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1.
Derek M. Dolney Gregory D. Hawkins Paul Winget Daniel A. Liotard Christopher J. Cramer Donald G. Truhlar 《Journal of computational chemistry》2000,21(5):340-366
Atomic surface tensions are parameterized for use with solvation models in which the electrostatic part of the calculation is based on the conductor‐like screening model (COSMO) and the semiempirical molecular orbital methods AM1, PM3, and MNDO/d. The convergence of the calculated polarization free energies with respect to the numerical parameters of the electrostatic calculations is first examined. The accuracy and precision of the calculated values are improved significantly by adjusting two parameters that control the segmentation of the solvent‐accessible surface that is used for the calculations. The accuracy of COSMO calculations is further improved by adopting an optimized set of empirical electrostatic atomic radii. Finally, the electrostatic calculation is combined with SM5‐type atomic surface tension functionals that are used to compute the nonelectrostatic portions of the solvation free energy. All parameterizations are carried out using rigid (R) gas‐phase geometries; this combination (SM5‐type surface tensions, COSMO electrostatics, and rigid geometries) is called SM5CR. Six air–water and 76 water–solvent partition coefficients are added to the training set of air–solvent data points previously used to parameterize the SM5 suite of solvation models, thereby bringing the total number of data points in the training set to 2266. The model yields free energies of solvation and transfer with mean unsigned errors of 0.63, 0.59, and 0.61 kcal/mol for AM1, PM3, and MNDO/d, respectively, over all 2217 data points for neutral solutes in the training set and mean unsigned errors of 3.0, 2.7, and 3.1 kcal/mol, respectively, for 49 data points for the ions. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 340–366, 2000 相似文献
2.
The stability of various conformers of myrsinone (2,3-dihydroxy-5-undecyl-1,4-benzoquinone) and its tautomer [2,3-dihydroxy-5-(undec-1-ene)-cyclohex-2-ene-1,4-dione]
has been studied in vacuo and in solution on model systems with the long alkyl side chains replaced with ethyl and eth-1-ene
groups, respectively. Ab initio Hartree–Fock (HF) calculations in vacuo and free-energy calculations either in chloroform
or in water solution, in the framework of the polarizable continuum model, have been carried out on the 6-31G* optimized geometries
obtained in vacuo and in solution (water or chloroform). The nature of the stationary points found was investigated using
normal mode analysis. The energy gap between the two tautomeric forms turns out to be about 1.3 kcal/mol in vacuo and becomes
about 0.3 kcal/mol in chloroform solution, whereas in water the second tautomer is favored by about 1.2 kcal/mol. The effect
of second-order M?ller–Plesset (MP2) correlation corrections has been considered on both the energy and the geometry of the
tautomers in vacuo, whereas in solution only their effect on the energy was taken into account. The contribution to the stability
in the gas phase and in solution of the intramolecular hydrogen bonds between the hydroxy hydrogen and the quinonoid oxygen
is larger at the MP2 level than at the HF one. The directionality of the hydrogen bonds between the hydroxy groups affects
considerably only the stability of the isomer with the exocyclic double bond. The computed chemical shifts for the two tautomers
were compared to the experimental ones. In addition the solvation properties of 2,5-dihydroxy-3-ethyl-1,4-benzoquinone and
of an ortho-quinonoid system were considered in order to evaluate the effect of the repulsion between vicinal hydroxy or quinonoid groups.
Received: 14 September 1999 / Accepted: 3 February 2000 / Published online: 2 May 2000 相似文献
3.
