Intermolecular interaction coefficients using point charge models

Values of the two-body interaction coefficientsC6 andd4 between like species, as well as the three-body term 3, are obtained from point charge model formulae. Lewis set results may be compared with atomically centered wave-function values as well as results obtained using an experimental point charge model. Generally results are in good agreement with experiment and when theoretical values differ wildly from experiment they may be normalized using theoretical and experimental static polarizability values.     

Electric polarizabilities using point charge models

The formula for the electric polarizability of a molecule using a Frost model Lewis basis set, that is one orbital per electron pair, can be extended to include wavefunctions containing additional Gaussians using point charge models. Using these alternative formulae, results for hydrocarbon Lewis sets and molecular fragment wavefunctions are in good agreement with experiment and with the results obtained using the original formula. In addition results for atomically centred wavefunctions for molecules containing lone pairs are also good and show an improvement over the Lewis set results for these species.     

FSGO point charge models—Their accuracy and extension to higher gaessians

An FSGO model of ethane demonstrates how currently used point charge models can give the wrong sign for the potential. The Hall point charge potential on the other hand is asymptotically accurate.An extension of the Hall point charge model to higher Gaussians is demonstrated. This extended model potential is shown to be of comparable accuracy to that of the spherical Gaussian model.     

The currant bun model of simple molecules

The model of a molecule previously introduced by the authors, in which most of the electronic charge is represented by point charges on the nuclei, in the lone pairs and bonds, and the remainder is in a diffuse function which represents the outer electrons, is here extended. Molecules with several heavy atoms require several diffuse functions, one for each heavy atom, in addition to one at the centre of nuclear charge. In an electric field the centre of each diffuse function moves against a harmonic restoring force and this gives rise to the polarizability of the molecule. When two molecules interact the movement of this function produces the dispersion force. The model thus embodies all the long range forces between molecules in a more accurate and simpler way than was possible earlier.     

Long-range molecular interaction coefficients computed from frost-model wave functions

The average long-range interaction energy between two molecules can be written as an inverse asymptotic series in the intermolecular separation distanceR. Using Frost-model wave functions, the dispersion coefficients of the first three (R ^–6,R ^–8,R ^–10 terms in the series are obtained. Coefficients of three- and four-body non-additive interaction energies are also calculated and the form of the dispersion interaction when retardation effects are included is examined.     

Point charge models for LiH,CH4, and H2O

Point charge models for LiH. CH₄, and H₂O are presented. The models preserve the correct total charge and dipole moment of the molecules. Relations between spherical Gaussian wave function values and point charge model values of a variety of one-electron molecular properties are derived. The errors inherent in some of the point charge model values are of two types: those which may be large but are easily evaluated and those which are small and diminish rapidly as the distance from the molecule increases. The models are shown to be a reliable means of calculating one-electron properties and possible uses of the models are suggested.     

The random contact point model for pair interaction coefficients of unlike solute molecules

The random contact point model described in previous papers is extended to include like-unlike pair interaction coefficients. On the basis of this extension we present the thermodynamics of group interactions involving alkyl, hydroxyl and amide (peptide) groups and water molecules. The values obtained for the Gibbs energy of group interaction (absolute values ranging from 1.5 to 9.8 kJ-mol^–1) indicate that all these groups attract or repel each other in aqueous solutions with comparable strength. Group interaction parameters obtained from aqueous and non-aqueous systems, and based on interaction coefficients and other thermodynamic quantities, agree well. The model also allows for the quantification, though not for the prediction, of the cooperativity of hydrophobic interaction.     

Additivity of atomic static polarizabilities and dispersion coefficients

A new empirical method is proposed to evaluate the average molecular polarizabilities assuming the additivity of atomic static polarizability. Atomic static polarizability for each atom in a particular valence state is obtained. Calculated molecular polarizabilities of 94 non-halogenated compounds and of the bases in nucleic acids show the excellent agreement with experimental data.To check the further validity of this method, dispersion coefficients for CH₄, C₂H₆, C₃H₈,n–C₄H₁₀,n–C₅H₁₂,n–C₆H₁₄,n–C₇H₁₆,n–C₈H₁₈, H₂, H₂O and NH₃ are obtained from a sum of atomic terms using a London-type formula, and are compared with the accurate values of dipole oscillator strength distribution (DOSD) method. The results show the excellent agreement between theory and experiment.     

The significance of the difference in the point of zero charge between rutile and anatase

The points of zero charge (PZC) of titanium dioxide reported in the literature range from 2 to 8.9. A set of 138 PZC of titanium dioxide was used to explore the effect of the crystalline structure on the PZC. The average and median PZC at pH 5.6 and 5.8, respectively, was found when the entire data set was taken into account. The PZC of anatase (31 entries, average and median 5.9 and 6, respectively) is slightly higher than that of rutile (49 entries, average and median 5.4 and 5.5, respectively), and the difference between the polymorphs corresponds to half of a standard deviation in each set of PZC.     

Bond and inner shell interaction coefficients for hydrocarbons

Molecular interactions may be broken up into the interaction of bonds and inner shells. Results for bond and inner shell interaction coefficients for pairwise additive termsC ₆,C ₈,C ₁₀ andd ₄ are given, as well as the non-(pairwise) additive coefficient ₃. Simple combination rules may be used to estimate these terms with good results. Parameters should be applicable to macromolecules.     

