Outermost Valence Electrons Research Articles

An environmental impact statement for molecular anions.

A molecular anion's (MA's) chemical reactivity and physical behavior can be quite different when it is surrounded by other molecules than when it exists in isolation. This sensitivity to the surrounding environment is especially high for anions because their outermost valence electrons are typically loosely bound and exist in rather spatially diffuse orbitals, allowing even weak intermolecular interactions arising from the environment to have strong effects. This Perspective offers illustrations of such sensitivity for a variety of cases including (i) the effect of solvation on electron binding energies, (ii) how some "well known" anions need to have solvent molecules around to even exist as stable species, (iii) how internal Coulomb repulsions within a multiply charged MA can provide temporary stability toward electron loss, (iv) how MAs arrange themselves spatially near liquid/vapor interfaces in manners that can produce unusual reactivity, (v) how nearby cationic sites can facilitate electron attachment to form a MA site elsewhere, (vi) how internal vibrational or rotational energy can make a MA detach an electron.

SnTe monolayer: Tuning its electronic properties with doping

Abstract Two-dimensional (2D) group IV monochalcogenides have attracted extensively attention due to their specific electronic properties and promising applications. The atomic-thick SnTe is an exemplar in 2D group IV monochalcogenides and shows robust ferroelectric properties at room-temperature. Using first principles calculation based on the density functional theory, we investigated the effects of doping on geometric structure, formation energy, band structure and carrier effective mass of SnTe monolayer. The results indicate that the radii and outermost valence electrons of doped atoms significantly affect the geometric structure and electronic structure of the doped compounds. The SnTe monolayer with donor (acceptor) impurities could be transformed into n-type (p-type) degenerate semiconductors with smaller electronics (holes) effective mass. Moreover, the direct bandgap semiconductors were obtained in this study by doping MIV or XVI in SnTe monolayer. The electrical conductivity and photoelectric conversion efficiency can also be improved by doping atoms in SnTe monolayer. These intriguing results make doped SnTe monolayers promising candidate materials for thermoelectric or photoelectric devices.

Experimental charge-density studies: data reduction and model quality: the more the better?

In this review, recent developments concerning data and model quality in experimental charge-density investigations from a personal view-point are described. Data quality is not only achieved by the high resolution, high I/σ(I) values, low merging R values and high multiplicity. The quality of the innermost reflections especially is crucial for mapping the density distribution of the outermost valence electrons and can be monitored by (I/σ)asymptotic. New detector technologies seem to be promising improvements. Empirical corrections to correct for low-energy contamination of mirror-focused X-ray data and for resolution- and temperature-dependent errors caused by factors such as thermal diffuse scattering are described. Shashlik-like residual density patterns can indicate the need for an anharmonic description of the thermal motion of individual atoms. The physical reliability of the derived model must be thoroughly analysed. The derived probability density functions for the mean-squared atomic vibrational displacements especially should have only small negative values. The treatment of H atoms has been improved by methods to estimate anisotropic thermal motion. For very high resolution data, the polarization of the core density cannot be neglected. Several tools to detect systematic errors are described. A validation tool is presented that easily detects when the refinement of additional parameters yields a real improvement in the model or simply overfits the given data. In all investigated structures, it is proved that the multipole parameters of atoms with a comparable chemical environment should be constrained to be identical. The use of restraints could be a promising alternative.

The Analysis of Element and Measure Analysis of NMR Spectrum and XRD Spectrum for High Purity 3,4,9,10 Perylenetetracarboxylic Dianhydride-PTCDA

