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Showing papers on "Chemical binding published in 1981"


Journal ArticleDOI
TL;DR: A review of single-particle (electron) density in terms of quantum subspaces can be found in this article, where the success and drawbacks of earlier models, such as Thomas-Fermi-Dirac, incorporating density matrices are examined.
Abstract: The definition, properties, and applications of the single-particle (electron) density $\ensuremath{\rho}(\mathrm{r})$ are discussed in this review. Since the discovery of Hohenberg-Kohn theorem, which gave a theoretical justification for considering $\ensuremath{\rho}(\mathrm{r})$, rather than the wave function, for studying both nondegenerate and degenerate ground states of many-electron systems, $\ensuremath{\rho}(\mathrm{r})$ has been acquiring increasing attention. The quantum subspace concept of Bader et al. has further highlighted $\ensuremath{\rho}(\mathrm{r})$ since a rigorous decomposition of the three-dimensional (3D) space of a molecule into quantum subspaces or virial fragments is possible, the boundaries of such subspaces being defined solely in terms of $\ensuremath{\rho}(\mathrm{r})$. Further, $\ensuremath{\rho}(\mathrm{r})$ is a very useful tool for studying various chemical phenomena. The successes and drawbacks of earlier models, such as Thomas-Fermi-Dirac, incorporating $\ensuremath{\rho}(\mathrm{r})$ are examined. The applications of $\ensuremath{\rho}(\mathrm{r})$ to a host of properties---such as chemical binding, molecular geometry, chemical reactivity, transferability, and correlation energy---are reviewed. There has been a recent trend in attempting to bypass the Schr\"odinger equation and directly consider single-particle densities and reduced density matrices, since most information of physical and chemical interest are encoded in these quantities. This approach, although beset with problems such as $N$-representability, and although unsuccessful at present, is likely to yield fresh concepts as well as shed new light on earlier ideas. Since charge density in 3D space is a fundamental quantum-mechanical observable, directly obtainable from experiment, and since its use in conjunction with density-functional theory and quantum fluid dynamics would provide broadly similar approaches in nuclear physics, atomic-molecular physics, and solid-state physics, it is not unduly optimistic to say that $\ensuremath{\rho}(\mathrm{r})$ may be the unifying link between the microscopic world and our perception of it.

132 citations


Journal ArticleDOI
TL;DR: In this article, the infrared spectra in the NH, OH, and CO stretching regions are reviewed for various methylated and halogenated 2(4)-oxopyrimidines and uracils, as well as other nucleic acid derivatives, in the vapor phase and in low-temperature matrices.
Abstract: Infrared spectra in the NH, OH, and CO stretching regions are reviewed for various methylated and halogenated 2(4)-oxopyrimidines and uracils, as well as other nucleic acid derivatives, in the vapor phase and in low-temperature matrices. The 2-oxopyrimidines are predominantly in the enol form both in the vapor phase and in low-temperature matrices. By contrast, the 4-oxopyrimidines exhibit comparable populations of the keto and enol forms, with KT ≈ 1–2, and a difference in chemical binding energy between the two forms in the gaseous phase of the order of 1–2 kcal/mol. The observed tautomeric equilibria in the gas phase point to the need for a drastic revision of interpretations of theoretical methods, and simultaneously provide the appropriate quantitative data necessary for testing the results of quantum mechanical calculations. In sharp contrast to other heterocyclic systems, several of the bases found in natural nucleic acids were found to exist predominantly in the keto or amino forms, as in the solution phase. In particular, uracils exist predominantly in the keto form. This has made possible, for this class of compounds, to evaluate the heats of sublimation and vaporization, and to relate these data to hydrogen bonding and stacking interactions in the condensed phases. Examples are presented of base analogs which do exhibit appreciable tautomerism in solution. Some biological implications of the foregoing are presented in relation to the types of heterocyclic bases found in natural nucleic acids, and to concepts of spontaneous and induced mutagenesis.

