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1.
Maria Bykhovskaia Boris Gelmont Tatiana Globus Dwight L. Woolard Alan C. Samuels Tap Ha Duong Krystyna Zakrzewska 《Theoretical chemistry accounts》2001,106(1-2):22-27
We present a computational method which couples normal mode analysis in internal coordinates of a molecule with very far
IR spectroscopy. The analytical expression for the dependence of IR absorption on frequency incorporates frequencies and optical
activities of each normal mode. In order to predict far-IR spectra of a molecule we evaluate the optical activity of each
normal mode. This optical activity is determined by the vibration amplitude of the dipole moment produced by a normal mode.
We calculated normal modes of DNA double-helical fragments (dA)12 · (dT)12 and (dA-dT)6 · (dA-dT)6 and evaluated their optical activities. These were found to be very sensitive to the DNA base-pair sequence. The positions
of the resonance peaks in the calculated absorption spectrum of (dA)12 · (dT)12 are in a good agreement with those obtained by Fourier transform IR spectroscopy (Powell JW et al. 1987 Phys Rev A 35: 3929–3939).
Received: 20 June 2000 / Accepted: 5 January 2001/ Published online: 3 May 2001 相似文献
2.
Sándor Kristyán Adrienn Ruzsinszky Gábor I. Csonka 《Theoretical chemistry accounts》2001,106(5):319-328
Gaussian-3 ground-state total electronic energies have been approximated using single point 6-31G(d) basis set Harteee–Fock
self-consistent-field (HF-SCF) total energies and partial charges based on our earlier rapid estimation of correlation energy
from partial charges method. Sixty-five closed-shell neutral molecules (composed of H, C, N, O, and F atoms) of the G2/97
thermochemistry database were selected for the present study. The main feature in this work is that the␣basis set error has
been treated by the least squares fit of rapid estimation of basis set error and correlation energy from partial charges (REBECEP)
atomic parameters. With these parameters a rather accurate closed-shell ground-state electronic total energy can be obtained
from a small basis set HF-SCF calculation in the vicinity of stationary points. The average absolute deviation of the best
REBECEP enthalpies of formation from the experimental enthalpies of formation is 1.39 kcal/mol for the test set of 65 enthalpies
of neutral molecules.
Received: 11 December 2000 / Accepted: 6 February 2001/Published online: 11 October 2001 相似文献
3.
We propose a method to locate saddle points that is based on the interplay between the driving coordinate and the restricted
quasi-Newton algorithm. The method locates the transition state using a reduced potential-energy surface. The reduced potential-energy
surface is characterized by the set of driving coordinates. The proposed algorithm starts at a point on the surface that is
slightly perturbed from either reactant or product and, in principle, converges to the transition state. Finally we give a
special type of update Hessian matrix formula that should be applied in optimizations carried out on reduced potential-energy
surfaces.
Received: 29 September 2000 / Accepted: 3 January 2001 / Published online: 3 April 2001 相似文献
4.
W. A. de Jong R. J. Harrison J. A. Nichols D. A. Dixon 《Theoretical chemistry accounts》2001,107(1):22-26
A complete set of fully-relativistic benchmark results for the bond lengths and vibrational frequencies of uranyl at various
levels of correlation treatment are presented. It is shown that the relativistic and correlation effects are of the same magnitude
and should be treated on an equal footing. Results of uranyl calculations using various relativistic effective core potentials
(RECP) are presented. Appropriate choices for RECPs for use in density functional theory (DFT) calculations in the local density
approximation (LDA) and with the gradient-corrected B3LYP exchange-correlation functional are discussed. The conclusion is
reached that small-core RECPs need to be used and that the best results as compared to the benchmark values are obtained by
using a DFT functional that includes some fraction of Hartree-Fock Exchange.
Received: 18 May 2001 / Accepted: 25 July 2001 / Published online: 30 October 2001 相似文献
5.
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
6.
7.