vacuum sublimation method was used to purify the homegrown 3,4,9,10 perylenetetracarboxylic dianhydride(PTCDA)powder with a purity of 98% in its sublimation point of 450 ℃. With Bill’s law and ultraviolet-visible spectrophotometer testing analysis, its purity reached to 99.8%. Meanwhile, the contents of C and H elements in the pre-and post-purified molecules were also measured by using elemental analyzer. The measured results indicate that the contents of C and H elements in the post-purified the molecules are very close to the theoretical value. H element in the molecular structure was investigated with nuclear magnetic resonance (NMR) spectroscopy and the results demonstrated that there are an equal number of H atoms in two different chemical environments and it can only be located on the aromatic ring. By discussing the chemical bond formation of PTCDA molecules, the C, H and O atoms in high purity PTCDA molecules are mainly covalent bonds. The crystalline state and crystal structure of this organic material were tested and analyzed by X-ray diffractometer. The results suggest that the post-purified PTCDA power existed α-PTCDA and β-PTCDA two phases, in which α-PTCDA phase is major component while β-PTCDA phase accounts for about one five of the total ingredients. Besides, the crystal cell belonged to bottom-centered monoclinal structure. Meanwhile, the crystal state, grain size and band structure of PTCDA single crystal thin films formed on the surface of p-type silicon in its sublimation point are investigated in detail. During the high-purity α-PTCDA forming organic single thin film on the surface of p-type single silicon, the π-electron cloud covered on the top, bottom and two sides of its thin film’s molecular layer plane. Due to the formation of delocalized bond that attributed to the overlap of the outermost valence electron orbital of C, H, O atom, the valence electrons generate co-movement and the energy level splitting for the band. The energy difference between valence band and the first tight binding is 2.2 eV which lead to this organic material possessing the properties of semiconductor conduction. In addition, this organic material with the intrinsic carrier concentration for 1014 cm-3 belong to weak p-type organic semiconductor material. This organic material combines with the surface of p-type silicon to form hetehomo-type heterojunction which is provided with excellent response for visible light to near infrared wavelengths of light.

A Quantitative Scale of Oxophilicity and Thiophilicity.

Oxophilicity and thiophilicity are widely used concepts with no quantitative definition. In this paper, a simple, generic scale is developed that solves issues with reference states and system dependencies and captures empirically known tendencies toward oxygen. This enables a detailed analysis of the fundamental causes of oxophilicity. Notably, the notion that oxophilicity relates to Lewis acid hardness is invalid. Rather, oxophilicity correlates only modestly and inversely with absolute hardness and more strongly with electronegativity and effective nuclear charge. Since oxygen is highly electronegative, ionic bonding is stronger to metals of low electronegativity. Left-side d-block elements with low effective nuclear charges and electronegativities are thus highly oxophilic, and the f-block elements, not because of their hardness, which is normal, but as a result of the small ionization energies of their outermost valence electrons, can easily transfer electrons to fulfill the electron demands of oxygen. Consistent with empirical experience, the most oxophilic elements are found in the left part of the d block, the lanthanides, and the actinides. The d-block elements differ substantially in oxophilicity, quantifying their different uses in a wide range of chemical reactions; thus, the use of mixed oxo- and thiophilic (i.e., "mesophilic") surfaces and catalysts as a design principle can explain the success of many recent applications. The proposed scale may therefore help to rationalize and improve chemical reactions more effectively than current qualitative considerations of oxophilicity.

Resonance Effects in Photoemission Time Delays.

We present measurements of single-photon ionization time delays between the outermost valence electrons of argon and neon using a coincidence detection technique that allows for the simultaneous measurement of both species under identical conditions. The analysis of the measured traces reveals energy-dependent time delays of a few tens of attoseconds with high energy resolution. In contrast to photoelectrons ejected through tunneling, single-photon ionization can be well described in the framework of Wigner time delays. Accordingly, the overall trend of our data is reproduced by recent Wigner time delay calculations. However, besides the general trend we observe resonance features occurring at specific photon energies. These features have been qualitatively reproduced and identified by a calculation using the multiconfigurational Hartree-Fock method, including the influence of doubly excited states and ionization thresholds.

Electronic Structure of LaF+ and LaF from Frozen-Core Four-Component Relativistic Multiconfigurational Quasidegenerate Perturbation Theory

The electronic structure of the molecules LaF+ and LaF was studied using frozen-core four-component multiconfigurational quasidegenerate perturbation theory. To obtain proper excitation energies for LaF+, it was essential to include electronic correlations between the outermost valence electrons (4f, 5d, and 6s) and ionic core electrons composed of (4s, 4p, 4d, 5s, and 5p). The lowest-lying 16 excited states were examined for LaF+, and the lowest 30 states were examined for LaF. The excitation energies calculated for LaF+ agree with the available experimental values, as well as with values from ligand field theory. Errors are within 0.4 eV; for example, the highest observed state 2Pi is 3.77 eV above the ground state, and the present value is 4.09 eV. For LaF, agreement between the experimental and theoretical state assignments and between the experimental and calculated excitation energies was generally good, except for the electron configurations of certain states. Errors are within 0.4 eV except for a single anomaly; for example, the highest observed excited-state discussed in this work is 2.80 eV above the ground state, and the present value is 2.42 eV. We discuss the characteristics of the bonding in LaF+ and LaF.