45 citations


Journal ArticleDOI
01 Jan 1981
TL;DR: In this article, flash-induced transients in the near-infrared scattering of bovine rod outer segments and isolated discs are investigated, using the angular dependence of the relative intensity change (difference scattering curve), changes of the polarizability (refractive index) and particle shape.
Abstract: Flash-induced transients in the near-infrared scattering of bovine rod outer segments and isolated discs are investigated. Their common characteristic is the saturation at a rhodopsin bleaching of ca. 10%, which was previously described for the so-called “signalP”. The theory is based on the Rayleigh-Gans-approximation and on a cylindrical particle shape. This treatment is shown to be applicable in the measured angular range (in generalθ≤30‡), in spite of the polydisperse shape of the real particles. Using the angular dependence of the relative intensity change (difference scattering curve), changes of the polarizability (refractive index) and of the particle shape can be distinguished. Model difference scattering curves are calculated for the dimensions of the rod outer segments. Static scattering measurements are used for an estimation of the average particle shape: the isolated disc samples appear to contain flat discs as well as an admixture of rod-like structures (ca. 1% of the total scattering mass); in rod outer segment preparations, a contribution of non-rodlike scattering is found which is strongly dependent on the treatment of the sample. The flash induced transients were measured using randomly oriented particles (discs and rod outer segments) and axially oriented rod outer segments. The angular dependence of the amplitude yields its difference scattering curve. On suspensions of isolated discs, which were re-loaded with the proteins extracted at low ionic strength, one single signal is observed (termedPD, first order,Τ=0.6–1.2 s). Using randomly oriented rod outer segments, a signal with complex millisecond kinetics (termed signalP) and a slow signal (termedPS, first order,Τ=5–25 s) can be distinguished kinetically. In the axially oriented rod outer segments, theP-signal splits into a fast axial (10 ms) and a slower radial component (50–100 ms). The slow signalPS observed in ROS and the signalPD in discs have one common physical interpretation as local changes of the polarizability, directly observed in light-scattering as a change of the refractive index. The fast signalP in ROS, however, has no detectable local component but represents a pure shrinkage effect. On the axially oriented system, this shrinkage turns out to be axial and radial with different kinetics. Only rough estimations for the relative shrinkage effects and refractive index changes can be given. One obtains for 1% rhodopsin bleaching:δn/n≈10−4,δL/L≈10−2,δR/R≈5×10−4. Assuming a fluid plane for the disc membrane, the planar shrinkage induced by one bleached rhodopsin is estimated from the radial shrinkage as ca. 300 a2. This high value is discussed in relation to the binding of rhodopsin to the GTP-binding protein which is involved in comparable effects described by Kuhn et al. (1981). According to our data, a chemical binding process in milliseconds is only indicated in the isolated disc; in the closed disc stack of the rod outer segment, only weak (fast) local interactions are consistent with the difference scattering data. A turn or lift of the GTPase would better satisfy this condition and explain the above high value for the individual shrinkage effect.

34 citations


Patent
08 Sep 1981
TL;DR: In this article, the authors describe a method and apparatus for cooling the solid residue of gasification of a reactor operated at a pressure above atmospheric for the hydrogenation of carbonaceous materials, where the residual heat which corresponds to the difference between the desired final temperature and the temperature after cooling by the liquid, is eliminated by a gas blown into the bottom region of the cooling apparatus and/or by indirect heat exchange.
Abstract: The present specification describes and claims a method and apparatus for use in cooling the solid residue of gasification of a reactor operated at a pressure above atmospheric for the gasification of carbonaceous materials The residue of gasification is conducted out of the reactor into a cooling apparatus located therebelow and flows through the cooling apparatus from the top to the bottom thereof A cooling liquid is introduced into the solid residue in the upper region of the cooling apparatus and is metered such that the greater portion of the heat contained in the residue is eliminated in the form of heat of vaporization, sensible heat and chemical binding energy with the resultant steam and reaction products produced The remaining residual heat which corresponds to the difference between the desired final temperature and the temperature after cooling by the liquid, is eliminated by a gas blown into the bottom region of the cooling apparatus and/or by indirect heat exchange

20 citations


Journal ArticleDOI
TL;DR: In this article, the authors used a generalized tight-binding Slater-Koster fit to the band structure calculated from first principles for the host combined with one extra s orbital for each isolated impurity atom to estimate the heats of formation of hydrogen atoms at octahedral and tetrahedral interstitial sites in f.c.
Abstract: The heats of formation of hydrogen atoms at octahedral and tetrahedral interstitial sites in f.c.c. transition metals (palladium and nickel) were estimated using a generalized tight-binding Slater-Koster fit to the band structure calculated from first principles for the host combined with one extra s orbital for each isolated impurity atom. The chemical binding energies of nearest-neighbouring and next-nearest-neighbouring pairs of hydrogen atoms in α-Pd hydride were found to be repulsive. We also calculated chemical binding energies between d element substitutional impurities in palladium (such that δZ ⩽ −1) and hydrogen atoms as the first and the second neighbours. These theoretical studies are compared with existing experimental results.

