Valence Shell Research Articles

Cyclic M(SO2) (M=Zn, Cd) and its Anions: Matrix Infrared Spectra and DFT Calculations

Reaction of laser ablated zinc and cadmium atoms with SO2 molecules was studied by low temperature matrix isolation infrared spectroscopy. Cyclic M(SO2) and anion M(SO2)− (M=Zn, Cd) were produced in excess argon and neon, which were identified by 34SO2 and S18O2 isotopic substitutions. The observed infrared spectra and molecular structures were confirmed by density functional theoretical calculations. Natural charge distributions indicated significant electron transfer from s orbitals of zinc or cadmium metal atom to SO2 ligand and cyclic M(SO2) complexes favored “ion pair” M+(SO2)− formation, which were trapped in low temperature matrices. In addition Zn-O or Cd-O bond in M(SO2) exhibited strong polarized covalent character. Reaction of Hg atom with SO2 was also investigated, but no reaction product was observed, due to the relativistic effect that resulted in the contraction of 6s valence shell and high ionization potential of Hg atom.

Charge Density Analysis and Bond Characterization of 3d‐Transition Metal Complexes

AbstractChemical bond, which makes molecules from independent atoms, is a very important concept in understanding the molecular behavior. Bond characterization is therefore becoming ever needed for predicting the physical and chemical properties of the molecule and the material. Combined experimental and molecular orbital calculation studies have been used for such purpose in terms of deformation density distribution, natural bond orbital analysis and the topological analysis on the total electron density. These analyses provide not only the distribution of density accumulation and density depletion, but also the information such as bond order, bond type etc. Atom domain in molecule or compound can be defined uniquely. Therefore the combined quantum mechanical calculation and high resolution X‐ray crystallography studies will be extremely useful for characterizing the chemical bond precisely. Bonding in 3d‐transition metal complexes is a major topic in coordination chemistry. The complexity of the 3d orbital splittings subjected in the ligand field is well known and this makes the bonding characterization more interesting and challenging. Charge density analysis on a series of Cr compounds, which contain a Cr‐L multiple bond, will be presented. Detail characterization of Cr‐nitrido, ‐oxo, ‐imido, ‐carbyne and ‐carbene bonds will be described. Metal squarates of late 3d transition elements have been investigated systematically. The valence shell charge concentration (VSCC) around metal ion will be demonstrated by the Laplacian topology, which provides the physical basis for the Lewis structure and valence shell electron pair repulsion (VSEPR). The metal‐metal bond is extremely important for metal clusters; the metal‐metal quintuple bond is rare and a detail analysis will be described. Electronic configurations of the 3d metal ion play important role in coordination chemistry; taking the advantage of the x‐ray absorption spectroscopy available in synchrotron radiation facility, the exact electronic configuration can be monitored. The great recent improvements on both experiment and theory have made the charge density analysis more accessible to even more complicated system, for example, the excited state or the meta‐stable states.

Pnicogen Bonds: A Theoretical Study Based on the Laplacian of Electron Density

Although, most of the authors classify the pnicogen bonds as σ-hole bonding, there are some evidence that show they do not require any positive electrostatic potential around interacting molecules. In this work, the Laplacian of electron density is used to study pnicogen bonds in different dimer and trimer complexes. It is shown that the noncovalent P···P, P···N, and N···N bonds can be categorized as lump-hole interactions; a region of charge depletion and excess kinetics energy (hole) in the valence shell charge concentration (VSCC) of pnicogen atom combines with a region of charge concentration and excess potential energy (lump) in the VSCC of another molecule and form a pnicogen bond. In fact, since the full quantum potential (according to the local statement of virial theorem) has been used in the definition of the Laplacian, the lump-hole concept is more useful than the σ-hole in which the electrostatic part of potential is only considered. It is shown that the existence of hole in the VSCC of pnicogen atom is responsible for formation and (in the absence of other interactions) geometry of pnicogen bonded complexes. Because there is (at least) one hole in their VSCC, the pnicogen atoms in PH3, PH2F, H2C═PH, H2C═PF, and NH2F can engage in direct pnicogen-pnicogen interactions. However, the VSCC of nitrogen atom in the NH3 is devoid of hole and hence cannot act as an electron acceptor in pnicogen-bonded complexes.

