Showing papers on "Valence (chemistry) published in 1994"
TL;DR: In this article, a triple zeta valence (TZV) basis set is presented for Li to Kr. The TZV bases are characterized by typically including a single contraction to describe inner shells and three basis functions for valence shells.
Abstract: Contracted Gaussian basis sets of triple zeta valence (TZV) quality are presented for Li to Kr. The TZV bases are characterized by typically including a single contraction to describe inner shells and three basis functions for valence shells. All parameters—orbital exponents and contraction coefficients—have been determined by minimization of atomic self‐consistent field ground state energies. Advantages and necessary modifications of TZV basis sets are discussed for simple test calculations of molecular energies and nuclear magnetic resonance (NMR) chemical shieldings in treatments with and without inclusion of electron correlation.
7,859 citations
TL;DR: In this paper, an all-atom CFF93 force field for polycarbonates based on ab initio calculations is reported, and the force field parameters are derived by fitting to quantum mechanical total energies, first and second derivatives of total energies and electrostatic potentials, all generated from AB initio quantum mechanical calculations on model compounds at HF/6-31GS level of theory.
Abstract: An all-atom CFF93 force field for polycarbonates based on ab initio calculations is reported. Force field parameters are derived by fitting to quantum mechanical total energies, first and second derivatives of total energies, and electrostatic potentials, all generated from ab initio quantum mechanical calculations on model compounds at HF/6-31GS level of theory. Valence parameters and ab initiocharges are then scaled to correct for differences between experiment and the Hartree-Fock approximation. The van der Waals parameters and the scaling factors for atomic partial charges are determined from crystal structures. Based on the force field, molecular mechanics calculations are performed for several model compounds, and the results are compared with experimental values and with the results of the ab initio calculations.
933 citations
TL;DR: The electronic structure of three phases of ZrO[sub 2], cubic, tetragonal, and monoclinic, is determined using vacuum-ultraviolet and x-ray-photoemission spectroscopies and the orthogonalized linear-combination-of-atomic-orbitals method, in an attempt to understand the complex interaction of the stabilizing dopants and associated atomic defects with the crystal structures of ZRO[ sub 2] and their phase transitions.
Abstract: The addition of suitable dopants to ZrO[sub 2] can induce dramatic phase stabilization, and this dopant-induced phase stabilization is the basis of transformation toughening of zirconia-based structural ceramics. We have determined the electronic structure of three phases of ZrO[sub 2], cubic, tetragonal, and monoclinic, using vacuum-ultraviolet and x-ray-photoemission spectroscopies, coupled with [ital ab] [ital initio] band-structure and optical property calculations using the orthogonalized linear-combination-of-atomic-orbitals method, in an attempt to understand the complex interaction of the stabilizing dopants and associated atomic defects with the crystal structures of ZrO[sub 2] and their phase transitions. The experimental samples were single or polycrystalline stabilized materials which contain atomic defects, while the calculations were performed for undoped idealized ZrO[sub 2] structures without atomic defects. Reasonable agreement is found between experiment and theory at this level. The primary difference among the three phases of ZrO[sub 2] is the hybridization or mixing of the Zr 4[ital d] ([ital x][sup 2][minus][ital y][sup 2] and [ital z][sup 2]) and the Zr 4[ital d] ([ital xy], [ital yz], and [ital zx]) bands, which form the conduction bands as the symmetry decreases from cubic to monoclinic. This leads to a complex evolution of the O 2[ital p] to Zr 4[ital d]more » and the O 2[ital s] to Zr 4[ital d] interband transitions. In addition, in the real materials, the presence of yttrium stabilizer introduces additional Y 4[ital p] valence bands and Y 4[ital d] conduction bands. The effective coordination of zirconium by oxygen is reduced from eightfold to sevenfold by the presence of the stabilizing ions and defects and this leads to the introduction of an occupied Zr 4[ital d] valence band suggestive of the presence of Zr[sup 2+].« less
456 citations
TL;DR: In this article, the authors discuss the theory of mixed-valence systems and theory of electron exchange, and the dependence of intervalence transition energy on the activation barrier to thermal electron transfer.
