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1.
《Tetrahedron: Asymmetry》2005,16(16):2764-2770
The replacement of the carboxylic acid functionality with a tetrazolic acid in the proline-catalyzed asymmetric aldol reaction between acetone and trimethylacetaldehyde has been studied using DFT methods at the B3LYP/6-31G** computational level. For the C–C bond-formation step, four reactive channels corresponding to the syn and anti arrangement of the enamine and the re and si attack modes have been analyzed. The B3LYP/6-31G** results allowed us to explain the stereoselectivity and the increase in reactivity relative to the proline-catalyzed process in DMSO. 相似文献
2.
P. Thanikaivelan J. Padmanabhan V. Subramanian T. Ramasami 《Theoretical chemistry accounts》2002,107(6):326-335
The use of Fukui functions for the site selectivity of the formaldehyde molecule for nucleophilic, electrophilic and radical
attacks has been made with special emphasis to the dependence of Fukui values on the basis sets as well as population schemes
in the framework of B3LYP theory. Out of the five population schemes selected viz., Mulliken population analysis, natural
population analysis, CHELP, CHELPG and atoms in molecules (AIM), it is found that the CHELPG and AIM schemes predict precise
reactive site with less dependency on the basis sets. Charges derived from Hirshfeld partitioning, calculated using the BLYP/dnd
method (implemented in the DMOL3 package), provide non-negative Fukui values for all the molecular systems considered in this study. Supporting results have
been obtained for acetaldehyde and acetone molecules at the 6-31+G** basis set level. These results support the fact that
high Fukui values correspond to soft–soft interaction sites. On the other hand, the correlation of the low Fukui value to
the hard–hard interaction site merits further investigation.
Received: 10 November 2001 / Accepted: 6 March 2002 / Published online: 13 June 2002 相似文献
3.
Friedrich Grein 《Theoretical chemistry accounts》2003,109(5):274-277
Using 6-31G and 6-311G basis sets to which diffuse and polarization functions were added in a stepwise fashion (a total of
16 basis sets), Hartree–Fock (HF), MP2 and B3LYP geometry optimizations were performed on biphenyl. With the MP2 method, diffuse
functions raise the dihedral angle φ, for example, from 46.3° for 6-31G to 54.1° for 6-311++G, while polarization functions
lower it, for example, from 54.1° for 6-311++G to 42.1° for 6-311++G(2d,2p). For a single set of polarization functions, φ(MP2)
lies close to or above φ(HF) (44–47°), but for a double set it is below φ(HF) and is close to B3LYP values (38–42°) which
show little basis set dependence. The most reliable value for φ, 42.1° [MP2/6-311++G(2d,2p)], is expected to increase slightly
by adding more diffuse functions. The corresponding best calculated energy barrier at 0° (coplanar conformation) is 2.83 kcal/mol,
much higher than the experimental estimate (1.4 ± 0.5 kcal/mol). The barrier at 90° is 1.82 kcal/mol, in line with the experimental
estimate (1.6 ± 0.5 kcal/mol) and with previous theoretical results.
Received: 9 September 2002 / Accepted: 15 November 2002 /
Published online: 1 April 2003
Correspondence to: Friedrich Grein e-mail: fritz@unb.ca
Acknowledgement. The author would like to thank NSERC (Canada) for financial support. 相似文献
4.
Davor Margetić Ronlad N. Warrener Mirjana Eckert-Maksić Ivana Antol Zoran Glasovac 《Theoretical chemistry accounts》2003,109(4):182-189
DFT calculations of 7′–oxasesquinorbornenes and 7,7′-dioxasesquinorbornenes using the B3LYP/6–31G* method are reported.
All the investigated structures (syn- and anti- derivatives) showed significant non-planarity of the central double bond, with the exception of those anti-derivatives possessing symmetrical structures. The influence of the replacement of the methylene groups at position 7- of
the norbornene fragment with oxygen and the introduction of second and third (peripheral) double bonds and benzene rings on
the molecular and electronic structures of these molecules have also been investigated.
