Solvent intramolecular hydrogen bonding

The NMR spectra of some 1- and 2-substituted perimidinium salts 139 are discussed and assigned. Several 2-substituted derivatives possess and H NMR spectra that reflect relatively slow prototropic tautomerism due to hydrogen bonding with a solvent, intramolecular hydrogen bonding, or enhanced delocalization of the nitrogen lone pair (88MRC191). 

Many heterocycles containing two or more heteroatoms can exist in more than one tautomeric form. Both external (polarity of the solvent, intramolecular hydrogen bonding) and internal (relative resonance stabilization of the conjugated system, stabilities of the functionalities) factors influence such tautomeric equilibria. For five- and six-membered rings, frequently more than one tautomer preserves the aromatic character. Comparisons of tautomeric equi-... 

Similar results were reported earlier for 164 the pyridoiie 164a is the major tautomer in polar solvents, while the pyridinol 164b predominates in nonpolar ones (67AC877). Tire hydroxy form can be stabilized by intramolecular hydrogen bonding [71JCS(B)2339],... 

Moreover, with a change of solvent, a new tautomeric form can arise owing to formation of intermolecular hydrogen bonds in place of the previously existent intramolecular hydrogen bonds. This situation is characteristic, for example, for pyrimidine derivatives 49, for which the use of polar (DMSO, DMF, MeOH, HMPT) solvents or specifically solvating cosolvents (S) (e.g., a small amount of water or A-methylpyrrolidinone) leads to the appearance of ylidene tautomer 49b with the p-quinonoid disposition of the double bonds (Scheme 18) [88KGS521 90UK457]. 

It has been pointed out321-324 that the two groups of solvents differ by some definite structural features. In particular, ED, 1,2-BD, and 1,3-BD possess vicinal OH groups that can form intramolecular hydrogen bonds. For these solvents, the ability of the organic molecule to interact with neighboring molecules is reduced. This results in the possibility of a different orientation at the interface because of different interactions of the OH groups with the Hg surface.323 The different molecular structure leads to different dipolar cooperative effects. As a result, the dependence of C on the bulk permittivity follows two different linear dependencies. 

Intramolecular hydrogen-bonds can increase the stability of certain conformations. For example, dianhydrides that contain fJ-L-Sorp or ct-D-Frup in the 5C2 conformation have the C-4 hydroxyl group in a 1,3-diaxial relationship with 0-2, which permits the formation of an intramolecular hydrogen bond. This might, in part, offset the destabilizing influence of three or two axial substituents, respectively. This effect is decreased in hydrogen-bonding solvents. 

Many synthetic water-soluble polymers are easily analyzed by GPC. These include polyacrylamide,130 sodium poly(styrenesulfonate),131 and poly (2-vinyl pyridine).132 An important issue in aqueous GPC of synthetic polymers is the effect of solvent conditions on hydrodynamic volume and therefore retention. Ion inclusion and ion exclusion effects may also be important. In one interesting case, samples of polyacrylamide in which the amide side chain was partially hydrolyzed to generate a random copolymer of acrylic acid and acrylamide exhibited pH-dependent GPC fractionation.130 At a pH so low that the side chain would be expected to be protonated, hydrolyzed samples eluted later than untreated samples, perhaps suggesting intramolecular hydrogen bonding. At neutral pH, the hydrolyzed samples eluted earlier than untreated samples, an effect that was ascribed to enlargement... 

II). From this it must be concluded that the long-wavelength absorption of curve I is due to the intra-molecular hydrogen bond of TIN in unpolar solvents as the intensity of this band is reduced in polar solvents (curve II) and disappears completely in the spectrum of MT (without intramolecular hydrogen bond), curve III. Curve IVa represents the fluorescence and IVb the phosphorescence emission of both TIN and MT in... 

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