3v Symmetry Research Articles

Photoluminescence and electron paramagnetic resonance studies of defect centers in porous silicon

Photoluminescence (PL) and electron spin resonance (ESR) spectrometers were exploited to study the P b defect centers in porous silicon (PS) under various heat treatments. The breaking of Si–H x and Si–O–H bonds in PS by thermal annealing changes the emission wavelength and increases the recombination centers resulting in degrading the PL intensity. Four kinds of dangling bonds on interfaces of silicon (1 1 1), (1,−1,−1), (−1,1,−1), and (−1,−1,1) faces and SiO 2 with C 3V symmetry were identified. The skeleton structure of PS collapsed under thermal annealing. Annealing in hydrogen gas, which is controvertible to the pervasive anticipation, cannot passivate the dangling bonds introduced at high temperatures.

Absorption spectrum analysis of uranium(III) formate

The absorption spectrum of a thin film of uranium(III) formate was measured at 4 and 300 K in the 4000–23 000 cm −1 range. The energy levels of U 3+ were assigned and fitted to a semiempirical Hamiltonian representing the combined atomic and crystal-field interactions at the C 3v symmetry site. Ten energy level parameters were varied simultaneously in least-squares adjustments yielding a mean error of 23 cm −1. The analysis enabled the determination of the crystal-field parameters and the assignment of 49 crystal-field levels. The calculated total splitting of the ground level is equal to 520 cm −1. An analysis of the 5f 3→5f 3 band intensities based on the Judd–Ofelt theory is presented.

Synthesis and structure of pseudotetrahedral Co(II) zwitterionic complexes: trichloro(1-methylpiperazin-1-ium-N 4)cobalt(II) and trichloro(1,4-dimethylpiperazin-1-ium-N 4)cobalt(II)

The pseudotetrahedral cobalt(II) zwitterionic complexes, [CoCl 3(H 2Meppz)] ( 1) [H 2Meppz +=1-methylpiperazin-1-ium cation] and [CoCl 3(HMe 2ppz)] ( 2), [HMe 2ppz +=1,4-dimethylpiperazin-1-ium cation] have been synthesized and characterized in the solid state by X-ray single crystal analysis, IR spectra, magnetic measurements and electronic spectra. In both the compounds the cobalt(II) center is coordinated in a distorted tetrahedral fashion by the three chloride ions and by one nitrogen of the piperazine ring that retains the more stable chair conformation. The distorted coordination polyhedron in complex 1 preserves the C 3v symmetry while in complex 2 it retains only the m symmetry. In complex 1, the (H 2Meppz) + cation binds the Co(II) ion in the equatorial position of the piperazine ring using the unmethylated N1–H nitrogen atom that is less hindered than the methylated one. Complex 2, on the contrary, is a novelty being the first example of a Co(II) ion bound in the axial position of a piperazine ring, this produces a long Co(II)–N bond, 2.108(4) Å. Electronic spectra in the solid state are in perfect accordance with the X-ray crystallographic results indicating a C 3v symmetry for complex 1 and a C s( m) symmetry for complex 2. These complexes present strong two-center and three-center hydrogen bonds of N +–H⋯Cl type.

Study of the structure of H<sub>2</sub>SO<sub>4</sub>-amine Primene 81R and Fe(III)-amine Primene 81R sulphate compounds by infrared spectroscopy

The primary amine Primene 81R extracts sulphuric acid from aqueous solutions to form the corresponding sulphate and bisulphate salts. The infrared study shows that in the sulphate salt, the sulphate group presents a T d symmetry, whereas the amine bisulphate presents a C 3v symmetry for the bisulphate group. The former of the above compounds, the amine sulphate, is complexed to Fe(III) in sulphate media, and two complexes are extracted depending on temperature. At 8°C or lower temperatures a complex with the stoichiometric formula 4(RNH 3 ) 2 SO 4 (Fe(OH)SO 4 ) 2 is formed, whereas at 50°C or higher temperatures the complex with the stoichiometric formula 3(RNH 3 ) 2 SO 4 (Fe(OH)SO 4 ) 2 is formed. The infrared spectrum of both complexes is very similar and it is consistent with a weaker interaction between the protons associated with the amine group and the anion, and with a change in the symmetry of the sulphate group respect to the symmetry presented in the amine sulphate salt.

