Tautomerization. hydrogen transfer

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

The photoinitiated colour change of aromatic nitro compounds such as the dinitrobenzyl-pyridine (140) is due to a hydrogen transfer from the methylene bridge to the oxygen of the nitro group (aci-nitro tautomerism, Scheme 14) (60JCP(32)1888). 

The keto-enol tautomerization in the excited triplet state of 2-methylacetophenone is associated with hydrogen transfer in the CH O fragment ... 

Charge density analysis has been carried out for three reaction paths involving intramolecular hydrogen transfer the keto-enol tautomerism of acetaldehyde, the pinacol rearrangement of protonated ethane-1,2-diol, and the unimolecular decomposition of methanediol, reactions involving H-transfer between C O, C C, and O O atoms.288... 

Figure 7 Intramolecular hydrogen transfer in ortho-hydroxy-benzoyl filters leading to an unstable photoenol which can stabilize itself by a tautomeric proton transfer.

Hydrogen transfer (tautomerism) (e.g., metal dithizonates, A-salicylidene-anilines, etc.)... 

In contrast to keto-enol tautomerism, such enol-enol tautomerism is characterized by extremely rapid hydrogen transfers. It was shown by ab initio calculations that structures 32a and 32b are more stable than the degenerate tautomeric forms 32c and 32e by 104.7 kJ mol as well as by 117 kl mol than symmetric structure 32d. According to these calculations, a synchronous tunneling of two protons must occur in the naphthazarine molecule 32 between the identical structures 32a and 32b with a frequency of 20 to 40 MHz, i.e. approximately 10" to 10" migrations of hydrogen from one oxygen atom to another per second take place. 

Photochromic isomerization of salicylaldehyde 2-quinolylhydrazone 80 involves both amino-imino tautomerism and E-Z isomerizations. When an ethanolic solution of 80a is irradiated in the UV region between 250 and 400 nm at room temperature, it is readily converted into the colored isomer 80b, which is stable in both protic and non-protic solvents at ambient temperature. This conversion presumably occurs via an intramolecular hydrogen transfer from the phenolic group, which precedes the double bond isomerization. In the dark, the reverse isomerizations take place (75JOC2512). No substituent effect on this tautomeric equilibrium has been observed (75JCS(P1)2036). 

The differences in barrier heights are even more dramatic. This is best exemplified by calculations on porphycenes 1 and la the inclusion of zero point energy results in the transition state energy being lower than that of the cis form in 1 and of both forms in la. These results show that the harmonic approximation is not appropriate for the vibrations involved in the hydrogen transfer path in porphycenes (NH stretch, in particular) and that the barriers to tautomerization must be very low. 

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