Proton tautomer

Optical detection of magnetic resonance (ODMR) was attempted for measurements of the pH effects on the triplet state of purine to investigate the protonation site of purine at low temperatures (78JA7131). The ODMR spectrum did not show the presence of more than one triplet state at liquid helium temperatures. Since the protonated tautomers 1H,9H (3a) and H,1H (3b) have similar bond structures, their triplets should have similar zero-field parameters and are thus not easy to distinguish by ODMR. 

An interesting example of the N(9)-C(8) prototropic tautomerism has been reported for the caffeine radical by pulse radiolysis studies in aqueous solution the transformation of the heteroatom-protonated electron adduct 25 into the carbon-protonated tautomer 26 occurred spontaneously in neutral media [95JCS(F)615]. 

Another system with a tendency to polymorphism and in which one might expect the different crystal forms to be composed of different proton tautomers is benzoquinone oxime-nitrosophenol (8c). Here all materials studied proved to... 

The gas-phase basicity (GB) of 3-thio-5-oxo 1, 5-thio-3-oxo 2, and 3,5-dithio 4 derivatives of 2,7-dimethyl-[l,2,4]-triazepine (Figure 1) has been measured by means of Fourier transform ion cyclotron resonance (FTICR) mass spectrometry and complemented with theoretical calculations. The experimental FTICR results are discussed in Section 13.14.4.1.l(i). The structures and vibrational frequencies of all stable protonated tautomers and all transition states connecting them have been obtained by means of the B3LPY density functional method, together with 6-31G basis set expansion. The final energies were obtained at the B3LYP/6-311 + G(3df,-2p) level (2002JPC7383). 

Unlike all the porphyrin isomers discussed above (including porphyrin itself), hemiporphycene 31 possesses two nonequivalent /ran.v-protonated tautomers, even in the case of symmetrical peripheral substitution. The two tautomers 31a-l (22H,24H) and 31a-2 (21/7,23//) were identified in the II NMR spectrum recorded at low temperatures, with a concentration ratio of 7 1. This was in agreement with theoretical calculations, which predicted a marginally higher energy for tautomer 31a-2. 

Figure 7. Structure and decay of 5-deazaflavin radicals. The 5-protonated tautomer, which would be analogous to the biologically essential blue flavo-semiquinone, is not formed, and disproportionation, which is characteristic for the flavin system, does not happen.

The results were compared with theoretical calculations for 4-methyl Imidazole (Fig. 8). At low pH corresponding to the protonated Imldazollum Ion, the agreement between theory and experiment Is remarkably good. At high pH corresponding to the neutral Imldeizole moiety, It becomes necessary to consider the two tautomeric forms of Imidazole. The observed coupling constant Is related to the fractional population of the N-1 protonated tautomer, f, by ... 

Consistent with the nucleophilic character of the sulfur in HS03, one might expect that the S-protonated tautomer should also be reasonably stable. Oxygen NMR studies strongly suggest the presence of both species in equilibrium in aqueous solutions ... 

In addition to proton tautomers shown here, other tautomers (broadly defined as positional isomers... 

It is argued that the intermediate bearing both mercury (Fig) and a hydroxyl group (OH) attached to the double bond then loses mercury (Hg ) to produce the enol, which is generally nonisolable and which rapidly undergoes proton tautomer-... 

Problem 8.3. Assume that any ketone can be simply represented by the carbonyl fragment shown in the figure and any enol by the corresponding two carbon fragment with which the former is in equilibrium. Using the bond energies of Chapter 1 (Table 1.1), estimate the relative stabilities (expressed as the difference in energy [A ]) of the two proton tautomers. 

Figure 8.9. Proton tautomers of hydroxybenzene (phenol), 1,3-dihydroxybenzene (resorcinol), and 1,3,5-trihydroxybenzene (phloroglucinol).

Scheme 8.102. The formation of furfural (furan 2-carboxaldehyde) from pentoses (typical examples, D-ribose and its proton tautomer D-ribulose, are shown) on treatment with sulfuric acid (Scheme 8.75).The conversion to furan 2-carboxylic acid,furan (oxa-2,4-cyclopentadiene), and tetrahydrofuran (THF, oxacyclopentane, oxolane) follows.

The mTMP anion has only two sites for protonation, namely, the oxygen atoms of carbonyl groups. However, each of these protonated tautomers possesses an additional degree of fieedom caused by rotation around the C-O bond, leading to existence of conformers with cis and trans orientation of the hydrogen atom of the protonated carbottyl group with respect to the N3 atom (Fig. 5.13). 

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