EPR study of gamma irradiated arsanilic acid single crystal

Abstract EPR spectra of gamma irradiated arsanilic acid single crystal shows rarely observed and unstable arsenate centred [(OH) 2 As=O] radical. There are two magnetically distinct sites in monoclinic lattice. The principal hyperfine and g values of both sites are determined from spectra taking the large hyperfine effects into account. The principal hyperfine values are found to be A x =602, A y =437 and A z =428 Gauss for one site and A x =602, A y =438 and A z =429 Gauss for the other site. The g values of both sites are the same with the values g x =2.048, g y =2.001 and g =2.000. The radical is in slightly distorted C 3v symmetry. The unpaired electron is mainly localised on s orbitals of O atoms and partly on p orbital of central As atom. Calculations of EPR parameters for large hyperfine values are discussed and some easily applicable numerical techniques are utilised.

Infrared studies of sub-monolayer methylamine and trimethylamine adsorption on Ni(111)

The adsorption of monomethylamine and trimethylamine on Ni(111) has been studied using reflection absorption infra-red spectroscopy (RAIRS). At 110 K, both molecules are found to chemisorb molecularly and to retain their gas-phase symmetries at the surface. Methylamine possesses an adsorption site symmetry of C s in which the NH 2 unit is held parallel to the surface plane and the methyl group is tilted with respect to the surface normal. Trimethylamine possesses C 3v symmetry upon adsorption, with the c 3 axis aligned along the surface normal. The high sensitivity RAIR data allow a peculiar feature of methylamines to be probed: the lone pair on the nitrogen gives rise to inequivalence in the CH bonds of the methyl unit, leading to the creation of one much weaker C–H bond that produces a decoupled vibration at an unusually low stretching frequency.

Synthesis and X-ray crystal structures of new trirhenium nonachloride chalcogenide clusters

The acetone adduct of trirhenium nonachloride, Re 3Cl 9 (acet) 3, where acet=acetone, reacts in acetone solution at room temperature with tetrabutylammonium chalcogenides [N(C 4H 9) 4] 2E, where E=S, Se and Te, producing the tetrabutylammonium salts of the new cluster anions [Re 3 ( μ 3-E) ( μ 2-Cl) 3Cl 6] 2−. In the crystallographically determined structures of these compounds, triangles of rhenium atoms are capped by single chalcogen atoms, giving anions of near C 3v symmetry. The compound where E=S is also formed in the reaction of Re 3Cl 9 with [N(C 4H 9) 4] 2 [MoS 4] by sulfide transfer.

Photoluminescence property and photocatalytic reactivity of V-HMS mesoporous zeolites Effect of pore size of zeolites on photocatalytic reactivity

The photoluminescence properties and photocatalytic reactivities of the V-HMS mesoporous zeolite have been investigated by means of in situ photoluminescence, UV-vis, EXAFS, XRD and ESR measurements. It was found that the V-HMS zeolite involves a vanadium species which is highly dispersed and incorporated in the framework of the mesoporous zeolites having a V=O bond and a C 3v symmetry. This species plays an important role as the emitting site of this zeolite. It was also found that the zeolite exhibits photocatalytic reactivity under UV irradiation in the presence of cis-2-butene, resulting in the formation of trans-2-butene and 1-butene. The photocatalytic reactivity of the V-HMS mesoporous zeolite was found to be much higher than that of the V-silicalite microporous zeolite (VS-1). The efficiency of the dynamic quenching of the photoluminescence of the V-oxide species in the excited state in the V-HMS mesoporous zeolite by smaller molecules such as O 2 was found to be the same as that of the VS-1 microporous zeolite. By comparing these results with results obtained on the VS-1 microporous zeolite photocatalyst, we can conclude that the pore size effect plays a significant role as one of the major factors which determine the photocatalytic reactivities of porous zeolite photocatalysts.

