Tautomers temperature dependence

Although not a heteroaromatic compound, the case of citrinin studied by Destro and Luz ([97JPC(A)5097] and references therein) is so significant that it deserves mention here. Citrinin exists in the crystal as a mixture of the p-quinone 5a and o-quinone 5b tautomers (Scheme 3). The equilibrium ii temperature dependent and by using CPMAS NMR (Section VI,F) and, more remarkably. X-ray crystallography, the authors were able to determine the AH and AS values (the rate is extremely fast on the NMR time scale, >10 s ). 

Figure 7. Temperature dependence of the Q(0,0) longwave bands optical densities of compound 10 NH-tautomers 1 and 2 (T = 77 - 500 K) and schematic diagrams of energies of the two lowest singlet (6o,Si) states of tautomers at 77 K according to experimental data.

Study of kinetically stable azetes (e.g. 43, R = Bu , Mes) reveals their capability to exist in the form of two valence tautomers differentiated by the position of ring double bonds. Their interconversion can be followed by temperature-dependent NMR spectroscopy (86AG(E)842, 88AG(E)1559). Interestingly, for 2,3-di-ferf-butyl-4-mesitylazete form (43a) is thermodynamically more stable. 

Dimethoxycarbonylbenzene oxide (98) exists in equilibrium with the oxepin form (159), the latter predominating at equilibrium at ambient temperature.8 The NMR spectrum shows temperature dependence, indicating that a considerable amount of the oxide tautomer exists at equilibrium. 4,5-Dimethylbenzene oxide (160) also exists in equilibrium with its corresponding oxepin tautomer (161). ... 

IEs were used to verify a tautomeric equilibrium in 2-phenylpropenal-c/ (47 + 48) and to distinguish this mixture from a symmetric structure with a hydrogen centered within the hydrogen bond.120 According to 13C and H chemical shifts, the equilibrium favors 48. From the temperature dependence AH° was estimated as -27 cm-1, or -0.08 kcal mol-1. Because both tautomers have the same conjugate base, 47, which is less stable, must be the stronger acid, owing to a secondary deuterium IE. 

Table 42 gives an overview of annular tautomerism data for azoles in the gas phase and in solution or crystals. In the gas phase the stability of alternative tautomers largely depends on their relative aromaticities. In Section 2 A.4.2.2 it was noted that 1,2-relationships between pyrrole- and pyridine-type nitrogen atoms favor aromaticity (Figure 21) and this is consistent with the relative stabilities of triazole and tetrazole tautomers in the gas phase (Table 42) <2010T2695>. In solution (and crystals) other factors such as solvent polarity, hydrogen bonding, and temperature become important and the relative stabilities can be reversed. Polar solvents tend to stabilize the tautomer with the largest dipole moment and this probably accounts for the observation of both 2H-1,2,3-triazole (p = 0.12D) and H-1,2,3-triazole (p = 4.55D) in...

Helene and Douzou concluded that tautomer 42 is more stable by 1 kcal/mole than tautomer 43 in aqueous solution. Morita and Nagakura determined the corresponding energy differences and the entropy change to be 1.3 kcal/mole and 4.7 cal/mole degree, respectively, on the basis of the temperature dependence of the absorption spectrum of the molecule. The ionic forms (cation and anion) of isocytosine have been shown to have the structures 53 and 54, respectively, by and NMR spectroscopic studies. The NMR study showed also that the dication of this molecule has a structure similar to 53 with a... 

Very recently, a low-temperature NMR study of tautomerism in purine derivatives has been conducted. The temperature dependences of the H NMR spectra were studied in DMF- /j solution. Two sets of signals were observed in the NMR spectra of the two purine derivatives at 213 K, whereas at laboratory temperature, there was only a single set of signals, reflecting the time-averaged contribution of the two components. Based on the characteristic values of the and N chemical shifts and the three-bond scalar coupling constants, the two components were determined to be the N -H and N -H tautomers. The NMR parameters obtained for the two tautomers of 6-methoxy purine are shown in Fig. 12. 

The ability to measure low temperatures using C is also important, and one very sensitive system was based on the temperature dependent C chemical shifts for certain substituted semibullvalenes that exist as a pair of different valence tautomers. Equilibration between tautomers occurs rapidly on the NMR timescale via non-degenerate Cope rearrangement, and this system is suitable for measurements from 100 to 300 K, using chlorodifluoromethane and perdeuterated dimethyl ether (3 1 or 4 1 v/v) as solvents. The two carbons 6 and 2, shown in Fig. 8, have very different chemical shifts, with the difference, in terms of an equilibrium constant K, given by ... 

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