Solvation effect, chemical shifts

In donating solvents the subtle effects determining the chemical shift in chloroform, benzene, and hexane are apparently masked. In hexane, which is considered a poor solvent, self-association is possible and would explain the appearance of the Sn spectrum. Chloroform and benzene are excellent solvents for organometallic polymers, and the structure and downfield position support a well-solvated, unassociated environment. 

A study39 of substituent effects on the 15N chemical shift (515N) (Table 10) for 4-substituted anilines in DMSO was interpreted in terms of substituent solvation-assisted resonance (SSAR) effects. Solvation of certain conjugated jr-electron-acceplor (+R) substituents has been found to give significant enhancements in the acidities of anilines, phenols and other acids40,41, and the magnitudes of these enhancements increase with... 

Attempts to obtain alkylcarbonium complexes by dissolving alkyl chlorides (bromides) in liquid Lewis acid halides (stannic chloride, titanium (IV) chloride, antimony pentachloride, etc.) as solvent were unsuccessful. Although stable solutions could be obtained at low temperature with, for example, t-butyl chloride, the observed N.M.R. chemical shifts were generally not larger than 0 5 p.p.m. and thus could be attributed only to weak donor-acceptor complexes, but not to the carbonium ions. The negative result of these investigations seems to indicate that either the Lewis acids used were too weak to cause sufficient ionization of the C—Cl bond, or that the solvating effect of the halides... 

The particular array of chemical shifts found for the nuclei of a given polymer depends, of course, on such factors as bond orientation, substituent effects, the nature of nearby functional groups, solvation influences, etc. As a specific example, derivatives of the carbohydrate hydroxyl moieties may give rise to chemical shifts widely different from those of the unmodified compound, a fact that has been utilized, e.g., in studies (l ) on commercially-important ethers of cellulose. Hence, as illustrated in Fig, 2, the introduction of an 0-methyl function causes (lU,15) a large downfield displacement for the substituted carbon. This change allows for a convenient, direct, analysis of the distribution of ether groups in the polymer. Analogously, carboxymethyl, hydroxyethyl and other derivatives may be characterized as well... 

From these investigations it is clear that the Li chemical shift gives a clear indication of the lithium cation position when there are direct effects from ring currents in delocalized anions. However, as shown for the quinuclidine CIP and THF SSIP fluorenyllithium complexes, the correct assignment cannot be reached solely based on the chemical shifts. Furthermore, there is no clear-cut information about solvation to be gained from the chemical shifts. As we discuss in the following Section, the quadrupolar interaction is much more sensitive to these effects. In order to obtain maximal structural information from Li NMR spectroscopy, the chemical shift should be determined and used in combination with the quadrupolar coupling constant. 

Variations in the absolute concentration of the carbocation solutions and temperature had minor effects on chemical shifts. The counter ion effect and the equilibrium could be minimized by going to higher superacidity systems with lower nucleophilicity counter ions. Resonances due to the PAH itself were considerably shielded. Solvation by FSO3H and the formation of ion pair-molecule clusters were suggested as possible reasons. 

Inspecting the table, we observe that some experimental trends are reproduced by the calculations (for instance, the small increase in the chemical shift of cis-UF4CI2 as compared to UFCI5). However, other experimental trends are not reproduced by either theoretical method. The reasons for this somewhat disappointing result are not clear in the moment. However, solvation effects are expected to have a non-negligible influence on the electronic structure, and hence on the calculated chemical shifts, and probably also on the optimized geometries. Note that some of the molecules in the... 

Solvent effects on nuclear magnetic resonance (NMR) spectra have been studied extensively, and they are described mainly in terms of the observed chemical shifts, 8, corrected for the solvent bulk magnetic susceptibility (Table 3.5). The shifts depend on the nucleus studied and the compound of which it is a constituent, and some nuclei/compounds show particularly large shifts. These can then be employed as probes for certain properties of the solvents. Examples are the chemical shifts of 31P in triethylphosphine oxide, the 13C shifts in the 2-or 3-positions, relative to the 4-position in pyridine N-oxide, and the 13C shifts in N-dimethyl or N-diethyl-benzamide, for the carbonyl carbon relative to those in positions 2 (or 6), 3 (or 5) and 4 in the aromatic ring (Chapter 4) (Marcus 1993). These shifts are particularly sensitive to the hydrogen bond donation abilities a (Lewis acidity) of the solvents. In all cases there is, again, a trade off between non-specific dipole-dipole and dipole-induced dipole effects and those ascribable to specific electron pair donation of the solvent to the solute or vice versa to form solvates. 

This method can be successfully applied to the case of a solvation effect on the proton chemical shift. However, the effect cannot always be explained by this method. The quantity is very sensitive to the solute-solvent interaction and a serious drawback inherent in the classical-quantum hybrid approach is revealed. The result of ab initio MO analysis for small clusters suggests that electron exchange between solute and solvent is crucial to compute correct values of the chemical shift. A few attempts have been made to overcome this deficiency [18]. 

Q-Chem (http //www.q-chem.com) This commercial package is able to perform energy calculations and geometry optimizations at ground state and excited states at various ab initio and DFT levels. It also performs calculations of NMR chemical shifts, solvation effects, etc. 

Since the two ions are essentially the same chemically but merely differ slightly in how they pack into the crystal, it is not surprising the average shifts do not differ significantly. For comparison, the chemical shift of a 1 M solution of calcium formate is 8 172 ppm. Again, we expect similarity, but not identity, with respect to the isotropic solid-state shift. In the solution case the ion is surrounded by a solvation sphere, whereas in the crystal, the solvent is other ions. It is not unreasonable to expect a moderate shift due to that effect. It is also very often the case that principal values are individually more sensitive to intra-and intermolecular effects than are isotropic shifts. ... 

Si NMR chemical shifts were calculated for each molecule relative to the theoretical shielding for tetramethylsilane (TMS), at the HF/6-311+G(2d,p)86 level using the GIAO method,94 as implemented in Gaussian 94 and Gaussian 98. Shifts for gas-phase molecules are reported because the inclusion of solvation via the SCRF method was found to have little effect on the predicted shifts.83 Comparison of calculated shifts with experimental values for compounds with well-known structures yielded an error estimate of about 1 to 8% for quadra-coordinated silicon and 2 to 9% for penta-coordinated silicon. 

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