Jason D. Thompson Christopher J. Cramer Donald G. Truhlar 《Theoretical chemistry accounts》2005,113(2):107-131
Hybrid density functional theory, which is a combined Hartree–Fock and density functional method, provides a simple but effective way to incorporate nonlocal exchange effects and static and dynamical correlation energy into an orbital-based theory with affordable computational cost for many important problems of gas-phase chemistry. The inclusion of a reaction field representing an implicit solvent in a self-consistent hybrid density functional calculation provides an effective and efficient way to extend this approach to problems of liquid-phase chemistry. In previous work, we have parameterized several models based on this approach, and in the present article, we present several new parameterizations based on implicit solvation models SM5.43 and SM5.43R. In particular, we extend the applicability of these solvation models to several combinations of the MPWX hybrid-density functional with various one-electron basis sets, where MPWX denotes a combination of Barone and Adamos modified version of Perdew and Wangs exchange functional, Perdew and Wangs correlation functional, and a percentage X of exact Hartree–Fock exchange. SM5.43R parameter optimizations are presented for the MPWX/MIDI!, MPWX/MIDI!6D, and MPWX/6-31+G(d,p) combinations with X=0 (i.e., pure density functional theory), 25, 42.8, and 60.6, and for MPWX/6-31G(d) and MPWX/6-31+G(d), with X=0, 42.8, and 60.6; this constitutes a total of 18 new parameter sets. [Note that parameter optimizations using MPW25/6-31G(d) and MPW25/6-31+G(d) were carried out in a previous SM5.43R parameterization.] For each of the five basis sets, we found no significant loss in the accuracy of the model when parameters averaged over the four values of X are used instead of the parameters optimized for a specific value of X. Therefore for each of the five basis sets used here, the SM5.43R and SM5.43 models are defined to have a single parameter set that can be used for any value of X between 0 and 60.6. The new models yield accurate free energies of solvation for a broad range of solutes in both water and organic solvents. On the average, the mean-unsigned errors, as compared with those from experiment, of the free energies of solvation of neutral solutes range from 0.50 to 0.55 kcal/mol and those for ions range from 4.5 to 4.9 kcal/mol. Since the SM5.43R model computes the free energy of solvation as a sum of bulk-electrostatic and non-bulk-electrostatic contributions, it may be used for detailed analysis of the physical effects underlying a calculation of the free energy of solvation. Several calculations illustrating the partitioning of these contributions for a variety of solutes in n-hexadecane, 1-octanol, and water are presented.Acknowledgement This work was supported by a Department of Defense Multidisciplinary University Research Initiative (MURI) grant managed by the Army Research Office and by the National Science Foundation. 相似文献
4.
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
5.
Ab initio HF/6-31G* and MP2/6-31G*//HF/6-31G* methods were used to calculate the structure optimization and conformational interconversion pathways for all-(Z )-cyclododeca-1,4,7,10-tetraene. This compound adopts the symmetrical crown (C 4v) conformation. Ring inversion takes place via symmetrical intermediates, such as boat-chair (BC, C s) and twist (C 2h) conformers and requires about 22.3 kJ · mol−1. The calculated strain energies for BC and twist conformers are 5.9 and 13.5 kJ · mol−1. The results of semiempirical AM1 calculations for structural parameters and relative energies of the important geometries of the title compound are in good agreement with the results of ab initio methods. 相似文献
6.
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
7.
Density functional theory transition structures were located for three concerted [6 + 4] cycloaddition reactions involving
cis-hexatriene and butadiene, cyclopentadiene and cycloheptatriene, and cyclopentadiene and tropone. Geometries, energies, and
entropies were computed at the Becke3LYP/6-31G* level. The activation energy of the concerted [6 + 4] cycloaddition of hexatriene
and butadiene is 33.3 kcal/mol, about 8 kcal/mol above the activation energy of the butadiene plus ethylene [4 + 2] cycloaddition.
The endo concerted [6 + 4] transition state is 1.1 kcal/mol higher than the exo. The [6 + 4] reaction of cyclopentadiene and
cycloheptatriene has a barrier of 25.9 kcal/mol, while the cyclopentadiene–tropone barrier drops to 20.7 kcal/mol.
Received: 3 December 1998 / Accepted: 18 February 1999 / Published online: 7 June 1999 相似文献
8.