Hemichelation, a Way To Stabilize Electron-Unsaturated Complexes: The Case of T-Shaped Pd and Pt Metallacycles.

A rational method of synthesis of stable neutral T-shaped 14 electron Pd and Pt complexes is proposed. It takes advantage of the ambiphilic character of the tricarbonyl(η(6)-indenyl)chromium anion, of which the main property is to behave as a hemichelating ligand, that is a nonconventional heteroditopic ligand capable of chelating a metal center by way of covalent and noncovalent bonding, thus preserving its unsaturated valence shell. The reaction of the in situ formed tricarbonyl(η(6)-2-methylindenyl)chromium anion with a series of Pd and Pt metallacycles afforded new air-stable and persistent synfacial heterobimetallic complexes in which the metallacycle binds the indenyl fragment via its metal in an η(1) fashion, leaving the fourth coordination site at the chelated metal virtually vacant. The structures of eight of these novel complexes are disclosed, and their bonding features are investigated by an array of theoretical methods based on the density functional theory (NBO, EDA, ETS-NOCV, AIM, NCI region analysis). Theory shows that the formation of these unusual structures of bimetallic synfacial η(1)-indenyl-Pd/Pt complexes is driven thermodynamically by attractive Coulombic occlusion of the fourth vacant coordination site at Pd/Pt centers by the Cr(CO)3 moiety.

Ionization photophysics and spectroscopy of dicyanoacetylene

Photoionization of dicyanoacetylene was studied using synchrotron radiation over the excitation range 8-25 eV, with photoelectron-photoion coincidence techniques. The absolute ionization cross-section and detailed spectroscopic aspects of the parent ion were recorded. The adiabatic ionization energy of dicyanoacetylene was measured as 11.80 ± 0.01 eV. A detailed analysis of the cation spectroscopy involves new aspects and new assignments of the vibrational components to excitation of the quasi-degenerate A(2)Πg, B(2)Σg(+) states as well as the C(2)Σu(+) and D(2)Πu states of the cation. Some of the structured autoionization features observed in the 12.4-15 eV region of the total ion yield spectrum were assigned to vibrational components of valence shell transitions and to two previously unknown Rydberg series converging to the D(2)Πu state of C4N2(+). The appearance energies of the fragment ions C4N(+), C3N(+), C4(+), C2N(+), and C2(+) were measured and their heats of formation were determined and compared with existing literature values. Thermochemical calculations of the appearance potentials of these and other weaker ions were used to infer aspects of dissociative ionization pathways.

Study of resonances in the photoionization of Ar@C60 and C60

The static-exchange method is applied to investigate photoionization of a confined system, Ar@C60. The realistic confinement potential employed in the present work explicitly includes the position of each carbon atom in the C60, which is a considerable improvement over methods to address the structure and photo dynamics of a confined system where the carbon shell is modelled by a spherically symmetric model potential. In the present study, photoionization from the deepest of the valence shells of C60 and Ar with ag symmetry is considered. There is a one-to-one correspondence between resonances that appear in the Ar@C60 system when the hole is in the 3s of the Ar, when the hole is in the C60 shell, or when the hole is in a free C60 system. Encapsulation of the Ar atom leads to some differences in the shapes and shifts in the energies of the resonant features in the cross-sections. However the qualitative features of the resonant state wave functions are very similar in the corresponding resonances of Ar@C60 and C60.