Abstract: Publisher Summary This chapter discusses the theory of mixed-valence systems and theory of electron exchange. In the case of strong coupling, for which it is no longer appropriate to use perturbation theory, the interaction of states is so great that bonding and antibonding potential energy curves result. Because the ground state is delocalized between metal ions, it is not strictly appropriate to describe the intervalence transition band of a class III complex as a metal-to-metal charge transfer transition. The metal-to-metal charge transfer (MMCT) designation is used for the sake of simplicity. The dependence of intervalence transition energy on the activation barrier to thermal electron transfer allows the solvent dependence of intervalence transitions to be understood in terms of relationships developed for the Marcus Theory of electron transfer. The Hush model is the preferred method of analysis of mixed valence complexes for the experimentalist because of its readily understandable derivation, its overlap with the Marcus theory of electron transfer, and the facility of its application. However, it is applicable only to weakly coupled class II complexes. A quantitative theory that is applicable to all mixed valence complexes is desirable.
408 citations
TL;DR: In this paper, the effect of rare-earth ion substitutions on magnetic and electrical properties of Ni-Zn ferrite was investigated. But the authors focused on the magnetic properties and not the electrical properties.
Abstract: The effect of Fe substitutions by rare-earth ions on magnetic and electrical properties of a Ni-Zn ferrite prepared by the classical method is investigated. A set of seven compounds with formula Ni0.7Zn0.3Fe1.98R0.02O4, where R identical to Yb, Er, Dy, Tb, Gd, Sm or Ce, was prepared. Emphasis is placed on current experimental results of bulk magnetic measurements and transport phenomena. The results obtained reveal that, by introducing a relatively small amount of R2O3 instead of Fe2O3, an important modification of both the structure and the magnetic and electrical properties can be obtained. We explain the influence of the rare-earth ions as an effect of the ionic radius. This assumption is supported by the lattice constant measurements. The best results from the viewpoint of magnetic and electrical characteristic acceptable for high frequencies were obtained for R ions with a large radius and with a stable valence of 3+ such as Gd in our work.
304 citations
277 citations
TL;DR: In this article, the Hartree-Fock and ML2 levels of theory using effective core potentials for the metal atoms were used to optimize the geometries of hexacarbonyls.
Abstract: The geometries of the hexacarbonyls and pentacarbonyls of chromium, molybdenum, and tungsten are optimized at the Hartree-Fock and ML2 levels of theory using effective core potentials for the metal atoms. The M-CO bond lengths of Mo(CO) 6 and W(CO) 6 predicted at the ML2 level using moderate valence basis sets are in excellent agreement with experimental values. The Cr-CO bond length in Cr(CO) 6 calculated at ML2 is too short. The total bond energies of the metal hexacarbonyls calculated at the CCSD(T) level of theory are slightly lower than the experimentally derived values
262 citations
TL;DR: The modified ionic and covalent valence indices as discussed by the authors are defined in the framework of the two-particle density matrix, with respect to the reference state of separated atoms or ions (SAL).
Abstract: The modified ionic and covalent valence indices are introduced, defined in the framework of the two-particle density matrix, with respect to the reference state of separated atoms or ions (SAL). They include only quadratic contributions in changes of the molecular charge-and-bond order matrix elements, relative to the SAL. General properties of the modified valence indices are examined and illustrative qualitative results for model systems are presented. Numerical UHF SCF MO valence data for selected diatomic and triatomic molecules are reported and interpreted in terms of the valence saturation effect and the ionic vs. covalent valence competition. A three-orbital valence model of a symmetric transition state of the bond-forming–bond-breaking reaction supports the BEBO model postulate of preservation of the total “bond order.” The model predictions are compared with the UHF numerical values. © 1994 John Wiley & Sons, Inc.
218 citations
TL;DR: A density functional study of the molecular structure and vibrational frequencies of the linear and planar monocyclic isomers of the C n (2≤n≤18) series was performed in this article.
