Showing papers on "Valence (chemistry) published in 1986"

Bond orders and valences from ab initio wave functions

Abstract: The definition and properties of the bond order and valence indices calculated from ab initio wave functions are summarized. Their physical interpretation relationships to the exchange effects in bonding and generalization to correlated wave functions are also discussed. Some examples with typical bond order and valence values are shown.

Looking at orbitals in the laboratory: The experimental investigation of molecular wavefunctions and binding energies by electron momentum spectroscopy

Abstract: The experimental technique of electron momentum spectroscopy (EMS) (i.e., binary (e, 2e) spectroscopy) is discussed together with typical examples of its applications over the past decade in the area of experimental quantum chemistry. Results interpreted within the framework of the plane wave impulse and the target Hartree—Fock approximations provide direct measurements of, spherically averaged, orbital electron momentum distributions. Results for a variety of atoms and small molecules are compared with calculations using a range of Fourier transformed SCF position space wavefunctions of varying sophistication. Measured momentum distributions (MD) provide a “direct” view of orbitals. In addition to offering a sensitive experimental diagnostic for semiempirical molecular wavefunctions, the MD's provide a chemically significant, additional experimental constraint to the usual variational optimization of wavefunctions. The measured MD's clearly reflect well known characteristics of various chemical and physical properties. It appears that EMS and momentum space chemistry offer the promise of supplementary perspectives and new vistas in quantum chemistry, as suggested by Coulson more than 40 years ago. Binding energy spectra in the inner valence region reveal, in many cases, a major breakdown of the simple MO model for ionization in accord with the predictions of many-body calculations. Results are considered for atomic targets, including H and the noble gases. The measured momentum distribution for H2 is also compared with results from Compton scattering. Results for H2 and H are combined to provide a direct experimental assessment of the bond density in H2, which is compared with calculations. The behavior of the outer valence MD''s for small row two and row three hydride molecules such as H2O and H2S, NH3, HF, and HCl are consistent with well known differences in chemical and physical behavior such as ligand-donor activity and hydrogen bonding. MD measurements for the outermost valence orbitals of HF, H2O and NH3 show significant differences from those calculated using even very high-quality wavefunctions. Measurements of MD's for outer σg orbitals of small polyatomic molecules such as CO2, COS, CS2, and CF4 show clear evidence of mixed s and p character. It is apparent that EMS is a sensitive probe of details of electronic structure and electron motion in atoms and molecules.

Valence electronic structure of AuZn and AuMg alloys derived from a new way of analyzing Auger-parameter shifts.

Abstract: A new model is presented that makes it possible to determine the degree of electron transfer in alloys from measurements of the Auger parameter This approach is superior to the use of core-ionization-energy shifts in conjunction with a potential model The Auger parameter does not depend on any reference level, whereas ionization energies measured with respect to the Fermi level must be corrected to the vacuum level before they can be used in the potential model Furthermore, the new model does not require inclusion of contributions from Madelung, surface-dipole, or other bulk contributions, which must be considered in the potential model This new technique is applied to the alloys AuZn and AuMg to show that approximately 01e is transferred to the gold in the first case and about 02e in the second The analysis shows that there is a small increase in the gold 5d population through the series Au, AuZn, AuMg The results are combined with data on M\"ossbauer isomer shifts and theoretical band-structure calculations for Au to give a description of the valence electronic configuration in these alloys The substantial transfer of electrons to the gold 6s orbital, inferred from the M\"ossbauer results, is partially offset by back transfer of 6p electrons from gold to the partner atom This conclusion is supported by band-structure calculations for AuCs

Carbon K‐shell excitation of gaseous and condensed cyclic hydrocarbons: C3H6, C4H8, C5H8, C5H10, C6H10, C6H12, and C8H8

Abstract: The carbon K‐shell excitation spectra of gaseous cyclic hydrocarbons, both saturated (cyclopropane, cyclobutane, cyclopentane, cyclohexane) and unsaturated (cyclopentene, cyclohexene, and cyclooctatetraene), have been recorded by electron energy loss spectroscopy under dipole‐dominated conditions. These are compared to the NEXAFS spectra of multilayers and monolayers of C4H8, C5H8, C6H12, and C8H8 on Pt(111). Multiple scattering Xα calculations of the spectra of cyclopropane, cyclobutane, and cyclohexane are also reported. In most cases the gas and solid spectra are essentially the same indicating that intramolecular transitions dominate in the condensed phase. The NEXAFS polarization dependence of the condensed phases has assisted spectral assignments and the determination of the molecular orientation in the monolayer phase. In the saturated species a sharp feature about 3 eV below the carbon 1s ionization threshold is identified as a transition to a state of mixed Rydberg/valence character with the π*(C...

