Nitrogen nuclear shielding

In this section we shall present and compare different computational strategies one can adopt to simulate the effect of the environment on spectroscopic properties of solvated systems. In particular, as a representative example, we shall summarize the results of two studies [30,31] we have published in the last years on the environment effects on the nitrogen nuclear shieldings of a specific class of molecular systems containing sp2-type nitrogens three diazines, also known as pyridazine (1,2-diazine), pyrimidine (1,3-diazine) and pyrazine (1,4-diazine). 

To try to reproduce the solvent effects on nitrogen nuclear shieldings of diazines and understand the physics beyond them we have applied the QM/continuum (PCM) and QM/MM(pol) (DPM) approaches described in the previous sections. 

Table 1-3. Diagonal components, and the corresponding isotropic value, of the nitrogen nuclear shielding tensor in pyrimidine calculated using either DPM or PCM. All calculations are at B3LYP/6-311++G(2d,2p) level. Results are in ppm...

We have presented and compared different solvation models (continuum, discrete, continuum + discrete) to study solvent effects on molecular properties. In particular, the nitrogen nuclear shielding, which is known to be very sensitive to even small modifications of electronic and/or nuclear charge distributions, has been analyzed. Such alternation/combination of different models has been required to study the complex nature of solute-solvent interactions when both long-range polar and shorter-range specific H-bond effects are active. 

Solvent effects can be produced by specific interactions, such as protonation or hydrogen bonding and nonspecific interactions which may arise from solvent polarity effects. Both of these types of interaction are found in studies of solvent effects on nitrogen nuclear shieldings. The extent of substituent effects depends upon the position of substitution and the electronic nature of the substituent. 

The clearest use of ONIOM-PCM is for solute-solvent clusters embedded in a continuum. The method can also be used to partition the solute itself into layers that are each treated at a different level of theory. An example is the study of NMR shielding in a merocyanine in solution (Figure 4.10) [41], We looked at the shielding on the nitrogen center. Nuclear shielding is a relatively local property, and previous gas-phase studies showed that ONIOM can accurately reproduce target values [43], We investigated several... 

In Table 1-1 we thus report the N nuclear shielding (with its paramagnetic and diamagnetic contributions) and nitrogen lone pair natural population (NP(lp)) for the diazine showing the largest solvent effect, i.e., pyridazine, and its clusters in vacuo (VAC) and in the presence of an external PCM continuum. 

The amino substituent increases the nucleophilicity of the /Lcarbon due to the electron-donating power of the nitrogen and makes it useful in alkylation reactions368,369. Isotopically labelled enamines have been utilized to study the conjugation of the double bond with nitrogen in these and related compounds by investigating the deuterium isotope effects on 13C and 15N nuclear shielding in enamine derivatives. Mechanistic... 

Recent ab initio type calculations of nuclear shielding of carbon, nitrogen and oxygen in small molecules by Ditchfield et al, [171], essentially show that the diamagnetic contribution to the total shielding for second row nuclei does not vary appreciably from one molecule to another. 

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