Valence Force Field Research Articles

Spectroscopic Characterization of Matrix-Isolated Phenylcarbene and Cycloheptatetraene

The full UV/VIS and IR spectra are presented for phenylmethylene (1) and its rearrangement product, cycloheptatetraene (2). The spectra are assigned and discussed on the basis of CASSCF/CASPT2 excited-state and of scaled B3LYP force-field calculations, respectively. The optical spectrum of 1 is very similar in appearance to that of the iso-π-electronic benzyl radical, which demonstrates that the unpaired σ-electron does not interfere noticeably with the π-electronic structure. The IR spectrum of 1 is not very well reproduced by the unscaled B3LYP calculations because of pronounced anharmonicity of the Ph−C−H bending mode, and even a differentiated scaling of the harmonic force field cannot fully remedy this. In contrast, the IR spectrum of 2 is very well reproduced and the corresponding valence force field shows the expected properties for this cyclic allene.

Fourier transform Raman and infrared spectra and normal coordinate analysis of organo-arsenic(III), -antimony(III) and -bismuth(III) thiolates

The FT-Raman and infrared spectra of (PhS)BiPh 2, (PhS) 2BiPh, (PhS) 3Bi, (PhS) 3Sb, (PhS) 3As and (PhSe)BiPh 2 were measured in the range 3600–100 cm −1. A normal coordinate analysis was performed for all substances in terms of the calculation and discussion of the force constants which are dependent on the element-sulphur and element-selenium group using a modified valence force field. Furthermore, for all compounds 1H-NMR, 13C-NMR and MS data were also given. The investigation of the microbiological activity of some substances against Escherichia coli was an additional aspect because of their strong bactericidal and fungicidal effects.

Structural interpretation of the infrared spectrum of a Sb 0.05As 0.45Se 0.50 glassy semiconductor

The infrared (IR) absorption spectrum for glassy Sb 0.05As 0.45Se 0.50 was measured in the wave number region 500–90 cm −1 at room temperature and in a continuously purged sample compartment, following the polyethylene pellet method. Main structural units in the glassy are considered to be AsSe 3 and SbSe 3 pyramids, Se 8 rings and Se n chains. By using X-ray diffraction data it was found a structural model with the assumption of tricoordinated As, tricoordinated Sb and dicoordinated Se [C. Corredor, J. Vazquez, R.A. Ligero, P. Villares, R. Jimenez-Garay, Materials Letters 36, 1998, 271; I. Quiroga, J. Vázquez, P. Villares, R. Jiménez-Garay, Materials Letters 17, 1993, 333.] which confirms the information obtained by means of far-infrared transmission (FIT). From the “Valence Force Field” (VFF) we can determine the vibrational frequencies, ω i , of possible structural units of alloy in this study by using the expressions derived by Herzberg. The results for this alloy are the following: 85, 110 and 230 cm −1, for AsSe 3, and 112, 215 and 315 cm −1, for SbSe 3. The comparison between the frequencies obtained in the far-IR spectrum and the values calculated of the normal frequencies let us assign bands to structural units.

Elastic properties of single-walled carbon nanotubes

Analytical expressions for the velocities of the longitudinal and the torsional sound waves in single-walled carbon nanotubes are derived using Born's perturbation technique within a lattice-dynamical model. These expressions are compared to the formulas for the velocities of the sound waves in an elastic hollow cylinder from the theory of elasticity to obtain analytical expressions for the Young's and shear moduli of nanotubes. The calculated elastic moduli for different chiral and achiral (armchair and zigzag) nanotubes using force constants of the valence force field type are compared to the existing experimental and theoretical data.

Microscopic Parameters of Heterostructures with Ge Nanoclusters and Thin Films in a Si Matrix

Microstructural parameters (interatomic distances, coordination numbers, and their anisotropy) of heterostructures with Ge nanoclusters and thin films were determined by EXAFS spectroscopy. The variation of these parameters is correlated to the morphology of the germanium quantum points on Si(001), and adequate structural models are suggested. It is found that the pseudomorphic four-monolayer germanium films buried in a Si matrix contain up to 50% Si atoms. The pyramidal germanium islands (lateral size ∼15 nm, height ∼1.5 nm) formed during further Stranski–Krastanov heteroepitaxial growth have a thinner Ge–Si intermediate boundary layer near two monolayers and Ge–Ge interatomic distances 0.04 A shorter than those in pure germanium in agreement with the results of bond length calculations in the framework of the valence force field (VFF) model.

