Tautomerism in the solid state

Short-lived organic radicals, electron spin resonance studies of, 5, 53 Small-ring hydrocarbons, gas-phase pyrolysis of, 4, 147 Solid state, tautomerism in the, 32, 129 Solid-state chemistry, topochemical phenomena in, 15, 63 Solids, organic, electrical conduction in, 16, 159 Solutions, reactions in, entropies of activation and mechanisms, 1, 1 Solvation and protonation in strong aqueous acids, 13, 83 Solvent effects, reaction coordinates, and reorganization energies on nucleophilic substitution reactions in aqueous solution, 38, 161 Solvent, protic and dipolar aprotic, rates of bimolecular substitution-reactions in,... 

Solid state, tautomerism in the, 32, 129 Solid-state chemistry, topochemical phenomena in, 15, 63 Solids, organic, electrical conduction in,... 

The tautomerism in the solid state and in solution of five 4-bromo-1 //-pyrazoles has been studied by multinuclear magnetic resonance spectroscopy <07T8104>. When there is a bromine atom at position 3(5), the tautomer present in all cases in the solid state and in the solution state is the 3-bromo one. 

Oxazolidines are subject to ring-chain tautomerism. A variety of substituted oxazolidines in the solid state exist in the chain form, based on C NMR experiments <85X5919,92X4979). In solution, the two forms are in equilibrium, the position of which depends on the solvent and the substituents. Oxazolidines prepared from meta- and para-substituted benzaldehydes and 2-amino-2-methyl-propanol, norephedrine, norpseudoephedrine, and serine esters all give good linear plots for the equation log -l-log Ax=h where a are the Hammett-Brown values <93X6701,93JOC1967). 

Tautomeric equilibria of substituted 9(10//)-acridinethiones 71 (R = H, 2-C1, 3-Cl, 4-C1, 2-OMe, 2-OEt, 4-OMe, 2-N02, 3-N02) have been studied by IR, UV, and mass spectroscopy. It has been shown that the thione form predominates both in the solid state and in the gas phase (only for 71 (R = H, 2-C1) the appreciable amounts of the thiol tautomers were detected in the gas phase) in solution, however, the position of the tautomeric equilibrium depends on the temperature and solvent... 

Tautomerism may be of many kinds as illustrated in this book and in Ref. [1]. However, tautomerism may be less obvious depending on the rates of interconversion or the equilibrium constant may be very much in favor of one form. A typical example of the latter is acetone and other ketones. Nevertheless, tautomerism plays a major role in the chemistry of alkyl ketones. Tautomerism may occur both in the gas phase, namely the liquid and condensed phases, and in the solid state, namely in the ground state as well as in the excited state. As tautomerization often involves the movement of light atoms, proton transfer is the usual case and hydrogen bonding will often be involved. Tautomerization can of course be both intra- and intermolecular, and in the latter case a solvent molecule may also be involved. For some characteristic cases, see Ref. [1, Chapter 3]. 

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). 

As already explained in the relevant section, the use of X-ray crystallography (Section V,D,2), the possibility to determine the molecular structure from powder diffraction without the need to obtain monocrystals, and the many variants of solid-state NMR (Section VI,F) have profoundly enhanced the study of tautomerism in the solid state. 

Five possible isomers may coexist in the tautomeric equilibrium involving 2,6-diazidopurine 343 (Scheme 135). It was concluded (75UK1028) that diazide 343AA is the sole form in the solid state and in solutions in acetic and trifluoroacetic acids. In DMSO solution, two additional tautomers, 343AT and 343ATS were also observed. 

Tliere are several reasons for this great interest in the tautomerism of porphyrins (which could justify its own review) (1) their biological significance, (2) their applications in material science ( hole burning is related to their tautomerism), (3) the simplicity of the system (annular tautomerism involving intramolecular proton transfer both in solution and in the solid state), and (4) the possibility of elucidating the kinetic processes in great detail. 

Tautomeric studies of equilibria between enamino and methylene imino form have been reported for several heterocycles. Tautomerism of 1,2,4-tri-azino[4,3- ]quinoxalin-5-ones was deduced to be solvent dependent the enamino form 170b is predominant in DMSO-dg ( H NMR), whereas 170a is the major tautomer in the solid state (IR in nujol) (90JHC691 95H2057) (cf. Section III,B). 

---