Decrease In Bond Length Research Articles

Vibronic coupling in molecules and in solids.

We utilize the experience gained in our previous studies on the "chemistry of vibronic coupling" in simple homonuclear and heteronuclear molecules to begin assembling theoretical guidelines for the construction of potentially superconducting solids exhibiting large electron-phonon coupling. For this purpose we analyze similarities between vibronic coupling in isolated molecules and in extended solids. In particular, we study vibronic coupling along the antisymmetric stretch coordinate (Q(as)) in linear symmetric AAA molecules, and along the optical phonon "pairing" mode coordinate (Q(opt)) in corresponding one-dimensional [A]( infinity ) chains built of equidistant A atoms. This is done for a broad range of chemical elements (A). The following similarities between vibronic coupling in molecules and phonon coupling in solids emerge from our calculations: 1) The HOMO/LUMO electronic energy gap in an AAA molecule increases along Q(as), and the highest occupied crystal orbital/lowest unoccupied crystal orbital gap in [A]( infinity ) chain increases along Q(opt). 2) The maximum vibronic instability is invariably obtained for a half-filled, singly occupied molecular orbital in AAA molecules, and for a corresponding half-filled band in [A]( infinity ) chains. 3) The vibronic stability of an AAA molecule increases with a decrease of the AA bond length, as does the vibronic stability of [A]( infinity ) chains (external pressure may lead to a reversal of a Peierls distortion). 4) The high degree of s-p mixing and ionic/covalent forbidden curve crossing dramatically enhance the vibronic instability of both AAA molecules and [A]( infinity ) chains. We also introduce one quantitative relationship: The parameter log(R) (where R is molar refractivity, a parameter used by Herzfeld to prescribe the conditions for the metallization of the elements) correlates with a parameter f(AA) (defined as twice the electronegativity of A, divided by the equilibrium AA bond length), used by two of us previously to describe vibronic coupling in AAA molecules for a broad range of elements (A=halogen, H, or an alkali metal). We hope to illustrate that key chemical aspects of vibronic coupling in simple molecules may thus be profitably transferred to corresponding materials in the solid state.

Effect of Fe substitution in the structure and superconducting properties of the (Y 0.8Pr 0.2)Ba 2Cu 4− xFe xO 8 system

The effects of iron substitutions on the superconducting and structural properties of (Y 0.8Pr 0.2)Ba 2Cu 4− x Fe x O 8 were studied. As iron concentration increases, the rate at which T c diminishes with x is the same as for the YBa 2Cu 4− x Fe x O 8 system, but faster than in the YBa 2Cu 3− x Fe x O 7− δ system. The Mössbauer spectra of the different samples, together with Rietveld refinements of their corresponding X-ray spectra, indicate that the iron atoms occupy preferably the Cu(1) sites of the double (CuO) 2 chains in fivefold coordination; that is, due to their tri-valency, the iron atoms attract extra oxygen atoms, named O(5), that place themselves along the a-axis in between two adjacent planes of the double (CuO) 2 chains. As x is increased, the Cu(1)–O(4) bond length decreases, whereas the Cu(2)–O(4) one increases. Also, a reduction of the occupation factors of the oxygen atoms in the CuO 2 planes is observed. Some of these changes may be due to the presence of O(5) atoms, which in turn affect the charge-transfer mechanism between the (CuO) 2 double chains and the CuO 2 planes where superconductivity takes place, and could be responsible of the rate at which T c decreases.

The interfacial structure of PPV/TiO2 nanocomposite

The aim of this work is to investigate the interfacial structure of poly(phenylenevinylene)(PPV)/TiO 2 nanocomposite by analyzing X-ray absorption spectroscopy at the Ti K edge. The pre-edge regions of X-ray absorption near-edge structure (XANES) spectra displayed distortions of octahedral geometry (TiO 6 ) in the surface of TiO 2 nanoparticles, while the post-edge regions of XANES showed PPV could modify the surface of nano-TiO 2 . From analyzing extended X-ray absorption fine structure spectra of the nanoparticles and comparing them with bulk TiO 2 , it was found that bond length (Ti-O) and coordination number decrease in nano-TiO 2 , which is attributed to coordinatively unsaturated TiO x surface species (x < 6). In PPV/TiO 2 nanocomposite, PPV molecule can bond to the surface of TiO 2 nanoparticles, leading the interfacial structure reconstructed.