Calculated Electron Impact Ionization Cross Sections of Excited Ne Atoms Using the DM Formalism

AbstractWe used the semi‐classical Deutsch‐Märk (DM) formalism to calculate absolute electron‐impact ionization cross sections of excited Ne atoms from threshold to 1000 eV. Excited states of Ne where the outermost valence electron is excited to states with principal quantum numbers up to n = 7 and orbital angular momentum quantum numbers up to l = 2 have been considered and systematic trends in the calculated cross section data are discussed. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Calculated cross sections for the electron-impact ionization of excited argon atoms using the DM formalism

The semiclassical Deutsch-Märk (DM) formalism was used to calculate absolute cross sections for the electron-impact ionization of excited argon atoms from threshold to 1000eV. Excited states of Ar where the outermost valence electron is excited to states with principal quantum numbers up to n=7 and orbital angular momentum quantum numbers up to l=2 have been considered. Systematic trends in the calculated cross section data are discussed.

Imaging of orbital electron densities by electron momentum spectroscopy – a chemical interpretation of the binary (e,2e) reaction

It is shown that electrons in molecules behave in an essentially orbital-like fashion and that imaging of the spherically averaged orbital electron density can be achieved in momentum space by electron momentum spectroscopy (EMS). EMS measurements, using the binary (e,2e) reaction under binary encounter collision conditions, are demonstrated to effectively probe valence electron (frontier orbital) electron transfer out of a molecule by providing imaging of the spherically averaged Dyson orbital electron momentum density distribution corresponding to the ionization process. Experimental EMS cross-sections for the outermost valence (frontier) electrons of HF, H 2O, NH 3, CH 4, H 2S and a range of other molecules are found to be in excellent agreement with MRSD-CI calculations of the respective Dyson orbital densities using the plane wave impulse approximation. High level Hartree–Fock and density functional theory (DFT) (B3LYP and B3PW91) calculations, with large, saturated and diffuse basis sets, demonstrate that the Dyson orbital densities and thus the EMS experiments are also extremely well described by the respective initial state (i.e. neutral molecule) canonical molecular orbital (CMO) Hartree–Fock independent particle electron density distributions, with as good or sometimes an even better description being given by the respective (correlated) Kohn–Sham orbital (KSO) densities of DFT. In this sense EMS is shown to provide experimental imaging of orbital electron densities, with the emerging electrons having a delocalised orbital character immediately prior to `knock-out'. The present experimental and theoretical findings also lend support to the earlier predictions of Fukui as to the possibility of observing the `orbital pattern' experimentally [Int. J. Quant. Chem. 12 (Suppl. 1) (1977) 277] as well as to the recent views of Stowasser and Hoffmann [J. Am. Chem. Soc. 121 (1999) 3414] concerning the `reality' of KSOs. Further supporting evidence for the present findings is provided by a consideration of the results of frontier orbital theory applied to chemical reactions involving electron transfer, such as electrophilic attack [J. Chem. Phys. 20 (1952) 722]. The present results and interpretation are also strongly supported by scanning tunneling microscopy (STM) theory [Phys. Rev. B 31 (1985) 805] and in particular by recent STM experiments on adsorbed C 60 molecules [Chem. Phys. Lett. 321 (2000) 78] which show images which correspond very closely with DFT calculations of the electron density distribution in the HOMO orbital of the neutral molecule. The EMS measurements and associated theoretical calculations, together with the evidence from frontier orbital theory and the STM experiments, strongly suggest that delocalised CMO, or often, even better, the KSO, densities provide an operational definition of orbital electron densities, and thus of orbitals, appropriate for use in discussions of chemical bonding as well as for predicting the outcome of chemical reactions and physical processes involving electron transfer.

Effects of electron correlation on the level widths, the photoelecton, Auger electron and ion-yield spectra above the 4d threshold in Ba

Electron correlation has been studied in the singly ionized 5p-1, 5s-1 and 4d-1 states and the double ionized (5s, 5p, valence)-2 states of Ba by carrying out extensive configuration interaction Hartree-Fock calculations including the mixing of configurations with outermost valence electrons (nl = 6s,5d,6p,4f, ... ). The results of calculations have been compared with existing experimental photoelectron, ion-yield, electron-ion coincidence, and Auger electron spectra.

