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1.
This is an overview of the use of empirical force fields in the study of reaction mechanisms. Empirical-valence-bond-type
methods (including reactive force field and multiconfigurational molecular mechanics) produce full reaction surfaces by mixing,
in the simplest case, known force fields describing reactants and products. The SEAM method instead locates approximate transition
structures by energy minimization along the intersection of the component force fields. The transition-state force-field approach
(including Q2MM) designs a new force field mimicking the transition structure as an energy minimum. The scope and applicability
of the various methods are compared.
Received: 17 April 2002 / Accepted: 26 July 2002 / Published online: 4 November 2002
Correspondence to: P.-O. Norrby e-mail: pon@kemi.dtu.dk 相似文献
2.
Victor M. Anisimov Nikolay Anikin Vladislav Bugaenko Vladimir Bobrikov Alexey Andreyev 《Theoretical chemistry accounts》2003,109(4):213-219
Accurate electrostatic maps of proteins are of great importance in research of protein interaction with ligands, solvent
media, drugs, and other biomolecules. The large size of real-life proteins imposes severe limitations on computational methods
one can use for obtaining the electrostatic map. Well-known accurate second-order M?ller–Plesset and density functional theory
methods are not routinely applicable to systems larger than several hundred atoms. Conventional semiempirical tools, as less
resource demanding ones, could be an attractive solution but they do not yield sufficiently accurate calculation results with
reference to protein systems, as our analysis demonstrates. The present work performs a thorough analysis of the accuracy
issues of the modified neglect of differential overlap type semiempirical Hamiltonians AM1 and PM3 on example of the calculation
of the molecular electrostatic potential and the dipole moment of natural amino acids. Real capabilities and limitations of
these methods with application to protein modeling are discussed.
Received: 26 April 2002 / Accepted: 19 September 2002 /
Published online: 14 February 2003 相似文献
3.
Transition structures associated with the C-C bond-formation step of the proline-catalyzed intermolecular aldol reaction
between acetone and isobutyraldehyde have been studies using density functional theory methods at the B3LYP/6-31G** computational
level. A continuum model has been selected to represent solvent effects. For this step, which is the stereocontrolling and
rate-determining step, four reactive channels corresponding to the syn and anti arrangement of the active methylene of the
enamine relative to the carboxylic acid group of l-proline and the re and si attack modes to both faces of the aldehyde carbonyl group have been analyzed. The B3LYP/6-31G**
energies are in good agreement with experiment, allowing us to explain the origin of the catalysis and stereoselectivity for
these proline-catalyzed aldol reactions.
Received: 2 April 2002 / Accepted: 18 July 2002 / Published online: 11 October 2002
Acknowledgements. This work was supported by research funds provided by the Ministerio de Educación y Cultura of the Spanish Government by
DGICYT (project PB98–1429). All the calculations were performed on a Cray–Silicon Graphics Origin 2000 of the Servicio de
Informática de la Universidad de Valencia. We are most indebted to this center for providing us with computer capabilities.
Correspondence to: L. R. Domingo e-mail: domingo@utopia.uv.es 相似文献
4.
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
5.
Concerted as well as stepwise reaction pathways for cyclization of 3-azido-propenal to isoxazole have been investigated by
density functional (B3LYP) and ab initio methods up to CCSD(T)/cc-pVQZ methods. These calculations clearly establish the pathway
with concerted albeit asynchronous nitrogen extrusion and ring closure as the most feasible mechanism. Barriers for cyclization
increase in the order Hartree–Fock<B3LYP<ACPF<CCSD(T). According to the geometrical parameters and the electronic structure
of the TS as evidenced by natural bond order analysis this cyclization can be interpreted as a pseudopericyclic (heteroelectrocyclic)
reaction.
Received: 26 May 2002 / Accepted: 18 June 2002 /
Published online: 14 February 2003 相似文献
6.
Soluble polymers have been prepared that are designed to undergo enhanced rates of hydrolysis at pH values less than that
observed in blood circulation. The degradable element in the polymer mainchain is derived from cis-aconityl acid and is defined by a carboxylic acid pendent functionality (C-4) that is cis across a double bond to an amide
at C-1 in the polymer mainchain. While degradation studies in vitro have confirmed these polymers do undergo enhanced rates
of degradation at acidic pH values, there is also increasing evidence that during the degradation process the double bond
isomerises to the trans configuration and thus prevents the full degradation of a polymer. From a molecular modelling perspective
we are seeking to understand the propensity for this cis–trans isomerisation and the mechanism of this cis–trans isomerisation
is discussed.