Fernando R. Ornellas 《Theoretical chemistry accounts》2000,103(6):469-476
The [H,S,Cl] potential-energy surface has been investigated at the self-consistent field (SCF), complete active space self-consistent
field (CASSCF), second-order M?ller–Plesset, coupled-cluster single-double and perturbative triple excitation, [CCSD(T)]/6-31G(d,p),
6-31G(2df,2pd), and correlation-consistent polarized valence triple zeta (cc-pVTZ) levels of theory. CCSD(T)/ cc-pVTZ results
predict a very stable HSCl species, an isomer HClS, 51.84 kcal/mol higher in energy, and a transition state 57.68 kcal/mol
above HSCl. Independent of the level of theory, results with the smaller 6-31G(d,p) basis set turned out to be poor, especially
for HClS. Vibrational analysis indicates that both species can be easily differentiated if isolated. Bonding differences between
these molecules are illustrated by contour plots of valence orbitals. Viewed classically, bonding in HClS involves a dative
bond. Transition-state rate constants, and equilibrium constants for the HSCl ↔ HClS isomerization have been estimated for
various temperatures (200–1000 K). At 298.15 K, the forward rate is predicted to be 7.95 × 10−29 s−1, and the equilibrium constant to be 2.31 × 10−38. Tunneling corrections vary from 1.57 at 298.15 K to 1.05 at 1000 K. Activation energies have been obtained by a two-points
linear fit to the Arrhenius equation.
Received: 7 May 1999 / Accepted: 22 July 1999 / Published online: 4 October 1999 相似文献
8.
In order to understand the structure of DNAs and their interactions when on microarray surfaces, we performed the first all-atom
molecular dynamics simulation of DNA tethered to a surface. On the surface, the binding of the DNA was enhanced, and its average
equilibrium conformation was the B form. The DNA duplex spontaneously tilted towards its nearest neighbor and settled in a leaning position with a interaxial
distance of 2.2 nm. This close packing of the DNAs, which affects both in situ synthesis and deposition of probes on microarray
surfaces, can thus be explained by salted-induced colloidlike DNA–DNA attractions.
Received: 30 November 2000 / Accepted: 7 February 2001 / Published online: 22 May 2001 相似文献
9.
Monte Carlo simulations have been carried out for 2-methoxyethanol in an isothermal–isobaric ensemble (NPT) at 298.15 K and
1 atm pressure. The optimized potential for liquid simulation force field parameters has been used for modeling 2-methoxyethanol
and the TIP4P model for water. Intramolecular rotations are described by an analytical potential function fitted to ab initio
energies. It has been shown that the water molecules can form hydrogen bonds between adjacent O atoms of CH3OCH2CH2OH in aqueous media. The self-association of 2-methoxyethanol in aqueous media has been studied by statistical perturbation
theory.
Received: 9 October 2000 / Accepted: 5 January 2001 / Published online: 3 May 2001 相似文献
10.
Density functional theory, in combination with a) a careful choice of the exchange-correlation part of the total energy and b) localized basis sets for the electronic orbitals, has become the method of choice for calculating the exchange-couplings
in magnetic molecular complexes. Orbital expansion on plane waves can be seen as an alternative basis set especially suited
to allow optimization of newly synthesized materials of unknown geometries. However, little is known on the predictive power
of this scheme to yield quantitative values for exchange coupling constants J as small as a few hundredths of eV (50–300 cm−1). We have used density functional theory and a plane waves basis set to calculate the exchange couplings J of three homodinuclear Cu-based molecular complexes with experimental values ranging from +40 cm−1 to −300 cm−1. The plane waves basis set proves as accurate as the localized basis set, thereby suggesting that this approach can be reliably
employed to predict and rationalize the magnetic properties of molecular-based materials.
Corresponding author. E-mail: Carlo.Massobrio@ipcms.u-strasbg.fr
Received August 5, 2002; accepted August 9, 2002 相似文献
11.
Experimental tie-line data for two quaternary systems, water + diisopropyl ether + 2,2,4-trimethylpentane + methyl tert-butyl ether or toluene, were investigated at 298.15 K and atmospheric pressure. The experimental liquid–liquid equilibrium
data were correlated using a modified UNIQUAC activity coefficient model with ternary and quaternary parameters, in addition
to the binary ones. The calculated results were further compared with those obtained with an extended UNIQUAC model from Nagata
[Fluid Phase Equilib. 54, 191–206 (1990)]. 相似文献
12.