Point of zero charge/isoelectric point of exotic oxides: Tl2O3

The pristine point of zero charge of Tl(2)O(3) determined in the presence of NaNO(3) using the inert electrolyte titration method and confirmed by microelectrophoresis is at pH 7.9.     

Retrieval of interaction coefficients from total vapor pressure-composition data

A numerical-derivative method, employing a running cubic spline, has been developed to infer coefficients in power series expansions of total vapor pressure vs. composition data. The technique makes it possible to determine values of the coefficients individually, avoiding a generic problem in least squares analysis, viz., that the regression parameters are highly coupled and that their derived values depend strongly on the number of constants used in fitting data. The numerical derivative method also indicates when errors of measurements will preclude the determination of meaningful values of higher-order terms in polynomical representations of data.     

Point charge representation of multicenter multipole moments in calculation of electrostatic properties

Summary Distributed Point Charge Models (PCM) for CO, (H₂O)₂, and HS-SH molecules have been computed from analytical expressions using multicenter multipole moments. The point charges (set of charges including both atomic and non-atomic positions) exactly reproduce both molecular and segmental multipole moments, thus constituting an accurate representation of the local anisotropy of electrostatic properties. In contrast to other known point charge models, PCM can be used to calculate not only intermolecular, but also intramolecular interactions. Comparison of these results with more accurate calculations demonstrated that PCM can correctly represent both weak and strong (intramolecular) interactions, thus indicating the merit of extending PCM to obtain improved potentials for molecular mechanics and molecular dynamics computational methods.Dedicated to Prof. Alberte PullmanPacific Northwest Laboratory is operated for the US Department of Energy by Battelle Memorial Institute under contract DE-ACO6-76RLO 1830     

Oil-water partition coefficients from solvent activities

The activity coefficients of 1-octanol and 2-octanol in benzene at 20°C are reported here. These have been combined with literature data and used to estimate the 1-octanol-water partition coefficient for benzene. The value of 1200±100 on the mole-fraction scale agrees well with the literature value of about 1200.     

氢氧化铝镁钠米颗粒的零电荷点及电荷密度研究

探讨了电位滴定(PT)法测定的零电荷点的物理意义, 认为是零净电荷点(ZPNC)。并对零净电荷点pH(pHZPNC), 零可变电荷点pH(pHZPVC), 永久电荷密度(σP), 可变电荷密度(σV)和净电荷密度(σT)之间的关系进行了理论分析。用PT法测定了氢氧化铝镁纳米颗粒的pHZPNC和σP, 探讨了电解质浓度和pH对各电化学性质的影响规律。另外, 还考察了CO3^2^-对PT法测定结果的影响。     

Dispersion coefficients for first hyperpolarizabilities using coupled cluster quadratic response theory

The frequency dependence of third-order properties can in the normal dispersion region be expanded in a Taylor series in the frequency arguments. The dispersion coefficients thus obtained provide an efficient way of expressing the dispersion of frequency-dependent properties and are transferable between different optical processes. We derive analytic expressions for the dispersion coefficients of third-order properties in coupled cluster quadratic response theory and report an implementation for the three coupled cluster models CCS, CC2, and CCSD. Calculations are performed for the first hyperpolarizability of the NH₃ molecule. The convergence of the dispersion expansion with the order of the coefficients is examined and we find good convergence up to about half the frequency at which the first pole in the hyperpolarizability occurs. Padé approximants improve the convergence dramatically and extend the application range of the dispersion expansion to frequencies close to the first pole. The sensitivity of the dispersion coefficients on the dynamic correlation treatment and on the choice of the one-electron basis set is investigated. The results demonstrate that, contrary to presumptions in the literature, the dispersion coefficients are sensitive to basis set effects and correlation treatment similar to the static hyperpolarizabilities. Received: 26 March 1998 / Accepted: 21 July 1998 / Published online: 19 October 1998     

Activity coefficients of single electrolytes in concentrated solutions derived from a quasi-lattice model

This paper describes a new model to calculate the mean activity coefficients of dissociated electrolytes in concentrated solutions. It is based on three assumptions: (i) a quasi-lattice arrangements of ions in solution; (ii) a contribution from ion-water interactions to the mean activity coefficients; (iii) a concentration dependence of the dielectric constant. The mean activity coefficients of thirteen strong electrolytes from moderately dilute solutions to saturated solutions are found to correlate well by this model. For dilute solutions, a limiting equation in which only ion-specific parameters are required is proposed. It is suggested that specific ionwater interactions might be the major source of the nonideality of strong electrolyte solutions at high concentrations.     

The approximation of electron densities

This paper discusses the approximate representation of the electron density produced by an ab initio calculation. A linear combination of Gaussians is fitted to the density by minimizing a functional which is the consequent error in field-energy. The practical implementation of the procedure, following a Gaussian 80 calculation, is described and some of the complications are analysed.     

The electron density of the water molecule

The electron density of the water molecule, as calculated by a standard program, is approximated by linear combinations of spherical Gaussians. The accuracy of the result is studied as a function of the numbers and positions of the Gaussians. Since this shows where the charge is located in the molecule it has immediate physical significance. The building-up of the density can be followed in more and more detail. From these expansions, point charge models of water are readily deduced. These are compared with models of similar kinds used by other authors. Some of the calculations have been repeated with a wavefunction of higher accuracy to investigate the stability of the results. Results show that the more accurate density requires more Gaussians to represent its greater complexity.