Electronic structure and geometry

Examine the geometry of the most stable radical. Is the bonding in the aromatic ring fuUy delocalized (compare to model alpha-tocopherol), or is it localized Also, examine the spin density surface of the most stable radical. Is the unpaired electron localized on the carbon (oxygen) where bond cleavage occurred, or is it delocalized Draw all of the resonance contributors necessary for a full description of the radical s geometry and electronic structure. 

The complexity of the particle size related changes in the cluster geometry and electronic structure renders the results of the theoretical calculations extremely useful when analyzing the experimentally observed trends. 

Lamoen, D., and M. Parrinello. 1996. Geometry and electronic-structure of porphyrins and porhyrazines. Chem. Phys. Lett. 248, 309. 

An electron-gain centre of similar geometry and electronic structure is generated (43) by radiolysis of the nitrosocarbonyl Mn(C0)4N0. Spectra associated with the electron-loss centres Mn(C0)nX+ (n=4 or 5) are less well-defined and pose analytical difficulties (41). However, there is little doubt that these are high-spin radicals, probably electronic sextets. 

As for the possible correlation between geometry and electronic structure, consider the variation of ionic radii with atomic number in the first row transition metal series If the points for Ca, Mn, and Zn are connected, i. e., for atoms with a spherically symmetrical distribution of d electrons, the ionic radii of the other atoms are smaller than interpolation would yield from the Ca-Mn-Zn line. The nonuniform distribution of d electrons around the nuclei is assumed to be the reason for this contraction of the ionic radii. The data available so far on the bond lengths for the vapor-phase dichlorides are seen in Fig. 8. 

Experimentally, XANES spectta are found to be extremely sensitive to Ae oxidation sAte, geometry, and electronic structure of Ae absorbmg site. XANES features, especi y Ae bound- bound transitions can be sufficienfly well resolved Aat it is possible to correlate a specific transition wiA a specific structure feature. An example, involvmg Ae Astinction between Cu(I) and Cu(II) is Ascussed below. 

The geometry and electronic structure of the azide ion, Nj, is quite similar to that of allene. The rotational strength observed for the antisymmetric stretching vibration of the azide (2025 cm ) in azidomethemoglobin A (Figure 25), measured by Marcott, et al., is —3 x 10 esu cm, with an anisotropy ratio 0.02 (115). These values are two orders of magnitude or more greater than normally observed for VCD. 

Scheme 16.27. Orbital interactions that determine the geometry and electronic structure of 28 and 99. ...

The concept of electronic delocalization has germinated in the pre-electron period to Kekule s structural theory and its application to benzene as a prototype of a family of compounds so-called aromatics . Kekule had to address two major properties of benzene revealed from substitution experiments. The first was the empirical equivalence of all positions of benzene, what is called today the Dfjh symmetry of both geometry and electronic structure, and second the persistence of the aromatic essence in chemical reactions, what we recognize today as aromatic stability . Thus, Kekule postulated that there is a Ce nucleus and the four valences of the carbons are distributed to give two oscillating structures, which when cast in our contemporary molecular drawings look like part a in Scheme 2.39-44 One of the many alternative hypotheses on the nature of... 

Systematic Hiickel calculations on these and other series reveal the connection between geometry and electronic structure that is summarized in two rules. The... 

If the purpose of a calculation is to probe the inherent properties of a molecule as a thing in itself, or of a phenomenon centered on isolated molecules, then we do not want the complication of solvent. For example, a theoretically oriented study of the geometry and electronic structure of a novel hydrocarbon, e.g. pyramidane [6], or of the relative importance of diatropic and paratropic ring currents [7], properly examines unencumbered molecules. On the other hand, if we wish, say, to calculate from first principles the pZa of acids in water, we must calculate the relevant free energies in water [8]. Noteworthy too is the fact that solvation, in contrast to gas phase treatments, is somewhat akin to molecules in bulk, in crystals [9]. Here a molecule is solvated by its neighbors in a lattice, although the participants have a much more limited range of motion than in solution. Rates, equilibria, and molecular conformations are all affected by solvation. Bachrach has written a concise review of the computation of solvent effects with numerous apposite references [10]. 

A. Dipole Moments, Molecular Geometry, and Electronic Structure... 

S. S. Shaik, E. Duzy, A. Bartuv, J. Phys. Chem. 94, 6574 (1990). The Quantum Mechanical Resonance Energy of Transition States An Indicator of Transition State Geometry and Electronic Structure. 

Molecular modeling includes a collection of computer-based tools of varying theoretical soundness, which make it possible to explain, and eventually predict, the properties of molecular systems on the basis of their composition, geometry, and electronic structure. The need for such modeling arises while studying and/or developing various chemical products and/or processes. The raison d etre of molecular modeling is provided by chemical thermodynamics and chemical kinetics, the basic facts of which are assumed to be known to the reader.1... 

This performs the announced program of obtaining a closed expression for the energy of the CC (or at least of its CLS) in terms of its geometry and electronic structure variables. 

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