17 citations


Journal ArticleDOI
TL;DR: In this article, a self-consistent method to determine a cluster charge is offered in terms of which the value and direction of the charge transfer at forming of the chemical bond may be obtained.

15 citations


Journal ArticleDOI
TL;DR: In this article, an ab initio molecular calculation of the elastic scattering was performed in good agreement with the experiment, and thus the proportion of the electron distribution involved in the chemical binding was estimated.
Abstract: Differential cross sections have been measured for high-energy electron scattering from diborane. The effect of chemical binding was seen by comparing experiment with the calculation for an independent-atom model. An ab initio molecular calculation of the elastic scattering was in good agreement with the experiment, and thus the proportion of the electron distribution involved in the chemical binding was estimated.

12 citations


Journal ArticleDOI
TL;DR: In this article, solid state 23Na- and 13C-NMR spectra of alkali cellulose are presented as a function of NaOH-concentration of the steeping lye, steeping temperature and amount of adhering lye (press factor).
Abstract: Solid state 23Na- and 13C-NMR spectra of alkali cellulose are presented as a function of NaOH-concentration of the steeping lye, steeping temperature and amount of adhering lye (press factor). Results are discussed with regard to “chemical binding” of NaOH to the cellulose chain in the system cellulose/ NaOH/H2O.

9 citations


Journal ArticleDOI
TL;DR: In this paper, it was shown that the modification of the electron-electron term resulting from the binding of a pair of hydrogen impurities in α-palladium hydrides cancels the corresponding modification of oneelectron band energies.

9 citations


Book ChapterDOI
01 Jan 1981
TL;DR: In this article, a paramagnetic atom and its immediate neighbors have any effect on the observed spectrum, be it theg value, hyperfine splitting (hfs), superhyperfine splitting(shfs), zero field splitting (zfs), or anisotropy.
Abstract: Virtually every known physical technique has been employed in studies of biological macromolecules. For structural determination, X-ray crystallography can furnish the most detailed information. But sometimes this complex picture is hard to interpret unambiguously. Oftentimes a very sensitive probe of a particular atom or peptide will better serve the purpose of investigation. EPR has one great advantage when employed in the analysis of chemical binding and structure, in that only the paramagnetic atom concerned and its immediate neighbors have any effect on the observed spectrum, be it theg value, hyperfine splitting (hfs), superhyperfine splitting (shfs), zero field splitting (zfs), or anisotropy. Detailed information can therefore be obtained about this atom, such as its binding and orientation, without any doubt as to which atoms or groups are associated with the absorption lines. In addition, if the EPR data are combined with other measurements, such as X-ray determination of the polypeptide chain structure, then a very powerful means is at hand to obtain the greatest details of the structure of the molecule as a whole.

8 citations


Journal ArticleDOI
TL;DR: In this article, a simple equation has been derived for calculating accurate ZPE + HT − H 0 (zero point and heat content) energies of saturated hydrocarbons from their enthalpies of formation and carbon-13 nuclear magnetic resonance spectra.
Abstract: A simple equation has been derived for calculating accurate ZPE + HT − H0 (zero-point and heat content) energies of saturated hydrocarbons from their enthalpies of formation and carbon-13 nuclear magnetic resonance spectra. Applications to conformational analysis indicate a near invariance of vibrational energies with respect to chair–boat conformational changes of the cyclohexane ring, the loss in molecular stability arising then from a weakening of the chemical binding due to a reorganization of the electronic charges. The origin of the destabilizing effect of butane gauche interactions is found, in the cyclohexane series, in a weakening of the chemical binding (∼1.85 kcal/mol) which is partially compensated by a lowering (∼0.85 kcal/mol) vibrational energy, thus offering an explanation for the loss in molecular stability of ∼1.00 kcal/mol for one gauche interaction without invoking Coulomb-type repulsions between non-bonded atoms. Calculated enthalpies of formation are presentd for a number of cycloalk...

Journal ArticleDOI
TL;DR: In this article, chemical binding in the H 2 + molecule has been studied in terms of the variations, with respect to R, of electrostatic field and stress components at five selected points on the interuclear axis.
Abstract: Following an interpretive formalism presented earlier, chemical binding in the H 2 + molecule has been studied in terms of the variations, with respect to R, of electrostatic field and stress components at five selected points on the interuclear axis. At three points phenomena analogous to those recorded earlier for H 2 have been observed. In particular, the existence of extremal relationships for the difference density field and the total field, as well as for the corresponding stresses, at R ≈ R eq for certain specific points on the internuclear axis has been confirmed. As in the case of H 2 , chemical binding in H 2 + occurs due to local variations of electrostatic pressure from point to point in such a manner as to cause the vanishing of either the total electrostatic force density or the difference force density at certain points on the internuclear axis.