Coordination Polyhedra RhX6 (X = F, Cl, Br) in Crystal Structures

Voronoi–Dirichlet polyhedra (VDP) and the method of intersecting spheres were used to perform crystal-chemical analyses of compounds containing complexes [Rh a X n ] z– (X = F, Cl, Br). It was found that, irrespective of oxidation number (+3, +4, or +5), rhodium atoms always exhibit the coordination number 6 with respect to the halogen atoms and have octahedral coordination. The influence of site symmetry and the valence state of Rh on the distortion of RhX6 octahedra are considered. The electronic configuration of the Rh atoms is shown to influence the symmetry of their valence-force field within the crystal structure.

Theoretical and experimental vibrational study of polyaniline in base forms: non-planar analysis. Part I

Experimental and theoretical studies of basic forms of polyaniline are presented. Experimental spectra of emeraldine, leucoemeraldine and pernigraniline and their oligomer model compounds (diphenylamine, phenyl-end-capped dimer, phenyl-end-capped tetramer, phenyl-end-capped hexamer, imine form of phenyl-end-capped dimer, pernigraniline base phenyl-end-capped tetramer and emeraldine base phenyl-end-capped tetramer) were recorded in N-methyl-2-pyrrolidone solution and in the solid state. The calculations were performed using the valence force field model. A non-planar geometry of the polymers studied was taken into consideration and particular phenylene ring torsions rendering the system non-planar were analyzed. The assignments of the observed modes was made possible by comparison of the calculated and experimentally measured wavenumbers. Copyright © 2000 John Wiley & Sons, Ltd.

Interchain Interaction in Semiconducting Liquid and Amorphous Selenium

Semiconducting liquid and amorphous selenium (l-Se,a-Se), which are representative disordered materials, are regarded as assemblies of two-coordinated chains. The atomic and electronic structures have been studied theoretically using ab initio and tight-binding molecular-dynamics (MD) or Monte Carlo methods. However, the former is difficult to apply to a large system size, and the latter is difficult to deal with intermolecular interactions appropriately. A classical chain model 1) has been also applied to study the atomic structure and dynamics. It assumes, however, an isotropic Lenard-Jones potential (LJ) as the interaction between the nonbonded atoms. Since each Se atom in the chains has a p-lone pair (LP), the interaction between the nonbonded atoms is anisotropic in practice. We construct a chain model including the anisotropy based on an ab initio molecular-orbital (MO) method, and obtain the structural samples by MD simulations. We compute the intra and intermolecular potential of Se clusters using an ab initio MO method (MP2/CEP-31G(d)). Potentials of an isolated Se chain (HSe10H) are calculated around the optimized geometry, and fitted to a valence force field model. The bending force constant is similar to that of Ref. 1). The barrier of internal rotation is 163 meV that is about 2.4 times as large as the corresponding value of Ref. 1). In our MD simulations, we scale the calculated stretching force constant by 0.7 to explain the experimental stretching modes. Intermolecular potentials are calculated for (HSe3H)2 dimer. They are closely related to the way of facing between the LPs on the central Se atom in the monomer, which causes 0.3 A change of the atomic effective radius on the interaction. The interaction potential of the pair of nonbonded atoms i and j is represented by

A molecular mechanics study of the adsorption of ethane and propane on a V2O5(001) surface

Adsorption geometries (positions and orientations) and adsorption energies of ethane and propane molecules physisorbed on the (001) surface of vanadium pentoxide were determined by a molecular mechanics simulation. The adsorbate molecules were modeled both as rigid molecules and as flexible molecules described by the consistent valence force field. The energetically most favored sites for adsorption are located between two double rows of vanadyl oxygens. In this area the potential energy surfaces exhibit broad minima for ethane and propane adsorption energies with values of −22.8 and −33.3 kJ mol−1, respectively. The shortest distances between the alkane hydrogen and different surface lattice oxygen species increase in the following order: double-coordinated bridging oxygen<vanadyl oxygen<threefold coordinated oxygen. All C–H bonds of the adsorbates facing the surface are shortened and those facing outward are elongated. The physisorbed states may be tentatively considered as precursors to chemisorption and the subsequent catalytic reaction. Enthalpies of physisorption of ethane and propane on crystalline V2O5 have been determined by microcalorimetry. These experimental values agree well with the adsorption energies obtained from the simulation.