Received: 11 November 2002 / Accepted: 6 June 2002 /
Published online: 29 April 2003 相似文献
5.
Nucleophilic vinylic substitutions of 4H-pyran-4-one and 2-methyl-4H-pyran-4-one with ammonia were calculated by the B3LYP method using the 6-31G(d,p) basis set. Bulk solvent effects of aqueous
solution were estimated by the polarized continuum and Poisson–Boltzmann self-consistent reaction field models using the 6-311+G(d,p)
basis set. In the gas phase different mechanisms were found for the two reaction systems calculated. The reaction of 4H-pyran-4-one proceeds through enol, whereas a feasible path for the less reactive 2-methyl-4H-pyran-4-one is the mechanism through a keto intermediate. Addition of ammonia in concert with proton transfer is the rate-determining
step ofthe reaction. The mechanism proceeding either by a bimolecular nucleophilic substitution (SN2) or by one involving a tetrahedral zwitterionic intermediate is shown to be unlikely in the gas phase or nonpolar solution.
The effects of bulk solvent not only consist in a reduction of the various activation barriers by about 25–40 kJ mol−1 but also in a change in the reaction mechanism.
Received 26 May 2002 / Accepted 26 July 2002 /
Published online: 14 February 2003 相似文献
6.
S. Calvo-Losada T. L. Sordo F. J. López-Herrera J. J. Quirante 《Theoretical chemistry accounts》2000,103(5):423-430
The influence of protecting the hydroxyl group of a β-oxy-α-diazo carbonyl compound on the competition between the Wolff
rearrangement (WR) and the [1,2]-hydrogen shift (HS) was investigated theoretically. Stationary points on the potential-energy
surface were located with the B3LYP density functional and the 6-31G** basis set. For the basic system geometry optimisations
at B3LYP/6-311+G** were performed to validate the reliability of the B3LYP/6-31G** calculations. Single-point energy calculations
were carried out at the B3LYP/6-311+G** level on the B3LYP/6-31G**-optimised geometries. Further insight into the processes
was achieved with the aid of the theory of “atoms in molecules” of Bader. The calculated energy barriers qualitatively predicted
the yields of HS and WR obtained experimentally. In order to rationalise the calculated energy barriers, it was necessary
to take into account not only the electronegativity of the protective groups but also the alignment of the migrating groups
with the depletion sites at the carbene centre. Further, when the hydroxyl group was not protected the existence of an intramolecular
hydrogen bond played an important role in both HS and WR.
Received: 30 December 1998 / Accepted: 7 May 1999 / Published online: 4 October 1999 相似文献
7.
Addition–elimination reactions involving a nucleophile and a remote leaving group [SH
N(AE)tele] are well-known under basic conditions, especially amongst electron-poor six-membered heterocycles, but are less commonly
encountered for five-membered heterocycles and are rare under acidic conditions. Concentrated HCl converts 1-hydroxy-1H-pyrazolo[3,4-c] isoquinoline and 1-hydroxy-1H-pyrazolo[3,4-c]quinoline into 3-chloro-1H-pyrazolo[3,4-c]isoquinoline and 3-chloro-1H-pyrazolo[3,4-c]quinoline, respectively. However, apparently neither the isomeric 1-hydroxy-1H-pyrazolo[4,3-c](iso)-quinolines nor the parent 1-hydroxypyrazole undergo this reaction. Additionally, all these systems are refractory under
basic conditions. We present a plausible mechanism for the reaction, involving the 3-addition of Cl- to the diprotonated heterocycle, followed by the elimination of water. Calculations of the initial transition states and
intermediates, using optimisation at B3LYP/6-311+G(d,p), including thermochemistry [HF/6-31+G(d)], and single-point Poisson–Boltzmann
self-consistent reaction field determination of the free energy of solvation (Jaguar Poisson–Boltzmann self-consistent reaction
field), support this mechanism and reproduce the observed order of reactivity, the addition step being 2–4 kcal less favourable
for the isomeric 1-hydroxy-1H-pyrazolo[4,3-c](iso)quinolines and provide a rationalisation for the role of strong acid.