Momentum profiles for open shell molecules: studies of the HOMOs of NO, O2 and NO2 by electron momentum spectroscopy and SCF, post-Hartree-Fock and DFT calculations

Measurements of the outermost valence electron orbital momentum profiles of the open shell molecules NO, O2 and NO2 have been obtained using electron momentum spectroscopy (EMS). The presently reported experimental momentum profiles of NO and O2 display much improved statistics compared with previously published EMS results while the data for the HOMO of NO2 is the first reported. In the case of NO, the present measurements are considerably different from previous results and these differences appear to be due to the presence of NO2 impurities in the previous work. The EMS measurements provide a stringent test of basis set effects and the quality of ab initio methods in the description of these open shell systems. The experimental momentum profiles have been compared with theoretical spherically averaged momentum profiles from several basis sets calculated at the level of the target Hartree-Fock approximation (THFA) with a range of basis sets using both unrestricted Hartree-Fock (UHF) and restricted open shell Hartree-Fock (ROHF) methods. Various configuration interaction calculations such as multi-reference singles and doubles configuration interaction (MRSD-CI), averaged coupled pair functional (ACPF) and quasi-degenerate variational perturbation theory (QDVPT) calculations of the full ion-neutral overlap amplitude have also been compared to experiment to investigate the effects of electron correlation and relaxation. The experimental momentum profiles have further been compared to calculations at the level of the target Kohn-Sham approximation (TKSA) using density functional theory (DFT) with the local density approximation and also with gradient corrected exchange-correlation potentials. In addition to momentum profiles, other electronic properties such as total energies, dipole moments, quadrupole moments and values of the electronic spatial extent have been calculated by the various theoretical methods and compared to experimental values.

An investigation of the outermost orbital momentum distributions of formaldehyde, acetaldehyde and acetone, by electron momentum spectroscopy and quantum chemical calculations at the SCF and MRSD-CI levels

The outermost valence electron momentum profiles have been obtained at high-momentum resolution for acetone (5b2) and acetaldehyde (10a′). These are compared with calculated profiles of wide ranging quality from minimal basis STO-3G to near Hartree-Fock limit SCF treatments, together with the outermost momentum profile of formaldehyde (2b2) measured previously by Bawagan et al. (Chem. Phys. 128 (1988) 439). The effects of the addition of diffuse functions to the basis sets are examined for a number of properties. Many-body corrections to the independent particle SCF model are evaluated by multi-reference singles and doubles configuration interaction calculations of all three molecules. These are found to give large improvements for calculated total energy but only modest gains in agreement with the observed momentum profiles. The implications for the nature of chemical bonding in these orbitals is discussed in the context of the theoretical and experimental results.

Theory of inelastic scattering of low energy electrons by lanthanide and actinide atoms

A theory is proposed for transitions between low-lying energy levels of lanthanide and actinide atoms. Inelastic electron scattering is studied for the range of impact energies below a few electron volt. The quadrupole interaction between the incident and the atomic valence electrons plays an important role. It changes the manner of angular momentum couplings between the outermost valence electron shell and the incompletely occupied f-electron subshell, resulting in atomic inelastic transitions. This special circumstance allows us to formulate the process in analogy with the theory of molecular rotational excitation by low energy electron impact.

Ionic compounds

From the insulator character of oxides like Ca 0.86 Sr 0.14 CuO 2 , CuO, NiO, the purpose of this work is to determine which are the conduction electrons of high-Tc copper based superconductors, supposing that they are not the 3d copper electrons. The ionic and covalent bonds in La 1.85 Sr 0.15 CuO 4 and in the monochalcogenides of rare earth LnX as their electrical properties are discussed. This discussion leads to the hypothesis that the outermost valence electrons of valence “6s” for the rare earths elements give rise to ionic bond but with neutral atoms and not with charge transfer. It is one of the “6s”s electrons of Ln which gives the metallic bond of the LnX. It is this electron “6s” of La which is the conduction electron of La 1.85 Sr 0.15 CuO 4 . This approach allows an explanation of the electrical properties of the 3d metal oxides.

