Dipole moment derivatives,polar tensors and effective atomic charges for the dihalogenomethanes,CH2X2 (X = F,CL, Br)

Dipole moment derivatives have been calculated for difluoro-, dichloro- and dibromomethane using the approaches outlined in an earlier publication [1]. Atomic polar tensors and effective atomic charges for the “best” sign combination are also presented for these molecules.     

Interpretation of vibrational absorption intensities: Effective bond charges from rotation free atomic polar tensors

In the present paper a new theoretical framework for analysis of vibrational intensities is presented. In a mathematically straightforward procedure corrected for various rotational contributions, atomic polar tensors are transformed into quantities termed effective bond charges. All rotational contributions to dipole moment derivatives are appropriately considered and eliminated from the equations. The effective bond charges are expected to reflect in a generalized manner polar properties of the valence bonds in molecules. Aside from the usual harmonic approximation no other constraints are imposed on the dipole moment function. The application of the formulation developed is illustrated with calculations employing atomic polar tensors for H₂O, NH₃, CH₄, C₂H₄, C₂H₂ and CH₂O obtained from RHF/6-31G** ab initio calculations.     

The infrared intensities and polar tensors of the fluorochloromethanes

The polar tensors of CF3Cl, CF2Cl2 and CFCl3 have been calculated using recent measurements of their gas phase infrared fundamental intensities. The polar tensors obtained for CF2Cl2 and CFCl3 are in very good agreement with those obtained previously since the more recent experimental intensity results are in good agreement with those reported earlier. For CF2Cl2 rhoC = +1.626, rhoF = -0.577 and rhoCl = -0.26e whereas rhoC = +1.369, 0.478 and rhoCl = 0.297e for CFCl3. However, two sets of significantly different mean dipole moment derivatives are obtained from the experimentally measured intensities of CF3Cl reported by two different laboratories. On the other hand, the differences in the mean derivatives of these two sets are not large enough so that results from electronegativity models, potential models for core ionization energies and quantum chemical calculations at the Moller-Plesset 2 and B3LYP density functional levels are sufficient to indicate which set is the correct one. As such average values of rhoC = +1.907+/-0.178e, rhoF = -0.590+/-0.056e and rhoCl = -0.139+/-0.013e obtained from both sets of polar tensor elements are recommended for the CF33Cl mean dipole moment derivatives.     

The infrared vibrational intensities and polar tensors of HFCO and DFCO

The infrared vibrational intensities of HFCO and DFCO have been calculated at the B3LYP/cc-pVTZ, MP2(FC)/6-311++G(3d,3p) and QCISD(FU)/aug-cc-pVTZ levels. All calculations predict the isotopomers to have identical intensity sums, within about 1 km mol(-1). This is in contrast with experimental intensity sum results reported in the literature. Dipole moment derivative directions calculated by the three methods are in excellent agreement for all in-plane normal coordinates. All the theoretical polar tensor elements are also in good agreement with each other having standard deviations varying between 0.003 and 0.043 e. The oxygen and fluorine atoms have negative mean derivatives (approximately -0.6 e), whereas the carbon mean derivative is very positive (approximately +1.1e) and the hydrogen one is almost zero (approximately +0.03 e). The HFCO theoretical intensity sums calculated by all three methods as well as their carbon and oxygen mean dipole moment derivatives are in good agreement with those estimated from the experimental intensities and atomic mean derivatives of H2CO and F2CO.     

Vibrational infrared intensities and polar tensors of 1,2-difluoroethylenes

The polar tensors of cis and trans-1,2-difluoroethylenes have been determined with new normal modes based in a reassignment of nu(7) and nu(12) bands of the trans isomers and frequency values corrected for Fermi resonances and phase shifts. The signs of the dipole moment derivatives (and its directions, for B(u) symmetry species) were considered to be those of MP2/6-31G** estimates. Root mean square errors calculated for the new tensor element values from each pair of isotopomers (trans-1,2-C(2)H(2)F(2)/trans-1,2-C(2)D(2)F(2) and cis-1,2-C(2)H(2)F(2)/cis-1,2-C(2)D(2)F(2)) show that the new polar tensor sets fit the isotopic invariance criterion better than previously reported sets. The accuracy of polar tensor transference procedures was tested by calculating the infrared intensities of trans-1,2-C(2)H(2)F(2) through the new polar tensors of the cis isomer. The resulting estimates are very accurate and also support the new band assignment, though the A(6) intensity remains still somewhat underestimated.     

The charge-charge flux-overlap model for the interpretation of atomic polar tensors and infrared intensities

The interpretation of infrared intensities using the concepts of atomic polar tensors divided according to contributions from charge-charge flux-overlap (CCFO) terms appears in general to be quite useful. Here we shall illustrate this analysis for the F and H atoms in chemically related molecules to show how their properties vary for CF, CH, OF and OH bonds as R varies in RCF, RCH and ROH molecules.     