Received: 27 June 2002 / Accepted: 6 September 2002 /
Published online: 14 February 2003 相似文献
8.
Steven M. Bachrach Joseph M. Hayes Trang Dao Justin L. Mynar 《Theoretical chemistry accounts》2002,107(5):266-271
Density functional calculations indicate that nucleophilic substitution in the thiolate–disulfide and thiolate–trisulfide
exchange reactions proceeds by an addition–elimination pathway. Solution calculations were performed using B3LYP/6-31+G* and
the polarized continuum method. These solution-phase calculations indicate that for the reactions where the sulfur under attack
bears a hydrogen atom, the substitution proceeds via an addition–elimination mechanism; however, when a methyl group is attached
to the sulfur under attack, the SN2 mechanism is predicted.
Received: 12 October 2001 / Accepted: 28 November 2001 / Published online: 8 April 2002 相似文献
9.
Xiang-Yuan Li Quan Zhu Lin-Lin Zhao Shun-Qing Xiao Feng Liu 《Theoretical chemistry accounts》2002,107(5):282-290
Based on the spherical cavity approximation and the Onsager model, a dipole–reaction field interaction model has been proposed
to elucidate the solvent reorganization energy of electron transfer (ET). This treatment only needs the cavity radius and
the solute dipole moment in the evaluation of the solvent reorganization energy, and fits spherelike systems well. As an application,
the ET reaction between p-benzoquinone and its anion radical has been investigated. The inner reorganization energy has been calculated at the level
of MP2/6–31+G, and the solvent reorganization energies of different conformations have been evaluated by using the self-consistent
reaction field approach at the HF/6–31+G level. Discussions have been made on the cavity radii and the values are found to
be reasonable when compared with the experimental ones of some analogous intramolecular ET reactions. The ET matrix element
has been determined on the basis of the two-state model. The fact that the value of the ET matrix element is about 10 times
larger than RT indicates that this ET reaction can be treated as an adiabatic one. By invoking the classical Marcus ET model, a value of
4.9 × 107M−1s−1 was obtained for the second-order rate constant, and it agrees quite well with the experimental one.
Received: 19 October 2001 / Accepted: 17 January 2002 / Published online: 3 May 2002 相似文献
10.
Ab initio molecular orbital theory and density functional theory have been used to study nine isomers of N7 ionic clusters with low spin at the HF/6-31G*, MP2/6-31G*, B3LYP/6-31G*, and B3LYP/6-311(+)G* levels of theory. All stationary
points are examined with harmonic vibrational frequency analyses. Four N7
+ isomers and five N7
− isomers are determined to be local minima or very close to the minima on their potential-energy hypersurfaces, respectively.
For N7
+ and N7
−, the energetically low lying isomers are open-chain structures (C
2
v
and C
2
v
or C2). The results are very similar to those of other known odd-number nitrogen ions, such as N5
+, N9
+, and N9
−, for which the open-chain structures are also the global minima. This research suggests that the N7 ionic clusters are likely to be stable and to be potential high-energy-density materials if they could be synthesized.
Received: 16 July 2001 / Accepted: 8 October 2001 / Published online: 21 January 2002 相似文献
11.
Xavier Fradera Jordi Poater Sílvia Simon Miquel Duran Miquel Solà 《Theoretical chemistry accounts》2002,108(4):214-224
This article presents an overview of recent advances in the study of electron pairing through the use of localization and
delocalization indices obtained from double integration over atomic basins of the exchange–correlation density in the framework
of the atoms-in-molecules theory. These localization and delocalization indices describe the intra- and interatomic distribution
of the electron pairs in a molecule. The main results of the application of these second-order indices to the analysis of
molecular structure and chemical reactivity are briefly reviewed. It is shown that localization and delocalization indices
represent a powerful tool to describe the electron-pair structure of molecules, which, in turn, provides deeper insight into
relevant chemical phenomena such as electron correlation effects and the formation of localized α, β electron pairs.