Valence competition in high-T c superconductors

We presente a description of copper valence in superconductors based on the La 2O 3 crystal structure. It is supposed that the two outermost valence electrons of La lead to more ionic bonds than those obtained from the third deep valence electron. Consequently the two oxygen atoms of the tetrahedral site form covalent bonds with the deep valence electron of La. The oxygen of the octahedral site is found to have bonds with just the outermost valence electron of La. Thus it provides an explanation for oxygen holes found in all the new High-Tc superconductors. The above distinction between valence electrons leads to covalent bonds between Cu and O for the CuO 2 planes of La 2CuO 4. For La 2 − xSr xCuO 4 compounds, copper has been supposed to be trivalent, but spectroscopic studies point out bivalent copper. We propose that the krypton shell of Sr is responsible for the lack of one unit of valence, an hypothesis supported by the existence of rare gas compounds. We further suppose that the competition of valence between the contribution from Cu valence electrons and those of Sr-Kr shell might explain the conductivity in these materials.

Superconducting and optical properties of α-zirconium from its augmented-plane-wave band structure

The detailed electronic energy band structure of hexagonal close-packed alpha -zirconium, corresponding to the atomic configuration 4d25s2 of its four outermost valence electrons, has been computed by the composite-wave variational version of the augmented-plane-wave (APW) method in conjunction with the X alpha ( alpha =0.70424) exchange approximation for obtaining the potentials. From these data the electronic density of states and its angular-momentum-decomposed components have been obtained by the Raubenheimer-Gilat method. These quantities are required in order to calculate the electron-phonon interaction parameter ( lambda ) and the superconducting transition temperature (Tc) within the framework of the theories of Gaspari and Gyorffy (1972) and McMillan (1968). A study of the variation of Tcd with the Coulomb pseudopotential ( mu *) revealed that mu *=0.1 yields the best agreement between theory and experiment for alpha -Zr. Also studied from the energy band data are the optical properties of alpha -Zr via the variation of the imaginary part of the interband dielectric function for bound electrons and the corresponding optical conductivity ( sigma ) as a function of the photon energy ( omega ). The results indicate five peaks in the sigma - omega curve, the positions of which on the energy scale in fair agreement with experiment.

Electronic, superconducting, and optical properties of technetium from its augmented-plane-wave band structure

The detailed energy-band structure of hexagonal-close-packed technetium, corresponding to the atomic configuration 4d/sup 5/5s/sup 2/ of its seven outermost valence electrons, has been obtained throughout the Brillouin zone using the composite-wave variational version of the augmented-plane-wave (APW) method in conjunction with the X..cap alpha.. (..cap alpha.. = 0.702 99) exchange approximation for obtaining the potentials. From the band-structure data the electronic density of states (DOS) and the angular-momentum--decomposed DOS were calculated by the accurate Gilat-Raubenheimer method. These quantities were used to calculate the electron-phonon coupling constant and the transition temperature (T/sub c/) using the theories of Gaspari and Gyorffy and of McMillan. Also studied were the Fermi surface and the optical properties of Tc via the imaginary part of the interband dielectric constant for bound electrons, the latter being the first of such a study on Tc to date. The superconducting properties and the Fermi surface which could be compared to experiments show, in general, satisfactory agreement.

Calculation of the excited states of H2S, PH3, and SiH4 by the one-center expansion approximation

Excitation energies are calculated for the states arising from excitation of an outer-most valence electron of H2S, PH3 and SiH4 by the use of the one-center expansion SCF MO and the improved virtual orbital approximations. The results are compared with experimental data.

A new method for studying ionising potentials

It is evident that an exact knowledge of the energy necessary to remove the electrons from a molecule is of great value in formulating definite ideas of atomic structure and in testing the correctness of any quantitative theory. Not only is it desirable to know the work required to detach the outermost valence electron but also the further energy steps corresponding to the removal of more closely bound electrons. Moreover, in the case of polyatomic molecules, it would be most interesting to learn what the actual products of ionisation are, whether it is necessarily accompanied by dissociation and, if so, what is the state of the dissociated components. In ionisation of a gas by the impact of slowly moving electrons we have a method of disrupting molecules under known energy conditions, and therefore turn to experiments of this type for the solution of the problems just propounded. The first important quantitative work was that of Franck and Hertz in 1913 using a method originally due to Lenard. Modifications and improvements in this general mode of attack have enabled subsequent investigators to contribute greatly to knowledge of atomic structure. For a detailed discussion of methods and results reference can be made to Hughes’ “Report on Photo-electricity” or to a recent series of papers by Franck, but for the purpose of the present paper it will be sufficient to state the principal results as briefly as possible.