Subvalent Group 4B metal alkyls and amides. Part 8. Germanium and tin carbene analogues MR2[M = Ge or Sn, R = CH(SiMe3)2]: syntheses and structures in the gas phase (electron diffraction); molecular-orbital calculations for MH2 and GeMe2

Abstract: Bis[bis(trimethylsilyl)methyl]germanium, GeR2[R = CH(SiMe3)2], is conveniently prepared from GeCl2(diox)(diox = 1,4-dioxane)(for which an improved synthesis, from GeCl4 and SnHBun3, is reported) and 2MgCl(R)(OEt2), MgR2(diox)0.5, or MgR2(OEt2) in OEt2. The corresponding tin(II) alkyl is accessible from SnCl4 and successively 2LiR (to yield SnCl2R2) and Li2(cot)(cot = cyclooctatetraene) in OEt2. Gas-phase electron diffraction (g.e.d.) patterns, recorded with nozzle temperatures of 155 °C for GeR2 and 120 °C for SnR2, show that the gas consists of V-shaped monomers. Least-squares refinements of models of C2 symmetry yielded the bond distances Ge–C 203.8(15) and Sn–C 222(2) pm and the angles CGeC 107(2) and CSnC 97(2)°. In GeR2 the –CH(SiMe3)2 ligands are oriented so that the HCiMCiH moiety (Ci= inner, or methine, C) has a nearly planar syn,syn conformation but in SnR2 the dihedral angles CiMCiH are ca. 15°. Ab initio molecular-orbital calculations with a better than DZ (double zeta) basis were carried out on the model compounds MH2 and GeMe2, and yielded the optimal bond distances Ge–H 158, Ge–C 202, and Sn–H 177 pm, and valence angles HGeH 93, CGeC 97, and HSnH 93°. Correlation of experimental and calculated structures shows that MII–H and MII–C bond distances are significantly larger (by 4–10 pm) than their MIV–H and MIV–C counterparts, and valence angles at M increase in the series GeH2= SnH2 < GeMe2= SnMe2 < GeR2 < SnR2. Both the long bond distances and the less-than-tetrahedral valence angles are rationalised by assuming the metal lone pair to occupy atomic orbitals of predominant s character and the M–H and M–C σ-bonding orbitals to be formed from metal atomic orbitals of predominant p character.

Mixed-valence 1',6'-dihalobiferrocenium salts: the effects of the solid-state environment on electron-transfer rates

Abstract: Determination des structures cristallines du triiodure de diiodobiferrocenium et de l'octaiodure de dichlorobiferrocenium. Spectres Mossbauer, RPE et IR

The exact one-electron model of molecular structure

Abstract: The Schrodinger equation satisfied by the electron density is derived without approximation from the theory of marginal and conditional amplitudes. The equation arises from a factorization of the total N-electron wavefunction defined by the normalization appropriate to a conditional amplitude. This Schrodinger equation is an exact dynamical model for computing effective one-electron potentials from known N-electron wavefunctions. Results are presented for several small molecules. They display the shell structure of atoms, and the valence structure of both ionic and covalent molecules.

The electronic structures of platinum dichalcogenides: PtS2, PtSe2 and PtTe2

Abstract: The band structures and density of states functions of platinum dichalcogenides, calculated using the LMTO-ASA method in conjunction with muffin-tin potentials, are reported for the first time. The results predict that PtS2 is a semiconductor with an energy gap of about 1.2 eV, PtSe2 is a semimetal with a slight indirect overlap of the conduction and valence bands, the PtTe2 is a metal with a complex Fermi surface. The electronic structures of these compounds are found to be substantially different from those of group IVb, Vb, and VIb TMDC having the same crystal structure. The results are discussed in terms of atomic orbital binding energies and the local coordination of the constituent atoms. The band structures obtained are generally in good agreement with the available experimental measurements.