Laser spectroscopy of metallic free radicals: the observation of the C̃–X̃ vibronically allowed electronic transition for Ca–OCH 3, Ca–OC 2H 5 and Ca–CCH

Observation of the forbidden transition C̃ 2Δ–X̃ 2Σ + for the linear calcium radical CaCCH as well as the corresponding band systems C̃ 2E 1–X̃ 2A 1 for CaOCH 3 (C 3v symmetry) and C̃ 2A′–X̃ 2A′ for CaOCH 2CH 3 (C s symmetry) has been achieved using laser vaporisation, supersonic jet and laser spectroscopy. These molecules have a local linear geometry near the metal center. The C̃–X̃ transition is electronically forbidden because of the Δ Λ=0, ±1 selection rule and becomes allowed through coupling the vibrational angular momentum ℓ associated with the Ca–X–Y bends to the electronic angular momentum Λ of both the C̃ and X̃ states. The vibronic bands 2 0 1, 2 0 2, 2 0 13 0 1, 2 0 23 0 1, 2 0 14 0 1 for CaCCH and 2 0 1, 2 0 2, 2 0 13 0 1, 2 1 0 for CaOCH 3 and 2 0 1, 2 0 2, 2 0 13 0 1, 2 0 14 0 1, 2 1 0 for CaOCH 2CH 3 have been assigned. Vibration frequencies and spin–orbit splitting in the excited C̃ state are derived for the radicals. The Renner–Teller parameter has been estimated for CaCCH and CaOCH 3. The vibronic bands structure and the value of the spin–orbit constant show that the M–X bond is highly ionic and the local symmetry near the calcium ion is linear.

O-Ru(0001) Surface Bond and Band Formation

Incorporating the bond-to-band and the potential barrier model [J. Phys. Chem. Solids58, 903 (1997); J. Phys.: Cond. Matt.27, 5823 (1997)] into the existing database reveals that the O-Ru(0001) triphase results from the formation of O -1, hybridized O -2 and the addition of a virtual bond, respectively. Oxygen is located at the center of a tetrahedron, forming a Ru 4 O cluster. Initially (<1/4 ML), the Ru 4 O ( O -1+ Ru ++ 3Ru dipole) forms, retaining C 3v symmetry. Then (1/2 ML), the Ru 4 O develops into a Ru 2 O ( O -2+ 2Ru + + 2Ru dipoles) by forming another bond with a surface Ru atom; as a result, a dipole-ion pairing row forms. Finally (1.0 ML), a virtual bond [2Ru(dipoles)+] forms by redistributing the dipole electrons; this process not only reduces the dipole moment but also narrows the antibond subband. The first layer spacing depends upon bond geometry and the second layer spacing contracts due to charge redistribution.

Vibrational spectra and structural features of noble gas fluorides in cryogenic and nonaqueous solutions

Data on the vibrational spectra of noble gas fluorides in the gas phase and in cryogenic and nonaqueous solutions are considered in detail. Based on analysis of the IR spectra of xenon fluorides dissolved in liquid Kr and Xe, it is concluded that the XeF6 molecule possesses the geometry of a distorted octahedron withC 3v symmetry. The contours of spectral lines of totally symmetric stretching modes in the Raman spectra of noble gas fluorides in nonaqueous solutions are considered; the mechanisms of formation contours of these lines, the dynamic parameters of XeF n (n=2, 4, 6) and KrF2, and the characteristic times of intramolecular rearrangements in the nonrigid XeF6 molecule are analyzed. It is concluded that in the XeF2-HF and XeF6-HF systems, a number of associates and ionic clusters are formed due to the donor-acceptor interaction of the Lewis bases and acids.

Laser-selective excitation spectra of Ho 3+ ions in BaF 2 crystals

Crystal-field energy levels for five multiplets of the dominant Ho 3+ centre in BaF 2 : Ho 3+ crystals have been derived from the centre’s selective excitation and fluorescence spectra. The centre is deduced to be of trigonal symmetry and irrep labels of the C 3v point group are assigned to the energy levels. The splittings of up to 5.5 cm −1 observed for transitions involving the γ 3 (doublet) symmetry levels are evidence of some distortion at the Ho 3+ site from exact C 3v symmetry. The centre exhibits strong up-conversion fluorescence in the 0.1% Ho 3+-doped crystals. A minor centre, characterised by fluorescence transitions of much lower energies compared to the dominant centre, is also present.