Photoelectron Spectroscopy of Nickel, Palladium, and Platinum Oxide Anions

The 364-nm negative ion photoelectron spectra of XO and OXO molecules (X=Ni, Pd, and Pt) are reported. The spectra yield the electron affinities (EAs): EA(NiO)=1.455±0.005 eV; EA(PdO)=1.672±0.005 eV; EA(PtO)=2.172±0.005 eV; EA(ONiO)=3.043±0.005 eV; EA(OPdO)=3.086±0.005 eV; EA(OPtO)=2.677±0.005 eV. In addition, for the diatomics, transitions from the anion X̃ 2Π 3/2 and X̃′ 2Π 1/2 states into neutral X̃ 3Σ −, 3Π, and for NiO and PdO, 1Π, are assigned. Several states have been reassigned from those in the existing literature. Anion 2Π 3/2– 2Π 1/2 spin–orbit splittings are measured, as are neutral 3Π 2– 3Π 1 spin–orbit splittings: the XO 3Π 2– 3Π 1 splittings increase from 405±30 cm −1 (NiO) to 805±30 cm −1 (PdO) to 3580±40 cm −1 (PtO). A bond length shortening of 0.03±0.01 Å is measured upon electron detachment from NiO −, resulting in an anion bond length of 1.66±0.01 Å. The bond length does not change upon electron detachment from PdO − using 3.4-eV photons. The Pt–O bond length decreases by 0.035±0.010 Å in the 3Π 1← 2Π 3/2 transition. The spectrum of OPtO displays a significantly more extended vibrational progression than those of ONiO or OPdO, and the O–Pt bond length is found to decrease by 0.07±0.01 Å upon electron detachment. The spectra support the view that the Ni–O bond is largely ionic, the Pd–O bond is somewhat less so, and the Pt–O bond displays a substantial covalent character.

151Eu Mössbauer Spectroscopic Study on the EuyM1-yO2-x (0 ≤ y ≤ 1.0) (M=Th, U)

151Eu-Mössbauer spectroscopic and powder X-ray diffraction (XRD) study has been performed for the EuyU1-yO2-x (M=Th, and U) systems over the entire composition range of 0 ≤ y ≤ 1.0. The XRD results of the Eu-Th system showed that a very wide defect-fluorite (DF) type phase in which oxygen-vacancies (Vo) are disordered (x=y/2) is formed for 0 ≤ y ≤ 0.5 and that two-phases regions sandwitching a narrow C-type (C) single phase around y ≈ 0.8 appear for 0.5 < y < 0.8 (DF + C) and 0.82 < y < 1.0 (C + B-type (monoclinic) Eu2O3). The Mossbauer results show that the isomer shifts (ISs) of Eu3+ in this system smoothly increase with Eu composition, y. The decrease of average coordination number (CN) of O2− around Eu3+ with y (CN=8-2y)(x=y/2) results in the decrease of the average Eu-O bond length, which is due to the decrease of repulsion force between O2- anions. This result confirms that the IS of Eu3+ correlates well with the average Eu-O bond length in oxide systems. For the Eu-U system, also the defect-fluorite (DF)-type solid solution phase is formed for y < 0.5, but the lattice parameter (a0) of the system decreases almost linearly with y, in accordance with the calculated a0 vs. y curve for the oxygen-stoichiometric (i.e. x=0) fluorite dioxide (CN=8). Consistent with this XRD result, the ISs of Eu3+ in this composition range are smaller than those of the Eu-Th system and remain almost constant around 0.5 mm/s, which is comparable to those of pyrochlore oxides (Eu2Zr2O7 and Eu2Hf2O7 (y=0.5)) with O2--8 fold coordinated Eu3+(CN=8).