Products of dissociative recombination of polyatomic ions

Abstract: On discute de la recombinaison dissociative d'ions polyatomiques. Les processus qui impliquent le moins de rearrangement des liaisons de valence sont favorises

Valence bands of oriented finite linear chain molecular solids as model compounds of polyethylene studied by angle-resolved photoemission

Abstract: Angle-resolved photoemission spectra were measured using synchrotron radiation of two kinds of oriented model compounds of polyethylene with their molecular axes perpendicular to the substrate surface, i.e. evaporated films of hexatriacontane CH3(CH2)34CH3 and Langmuir-Blodgett films of Cd arachidate (CH3(CH2)17COO)2Cd. Both films show similar photoelectron energy distribution curves determined by the long-alkyl chain. The intramolecular energy-band dispersion of polyethylene was determined from the photon-energy dependence of the normal-emission spectra. This is the first direct observation of an energy-band dispersion in organic solids. The upper bands formed by C 2p and H 1s electrons extend from 8.8 to 15.5 eV below the vacuum level, and the deeper-lying bands originating from C 2s electrons lie from 17.5 to 24.7 eV. The band structure obtained is compared to results from XPS and ESR studies. Furthermore, the experimentally determined band structure is discussed in detail in view of theoretical calculations for polyethylene. Ab initio and extended Huckel calculations give a good description of the experimental results.

The generalized resonating valence bond description of cyclobutadiene

Abstract: The low-lying electronic states of square and rectangular cyclobutadiene (CBD) are calculated by using the generalized resonating valence bond (GRVB) method and compared with the results from Hartree-Fock and configuration interaction wavefunctions. We find that simple valence bond concepts correctly predict the sequence of excited states (including ground-state singlet) and the distortion to a rectangular geometry for the ground state. Contrary to common expectation, we find that the singlet ground state of square CBD has 22 kcal of resonance energy (relative to a single valence bond structure). Thus, CBD is not antiresonant, though it is much less stable than normal conjugated systems.

Distinguishing Atom Differences in a Molecular Graph Shape Index

Abstract: The molecular shape index, previously described, has been extended to take into account the size differences among heteroatoms and carbon atoms in different valence states. In the algorithm for the shape index, 2K, calculated as a function of the non-hydrogen atom count, the count value of non-C sp3 atoms is modified by a term α. The value of α is derived from the covalent radius of an atom relative to the radius of C sp3. Several examples of the use of the atom-specified shape index, 2Kα, in biological studies are presented.

Coloured coatings prepared by the sol-gel process

Abstract: Homogeneous clear transparent coloured coatings can be produced by using sol-gel processes. Transition metal elements are incorporated into the glass network as ions showing a certain valence and coordination. The oxidation-reduction equilibrium moves towards the oxidized state in these low temperature glassy films. It was found that cobalt exists as Co 2+ in tetrahedral coordination, chromium as Cr 6+ in tetrahedral symmetry and manganese as Mn 3+ in octahedral coordination. Crystalline segregations of Co 3 O 4 , Mn 2 O 3 , CuO and Cr 2 O 3 were also observed. The spectra for iron and copper coatings change with coating thickness. Both light scattering and absorption mechanism exist. These thin coating films give absorption colours which are too weak for application. Only colours due to light scattering (manganese, iron and copper ruby) were effective.

k⋅p theory of semiconductor superlattice electronic structure. I. Formal results

Abstract: This is the first of a two-paper series in which we present a complete k\ensuremath{\cdot}p theory of semiconductor superlattices. Here we present the formal theoretical results. In the second paper, numerical impleed to derive an eigenvalue equation for the superlattice wave vector and eigenfunctions. The formalism has the advantage of involving only small-dimensionality matrices (typically, 12\ifmmode\times\else\texttimes\fi{}12). It is well suited to optical and transport property calculations.

Rydberg–valence interactions in the Πg states of O2

Abstract: Multiphoton ionization spectroscopy is used to characterize the (3sσ) 3Πg Rydberg state of O2. It is found that the 3Πg Rydberg state is very diffuse and has rotationally resolved structure for the v’=2 level only. The FWHM of rotational lines for both the (3sσ) 3,1Πg states is greater than the excitation bandwidth, indicating strong radiationless decay of the two states. Interactions with the nearby 3Πg and 1Πg valence states are invoked to explain the observed behavior.

Fragmentation of nitrous oxide by monochromatic soft x rays

Abstract: Studies of the fragmentation of nitrous oxide using time‐of‐flight mass spectroscopy following excitation by monochromatic synchrotron radiation are reported. The consequences of core electron excitation of all three atoms to the lowest unoccupied molecular orbital are described. Clear changes in the mass spectra with the atomic site of excitation are observed. These changes can be understood qualitatively in terms of the atomic populations, which govern the Auger decay channels, and the overlap populations, which determine the bond structure, of the valence molecular orbitals. Mass spectra correlated with Auger electrons are very similar to mass spectra correlated with events producing low energy electrons. This result may indicate that the selectivity of the primary Auger decay process is not affected by additional excitation events.