Crystal structure of K 18.5H 1.5[Ce 3(CO 3)(SbW 9O 33)(W 5O 18) 3]·14H 2O

Crystal structure of the title compound, which was isolated from a neutral solution containing HCO 3 −, was determined by X-ray diffraction analysis. It crystallized in monoclinic (C2/m) with crystallographic parameters of a=28.979(5), b=18.379(4), c=19.461(3) Å, β=100.21(1)°, Z=4, and V=10201(3) Å 3. The [Ce 3(CO 3)(SbW 9O 3 3)(W 5O 1 8) 3] 20− anion possesses a trinuclear [Ce 3(CO 3)] 7 + core attached by one B-α type [SbW 9O 3 3] 9 − and three [W 5O 1 8] 6 − groups with C 3v symmetry. The [Ce 3(CO 3)] 7 + core has the carbonate group which links three Ce 3+ cations. A groove is formed between adjacent [W 5O 1 8] 6 − groups in the anion, resulting in three grooves in the anion. Each groove contains two K + cations contacting the O atoms of the [Ce 3(CO 3)] 7 +, [SbW 9O 3 3] 9 −, and three [W 5O 1 8] 6 − groups. Other K + cations form O–K–O linkages among the anions in the lattice.

The trimethylamine adduct of trimethylindium in the vapour phase: structural characterization and dissociation energies from electron diffraction and infrared spectroscopy

The electron diffraction structure of the trimethylamine adduct of trimethylindium is reported. As far as we know this is the first reported gas–phase study of an indium adduct. The molecule has C 3v symmetry and an indium–nitrogen bond length of 237.8(12) pm. The small changes in the geometry of the trimethylindium moiety on coordination show that the adduct bond is weak. Similarly, the infrared spectra show only small shifts in frequencies for modes of the adduct compared to those of the acceptor and donor. The infrared spectrum of trimethylindium has been re–examined and reassignments made on the basis of ‘hot’ molecules. The adduct is almost wholly associated in the vapour at room temperature. Dissociation of the adduct at elevated temperatures has been studied by electron diffraction and infrared spectroscopy. The former, measured at constant pressure, has yielded an enthalpy of dissociation of 81 plus or minus 6 kJ mol −1 , the latter, measured at constant volume, gives an internal energy of dissociation of 84 plus or minus 2 kJ mol −1 . The difference between these two terms is discussed.

On the nature of the [formula omitted] and [formula omitted] surface bonding

The nature of sulfate-Ag(111) and sulfate-Au(111) surface bonding has been investigated at the SCF + MP2 level of theory. Convergence of binding energy with cluster size is investigated and, unlike neutral adsorbates, large clusters are required in order to obtain reliable binding energies. In the most stable adsorption mode, sulfate binds to the surface via three oxygen atoms (C 3v symmetry) with a binding energy of 159.3 kcal/mol on Ag(111) and 143.9 kcal/mol on Au(111). The geometry of adsorbed sulfate was optimized at the SCF level. While the bond length between sulfur and the oxygens coordinated to the surface increases, the sulfur-uncoordinated oxygen bond length decreases. This weakening and strengthening of the bonds, respectively, is consistent with bond order conservation in adsorbates on metal surfaces. Although a charge transfer of 0.4 electrons towards the metal is observed, the adsorbate remains very much sulfate-like. The molecular orbital analysis indicates that there is also some charge back-donation towards unoccupied orbitals of sulfate. This results in an increased electron density around sulfur as revealed in the electron density difference maps. Analysis of the Laplacian of the charge density of free sulfate provides a suitable framework to understand the nature of the different charge transfer processes and allows us to establish some similarities with the CO- and SO 2-metal bondings.

Microoptical spectroscopy of BiI 3 molecules adsorbed in nano-channels of zeolite single crystals

Absorption spectra are reported for BiI 3 molecules incorporated into channels of zeolite MOR and AFI. Polarized absorption spectra for the zeolite single crystals, of size about 50 μm, were measured by means of microoptical spectroscopy. The absorption spectrum of BiI 3 in MOR is different from that observed in LTA, and also shows a polarization dependent on the direction of the channel. The BiI 3 molecule is expected to have nearly C 3v symmetry even in the MOR channels, and has a preferred orientation of the molecular axis along the channel. On the other hand, no polarization dependence of BiI 3 is observed in the AFI channel, although AFI has a channel size similar to that of MOR. The cations and the framework induce the electric field inside the MOR channels, which acts as an adsorption potential for molecules. The experimental results indicate that the electric field plays an important role in the molecular orientation.

