共查询到20条相似文献,搜索用时 15 毫秒
1.
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
2.
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
3.
Contracted Gaussian-type function sets are developed for correlating p, d, and f functions for a valence electron of the
hydrogen atom and alkali-metal atoms from Li to Rb. A segmented contraction scheme is used for its compactness and efficiency.
Contraction coefficients and exponents are determined by minimizing the deviation from the K orbitals of the atoms. The present basis sets yield an accuracy comparable to the correlation-consistent basis set for the
hydrogen atom and also give a similar high accuracy for the alkali-metal atoms. In the calculations of spectroscopic constants
of alkali hydrides, the decontraction of the p function plays an important role, especially for LiH. The contributions of
d and f functions are nontrivial for KH and RbH.
Received: 6 September 2002 / Accepted: 13 November 2002 / Published online: 19 March 2003
Acknowledgements. This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education of Japan.
Correspondence to: T. Noro e-mail: tashi@sci.hokudai.ac.jp 相似文献
4.
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
5.
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
6.
The radial electron-pair intracule (relative motion) H(u) and extracule (center-of-mass motion) D(R) densities in position space were known to reveal four types of maxima which are related to the four inner electron shells,
K, L, M, and N, of atoms. The corresponding radial electron-pair intracule Hˉ(v) and extracule Dˉ(P) densities in momentum space are studied for the 102 atoms from He (atomic number Z=2) to Lr (Z=103). The densities Hˉ(v) and Dˉ(P) are found to have either one maximum or two maxima, and the numbers of maxima in Hˉ(v) and Dˉ(P) are the same for 98 atoms. For these atoms, the locations υ
max and P
max and the heights Hˉ
max
and Dˉ
max
of the corresponding maxima satisfy the approximate relations υ
max ≅ 2P
max and Hˉ
max
≅Dˉ
max
/2. On the basis of their Z-dependence, the maxima in Hˉ(v) and Dˉ(P) of the 102 atoms are classified into five types. Shell-pair decompositions of the radial densities show that these maxima
reflect five outer electron shells of atoms.
Received: 24 January 2001 / Accepted: 12 March 2001 / Published online: 13 June 2001 相似文献
7.
8.
Adding the tight and diffuse Gaussian-type functions (GTFs), Faegri's variationally determined double-zeta-quality basis
sets for molecular relativistic calculations are examined. An example atom is Cm. When the tight s-type GTF is added the total
energy increases, whereas when diffuse GTFs are added the total energy decreases. The reasons for these findings are clarified.
It is also pointed out that not only the Faegri's sets but also other variationally determined basis sets would show similar
behavior so far as the expansion terms are not sufficient.
Received: 22 July 2002 / Accepted: 21 October 2002 / Published online: 31 January 2003
Correspondence to: H. Tatewaki e-mail: htatewaki@nsc.nagoya.cu.ac.jp 相似文献
9.
The recent “chemical energy component analysis” permits the total energy of a molecule to be presented approximately but
to good accuracy as a sum of atomic and diatomic energy contributions. Here the diatomic energy components are further decomposed
into terms of different physical origin: electrostatics (in point-charge approximation and the distributed charge corrections),
exchange effects, diatomic overlap and atomic basis extension terms. This analysis may provide us with a deeper insight into
the factors influencing both the chemical bonds and the nonbonded interatomic interactions.
Received: 6 May 2002 / Accepted: 13 November 2002 / Published online: 19 March 2003
Acknowledgements. The authors are indebted to the Hungarian Scientific Research Fund for partial financial support (grant no. OTKA T29716).
Correspondence to: I. Mayer e-mail: mayer@chemres.hu 相似文献
10.
Starting with the Levinthal paradox, a brief introduction to the protein folding problem is presented. The existing theories
of protein folding, including the folding funnel scenario, are discussed. After briefly discussing different simulation studies
of model proteins, we discuss our recent work on the dynamics of folding of the model HP-36 (the chicken villin headpiece)
protein by using a simplified hydropathy scale. Special attention has been paid to the statics and dynamics of contact formation
among the hydrophobic residues. The results obtained from this simple model appear to be surprisingly similar to several features
observed in the folding of real proteins. The account concludes with a discussion of future problems.
Received: 28 April 2002 / Accepted: 11 August 2002 / Published online: 13 November 2002
Correspondence to: B. Bagchi e-mail: bbagchi@sscu.iisc.ernet.in
Acknowledgements. We thank Samir Pal and Arnab Mukherjee for many discussions and Arun Yethiraj for helpful suggestions. The financial support
from DST, India, is gratefully acknowledged. G. S. thanks CSIR for a research fellowship. 相似文献
11.
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
12.
13.
Recent advances in the study of short-lived reactive resonances are reviewed. Special emphasis is given to addressing the
question of how reactive resonances might be observed in molecular beam scattering experiments. Three case studies are presented
for simple triatomic systems that are believed to exhibit resonance phenomena: F + HD → D + HF, F + H2 → H + HF, and H + HD → D + H2. It is seen that reactive resonances do strongly influence collision observables, but in a different way for each case. At
this stage, there does not appear to be a unique resonance signature that can be applied to all reactions.
Received: 2 March 2002 / Accepted: 2 June 2002 / Published online: 17 October 2002
Acknowledgements. For their valuable input into the research discussed here, we are grateful to our collaborators, K. Liu, S.H. nk;Lee, F.
Dong, D.E. Manolopoulos, D, Skouteris, X. Yang, S. Harich, D. Dai, and C.C. Wang. This work was supported by the National
Science Foundation.
