Covalent States Research Articles

Sensitivity of doping states in the copper oxides to electron-lattice coupling.

Doping states in a two-dimensional three-band Peierls-Hubbard model are investigated with inhomogeneous Hartree-Fock and random phase approximations. They are sensitive to small changes of electron-lattice and electron-electron interactions. For parameters relevant to the insulating copper oxides a small ferromagnetic polaron is found. Moderate intersite electron-lattice coupling triggers a nonlinear feedback mechanism resulting in a rapid crossover from a Zhang-Rice regime to a covalent molecular singlet state in which the local magnetic moment is quenched and local lattice distortion is large. Various states are characterized by distinct optical and infrared absorption spectra.

Collision energy dependence of the cross sections for production of CN(B 2Σ+) in the reactions of Ar(3P0,2) and Kr(3P0,2) with BrCN

Cross sections for the formation of CN(B 2Σ+) in collisions of Ar(3P0,2) and Kr(3P0,2) with BrCN were measured in a crossed beam geometry as a function of the collision energy. An arc-heated metastable rare-gas beam was velocity selected by a time-of-flight method, and the collision energy Ec was varied in the range of 0.7–5.2 eV for Ar(3P0,2) and 1.0–5.2 eV for Kr(3P0,2). The cross sections were found to be proportional to E−mc, with m=0.355±0.008 for Ar(3P0,2) and 0.124±0.010 for Kr(3P0,2). The relative cross sections were normalized against the known absolute cross section for collisional excitation transfer from Ar(3P0,2) to N2. The absolute cross sections thus determined were, for example, 16.0±4.9 Å2 for the Ar(3P0,2)+BrCN system and 42.3±14.8 Å2 for the Kr(3P0,2)+BrCN system at the collision energy of 1.0 eV. The results were explained in terms of the curve-crossing mechanism that an ion-pair [Rg+BrCN−] intermediate state intervenes between the entrance and the exit covalent states.

Highly Correlated Electron State in Oxide Superconductors

The superconductivity in the cuprate oxide superconductor appears in metallic region close to the metal-insulator transition. Nature of the highly correlated electron state realized in this region is one of the central subjects to understand the mechanism of high T c superconductivity. A new method is proposed in which electrons near the Fermi level are described by a kind of covalent electronic states associated with Cu-O bonds

Studies of the cyclic adenosine monophosphate chemoreceptor ofParamecium

A doublet of proteins (approximately 48,000 Mr) from the Paramecium cell body membrane fits several criteria for the external cAMP chemoreceptor. These criteria include: (i) selective elution from a cAMP affinity column, matching a specificity that could be predicted from the behavioral response and whole-cell binding; (ii) binding to wheat germ agglutinin indicating the presence of carbohydrate moieties indicating surface exposure; and (iii) selective inhibition of the intact cells' chemoresponse to cAMP by antibodies against the doublet. Additional evidence for the existence of a receptor, in general, comes from selective elimination of the cAMP chemoresponse by photoaffinity labeling of while cells with 8-N3-cAMP. The doublet proteins are not identical to the regulatory subunit of a cAMP-dependent protein kinase from Paramecium, the Dictyostelium cAMP chemoreceptor, or the 42-45 kDa range proteins related to the large surface glycoprotein in Paramecium. The doublet proteins are not readily separable and, as in Dictyostelium, may represent two different covalent modification states of the same protein. Amino acid analysis indicates that the proteins are similar, but does not distinguish between the possibilities of proteolysis and covalent modification. Once cloned, this doublet may prove to be only the fifth external, eukaryotic chemoreceptor to be identified.

High-resolution spectroscopy of NaI around the atomic asymptotes Na (3p 2P1/2,3/2)+I ( 2P3/2)

Excitation spectra of NaI were recorded in the wavelength range 239–256 nm with a frequency doubled pulsed dye laser. The assignment of 27 vibrational bands shows that the observed spectra belong to a new bound covalent state below the atomic asymptotes Na(2P1/2,3/2) + I(2P3/2) and above the predissociating state A0+ [S. H. Schaefer, D. Bender, and E. Tiemann, Chem. Phys. 89, 65(1984)]. The Dunham parameters and the potential curve for the new state are given. The potential curve is compared with that derived by the analysis of a fluorescence progression from a broadband excitation that was published recently by Bower et al. [J. Chem. Phys. 89, 4478 (1988)].

