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1.
We propose an analytic form to represent the intersecting potential energy surfaces (PES) of the first two singlet states
of azomethane. The aim is to run semiclassical simulations of photochemical events such as fragmentation and isomerization.
The PES are based on ab initio calculations and corrected on the basis of available experimental data. We resort to a quasi-diabatic
representation, suitable to deal with the S0-S1 conical intersection and to include the essential information about electronic couplings in a 2 × 2 effective hamiltonian
matrix.
Received: 18 February 1999 / Accepted: 12 April 1999 / Published online: 14 July 1999 相似文献
2.
Sequential Monte Carlo/quantum mechanical calculations are performed to study the solvent effects on the electronic absorption
spectrum of formamide (FMA) in aqueous solution, varying from hydrogen bonds to the outer solvation shells. Full quantum-mechanical
intermediate neglect of differential overlap/singly excited configuration interaction calculations are performed in the supermolecular
structures generated by the Monte Carlo simulation. The largest calculation involves the ensemble average of 75 statistically
uncorrelated quantum mechanical results obtained with the FMA solute surrounded by 150 water solvent molecules. We find that
the n → π* transition suffers a blueshift of 1,600 cm−1 upon solvation and the π → π* transition undergoes a redshift of 800 cm−1. On average, 1.5 hydrogen bonds are formed between FMA and water and these contribute with about 20% and about 30% of the
total solvation shifts of the n → π* and π → π* transitions, respectively. The autocorrelation function of the energy is used
to sample configurations from the Monte Carlo simulation, and the solvation shifts are shown to be converged values.
Received: 14 March 2002 / Accepted: 3 April 2002 / Published online: 24 June 2002 相似文献
3.
4.
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
5.
ZDENĚK TOŠNER SAHAR NIKKHOU ASKI JOZEF KOWALEWSKI 《Journal of inclusion phenomena and macrocyclic chemistry》2006,55(1-2):59-70
The reorientation of 1-adamantanecarboxylic acid (AdCA) within the β-cyclodextrin (β-CD) cavity is investigated by means of multiple-field 13C NMR relaxation. The dissociation constant describing the complexation equilibrium is determined using translational diffusion measurements for the guest during a titration by the host in D2O/DMSO solvent mixture. The changes in apparent diffusion properties of AdCA during the titration are at 25 °C well described assuming the formation of a 1:1 complex, whereas at 0 °C the data indicate the presence of a 2:1 (guest:host) complex. The 13C NMR relaxation parameters for the AdCA molecule bound inside the β-CD cavity are extracted. Despite the high association constant, indicating a strong interaction between the two molecules, the guest molecule is quite mobile. The reorientation of the bound AdCA at 25 °C can be described by either the Lipari–Szabo or the axially symmetric rotational diffusion model. The motion is extremely anisotropic: the adamantyl group rotates fast around the β-CD symmetry axis, inside its cylindrical cavity. At lower temperature, the relaxation properties are no longer possible to explain using these models. Instead, the data are analyzed using extended, three-step spectral density of Clore et al. [J. Am. Chem. Soc. 112, 4989 (1990)]. 相似文献
6.
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
7.
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
8.
9.
The improved generator coordinate Hartree–Fock (GCHF) method is extended to molecular systems. The Griffin–Hill–Wheeler–HF
equations were solved by an integral discretization technique. The method is then implemented with the use of the GAMESS program
and applied to the H2, Li2, and LiH molecules. For these molecules, sequences of basis sets of atom-centred Gaussian-type functions are employed to
explore the accuracy achieved with our approach. For all systems studied, our ground-state HF total energies are better than
those obtained with basis sets generated with the original GCHF method for molecules and larger even-tempered basis sets.
For H2, Li2, and LiH, the differences between our best energies and the corresponding numerical HF results are about 2 × 10−2, 1, and 4 × 10−1 μhartree, respectively. The dipole, quadrupole, and octupole moments at the center of mass and electric field, the electric
field gradient, the electrostatic potential, and the electron density at the nuclei were evaluated and compared with results
reported in the literature.
Received: 4 May 1999 / Accepted: 22 July 1999 / Published online: 2 November 1999 相似文献
10.
Monte Carlo simulations have been carried out for 2-ethoxyethanol (C2E1) in isothermal-isobaric ensemble (NPT) at different temperatures and 1 atm pressure with a continuum configurational biased
procedure in water and chloroform media. Hydrogen bond bridges were formed between adjacent oxygen atoms in C2E1 (CH3CH2OCH2CH2OH) through water molecules. We also found that the stable conformers of C2E1 in water and CHCl3 are different and the effect of temperature on solute-solvent interaction energies is considerable. The self-association
of C2E1 in aqueous and nonaqueous media has been studied by statistical perturbation theory, and the relative free energy has been
obtained at different reaction coordinates by double-wide sampling method. Two minima were found in water solvent in the potential
of mean force (PMF), corresponding to the contact and solvent-separated pairs, but only one minimum was found in CHCl3 solvent corresponding to a contact pair complex.