Abstract: A density functional study of the molecular structure and vibrational frequencies of the linear and planar monocyclic isomers of the C n (2≤n≤18) series was performed. A gradient corrected energy functional of the Becke-Perdew type and a triple-ζ valence basis with a polarization function was employed. The results show that for n≤9 the linear chains are generally more stable than the ring structures. A possible exception is C 6 where, in contrast to previous studies, the two forms were computed to be near isoenergetic. For n≥10 the cyclic structures dominate; however, only the C 10 , C 12 , and C 14 monocyclic planar rings were found to represent minimas on the energy surface. Excellent agreement for the computed vibrational frequencies with experiment-whenever available-was found
191 citations
TL;DR: In this paper, the interplanar distance and the electronic properties of different stackings of graphene sheets have been studied in the framework of density functional theory, and the valence charge densities and the densities of states of these three graphitic structures are compared, as well as their respective band structures.
188 citations
TL;DR: In this paper, a simple valence-bond charge transfer model is proposed to describe the nonlinear optical properties of charge transfer organic materials, including the absorption frequency, hyperpolarizabilities, and bond length alternation, all of which are described in terms of three parameters related to the band gap, bandwidth, and dipole moment of the charge transfer state.
Abstract: The nonlinear optical properties of charge-transfer organic materials are discussed in the framework of a simple valence-bond charge-transfer model. This model leads to analytic formulas for the absorption frequency, hyperpolarizabilities, and bond length alternation, all of which are described in terms of three parameters, V, t, and Q related to the band gap, bandwidth, and dipole moment of the charge-transfer state. These parameters are derivable from experiment or from theory. The valence-bond charge-transfer model provides a clear physical picture for the dependence of the hyperpolarizabilities on the structure of charge-transfer molecules and leads to good agreement with the trends predicted by the AM1 calculations.
TL;DR: In this paper, D. K. Multani and R. R. Kalsotra discuss their work in the context of the KALOTRA project, which is an extension of the work of B. D. Jam.
Abstract: [2] D. A. Johnson. Adv. h r g . Chem. Rur/folioc.hrm. 1977, 20. 1. [3] B L. Kalsotra. S . P. Anand. R. K. Multani. B. D. Jain. 1 Orgonome/. Chrm. 1971. 28. X7. 141 B L.. Kalsotra. R . K . Multani. B D. Jam. J Inorg. Nircl. Citein. 1972, 34. 2265. [ 5 ] S K:ipur. B. L. Kalsotra. R. K. Multani. B. D. Jain. J I n ~ r g . Nucl. C/irm. 1973. 35. 16x9. (61 S. Kapur. B L. Kakotra. R. K. Multani. J Inurg. Nucl. Chem 1973, 35. 3966. [7] G B. Deacon. T. D. Tuong. D. G. Vince. Polyhedro:~ 1983. 2, 969. [S] N. Rhsch. A Streitweiser. J A M . Che~?i. Six. 1983. 2, 969: N. Rosch. Iriorg.
TL;DR: In this paper, the same vacancy defect can be present in all the CdTe films grown by vacuum evaporation at 220°C on both plain glass and indium-tin-oxide-coated glass substrates.
Abstract: We have performed positron annihilation experiments on CdTe films grown by vacuum evaporation at 220 °C on both plain glass and indium‐tin‐oxide‐coated glass substrates. By checking the linearity of the valence annihilation parameter S versus the core annihilation parameter W we introduce a method to analyze the data which directly shows that the same vacancy defect can be present in all the films. By comparing the core annihilation parameter at the defect to that at the VCd vacancy we can identify this defect as the divacancy VCd‐VTe. Its concentration in the films decreases from about 1018 to less than 1016 cm−3 after annealing in air at 400 °C for about 30 min. Chlorine doping seems to stabilize the divacancies.
TL;DR: The properties of PdZn surfaces have been examined using thermal desorption mass spectroscopy, core-and valence-level photoemission, and CO chemisorption as mentioned in this paper.