Using a recently proposed orbital deletion procedure and the block-localized wavefunction method, the rotational barriers
in H2BNH2 and H2BPH2 are analyzed in terms of conjugation, hyperconjugation, steric effect and pyramidalization. With the zero-point energy corrections,
the π-binding strengths in the planar H2BNH2 and H2BPH2 are both around 20 kcal/mol at the HF level using the 6-311+G** basis set. With the deactivation of the π atomic orbitals
on the boron atom and the evolution from a planar structure to a 90°-twisted structure, the steric repulsion between the B‐H
and the N‐H or P‐H is relieved and moreover, the negative hyperconjugation from the lone electron pair or pairs on the nitrogen
or phosphorus atoms to the antibonding orbital χ*
B
H
2 of the BH2 group stabilizes the twisted structure by 7.4(8.8) or 4.0(5.0) kcal/mol at the HF/6-31G*(6-311+G**) level. However, the repulsive
interaction between the lone pair(s) and the two BH σ bonds is so prominent that the overall steric effect contributes 20.3(22.9)
and 19.3(19.8) kcal/mol to the rotational barriers in H2BNH2 and H2BPH2 with the 6-31G*(6-311+G**) basis set. The present techniques and analyses may also give some clues to justify the parameterization
in the empirical molecular mechanics methods.
Received: 17 April 1998 / Accepted: 17 September 1998 / Published online: 1 February 1999 相似文献
9.
Summary. Ab initio HF/6-31G* and MP2/6-31G*//HF/6-31G* methods were used to calculate the structure optimization and conformational interconversion pathways for all-(Z )-cyclododeca-1,4,7,10-tetraene. This compound adopts the symmetrical crown (C
4v) conformation. Ring inversion takes place via symmetrical intermediates, such as boat-chair (BC, C
s) and twist (C
2h) conformers and requires about 22.3 kJ · mol−1. The calculated strain energies for BC and twist conformers are 5.9 and 13.5 kJ · mol−1. The results of semiempirical AM1 calculations for structural parameters and relative energies of the important geometries
of the title compound are in good agreement with the results of ab initio methods.
Received July 9, 2001. Accepted September 26, 2001 相似文献
10.
Ab initio calculations were performed to investigate the structure and bonding of the phenol dimer and its cation, especially
the OH stretching frequencies. Some stable structures of the phenol dimer and its cation were obtained at the Hartree–Fock
level and were found to be in agreement with predictions based on spectroscopic investigations. In these dimers the phenol
moieties are bound by a single OH⋯O hydrogen bond. The hydrogen bond is much stronger in the dimer cation than in the neutral
dimer. The calculated binding energy of the phenol dimer in the most stable structure was 6.5–9.9 kcal/mol at various levels
of calculation, compared with the experimental value of 5 kcal/mol or greater. The binding energy of the phenol dimer cation
is more than 3 times (24.1–30.6 kcal/mol) as large as that of the neutral dimer. For the phenol dimer the OH stretching frequency
of the proton-accepting phenol (PAP) is 3652 cm−1 and that of the proton-donating phenol (PDP) is 3516 cm−1; these are in agreement with observed values of 3654 and 3530 cm−1, respectively. For the phenol dimer cation the OH stretching frequency of the PAP is 3616–3618 cm−1 in comparison with an observed value of 3620 ± 3 cm−1. That of the PDP in the dimer cation is calculated to be 2434–2447 cm−1, which is 1210–1223 cm−1 lower than that of the bare phenol. The large reduction in the OH stretching frequency of the PDP in the phenol dimer cation
is attributed to the formation of a stronger hydrogen bond in the cation than in the neutral dimer.
Received: 24 March 2000 / Accepted: 26 April 2000 / Published online: 11 September 2000 相似文献
11.
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
12.
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
13.
N. M. Klimenko E. A. Rykova M. L. McKee I. N. Senchenya 《Russian Chemical Bulletin》1999,48(3):417-427
Ab initio MP2/6-31G*//HF/6-31G*+ZPE(HF/6-31G*) calculations of the potential energy surface in the vicinity of stationary points and the pathways of intramolecular rearrangements
between low-lying structures of the OBe3F3
+ cation detected in the mass spectra of μ4-Be4O(CF3COO)6 were carried out. Ten stable isomers with di- and tricoordinate oxygen atoms were localized. The relative energies of six
structures lie in the range 0–8 kcal mol−1 and those of the remaining four structures lie in the range 20–40 kcal mol−1. Two most favorable isomers, aC
2v isomer with a dicoordinate oxygen atom, planar six-membered cycle, and one terminal fluorine atom and a pyramidalC
3v isomer with a tricoordinate oxygen atom and three bridging fluorine atoms, are almost degenerate in energy. The barriers
to rearrangements with the breaking of one fluorine bridge are no higher than 4 kcal mol−1, except for the pyramidalC
3v isomer (∼16 kcal mol−1). On the contrary, rearrangements with the breaking of the O−Be bond occur with overcoming of a high energy barrier (∼24
kcal mol−1). A planarD
3h isomer with a tricoordinate oxygen atom and linear O−Be−H fragments was found to be the most favorable for the OBe3H3
+ cation, a hydride analog of the OBe3F3
+ ion; the energies of the remaining five isomers are more than 25 kcal mol−1 higher.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 420–430, March, 1999. 相似文献
14.
