Valence Force Field Research Articles

A theoretical study of vibrational spectra of tetranitrate-methyl-β-D-glucopyranoside and 2,3-di-O-nitro-methyl-β-D-glucopyranoside

Coupled calculations were carried out of normal vibration frequencies from the point of view of the valence-force field scheme and of absolute IR band intensities by the CNDO 2 method for the tetranitrate-methyl-β-D-glucopyranoside molecule. A good agreement was achieved with the experiments. Normal coordinate analysis was made for 2,3-di-O-nitro-methyl-β-D-glucopyranoside molecule with force constants obtained for the tetra-nitrate-methyl-β-D-glucopyranoside. Before proceeding to the calculation of the spectra of such complex molecules of nitrates of monosaccharides, a complete experimental and theoretical investigation was performed of the vibrational spectra of methylnitrate, which made it possible to determine the deficit of force constants for the calculations of the spectra of nitrosubstituted glucopyranosides. Detailed interpretations of the observable IR spectra of both the nitro-glucopyranosides compounds considered are given. The absorption spectra sensitivity to the spot of the nitrate group localization was discovered. Special attention was focused on analyzing the spectra of nitrates of saccharides for the characteristic split of the band due to the asymmetric stretching vibrations of the ONO 2 groups in the region of 1600–1700 cm −1.

Structure and bonding in cerium oxysulfide compounds. III—Electronic, infrared and resonance Raman spectra of and lattice dynamics calculations on mixed‐valence Ce4O4S3

The electronic (350–900 nm), infrared and Raman (700–30 cm-1) spectra of polycrystalline samples of the mixed-valence (Ce3+/Ce4+) oxysulfide compound Ce4O4S3 were investigated. The compound is a low-gap semiconductor with intervalence transitions in the near-infrared (ca. 850 nm) and visible (ca. 540 nm) regions, so that resonance-enhanced Raman spectra were obtained in the 450–650 nm range. From some polarized Raman data and a comparison with the vibrational results for Ce2O2S and Ce2.0O2.5S reported in Parts I and II, tentative infrared and Raman assignments are proposed. These results allow one mainly to localize the totally symmetry Ag modes and to suggest that the higher wavenumber signals in the 580–410 cm-1 range are due to the stretching vibrations of the shortest Ce4+—O bonds. In addition, complete lattice dynamic calculations on the orthorhombic structure (D2h9, Z=2) of Ce4O4S3 were performed by using a valence force field potential function transferred from the force fields previously obtained for Ce2O2S and Ce2.0O2.5S. With these calculations one can satisfactorily reproduce the whole experimental wavenumbers, propose more confident vibrational assignments and confirm that the Ce4+—O bond strengths are definitively stronger than the Ce3+—O counterparts. Such conclusions are corroborated by the establishment of the Raman excitation profiles which maximize near 568 nm. The largest enhancements are obtained for the totally symmetric modes at 509, 412 and 350 cm-1, which contain important potential energy distributions in the Ce4+–O stretching force constants, and at 132 cm-1, which corresponds to a deformation or compressional motion (along b) within the (Ce4O4)6+ units. These vibrations are thus the more effective coupling modes in the intervalence charge-transfer mechanisms. © 1997 John Wiley & Sons, Ltd.

Laser Raman and IR spectra and force fields for 2,6-difluorobenzonitrile

Raman spectrum of 2,6-difluorobenzonitrile in the powder form has been recorded in the region 50–4000 cm −1 on a Jasco K-500 spectrophotometer using the 488.0 nm radiation from an argon laser. IR spectra in the region 200–4000 cm −1 have been recorded in KBr pellet and nujol mull on a Nicolet DX spectrometer and in liquid and vapour phases on a Jasco FTIR/7000 spectrometer. Using these observed frequencies force field calculations for the planar and non-planar modes have been carried out assuming a general valence force field.

Fourier-transform Raman and infrared spectra and normal coordinate analysis of the triphenyl compounds and their methyl-, methoxy- and fluoro-substituted derivatives of arsenic, antimony and bismuth

The Fourier transform (FT)-Raman and infrared (IR) spectra of triphenyl-, perdeuterated triphenyl-, tris(2-methylphenyl)-, tris(3-methylphenyl)-, tris(2,4,6-trimethylphenyl)-, tris(2-methoxyphenyl)-, tris(3-methoxyphenyl)-, tris(3-fluorophenyl)- and tris(4-fluorophenyl)-arsine, -stibine and -bismuthine were measured in the range 3600-100 cm −1. A normal coordinate analysis was performed for all substances using a modified valence force field. This leads to an unique force field for these compounds, where only the metal depending force constants vary noticeably. It was also possible to find a relationship between the chemical nature and the position of the different substituent and the metal-carbon stretching force constant. For all substances 1H-NMR, 13C-NMR and MS data were also given.