The “hybrid protein model” is a fuzzy model for compacting local protein structures. It learns a nonredundant database encoded
in a previously defined structural alphabet composed of 16 protein blocks (PBs). The hybrid protein is composed of a series
of distributions of the probability of observing the PBs. The training is an iterative unsupervised process that for every
fold to be learnt consists of looking for the most similar pattern present in the hybrid protein and modifying it slightly.
Finally each position of the hybrid protein corresponds to a set of similar local structures. Superimposing those local structures
yields an average root mean square of 3.14 ?. The significant amino acid characteristics related to the local structures are
determined. The use of this model is illustrated by finding the most similar folds between two cytochromes P450.
Received: 13 June 2000 / Accepted: 18 September 2000 / Published online: 19 January 2001 相似文献
13.
Maria Wiśniewska Sławomir J. Grabowski 《Monatshefte für Chemie / Chemical Monthly》2002,133(3):305-312
Summary. The complexes of formaldehyde and some of its derivatives with HF and HCl were investigated at HF/6-311 + +G** and MP2/6-311 + +G** levels of theory. Interaction energies were corrected for the basis set superposition error (BSSE). The full optimizations
of dimers and monomers were performed during calculations. The Bader theory of atoms-in-molecules (AIM) was also applied for the localization of bond critical points (BCP) and for the calculation
the electron densities and their Laplacians at these points. The relationships between H-bond energy and parameters obtained from calculations were also studied.
Received June 29, 2001. Accepted (revised) October 29, 2001 相似文献
14.
Hélio F. Dos Santos Patrick J. O'Malley Wagner B. De Almeida 《Theoretical chemistry accounts》1998,99(5):301-311
In the present work, the conformational equilibrium for the herbicide diuron (DCMU) has been investigated using high level
ab initio calculations. The solvent effect was included through two different continuum models: (1) the real cavity IPCM method
and (2) the standard dipole Onsager model SCRF. The effect due to solute-solvent hydrogen-bond interactions was analyzed considering
a hybrid discreet-continuum model. At the Hartree-Fock level, the gas phase results showed that only the trans forms (A and B) are present in the equilibrium mixture, with the relative concentrations found to be 33% (A) and 67% (B)
(HF/6-311+G**//6-31G**). When the electronic correlation effect is included (MP2/6-31G*//HF/6-31G*), a relative stabilization
of the cis forms was observed, with the conformational distribution calculated as 38% (A), 50% (B), 6% (C) and 6% (D). The trans conformations were found to be completely planar, which has been considered to be a prerequisite for the herbicide binding.
In water solution, the trans conformation A should be the most abundant conformer, the IPCM and SCRF values being ca. 100% and ca. 85% respectively. The
IPCM calculations with the isodensity level set to 0.0005 present a conformational distribution close to that obtained from
the hybrid model [92% (A) and 8% (B)], which has been considered our best solvent approach. Regarding the biological action
of urea-type herbicides, the results presented here are important, because some QSAR studies have suggested that the partition
coefficient is related to the herbicide activity, so the conformational equilibrium may play a role in the biological action.
Received: 23 February 1998 / Accepted: 28 May 1998 / Published online: 19 August 1998 相似文献
15.
Ab initio calculations have been performed to study the molecular structures and vibrational levels of the four low-lying
ionic states (1, 22Π, and 1, 22Σ+) of carbonyl sulfide. The global regions of the potential-energy surfaces have been obtained by multireference single and
double excitation configuration interaction calculations. Vibrational calculations using explicit vibrational Hamiltonians
have been used for vibrational analysis. The equilibrium molecular structures and a vibrational analysis of the four ionic
states are presented. The theoretical ionization intensity curves including the vibrational structures of the ionic states
are also presented and are compared with the photoelectron spectrum.
Received: 20 January 2001 / Accepted: 22 August 2001 / Published online: 30 October 2001 相似文献
16.
