On exchange-repulsion non-additivity in the interaction of three argon atoms

The non-additive contribution to the exchange repulsion for the argon trimer in the equilateral-triangle configuration has been calculated in the first order of symmetry-adapted perturbation theory. Approximation of the non-additivity by the interaction of outer-shell electron is as efficient as for Ne₃. The role of higher-order contributions to the non-additivity is discussed.     

Perturbation calculations of the non-additivity of the exchange-dispersion energy for the Be3 system

The non-additivity of the exchange-dispersion energy for the Be trimer in the region of small overlap is calculated using UCHF wavefunctions and an outer-shell approximation. It is shown that this effect, unlike in He₃, cannot be neglected even for separations twice as large as the nearest-neighbour distance in the beryllium crystal. The role of various contributions to the total non-additivity of the interaction energy is discussed.     

Intermolecular SCF method without bsse: the closed-shell case

The BSSE-free SCF theory ofintermolecular interactions, based on the “chemical Hamiltonian approach” has been developed and realized for closed-shell systems. Sample calculations are presented for the H₂O, HF, LiH and He dimers. The results stress the non-additivity of the BSSE pointed out recently.     

Hydration reactivity of ketenes. Non-additive effect of alkyl substituents upon the rate of spontaneous hydrolysis

The kinetics of spontaneous hydrolysis of dialkylsubstituted ketenes in water-acetonitrile mixtures at 20°C has been studied. An increase of the total volume of substituents at the terminal carbon atom of the ketenes leads to a sharp decrease of hydration rate, which is connected with the non-additivity of substituent effects. The possible reasons are discussed in terms of the reaction mechanism.     

A cooperative calculation of geometries,energetics and electric properties of water trimers and tetramers

The cooperative model of Campbell and Mezei reproduces Kistenmacher, Lie, Popkie and dementi's Hartree-Fock energy for (H₂O)₄ with only a 3% error. Permanent multipole electric fields at the O nuclei and induced dipole vectors have been calculated. Analysis of a redetermination of the geometry of (H₂O)₄ suggests the probable symmetry of the structure at the energy minimum. The non-additive and the cooperative energies (defined below) are reported. A non-cooperative calculation of the non-additivity introduces a substantial error.We express our appreciation for the support of this research by a grant of computer time from the Courant Institute of Mathematical Sciences, under Contract EY-76-C-02-3077 with the Department of Energy     

Substituent effects on carbon-13 chemical shifts in 2,6-disubstituted adamantanes

Carbon-13 NMR spectral data for a series of symmetrical 2,6-disubstituted adamantanes (O?, CH₂?, CH₃, OH, OCOCH₃) are presented. The substituent effects on ¹³C chemical shifts are additive, except for carbons 2 and 6 in 2,6-adamantanedione. The non-additivity of the substituent effect in the diketone can be interpreted in terms of a through-space interaction of the carbonyl π-electrons; the additivity in the other derivatives indicates that there is no appreciable interaction between the substituents.     

The preferred conformations of 2,4-dichlorophenol, 2,4-dichloroanisole and their sulphur analogues

Analysis of the benzene dipole moments of 2-chloroanisole and 2,4-dichloroanisole, 2-chlorothioanisole and 2,4-dichlorothioanisole shows these compounds to exist as trans. 2-Chlorophenol and 2,4-dichlorophenol, and their sulphur analogues, are mixtures of Cis and trans conformers. The populations of the cis conformers are, at 25°C, 86 and 78% for the phenols, and ca. 70 and 50% for the corresponding thiophenols. The influence of various factors, such as the non-additivity of the (2-ClC₆H₄X)-group moment and torsional oscillation of the H-X bond (especially the H-S bond), on the calculated populations has been discussed. Incidentally it has been shown that p-dimethoxybenzene and p-di-(methylthio)benzene, as well as p-phenylenediamine and p-diformylbenzene, are equimolecular mixtures of cis and trans conformers.     