Received: 29 April 2002 / Accepted: 6 September 2002 /
Published online: 14 February 2003 相似文献
7.
Rajeev Prabhakar Margareta R. A. Blomberg Per E. M. Siegbahn 《Theoretical chemistry accounts》2000,104(6):461-470
A concerted mechanism for proton exchange between water and the amino acid side chains of cysteine, serine, arginine and
glutamic acid has been investigated with hybrid density functional theory. The models used include, besides the amino acid
side chain, a number of water molecules ranging from one to five in some cases. The modeling of the amino acids without their
backbones is shown to be an excellent approximation. Long-range polarization effects were incorporated through a dielectric
cavity method allowing a better comparison to existing measurements for free amino acids in water. The barriers converge rather
fast with the number of water molecules for all the present amino acids and the converged values are in reasonable agreement
with experiments with discrepancies in the range 2–6 kcal/mol. The dielectric effects were found to be small for all systems
except cysteine, where there is a lowering of the barrier by 3–5 kcal/mol. The transition states for these concerted pathways
form rings in which the separated charges can be stabilized.
Received: 25 October 1999 / Accepted: 5 April 2000 / Published online: 21 June 2000 相似文献
8.
Xavier Prat-Resina Mireia Garcia-Viloca Gerald Monard Angels González-Lafont José M. Lluch Josep Maria Bofill Josep Maria Anglada 《Theoretical chemistry accounts》2002,107(3):147-153
We propose a methodology to locate stationary points on a quantum mechanical/molecular mechanical potential-energy surface.
This algorithm is based on a suitable approximation of an initial full Hessian matrix, either a modified Broyden–Fletcher–Goldfarg–Shanno
or a Powell update formula for the location of, respectively, a minimum or a transition state, and the so-called rational
function optimization. The latter avoids the Hessian matrix inversion required by a quasi-Newton–Raphson method. Some examples
are presented and analyzed.
Received: 16 July 2001 / Accepted: 9 October 2001 / Published online: 9 January 2002 相似文献
9.
10.
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
11.
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
12.
This work deals with theoretical investigations on the oxygen–transition metal bond in systems containing linear chains of
Ti–O units. From an experimental point of view, in the recent past a number of systems containing linearly arranged Ti–O units
were synthesized, in which the Ti atom is complexed with Schiff bases such as acacen and salen. The theoretical study presented
here has been carried out applying the density functional theory to model compounds of these systems, in order to shed light
on the interactions between the transition metal and oxygen. Calculations have been performed on Ti–O oligomers (dimers, trimers
and tetramers) by means of density functional theory at the gradient-corrected level of theory, optimizing the molecular geometries.
Calculations have also been performed on linear polymers of the same systems, applying periodic boundary conditions, in order
to compare the results with those of oligomeric analogues.
Received: 12 January 2002 / Accepted: 16 April 2002 / Published online: 5 July 2002 相似文献
13.
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
14.
R. Q. Topper K. Chung C. M. Boelke D. Louie J. S. Kang R. Hannan T. Kiang L. H. Chan 《Theoretical chemistry accounts》2003,109(4):233-238
2-(Acetylamino)fluorene (AAF), a potent mutagen and a prototypical example of the mutagenic aromatic amines, forms covalent
adducts to DNA after metabolic activation in the liver. A benchmark study of AAF is presented using a number of the most widely
used molecular mechanics and semiempirical computational methods and models. The results are compared to higher-level quantum
calculations and to experimentally obtained crystal structures. Hydrogen bonding between AAF molecules in the crystal phase
complicates the direct comparison of gas-phase theoretical calculations with experiment, so Hartree–Fock (HF) and Becke–Perdew
(BP) density functional theory (DFT) calculations are used as benchmarks for the semiempirical and molecular mechanics results.
Systematic conformer searches and dihedral energy landscapes were carried out for AAF using the SYBYL and MMFF94 molecular
mechanics force fields; the AM1, PM3 and MNDO semiempirical quantum mechanics methods; HF using the 3-21G*and 6-31G* basis
sets; and DFT using the nonlocal BP functional and double numerical polarization basis sets. MMFF94, AM1, HF and DFT calculations
all predict the same planar structures, whereas SYBYL, MNDO and PM3 all predict various nonplanar geometries. The AM1 energy
landscape is in substantial agreement with HF and DFT predictions; MMFF94 is qualitatively similar to HF and DFT; and the
MNDO, PM3 and SYBYL results are qualitatively different from the HF and DFT results and from each other. These results are
attributed to deficiencies in MNDO, PM3 and SYBYL. The MNDO, PM3 and SYBYL models may be unreliable for compounds in which
an amide group is immediately adjacent to an aromatic ring.
