Water shielding polarizabilities

Nymand et al. ° performed molecular dynamics simulations on liquid water, and they used the electric field effect formalism [Eq. (6)] to explain the gas to liquid shifts of the and O nuclei. For the proton it turned out that the resulting gas to liquid shift of — 3.86 ppm at 300 K compared well with the experimental value of —4.70 ppm, whereas for O the method failed to reproduce the experiment. Even if electric field gradient terms are introduced, requiring additional quadrupolar shielding polarizabilities, no better results could be obtained for the O gas to liquid shifts. Isotropic proton chemical shifts are obviously a special case where many higher order terms cancel, hence it is justified to use the simple electric field equations in these chemical shift calculations. 

The concept of dipole moment and its relationship to ion polarizability were discussed in section 1.8. Section 1.19 introduced the concept of dielectric constant of a crystalline solid and its relationship with the polarizability of its constituting ions (see eq. 1.168). The dielectric constant of a liquid solvent such as water represents the capacity of the solvent s molecules to shield the charges of ion... 

Table 6 Isotropic shielding constant (in ppm) for X being oxygen or carbon and the shift compared to the gas-phase value. QM denotes the system that is treated quantum mechanically, and the water part describes the approach for treating the solvent. For the latter, PCMl and PCM2 denotes polarizable continuum models and TIP3P and SPC two different force helds. - marks gas-phase calculations. All results are from ref. 33... 

N chemical shielding in peptides and proteins is known to be sensitive to secondary structure as well as noncovalent interactions. Cai et al have recently employed DFT calculations with a polarizable continuum solvent model and explicit water molecules in the first solvation shell for A-formyl-alanyl-X amides, where X is one of the 19 naturally occurring amino acids excluding proline. This recent work suggests that the explicit water molecules incorporated in the calculations affect the isotropic amide N chemical shift, but not its anisotropy. A solvation model likewise appears to improve the correlation between calculated and observed C chemical shifts in the complex formed by piperidine-4-carboxylic acid and chlor-oacetic acid. Ksiazek et have evaluated the performance of the... 

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