Infrared, Raman and resonance Raman spectra of the solid nickel(II) thiophosphate compound (PPh4)3 [(NiPS4)3] and force constants in the trinickel [Ni3P3S12]3− cyclic anion

The UV–visible (350–900 nm), infrared and Raman (30–800 cm−1) spectra of a new transition metal thiophosphate solid compound, (PPh4)3 [(NiPS4)3], were analyzed using powder samples. Electronic transitions of the 1F2 1A1 type originating in the (PS4)3− groups were detected from visible absorbance measurements in the 440–520 nm range and most of the expected infrared and Raman vibrational modes of the trimetallic ring anion [Ni3P3S12]3− were observed in the 600–30 cm−1 region. Also, resonance Raman spectra were recorded using exciting radiation of several wavelengths in the 680–450 nm range and the Raman excitation profiles of several fundamentals were established: they all peak near 488 nm and exhibit the strongest enhancements for totally symmetric and non-totally symmetric stretching and bending modes of (NiS4) groups. It is therefore concluded that several ligand to metal charge-transfer electronic transitions are overlapped and involved in vibronic coupling mechanisms. Furthermore, the vibrational assignments were checked by a valence force field calculation for an isolated [Ni3P3S12]3− anion of C3v symmetry. The potential energy distributions and mean squared vibrational amplitudes revealed strong ν(Ni–S) and δ(S–P–S) couplings and relatively high ν(Ni–S) stretching modes at near 310 cm−1. Compared with results in the one-dimensional KNiPS4 compound containing infinite anionic chains [NiPS4]−, one notes a force constant increase on the P—S exocyclic (µ1) bonds, no change on the bridged (µ2 and µ3) P—S bonds and a slight increase in the Ni–S stretching force constants varying from 75 to 85 N m−1 in accordance with a localized ligand to metal charge-transfer mechanism. Hence a comparison of the induced Raman scattering processes in the (PPh4)3 [(NiPS4)3] and KNiPS4 solid compounds shows that the ligand to metal charge transfers and the crystal field effects have a strong influence on the vibrational properties in these thiophosphate compounds. Copyright © 1999 John Wiley & Sons, Ltd.

Atomic-resolution study of lattice distortions of buriedInxGa1−xAsmonolayers in GaAs(001)

X-ray standing wave measurements were used to study the strain in one monolayer of pseudobinary alloy In{sub x}Ga{sub 1-x}As buried in GaAs(001) by molecular-beam epitaxy. The measured In position along the [001] direction exhibited a nearly linear dependence on the In concentration x, thus supporting the validity of macroscopic continuum elasticity theory at the one-monolayer limit. A random-cluster calculation using a valence force field was performed to explain microscopically the origin of the vertical expansion of the strained monolayer observed by the experiment. The calculated As-In-As bond angle and the positions of the first-nearest-neighbor As atoms of In suggest that the nearly linear dependence of the In height on the alloy composition is a combined result of the As-In-As bond bending and the local lattice distortion at the GaAs/In{sub x}Ga{sub 1-x}As interface. The calculated In-As and Ga-As bond lengths were found to depend weakly on the In concentration, consistent with an earlier calculation for the case of a thick In{sub x}Ga{sub 1-x}As film on GaAs(001) and the available x-ray absorption fine-structure data.