Sudden polarization in interacting model π systems: An exact study

The twisted excited state of polyenes has attracted much theoretical and experimental attention due to the postulate that it exhibits sudden polarization for twist angle θ= 90°. We examine this phenomenon in long chain polyenes using correlated model Hamiltonians. While the Hubbard model is employed to study this process as the correlations are gradually turned on, the Pariser-Parr-Pople (PPP) Hamiltonian is employed for a realistic study of long chain polyenes. Exact polarizabilities of the ground and excited states of polyenes with upto ten carbon atoms are calculated within the Hubbard and PPP models as a function of twist angle. We find that the dipole allowed excited states exhibit sudden polarization in all the cases. Besides, the 3 1A g state in the covalent subspace which is a two-photon state also exhibits this phenomenon as it becomes degenerate with the dipole allowed excited state. It is suggested that photochemical processes involving zwitterionic intermediates should be possible through two-photon excitation to this covalent excited state.

Laser spectroscopy on the A0 +-X1Σ + transition of T1I

The first excited electronic state of TII was investigated by laser excitation and laser fluorescence spectroscopy. Almost completely resolved rotational fine structure was observed with the help of a collimated molecular beam and a single-mode cw dye laser. From the derived Dunham parameters Y lk a RKR potential for the excited state is calculated. The unusual shape of the potential energy curve can be understood as originating from an avoided crossing of the ionic ground-state potential and a non-bonding covalent state of the atomic asymptote (6p) 2P 1 2 (TI)+(5p) 5 2P 3 2 (I).

Formation and characterization of a low-lying electronic state of the alkali monoxides LiO…CsO in the red and near infrared

In studying a variety of highly exothermic alkali oxidation chemiluminescent processes, a significant body of evidence has been obtained to indicate that the alkali monoxides LiO, NaO, KO, RbO, and CsO possess a weakly bound low-lying predominantly covalent excited electronic state thought to be of 2Π symmetry (B 2Π). The emission from this state for the heavier alkali oxides lies at the fringes of the visible region whereas the LiO emission system extends through the entire visible range to the near infrared. Alkali atoms react with ozone (O 3) and nitrous oxide (N 2O) under both single collision beam-gas and multiple collision conditions to yield the chemiluminescent spectra ascribed to the alkali monixides. Under single collision conditions, the observed emission spectra resulting from an oxidation process first order in metal and first order in oxidant are virtually continuous. The single collision studies are extended in a controlled manner to higher pressure by entraining the alkali atoms in Ar, He, N 2, or CO and subsequently carrying out the oxidation of this mixture at background pressures ranging from 0.15 to 1 Torr in order to study the internal relaxation and rapid intramolecular energy transfer characteristic of the metal monoxide. Partially rotationally relaxed spectra obtained for LiO, NaO, and KO demonstrate considerable structure associated predominantly with the vibrational levels of the ground and very low-lying ionic states of the alkali oxides. Temperature dependence studies ( T beam) under single collision conditions demonstrate that the O 3 reactions proceed with much lower activation energy for excited state formation versus the N 2O reactions. Through energy conservation, the bond energies for LiO, NaO, KO, and CsO are extracted. While those bond energies determined for NaO, KO, and CsO are in good agreement with previous workers, the determined bond energy for LiO is notably higher than that determined mass spectrometrically. This study correlates well with the recent evaluation of M 2+O 2 alkali oxidation processes where weak emission from an MO 2 (M=Na…Cs) species appears to be observed. The significance of the low-lying alkali oxide state as it pertains to the detection of the alkali oxides in combustion streams is considered.

Determination of an Adlayer Bonding Transition by Surface Extended X-Ray-Absorption Fine-Structure Spectroscopy: Cesium Adsorbed on Ag {111}

As the Cs coverage of Ag{111} is increased from approximately 0.15 to 0.3 monolayer the Cs-Ag bond length is found to increase from 3.20 \ifmmode\pm\else\textpm\fi{} 0.03 to 3.50 \ifmmode\pm\else\textpm\fi{} 0.03 \AA{}. This dramatic change is associated with a transition in the cesium bonding from predominantly ionic at low coverage towards a more covalent state at high coverage.