Abstract: The properties of PdZn surfaces have been examined using thermal desorption mass spectroscopy, core- and valence-level photoemission, and CO chemisorption. The formation of Pd-Zn bonds increases the binding energy of the core and valence levels of Pd and reduces the binding energy of the core and valence levels of Zn. An equiatomic PdZn alloy exhibits a large depletion of Pd(4d) states near the Fermi level and a shift of [approximately] +0.7 eV in the binding energy of the Pd(3d) levels. Ab initio SCF calculations for the PdZn molecule and Pd[sub n]Zn[sub n] (n = 2-4) clusters show a decrease in the electron population of the Pd(4d) orbitals as a consequence of Pd(4d) [yields] Zn(4p) charge transfer and Pd(4d) [yields] Pd(5s,5p) rehybridization. These electron transfers reduce electron-electron repulsion within a Pd atom, shifting its core and valence levels toward higher binding energy. The electronic perturbations induced by Zn on Pd reduce the CO-chemisorption ability of Pd by weakening the Pd(4d)-CO(2[pi]) bonding interactions. PdZn surfaces that are rich in Zn show Pd-CO bonds that are 12-16 kcal/mol weaker than those seen on pure Pd surfaces. 57 refs., 12 figs., 1 tab.
TL;DR: The valence is a tool of pinpoint accuracy for locating cation-binding sites, which can also be exploited in engineering high-affinity binding sites and characterizing the linkage between structural components and functional energetics for molecular recognition of metal ions by proteins.
Abstract: The coordination shell of Ca2+ ions in proteins contains almost exclusively oxygen atoms supported by an outer shell of carbon atoms. The bond-strength contribution of each ligating oxygen in the inner shell can be evaluated by using an empirical expression successfully applied in the analysis of crystals of metal oxides. The sum of such contributions closely approximates the valence of the bound cation. When a protein is embedded in a very fine grid of points and an algorithm is used to calculate the valence of each point representing a potential Ca(2+)-binding site, a typical distribution of valence values peaked around 0.4 is obtained. In 32 documented Ca(2+)-binding proteins, containing a total of 62 Ca(2+)-binding sites, a very small fraction of points in the distribution has a valence close to that of Ca2+. Only 0.06% of the points have a valence > or = 1.4. These points share the remarkable tendency to cluster around documented Ca2+ ions. A high enough value of the valence is both necessary (58 out of 62 Ca(2+)-binding sites have a valence > or = 1.4) and sufficient (87% of the grid points with a valence > or = 1.4 are within 1.0 A from a documented Ca2+ ion) to predict the location of bound Ca2+ ions. The algorithm can also be used for the analysis of other cations and predicts the location of Mg(2+)- and Na(+)-binding sites in a number of proteins. The valence is, therefore, a tool of pinpoint accuracy for locating cation-binding sites, which can also be exploited in engineering high-affinity binding sites and characterizing the linkage between structural components and functional energetics for molecular recognition of metal ions by proteins.
TL;DR: The differences between L3 and L2 edges of 3D and 4D transition metal complexes and compounds in octahedral symmetry are discussed in this paper, where the main origin of these differences are the multiplet effects due to the coupling of the 2p core wave function and the 3d and 4d valence wave functions.
Abstract: The differences between L3 and L2 edges of 3d and 4d transition metal complexes and compounds in octahedral symmetry are discussed. The main origin of these differences are the multiplet effects due to the coupling of the 2p core wave function and the 3d and 4d valence wave functions. The 3d and 4d spin–orbit coupling is a second origin of difference. For 3d systems the multiplet effects dominate all other interactions and the L3 and L2 edge are completely mixed and reordered. For 4d systems the core hole spin–orbit coupling is large and the L3 and L2 are separated by about 100 eV with a ratio close to 2:1. The differences between the L3 and L2 edge originate from the weight transfer between the t2g and eg peaks due to the multiplet effect. This weight transfer is about 25% for the L3 edge and about 5% for the L2 edge, which implies that for a comparison to single‐particle calculations the L2 edge is preferable to use. Partly filled 4d systems are low‐spin and the occupation of the t2g states implies a de...
TL;DR: In this paper, the complete valence shell photoelectron spectrum of sulphur dioxide has been studied using HeI, HeII and synchrotron radiation, and it has been shown that the inner valence region contains several complicated structures and is dominated by a prominent feature centred at a binding energy around 34 eV.
01 Jan 1994
TL;DR: In this article, parallel electron energy loss spectroscopy (PEELS) was used to record the Mn L2,3-edges from a range of natural and synthetic manganese containing materials, covering valences 0, II, III, IV and VII, with an energy resolution of ca. 0.5 eV.