Non-Collinear Magnetic Moments of Five-Atom Transition-Metal Clusters

Non-collinear magnetic moments are calculated for trigonal bipyramid clusters, V 5 , Cr 5 , Mn 5 , Fe 5 , Co 5 , and Ni 5 by a simple method employing the locality of the local spin-density functional approximation and the discrete variational scheme. We discuss the magnetic properties of the transition-metal clusters with the bond lengths of 80%–100% of the crystalline interatomic distance. Parallel magnetic moments appear for the Ni, Co, and Fe clusters with almost all the bond lengths. Tilting magnetic moments also appear for the Co and Fe clusters in a certain bond length. Non-collinear magnetic arrangements appear for the Mn and Cr clusters, which change to antiparallel arrangements with decreasing bond length. Only an antiparallel alignment appears for the V cluster.

Lewis acidity and basicity of cation-exchanged zeolites: QM/MM and density functional studies

The structure of Li +, Na + and K +-exchanged faujasite (FAU) zeolite is studied by means of cluster model and QM/MM approach at HF/6-31G ∗∗ level of calculations. When CO molecule interacts with cation, “hard” site present in the zeolite channel, the CO bond length decreases compared to the free CO molecule and this decrease in CO bond length depends on Lewis acid strength of the cation. The strength of Lewis acidity of the cations and basicity of framework oxygen atoms is investigated using density functional (DF) methods. We used local reactivity descriptors based on hard–soft acid–base concept as acidity and basicity parameters. The calculated ‘relative electrophilicity’ and ‘relative nucleophilicity’ values are found to be better descriptors of acidic and basic sites, respectively. From relative electrophilicity ( s k +/ s k −) value the Lewis acidity of the cation-exchanged faujasite zeolite is found to decrease in the order: [Li-FAU]>[Na-FAU]>[K-FAU], and from relative nucleophilicity ( s k −/ s k +) value of the framework oxygen atom, the Lewis basicity is found to increase in the order: [Li-FAU]<[Na-FAU]<[K-FAU].

Structural variations in the aegirine solid-solution series (Na,Li)FeSi2O6 at 298 K and 80 K

Abstract The structural variations within the aegirine (NaFeSi2O6) – Li–aegirine (LiFeSi2O6) solid-solution series have been determined from structure refinements of single–crystal X-ray diffraction data of synthetic compounds at 298 K and 85 K. At 298 K, all compounds have space group C2/c. Replacing Na+ by Li+ in aegirine causes a distinct decrease of the b lattice parameter by 1.54% and an increase of the monoclinic angle β by 2.60%. The oxygen coordination of the M2 cation decreases from 6 + 2–fold in aegirine to 6–fold in pure LiFeSi2O6, the bond length decreases by 7.1% with increasing replacement of Na+ by Li+. The decrease of the M2–oxygen coordination number correlates with a straightening of the tetrahedral chains which are slightly kinked in aegirine and fully extended in Li–aegirine (O3-O3-O3 bridging angle = 174.2(1)° and 180.8(1)° respectively). Small variations of individual and mean Si-O bond lengths and distinct changes in O-Si-O bond angles were also found. Within the Fe3+O6 – M1 octahedra, structural rearrangements mainly affect the O-O interatomic distances (= edges of the octahedra). At 85 K, the compounds Na1- x Li x FeSi2O6 with Li+ concentrations x &gt; 0.75 crystallize in the space group P21/c. For the compounds with x &lt; 0.75, no phase transition from C2/c → P21/c was observed down to low temperatures (30 K). A x – T phase diagram for the stability of the low temperature P21/c structure is provided. The characteristica of the P21/c low temperature form of the (Na,Li)FeSi2O6 compounds agree with those of pure LiFeSi2O6, recently described by Redhammer et al. (2001).

