Simple Valence Research Articles

A transferable force field for a series of molecules containing CC groups

A simplified valence force field has been developed for some molecules containing the CC group by using the overlay technique. In constructing the force field special attention was paid to the transferability of the force constants since they are intended to be used in large molecules. The force field contains 57 parameters out of which 22 are transferred from other overlay force fields and 35 are optimized using 200 experimental frequencies. The root-mean-square frequency error is 10.5 cm −1 which includes the CH strecthing vibrations.

Raman cross section measurements in the visible and ultraviolet using an integrating cavity: Application to benzene, cyclohexane, and cacodylate

A new technique has been developed for the measurement of absolute Raman scattering cross sections. The Raman scattering of a sample in an integrating cavity is compared to the elastic scattering from a reference scatterer of unit yield. The fundamental advantage of this approach is that it permits the measurement of absolute cross sections throughout the visible and UV regions with a simple experimental apparatus and procedure. The 992 cm−1 mode of benzene was measured in the visible region and compared satisfactorily to the values obtained by Kato and Takuma [J. Chem. Phys. 54, 5398 (1971)] and Abe et al. [J. Raman Spectrosc. 6, 38 (1977)]. The scattering cross section of the 608 cm−1 mode of sodium cacodylate, an aqueous intensity standard, was measured from 647 to 240 nm. Analysis within the A-term formalism provided a functional form for interpolation between measured points and revealed that the 608 cm−1 cacodylate mode derives intensity from states in the vicinity of 83 kK (kilokaiser). The 802 cm−1 skeletal mode of cyclohexane was measured over the same spectral region to provide a nonaqueous intensity standard. In this case the principal contribution to the preresonance scattering arises from electronic states near 115 kK and not from the lowest allowed σ → σ* (CC) states. To compare scattering mechanisms, the CH stretching modes of cyclohexane were referenced to the 802 cm−1 mode. The CH stretching modes of cyclohexane derive A-term intensity from the strongly allowed σ → σ*(CH) state near 85 kK, as expected from simple valence arguments.

Interpretation of DNA vibration modes: I--The guanosine and cytidine residues involved in poly(dG-dC).poly(dG-dC) and d(CG)3.d(CG)3.

A normal coordinate analysis has been carried out on guanosine and cytidine residues appearing in oligo and polynucleotides by using a simplified valence force field that allows the vibrational spectra of 5'-dGMP and 2'-deoxycytidine molecules to be reproduced. The role of both C2'-endo and C3'-endo conformations on sugar pucker, as well as that of glycosidic torsion angle (X), on several characteristic vibration modes of these residues have been studied. The present calculations based on a non-redundant set of internal coordinates preserving the harmonic approximation of the potential field, allows us to explain quite satisfactorily the modifications of the vibrational spectra in the 1550-1250 cm-1 and 785-500 cm-1 regions, when the right----left-handed conformational transition occurs.

XPS spectra within a simple intermediate valence model with plasmons

The XPS spectra from the core level of an intermediate valence impurity in the necklace of a finite number (from one to six) of atoms are considered. We discuss the changes in the system caused by the core hole which is created in the photoemission process, comparing the average numbers of f-electrons before and after the experiment. The dependence of the shape of the core spectra on the electron-plasmon coupling λ is considered as well. Inclusion of the larger number of host atoms in the necklace leads to quantitative changes in the spectra only. The qualitative changes are not observed in practice.

A simple electron gas treatment of the magnetic susceptibility tensor of the lowest 3Σ+u state of H2

The incremental diamagnetic susceptibility tensor of the 3Σ+u state of H2 is obtained using an electron gas theory. This state of the hydrogen molecule is a model for the ground state of a system of interacting closed shell species. Because the energy functional is a gauge independent functional of the electron density, there is no separation of the susceptibility tensor into paramagnetic and diamagnetic portions. The results agree in sign, but not magnitude, with a simple valence bond calculation using London orbitals.

3-Chloro-1-propene (allylchloride): gas-phase molecular structture and conformation as determined by electron diffraction

A gas-phase study of 3-chloro-1-propene by electron diffraction showed that the most abundant conformer was gauche with a torsional angle τ=120.8(5.4)°, relative to τ = 0 for syn from when all heavy atoms are in the same plane and the chlorine atom is eclipsing the double bond. At 20 and 90°C the amount of the gauche conformer was found to be 82(9) and 78(15)%, respectively. The least-squares refinements of the molecular structure were assisted by a few constraints taken from ab intio calculations. At 90°C the following gauche structure was obtained in terms of r a and L α parameters: r(C=C) = 1.341(5) Å, r(=CC) = 1.506(6) Å, r(CCl) = 1.803(5) Å, ( r(CH)) = 1.081(17) Å, LC=CC = 122.6(1.3)°, LCCCl = 110.0(1.0)°, LClCH = 106.6(2.4)°. Error limits are given as 2σ (σ includes uncertainty due to correlation and systematic errors in the data). The syn form was found to have 2 large root-mean-square torsional amplitude, 20–25°. In connection with the structure analysis a relatively simple valence force field was developed.