Spectroscopy and potential inversion for the predissociated C 1Π state of AlBr. Electronic structure of group IIIa halides

Using frequency-doubled pulsed dye-laser radiation, the spectra of the C 1Π-X 1Σ + system of AlBr have been recorded. Dunham parameters, RKR curves and Franck-Condon factors are calculated from transitions to not significantly predissociated levels of the C 1Π state. The quantitative description of predissociation is based on tunneling through a potential barrier. The full potential curve of the C 1Π state is derived directly by a nonlinear fit procedure from measured energy positions of bound and quasi-bound levels and predissociation rates which were determined directly from linewidths and from fluorescence intensities of the investigated fine structure. The potential curve clearly shows a maximum at medium internuclear distance. In a systematic context of the group IIIa metal halides it is qualitatively shown that strong avoided crossings of bound ionic states arising from different ionic asymptotes and repulsive covalent states arising from the asymptotes of the atomic ground states, e.g. metal 2P, halogen 2P, create potential maxima, which are responsible for predissociation by tunneling.

Electronic properties and superconductivity of rapidly quenched Al-Si alloys.

The authors present detailed studies of electronic properties of Al-Si alloys prepared in a nonequilibrium state by means of rapid solidification. The quenched alloys exhibit an enhanced superconducting transition temperature up to 6.2 K in an Al-Si 30 at.% alloy as well as an increased thermal slope of resistivity. Using differential scanning calorimetry, a large enthalpy variation (ΔH=4.1 kJ/mole for Al-Si 30 at.%) has been measured during the irreversible transition from the nonequilibrium state to the equilibrium one. This is mainly attributed to the energy difference between the metallic state of silicon atoms trapped in FCC aluminum matrix during quenching and the usual covalent state of silicon precipitates in an equilibrium state. This large energy difference is presented as the origin of a lattice instability which softens the phonon spectrum and gives rise to a stronger electron-phonon coupling. This appears to be a characteristic property of nonequilibrium Al-Si solid solutions, which is associated with the metallic state of silicon atoms. An interpretation of the Tc enhancement is proposed for both Al-Si and Al-Ge alloys based on the phonon softening in these nonequilibrium crystalline alloys

Initiation factor protein modifications and inhibition of protein synthesis.

The protein covalent modification state of eucaryotic initiation factors eIF-2 and eIF-4B in HeLa cells was examined after they were exposed to a variety of conditions or treatments that regulate protein synthesis. A few factors (e.g., variant pH and sodium fluoride) altered the phosphorylation state of the initiation factor proteins, but the majority (hypertonic medium, ethanol, dimethyl sulfoxide sodium selenite, sodium azide, and colchicine) had no effect on either protein. While initiation factor phosphorylation may regulate protein synthesis in response to many physiological situations, other pathways can regulate protein synthesis under nonphysiological circumstances.

Initiation factor protein modifications and inhibition of protein synthesis.

The protein covalent modification state of eucaryotic initiation factors eIF-2 and eIF-4B in HeLa cells was examined after they were exposed to a variety of conditions or treatments that regulate protein synthesis. A few factors (e.g., variant pH and sodium fluoride) altered the phosphorylation state of the initiation factor proteins, but the majority (hypertonic medium, ethanol, dimethyl sulfoxide sodium selenite, sodium azide, and colchicine) had no effect on either protein. While initiation factor phosphorylation may regulate protein synthesis in response to many physiological situations, other pathways can regulate protein synthesis under nonphysiological circumstances.

A CASSCF-CCI study of the valence and lower excited states of the benzene molecule

A b initio complete active space (CAS) SCF and contracted CI calculations have been carried out for all valence and the lower Rydberg states of the benzene molecule. The CASSCF active space comprised 12 π-type molecular orbitals and the basis set included both polarization functions and diffuse functions in order to describe properly both valence and Rydberg type orbitals. Resulting excitation energies for the Rydberg states are in close agreement with experiment. CASSCF results for the valence states give errors ranging from 0.0 for the covalent states up to more than 1.0 eV for the most ionic states. Inclusion of σ–π correlation effects reduces the errors in the ionic states to less than 0.6 eV. The 1E1u state is computed to lie 7.4 eV above the ground state with a transition moment of 1.70 a.u., experimental values are 7.0 eV and 1.61 a.u., respectively.

A stopped-flow apparatus for photoaffinity labeling studies in the milliseconds time range. Application in investigations of the nicotinic acetylcholine receptor

A photoaffinity labeling method is described to label a protein covalently in various transient covalent states. The method uses a combination of an especially adapted stopped-flow apparatus with a Q-switched Nd:YAG DCR-2A laser (wavelength 266 nm, i.e. four-fold the primary frequency, pulse duration 4 ns, pulse energy 15 mJ). The construction of the mixing cell, the triggering device and the set-up for determining the dead time of the stopped-flow apparatus is described. The dead time is 2.4 ms. In combination with a specific photolabel the method has been used for labeling functional states (resting, activated, desensitized, antagonist-blocked) of the nicotinic acetylcholine receptor from Torpedo marmorata electric tissue.

