Tautomerization rates

It is well-known that para substituents on the phenyl groups of H2TPP have no influence on the tautomerism rates in the ground state (see Section III,A,1). In the case of PHB, there seems to be only a small substituent effect on <1>phb (the quantum efficiency for hole burning) through modification of the relative energy of Ti (93CM366). 

Apart from the obvious use of proton NMR spectroscopy as an aid in structure elucidation, the technique has also been used to probe the phenomena of intermolecular aggregation, apical ligand binding, NH tautomerism, rates of protonation, and a host of theoretical aspects. An extensive review of porphyrin and chlorin NMR spectroscopy has appeared... 

If this reaction is observed polarimetrically, the equilibrium rate constant obtained is equal to the sum of the tautomerization rate constants (Ki -f- K. i) for the overall reaction, as described above (20). Therefore, no matter what the path or the mechanism of the mutarotation phenomenon, the data obtained polarimetrically are completely explained by the above overall configuration change. 

The keto-enol tautomerism rates for oxaloacetate and zinc(II)-oxaloacetate have also been investigated using acetate buffer solutions.339,357 Thus for the equilibrium shown in equation (24) the value of fcf for the reaction oxacketI12 +OAc is 6.6x 10 3 M"1 s l, while for Zn(oxac)keto+ OAc , fcf= 25 M-1 s 1. The rate acceleration at 25 °C is ca. 4 x 103. Metal ion-promoted enolization is considered in detail in Section 61.4.20. 

Covey and Leussing330,357 have studied keto-enol tautomerism rates for oxaloacetate (oxac2-) and zinc(II) oxaloacetate complexes in acetate buffer solutions at 25 °C. Thus for the equilibrium oxacketo2 oxaceno,2- the value of fcf, the rate constant for the forward reaction, for the reaction oxacketo2 + OAc is 6.6 x 10 3 M-1 s 3 while for Zn(oxac)kcto) + OAc, kf is 25 M 1 s 1. The rate... 

Watarai H, Suzuki N. Keto-enol tautomerization rates of acetylacetone in mixed aqueous media. J Inorg Nucl Chem 1974 36 1815-1829. Great Britain Pergamon Press. 

Determination of Tautomerization Rates from Anisotropy Measurements... 

Fluorescence anisotropy measurements can also be used to obtain the rates of the excited state tautomerization. Two variants can be applied. The first is based on the analysis of time-resolved anisotropy curves. These are constructed from measurements of the fluorescence decay recorded with different positions of the polarizers in the excitation and emission channels. The anisotropy decay reflects the movement of the transition moment and thus, the hydrogen exchange. For molecules with a long-lived Sj state, the anisotropy decay can also be caused by rotational diffusion. In order to avoid depolarization effects due to molecular rotation, the experiments should be carried out in rigid media, such as polymers or glasses. When the Sj lifetime is short compared to that of rotational diffusion, tautomerization rates can be determined even in solution. This is the case for lb, for which time-resolved anisotropy measurements have been performed at 293 K, using a... 

Figure8.16 Arrhenius plot for tautomerization rates obtained fori in polyvinyl butyral (see also Fig. 8.15).

The data presented in this work demonstrate large differences between the tautomerization characteristics in porphycene and its parent isomer, free base porphyrin. The origin for these dissimilarities may be due to different electronic structure or to distinct geometries. It seems at present that the latter are mainly responsible for the different patterns of tautomerism in 1 and 2. In particular, strong, nearly linear hydrogen bonds in porphycenes lead to low barriers and rapid tautomerization rates. 

Covey and Leussing have studied keto-enol tautomerism rates for oxaloacetate (oxac ) and zinc(II) oxaloacetate complexes in acetate buffer solutions at 25 °C. Thus for the equilibrium... 

The first step appears to be the conversion of the nitro compound 4 into the tautomeric nitronic acid 6a. Base catalysis in the tautomerization rate between nitro compound and nitronic acid is well established. It might be argued that, as the case of p-diketones, the tautomerization rate could be accelerated also in acid, as assumed in the case of p-tolue-nesulfonic acid [21]. However, there is evidence that acids do not modify the conversion rate in this tautomerism [89,90]. 

A second method for studying tautomerization rates makes use of the fact that in vapor or in different solvents the equilibrium constant between the tautomers can be vastly different. Watarai et al. ]71, 72, 73, 74] studied the tautomerization rate of acetelacetone (Figure 1.9) in a variety of solvents and solvent mixtures. Their method makes use of the fact that, in the vapor, the enol content is 93.3%, whereas in water the enol fraction is only 0.15. The reaction can be followed by UV spectroscopy, since the enol has an absorptivity s = 11 000 at 273 run, whereas the... 

Watarai, H. and Suzuki, N. (1976) Keto-enol tautomerization rates of acetylacetone in water-acetonitrile and water-dimethyl sulfoxide mixtures. J. Inorg. Nucl. Chem., 38, 1683-1686. 

The corresponding tautomerism of 2-hydroxy thiophenes and 2- and 3-hydroxy selenophenes has been compared by Gronowitz [93]. The tautomerization rates were measured and compared with those of the hydroxy furan derivatives. Tautomerization is faster in the selenophenes than in thiophenes and especially when compared to the corresponding furans. 

STM tip during a voltage pulse to induce the tautomerization. (b) Ball-and-stick model of the inner cavity, (c) Voltage dependence of the tautomerization rate. (Reproduced from Liljeroth et al. [27]. Copyright (2007), with permission of AAAS.)... 

Figure 7.11 (a-c) STM images of a single porphycene molecule on Cu(110) at different temperatures (as indicated). The fuzzy appearance results from switching between two orientations [cis-cis tautomerization). (d) Tunneling current dependence of the tautomerization rate measured at different... 

Figure 7.15 (a) Voltage dependence of the cis-cis tautomerization yields measured for a H- (circles) and D-porphycene (squares) molecule, respectively, at 5 K (the latter molecule is sketched in the inset). Yields beyond 380 mV cannot be measured, due to the very high tautomerization rates and the STM time-resolution of approximately a few hundred microseconds. The inset shows the voltage dependence around the threshold in both bias polarities, (b) Tunneling current... 

---