The vibrational spectra and molecular structure of 3,3′-dibromo-, 3,5,4′-tribromo- and 3,5,3′,5′-tetrabromobiphenyl

The i.r. spectra of the title compounds were recorded between 4000 and 200 cm −1 as melts and as polycrystalline solids between CsI plates. Pellets in KI and polyethylene matrices were studied in the regions 4000-200 and 500-30 cm −1, respectively. Raman spectra of the compounds as crystalline solids were obtained, 3,3′-dibromobiphenyl was also studied as a melt, whereas 3,5,4′-tribromobiphenyl was investigated as a solute in CS 2 and C 6H 6. solutions. The spectra indicate that the three brominated biphenyls are nonplanar in the melt, in solution and in the crystalline state, as independently observed in the vapour phase by electron diffraction technique. A few i.r. and Raman bands observed in the melt and in solution of 3,3′-dibromobiphenyl, vanishing in the crystal spectra, may suggest a second conformer in the melt. Force constant calculations have been carried out, adopting a simple valence force field derived for condensed aromatics and previously applied to biphenyl. Tentative assignments of practically all the fundamentals in the title compounds were made, based upon the Raman polarization measurements, i.r. dichroism and the results of the force constant calculations.

Vibrational spectra of C 6H 5CHCl 2

This paper presents a vibrational assignment for C 6H 5CHCl 2 based essentially on spectral comparisons, in the series of α-chlorinated toluenes, with a few deuterated analogues. A normal coordinate analysis, performed with a simplified valence force field, asseses the extent of vibrational splitting caused by the CH | and CH ⊥ conformations of the CHCl 2 group. The relative importance of geometrical and force field parameters on the internal rotation of this group is considered. Upper limits to the rotational barriers of mono-, di- and trichloromethylbenzene are also calculated and discussed.

ChemInform Abstract: ELECTRONIC STRUCTURE AND FORCE CONSTANTS OF THE DITHIONITRONIUM CATION NS2‐

Abstract The standard GF method was employed to determine the valence force constants of the dithionitronium cation (NS 2 + ), using the experimental IR and Raman frequencies. The calculation force constants ( f NS = 8.256, f NS,NS = 0.663 and f SNS = 0.236 mdyn A −1 ) reproduce well the 32 SN 34 S + frequencies and suggest a bond order greater than 1, as well as a delocalization of the π electrons in the trinuclear system. CNDO/2 calculations reveal positive charges on sulfur atoms, contrary to simple valence electron count rules. The CNDO/2 preferred geometry is D ∞h rather than a C 2V cyclic structure, in agreement with the X-ray diffraction data. A very important S⋯S interaction has been found.

Electronic structure and force constants of the dithionitronium cation NS 2+

The standard GF method was employed to determine the valence force constants of the dithionitronium cation (NS 2 +), using the experimental IR and Raman frequencies. The calculation force constants ( f NS = 8.256, f NS,NS = 0.663 and f SNS = 0.236 mdyn Å −1) reproduce well the 32SN 34S + frequencies and suggest a bond order greater than 1, as well as a delocalization of the π electrons in the trinuclear system. CNDO/2 calculations reveal positive charges on sulfur atoms, contrary to simple valence electron count rules. The CNDO/2 preferred geometry is D ∞h rather than a C 2V cyclic structure, in agreement with the X-ray diffraction data. A very important S⋯S interaction has been found.

Calculation of low vibration frequencies of α-helix B 9—B 19 of insulin

Using a simplified model consisting of only two masses for each peptide residue and a simple valence force field we have calculated the low skeletal frequencies of α-helix B 9—B 19 of the insulin molecule; we have also treated the influence of the values of hydrogen bond force constants on that of frequencies. Although there is a lack of experimental data in the low frequency region, a few values of our calculated frequencies agree well with the experimental Raman lines.

Estimating the hessian for gradient-type geometry optimizations

Optimization methods that use gradients require initial estimates of the Hessian or second derivative matrix; the more accurate the estimate, the more rapid the convergence. For geometry optimization, an approximate Hessian or force constant matrix is constructed from a simple valence force field that takes into account the inherent connectivity and flexibility of the molecule. Empirical rules are used to estimate the diagonal force constants for a set of redundant internal coordinates consisting of all stretches, bends, torsions and out-of-plane deformations involving bonded atoms. The force constants are transformed from the redundant internal coordinates to Cartesian coordinates, and then from Cartesian coordinates to the non-redundant internal coordinates used in the specification of the geometry and optimization. This method is especially suitable for cyclic molecules. Problems associated with the choice of internal coordinates for geometry optimization are also discussed.

Spectroscopic investigation of organosilicon polymers. I. Vibrational analysis of poly(tetramethyl-p-silphenylene-siloxane)

This article is a detailed, normal coordinate analysis of an isolated chain of poly(tetramethyl-p-silphenylene-siloxane), based on results obtained for the corresponding monomer and a recent x-ray structure. Predicted vibrational frequencies are compared with new experimental infrared (IR) and Raman data obtained from powder and well oriented films. Vibrational spectra were also obtained for a deuterio derivative in which all hydrogens in the benzene rings were replaced with deuterium atoms. The results indicate that an intermode mixing occurs between vibrations of the ring and the saturated part which lead to low frequency normal modes characteristic of the chain. The simplified valence force field obtained for this polymer should provide a useful tool for understanding the vibrational spectra of other members of the class of polysiloxanes.