Rita Prosmiti Alexei A. Buchachenko Pablo Villarreal Gerardo Delgado-Barrio 《Theoretical chemistry accounts》2001,106(6):426-433
Ab initio molecular electronic structure calculations are performed for H5
+ at the QCISD(T) level of theory, using a correlation-consistent quadruple-zeta basis set. Structures, vibrational frequencies
and thermochemical properties are evaluated for ten stationary points of the H5
+ hypersurface and are compared with previous calculations. The features of the H3
+…H2 interaction at intermediate and large intermolecular distances are also investigated. Furthermore, an analytical functional
form for the potential-energy surface of H5
+ is derived using a first-order diatomics-in-molecule perturbation theory approach. Its topology is found to be qualitatively
correct for the short-range interaction region.
Received: 15 March 2001 / Accepted: 5 July 2001 / Published online: 11 October 2001 相似文献
17.
Liquid–liquid equilibrium tie line data were determined for three quaternary systems water + ethanol + diethyl carbonate+n-heptane, water + ethanol + 1,1-dimethylethyl methyl ether + diethyl carbonate, and water + 1,1-dimethylethyl methyl ether + diethyl
carbonate+n-heptane at 298.15 K and atmospheric pressure. The experimental liquid–liquid equilibria results have been correlated using
a modified UNIQUAC model and an extended UNIQUAC model, both with multicomponent interaction parameters in addition to the
binary ones. 相似文献
18.
A model of low-barrier hydrogen bonds (LBHBs) in enzymes has been studied by ab initio quantum mechanical calculations including
the self-consistent reaction field solvent model. The hydrogen-bond strengths and the deprotonation energies for the hydrogen-bonded
and non-hydrogen-bonded cis-urocanic acid were calculated at the HF/6-31 + G(d,p) level at various dielectric constants. The same calculations were performed
for the α,β-dihydrourocanic acid to model the catalytic dyad of serine protease. The deprotonation energy of Nɛ2 in α,β-dihydrourocanic acid is increased by formation of LBHBs and depends very much on the dielectric constant. This study
suggests that the formation of LBHBs increases the basicity of the dyad, and the polarity change near the reaction center
in the active site could help the proton abstraction from Ser 195 and the donation to the leaving group. Both the LBHBs and
the environment can play crucial roles in the enzyme catalysis.
Received: 8 March 2000 / Accepted: 3 January 2001 / Published online: 3 May 2001 相似文献
19.
The numerical properties of the radial part of overlap integrals with the same screening parameters in the form of polynomials
in p = ξR over Slater-type orbitals have been studied and obtained by using three different methods. For that purpose, the characteristics
of auxiliary functions were used first, then Fourier transform convolution theorem, and recurrence relations for the basic
coefficients of A
s
n
l
λ,
n
′
l
′λ were used. The calculations of the radial part of overlap integrals with the same screening parameters were made in the range
1 ≤ n ≤ 75, 1 ≤ n′ ≤ 75, and 10−6 ≤ p.
Received: 18 January 2001 / Accepted: 5 April 2001 / Published online: 27 June 2001 相似文献
20.
Andrea D. Magrí A. L. Magrí Fabrizio Balestrieri Amalia Sacchini D. Marini 《Analytical and bioanalytical chemistry》1997,357(7):985-988
A sensitive spectrophotometric method for the determination of ethanol with potassium dichromate was developed. Chromium(VI),
in 7 mol L-1 perchloric acid, reacts quantitatively with ethanol to form chromium(III) and acetic acid. The reaction is complete in about
15 min at room temperature and the chromium(VI) consumed may be determined by the decrease of the absorbance at 267 nm. The
analytical working parameters (such as acidity and ionic strength of the solution, temperature and time of the reaction, chromium(VI)
concentration, matrix interferences) were optimized. The apparent molar absorptivities of chromium(VI), in the monomer and
dimer form, and the equilibrium quotient for the dimerization reaction were determined in 3 mol L-1 perchloric acid solution. The method was applied to the analysis of micro samples of commercial beverages (beers, wines and
spirits) without the previous distillation of ethanol.
Received: 9 April 1996/Revised: 3 June 1996/Accepted: 11 June 1996 相似文献