Correspondence to: R. T. Skodje e-mail: skokje@spot.colorado.edu 相似文献
14.
15.
Martin Gruebele 《Theoretical chemistry accounts》2003,109(2):53-63
Vibrational energy flow in organic molecules occurs by a multiple-time-scale mechanism that can be modeled by a single exponential
only in its initial stages. The mechanism is a consequence of the hierarchical structure of the vibrational Hamiltonian, which
leads to diffusion of vibrational wavepackets on a manifold with far fewer than the 3N−6 dimensions of the full vibrational state space. The dynamics are controlled by a local density of states, which does not
keep increasing with molecular size. In addition, the number of vibrational coordinates severely perturbed during chemical
reaction is small, leading to preservation of the hierarchical structure at chemically interesting energies. This regularity
opens up the possibility of controlling chemical reactions by controlling the vibrational energy flow. Computationally, laser
control of intramolecular vibrational energy redistribution can be modeled by quantum-classical, or by purely quantum-mechanical
models of the molecule and control field.
Received: 26 July 2002 / Accepted: 30 September 2002 / Published online: 2 December 2002
Electronic Supplementary Material to this paper can be obtained by using the Springer Link server located at http://dx.doi.org/10.1007/s00214-002-0394-2.
Acknowledgements. This work was supported by NSF grant CHE 9986670.
Correspondence to: M. Gruebele e-mail: gruebele@scs.uiuc.edu 相似文献
16.
Toshikatsu Koga 《Theoretical chemistry accounts》2002,107(4):246-249
On the basis of multiconfiguration Hartree–Fock calculations, correlated electron-pair intracule (relative motion) and extracule
(center-of-mass motion) properties are reported for the Li atom in momentum space. The present results are more accurate and
consistent than those in the literature.
Received: 10 September 2001 / Accepted: 11 December 2001 / Published online: 22 March 2002 相似文献
17.
Computational schemes are presented with which to evaluate the electrostatic Coulomb energy in relativistic molecular electronic
structure calculations using a basis of four-component Dirac spinor amplitudes. We demonstrate that algorithms may be constructed
and implemented which differ only in minor details from those in common use in nonrelativistic quantum chemistry, and that
the four-component formalism is neither as complicated nor as expensive as has been suggested recently in the literature.
Spherically symmetrical atomic basis sets are presented which indicate that accurate representations of the Coulomb energy
may be obtained using modest expansions of the electronic density in a scalar auxiliary basis set of spherical harmonic Gaussian-type
functions.
Received: 15 April 2002 / Accepted: 15 May 2002 / Published online: 29 July 2002 相似文献
18.
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
19.
Jordi Poater Miquel Solà Miquel Duran Xavier Fradera 《Theoretical chemistry accounts》2002,107(6):362-371
Localization, λ(A), and delocalization indices, δ(A,B), as defined in the atoms in molecules theory, are a convenient tool for the analysis of molecular electronic structure from
an electron-pair perspective. These indices can be calculated at any level of theory, provided that first- and second-order
electron densities are available. In particular, calculations at the Hartree–Fock (HF) and configuration interaction (CI)
levels have been previously reported for many molecules. However, λ(A) and δ(A,B) cannot be calculated exactly in the framework of Kohn–Sham (KS) density functional theory (DFT), where the electron-pair
density is not defined. As a practical workaround, one can derive a HF-like electron-pair density from the KS orbitals and
calculate approximate localization and delocalization indices at the DFT level. Recently, several calculations using this
approach have been reported. Here we present HF, CI and approximate DFT calculations of λ(A) and δ(A,B) values for a number of molecules. Furthermore, we also perform approximate CI calculations using the HF formalism to obtain
the electron-pair density. In general, the approximate DFT and CI results are closer to the HF results than to the CI ones.
Indeed, the approximate calculations take into account Coulomb electron correlation effects on the first-order electron density
but not on the electron-pair density. In summary, approximate DFT and CI localization and delocalization indices are easy
to calculate and can be useful in the analysis of molecular electronic structure; however, one should take into account that
this approximation increases systematically the delocalization between covalently bonded atoms, with respect to the exact
CI results.
Received: 13 February 2002 / Accepted: 24 April 2002 / Published online: 18 June 2002 相似文献
20.
Evaluation of an ab initio quantum mechanical/molecular mechanical hybrid-potential link-atom method
Hybrid potentials have become a common tool in the study of many condensed-phase processes and are the subject of much active
research. An important aspect of the formulation of a hybrid potential concerns how to handle covalent bonds between atoms
that are described with different potentials and, most notably, those at the interface of the quantum mechanical (QM) and
molecular mechanical (MM) regions. Several methods have been proposed to deal with this problem, ranging from the simple link-atom
method to more sophisticated hybrid-orbital techniques. Although it has been heavily criticized, the link-atom method has
probably been the most widely used in applications, especially with hybrid potentials that use semiempirical QM methods. Our
aim in this paper has been to evaluate the link-atom method for ab initio QM/MM hybrid potentials and to compare the results
it gives with those of previously published studies. Given its simplicity and robustness, we find that the link-atom method
can produce results of comparable accuracy to other methods as long as the charge distribution on the MM atoms at the interface
is treated appropriately.
Received: 27 September 2002 / Accepted: 21 October 2002 / Published online: 8 January 2003
Correspondence to: M. J. Field e-mail: mjfield@ibs.fr
Acknowledgements. The authors thank the Institut de Biologie Structurale – Jean-Pierre Ebel, the Commissariat à l'Energie Atomique and the
Centre National de la Recherche Scientifique for support of this work. 相似文献