Abstract: Parallel electron energy-loss spectroscopy (PEELS) in a scanning transmission electron microscope (STEM) was used to record the Mn L2,3-edges from a range of natural and synthetic manganese containing materials, covering valences 0, II, III, IV and VII, with an energy resolution of ca. 0.5 eV. The Mn L2,3 electron-loss near-edge structure (ELNES) of these edges provided a sensitive fingerprint of its valence. The Mn2+ L2,3-edges show little sensitivity to the local site symmetry of the ligands surrounding the manganese. This is illustrated by comparing the Mn L2,3-edges from 4-, 6- and 8-fold coordinated Mn2+. In contrast, the Mn L3-edges from Mn3+ and Mn4+ containing minerals exhibited ELNES that are interpreted in terms of a crystal-field splitting of the 3d electrons, governed by the symmetry of the surrounding ligands. The Mn L3-edges for octahedrally coordinated Mn2+, Mn3+ and Mn4+ showed variations in their ELNES that were sensitive to the crystal-field strength. The crystal-field strength (10Dq) was measured from these edges and compared very well with published optically determined values. The magnitude of 10Dq measured from the Mn L3-edges and their O K-edge prepeaks of the manganese oxides were almost identical. This further confirms that the value of 10Dq measured at the Mn L3-edge is correct. Selected spectra are compared with theoretical 2p atomic multiplet spectra and the differences and similarities are explained in terms of the covalency and site symmetry of the manganese. The Mn L3-edges allow the valence of the manganese to be ascertained, even in multivalent state materials, and can also be used to determine 10Dq.
TL;DR: In this paper, the crystal structures of RBa2Cu3Ox (R=Y and rare earths; x=6 and 7) ceramic materials were investigated at 10 K by neutron diffraction and consistently analyzed concerning systematic trends.
Abstract: The crystal structures of RBa2Cu3Ox (R=Y and rare earths; x=6 and 7) ceramic materials were investigated at 10 K by neutron diffraction and consistently analysed concerning systematic trends. Other than for non-superconducting PrBa2Cu3O7 the lattice parameters and most interionic distances exhibit the well known lanthanide contraction behaviour, i.e., a linear relationship with the ionic radii of trivalent rare-earth ions. The only exceptions are associated with the apex oxygen O(1) ions: the chain copper Cu(1)-O(1) distances are constant within error limits, and the plane copper Cu(2)-O(1) distances are increasing across the rare-earth series. The much stronger increase of the distance Cu(2)-O(1) in the RBa2Cu3O6 series compared to the RBa2Cu3O7 series can be explained by the increase of Tc from 90 K for YbBa2Cu3O7 to 96 K for NbBa2Cu3O7. The smaller distance Cu(1)-O(1) for the RBa2Cu3O6 series compared to the RBa2Cu3O7 series may be related to the suggested double-well potential of the apical oxygen ion. For some interionic distances of PrBa2Cu3O7 approximately parallel to the h direction (i.e., the chain direction) we determine by extrapolation a valence of +3.4 for the Pr ions. This indicates for PrBa2Cu3O7 a highly anisotropic 4f-CuO2 valence band hybridization. An important structural property with respect to the superconductivity is the puckering of the CuO2 planes: the superconductivity is lost when the puckering angle exceeds a critical value of about 167.3.
TL;DR: In this paper, the synthesis of extremely large nanoscaled anionic species is discussed, together with the occurrence of a large transferable {Mo17} building block in the compounds 3a, 3b and 4a which also exists in the corresponding iron compound Na3(NH4)12[H15Mo57Fe6(NO)6O183(H2O)18].