Molecular and Electronic Structure of C5H5N−SO3: Correlation of Ground State Physical Properties with Orbital Energy Gaps in Partially Bound Lewis Acid−Base Complexes

The donor−acceptor complex C5H5N−SO3 has been studied in the gas phase using rotational spectroscopy. The adduct has the expected geometry in which the nitrogen lone pair is directed along the C3 axis of the SO3 and the SO3 undergoes free or nearly free internal rotation within the complex. The N−S bond length is 1.915(1) A, and the NSO angle is 98.91(2)o, indicating that the formation of the dative bond is nearly, but not entirely, complete. Small but significant changes in the heavy-atom ring structure of the pyridine upon complexation are also measured by a series of 13C substitution experiments. Analysis of the 14N quadrupole coupling constants indicates a transfer of approximately 0.54 electrons away from the pyridine upon formation of the dative bond. In the series of complexes of SO3 with HCN, CH3CN, H3N, C5H5N, and (CH3)3N, electron-transfer values increase as the bond length decreases, and these changes are shown to accompany a gradual decrease in the energy gap between the lone pair orbital of t...

Molecular scale characteristics of Cu(II) bonding in goethite–humate complexes

—Interactions between oxide minerals and natural organic matter affect metal adsorption properties of mineral surfaces, but the mechanisms of metal bonding are not well understood. Extended X-ray absorption fine structure (EXAFS) spectroscopy analyses were performed on aqueous pastes containing Cu(II) and goethite (α-FeOOH) with humic acid adsorbed at 0, 14, 28, 57, 88, 216, and 236 g kg −1 goethite, and with aqueous suspensions of humic acid only. Analyses were conducted at the Cu K edge with 40 mmol Cu(II) kg −1 goethite or 2 mmol Cu(II) kg −1 suspension for the humic acid system. Samples were equilibrated at pH 5.6 in a 0.1 M NaNO 3 background electrolyte. For all systems, analysis of EXAFS results suggests that Cu(II) is present in a distorted octahedral configuration containing four short equatorial (1.94–1.97 Å) and two longer axial bonds with oxygen. When the concentration of adsorbed humic acid on goethite was increased from 0 to 28 g kg −1, the axial Cu-O bond length decreased to 2.24 ± 0.03 Å, which was less than for individual humic acid (2.32 ± 0.02 Å) or goethite (2.29 ± 0.03 Å) samples. The apparent decrease in the axial Cu-O bond length was attributed to a decrease in the ligand field splitting energy. When humic acid ligands replace equatorial water molecules in the Cu(II) coordination sphere, a weaker ligand field strength occurs. For absorbed humic acid up to 88 g kg −1 goethite, second-shell iron neighbors were observed between 3.17 and 3.20 Å, suggesting that Cu(II) was bonded on average to both inorganic (goethite) and organic (humic acid) functional groups. In addition, derivative X-ray absorption near edge structure (XANES) spectra for Cu(II) on goethite–humate complexes (<88 g kg −1) were poorly fit by use of a linear combination of spectra for Cu(II) on goethite or humate alone. At humate concentrations between 216 and 236 g kg −1 goethite, second-shell iron neighbors could not be identified, and the Cu-O axial distance (2.32 ± 0.02 Å) and derivative XANES spectra strongly resembled that of Cu(II) adsorbed to humate only. Analysis of the XANES and EXAFS data suggested that Cu(II) was bonded on average to both inorganic and organic functional groups as a type A ternary complex at lower levels of adsorbed humate, and to organic groups as a type B complex at higher levels.

Crystal structure of ZnO:Li at 293 K and 19 K by x-ray diffraction

Crystal structures of single crystals of ferroelectric Zn1−xLixO and paraelectric ZnO were studied by X-ray diffraction at 293K and 19K. At 293 K, the Zn-O bond length decreases by an amount of 0.007 Å along c-axis associated with Li-substitution. It is about two orders smaller than the changes in typical ferroelectrics. At 19 K, the clear change of Zn-3d z2 electron is observed in the difference Fourier maps. This change in electronic distribution is important in the ferroelectricity in Zn1−xLixO.