The first metals in Mendeleiev’s table: further arguments to place He above Ne and not above Be

In a recent paper in this Journal, one of us argued against placing He above Be in Mendeleiev’s system of the elements. In it the goal was to dispute the notion that in Mendeleiev’s system of the elements the location of He should in fact lie above Be, which has a very similar electronic configuration, rather than above the noble gas column. That paper was based on rather old, Hartree–Fock limit studies on the strikingly limited non-additive contributions in the He₃ and He₄ systems in contrast with the much larger non-additivity obtained for the Be₃, Be₄ and Be₅ oligomers. In a recent benchmark multireference Averaged Quadratic Coupled Cluster results on Be₂ and Be₃ we showed that the delocalized non-additive contribution comprises 94 % of the binding energy of Be₃. Here we use this and other pertinent information (drawn from the same paper) to conclude that He may not be associated with Be in Mendeleiev’s Table, despite their quite similar spectroscopic ground states. Furthermore, we use the new results to show that the large non-additivity implies that less than 2 % of the Be₃ binding is located in each Be pair contained within the Be trimer. The rest of the interaction energy is necessarily delocalized over all three Be atoms. This might actually announce the bulk properties (i.e. “the electron gas”) that in solid-state physics explain the large electric and heat conduction for the solid Be metal. Thus, in the case of beryllium the metallic characteristics are already evident in Be₃, a far cry from the monoatomic helium gas.     

Clusters of hydrogen molecules: Ab initio SCF calculations corrected semiempirically for correlation energies

Stabilization energy of the (H₂) _n clusters (n = 2–8) was calculated as a sum of the SCF interaction energy and the semiempirical interaction correlation energy estimated according to Sinanolu and Pamuk. Optimum successive attachment of hydrogen molecules leads to the formation of a gas-phase solvation shell consisting of seven hydrogen molecules. Basis set effect has been found to be important with all clusters under study. The non-additivity effect was investigated with the (H₂)₄ cluster. Vertical ionization potentials of the clusters considered are predicted to be 0.4–0.6 eV lower than the ionization potential of the parent H₂ molecule.     

The 13C n.m.r. spectra of polysubstituted benzenes. 1—the use of hexachlorobenzene as a reference for the structure elucidation of multisubstituted polychlorobenzenes

The use of ¹³C n.m.r. for structure elucidation of a series of products obtained by reaction of dialkylamines with 1,2-dinitrotetrachlorobenzene has shown that the general problem of non-additivity of substituent effects of contiguously substituted benzenes may be overcome, with these products, by the choice of a suitable standard. In this case hexachlorobenzene was used and the difference between the actual and calculated chemical shifts was small, allowing unambiguous structural assignment.     

Theoretical study of hydrated Be2+ ions

Hydrated Be²⁺ ions [Be(H₂O)_n]²⁺, n = 1−4 and 6, were examined theoretically. The structure of the hydrated ions was determined and the hydration energy estimated with and without electron correlation. The bond between the Be²⁺ ion and the oxygen of water is very strong and has the nature of a dative bond. The non-additivity of the binding energy is so profound that without taking it into account the structure and dynamics of Be²⁺ ions cannot be explained. The hydration number in water is found to be 4. The fifth and sixth water molecules prefer forming the second coordination shell to the Be²⁺ ion. The result is in agreement with X-ray analysis of the aqueous solution, but not with a recent molecular dynamics simulation. In addition, the harmonic vibrational frequencies for the complexes are evaluated and compared with some experiments.     

Studies on the behavior of liquid hydrocarbon systems

A method for estimating the effect of intermolecular interaction and component structure on oil system properties is suggested. The results of calculations via the models of oil system properties taking into account their non-additivity are presented.     

Theoretical ab initio study of the interplay between hydrogen bonding, cation–π and π–π interactions

The interplay among three important non-covalent interactions involving aromatic rings is studied by means of ab initio calculations (RI-MP2/aug-cc-pVTZ//RI-MP2/aug-cc-pVDZ, and RI-MP2/CBS levels of theory). They demonstrate that synergetic effects are present in complexes where hydrogen bonding interactions, cation–π, and π–π interactions coexist. These synergetic effects have been studied using the genuine non-additivity energies and the molecular interaction potential with polarization partition scheme.     

The one-dimensional fully non-additive binary hard rod mixture: exact thermophysical properties