Novel Silver Cobaltacarborane Complexes with a Linearly Bridging Halide

Great attention has been paid to halide coordination chemistry, not only because it constitutes the fundamentals of inorganic chemistry, but because the versatile coordination abilities of halides can be used to construct many interesting polynuclear complexes, which include halidebridged inorganic polymers, halide-capped metal clusters, halide-templated polynuclear assemblies, and anionic guests. The structural versatility of halides mainly originates from their coordinating abilities of adopting a bridging bond between two or more metal atoms, as well as a terminal bond. Moreover, a halide bridging bond angle is so flexible that thermodynamic stability can be endowed with proper geometry, which conceptually varies from acute to right, obtuse, and linear. In spite of innumerable reports on molecular metal halides, examples of the linearly bridging fashion are very scarce. The reason for the rarity of the linear M–X–M arrangement can be easily explained by the VSEPR (Valence Shell Electron Pair Repulsion) concept. The linear M–X–M formation has only been achieved by adopting a macrocyclic chelate ligand, which is structurally demanding, so that the VSEPR repulsions among lone-pair electrons on the halide atom could be overcome. In the synthetic exploration of new cobaltacarboranes, we found an unexpected ionic product comprising a cation with a linear Ag–X–Ag arrangement. At first, we attempted a reaction between Tl2C2B9H11 and CoCl2(PPh3)2, mimicking the previously reported preparation of a nickellacarborane, [3,3-(PPh3)2–3,1,2–NiC2B9H11]. We found that a new cobalt complex of dicarbollide was generated, but no more information could be obtained. Treatment of the product mixture with AgBr(PPh3)3 afforded a cationic [L3Ag–X–AgL3] (L = triphenylphosphine) with CoSAN as a counterion (CoSAN = [3,3-commo-3,1,2-Co(C2B9H11)2]). Although linear Ag–X–Ag could be achieved, the reaction result was not well reproducible, nor was its mechanism reasonable. Hence, we directed the preparation of the linear complexes in a rationalized synthetic manner, as depicted in the following reaction equation:

Chemical Bond Characterization of a Mixed-Valence Tri-Cobalt Complex, Co3(μ-admtrz)4(μ-OH)2(CN)6·2H2O

Charge density study of a mixed-valence tri-cobalt compound, Co3(μ-admtrz)4(μ-OH)2(CN)6·2H2O (1) (admtrz = 3,5-dimethyl-4-amino-1,2,4-triazole), is investigated based on high resolution X-ray diffraction data and density functional theory (DFT) calculations. The molecular structure of this compound contains three cobalt atoms in a linear fashion, where two terminal ones are Co(III) at a low-spin (LS) state and a central one is Co(II) at a high-spin (HS) state with a total spin quantum number, S(total), of 3/2. It is centrosymmetric with the center of inversion located at the central Co atom (Co2). The Co2 ion is linked with each terminal cobalt (Co1) ion through two μ-admtrz ligands and a μ-OH ligand in a CoN4O2 coordination, where the Co1 is bonded additionally to three CN ligands with CoN2OC3 coordination. The combined experimental and theoretical charge density study identifies the different characters of two types of cobalt ions; more pronounced charge concentration and depletion features in the valence shell charge concentration (VSCC) are found in the Co(III) ion than in the Co(II) ion, and d-orbital populations also show the difference. According to topological properties associated with the bond critical point (BCP), the Co1-C(N) bond is the strongest among all the Co-ligand bonds in this compound; the Co-O is stronger than Co-N bond. Again Co1-O is stronger than Co2-O, so as the Co1-N being stronger than Co2-N bond. The electronic configuration of each type of Co atom is further characterized through magnetic measurement, Co-specific X-ray absorption near edge spectroscopy (XANES), and X-ray emission spectra (XES).