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
15.
Susannah Sigurdsson Roger Strömberg 《Phosphorus, sulfur, and silicon and the related elements》2013,188(11):2711-2724
Several economical methods for geometry optimization, that should be applicable to larger molecules, have been evaluated for 19 phosphorus acid derivatives. MP2/cc-pVDZ geometry optimizations are used as reference points and the geometries obtained from the other methods are evaluated with respect to deviations in bond lengths and angles, from the reference geometries. The geometry optimization methods are also compared to the much used B3LYP/6-31G(d) method. Single point energies obtained by subsequent EDF1/6-31+G(d) or B3LYP/6-31+G(d,p) calculations on the respective equilibrium geometries are also reported relative to the energies obtained from the reference geometries. The geometries from HF/MIDI! optimizations were closer to those of the references than the geometries of the HF/3-21G(d), HF/6-31G(d), and B3LYP/MIDI! optimizations. The EDF1/6-31+G(d) or B3LYP/6-31+G(d,p) single point energies obtained from the HF/3-21G(d), HF/6-31G(d), and B3LYP/MIDI! geometries gave a mean absolute deviation (MAD) from that of the reference geometries of 1.4-3.9 kcal mol m 1 . The HF/MIDI! geometries, however, gave EDF1/6-31+G(d) and B3LYP/6-31+G(d,p) energies with a MAD of only about 0.5 and 0.55 kcal mol m 1 respectively from the energies obtained with the reference geometries. Thus, use of HF/MIDI! for geometry optimization of phosphorus acids is a method that gives geometries of near-MP2 quality, resulting in a fair accuracy of energies in subsequent single point calculations, at a much lower computational cost other methods that give similar accuracies. 相似文献
16.
Xianyang Chen Chongde Li Tao Wu Tianyang Yao Guanzhi Ju 《Theoretical chemistry accounts》1998,99(4):272-276
Ab initio molecular electronic structure methods have been used to study the relative stability of the planar inorganic ring
(HAlNH)n (n = 2–4) during homodesmotic and monomer polymerization reactions. Optimized geometries, frequencies and energies through
restricted Hartree-Fock/6-31G* are reported, and energies at the self-consistent field optimized geometries including M?ller-Plesset
fourth perturbation theory with single, double and quadruple excitation (MP4SDQ) corrections are also reported for both reactions.
Homodesmotic reactions with MP4SDQ −28.5 kcal/mol for (AlN)2, 1.9 kcal/mol for (AlN)3 and −0.97 kcal/mol for (AlN)4. On analysing a π-molecular orbitals diagram, only one, three and three strongly bonding π-molecular orbitals exist for the
planar four-, six- and eight-membered AlN rings, respectively.
Received: 9 March 1998 / Accepted: 19 March 1998 / Published online: 23 June 1998 相似文献
17.
William T. G. Johnson David A. Hrovat Anne Skancke Weston Thatcher Borden 《Theoretical chemistry accounts》1999,102(1-6):207-225
Ab initio calculations on the lowest singlet and triplet states of 2,2-disilylcyclopentane-1,3-diyl find that the singlet
lies well below the triplet. The C
2 singlet diradical is calculated to be a minimum on the potential energy surface with an enthalpic barrier to ring closure
of ΔH
‡
298 = 13.5 kcal/mol at the CASPT2/6-31G* level of theory. The energy of the 1,3-divinyl-substituted singlet diradical is calculated
to be only 0.8 kcal/mol higher than that of 5,5-disilyl-1,3-divinylbicyclo[2.1.0]pentane at this level of theory, but the
transition state for their equilibration is computed to be 12.8 kcal/mol above the diradical in energy.
