Distorted Geometries

These results implied that N-heterocyclic carbenes bound via polarized electrostatic interactions to the electropositive metals and that in these compounds bond strengths did not depend on the bending angle imposed by steric factors.  

Although early transition and rare earth metal complexes with N-heterocyclic ligands are no longer laboratory curiosities, studies on their reactivity are still extremely rare. 

59 with electrophilic McsSiCl in THF afforded monomeric 62 via backbone silylation and simultaneous KCl elimination. 

Another example of regioselective C4 silylation was observed with the neodymium analogue to 57 and 58, compound 63. Addition of Me3SiI to 63 yielded the backbone-functionalized dimer 40 (Equation (6.11)), which showed interesting reactivity with other metals (see Section 6.2.3.). For this regioselective silylation, a reaction mechanism starting with nucleophilic 

Maron and Bourissou observed in their calculations with different metal complexes bearing a tridentate 2,6-bis(methylimidazol-2-ylidyl)pyridine ligand an intrinsic preference of the ligand for La over U and for Sm over Am (predicted for bare ions). Such differentiation was also calculated for the tris-amido fragments, where lanthanum was preferred over uranium. In contrast to the expected reactivity, Mehdoui et al. were able to synthesize two analogous cerium(lll) 64 and uranium(lll) 65 compounds [( lMe)M(Cp )2l] by addition of IMe to an equimolar solution of [M(Cp )2l] (Equation (6.12)).  

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Cycloalkane properties depend on ring size. Strained molecules, i.e., moleeules with distorted geometries, tend to be more reaetive in ring-breaking ehemieal reaetions. For example, eombustion of a strained eyeloalkane should release more energy per CH2 group than eombustion of an unstrained moleeule. 

The less strain energy inherent in the four-membered ring sulfoxides and sulfones, their less distorted geometries and the lack of potential aromatic -type conjugation effects make the comparison of their physical and chemical properties with other cyclic and... 

Interesting variations of this mns-R2SnX4 structure exist they usually take the form of a distorted geometry in which the C-Sn-C... 

These studies discuss vertical and adiabatic excitation energies but the photophysical behavior requires calculations along the PES and at highly distorted geometries, which are more difficult to carry out in the presence of solvent. Some theoretical work has been done in this area, but it is quite limited. 

The effect of solvation on uracil and thymine photophysics has been studied by Gustavvson and coworkers, who have studied uracil with four explicit water molecules and PCM to study distorted geometries [92,93,149], The conical intersection connecting Si to the ground state that was found in the gas phase is also present in solution. The barrier connecting the Si minimum to the conical intersection is lower in solution, however, causing much shorter lifetimes. So the nanosecond lifetime which is observed in the gas phase is not observed in solution but a picosecond lifetime is observed. 

The dynamics of polysilane dendrimer excited states were also studied by Watanabe as mentioned above in a comparison with polysilynes,360,364 and suggested that a configuration coordinate model is applicable to the photophysics of branched silicon chains. Calculations showed a distorted geometry of the excited state localized at a branching point. 

In this approach, the potential energy V is a function of the reduced coordinates and the -matrix. For the kinetic energy, one would only be interested in the motion of the particle relative to the distorted geometry so that a suitable Lagrange function Lq for the system would read as follows ... 

Nonbonded interactions may obtain in halosubstituted benzenes. For example, consider the model compound o/T/io-difluoro-benzene which can exist in either a planar geometry or a distorted geometry in which the C-F bonds are bent, alternately above and below the mean plane as a result of severe dipolar and steric repulsion. 

C-S stretch, a coupling phenomenon facilitated by the distorted geometry of the blue copper site (20, 25). A vibration near 750... 

Figure 6.9. Top to bottom Two possible Jahn-Teller distorted geometries for the methane radical cation and calculated geometry for its energy minimum schematic representation of the SOMO for ethane radical cation SOMOs of three different propane radical cations observed by ESR the SOMO of butane radical cation and SOMOs for two conformers of pentane radical cation.

From electronic spectra it is inferred that the trigonal bipyramidal geometry of [NiX(Me6tpt)]X is distorted towards a tetrahedron. In this distorted geometry the steric constraints resulting from the presence of six-membered chelate rings are, at least in part, relieved.735-737... 

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