Intensity-dependent multiorbital effect in high-order harmonics generated from aligned O2molecules

High-order harmonics generated from multiple orbitals of aligned O${}_{2}$ molecules are studied theoretically and we focus on the effect of two-center interference during photoexcitation. We find that the contribution of the harmonics generated from different molecular orbitals to the total harmonics varies distinctively with the laser intensity. The total harmonic spectra are dominated by the harmonics generated from a 1${\ensuremath{\pi}}_{g}$ valence shell at lower intensities, but are dominated by those generated from a 1${\ensuremath{\pi}}_{u}$ valence shell at higher intensities. This change is a sequence of the structure-induced two-center interference during photoexcitation of harmonic generation. This study indicates that by properly choosing the intensity of driving laser pulses, one can extract the orbital information of both the 1${\ensuremath{\pi}}_{g}$ and the 1${\ensuremath{\pi}}_{u}$ valence shells by the generated harmonics. This provides a potential way to tomographically reconstruct the low-lying 1${\ensuremath{\pi}}_{u}$ valence shell.

Understanding the structure and electronic properties of Th4+-water complexes

We present a systematic quantum chemistry study of the [Th(H2O)n]4+(n = 1 to 10) complexes to gain insight into their electronic structure and properties: the effect of the ligand distribution on the valence shells of the thorium(IV) ion is studied by means of the electron localization function (ELF) topological analysis. Particular care is given to the study of the mono-aqua complex both at its equilibrium geometry, using various tools such as energy decomposition analyses (EDA), and along its dissociation pathway. Indeed, as several electronic states cross the Th4 +-H2O0ground state along the minimum energy path, we demonstrate that the diabatic representation implemented in MOLPRO is able to generate reference potential energy surfaces that will lead to the evaluation of diabatic dissociation curves. The calculated diabatic interaction energy curve will allow for a consistent parameterization of new generation force fields dedicated to heavy metals based on quantum chemistry.

A study of the valence shell electronic states of pyridazine by photoabsorption spectroscopy and time-dependent density functional theory calculations

The valence shell electronic states of pyridazine have been studied experimentally, by recording the photoabsorption spectrum, and theoretically, by calculating oscillator strengths and excitation energies. The absolute photoabsorption cross section has been measured between 4 and 40 eV, using synchrotron radiation, and is dominated by prominent bands associated with intravalence transitions. In contrast, structure due to Rydberg excitations is weak. One Rydberg state, belonging to a series converging onto the state limit has been observed and assigned. The accompanying vibrational structure has been characterized by analogy with that in the corresponding photoelectron band. Vibrational progressions associated with Rydberg states belonging to one or more series converging onto the state limit have also been observed. The absorption structure associated with these series is complex and only tentative assignments have been proposed for the Rydberg states. The time-dependent version of density functional theory has been used to calculate oscillator strengths and excitation energies for the optically allowed singlet–singlet valence transitions and also to obtain the excitation energies for electric-dipole-forbidden and/or spin-forbidden transitions. The valence shell photoionization dynamics have been investigated theoretically by calculating photoelectron angular distributions and photoionization partial cross sections of the four outermost orbitals. In addition, the ground state outer valence electronic configuration has been obtained at the complete active space self-consistent field and the N-electron valence state perturbation theory to second-order levels of theory.

Seventeen-electron compounds in an eighteen-electron world – An historical perspective

Beginning in about the 1920s, it was realized that the structures and stoichiometries of all known, structurally characterized metal carbonyls obeyed what came to be known as the effective atomic number (EAN) rule, whereby the sum of the electrons donated by the carbonyl ligands plus the number of electrons contained in the metal valence shell equals the number of valence electrons in the next inert (noble) gas element. While extraordinarily useful for predicting the constitutions of new metal carbonyls and their derivatives, universal acceptance of the validity of the EAN (or inert gas, noble gas, and, eventually 18-electron) rule resulted in a number of rule-breaking compounds being misrepresented. This historical perspective will show how several such organometallic compounds, initially formulated as obeying the 18-electron rule, were eventually recognized as 17-electron species with radical-like properties, the first to be known in what became an important branch of organotransition metal chemistry.