Ge clusters in Si matrix: structure and dynamics

We have determined by computer simulations, some structural properties of Ge clusters embedded in a Si crystalline host matrix for cluster sizes varying from \( \approx \) 0.5 to 1.5 nm. In order to describe inter-atomic forces we have chosen a Valence Force Field (VFF) semi-empirical potential. Next we have calculated the density of vibrational states by diagonalization of the dynamical matrix defined with the same potential. The influence of the volume/interface ratio of Ge on the vibrational properties is discussed.

Common force field for graphite and polycyclic aromatic hydrocarbons

A valence force field based on H\"uckel's theory has been developed, which allows us to establish a close correlation between phonons of graphite and the normal modes of small polycyclic aromatic hydrocarbons (such as coronene and hexabenzocoronene). The results show that in these systems two kinds of motions dominate the Raman spectrum: the \cyrchar\CYRYA{} mode and the ``breathing'' A mode. These modes are the equivalent, in a finite domain, of the ${E}_{2g}$ phonon of graphite at the \ensuremath{\Gamma} point and the ${A}^{\ensuremath{'}}$ phonon at the K point of the first Brillouin zone. This study provides a useful basis for the understanding of the Raman spectra of any material containing ${\mathrm{sp}}^{2}$ carbon domains.

FT-Raman and infrared spectra of silver(I) complexes with glutarimidate and 3,3-dimethylglutarimidate anions

FT-Raman and FT-IR spectra are reported for glutarimidato-Ag(I) and 3,3-dimethyl-glutarimidato-Ag(I) complexes, which show very close structural similarity to the corresponding Ag(I) complexes with pyrimidine nucleobases. The spectra indicate that in both complexes the deprotonated nitrogen atom and two oxygen atoms of the glutarimide ring are involved in binding with silver atoms. Vibrational assignment is supported by normal coordinate analysis based on a general valence force field. The linear NAgN stretching vibrations are assigned to the bands at 479 cm −1 (infrared) and 485 cm −1 (Raman). The silver–oxygen stretching vibrations are assigned to the bands in the range 253–206 cm −1.

A New Method for Evaluating the Conformations and Normal Modes of Macromolecule Vibrations with a Reduced Force Field. 2. Application to Nonplanar Distorted Metal Porphyrins

We have developed a novel method for molecular mechanics calculations and normal-mode analysis. It is based on symmetry of local units that constitutes the given molecule. Compared with general valence force field calculations, the number of free parameters is reduced by 40−80% in our procedure. It was found to reproduce very well the vibrational frequencies and mode compositions of aromatic compounds and porphyrins, as shown by comparison with DFT calculations. A slightly altered force field obtained from Ni(II) porphin was then used to calculate the structure and the normal modes of several meso-substituted Ni(II) porphyrins which are known to be subject to significant ruffling and/or saddling distortions. This method satisfactorily reproduces their nonplanar structure and Raman band frequencies in the natural abundance and isotopic derivative spectra. The polarization properties of bands from out-of-plane modes are in accordance with the predicted nonplanar distortions. Moreover, we found that some of the modes below 800 cm-1 which appear intense in the Raman spectra contain considerable contributions from both in-plane and out-of-plane vibrations, so that the conventional mode assignments become questionable. We also demonstrate that the intensity and polarization of some low-frequency Raman bands can be used as a (quantitative) marker to elucidate type and magnitude of out-of-plane distortions. These were recently shown to affect heme groups of hemoglobin, myoglobin, and, in particular, of cytochrome c.

Strained semiconductors structures: simulation of the thin films heteroepitaxial growth

We have performed an atomic-scale simulation of (100) thin films heteroepitaxial growth of semiconductors with zincblende structure by associating a Monte Carlo technique with an energy model based on the valence force field approximation. It is shown that grooves showing (111) facets are formed in the early stages of the growth owing to enhanced interlayer migrations. The strain relief can be achieved within a few atomic layers, in agreement with experimental observations.