Received: 8 April 2002 / Accepted: 26 June 2002 / Published online: 6 September 2002
Acknowledgements. Financial help was furnished by the Spanish DGES projects no. PB98-0457-C02-01 and BQU2002-04112-C02-02. J.P. thanks the
Departament d'Universitats, Recerca i Societat de la Informació de la Generalitat de Catalunya for benefiting from a doctoral
fellowship, no. 2000FI-00582. M.S. is indebted to the Departament d'Universitats, Recerca i Societat de la Informació of the
Generalitat de Catalunya for financial support through the Distinguished University Research Promotion, 2001. We also thank
the Centre de Supercomputació de Catalunya for providing us with computing facilities.
Correspondence to: M. Solà e-mail: miquel.sola@udg.es 相似文献
12.
Binding energies of helium, neon and atomic hydrogen encapsulated inside a C20 cage were calculated using an ab initio method at the B3LYP/6-31+G⋆ level of theory. The standard equilibrium constants for
the reactions of noble-gas atoms going into the C20 molecular cage have also been studied. The transition states for the reactions of C20 with hydrogen and helium were further obtained with an ab initio method at the B3LYP/6-31+G⋆ level and the rate constants
were estimated by using conventional transition-state theory. It was found that the hydrogen and helium atoms are extremely
difficult to put into the C20 cage. Once inside the cage, a helium atom can hardly get out, while a hydrogen atom can easily escape from the cage. The
results are expected to enrich fullerene science and be helpful for fullerene applications such as storage.
Received: 2 November 2002 / Accepted: 19 December 2002 /
Published online: 30 April 2003
Correspondence to: R. Q. Zhang, e-mail: aprqz@cityu.edu.hk
Acknowledgements. The work described in this paper was jointly supported by a grant from the City University of Hong Kong (project no. 7001222)
and a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (project no. 9040633/CityU,
1011/01P]. 相似文献
13.
The most stable structures of V
x
O
y
+/V
x
O
y
(x=1, 2, y=1–5) clusters and their interaction with O2 are determined by density functional calculations, the B3LYP functional with the 6-31G* basis set. The nature of the bonding
of these clusters and the interaction with O2 have been studied by topological analysis in the framework of both the atoms-in-molecules theory of Bader and the Becke–Edgecombe
electron localization function. Bond critical points are localized by means of the analysis of the electron density gradient
field, ∇ρ(r), and the electron localization function gradient field, ∇η(r). The values of the electron density properties, i.e., electron density, ρ(r), Laplacian of the electron density, ∇2ρ(r), and electron localization function, η(r), allow the nature of the bonds to be characterized, and linear correlation is found for the results obtained in both gradient
fields. Vanadium-oxygen interactions are characterized as unshared-electron interactions, and linear correlation is observed
between the electron density properties and the V–O bond length. In contrast, O2 units involve typical shared-electron interactions, as for the dioxygen molecule. Four different vanadium–oxygen interactions
are found and characterized: a molecular O2 interaction, a peroxo O2
2− interaction, a superoxo O2
− interaction and a side-on O2
− interaction.
Received: 15 October 2001 / Accepted: 30 January 2002 / Published online: 24 June 2002 相似文献
14.