Received: 26 May 2002 / Accepted: 12 December 2002 /
Published online: 14 February 2003 相似文献
15.
A simple implementation of third-order perturbation theory applied to a multireference zero-order wavefunction is presented.
Two different partitions of the Hamiltonian (M?ller–Plesset baricentric and Epstein–Nesbet) are considered. Two test cases,
CH2 and N2, are examined. The third-order results are shown to be in good agreement in either partition and are generally an improvement
with respect to the second-order results. The phenomenon of intruder states, absent in Epstein–Nesbet, appears to be magnified
in the M?ller–Plesset partition.
Received: 22 November 2001 / Accepted: 22 February 2002 / Published online: 3 May 2002 相似文献
16.
We present a method to treat the solvent efficiently in hybrid quantum mechanical/molecular mechanical simulations of chemical
reactions in enzymes. The method is an adaptation of an approach developed for molecular-mechanical free-energy simulations.
The charges of each of the exposed ionizable groups are scaled, and the system is simulated in the presence of a limited number
of explicit solvent molecules to obtain a reasonable set of structures. Continuum electrostatics methods are then used to
correct the energies. Variations in the procedure are discussed with an emphasis on modifications from the original protocol.
We illustrate the method by applying it to the study of a hydrolysis reaction in a highly charged system comprising a complex
between the base excision repair enzyme uracil-DNA glycosylase and double-stranded DNA. The resulting adiabatic reaction profile
is in good agreement with experiment, in contrast to that obtained without scaling the charges.
Received: 5 October 2001 / Accepted: 6 September 2002 / Published online: 28 February 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: M. Karplus e-mail: marci@tammy.harvard.edu 相似文献
17.
18.
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
19.
Olga Dmitrenko Robert D. Bach Rafal R. Sicinski Wolfgang Reischl 《Theoretical chemistry accounts》2003,109(4):170-175
B3LYP calculations in conjunction with natural bond orbital population analysis have been performed for a previtamin D model
and corresponding transition structures for the [1,7]-hydrogen migration. In addition the 19,19-difluoro, 19-methoxy and 19-fluoro
substituted analogs were investigated. The calculated activation barriers decrease in the following order: CHF2>CH3>CH2OCH3 (24.8, 23.5 and 20.1 kcal/mol). This is in qualitative agreement with experiments. It has been suggested that a decrease
of the barrier by a 19-methoxy substituent and its increase by a 19,19-difluoro substituent are phenomena of different origin.
In the case of 19-methoxy substitution, the effect is due to the charge redistribution in the triene system and the decrease
of the C(19)–H bond energy. The effect of two fluorine substituents at C-19 on the activation barrier is suggested to originate
from the combination and balance of several factors: electrostatic repulsion between the negative fluorine atom and the π-electron
cloud over the conjugated system, an increase of the HOMO–LUMO gap, and geminal difluoro substitution affecting C–F and C–C
bond energies.
Received: 17 May 2002 / Accepted: 11 September 2002 /
Published online: 14 February 2003 相似文献
20.
Amy J. Austin Michael J. Frisch John A. Montgomery George A. Petersson 《Theoretical chemistry accounts》2002,107(3):180-186
An overlap criterion is defined that connects the identification of core orbitals in a molecular system, which can be problematic,
to that in isolated atoms, which is well defined. This approach has been tested on a variety of troublesome systems that have
been identified in the literature, including molecules containing third-row main-group elements, and is shown to remove errors
of up to 100 kcal/mol arising from an inconsistent treatment of core orbitals at different locations on a potential-energy
surface. For some systems and choices of core orbitals, errors as large as 19 kcal/mol can be introduced even when consistent
sets of orbitals are frozen, and the new method is shown to identify these cases of substantial core–valence mixing. Finally,
even when there is limited core–valence mixing, the frozen-core approximation can introduce errors of more than 5 kcal/mol,
which is much larger than the presumed accuracy of models such as G2 and CBS-QB3. The source of these errors includes interatomic
core–core and core–valence dispersion forces.
Received: 31 August 2001 / Accepted: 11 October 2001 / Published online: 9 January 2002 相似文献