Intramolecular tautomerizations environment

Examples of Intramolecular Tautomerizations without Requiring Reorganization of the Environment... 

In this section we will discuss some typical examples exhibiting intramolecular tautomerizations whose kinetics have been studied by dynamic NMR in various environments. Often, the gas phase and liquid state degeneracies were found to be slightly disturbed in the solid state. In most cases, nonlinear Arrhenius curves were observed, which indicated the role of vibrationally assisted tunneling, supported in almost all cases by the observation ofnonclassical kinetic isotope effects. 

Optical properties of 3-hydroxyflavone (3-HF) doped sol-gel derived glass were studied (Carturan, 2003). The intramolecular excited-state proton transfer process is strongly affected by the chemical environment. The main results are that the Stokes-shifted emission from 3-HF tautomeric form is enhanced at increasing trifunctional alkoxide amount and at decreasing polarity of the non-hydrolizable groups. 

Phenyldiazenyl)naphthalen-l-ol (2) is one of the first tautomeric compounds discovered [39] and, in spite of its limited practical applicability, is one of the most studied [40]. Its attractiveness is due to the lack of intramolecular hydrogen bonding, as for instance, in 4, which makes the tautomeric equilibrium very sensitive to changes in the solvent environment. Hence, this compound is a suitable model, as shown in Chapters 11 and 13, to test the description of solvent effects both theoretically and experimentally. At the same time, the shift in the position of the equilibrium, caused by the temperature, is, in most solvents, very small slight and relatively difficult to be processed [13]. 

The existence of intramolecular hydrogen bonding in compounds l-(phenyl diazenyl)naphthalen-2-ol (4) and l-((phenylimino)methyl)naphthalen-2-ol (5) makes the tautomeric equilibrium less sensitive to changes in the solvent environment However, two main differences have to be outlined (i) the tautomeric equilibrium in the azo naphthol 4 is less influenced by the solvent, a phenomenon which will be discussed later in Section 2.3.3 and (ii) the change in the solvent leads to solvatochromic shift in the spectra of 4 [41] and not in these of 5 [32]. 

Keto-enol (quinonehydrazone-azo) tautomerism in 4-(phenyldiazenyl)naph-thalen-l-ol (25a) has been known for more than a century [62] and has been very intensively investigated both theoretically and experimentally [63,64]. Due to the lack of intramolecular hydrogen bonding, the tautomeric equilibrium is very sensitive to the changes in the environment [65] and, for a long period of time no experimental conditions have been described where only one of the tautomers are presented individually in solution (Figure 12.13). 

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