Recording medium for ink-jet printing

Abstract: A recording medium for ink-jet printing comprising a support material containing at least in the surface portion thereof a water-soluble metal salt with the ion valence of the metal thereof being 2 to 4 and a cationic organic material.

Effects of substituent-induced strain on the electronic structure of polydiacetylenes

Abstract: The valence effective Hamiltonian technique (VEH), and modified neglect of differential overlap (MNDO) calculations are used to study the influence of strain induced by side chains on the geometry of polydiacetylene backbones and the resulting polymer band structure, band gap, and ionization potential. Simulations of strain effects on the polymer backbone yield variations in optical properties which are similar to those observed experimentally during thermochromic phase changes in urethane‐substituted polydiacetylenes. Our results suggest that these changes in optical properties are related to strain at points of substituent attachment and not to fundamental changes in the backbone geometry such as an acetylenic‐to‐butatrienic transformation.

Inner shell electron-energy loss spectroscopy of some heterocyclic molecules

Abstract: The carbon K-shell spectra of gaseous furan, pyrrole, tetrahydrofuran, pyrrolidine, tetrahydropyran, and piperidine have been recorded by electron energy loss spectroscopy (ISEELS) under electric dipole scattering conditions (25 keV impact, small angle) The spectra are dominated by transitions to unoccupied valence states of π and σ symmetry Features attributed to transitions to π*(CH2) levels are consistently observed below the ionization threshold in the spectra of the saturated species the positions of continuum features are generally in agreement with a previously documented correlation with bond lengths Additional weak continuum features are observed in the smaller saturated heterocyclic species which are ascribed to delocalized σ* states

The structures of binary compounds. II. Theory of the pd-bonded AB compounds

Abstract: For pt.I see ibid., vol.19, p.285 (1986). The relative stability of the seven most common crystal structures of the pd-bonded AB compounds is examined within a tight-binding model which allows the influence of atomic size, p and d energy level separation, and band filling to be studied directly. The simple theory, which neglects the valence s electrons, accounts qualitatively for the observed domains of NaCl, CsCl, NiAs, MnP and boride stability, only the narrow FeSi domain remaining unaccounted for.

Autoionization and isotope effect in the threshold photoelectron spectrum of 12CO2 and 13CO2

Abstract: Threshold photoelectron spectra of 12CO2 and 13CO2 have been obtained from the onset at 900 A (13.75 eV) to 620 A (20 eV) at a 9 meV resolution using dispersed synchrotron radiation with a 0.3 A bandpass. A number of forbidden and/or previously unobserved transitions have been assigned for the X, A, B, and C states of CO+2. Photoelectron spectra by electron time of flight at selected excitation wavelengths in the Franck–Condon gap region below the A state, reveal the autoionization of neutral valence states as well as Rydberg series converging to the A 2Πu and B 2Σ+u states. The photoelectron energy distribution obtained upon excitation of the Rydberg states is consistent with that given by the Bardsley–Smith model for autoionization, while the superexcited valence states decay via a resonance autoionization process proposed previously by Guyon, Baer, and Nenner. These valence states are also responsible for the strong enhancement of vibrationally forbidden transitions, such as (000)→(010). Finally...

Initial formation of the interface between a polar insulator and a nonpolar semiconductor: CaF2 on Si(111)

Abstract: The initial stages of interface formation between CaF2 and Si(111) have been studied with both core‐level and angle‐resolved valence band photoemission spectroscopy. Both the Si 2p and Ca 3p core levels indicate that a Si–Ca bond is present at the CaF2–Si(111) interface. Annealing of thin CaF2 films at 700–800 °C results in preferential evaporation of F, and the surface undergoes a number of reconstructions until a stable Si(111):Ca 3×1 reconstruction is obtained on which there is calcium, but no fluorine. This surface exhibits a surface state with an upward dispersion of 0.4 eV towards the K point of the 1×1 surface Brillouin zone. Our results show that the CaF2 molecule can be dissociated on the Si(111) surface at typical epitaxial growth temperatures, with the Ca being more strongly adsorbed to the Si than is the F.