Solvent Effects, Crystal Fields

6 Solvent Effects, Crystal Fields. - This report is concerned with molecular properties and full coverage of intermolecular effects and solid state susceptibilities is not attempted. The papers reviewed in this section have been selected because they contain material closely related to the calculated properties of individual molecules. For example, calculations based on the electronic band structures of semiconductors etc. are excluded, but a few papers relating molecular crystal susceptibilities to the molecular hyperpolarizabilities are included. 

A computer simulation of liquid benzene by Janssen et al.245 using molecular dynamics based on input from gas phase ab initio static hyperpolarizabilities has successfully reproduced measured values of the refractive index and y-hyper-polarizability. 

Nakano et al246 have calculated the static a and tensors for linear H2NO dimers using DFT as part of a study of molecular clusters. Papadopoulos and Sadlej247 find, through CCSD calculations, that the polarizability and second hyperpolarizability of a Be atom are significantly reduced when it is embedded in a cluster of He atoms. 

Kirtman et al.248 use ab initio methods to benchmark model calculations in which a bundle of hexatriene molecules is intended to simulate polyacetylene. They find that the effect of the medium can be accurately reproduced by 

Semi-empirical AM 1 calculations of the dipole moments and polarizabilities of benzalazine and benzopyran derivatives have been carried out by Kodaka et al.249 as part of a study of their liquid crystal properties. 

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The magnetic moments rise only slightly at elevated temperatures (see Table 5), which led the authors to conclude that some population of the higher sT2(Oh) state is possible. No clear distinction can be made as to which of the influencing factors, viz. electronic effects, steric hindrance, and crystal solvent effects, plays the dominant role here, because all of these are operative to some extent. Data from the UV-vis spectra of the nickel(II) complexes indicate that the ligands have field strengths in the iron(II) crossover region. 

Crystal field theory, intensities of 4f-4f transitions, Judd-Ofelt theory of electric-dipole transitions, covalency model of hypersensitivity, dynamic coupling mechanism, solution spectra, spectral data for complexes, solvent effects, fluorescence and photochemistry of lanthanide complexes are dealt with in spectroscopy of lanthanide complexes. 

For d-transition-metal ions, the number of water molecules in the primary coordination sphere (A-zone) is in most cases determined by the strength of orbital overlap between the metal ion and H2O molecules, crystal field stabilization effects, and cationic charge. Other species (e.g., alkaline earths, rare earths) interact with solvent molecules via ion-dipole forces with minimal orbital overlap conhibution to the bonding. Their solvation numbers are determined by a combination of coulombic attraction between cations and water molecules, steric fiictors, and van der Waals repulsion between the bound water molecules. The larger size and high charge of the lanthanides combine with the absence of directed valence effects to produce primary-sphere hydration numbers above eight for these metal ions. 

There are some disturbing features of the activation parameters with regard to the conventional interpretations of the data in water. For example, the order of Dq values for the solvents is NHj > CHjCN > DMF > HjO CH3OH. If crystal field effects are the determining factor for the Alt values, then one should expect these to be in the same order for the various solvents. In fact, the order of Alt values for Ni(II) is CH3OH > DMF > CH3CN NH3 > HjO and there seems to be no relationship to the Dq values. The fact that the AV values for a particular metal ion are rather insensitive to the solvent will be discussed in the next section. 

Solvent potential. The averaged solvent electrostatic field, , is important for inhomogeneous media, such as enzymes, membranes, miscelles and crystalline environments systems. Due to the existence of strong correlations, such a field does not cancel out. This factor becomes an important contribution to solvent effects at a microscopic level. In a study of non-rigid molecules in solution, Sese et al. [25] constructed a by using the solute-solvent atom-atom radial distribution function. Electrostatic interactions in three-dimensional solids were treated by Angyan and Silvi [26] in their self-consistent Madelung potential approach such a procedure can be traced back to a calculation of . An earlier application of the ISCRF theory to the study of proton mechanisms in crystals of hydronium perchlorate both [Pg.441]

The ability to control crystallization is a critical requirement in many technologies such as in the food, pharmaceutical and chemical industries. Crystallization parameters such as particle size, particle shape, particle morphology and polymorph selectivity determine the crystal properties and uses. The solution concentration, crystallization time, crystallization temperature, solvent and crystallization vessel all have an effect on the crystal parameters. In the past few decades, researchers have been busy searching for new ways to control crystallization. Self assembled monolayers are showing great promise in this field. The... 

Theoretical Chemical Physics encompasses a broad spectrum of Science, where scientists of different extractions and aims jointly place special emphasis on theoretical methods in chemistry and physics. The topics were gathered into eight areas, each addressing a different aspect of the field 1 - electronic structure of atoms and molecules (ESAM) 2 - atomic and molecular spectra and interactions with electromagnetic fields (AMSI) 3 - atomic and molecular interactions, collisions and reactions (AMIC) 4 - atomic and molecular complexes and clusters, crystals and polymers (AMCP) 5 - physi / chemi-sorption, solvent effects, homogeneous and heterogeneous catalyses (PCSE) 6 - chemical thermodynamics, statistical mechanics and kinetics, reaction mechanisms (CTRM) 7 -molecular materials (MM), and 8 - molecular biophysics (MB). There was also room for contributions on electrochemistry, photochemistry, and radiochemistry (EPRC), but very few were presented. 

Abstract - The temperature dependence of the proton nmr spectra of dithiocarbamato iron(III) complexes is markedly solvent dependent. A study is made of the temperature dependence of the nmr shifts for the N-CH2 protons in tris(N,N-dibutyldithiocar-bamato) iron(III) in acetone, benzene, carbon disulfide, chloroform, dimethyIformamide, pyridine and some mixed solvents. This contribution shall outline first how the nmr shifts may be interpreted in terms of the Fermi contact interaction and the dipolar term in the multipole expansion of the interaction of the electron orbital angular momentum and the electron spin dipol-nuclear spin angular momentum. This analysis yields a direct measure of the effect of the solvent system on the environment of the transition metal ion. The results are analysed in terms of the crystal field environment of the transition metal ion with contributions from (a) the dithiocarbamate ligand (b) the solvent molecules and (c) the interaction of the effective dipole moment of the polar solvent molecule with the transition metal ion complex. The model yields not only an explanation for the unusual nmr results but gives an insight into the solvent-solute interactions in such systems. 

In this paper we shall extend our earlier interpretation of the redox results to the nmr data for the N - CH2 protons in tris(N,N-diethyldithiocarbamato) iron(III). We shall show that the solvent dependence of the nmr shifts can be interpreted as arising from solvent interactions with the iron(III) dithiocarbamate system. Although the solvent interactions are small compared with the electronic interactions within the transition metal iron complex the effect is marked since in these cases for the d iron system there are low lying electronic states where the energy separation is sensitive to small changes in the crystal field environment of the transition metal ion. 

In this analysis of the nmr data we have that the results for a variety of solvents and over a wide temperature range may be interpreted as arising from small changes in the crystal field environment of the iron atom due to two solvent interactions - a term which is an intrinsic property of the solvent and a second term arising from a solute-solvent interaction. Although the application of the model has been simplified the results nevertheless give an insight into the effect of the solvent on the nmr shifts of these iron dithiocarbamate complexes. 

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