Pere M. Deyà Antonio Frontera Guillem A. Suñer David Quiñonero Carolina Garau Antoni Costa Pau Ballester 《Theoretical chemistry accounts》2002,108(3):157-167
The structural and energetic changes associated with C–N bond rotation in a squaric acid derivative as well as in formamide,
3-aminoacrolein and vinylamine have been studied theoretically using ab initio molecular orbital methods. Geometry optimizations
at the MP2(full)/6-31+G* level confirmed an increase in the C–N bond length and a smaller decrease in the C=O length on going
from the equilibrium geometry to the twisted transition state. Other geometrical changes are also discussed. Energies calculated
at the QCISD(T)/6-311+G** level, including zero-point-energy correction, show barrier heights decreasing in the order formamide,
squaric acid derivative, 3-aminoacrolein and vinylamine. The origin of the barriers were examined using the atoms-in-molecules
approach of Bader and the natural bond orbital population analysis. The calculations agree with Pauling's resonance model,
and the main contributing factor of the barrier is assigned to the loss of conjugation on rotating the C–N bond. Finally,
molecular interaction potential calculations were used to study the changes in the nucleophilicity of N and O (carbonyl) atoms
upon C–N rotation, and to obtain a picture of the abilities of the molecules to act in nonbonded interactions, in particular
hydrogen bonds. The molecular interaction potential results confirm the suitability of squaramide units for acting as binding
units in host–guest chemistry.
Received: 13 March 2002 / Accepted: 23 June 2002 / Published online: 21 August 2002 相似文献
15.
Sándor Kristyán Adrienn Ruzsinszky Gábor I. Csonka 《Theoretical chemistry accounts》2001,106(6):404-411
Experimental enthalpies of formation have been approximated using single-point Hartree–Fock (HF)–self-consistent-field (SCF)
total energies plus the rapid estimation of basis set error and correlation energy from partial charges (REBECEP) energy corrections.
The energy corrections are calculated from the HF–SCF partial atomic charges and optimized atomic energy parameters. The performance
of the method was tested on 51 closed-shell neutral molecules (50 molecules from the G3/99 thermochemistry database plus urea,
composed of H, C, N, O, and F atoms). The predictive force of the method is demonstrated, because these larger molecules were
not used for the optimization of the atomic parameters. We used the earlier RECEP-3 [HF/6-311+G(2d,p)] and REBECEP [HF/6-31G(d)]
atomic parameter sets obtained from the G2/97 thermochemistry database (containing small molecules) together with natural
population analysis and Mulliken partial charges. The best results were obtained using the natural population analysis charges,
although the Mulliken charges also provide useful results. The root-mean-square deviations from the experimental enthalpies
of formation for the selected 51 molecules are 1.15, 3.96, and 2.92 kcal/mol for Gaussian-3, B3LYP/6-11+G(3df,2p), and REBECEP
(natural population analysis) enthalpies of formation, respectively (the corresponding average absolute deviations are 0.94,
7.09, and 2.27 kcal/mol, respectively). The REBECEP method performs considerably better for the 51 test molecules with a moderate
6-31G(d) basis set than the B3LYP method with a large 6-311+G(3df,2p) basis set.
Received: 10 March 2001 / Accepted: 5 July 2001 / Published online: 11 October 2001 相似文献
16.
Ab initio calculations with inclusion of correlation effects at the MP2/6-31G* level have been used to predict the interaction
energy of stacked cytosine dimer (C/C) as a function of twisting and sliding in the gas phase. Systematic calculations have
also been carried out on the solvation free energies of various rotated and translated C/C dimers using a polarized continuum
model approach at the HF/6-31G* level with a view to probe the role of various degrees of freedom on the free energy of solvation
of the C/C dimer. The interaction energy of the C/C dimer decreases upon changing from a parallel to an antiparallel conformation
in the gas phase. The 180°-rotated conformation has been found to be the most stable arrangement when compared to other rotated
positions. The rotated and translated dimers exhibit lower solvation free energy than the parallel conformation. The decrease
in the dipole moment upon rotation from the parallel to the antiparallel conformation indicates the cancellation of charge
distribution upon rotation in the z direction of one cytosine base with respect to the other. The calculation reveals that the present approach could not yield
association energy, ΔΔG
Asso, in a solvent medium. This may be due to the fact that in the case of floppy molecules the contribution from translational,
rotational and vibrational free energies plays a significant role in the calculation of ΔΔG
Asso.
