SOLVENT - INDUCED SHIFTS

Table I) is due to the so1ute-s01vent interactions that result in solvent induced shifts of the singlet-singlet excitation energies readily observable in solution absorption s p ec t r a (J 2). As expressed in... 

Convincing evidence was found that the majority of acyclic aldo-nitrones exist in the Z-form, by investigating the ASIS-effect (aromatic solvent induced shift effect) (399). However, in some cases, specified by structural factors and solvent, the presence of both isomers has been revealed. Thus, in C -acyl-nitrones the existence of Z -and -isomers was detected. Their ratio appears to be heavily dependant on the solvent polar solvents stabilize Z-isomers and nonpolar, E-isomers (399). A similar situation was observed in a- methoxy-A-tert-butylnitrones. In acetone, the more polar Z-isomer was observed, whereas in chloroform, the less polar E-isomer prevailed. The isomer assignments were made on the basis of the Nuclear Overhauser Effect (NOE) (398). /Z-Isomerization of acylnitrones can occur upon treatment with Lewis acids, such as, MgBr2 (397). Another reason for isomerization is free rotation with respect to the C-N bond in adduct (218) resulting from the reversible addition of MeOH to the C=N bond (Scheme 2.74). The increase of the electron acceptor character of the substituent contributes to the process (135). 

In this context, it should be pointed out that the correlation between aromatic solvent-induced shifts (ASIS) and the axial or equatorial orientation of protons in cyclic sulfoxides and sulfites is quite distinct (211-213) and may be utilized in the assignment of configurations. For instance, the absolute configuration at sulfur was assigned to the penicillin sulfoxide 202 based on analysis of the effect of aromatic solvents on the chemical shifts of protons of the thiazolidine ring (214,215). 

Generally solvents chosen for NMR spectroscopy do not associate with the solute. However, solvents which are capable of both association and inducing differential chemical shifts in the solute are sometimes deliberately used to remove accidental chemical equivalence. The most useful solvents for the purpose of inducing solvent-shifts are aromatic solvents, in particular hexadeuterobenzene (CgDg), and the effect is called aromatic solvent induced shift (ASIS). The numerieal values of ASIS are usually of the order of 0.1 - 0.5 ppm and they vary with the moleeule studied depending mainly on the geometry of the complexation. 

This aromatic solvent induced shift (ASIS) is explained by the formation of preferential collision complexes or clusters of aromatic solvent molecules in the vicinity of the polar groups of the solute, so that the effect of the solvent magnetic anisotropy within the space occupied... 

In addition, aromatic solvent induced shifts (ASIS, see Section 4.1.1.4.) can be useful for structural assignments in this class of compounds 286. 

Lanthanide shift reagents have been used to differentiate diastereomeric oximes, dinitrophenyl-hydrazones 304,305, and nitrones 306. A collision complex model to rationalize aromatic solvent induced shifts in the spectra of oximes and hydrazones has been developed305,307. 

Aromatic solvent induced shifts assisted in the stereochemical assignment of highly substituted cyclobutanes470. NMR measurements in the presence of lanthanide shift reagents are useful for azetidines471 and cis- and rra ,v-2-ethenylcyclobutanecarbonitrile472. 

There are marked solvent effects as illustrated in Table 12. Especially the ASIS (aromatic solvent induced shift, 5CDC 3 — 8< d6) f°r H(l) is large. Besides, concen-... 

Figure 1 Solvent-induced shifts in the H NMR spectrum of furan and tetrahydrofuran. Positive shifts (p.p.m.) are upheld from values in solvent tetrachloromethane (TMS internal reference)...

Concentration effects on 13C shifts of alcohols are small [271]. Solvent-induced shifts are enhanced on going from primary to tertiary alcohols and may be as high as 2 ppm [271]. Protonation shifts are much larger. The methanol carbon, for example, is shielded by —14.6 ppm relative to the neat liquid value when dissolved in magic acid [272], Protonation shifts of 1-alkanols in trifluoroacetic acid are shieldings for C-1 and alternating deshieldings for all other carbon atoms, e.g. <5 2 > > <5C 3 for 1-butanol [272],... 

Table 4.36. Solvent-Induced Shifts of Acrylic Acid Carbons (<5C in ppm) [282].

Models to describe frequency shifts have mostly been based on continuum solvation models (see Rao et al. [13] for a brief review). The most important steps were made in the studies of West and Edwards [14], Bauer and Magat [15], Kirkwood [16], Buckingham [17,18], Pullin [19] and Linder [20], all based on the Onsager model [21], which describes the solvated solute as a polarizable point dipole in a spherical cavity immersed in a continuum, infinite, homogeneous and isotropic dielectric medium. In particular, in the study of Bauer and Magat [15] the solvent-induced shift in frequency Av is given as ... 

In Table 2.5 a comparison between equilibrium (eq) and nonequilibrium (neq) IR intensity shifts (solvent-gas) is reported for some methylketones in a medium polarity solvent (1,2-dichloroethane) and in a polar solvent (acetonitrile). Data are taken from ref. [42], Nonequilibrium shifts are in very good agreement with experimental measurements, whereas a pure equilibrium model fails in reproducing the solvent-induced shifts. 

Moving now to the comparison with experiments, it is more correct to analyze the solvent induced shifts instead of the absolute energies in such a way, the analysis... 

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