Abstract: The compounds (NMe4)5[As2Mo8V4AsO40] . 3 H2O 2a, (NH4)21[H3Mo57V6(NO)6O183(H2O)18] . 65 H2O 3a, (NH2Me2)18(NH4)6[Mo57V6(NO)6O183(H2O)18] . 14 H2O 3b and (NH4)12[Mo36(NO)4O108(H2O)16] . 33 H2O 4a (3a and 4a were not correctly reported in the literature regarding to their composition, structures and the oxidation states of the metal centres) which contain large isolated anionic species, have been prepared (among them 3 a, 3 b, and 4 a in rather high yield) and characterized by complete crystal structure analysis as well as IR/Raman, UV/VIS/NIR, ESR spectroscopy and magnetic susceptibility measurements, redox titrations, bond valence sum calculations, elemental analyses and thermogravimetric studies. Perspectives for polyoxometalate chemistry referring to the synthesis of ''extremely'' large nanoscaled species are discussed, together with the occurrence of a large transferable {Mo17} building block in the compounds 3a, 3b and 4a which also exists in the corresponding iron compound Na3(NH4)12[H15Mo57Fe6(NO)6O183(H2O)18] . 76H2O 7a.
TL;DR: In this paper, the optical absorption spectra of a series of small gold clusters and their cations have been measured, between 1.9 and 5.6 eV, using a method based upon the photodepletion of a molecular beam of their van der Waals complexes containing one and two xenon atoms.
Abstract: The optical absorption spectra of a series of small gold clusters and their cations have been measured, between 1.9 and 5.6 eV, using a method based upon the photodepletion of a molecular beam of their van der Waals complexes containing one and two xenon atoms. This method provides size‐specific information even though the molecular beam contains a wide range of cluster sizes. There is little difference between the spectra of complexes containing one or two xenon atoms. However there is a pronounced odd–even alternation in the spectra of gold clusters with differing numbers of valence s electrons. This alternation is described in terms of a simple electron pairing scheme. The spectrum for Au13 is in reasonable agreement with Dirac scattered‐wave molecular orbital considerations for icosahedral Au13 [A. F. Ramos, R. Arratia‐Perez, and G. L. Malli, Phys. Rev. B 35, 3790 (1987)]. This description of the molecular and electronic structure of small gold clusters in terms of localized molecular orbitals is contrasted with other models based upon jellium potentials and delocalized excitations that have been used to describe small clusters of alkali metals and silver. The bonding in gold clusters is influenced by relativistic effects that increase the degree of sd hybridization in the molecular orbitals. Even though gold clusters can be described in this way, some evidence for electron shells is also presented. Thus, it is concluded that structural motifs other than jellium potentials can lead to shell structure in cluster properties.
TL;DR: In this article, a large intrinsic magnetoresistance has been found near the ferromagnetic transition of metallic manganese oxides with perovskite-type crystal structure, and the magnetic and transport properties were measured on bulk and thin-film La1−xAxMnO3+δ with A=Ca,Sr,Ba.
Abstract: A large intrinsic magnetoresistance has been found near the ferromagnetic transition of metallic manganese oxides with perovskite‐type crystal structure. The magnetic and transport properties were measured on bulk and thin‐film La1−xAxMnO3+δ with A=Ca,Sr,Ba. Assuming the double‐exchange model proposed by Zener [Phys. Rev. 81, 440 (1951); 82, 403 (1951)], the strong dependence of the transport properties on the magnetic field and also on the chemical composition is attributed to the mixed Mn3+/Mn4+ valence.
TL;DR: In this article, the quadratic, two-electron covalent and ionic valence indices are used to investigate the bond-breaking-bond-forming (BB-BF) process in an atom exchange reaction between H2 and X (X = H, F-I) as well as in the O2-H system Valence changes accompanying selected charge reorganizations are examined within the three-orbital model and valence diagrams for symmetric transition states (TSs) are given.