Studies of fullerenes and carbon nanotubes by an extended bond order potential

We present a novel approach to combine bond order potentials with long-range nonbond interactions. This extended bond order potential consistently takes into account bond terms and nonbond terms. It not only captures the advantages of the bond order potentials (i.e. simulating bond forming and breaking), but also systematically includes the nonbond contributions to energy and forces in studying the structure and dynamics of covalently bonded systems such as graphite, diamond, nanotubes, fullerenes and hydrocarbons, in their crystal and melt forms. Using this modified bond order potential, we studied the structure and thermal properties (including thermal conductivity) of C60 crystal, and the elastic properties and plastic deformation processes of the single-walled and double-walled nanotubes. This extended bond order potential enables us to simulate large deformations of a nanotube under tensile and compressive loads. The basic formulation in this paper is transferable to other bond order potentials and traditional valence force fields.

Surface morphology in InAs/GaAs(111)A heteroepitaxy: Experimental measurements and computer simulations

The surface morphology of InAs films grown on GaAs(111)A has been studied by scanning tunneling microscopy. The vertical surface displacement on the InAs films has been found to depend on the underlying GaAs buffer layer thickness: specifically, thin GaAs layers are observed to behave mechanically similar to compliant substrates. Atomistic simulations within a valence force field model have been used to compare quantitatively how the InAs surface morphology depends on film thickness and the underlying GaAs layer thickness. The experimental and theoretical results are in excellent agreement over a range of film thicknesses where the misfit dislocation network at the semicoherent InAs/GaAs interface is fully developed.

Infrared, Raman and resonance Raman spectra and lattice dynamics calculations of the solid potassium(I) palladium(II) thiophosphate compound KPdPS4

The UV–Visible (350–900 nm), infrared and Raman (30–800 cm−1) spectra of a new transition metal thiophosphate solid compound, KPdPS4, were analyzed using powder samples and single-crystal platelets. From polarized absorbance measurements, the main electronic transitions of the 1F2 ← 1A1 type due to the (PS4)3− groups are localized in the 400–520 nm range and all the expected infrared vibrational modes were observed in the 600–30 cm−1 region; various polarized Raman measurements were also performed. The vibrational assignments were then checked by complete valence force field lattice dynamics calculations in the different symmetry blocks of the tetragonal crystal structure (D4h14). Potential energy distributions and mean squared vibrational amplitude calculations revealed strong ν(Pd–S) and δ(S–P–S) couplings in the one-dimensional inorganic polymer anionic 1∞[PdPS]4]− chains and remarkably high ν)Pd–S) stretching modes near 350 cm−1. Compared with results for the analogous Ni(II) compound, there was a significant increase in the M– S force constants, varying from 75 N m−1 (Ni– S) to 90 N m−1 (Pd– S) in accordance with the Pauling electronegativity scale. Furthermore, various resonance Raman spectra of micrometer-sized samples were recorded using a confocal microscope instrument in order to characterize not only the existence of chains having a distribution of length but also some Raman impurity bands coming from the dithiophosphate compound, K4Pd(PS4)2. Also, resonance Raman spectra of powder samples were recorded using several exciting radiation wavelengths over the range 800–500 nm in order to probe the nature of the low-energy electronic transition at 658 nm; the Raman excitation profiles of several fundamentals were thus established and they demonstrate that both totally symmetric and non-totally symmetric stretching and bending modes of the PS4 and PdS4 groups are markedly enhanced. We therefore conclude that several ligand to metal charge-transfer electronic transitions are overlapped and involved in vibronic coupling mechanisms with higher energy bands. Finally, a comparison of the induced Raman scattering processes in the Cu3PS4, KNiPS4 and KPdPS4 solid compounds showed that the ligand to metal charge transfers and the crystal field effects have a strong influence on the optical and vibronic properties of these thiophosphate compounds. Copyright © 1999 John Wiley & Sons, Ltd.

Vibrational spectra and normal coordinate analysis of 4,13-diaza-18-crown-6-calcium chloride and bromide complexes

Calcium chloride ( 40CaCl 2 and 44CaCl 2) and bromide ( 40CaBr 2) complexes of 4,13-diaza-18-crown-6 (DA-18C6) were investigated using IR and Raman spectroscopy and normal coordinate analysis (NCA). Calcium isotopic substitution was used to determine the valence force field of the DA–18C6–CaCl 2 complex ( C 2h symmetry). Practically no change in the vibrations of the macrocyclic ligand were observed upon complex formation, indicating the absence of conformational distortion.