Journal ArticleDOI
TL;DR: In this paper, both harmonic and damped vibrational models of chemical binding in the resonance treatment of uranium compounds were considered, and it was found that the line shape obtained with the harmonic approximation relative to the free-gas, Maxwell model, is shorter at the peak and broader in the wings.

Journal ArticleDOI
TL;DR: In this paper, a closed-form calculation of the angular momentum distribution of mesonic particles resulting from Coulomb capture is presented, based on the semiclas· sical model, with the energy loss being due to a frictional force the mesonic particle experiences in the atomic electron gas.
Abstract: The gross features of the angular momentum distribution of mesonic particles resulting from Coulomb capture are calculated in closed form. The calculation is based on the semiclas· sical model of Coulomb capture with the energy loss being due to a frictional force the mesonic particle experiences in the atomic electron gas. Atomic structure effects other than the atomic radius, and chemical bonding are neglected. There are two basic questions to be answered by any theory of Coulomb capture of mesonic particles; capture ratio for capture in the individual elements (in the case of a compound, etc.) and angular momentum distribution for capture in a given element. The experimental capture ratio is more directly correlated to the yet unobserved Coulomb capture process, i.e., the transition from an unbound state to the first bound state, than the experimental X-ray intensity pattern: In all theories or models except the large mesic molecule modeF) the capture ratio is only determined by the Coulomb capture processes while there are a large number of yet unobserved Auger transitions between capture and X-ray emission. Although it is not easy to treat, theory and experiment are simplest in the case of muons. Fair agreement between a semiclassical theory and experiment has been obtained for the capture ratio. The situation is worse in the case of the X-ray intensity pattern: There are only some numerical values available as theoretical results, and one cannot speak of good agreement between these values and the experimental results. The field is reviewed, for example, in Refs. 2) ~ 4). It is the aim of the present paper to give a closed-form calculation of the gross features of the initial distribution immediately after Coulomb capture. Detailed effects, such as electron configurations and chemical binding will not be taken into account, but the effect of the atomic radius will be taken into account to some extent. The calculation is based on a semiclassical description of the capture process. 4 )·'9) In this model the mesonic particle is treated classically while the electrons are assumed to be a Fermi gas, with corrections for the atom's finite size in condensed matter. The energy loss and the resulting angular momentum

Patent
17 Jun 1981
TL;DR: In this article, the authors describe a method and apparatus for cooling the solid residue of gasification of a reactor operated at a pressure above atmospheric for the hydrogenation of carbonaceous materials, where the residual heat is eliminated by a gas blown into the bottom region of the cooling apparatus and/or by indirect heat exchange.
Abstract: The present specification describes and claims a method and apparatus for use in cooling the solid residue of gasification of a reactor operated at a pressure above atmospheric for the gasification of carbonaceous materials. The residue of gasification is conducted out of the reactor into a cooling apparatus located therebelow and flows through the cooling apparatus from the top to the bottom thereof. A cooling liquid is introduced into the solid residue in the upper region of the cooling apparatus and is metered such that the greater portion of the heat contained in the residue is eliminated in the form of heat of vaporization, sensible heat and chemical binding energy with the resultant steam and reaction products produced. The remaining residual heat which corresponds to the difference between the desired final temperature and the temperature after cooling by the liquid, is eliminated by a gas blown into the bottom region of the cooling apparatus and/or by indirect heat exchange.

Book ChapterDOI
01 Jan 1981
TL;DR: In this article, the chemical binding energy of a pair of hydrogen atoms in palladium within the infinite dilution limit was calculated using an extra-orbital model within a generalized spd tight-binding approximation.
Abstract: Using an extra-orbital model within a generalized “spd” tight-binding approximation, we calculate the chemical binding energy of a pair of hydrogen atoms in palladium within the infinite dilution limit. The parameters of the calculation are based on an electronic structure model of hydrogen in palladium built from a Slater-Koster fit to first principles bands structure for the host, combined with one s extra-orbital for each isolated hydrogen atom. We find that cluster of hydrogen cannot form in agreement with inelastic neutron-scattering in α-palladium hydride.