The electronic structure of conjugated systems with large effective charges

Pariser-Parr-Pople (PPP) calculations have been carried out for the sydnone, sydnoneimine, and sydnoneimine cation molecules. Different possibilities of selecting Coulomb two-electron integrals _v were examined. Characteristics of the electronic structure of the five-membered rings and of the exocyclic bonds of the computed molecules were examined. The energies of the lower excited states were calculated by the method of interaction of a finite number of configurations. It was shown that the energy of the lowest triplet level of these molecules must lie close to the energy of the ground state. To some extent this fact explains the considerable reactivity of the given compounds.     

Interaction potentials, structural ordering and effective charges in dispersions of charged colloidal particles

As colloidal dispersions of charged particles exhibit a wide variety of commercial, technological and scientific applications, a considerable theoretical effort has been devoted to finding an effective interaction potential from primitive models. The forces derived from this potential should justify the spatial ordering experimentally observed under certain conditions. This paper reviews the advances in these theoretical studies as well as some experiments (based on the mentioned order) that try to corroborate them. Special attention has been paid to the Derjaguin-Landau-Verwey-Overbeek (DLVO) potential. Nowadays, many of these theoretical investigations suggest that it could be applied if some of its parameters are renormalized. Nevertheless, to achieve a renormalization procedure in a strict way (from a primitive model) is a difficult task as a result of the size and charge asymmetries between small ions and macroions. Thus, several procedures for computing renormalized charges in a simple way have been developed. However, the notion of effective charge has also been widely used (as a adjustable parameter) in order to justify results found for several kinds of colloids (like solid particle dispersions or micellar systems) by means of quite different experimental techniques. Renormalization (as well as ion condensation) approaches, experiments and the controversial relationship between theoretical and phenomenological effective charges are also reviewed in this work.     

New Method for calculating effective charges on atoms in molecules, clusters, and solids

A procedure for calculating effective charges on atoms in an arbitrary polyatomic system has been developed on the basis of the one-electron DV cluster method. Effective charges are calculated as three-dimensional integrals of electron density over the space around atomic nuclei in the object under investigation. An original procedure is suggested to assign the charge density to a particular center at every point of space. Translated fromZhurnal Struktumoi Khimii, vol. 39, no. 6, pp. 1134–1140, November–December, 1998.     

The conjugation and effective charges of the atoms at the triple bond in germylacetylenes

The integrated extinction coefficients (A) of the C≡C stretching modes in the IR spectra of 12 germylacetylenes Me₃GeC≡CR are determined by the resonance interactions of substituents with the triple bond. TheA ^1/2 values change linearly with change in the difference between the effective π-electron charges on the atoms at the triple bond and σ⁰ _R constants of organic substituents R. The average value of the σ⁰ _R constant of the Me₃Ge substituent in the compounds studied is +0.06. The resonance acceptor effect of the Me₃Ge substituent toward the triple bond (d,π-conjugation) is stronger than the donor effect (σ,π-conjugation). Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1569–1574, August, 1998.     

Comparative analysis of effective atomic charges in the N-methylmorpholine-N-oxide molecule

The literature data and our quantum-chemical electron density distribution calculations were compared with experiment for the N-methylmorpholine-N-oxide molecule     

Intensities of IR bands of CH stretching vibrations and atomic charges in saturated three-membered rings

The intensities of the absorption bands of the CH stretching vibrations in the IR spectra of solutions of cyclopropane, aziridine, oxirane, and thiirane in CCl₄ were measured. Quantum-mechanical calculations of these intensities were performed.Translated from Khimiya Geterotsikhlicheskikh Soedinenii, No. 12, pp. 1612–1619, December, 1989.     

The separation and detection of alkaline oligoclonal IgG bands in cerebrospinal fluid using immobilised pH gradients

A method is described for the separation and detection of highly alkaline IgG bands in unconcentrated cerebrospinal fluid (CSF). These bands are frequently found in the cerebrospinal fluid of patients with inflammatory diseases of the central nervous system, particularly in the case of multiple sclerosis, and their detection is an important aid in clinical diagnosis. An isoelectric focusing technique using an immobilised pH gradient in polyacrylamide gel has been developed over the pH range 7-10, producing a linear and stable pH gradient with excellent resolution. After electrofocusing, the protein patterns were blotted onto polyvinylidene difluoride membranes and visualised using anti-human IgG followed by an enzyme-labelled second antibody. Blotting could be carried out by capillary diffusion for up to 16 h duration without any loss in resolution. Using this method, highly alkaline intrathecal IgG bands were found in the cerebrospinal fluid of all of the 14 multiple sclerosis patients. There were also 2 patients with alkaline IgG bands in their cerebrospinal fluid who were not diagnosed as multiple sclerosis. By contrast, no alkaline IgG bands with an isoelectric point (pI) greater than 8.6 were found in any of the serum samples studied (n = 50) from patients with various neurological disorders including multiple sclerosis.