Chemistry of vibronic coupling. Part 2. How to maximize the dynamic diagonal vibronic coupling constant for T1 states in AB systems (A, B=H, Li, Na, K, Rb, Cs, F, Cl, Br or I)?

The dynamic diagonal vibronic coupling constant (VCC) in several series of AB and AA molecules (A, B = H, Li, Na, K, Rb, Cs, F, Cl, Br or I) has been investigated. The electronic states considered are the singlet ground state (“ionic ” for heteronuclear AB species) and first excited singlet or triplet states (“covalent”). The VCC is thus studied for a charge transfer lowest lying triplet state. Qualitative trends in the VCC within the families of systems studied have been sought, with the aim of finding “ a chemistry of vibronic coupling”. Two interesting correlations emerge: the VCC for the charge transfer states in an AB system grows as the sum of the electronegativities of the A and B elements increases, as well as with decreasing AB bond length. A parameter f was defined as the sum of the electronegativities of the A and B elements divided by the AB bond length. This leads to a nearly monotonic correlation between computed values of VCC and f for 55 molecules originating from three distinct classes with a formal single bond: intermetallic species M1M2 (M = alkali metal), interhalogens X1X2 (X = halogen) and salt-like compounds MX. It emerges that contracted p-type orbitals making up the σ* MO (occupied by one electron in the excited state) seem to provide higher values of VCC than diffused s orbitals. The energy of the singlet–triplet gap is also correlated with the sum of the electronegativities of the A and B elements within two families of diatomics. Quantitative explanations of these two trends are still sought.

Ethanol Oxidation by Imidorhenium(V) Complexes: Formation of Amidorhenium(III) Complexes

The reaction of Re(NC6H4R)Cl3(PPh3)2 (R = H, 4-Cl, 4-OMe) with 1,2-bis(diphenylphosphino)ethane (dppe) is investigated in refluxing ethanol. The reaction produces two major products, Re(NC6H4R)Cl(dppe)(2)2+ (R = H, 1-H; R = Cl, 1-Cl; R = OMe, 1-OMe) and the rhenium(III) species Re(NHC6H4R)Cl(dppe)2+ (R = H, 2-H; R = Cl, 2-Cl). Complexes 1-H (orthorhombic, Pcab, a = 22.3075(10) A, b = 23.1271(10) A, c = 23.3584(10) A, Z = 8), 1-Cl (triclinic, P1, a = 11.9403(6) A, b = 14.6673(8) A, c = 17.2664(9) A, alpha = 92.019(1) degrees, beta = 97.379(1) degrees, gamma = 90.134(1) degrees, Z = 2), and 1-OMe (triclinic, P1, a = 11.340(3) A, b = 13.134(4) A, c = 13.3796(25) A, alpha = 102.370(20) degrees, beta = 107.688(17) degrees, gamma = 114.408(20) degrees, Z = 1) are crystallographically characterized and show an average Re-N bond length (1.71 A) typical of imidorhenium(V) complexes. There is a small systematic decrease in the Re-N bond length on going from Cl to H to OMe. Complex 2-Cl (monoclinic, Cc, a = 24.2381(11) A, b = 13.4504(6) A, c = 17.466(8) A, beta = 97.06900(0) degrees, Z = 4) is also crystallographically characterized and shows a Re-N bond length (1.98 A) suggestive of amidorhenium(III). The rhenium(III) complexes exhibit unusual proton NMR spectra where all of the resonances are found at expected locations except those for the amido protons, which are at 37.8 ppm for 2-Cl and 37.3 ppm for 1-H. The phosphorus resonances are also unremarkable, but the 13C spectrum of 2-Cl shows a significantly shifted resonance at 177.3 ppm, which is assigned to the ipso carbon of the phenylamido ligand. The extraordinary shifts of the amido hydrogen and ipso carbon are attributed to second-order magnetism that is strongly focused along the axially compressed amido axis. The reducing equivalents for the formation of the Re(III) product are provided by oxidation of the ethanol solvent, which produces acetal and acetaldehyde in amounts as much as 30 equiv based on the quantity of rhenium starting material. Equal amounts of hydrogen gas are produced, suggesting that the catalyzed reaction is the dehydrogenation of ethanol to produce acetaldehyde and hydrogen gas. Metal hydrides are detected in the reaction solution, suggesting a mechanism involving beta-elimination of ethanol at the metal center. Formation of the amidorhenium(III) product possibly arises from migration of a metal hydride in the imidorhenium(V) complex.