Electronic properties of carbon nanotubes containing defects

The electronic structure of carbon nanotubes containing defects is investigated within a tight-binding framework. The electronic properties of the following defects are examined: (a) a hemispheric cap ending a (10, 10) tubule, (b) a pair of pentagon and heptagon aligned along the same side of the tube, which widens the diameter, (c) a pair of diametrically-opposed heptagon and pentagon, which bends the structure, and (d) sp 3-like lines responsible for polygonization. In cases (a)–(c), a structural optimization was performed, either by minimization of a valence-force field adapted to the carbon network or by tight-binding molecular dynamics.

Cephalosporin 3'-phloroglucide esters and 7-(phloroglucidamido) cephalosporins as novel antibacterial agents.

Two series of new phloroglucide derivatives were synthesized that possessed antibacterial activities. The first series includes cephalosporin 3'-phloroglucide esters 19 and 20, which were obtained by condensation of cephalosporin 16 with bioactive phloroglucides 14 and 15, respectively. They exhibited a dual mode of antibacterial action. In comparison with cephalosporins 26 and 27, bearing an acetoxy unit at the C-3' position, the bifunctional cephalosporins 19 and 20 showed a broadened spectrum of activity. Results from the consistent valence force field (CVFF) calculations indicate that the most stable conformational isomer of phenolic acid 14, holding a cis-syn-syn geometry, possessed a cavity. It provides an ideal environment to accommodate metal ions of holoenzymes. Phenolic keto acid 15, however, possessed a trans-anti-syn conformation, which allowed chelation between metal ions and the phenolic hydroxyl groups as well as the carbonyl functionalities. Our biological results show that the cavity formed in phloroglucides plays an important role. The second series includes 7-(phloroglucidamido)cephalosporins 24 and 25, which were synthesized by condensation of cephalosporin 21 with 14 and 15, respectively. Results from the CVFF calculations indicate that cephalosporin 24 also possessed a cavity. Unlike cephalosporin 3'-phloroglucide esters 19 and 20, cephalosporins 24 and 25 were found resistant to beta-lactamases from Staphylococcus aureus 95 and Pseudomonas aeruginosa 18S-H. These new compounds, however, showed notable activities against S. aureus FDA 209P, S. aureus 95, Candida albicans, P. aeruginosa 1101-75, and P. aeruginosa 18S-H.

Half-Crystal Energy and Enthalpy of Sublimation of Diuron: 3-(3,4-Dichlorophenyl)-1,1-dimethylurea

The enthalpy of sublimation of diuron, 3-(3,4-dichlorophenyl)-1,1-dimethylurea, C 9 H 10 ON 2 Cl 2 , was measured at 393 K using a Tian-Calvet calorimeter. The corrected experimental value Δ sub H m o (298 K) = 133.87(74) kJ mol -1 was then compared to the half-crystal energy E k . This energy was calculated from structural data using the 'consistent valence force field', which consists of a 6-12 Lennard-Jones potential and a Coulomb term. The disagreement between Δ sub H m o (298 K) and E k = 152.6 kJ mol -1 is about 14%. It can be partially attributed to the inaccuracy of the atomic coordinates of the crystal structure and to the charges of the force field. For simple molecules such as normal octadecane (n-C 18 H 38 ), hexanedioic acid (C 6 H 10 O 4 ), diphenylmethanone (C 13 H 10 O) and anthracene (C 14 H 10 ), the agreement is very good, about 3%.

Strain distribution and electronic spectra of InAs/GaAs self-assembled dots: An eight-band study

Strained epitaxy has been shown to produce pyramidal-shaped quantum dot structures by single-step epitaxy. In this paper we examine the strain tensor in these quantum dots using a valence force field model. We use an eight-band $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ formalism to find the electronic spectra in the highly strained dots. Results obtained for the conduction-band spectra using the effective-mass approach are shown to have serious errors. This is particularly true for excited states in the conduction band. The dependence of the electronic spectra on the quantum dot size and shape is also reported along with comparisons with published experimental results.

Phonons in silver gallium diselenide

The dispersion curves along the directions [100] and [001] of have been measured by inelastic neutron scattering. The results are analysed with different rigid-ion models: Born - von Karman and valence force field models, which give a good agreement with experiments. Additional measurements of the acoustical branches allow us to determine elastic stiffness constants which are () , , , , , . The compressibility calculated from these constants is in accordance with the experimental value.

