Model molecules approach shift

Two theoretical approaches for calculating NMR chemical shift of polymers and its application to structural characterization have been described. One is that model molecules such as dimer, trimer, etc., as a local structure of polymer chains, are in the calculation by combining quantum chemistry and statistical mechanics. This approach has been applied to polymer systems in the solution, amorphous and solid states. Another approach is to employ the tight-binding molecular orbital theory to describe the NMR chemical shift and electronic structure of infinite polymer chains with periodic structure. This approach has been applied to polymer systems in the solid state. These approaches have been successfully applied to structural characterization of polymers... 

The light shift of the resonance line of a single terrylene molecule was measured with a near-resonant pump (A > F) by either varying the pump intensity at a iked detuning or vice versa. As can be derived from a perturbative approach as well as from the dressed atom model, the light shift d for a near-resonant ptimp can be approximated (A > Qi) by the simple expression,... 

When the relaxation mechanism is the modulation of the magnetic g and A components due to the rotational diffusion of the paramagnetic group (mainly for the nitroxide radicals, and for 5 = y paramagnetic ions), the analysis of the spectra in the fast-slow motion regime provides the correlation time for the rotational motion. An increase in the correlation time corresponds to a decrease in mobility of the paramagnetic probe or label. The evaluation of the correlation time for the rotational motion was performed by simple methods or by computation of the spectra. Different diffusion models can be considered, such as Brownian or jump diffusion models, and the rotational mobility may be considered isotropic or anisotropic. In this latter case, for nitroxide radicals, the main information was obtained from the perpendicular component of the correlation time. Furthermore, a shift of the main rotational axis accounts for the compression of the labels due to other molecules approaching the label at the dendrimer surface. 

The strategy included (1) the synthesis of model molecules representative of the monomeric units in the two different enchainments (1,2- and 1,4) (2) an in-depth characterization by mono- and two-dimensional NMR spectroscopy of model compounds and polymers (3) analysis of the chemical shifts by means of different approaches [21] ... 

Solvatochromic shifts for cytosine have also been calculated with a variety of methods (see Table 11-7). Shukla and Lesczynski [215] studied clusters of cytosine and three water molecules with CIS and TDDFT methods to obtain solvatochromic shifts. More sophisticated calculations have appeared recently. Valiev and Kowalski used a coupled cluster and classical molecular dynamics approach to calculate the solvatochromic shifts of the excited states of cytosine in the native DNA environment. Blancafort and coworkers [216] used a CASPT2 approach combined with the conductor version of the polarizable continuous (CPCM) model. All of these methods predict that the first three excited states are blue-shifted. S i, which is a nn state, is blue-shifted by 0.1-0.2 eV in water and 0.25 eV in native DNA. S2 and S3 are both rnt states and, as expected, the shift is bigger, 0.4-0.6eV for S2 and 0.3-0.8 eV for S3. S2 is predicted to be blue-shifted by 0.54 eV in native DNA. 

This approach has also been applied to saturated C(i) atoms. Due to the much lower polarizabilities of C-H and C-C single bonds, however, the effects are clearly smaller (82), though still detectable. Thus, some effects of substituents X on S-positioned carbon atoms in cyclohexanes and cholestanes were attributed to LEF effects of the C-X dipoles by Schneider and colleagues (76,77,85). The finding that only minor effects are to be expected in saturated molecules was confirmed by INDO-SCF calculations of electric-field effects on l3C chemical shifts of some model compounds performed by Seidman and Maciel (86). These authors conclude, further, that conformational studies on such systems are not promising (86). 

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