Gas phase tautomerism

Uracil, thymine, and cytosine have been studied using this technique (89JA2308 and references therein). For uracil and thymine, the dioxo tautomer predominates in the case of cytosine (70), three tautomers were detected, 70a, 70b, and 70c, the last one being the least abundant. The gas-phase tautomeric equilibrium of 2-pyridone 15a and 2-hydroxypyridine 15b has been studied by MW spectroscopy (93JPC46) using both a conventional spectrometer and a jet-cooled millimeter-wave spectrometer. The relative abundances are 3 1 in favor of the hydroxy form 15b, which exists in the Z conformation shown (Scheme 23). 

Salter LM, Chaban GM (2002) Theoretical study of gas phase tautomerization reaction for the ground and first excited electronic states of adenine. J Phys Chem A 106 4251... 

Tautomerism has been investigated in 3,5-dimethyl-2-(2 -pyridyl)pyrrole and 3,5-di-/< /t-butyl-2-(2 -pyridyl)pyrroles in the gas phase, in their alcohol complexes, and as dimers <2004PCP3938>. The compounds exist preferentially in the normal ry -conformation, which in the dimethyl compound is energetically favored over the /f-conformation by 4.3 kcal moP with a rotational barrier of 11.3 kcal mol , and are more stable than their tautomers in the gas phase. Tautomerism is observed in pyrrole-2,5-diacetic acid and its diethyl ester with the pyrrole form being favored for the free acid and the pyrrolidinediylidene tautomer for the diester <2003NJC1353>. 

A related study was undertaken by Zeng and Ren who examined the tautomer-ization of 2-aminothiazole in the gas phase and in solution. They considered more different scenarios, including the isolated molecules, the isolated molecules interacting with 1, 2, or 3 water molecules, and two interacting molecules. In all cases, they considered a gas phase tautomerization as well as reactions occurring in water or... 

The gas-phase tautomeric equilibrium of the selenium analog, 2,4-diselenouracil 171 (X = Se), was studied at the MP2 level of theory with a basis set of DZP quality. The relative energies, estimated at the MP4(SDQ) level, provided the same order of stability, predicted for 2,4-dithiouracil by the ab initio methods. The di(hydroselenyl) tautomer 171b (X = Se) was found to be less stable than diselenone tautomer 171a by 8.1 kcal/mol (96JST(388)237). 

Lin J, Yu C, Peng S, Akiyama I, Li K, Lee LK, Lebreton PR (1980) Ultraviolet photoelectrrai studies of the ground-state electronic-structure and gas-phase tautomerism of purine and adenine. J Am Chem Soc 102(14) 4627 631... 

The large influence of the solvent in the tautomerism of heterocycles has been stressed in a variety of compounds (for review see refs 20, 108 and references therein), and in some cases the gas phase tautomeric preferences are fully reverted in aqueous solution. A well known example is the keto/enol tautomerism in 4-piridone, for which the ratio in the population between keto and enol tautomers changes by more than 104 folds upon transfer from gas phase to aqueous solution [108 and references therein]. 

Proton transfer is another type of reaction of great interest to chemists, especially to biochemists, for it occurs quite often in biological systems. For an illustration, we chose the study of Zhang et al. on the intramolecular proton transfer for the tautomerization of foramidine NH2-CH=NH. The mechanism of the gas-phase tautomerization is shown in Figure 5. 

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