Kristallstrukturen, Schwingungsspektren und Normalkoordinaten- analysen von cis- und trans-[OsX2(acac)2], X = Cl, Br, I / Crystal Structures, Vibrational Spectra and Normal Coordinate Analyses of cis- and trans-[OsX2(acac)2], X = Cl, Br, I

The crystal structures of trans-[OsCl2(acac)2] (triclinic, space group P1̄, a = 7.4114(5), b = 7.6419(9), c = 7.9944(6) Å, α = 62.692(7), β = 87.687(6), γ = 60.667(6)°, Z = 1), trans-[OsBr2(acac)2] (triclinic, space group P1̄, a = 7.467(3), b = 7.621(3), c = 8.260(3) Å, α = 115.03(3), β = 92.78(3), γ = 117.91(3)°, Z = 1), cis-[OsCl2(acac)2] (monoclinic, space group C2/c, a = 13.8532(13), b = 7.7990(8), c = 13.6202(12) Å, β = 108.375(10)°, Z = 4) and cis-[OsBr2(acac)2] (monoclinic, space group C2/c, a = 13.944(2), b = 8.0347(13), c = 13.743(2) Å, β = 106.757(12)°, Z = 4) have been determined by single crystal X-ray diffraction analysis. To enhance the spectroscopic resolution, the IR and Raman spectra of the six complexes have been measured at low temperature (10 K). Using the molecular parameters of the X-ray determinations normal coordinate analyses based on a modified valence force field have been performed for trans-[OsCl2(acac)2], trans-[OsBr2(acac)2] and cis-[OsCl2(acac)2], and the normal modes of vibration assigned. With sets of 31 or 32 force constants, taking into account the innerligand vibrations, a good agreement between observed and calculated frequencies has been achieved. The valence force constants for the X-Os-X axes are fd(OsCl) = 1.81, fd(OsBr) =1.61 mdyn/Å, and for the Cl′-Os-O* axis are fd(OsCl’) = 1.94, fd(OsO*) = 2.81 mdyn/Å and fd(OsO) ranges from 3.27 to 3.31 mdyn/Å.

Vibrational analysis and spectra of cytidine 3′-monophosphate (3′-CMP)

The FTIR and FT-Raman spectra of cytidylic acid b (cytidine 3′-monophosphate, 3′-CMP) and its hexadeuterated derivative (deuteration in the amino and hydroxyl groups) in the solid phase and aqueous solutions have been assigned on the basis of normal coordinate analysis. This was carried out using a valence force field based on force fields for simpler molecules and modified for 3′-CMP, as well as applying the semiempirical PM3 method. Assignments are discussed in the light of observed and calculated wavenumbers and isotopic shifts. The wavenumbers calculated with a large set of force constants result in lower deviation than the wavenumbers calculated by the PM3 method. The results obtained indicate vibrational coupling between the cytosine and ribose moieties of the nucleotide for some frequencies below 1400 cm −1.

Shallow electron states of bounded intrinsic stacking faults in silicon

The electronic structure of bounded intrinsic stacking faults in silicon is studied. Especially the influence of the stacking fault width on the electronic states in the band gap is investigated. The extended defect studied comprises an intrinsic stacking fault with two reconstructed 90\ifmmode^\circ\else\textdegree\fi{} partials as boundaries. The atomic structure is determined by different valence force fields. These are the Keating potential, the bond-charge model, and an anharmonic version of the bond-charge model. The electronic structure is calculated by linear combinations of atomic orbitals. Ten Gaussian-type atomic orbitals of s-, p-, and d-type are used, and up to fourth-nearest-neighbor interactions are taken into account. The levels in the band gap are evaluated by the recursion method for nonorthogonal basis functions, and by a continued fraction representation of the local density of states.

Vibrational spectra and molecular dynamics study of the calcined (Na +, K +) EMT hexagonal forms of faujasitic zeolites

The Raman scattering and infrared absorption spectra of dehydrated bare NaEMT and KEMT zeolites were recorded in the mid- and low-frequency region. Molecular dynamics calculations based on a simplified general valence force field, the cation framework and cation—cation interactions reproduced the main structural and dynamic properties of the Na 20EMT and K 20EMT zeolites. The flexibility of the inner void space was studied by considering the diameter and area of the windows connecting two adjacent medium or large cages.