Crystal chemistry and classification of multinary semiconductors

On the basis of the simple valence rules and the geometrical Aufbau principle governing compositions and crystal structures of multinary semiconductors we have reviewed their crystal chemistry. We have discussed some chemical trends observed in them, which, although qualitative only, can provide interesting insights into the relation between bands and bonds in solids. We have also dwelled upon the recent development in structural sorting. Together with our rapidly growing knowledge of the electronic structure of solids, sorting may well contribute towards the solution of a hitherto unresolved problem: the theoretical prediction of the structure and the stability of phases as functions of chemical composition.

A theoretical investigation of molecular NaOH

GAUSSIAN-80 calculations utilizing a 6–21G basis with MP3 and CID electron correlation included show molecular NaOH to be a linear molecule with R OH = 0.9805 Å and R ONa = 1.8687 Å. The equilibrium rotational constant B eee is 13,590 MHz. A simple valence force field calculation is provided.

Lattice dynamics of polyacetylene as a one-dimensional system: Part I. The case of the trans perfect polymer

In this paper the lattice dynamics of trans-polyacetylene as a perfect one-dimensional crystal is treated. Dispersion curves, density of states and vibrational displacements are calculated. The results of the calculations are used to propose a vibrational assignment for (CH) x , (CD) x and ( 13CH) x . Calculations have been made with a quadratic simplified valence force field (SVFF). The relationship between SVFF and the electron-phonon coupling model for polyacetylene, generally adopted in the literature, is discussed. Comparison with the calculations of other authors is made.

The influence of the parallel magnetic field on the interband optical absorption in thin films

The effect of a parallel magnetic field on the interband optical absorption in size-quantized semiconductor layers is discussed on the basis of the effective mass approximation. The case of the simple and degenerate (germanium-like) valence band is considered.

An orthogonal orbital MCSCF plus CI treatment of molecular symmetry breaking. I. The lowest 3B2 and 3A2 states of CO2

A general, orthogonal orbital, multiconfuguration self-consistent field (MCSCF) plus configuration interaction (CI) procedure has been developed for treating open shell molecules subject to symmetry breaking (doublet instability, etc.) in the restricted Hartree–Fock approximation. The chemical origin of symmetry breaking was analyzed using a simple valence bond picture and the lowest 3A2 and 3B2 states of CO2 as examples. This analysis provided the necessary information to design MCSCF wave functions which possessed the symmetry of the nuclear frame, and changed smoothly between high and low symmetry nuclear configurations. Near qualitative potential energy surfaces near the potential minima of the two lowest triplet states of CO2 were obtained by CI calculations using the MCSCF orbitals and reference configurations. Both states were found to have C2v potential minima with the 3A2 state lying approximately 0.9 eV above the 3B2 state. The equilibrium CO bond lengths and OCO bond angles are 1.24 Å, 118° and 1.26 Å, 127° for the 3B2 and 3A2 states, respectively.

Electrical properties of narrow gap semiconductor PtSb 2

Results of measurements of conductivity, Hall and Seebeck coefficients of tellurium doped n-type crystals of platinum antimonide are presented. The Hall coefficient and the Seebeck coefficient undergo sign inversion twice, below and above room temperature. The detailed analysis of the experimental results revealed that below 200 K PtSb 2 can be described by a simple conduction and valence band model. The energy gap E g = (110−0.15 × T) (meV), the electron conductivity mass m n c / m 0 = 0.35, acoustic phonon limited electron mobility 〈μ a〉 n = 3 × 10 6 T − 3 2 ( cm 2 V · s ) and mobility ratio 〈μ a〉 n/〈μ a〉 p = 0.4 are determined. However, at higher temperatures a more complicated valence band model is needed to account for the experimental results. The arguments for the existence of subsidiary valleys in the valence band are presented.

Normal coordinate analysis of some 2,3-dihalo-1-propenes

Normal coordinate calculations, using relative simple valence force fields (29 force constants), have been carried out for the molecules 2-bromo-3-chloro-1-propene, 2,3-dichloro-1-propene and 2,3-dibromo-1-propene. The valence force fields of 2- and 3-halosubstituted propenes showed good transferability for the gauche conformers, torsion angle τ = 120° (τ = 0°, anti, when the C sp 2-halogen bond edipses the double bond). The average deviation between gauche calculated and observed frequencies was 10–15 cm −1 without adjustment of force constants. For the anti conformers, the C sp 2-halogen and C sp 3-halogen stretches were highly mixed in the normal modes. This required the introduction of interaction force constants which are different from the gauche ones, and also different from those present in the 3-halo-1-propene force fields. Altogether, this may be described as an increase in vibrational coupling between the saturated and the unsaturated parts of the molecule when passing from gauche to anti conformer. Relative values of torsional force constants as obtained from molecular mechanics calculations reproduced the observed gauche torsional frequencies very well.