Probing theoretical level effect on fluorine chemical shielding calculations

The extensive quantum‐chemical calculations are assessed for prediction of the 19F nuclear magnetic shielding constants. The methods within domain of density functional theory (DFT) and ab initio, Hartree–Fock (HF), and second‐order Møller–Plesset perturbation theory (MP2) are employed for calculation of the 19F chemical shielding constants for a series of 30 small fluorine‐containing molecules. The importance of inclusion of four factors, namely, electron correlation treatment, triple‐ξ valence shell, diffuse function, and polarization function on calculating the fluorine NMR chemical shieldings for a variety of Pople's standard basis sets at gauge invariant atomic orbital (GIAO) and continuous set of gauge transformation (CSGT) conditions are discussed by employing the linear regression analysis. The systematic investigation performed here on the influence of the method, level of theory, and basis set size on the accuracy of 19F chemical shielding demonstrates that the presence of a high level of electron correlation factor in level as well as the polarization function in basis set leads to an accurate wave function to compute the reliable fluorine chemical shielding. The results analysis illustrates that the polarization function has a significant effect on the basis sets to minimize the difference between theoretical and experimental values in several fluorine molecules. An additional comparison of the GIAO and CSGT results is used as a supplementary study to confirm the validity of the results. The basis set dependence of fluorine chemical shielding constants verifies that GIAO provides results that are often more accurate than those that are calculated with CSGT approach at the same basis set size, as expected. © 2013 Wiley Periodicals, Inc. Concepts Magn Reson Part A 42A: 140‐153, 2013.

On the Control Parameters of the Quasi‐One Dimensional Superconductivity in Sc3CoC4

AbstractWithin the series of ternary rare‐earth transition metal carbides Sc3TC4 (T = Fe, Co, Ni) only the Co congener displays a structural phase transition at 72 K and an onset of bulk superconductivity at 4.5 K. In this paper we present the results of a detailed analysis of the structural, electronic, and vibrational properties of the low‐temperature phase of Sc3CoC4 that represents one of the few well‐documented examples of a quasi one‐dimensional (1D) superconductor. Variable temperature neutron powder diffraction and low temperature X‐ray diffraction experiments were performed in order to confirm the subtle structural distortions during the phase transition. The results of periodic electronic structure calculations indicate, that the structural transition can clearly be identified as a Peierls‐type distortion and by a comparison with the isostructural carbide Sc3FeC4 we are able to identify the chemical, electronic, and the vibrational control parameters of the transition. Topological analyses of the electron density distribution and of the valence shell charge concentrations at the cobalt atom finally allow us to directly correlate the changes in the electronic structure due to the Peierls transition in reciprocal space with the according subtle changes in the real space properties of Sc3CoC4.

Experimental wavelengths for intrashell transitions in tungsten ions with partially filled3pand3dsubshells

Spectra and measured wavelengths of intrashell $n=3$ transitions in highly charged tungsten ions with partially filled $3p$ and $3d$ valence shells, Al-like W${}^{61+}$ through Fe-like W${}^{48+}$, are presented. The ions were created and excited at the electron-beam ion-trap facility at the Lawrence Livermore National Laboratory and measured with a high-resolution grazing-incidence spectrometer. The spectral lines were studied in the 27--41 \AA{} range and were analyzed by a comparison with synthetic spectra based on a collisional-radiative model. We determined that the emission includes not only electric-dipole-allowed transitions, but also several electric-quadrupole and magnetic-dipole transitions. Line-position uncertainties as low as 25 ppm were achieved. Thus, our measurements provide much-needed benchmarks for calculations of the atomic structure of highly charged ions with a partially filled subshell, since these ions are difficult to calculate due to electron-correlation effects.

Time delay in valence-shell photoionization of noble-gas atoms

We use the non-relativistic random phase approximation with exchange to perform calculations of valence shell photoionization of Ne, Ar, Kr and Xe from their respective thresholds to photon energy of 200 eV. The energy derivative of the complex phase of the photoionization matrix elements is converted to the photoelectron group delay that can be measured in attosecond streaking or two-photon transitions interference experiments. Comparison with reported time delay measurements in Ne and Ar at a few selected photon energies is made. Systematic mapping of time delay across a wide range of photon energies in several atomic targets allows to highlight important aspects of fundamental atomic physics that can be probed by attosecond time delay measurements.