Rare-earth elements in chlorapatite [Ca10(PO4)6Cl2]: Uptake, site preference, and degradation of monoclinic structure

Differences in the inter- and intracrystalline partitioning behavior of rare earth elements (REE) between chlorapatite (ClAp), fluorapatite (FAp), and hydroxylapatite (OHAp) are directly or indirectly related to substitution mechanism and spatial accommodation. The substitution of REE for Ca is charge compensated by Na in ClAp, Na and Si in FAp, and Si in OHAp. Twinned crystals of REE-substituted ClAp [La-ClAp, Nd-ClAp, Sm-ClAp, Dy-ClAp; Ca 10–2y Na y REE y (PO 4 ) 6 Cl 2 , with y = 0.05–0.09; space group P 2 1 / b ] have been grown from SiO 2 -bearing, H 2 O- and Na-rich phosphate-chloride melts, and their hexagonal ( P 6 3 / m ) subcell structures refined at room temperature with single-crystal X-ray intensities to R = 0.020–0.023. The crystal/melt partition coefficients for La, Nd, Sm, and Dy are 0.073, 0.128, 0.122, and 0.101. Thus, uptake of REE is up to two orders of magnitude lower in ClAp than in REE-substituted FAp and OHAp crystallized under equivalent conditions, but remains peaked at Nd. REE site occupancy ratios [(REE-Ca2)/(REE-Ca1)] obtained from direct refinement of electron densities are 0.71, 1.11, 0.21, and 0.09 for La-, Nd-, Sm-, and Dy-ClAp, respectively, and are consistent with relative change in Ca-O bond distances and sizes of Ca polyhedra. Thus, La, Sm, and Dy favor the Ca1 position of ClAp, not Ca2, as in FAp and OHAp; this unusual site preference is attributed to the large increase in size (6–8%) and distortion of the Ca2O 6 X polyhedron on substitution of Cl for (F,OH). The slight preference of Nd for Ca2 and the peaking of REE uptake at Nd are attributed to a 4 f crystal-field contribution, which is revealed by an anomalous decrease in the Ca2-O1 bond length and volume of the Ca2O 6 X polyhedron in Nd-ClAp, Nd-Fap, and Nd-OHAp. The P 2 1 / b structure of La-ClAp has been refined by detwinning the diffraction pattern, confirming that this monoclinic superstructure largely represents ordering of Cl atoms displaced along [001]. The P 2 1 / b structure of Dy-OHAp has been refined similarly. The intensity of superstructure reflections decreases abruptly with increasing substitution of REE for Ca, showing that REE cations interfere with ordering of Cl atoms(and OH − groups) during the P 6 3 / m → P 2 1 / b transition.

Spin transition in [Fe(PM-BiA)2(NCS)2] studied by theelectron paramagnetic resonance of the Mn2+ion