Spectra, potential curves and predissociation of LiI

High-resolution absorption and fluorescence spectra have been observed on the transition AO + −X 1Σ + on LiI for the first time. The analysis is performed along the lines of the semiclassical model of predissociation originating from the coupling of ionic and covalent states. Molecular parameters and potential curves are given for the limiting diabatic state which is a continuation of the ground state X 1 Σ +, and for the adiabatic state AO +. The coupling matrix element is in good agreement with the model of charge-transfer reactions by Grice and Herschbach.

Synthesis, spectroscopic properties and crystal structure of the eight-coordinated lead complex bis[thiocyanato]-1,4,7,10,13-pentaoxacyclopentadecano lead(II)

The complex [Pbc[15]crown-5](SCN) 2 was synthesized and characterized spectroscopically (IR and FIR range) and the crystal structure was determined by X-ray diffraction on a single crystal (orthorhombic space group Pmc2 1). The structure was solved by Patterson synthesis with least-square refinement. The final R-value is 0.0328. Two Pb-atoms and two unidentical halves of the ligand were found in the asymmetric unit. For each complexed lead cation and each half of the crown ether there exists a plane of reflexion, whereby C18 and C28 are disordered. There is, however, no centre of inversion between these unidentical halves. The elementary cell comprises four units and two different types of two centrally coordinated Pb cations. Both lead cations are eight-coordinated. In the case of Pb1 the eight donor centres are the five O-atoms of the ligand and three N-atoms of the thiocyanates. For Pb2 these are five oxygens of the ligand, one nitrogen and two sulphurs of the three thiocyanates. In this complex the thiocyanate ligands occur in both ionic and covalent states. The complexation of the crown ether and the two different species of thiocyanate anions are indicated too by typical shifted absorption bands in the IR spectra. The vibrations in the FIR region can be assigned to the interactions between lead and the donor groups.

The predissociation of NaI

The UV-absorption spectrum of NaI was measured in the range of 26900—33175 cm −1. Only rotational bandfragments were found which are assigned to the predissociation due to the interaction of the ionic ground state XO + and the covalent state O +. The spectral observation of the large energy range is completely describable with the analytic and semiclassical model of Child. Because of the symmetry of the equations involved for the weak and strong coupling case the analysis has to follow both ways and the final discrimination is done by comparing the derived potential functions of both solutions. NaI represents a predissociation of the strong coupling. The coupling coefficient, the molecular parameters of the diabatic state XO + and the adiabatic state AO + and their RKR-potential functions are given. The expected predissociation for similar alkalihalides like LiI, KI, and NaBr is discussed. First observations on LiI confirm the estimation.

Dynamic NMR as a Nondestructive Method for the Determination of Rates of Dissociation. V. Proton Affinities of p-Substituted N,N-Dibenzylanilines in Aprotic Solvents

Abstract Rates of proton exchange between p-substituted N,N-dibenzylanilinium ion and chloride ion in chloroform-d and acetonitrile-d3 were determined by the dynamic NMR technique. Comparison of the kinetic proton affinity of the anilines in the aprotic solvents with the proton affinity of anilines in the gas phase and the basicity in water revealed that they could be linearly correlated. The feature of the kinetic data was a large positive entropy of activation which can be ascribed to the change from an ionic ground state to a covalent transition state. The entropy of activation was somewhat smaller in acetonitrile-d3 than in chloroform-d. This was attributed to the change in the stage of the transition state for the exchange. A small amount of water which could not be removed easily from the aprotic solvents was proved not to harm the results.

Production of stable and autoionizing O2− and NO− ions in CsO2 and CsNO collisions

Abstract We present absolute total cross sections for electron and ion production in CsNO and CsO 2 collisions. The laboratory energy of the cesium atoms varied between 30 and 3000 eV. The branching ratio between electron and ion production in these systems provides a direct measure for the amount of molecular anions which are produced in stable or autoionizing vibrational states. The absolute magnitude of the observed cross sections shows that the ionization process is due to a curve crossing between the potentials of the covalent and ionic ground states. The experiment shows that CsNO collisions predominantly lead to production of NO − ions in the vibrational ground state. For both CsNO and CsO 2 collisions the distribution over vibrational states of the molecular anions is a function of the collision energy. The experimental results are discussed on the basis of several models.