Crystal field environments

Reactions in which isomerization of coordination compounds occur in solutions are common, and some reactions of this type in solid complexes have been studied. Generally, there is a change in color of the complex as the crystal field environment of the metal ion changes. Accordingly, some of the color changes that occur when complexes are heated may indicate isomerization, but very few geometrical isomerization reactions in solid complexes have been studied in detail. One such reaction is... 

Klebe, G. (1994) The use of composite crystal-field environments in molecular recognition and the de novo design of protein ligands. J. Mol. Biol. 237,221-235. 

Ideally, lanthanide ions occupy site with C3V symmetry when incorporated into the GaN lattice by substituting the Ga3+. However the actual site symmetry is often found lower than C3V due to strain and defects. So far no crystal field analysis has ever been reported for lanthanide-doped III-V QDs, due to the co-existence of multi-sites which complicates the energy level structure and makes crystal field analysis difficult. Site selective spectroscopy is a very useful tool thus proposed to investigate the different crystal field environments of lanthanide ions doped in III-V QDs. 

Energy level splitting for a d3 ion as its crystal field environment changes form octahedral (O symmetry) to tetragonal... 

This can be illustrated by a pair of hypothetical examples [22]. Suppose that we have four observations of a covalent bond length let them have e.s.d. s of 0.002, 0.008, 0.009 and 0.010 A, respectively, and assume, for the moment, that these e.s.d. s are reliable. Now, most bond lengths are hard parameters, i.e. considerable energy is required to distort them from their equilibrium values. They are therefore relatively insensitive to changes in the crystal-field environment. Thus, it is likely that the true value of the /th bond length (i.e. its value in the /th crystal structure, if we could measure it without experimental error u, in Equation 4.1), is virtually identical to that of the yth. Differences between the //, may therefore be assumed negligible compared with experimental errors, and (4.1) may be simplified to ... 

Fig. 13.8. Stereo-diagram of the preferred spatial location of hydrogen-bond donors about N in a pyridine ring, determined by mapping the composite crystal-field environment data taken from small molecule crystal structures containing this fragment. The data were expanded according to the C2v-symmetry of the pyridine ring...

Fiq. 26. Energy level diagram of Cr + in an octahedral and in an octahedral plus a trigonal crystal field environment. The two upper levels and Ti F) are... 

Taylor, R., Mullaley, A., and Mullier, G.W (1990) Use of crystallographic data in searching for isosteric replacements composite crystal field environments of... 

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. 

The crystal field environment of the d system in the series of iron(III) dithiocarbamate complexes may be expressed in the form of a potential with octahedral symmetry with a small trigonal component. Therefore, we may write for our purpose in this paper the effective crystal field potential, V(r), in terms of a series of spherical harmonics for the transition of metal ion as ... 

A previous proton nmr study(2) of tris(N,N diethyldithiocarba-mato)iron(III) indicated that the transition metal ion is an intermediate crystal field environment such that the A state and the lowest lying T2 state for the d ion are close in energy. Hence we need to consider both states. 

For the T2 level we need to consider the contribution to the nmr shift by the term proportional to X Xj the Fermi interac tion. Since in these compounds the effective crystal field environment of the iron atom is approximately of octahedral symmetry with a small most likely trigonal component(11,12) we shall assume that the form of the Fermi contact interaction is as given in reference (1). The magnetic susceptibility components Xi. and can be determined from the appropriate equations in ref.(13). (We shall neglect bonding in this paper determining x... 

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. 

Herein we present a set of crystal-field parameters for R ions in the two representative sites most commonly found in high-Tc superconductors. We focus on the R sites in R2 j Cej Cu04 and RBa2Cu30 c because they represent two distinctly different crystal-field environments. Most of the other series, including the Pb2Sr2Ri j-Ca cCu308,... 

The crystal field environment of iron switches from octahedral to square pyramid in both cases during reduction and vice versa dimng reoxidation. As... 

Site point connection methods include CL1X, LUDI, the linked-algorithm approach of Verlinde and co-workers, and Klebe s analysis of crystal field environments. ... 

G. Klebe,. Mol. Biol., 237,212 (1994). The Use of Composite Crystal-Field Environments in Molecular Recognition and the De Novo Design of Protein Ligands. 

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