Abstract: The recently introduced set of the quadratic, two-electron covalent and ionic valence indices is used to investigate the bond-breaking–bond-forming (BB-BF) process in an atom exchange reaction between H2 and X (X = H, F—I) as well as in the O2—H system Valence changes accompanying selected charge reorganizations are examined within the three-orbital model and valence diagrams for symmetric transition states (TSs) are given The UHF valence data for Li2O and CO2 and the H—H—X, O—O—H, and O—H—O (ABC) TSs (collinear and angular) are reported and compared to valence data in the separated fragments limits (SFL), AB and BC The overall valence, ν(ABC), and the total (ionic plus covalent) diatomic valences, νAB and νBC, are used as measures of the overall bond-order in a concerted BB–BF reaction, to test the postulate of the bond-energy–bond-order (BEBO) model In collinear TSs of H2X, ν ≊ −1, ie, one bonding electron pari, is found to be roughly preserved, whereas in the angular H2X and in collinear O—H—O TSs, the effect of increased valence at the saddle-point is observed, relative to that of diatomic fragments (reactiants or products) For the angular O—O—H TS, a similar increase in | ν (ABC)| relative to both O2 and OH SFLs is detected; smaller changes relative to the O2 data are found in the collinear TS This observation is in agreement with earlier predictions from the intersecting-state model The relative diatomic valences, ν/ν and ν/ν, are shown to conserve the overall relative bond multiplicity around 1 in both collinear and angular TSs of the H2—X systems © 1994 John Wiley & Sons, Inc
TL;DR: In this article, the effect of ionic strength on pHBC was investigated on a range of soils by establishing separate titration curves for each of 100 soils by adding incremental amounts of either Ca(OH)2, NaOH, HCl or H2SO4 to soil suspended (1 : 5) in water.
Abstract: Although the measurement of pH buffer capacity (pHBC) is used to determine lime requirement and acid addition rate in acidification studies, the experimental conditions under which pHBC is determined have not been studied. The effect of valence and ionic strength on the measurement of pHBC was investigated on a range of soils. The effect of the monovalent or divalent accompanying ion was examined by establishing separate titration curves for each of 100 soils by adding incremental amounts of either Ca(OH)2, NaOH, HCl or H2SO4 to soil suspended (1 : 5) in water. Linear regressions were fitted to the linear portion of each titration curve and the slopes of these lines were used as a measure of pHBC. For each soil, the pH buffer capacities were statistically compared. The pHBC determined with Ca(OH)2 was significantly (P = 0.05) greater than that determined with NaOH in 92 soils and, on average (all soils), was 2.2 times the pHBC in NaOH. The effect of ionic strength on pHBC was investigated in each of 20 soils by titrating with HCI in water and suspensions at nominal ionic strengths of 0.006, 0.03 and 0.3 m. In all soils there was a trend for increasing pHBC with increasing ionic strength (I) and, for I < 0.03 m, there was a marked increase in pHBC with increasing I. The results are discussed in relation to the effect of valence and ionic strength on pH buffer capacity mechanisms, and the implications with respect to calculating acidification rates and lime requirements.
TL;DR: In this paper, extended Huckel calculations were performed to study the rate-limiting step in the oxidative dehydrogenation of methanol as catalyzed by transition-metal oxides and polyoxometalates.
Abstract: Extended Huckel calculations were performed to study the rate-limiting step in the oxidative dehydrogenation of methanol as catalyzed by transition-metal oxides and polyoxometalates. The reaction was modeled as the transfer of hydrogen from a methyl adduct to the surface of a transition-metal oxide fragment whose valence electronic structure was shown to resemble those of the Keggin ions which are known to catalyze the reaction
TL;DR: In this paper, the structure of the high temperature polymorph of Bi2MoO6 has been determined by Rietveld powder refinement of high-resolution neutron diffraction data from the H1A diffractometer at the Brookhaven HFBR (high flux beam reactor).
TL;DR: In this paper, the ground and very low-lying excited states of all 120 first and second row diatomic molecules are surveyed and the best available experimental (where known) and theoretical values for the dissociation energies to ground-state atoms are also presented.
Abstract: : The ground and very low-lying excited states of all 120 first and second row diatomic molecules are surveyed. Three quarters of these molecules have had their ground state term symbols reliably experimentally determined. However, one quarter remain predicted only theoretically. For all 120 species, the best available experimental (where known) and theoretical values for the dissociation energies to ground-state atoms are also presented. The Aufbau principle, combined with standard energy ordering for the valence molecular orbitals, is able to properly account for the ground-state term symbols of all but twenty of the diatomics studies. The twenty exceptions produce higher than expected ground state spin multiplicity and arise when there are 4-5 or 7-8 valence electrons and group 3, 4, or 5 (but not group 6 or 7) atoms are involved.