants R, the title reactions may be important in high-temperature processes. The similarities and discrepancies among the CH(n)Cl(3-n) + NO(2) (n == 0-2) reactions are discussed in terms of the substitution effect. The present study may be helpful for further experimental investigation of the title reactions. 相似文献
13.
The title compounds were synthesized starting from homophthalic anhydride and an imine. The amides obtained showed unexpected values for 3J3,4 that cannot be used to deduce their configuration and conformation. This problem was resolved for one representative compound (the 4-(pyrrolidinylcarbonyl) derivative) by means of detailed NMR studies, X-ray diffraction and theoretical calculations. The compound has the trans configuration. In the solid state, its conformation is with dipseudoaxial (aa) oriented substituents at positions 3 and 4. In different solvents and in the gas-phase, the majority of the data reveal that the observed value of 3J3,4 results from an equilibrium of the ee and aa conformers. 相似文献
14.
Chanda‐Malis Ouk Natalia Zvereva‐Loëte Yohann Scribano Béatrice Bussery‐Honvault 《Journal of computational chemistry》2012,33(28):2211-2224
Multireference single and double configuration interaction (MRCI) calculations including Davidson (+Q) or Pople (+P) corrections have been conducted in this work for the reactants, products, and extrema of the doublet ground state potential energy surface involved in the N( 2 D) + CH4 reaction. Such highly correlated ab initio calculations are then compared with previous PMP4, CCSD(T), W1, and DFT/B3LYP studies. Large relative differences are observed in particular for the transition state in the entrance channel resolving the disagreement between previous ab initio calculations. We confirm the existence of a small but positive potential barrier (3.86 ± 0.84 kJ mol?1 (MR‐AQCC) and 3.89 kJ mol?1 (MRCI+P)) in the entrance channel of the title reaction. The correlation is seen to change significantly the energetic position of the two minima and five saddle points of this system together with the dissociation channels but not their relative order. The influence of the electronic correlation into the energetic of the system is clearly demonstrated by the thermal rate constant evaluation and it temperature dependance by means of the transition state theory. Indeed, only MRCI values are able to reproduce the experimental rate constant of the title reaction and its behavior with temperature. Similarly, product branching ratios, evaluated by means of unimolecular RRKM theory, confirm the NH production of Umemoto et al., whereas previous works based on less accurate ab initio calculations failed. We confirm the previous findings that the N( 2 D) + CH4 reaction proceeds via an insertion–dissociation mechanism and that the dominant product channels are CH2NH + H and CH3 + NH. © 2012 Wiley Periodicals, Inc. 相似文献
15.
The complete topological structure of the potential energy surface (PES) of methane in the inversion region was studied by
theab initio CCD(full)/6-311++G** method. The necessity of taking into account nuclear motions was shown. Penta- and hexacoordination of carbon atoms in boron-containing
organic compounds was investigated by theab initio MP2(full)/6-31G** and MP2(full)/6-311++G** methods. The CB4H4Li2, CB6N2H2, and CB6O2 systems containing hexacoordinated carbon atoms correspond to rather deep minima on the relevant PES and can be the subject
of synthetic studies. According to theab initio calculations, pyramidal boron-containing systems with hypercoordinated carbon atoms, which fulfill the “8e rule,” also correspond
to rather deep PES minima and can be detected experimentally.
Dedicated to Academician V. I. Minkin on the occasion of his 65th birthday.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 786–796, May, 2000 相似文献
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The geometric parameters, the charge distribution, and the energetics of N-methyl-2-(N-ethylanilino)-3-(indol-1-yl)-and N-methyl-2-(N-ethylanilino)-3-(indol-3-yl)maleimides and their conjugated acids were studied by density functional theory calculations
at the B3LYP/6-31G(d) level. The mechanism of the tandem hydride transfer/cyclization sequence, which occurs after protonation
of N-methyl-2-(N-ethylanilino)-3-(indol-1-yl)-and N-methyl-2-(N-ethylanilino)-3-(indol-3-yl)maleimides, was analyzed. The investigation of the potential energy surface for the tandem hydride
transfer/cyclization of the iminium cation that formed upon protonation revealed that the hydride transfer followed by intramolecular
cyclization at position 7 of the indole fragment in N-methyl-2-(N-ethylanilino)-3-(indol-1-yl)maleimide is the preferable process, unlike alternative intramolecular cyclization involving
the cationic center at the C(2) atom of the indole fragment and the benzene ring of the N-ethylaniline fragment of the indoleninium cation in N-methyl-2-(N-ethylanilino)-3-(indol-3-yl)maleimide. A study of the key intermediates of the assumed reaction mechanism demonstrated that
these intermediates are actually stationary points on the potential energy surface (minima and transition states).
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2069–2073, December, 2006. 相似文献
17.
Joaquin Espinosa-Garcia Cipriano Rangel Jose C. Corchado 《Molecules (Basel, Switzerland)》2022,27(12)
This paper is a detailed review of the chemistry of medium-size reactive systems using the following hydrogen abstraction reactions with ethane, X + C2H6 → HX + C2H5; X ≡ H, F(2P), Cl(2P), O(3P) and OH, and focusing attention mainly on the theoretical developments. These bimolecular reactions range from exothermic to endothermic systems and from barrierless to high classical barriers of activation. Thus, the topography of the reactive systems changes from reaction to reaction with the presence or not of stabilized intermediate complexes in the entrance and exit channels. The review begins with some reflections on the inherent problems in the theory/experiment comparison. When one compares kinetics or dynamics theoretical results with experimental measures, one is testing both the potential energy surface describing the nuclei motion and the kinetics or dynamics method used. Discrepancies in the comparison may be due to inaccuracies of the surface, limitations of the kinetics or dynamics methods, and experimental uncertainties that also cannot be ruled out. The paper continues with a detailed review of some bimolecular reactions with ethane, beginning with the reactions with hydrogen atoms. The reactions with halogens present a challenge owing to the presence of stabilized intermediate complexes in the entrance and exit channels and the influence of the spin-orbit states on reactivity. Reactions with O(3P) atoms lead to three surfaces, which is an additional difficulty in the theoretical study. Finally, the reactions with the hydroxyl radical correspond to a reactive system with ten atoms and twenty-four degrees of freedom. Throughout this review, different strategies in the development of analytical potential energy surfaces describing these bimolecular reactions have been critically analyzed, showing their advantages and limitations. These surfaces are fitted to a large number of ab initio calculations, and we found that a huge number of calculations leads to accurate surfaces, but this information does not guarantee that the kinetics and dynamics results match the experimental measurements. 相似文献