Another theoretical method, extensively exploited in chemical shift shielding calculation is gauge-including atomic orbitals (GIAO) [177] approach. That technique provides reasonable chemical shifts values for various nuclei. It should be noted that this approach does not take into account the periodicity of the crystal lattice and calculations are carried out for isolated molecules (gas phase). If analysis of intermolecular interactions is target of the project or there is its important part, then arbitrarily constructed cluster model has to be built up. [Pg.118]

Thus, the combined experimental and theoretical results indicate that the chemical shift observed for the S(2p) core level, of about 1.6 eV, should be due to a secondary effect from the attachment of Al atoms to the adjacent carbon atoms. Indeed, this is fully consistent with tib initio Hartree-Fock ASCF calculations of the chemical shifts in aluminum-oligolhiophene complexes 187], From calculations on a AI2/a-3T complex, where the two AI atoms are attached to the a-car-bons on the central thiophene unit, the chemical shift of the S(2p) level for the central sulfur atom is found to be 1.65 eV, which is in close agreement with the experimental value of about 1.6 eV [84]. It should be pointed out that although several different Al-lhiophene complexes were tested in the ASCF calculations, no stable structure, where an Al atom binds directly to a S atom, was found [87]. [Pg.396]

There have, however, been attempts to correlate Q-e values and hence reactivity ratios to, for example, c NMR chemical shifts 50 or the results of MO calculations 51153 and to provide a better theoretical basis for the parameters. Most recently, Zhan and Dixon153 applied density functional theory to demonstrate that Q values could be correlated to calculated values of the relative free energy for the radical monomer reaction (PA + Mn — PA ). The e values were correlated to values of the electronegativities of monomer and radical. [Pg.364]

Figure 2 compares the results of theory and experiment for the specific case of p-fluoronitrobenzene. Inspection of the calculated structure shows that the proton is still on the zeolite, and the F shifts are more like chloroform solution than superacid solution. Furthermore, when the l F chemical shift was calculated for the theoretical structure, it was found to agree with the experimental result. [Pg.576]

It is not only correlation effects which tend to be larger if lone pairs are present, one also finds larger solvent effects in these cases. The scatter of theoretical data compared with experimental solution data might thus not decrease distinctly if electron correlation is taken into account. For example, for selenium chemical shifts MP2 calculations do not perform much better on the whole than HF calculations. The situation is, however, different if comparisons can be made with gas phase chemical shifts. The deviation between theory and experiment (Figure 5) is significantly smaller in this case for correlated methods. [Pg.1833]

In addition to the obvious structural information, vibrational spectra can also be obtained from both semi-empirical and ab initio calculations. Computer-generated IR and Raman spectra from ab initio calculations have already proved useful in the analysis of chloroaluminate ionic liquids [19]. Other useful information derived from quantum mechanical calculations include and chemical shifts, quadru-pole coupling constants, thermochemical properties, electron densities, bond energies, ionization potentials and electron affinities. As semiempirical and ab initio methods are improved over time, it is likely that investigators will come to consider theoretical calculations to be a routine procedure. [Pg.156]

See also in sourсe #XX -- [ Pg.133 , Pg.134 , Pg.135 , Pg.136 ]

See also in sourсe #XX -- [ Pg.70 , Pg.77 ]

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