Dipoles s. Betaines

Dipolarophiles, strong A -azirines 26, 706 suppl. 28 Dipoles s. Betaines... 

Dipoles s. Betaines 2,2 Dipyridyl disaiade as reagent 26,175 Dipyrrylmethanes 27, 817 Direction (s. a. Orientation)... 

Figure 1.12 Reichardt s betaine dye in its zwitterionic ground state (a) and first excited state (b). The ground state has a larger dipole moment (15 D) than the excited state (6 D). Measurement of the energy of the transition between these two states (n - n ) is the basis for the Ej scale of solvent polarity...

Dipole Moments of the c/s-Conformers of Betaines of the First and the Third Group... 

The best known of the potential mercaptoimidazoles are the imidazoline-2- and benz-imidazoline-2-thiones, which resemble imidazolin-2-ones in that the tautomeric form (53 X = S) is the preferred form. The crystal structure and the HNMR spectrum of 1,3-dimethyl-3H-imidazoline-2-thione have been interpreted as showing partial double bond character in the N—C—N system, but no aromaticity (70CC56). However, the preference for a betaine structure (56) rather than (57) or (58) should be accepted with caution since it is really only a resonance structure similar to others which undoubtedly contribute to the overall structures of oxo-, thio- and amino-imidazoles. Measurement of the piSTa values for a series of imidazoline-2-thiones substituted variously on C-4, C-5, N-1 and N-3 by hydrogen, phenyl or methyl shows that all of the values are similar. Approximate Kr values calculated show that these compounds exist even more in the thione forms (53, X = S 58) than do the corresponding thiazoline-2-thiones and oxazoline-2-thiones. The UV spectra in aqueous solution support thione structures, as do dipole moment and X-ray studies (76AHC(S1)280, p. 400). 

Solvent polarity is one of the most common solvent characteristics that have been used for correlations of rate constants with the nature of the reaction medium. Since relative permittivities and dipole moments did not give satisfactory results, there was a strong need for a microscopic parameter and many attempts have been made to develop empirical solvent parameters that are based on a physical parameter which is sensitive to the solvent polarity. Solvatochromic dyes have been the most successful and particularly Reichardt s t(30) solvent polarity parameter. The latter is based on the transition energy of the longest wavelength solvatochromic absorption band of the betaine dye pyridinium-N-phenoxide (in fact, this is dye number 30 in the first publication ). Its value is given by... 

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