Tautomeric transition

Hendrickson DN, Pierpont CG (2004) Valence Tautomeric Transition Metal Gomplexes. 

Hemscheidt T (2000) Tropane and Related Alkaloids. 209 175-206 Hendrickson DN, Pierpont CG (2004) Valence Tautomeric Transition Metal Complexes. 234 63-95... 

Transition state for valence tautomerization. Transition state for inversion of the thiepine. 

Here we present the results of P3 studies of the VDEs of the most stable conformation of a dinucleotide—2, 2 -deoxyribodithymidine-3, 5 -monophosphate anion (dDTMP- or dTT-). The choice of this particular dinucleotide was dictated by (1) the availability of electrospray photodetachment photoelectron spectroscopy data, (2) the relatively small size of dTT-, and (3) the lack of tautomeric transitions in the parent nucleobase, thymine. The structure depicted in Figure 1 seems to be a global minimum. It was found among numerous conformations of dTT- by means of, first, molecular mechanics analysis and, second, reoptimization of the structures with the B3LYP/ 6-311 + + G model. The structure shown in Figure 1 is characterized by two H bonds between one of the phosphate oxygens and the two... 

Thus, the spectral data allow us to conclude80 that the presence of strong electron-withdrawing groups on the nitrogen atom stabilizes the quinonimine structures OSPRld-OSPRlf and hinders their tautomeric transition into spirocyclic analogs. 

It is well known that aliphatic carbonyl compounds with the hydrogens on C to the carbonyl group may undergo tautomeric transitions from the keto to the enol forms. The most stable tautomeric form of the o-state phenol molecule is in fact the enol form " . The reason why the enol form of phenol is favoured over the keto form is quite simple. ... 

It is based on the analysis of consequences of nucleobases protonation along with the details of intramolecular interactions in 2 -deoxyiibonucleotide anions. The results of om molecular simulations cast hght on relationship between the conformational namics of a molecule and the tautomeric transitions in the components of nucleotides. 

Carbonera C, Dei A, Letard J-F, Sangregorio C, Sorace L (2004) Thermally and light-induced valence tautomeric transition in a dinuclear cobalt-tetraoxolene complex. Angew Chem Int Ed 43 3136... 

The relative enei gies of n and k bases are given in Table lO. It is obvious that the canonical amino and keto forms of the new bases are exceptionally stable with respect to the amino/imino and ketofenol tautomeric transitions. It was suggested that this property makes both bases proper candidates for the species, reducing the probability of spontaneous mutations. However, calculations of the interaction energy for the jt-ac base pair reveal its notable propeller twist... 

Fast transient studies are largely focused on elementary kinetic processes in atoms and molecules, i.e., on unimolecular and bimolecular reactions with first and second order kinetics, respectively (although confonnational heterogeneity in macromolecules may lead to the observation of more complicated unimolecular kinetics). Examples of fast thennally activated unimolecular processes include dissociation reactions in molecules as simple as diatomics, and isomerization and tautomerization reactions in polyatomic molecules. A very rough estimate of the minimum time scale required for an elementary unimolecular reaction may be obtained from the Arrhenius expression for the reaction rate constant, k = A. The quantity /cg T//i from transition state theory provides... 

Since IR spectra are essentially due to vibrational transitions, many substituents with single bonds or isolated double bonds give rise to characteristic absorption bands within a limited frequency range in contrast, the absorption due to conjugated multiple bonds is usually not characteristic and cannot be ascribed to any particular grouping. Thus IR spectra afford reference data for identification of pyrimidines, for the identification of certain attached groups and as an aid in studying qualitatively the tautomerism (if any) of pyrimidinones, pyrimidinethiones and pyrimidinamines in the solid state or in non-protic solvents (see Section 2.13.1.8). 

We now discuss the annular tautomerism of azoles. For 5-phenyltetra-zole, the 2//-tautomer was found predominant in polyvinyl alcohol film (based on the determination of polarization angles of electronic transition... 

Tautomeric rearrangements of transition-metal complexes with azole ligands are relatively scarce. The fluxional behavior of the rhodium complex 43 with a neutral 3,5-dimethylpyrazole was explained as the result of rapid processes of metallotropy and prototropy occurring simultaneously (Scheme 24) [74JOM(C)51],... 

Flash vacuum thermolysis (FVT) of 2-substituted 4//-pyrido[l,2-n]pyrimidin-4-ones 126 above 800 °C afforded (2-pyridyl)iminopropadie-none (130) (99JCS(P2)1087). These reactions were interpreted in terms of reversible ring opening of 4//-pyrido[l,2-n]pyrimidin-4-ones to imidoyl-ketenes 127. A 1,5-H shift in 127 generated the N(l)H-tautomeric methylene ketene 128, in which facile elimination of HX took place via a six-membered cyclic transition state 129 to yield 130. In the case of 2-methoxy derivative 126 (X = OMe) another competing pathway was also identified at lower temperature, which resulted in the formation C3O2 and 2-methylaminopyr-idine via mesoionic isomer 131 (Scheme 9). The products were identified by IR spectroscopy. 

Many computational studies in heterocyclic chemistry deal with proton transfer reactions between different tautomeric structures. Activation energies of these reactions obtained from quantum chemical calculations need further corrections, since tunneling effects may lower the effective barriers considerably. These effects can either be estimated by simple models or computed more precisely via the determination of the transmission coefficients within the framework of variational transition state calculations [92CPC235, 93JA2408]. 

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