Received: 2 July 1998 / Accepted: 4 August 1998 / Published online: 16 November 1998 相似文献
18.
We have calculated the free energy differences between four conformers of the linear form of the opioid pentapeptide DPDPE
in aqueous solution. The conformers are Cyc, representing the structure adopted by the linear peptide prior to disulfide bond
formation, β
C
and β
E
, two slightly different β-turns previously identified in unconstrained molecular dynamics simulations, and Ext, an extended
structure. Our simulations indicate that β
E
is the most stable of the studied conformers of linear DPDPE in aqueous solution, with β
C
, Cyc and Ext having free energies higher by 2.3, 6.3, and 28.2 kcal/mol, respectively. The free energy differences of 4.0 kcal/mol
between β
C
and Cyc, and 6.3 kcal/mol between β
E
and Cyc, reflect the cost of pre-organizing the linear peptide into a conformation conducive for disulfide bond formation.
Such a conformational change is a pre-requisite for the chemical reaction of S–S bond formation to proceed. The relatively
low population of the cyclic-like structure agrees qualitatively with observed lower potency and different receptor specificity
of the linear form relative to the cyclic peptide, and with previous unconstrained simulation results. Free energy component
analysis indicates that the moderate stability difference of 4.0–6.3 kcal/mol between the β-turns and the cyclic-like structure
results from cancellation of two large opposing effects. In accord with intuition, the relaxed β-turns have conformational
strain 43–45 kcal/mol lower than the Cyc structure. However, the cyclic-like conformer interacts with water about 39 kcal/mol
strongly than the open β-turns. Our simulations are the first application of the recently developed multidimensional conformational
free energy thermodynamic integration (CFTI) protocol to a solvated system, with fast convergence of the free energy obtained
by fixing all flexible dihedrals. Additionally, the availability of the CFTI multidimensional free energy gradient leads to
a new decomposition scheme, giving the contribution of each fixed dihedral to the overall free energy change and providing
additional insight into the microscopic mechanisms of the studied processes.
Received: 20 April 1998 / Accepted: 9 September 1998 / Published online: 7 December 1998 相似文献
19.
Complete Basis Set and Gaussian-n methods were combined with CPCM continuum solvation methods to calculate pK(a) values for six carboxylic acids. An experimental value of -264.61 kcal/mol for the free energy of solvation of H(+), DeltaG(s)(H(+)), was combined with a value for G(gas)(H(+)) of -6.28 kcal/mol to calculate pK(a) values with Cycle 1. The Complete Basis Set gas-phase methods used to calculate gas-phase free energies are very accurate, with mean unsigned errors of 0.3 kcal/mol and standard deviations of 0.4 kcal/mol. The CPCM solvation calculations used to calculate condensed-phase free energies are slightly less accurate than the gas-phase models, and the best method has a mean unsigned error and standard deviation of 0.4 and 0.5 kcal/mol, respectively. The use of Cycle 1 and the Complete Basis Set models combined with the CPCM solvation methods yielded pK(a) values accurate to less than half a pK(a) unit. 相似文献
20.
Elmar Kaufmann Bruce Tidor Paul Von Ragu Schleyer 《Journal of computational chemistry》1986,7(3):334-344
Solvation energies of lithium first-row compounds LiX (X ? H, Li, BeH, BH2, CH3, NH2, OH, F) and of the lithium cation with the model solvents, water and ammonia, have been calculated ab inito (MP2/6-31 + G*//6-31G* with zero-point vibrational energy corrections at 3-21G//3-21G). The solvation energies are found to be remarkably constant: ?18.0 ± 1.2 and ?21.5 ± 1.3 kcal/mol for the hydrates and ammonia solvates, respectively. This independence on the nature of X is due largely to the ionic character of the LiX compounds (dipole moments 4.7–6.6 debye). The unexpectedly high solvation energies of the lithium molecule (?14.3 and ?17.8 kcal/mol, respectively) are due to the polarizability of Li2. At the same level, the lithium cation has interaction energies with H2O and NH3 of ?34.1 and ?39.7 kcal/mol, respectively. For the hydrates of LiOH and LiF cyclic structures with hydrogen bonds and somewhat increased solvation energies also are described. 相似文献