Received: 13 December 2001 / Accepted: 25 March 2002 / Published online: 13 June 2002 相似文献
17.
Ab initio molecular orbital calculations for N9, N−
9 and N+
9 isomers were carried out at the HF/ 6-31G*, B3PW91/6-31G*, B3LYP/6-31G* and MP2/ 6-31G* levels of theory. Stable equilibrium
geometric structures were determined by harmonic vibrational frequency analyses at the HF/6-31G*, B3PW91/6-31G* and B3LYP/6-31G*
levels of theory. The most stable free-radical N9 cluster is structure 1 with C
2
v
symmetry and that of anion N−
9 is structure 3 with C
s
symmetry. Only one stable structure of the N+
9 cation with C
2
v
symmetry was predicted. Their potential application as high-energy-density materials has been examined.
Received: 15 June 1999 / Accepted: 11 October 1999 / Published online: 14 March 2000 相似文献
18.
19.
This paper reports a new AM1/d model for phosphorus that can be used to model nucleophilic attack of phosphates relevant
for biological phosphate hydrolysis reactions. The parameters were derived from a quantum dataset calculated with hybrid density-functional
theory [B3LYP/6-311++G(3df,2p)//B3LYP/6-31++G(d,p)] of phosphates and phosphoranes in various charge states, and on transitions
states for nucleophilic attacks. A suite of non-linear optimization methods is outlined for semiempirical parameter development
based on integrated evolutionary (genetic), Monte Carlo simulated annealing and direction set minimization algorithms. The
performance of the new AM1/d model and the standard AM1 and MNDO/d models are compared with the density-functional results.
The results demonstrate that the strategy of developing semiempirical parameters specific for biological reactions offers
considerable promise for application to large-scale biological problems.
Received: 15 January 2002 / Accepted: 6 September 2002 / Published online: 28 March 2003
Contribution to the Proceedings of the Symposium on Combined QM/MM Methods at the 222nd National Meeting of the American
Chemical Society, 2001
Correspondence to: D.M. York e-mail: york@chem.umn.edu
Acknowledgements. D.M.Y. is grateful for financial support provided by the National Institutes of Health (grant 1R01-GM62248-01A1) and the
Donors of The Petroleum Research Fund, administered by the American Chemical Society, and the Minnesota Supercomputing Institute
through a 6-month research scholar award (X.L.). Computational resources were provided by the Minnesota Supercomputing Institute. 相似文献
20.
A method applying ab initio direct dynamics has been utilized in studying the hydrogen abstraction reaction HCN + OH → CN + H2O. The geometries of the reactants, products, and the transition state have been optimized at the QCISD/6-311G(d, p) level.
Single-point energies were further evaluated at the QCISD(T)/6-311+G(2df, 2p)//QCISD/6-311G(d, p) level. The barrier heights
for the forward and reverse reactions were predicted to be 15.95 and 7.51 kcal mol−1 at the QCISD(T)/6-311 + G(2df, 2p)//QCISD/6-311G(d, p) level, respectively. The reaction rate constants were calculated in
the temperature range from 298 to 4,000 K using the canonical variational transition-state theory with a small-curvature tunneling
correction. The results of the calculation show that the theoretical rate constants are in good agreement with experimental
data over the measured temperature range of 400–2,600 K.
Received: 18 August 2002 / Accepted: 30 August 2002 / Published online: 20 November 2002
Acknowledgements. Our thanks are due to D.G. Truhlar for providing the POLYRATE 8.2 program. This work was supported by the National Science
Foundation of China. We also thank D.C. Fang and Y. M. Xie for their valuable help, and P.R. Yan for reading our paper.
Correspondence to: Q. S. Li e-mail: qsli@mh.bit.edu.cn 相似文献