The non-additive binary hard particle mixture is characterized by a collision parameter between unlike hard particles that is not the arithmetic mean of the two pure component collision diameters. Instead, σ₁₂=(σ₁₁+σ₂₂)(1+Δ)/2, where σ₁₂ is the collision diameter between species 1 and 2 and Δ is a non-additivity parameter in which Δ≥−1 (Δ=0 corresponds to the additive mixture). For mixtures in which repulsive forces are mostly responsible for the observed behavior, the binary non-additive hard particle mixture provides a useful model for examining the thermophysical properties of a wide class of real fluid mixtures. To extend further the range of applicability of these mixtures, another layer of complexity can be considered by including potential interactions that exist past contact which themselves may also be defined as non-additive. For example, the well depth of the interaction of unlike species can be defined as ε₁₂=δ∣ε₁₁ε₂₂∣^1/2, where δ is another non-additivity parameter which may take on any real value (note that Δ=0 and δ=1 correspond to the Lorentz–Berthelot mixing rules).To gain insight into these complicated mixtures in higher dimensions, we consider the one-dimensional non-additive binary hard rod mixture with various attractive interactions. We generate the exact thermophysical properties of these mixtures, including a general form of the equation of state. Similarly, we also determine the radial distribution functions, g _ij(r), using the exactly known nearest-neighbor probability density distributions. Such relations enable one to determine the effects of the various non-additive parameters on the properties of the mixtures.     

Effect of polymerized silicic acid on mixed lipid-protein monolayers used as cell membrane models. I. Hemoglobin-stearoyl erythroceramide films

The effect of polysilicic acid on mixed monolayers of hemoglobin and stearoyl erythroceramide used as cell membrane models has been studied. The presence of the sphingolipid at the interface stabilizes the hemoglobin monolayer, hindering expulsion of its residues from the surface when the collapse pressure (ca. 23 mN/m) is reached. The interaction between the components of the mixed film results in non-additivity of their molecular areas to a degree depending on film composition and substrate pH. The presence of polysilicic acid in the substrate increases the miscibility of the components in the mixed monolayer as a result of ionic interaction between the silicic acid and the protein.     

Can lone pair-π and cation-π interactions coexist? A theoretical study

The interplay between two important noncovalent interactions involving different aromatic rings is studied by means of ab initio calculations (MP2/6-31++G**) computing the non-additivity energies. In this study we demonstrate the existence of cooperativity effects when cation-π and lone pair-π interactions coexist in the same system.     

Substituent effects in the 13C NMR chemical shifts of alpha-mono-substituted acetonitriles

13C chemical shifts empirical calculations, through a very simple additivity relationship, for the alpha-methylene carbon of some alpha-mono-substituted acetonitriles, Y-CH(2)-CN (Y=H, F, Cl, Br, I, OMe, OEt, SMe, SEt, NMe(2), NEt(2), Me and Et), lead to similar, or even better, results in comparison to the reported values obtained through Quantum Mechanics methods. The observed deviations, for some substituents, are very similar for both approaches. This divergence between experimental and calculated, either empirically or theoretically, values are smaller than for the corresponding acetones, amides, acetic acids and methyl esters, which had been named non-additivity effects (or intramolecular interaction chemical shifts, ICS) and attributed to some orbital interactions. Here, these orbital interactions do not seem to be the main reason for the non-additivity effects in the empirical calculations, which must be due solely to the magnetic anisotropy of the heavy atom present in the substituent. These deviations, which were also observed in the theoretical calculations, were attributed in that case to the non-inclusion of relativistic effects and spin-orbit coupling in the Hamiltonian. Some divergence is also observed for the cyano carbon chemical shifts, probably due to the same reasons.     

Scaled particle theory of mixtures of hard spheres with negatively non-additive diameters

The scaled particle theory is solved for mixtures of hard spheres with arbitrary negative non-additive diameters in three dimensions. The results obtained with the Gibbs—Duhem relation show excellent agreement with the Monte-Carlo calculations. The results from the virial relation for high densities and high non-additivity are less satisfactory.     

The combined effects of two chlorine substituents and the non-additivities of chemical shifts in aliphatic dichloro esters

Carbon-13 NMR spectra were measured for 25 methyl esters of aliphatic dichlorocarboxylic acids for substituted propanoic to hexanoic acids. Observed ¹³C shifts are compared with calculated shift values obtained from the shifts of the corresponding monochloro esters by applying a simple sum method. The greatest failures from additivity are observed for αβ and αα substituted carbons. The combined effects of two chlorine substituents were determined, and compared with those obtained from chlorine substituent effects in monochloro esters by assuming additivity. The trends displayed by the combined effects, and also by the non-additivity effects, are discussed.     

Determination of the true form of overlapping peaks, deformed by the base line in the case of stripping voltammetry

A way to determine the form of an analytical signal without accounting of the base line is described. It is based on subtraction of two peaks. The validity of correction of the base line inclination with the help of a linear function is indicated. The ability to determine the true form of the analytical signal in case of overlapping signals is proved. The approach was applied to define the degree of non-additivity of the analytical signal and the character of change of the peak shape as a function of concentration.