The spin transition of Fe2+ ions in the mononuclear compoundcis-bis(thiocyanato)-bis(N-2'-pyridylmethylene)-4-(aminobiphenyl)-iron(II) isstudied by electron paramagnetic resonance (EPR) of Mn2+. Incooling the compound down to the temperature range 174-168 K, the Fe2+ ions undergo a complete transition from the high spin (HS, S = 2) tothe low spin (LS, S = 0) state, occurring with a narrow, ~5 K andunusually sharp hysteresis loop. The temperature dependence of the unit cellparameters is almost linear on both sides of the spin transition; thevariation of the unit cell parameters at the spin transition is veryanisotropic. The EPR spectra, typical of the Mn2+ ion, onlygradually change with temperature in the two spin states of Fe2+but undergo a striking transformation in the spin transition range. This showsthat a considerable cooperativity exists between the metal ions. Computersimulations using a laboratory-developed simulation program indicatesignificant changes in the zero-field splitting (zfs) parameters in the courseof the spin transition. Lower-than-axial symmetry of the environment of Mn2+ persists in both spin states of Fe2+; however, a strongeraxial distortion arises in the HS state.The temperature variations of the zfs parameter D are related totransformations of the crystal structure using the Newman superposition modelamended for contributions of thermal expansion of the crystal lattice andlattice vibrations. Computer fits show a reduction of the modelparameter - power law exponent - in the LS state, t2 = 4, in comparison withthe HS state, t2 = 8. Such a tendency is consistent with the decrease of theFe-N bond lengths in the HS to LS transition.

Electron density distributions and bonded interactions for the fibrous zeolites natrolite, mesolite and scolecite and related materials

For the fibrous zeolites natrolite, Na2[Al2Si3O10]·2H2O, mesolite, Na2Ca2[Al2Si3O10]3·8H2O, and scolecite, Ca[Al2Si3O10]·3H2O, with topologically identical aluminosilicate framework structures, accurate single-crystal X-ray diffraction data have been analyzed by least-squares refinements using generalized scattering factor (GSF) models. The final agreement indices were R(F ) = 0.0061, 0.0165, and 0.0073, respectively. Ensuing calculations of static deformation [Δρ(r)], and total, [ρ(r)], model electron density distributions served to study chemical bonding, in particular by topological electron density analyses yielding bond critical point (bcp) properties and in situ cation electronegativities. The results for 32 SiO, 24 AlO, 14 CaO, and 12 NaO unique bonds are compiled and analyzed in terms of both mean values and correlations between bond lengths, bonded oxygen radii, bcp densities, curvatures at the bcps, and electronegativities. Comparison with recent literature data obtained from both experimental electron density studies on minerals and model calculations for geometry-optimized molecules shows that the majority of the present findings conforms well with chemical expectation and with the trends observed from molecular modeling. For the SiO bond, the shared interaction is indicated to increase with decreasing bond length, whereas the AlO bond is of distinctly more polar nature, as is the NaO bond compared to CaO. Also, the observed ranges of the Si and Al in situ electronegativities and their mean electronegativities agree well with both Pauling's values and model calculation results, and statistically significant correlations are obtained which are consistent with trends described for oxide and nitride molecules.

Resonance-enhanced multiphoton ionisation photoelectron spectroscopy of the ClO radical: the C 2 sigma- state

A (2 + 1) one-colour resonance-enhanced multiphoton ionisation study is carried out on the C 2 sigma- state of the ClO radical in the one-photon energy range 29,500-31,250 cm-1. The ClO radical is produced by one-photon photolysis of ClO2 employing 359.2 nm photons derived from a separate laser. In this way a significant concentration of vibrationally excited ClO in its spin-orbit split X 2 pi omega (omega = 3/2 or 1/2) electronic ground state is produced. In addition to mass-resolved excitation spectra, kinetic-energy resolved photoelectron spectra for the X 3 sigma-(v+)<--C 2 sigma-(v' = 3-5) transitions are measured. These transitions are not completely Frank-Condon diagonal, and indicate a decrease in bond length on removal of the Rydberg electron from the C 2 sigma- state. In addition to an unambiguous assignment of the C 2 sigma- state, valuable information is obtained on the degree of vibrational excitation with which the nascent ClO radical is formed in the photolysis of ClO2. Analysis of the photoelectron spectra is supported by Franck-Condon calculations based on potential energy curves either from experimental spectroscopic parameters, or obtained by theoretical ab initio methods.

