Chemical resonances

With counteranions of lower nucleophilicity the " Sn NMR chemical resonance of solutions of tributyl tin salts is further shifted to lower field. Lambert and Kuhlmann reported for the tiibutylstarmyl borate Bu3SnB(C6F5)3H 8" Sn = 360 in benzene and Kira and co-workers found for the related BusSnTFPB 8 Sn = 356 in CD2Cl2. In the latter case, F NMR spectroscopy indicates no cation-anion interaction. Finally, for Bu3SnTPFPB in toluene at —60 °C 5" Sn = 434.2 was reported by Piers et The solvent dependence of the Sn NMR chemical shift suggests... 

In solid-state NMR [1,51-64], the magnetic coupling between the fullerene anions has to be taken into accoimt. In the case of metal intercalated fullerides that have metallic properties a contribution from the conduction electrons must be added, a phenomenon called the Knight shift . Even if this additional shift affects the C-chemical resonance, the correspondence between extended and discrete systems of comparable Cjq oxidation state is quite close [1]. 

For quantum chemistry, first-row transition metal complexes are perhaps the most difficult systems to treat. First, complex open-shell states and spin couplings are much more difficult to deal with than closed-shell main group compounds. Second, the Hartree—Fock method, which underlies all accurate treatments in wavefunction-based theories, is a very poor starting point and is plagued by multiple instabilities that all represent different chemical resonance structures. On the other hand, density functional theory (DFT) often provides reasonably good structures and energies at an affordable computational cost. Properties, in particular magnetic properties, derived from DFT are often of somewhat more limited accuracy but are still useful for the interpretation of experimental data. 

To avoid problems in the laboratory, it is important to recognize that the Rhodonines are members of the indicator family of chemistry, related to phenolthalein. In dilute solution, the absorption spectrum is a function of the environment due to the chemical resonance associated with its structure. This feature requires that the parameters of the solution be specified when recording spectra of the Rhodonines. 

In contrast to the four tetrahedrally oriented elliptic orbits of the Sommer-feld model, the new theory leads to only three, mutually orthogonal orbitals, at variance with the known structure of methane. A further new theory that developed to overcome this problem is known as the theory of orbital hybridization. In order to simulate the carbon atom s basicity of four an additional orbital is clearly required. The only possible candidate is the 2s orbital, but because it lies at a much lower energy and has no angular momentum to match, it cannot possibly mix with the eigenfunctions on an equal footing. The precise manoeuvre to overcome this dilemma is never fully disclosed and appears to rely on the process of chemical resonance, invented by Pauling to address this, and other, problems. With resonance, it is assumed that, linear combinations of an s and three p eigenfunctions produce a set of hybrid orbitals with the required tetrahedral properties. 

The present section is concerned with the electron density distributions in monosubstituted cumulenes on a CNDO/S level (which is equivalent to a discussion on an ab initio STO-3G level (Section II.C.I)). Emphasis is on the ir electronic systems in allenes, ketenes, and butatrienes. Of particular importance with respect to ir electron densities is the fact that the cumulenic functionalities under consideration act as ir donors as may be deduced from the C chemical resonance positions of the para carbon atoms in phenyl substituted cumulenes (8,89fc,89[Pg.412]

Although the first reported signal enhancement in SERS was a factor of 10 -10, a further enhancement up to 10 becomes possible, which originates from the effect of shape and geometry together with chemical resonant mode (although why is not yet fully understood). Now, these additional factors are of practical importance because... 

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