Mean-square displacements of atoms in hexaborides

High-resolution neutron powder and single-crystal X-ray diffraction studies has been used to investigate the temperature dependencies of atomic mean-square displacements in rare-earth hexaborides. The experimental data on elastic constants and phonon frequencies have been used to improve the valence force field parameters for SmB 6 and the temperature dependencies of atomic mean-square displacements in SmB 6 is compared with theoretical values. The correlation between ionic radii and mean-square displacements of rare-earth atoms are shown.

Ab initio force field for aluminosilicates; molecular dynamics simulation of the infrared spectra of zeolites

A generalized valence force field for modelling the vibrational spectra of aluminosilicates is developed on the basis of ab initio calculations. The calculated frequencies in the infrared spectra of zeolites are in good agreement with the experimental measurements. The force field also explains the main features in the vibrational spectra of OH(OD) groups of protonated structures.

A normal mode analysis of form I syndiotactic poly(1-butene)

A normal mode analysis of syndiotactic poly(1-butene) of form I having (TTGG) 2 conformation was made in accordance with the valence force field derived by Holland-Moritz for isotactic poly(1-butene) of form I. The assignments were made successfully using polarized infrared and Raman spectra. Based on the assignments, spectral characters in the melt and the low temperature were investigated. The infrared and Raman spectra did not change in the melt but the CCH bendings of the spectra in the low temperature were sensitive to the conformation.

Force Fields for C&24XXXw;N-Substituted Benzenes. II—Planar and Non‐Planar Modes of the Three Isomeric Dicyanobenzenes

Planar and non-planar force field calculations for o–, m– and p–dicyanobenzenes were carried out using published data on the parent molecule and one isotope in each case. General valence force fields were determined using 61 (43 planar and 18 non-planar), 61 (43 planar and 18 non-planar) and 58 (41 planar and 17 non-planar) parameters for the o–, m– and p–isomers respectively. It was found that the earlier proposed assignments for some of the wavenumbers need to be revised. © 1997 by John Wiley & Sons, Ltd.

Semi-empirical calculation and normal coordinate study of the conformation and electronic and vibrational spectra of acrolein

Detailed quantum chemical and normal coordinate calculations have been conducted on acrolein and its deutero derivatives to understand the structural and spectroscopic characteristics of its rotational isomers, in particular the cis isomer. The results of molecular orbital studies in the MNDO (using the modified AM1 Hamiltonian), CS-INDO and CNDO approximations on potential energy curves, rotational barrier heights, conformations and physico-chemical properties of the ground and first excited ( nπ ∗ ) electronic states of acrolein are reported. Using multielectron configuration interaction based on the subroutine CIPSI, the electronic transitions have been explained. Generalized valence force fields have been obtained for the trans and cis conformers of acrolein and their monodeutero derivatives. A complete assignment of the in-plane and out-of-plane vibrational modes has been provided. It is inferred that while in the ground state the trans conformer of acrolein is more stable than the cis conformer. the reverse is the case in the first excited electronic state. In addition, a gauche conformer (Φ = 90°) may also exist in the excited state.

Force Fields for C&24XXXw;N-Substituted Benzenes. II—Planar and Non-Planar Modes of the Three Isomeric Dicyanobenzenes

Planar and non-planar force field calculations for o–, m– and p–dicyanobenzenes were carried out using published data on the parent molecule and one isotope in each case. General valence force fields were determined using 61 (43 planar and 18 non-planar), 61 (43 planar and 18 non-planar) and 58 (41 planar and 17 non-planar) parameters for the o–, m– and p–isomers respectively. It was found that the earlier proposed assignments for some of the wavenumbers need to be revised. © 1997 by John Wiley & Sons, Ltd.

Ab-initio-Rechnungen, Schwingungsspektren und Normalkoordinatenanalyse von closo-[B6H5F]2– / ab initio Calculations, Vibrational Spectra, and Normal Coordinate Analysis of closo-[B6H5F]2–

Abstract The structural parameters of closo-[B6H5F]2- with C4v symmetry have been determined by MP2/6-31G* optimization. They reveal typical B-F and B-B bond lenghts of 143.4 and 172.5 -173.9 pm, respectively. A frequency analysis at the same theoretical level has been performed. The vibrational spectra of [B6H5F]2- labelled with 10B, 11B and D have been additionally assigned by normal coordinate analysis based on a modified valence force field using the ab initio structure parameters. With a set of 10 force constants (e.g. fd(BB)=1.92, fd(BF)=5.25 mdyn/Å) a good agreement between observed and calculated frequencies can be achieved. The results correspond with the scaled (Fscale=0.97) ab initio frequencies and assignments.