Ab initio molecular orbital study on the structures and energetics of CH3OH(H2O)n and CH3SH(H2O)n in the gas phase

The purpose of this study was to calculate the structures and energetics of CH3OH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n and CH3SH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n in the gas phase: we asked how the CH3OH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document} and CH3SH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document} moieties of CH3OH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n and CH3SH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n change with an increase in n and how can we reproduce the experimental values ΔH°n−1,n. For this purpose, we carried out full geometry optimizations with MP2/6-31+G(d,p) for CH3OH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n (n=0,1,2,3,4,5) and CH3SH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n (n=0,1,2,3,4). We also performed a vibrational analysis for all clusters in the optimized structures to confirm that all vibrational frequencies are real. All of the vibrational frequencies of these clusters are real, and they correspond to equilibrium structures. For CH3OH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n, when n increases, (1) the CO bond length decreases, (2) the CH bond lengths do not change, (3) the OH bond lengths increase, (4) the OCH bond angles increase, (5) the COH bond angles decrease, (6) the charge on CH3 becomes less positive, and (7) these predicted values, except for the OH bond lengths of CH3OH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n, approach the corresponding values in CH3OH. The CO bond length in CH3OH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)5 is shorter than that in CH3OH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document} in the gas phase by 0.061 Å at the MP2/6-31+G(d,p) level. Except for the SH bond lengths in CH3SH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n, however, the structure of the CH3SH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document} moiety does not change with an increase in n. © 2000 John Wiley & Sons, Inc. J Comput Chem 22: 125–131, 2001

Structural Properties of Nonstoichiometric Barium and Strontium Peroxides: BaO2−x (1.97≥2−x≥1.72) and SrO2−x (1.98≥2−x≥1.90)

The crystal structures of eight single crystals of barium and strontium peroxide, prepared by high pressure–high temperature synthesis, were determined by X-ray diffraction. The crystallographic data were successfully refined in the calcium carbide structure (space group I4/mmm, Z=2). The results clearly indicate that the peroxides are nonstoichiometric compounds, MO2−x (M=Ba, Sr), with a peroxide deficiency and that oxide ions are present on interstitial lattice sites. The composition rage extends from near stoichiometry to compounds with a total oxygen content of 1.72 in the barium and 1.90 in the strontium peroxide. We find that the oxide ions, replacing the peroxide molecular ions, occupy an interstitial position that is in the middle of the replaced peroxide molecular ion. A decrease of the peroxide content in the crystals results in a decreased c-axis of the tetragonal unit cell of the structure, while the a-axis is almost constant. Furthermore, the peroxide bond length decreases, as was confirmed by Raman spectroscopy.

Bond Valence Analysis of Tetragonal Zirconias

In tetragonal zirconia, the cation is coordinated by two interpenetrating tetrahedra of oxygen ions, implying two different cation–oxygen bond lengths. On substituting the different tetravalent ions Ge, Ti, Sn, and Ce into tetragonal ZrO2–2 mol% Y2O3, the mean value of the shorter cation–anion bond length varies linearly with the concentration of the substituent ion where the bond length increases or decreases depending on whether the substituted ion is larger or smaller than the zirconium ion it replaces. It is argued in this paper that the length of the longer bond is determined by the requirement that the bond valence sum remains constant. In each case the length of the longer bond calculated on this basis is in good agreement with the measured bond length (from neutron diffraction), and following small adjustments of the bond valence constants, excellent agreement is obtained. The requirement for the bond valence sum evidently accounts for the physics of the situation, and at the same time the available bond length data allow very precise determination of the bond valence constants of the different ions in the tetragonal zirconia environment. It is shown how these bond length considerations provide an explanation for the variation with composition of oxygen position and lattice parameters in all of the materials considered. Among the interesting features accounted for by this analysis are the increase in cell volume occurring when Zr is replaced by the smaller Sn ion, and slight departures from Vegard's law observed in the substitution of Zr by Ti.