Doping dependence of second harmonic generation from native oxide/Si(111) interfaces

Optical second harmonic generation (SHG) was studied on native oxide/Si(111) substrates with both n- and p-type dopings of various doping concentrations. With the rotation about the (111) azimuth, six peaks were observed in SHG intensity at every 60° with C 3v symmetry for all substrates. However, the strong peaks appeared at 0, 120, and 240° on p-type Si substrates, whereas these peaks were small on n-type Si substrates. We found that the reduction of the peaks at 0, 120 and 240° had a correlation with the change of the doping type and the doping concentration from the p + to the n + region. Analysis of the azimuthal rotation dependence of the SH intensity indicates that the observed change is caused by the change of the 2nd order non-linear susceptibility tensor components, χ zxx. We think that this is due to the change of the interface dangling bond occupation and the resulting change of the interface dipole with the dopings.

Internal rotation and barrier height of CCl 3 group in trichloromethylbenzene as studied by gas-phase electron diffraction

The molecular structure of gaseous trichloromethylbenzene has been determined from electron diffraction data. An internal rotation of the CCl 3 group around the CC bond was treated as a precession motion. The rotational barrier height of the CCl 3 group is 1.3(6) kJ mol −1, and the minimum of the barrier is at the position such that one of the CCl bonds lies on the phenyl plane. The tilt angle of the C 3v symmetry axis is 4.6(14)° at the potential minimum position and 3.0(8)° at the potential maximum position. The molecular parameters and uncertainties are r g( CC) ring = 1.399(1) A ̊ , r g( CH) = 1.096(6) A ̊ , r g( CC) = 1.531(5) A ̊ , r g( CCl) = 1.784(2) A ̊ and r g( Cl⋯Cl) = 2.885(2) A ̊ , where the r g (CC) ring is the average value in the ring.

Internal rotation and barrier height of CC1 3 group in trichloromethylbenzene as studied by gas-phase electron diffraction

The molecular structure of gaseous trichloromethylbenzene has been determined from electron diffraction data. An internal rotation of the CC1 3 group around the CC bond was treated as a precession motion. The rotational barrier height of the CCl 3 group is 1.3(6) kJ mol −1, and the minimum of the barrier is at the position such that one of the CCl bonds lies on the phenyl plane. The tilt angle of the C 3v symmetry axis is 4.6(14)° at the potential minimum position and 3.0(8)° at the potential maximum position. The molecular parameters and uncertainties are r g (CC) ring = 1.399(1) Å, r g (CH) = 1.096(6) Å, r g (CC) = 1.531(5) Å, r g (CCl) = 1.784(2) Å and r g (Cl … Cl) = 2.885(2) Å, where the r g (CC) ring is the average value in the ring.

Effect of the Madelung potential value and symmetry on the adsorption properties of adsorbate/oxide systems

An ab initio Hartree-Fock self-consistent field cluster model approach has been used to analyze the effect of surface irregularities located at long distances of NO and CO adsorbed on a monocoordinated position of the Cu 2O(111) surface. To perform this study different surface cluster models were used. One of these clusters has C 3v symmetry as in the perfect surface. In the two other cluster models the adsorption site local symmetry is still C 3v but to simulate surface irregularities the array of point charges used to include the Madelung potential has C s symmetry. The NO Cu + interaction is always dominated by the electrostatic effects without important contributions from charge transfer mechanisms. The symmetry change between these two kinds of models does not practically alter the geometrical and vibrational parameters of the bond but affects the strength of the NO Cu 2 O interaction in a non-negligible way. On the other hand the CO Cu + interaction is much larger with substantial contributions from σ-donation and π-backdonation. In this case the symmetry change does not have noticeable effects. The present study shows that, on adsorbate/oxide systems with electrostatic bonds, the energy of interaction may depend not only on the local electrostatic field strength felt by the adsorbate but also on the degree of perfection of the oxide surface. The implications of this result for the analysis of experimental data are discussed.