Received: 18 January 2001 / Accepted: 22 October 2001 / Published online: 21 January 2002 相似文献
11.
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
12.
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
13.
Herschel Rabitz 《Theoretical chemistry accounts》2003,109(2):64-70
Interest in the control of quantum dynamics phenomena has grown in recent years, with laboratory studies showing increasing
successes. The role of theory in the control of quantum phenomena encompasses the design of laser controls, the development
of algorithms to guide the laboratory studies, and the means to analyze the ensuing dynamics observations. Laboratory laser
control instrumentation has the special capability of performing massive numbers of experiments in a short period of time,
to rapidly search for controls that meet the objectives. This unique laboratory feature needs to be factored in when considering
how to best utilize theoretical analyses. The present paper reviews the role that theory is playing, as well as suggests some
future avenues for theory in the laser control of quantum phenomena.
Received: 8 June 2002 / Accepted: 7 October 2002 / Published online: 10 March 2003
Acknowledgements. The author acknowledges support from the National Science Fund and the US Department of Defense. 相似文献
14.
Jing-yao Liu Ze-sheng Li Zhen-wen Dai Xu-ri Huang Chia-chung Sun 《Theoretical chemistry accounts》2002,108(3):179-186
The hydrogen-abstraction reaction C2H5F+O → C2H4F+OH has been studied by a dual-level direct dynamics method. For the reaction, three reaction channels, one for α-abstraction
and two for β-abstraction, have been identified. The potential-energy surface information is obtained at the MP2(full)/6-311G(d,p)
and PMP2(full)/6-311G(3df,3pd) (single-point) levels. By canonical variational transition-state theory, rate constants for
each reaction channel are calculated with a small-curvature tunneling correction. The total rate constant is calculated from
the sum of the individual rate constants and the temperature dependence of the branching ratios is obtained over a wide range
of temperatures from 300 to 5,000 K. The agreement of the rate constants with experiment is good in the experimental temperature
range from 1,000 to 1,250 K. The calculated results indicate that at low temperatures α-abstraction is most likely to be the
major reaction channel, while β-abstraction channels will significantly contribute to the whole reaction rate as the temperature
increases.
Received: 23 January 2002 / Accepted: 23 June 2002 / Published online: 20 September 2002 相似文献
15.
Multicenter integrals appearing in the Hartree–Fock–Roothaan equations for molecules are calculated using different kinds
of series expansion formulas obtained from the expansions of integer and noninteger n Slater-type orbitals, in terms of Ψ
α
-exponential-type orbitals (where α=1, 0, –1, –2,...) at a displaced center, that form complete orthonormal sets and are represented by linear combinations of
integer n Slater-type orbitals. The convergence of these series is tested by calculating concrete cases. The accuracy of the results
is quite high for quantum numbers, screening constants, and location of orbitals.
Received: 13 February 2002 / Accepted: 11 March 2002 / Published online: 4 July 2002 相似文献
16.
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
17.
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
18.
L. Delle Site 《Theoretical chemistry accounts》2002,107(6):378-380
In this work we comment on the statement about the nonuniqueness of the solution of Bader's equation for defining atoms in
molecules reported in the article of P. Cassam-Chena? and D. Jayatilaka in Theoretical Chemistry Accounts (2001) 105: 213–218
Received: 10 May 2001 / Accepted: 7 September 2001 / Published online: 3 June 2002 相似文献
19.
Relativistic energy-consistent small-core lanthanide pseudopotentials of the Stuttgart–Bonn variety and extended valence
basis sets have been used for the investigation of the dimers La2 and Lu2. It was found that the ground states for La2 and Lu2 are most likely 1∑
g
+ (σ
g
2π
u
4) and 3∑
g
− (4f
144f
14σ
g
2σ
u
2πu
2), respectively. The molecular constants including error bars were derived from multireference configuration interaction as
well as coupled-cluster calculations, taking into account corrections for atomic spin–orbit splitting as well as possible
basis set superposition errors. The theoretical values for La2 (R
e=2.70±0.03 ?, D
e=2.31±0.13 eV, ωe=186±13 cm−1) show good agreement with the experimental binding energy (D
e=2.52±0.22 eV), but the experimental vibrational constant in an Ar matrix (ωe=236±0.8 cm−1) is significantly higher. For Lu2 the theoretical values (R
e=3.07±0.03 ?, D
e=1.40±0.12 eV, ωe=123±1 cm−1) are in overall excellent agreement with experimental data (D
e=1.43±0.34 eV, ωe=122± 1 cm−1). The electronic structures of La2 and Lu2 are compared to those other lanthanide dimers and trends in the series are discussed.
Received: 25 March 2002 / Accepted: 2 June 2002 / Published online: 21 August 2002 相似文献
20.
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