The atomic orbitals of the topological atom

The effective atomic orbitals have been realized in the framework of Bader's atoms in molecules theory for a general wavefunction. This formalism can be used to retrieve from any type of calculation a proper set of orthonormalized numerical atomic orbitals, with occupation numbers that sum up to the respective Quantum Theory of Atoms in Molecules (QTAIM) atomic populations. Experience shows that only a limited number of effective atomic orbitals exhibit significant occupation numbers. These correspond to atomic hybrids that closely resemble the core and valence shells of the atom. The occupation numbers of the remaining effective orbitals are almost negligible, except for atoms with hypervalent character. In addition, the molecular orbitals of a calculation can be exactly expressed as a linear combination of this orthonormalized set of numerical atomic orbitals, and the Mulliken population analysis carried out on this basis set exactly reproduces the original QTAIM atomic populations of the atoms. Approximate expansion of the molecular orbitals over a much reduced set of orthogonal atomic basis functions can also be accomplished to a very good accuracy with a singular value decomposition procedure.

Valence shell direct double photodetachment in polyanions

A valence shell study of electrosprayed insulin protein polyanion photodetachment was carried out on a vacuum ultra-violet synchrotron radiation beamline coupled to a radiofrequency ion trap, for both close- and open-shell species. A two-electron photodetachment is observed, which arises from two different mechanisms that are disentangled: a sequential multi-photon absorption and a direct one-photon two-electron process. The threshold for the direct double-electron ejection is measured at 11.4 eV and corresponds to electronic excitation in the valence shell, which makes it the first observation of direct double photodetachment in the valence shell. The results are discussed in the light of previous knowledge from multiple photoionization and ab initio calculations on model polyanions. Double photodetachment appears to be a relaxation mechanism that leads to oxidized anions of striking stability, a feature of high relevance in radiobiology.

Calculation of valence electron motion induced by sequential strong-field ionisation

Strong-field ionisation leads to the formation of electron wave packets in the valence shell of the resulting ion under appropriate experimental conditions. Ab-initio calculations of the population and coherence dynamics are challenging and are usually limited to single ionisation of simple systems by linearly polarised fields. Here, a calculation based on static-field rate equations is presented to obtain the density matrix for sequential double ionisation. The results for single ionisation of neon and xenon by linearly polarised pulses are in satisfactory agreement with ab-initio calculations. For double ionisation of neon and xenon by elliptically polarised fields, five coherence channels with recurrence times between 1.1 fs and 51.9 fs are predicted to exhibit a significant degree of coherence. The degree of coherence is affected by the polarisation of the laser pulse and decreases in general for elliptical polarisation, but for the investigated cases a significant degree of coherence is predicted to occur up to the regime of close-to-circular polarisation.

The low-symmetry lanthanum(III) oxotellurate(IV), La10Te12O39

Single crystals of deca­lanthanum(III) dodeca­oxotellurate(IV), La10Te12O39, were obtained by reacting La2O3 and TeO2 in a CsCl flux. Its crystal structure can be viewed as a three-dimensional network of corner- and edge-sharing LaO8 polyhedra with TeIV atoms filling the inter­stitial sites. The TeIV atoms with their 5s 2 electron lone pairs distort the LaO8 polyhedra through variable Te—O bonds. Among the six unique Te sites, four of them define empty channels extending parallel to the a axis. The formation of these channels is a result of the stereochemically active electron lone pairs on the TeIV atoms. The atomic arrangement of the Te—O units can be understood on the basis of the valence shell electron pair repulsion (VSEPR) model. A certain degree of disorder is observed in the crystal structure. As a result, one of the five different La sites is split into two positions with an occupancy ratio of 0.875 (2):0.125 (2). Also, one of the oxygen sites is split into two positions in a 0.559 (13):0.441 (13) ratio, and one O site is half-occupied. Such disorder was observed in all measured La10Te12O39 crystals.