Solid state proton transfer

Figure 10.7 Arrhenius plot for solid-state proton transfer in dihydrogen-bonded complex LiBFLj triethanolamine. The data are obtained in the framework of the Avrami-Erofeev model. (Reproduced with permission from ref. 14.)...

Naumov P, Sekine A, Uekusa H, Ohashi Y (2002) Structure of the photocolored 2-(2 ,4 -dinitrobenzyl)pyridine crystal two-photon induced solid-state proton transfer with minor stmctural perturbation. J Am Chem Soc 124 8540-8541... 

By a combination of NMR data in the solid state and GIAO calculations, the existence of a proton transfer in hemi-salt 78 (solid state proton transfer, SSPT) has been established. This is an important result because SSPT remains an infrequent phenomenon (see Sect. 3.4). 

We have published some papers concerning some less common heterocycles such as 1,2,4-diazaphospholes 257 and 258 [203], Using X-ray crystallography, CPMAS NMR, and GIAO-type calculations we have found that 257 is a cyclic dimer with localized N-H protons (similar to pyrazole dimers) while 258 is probably a tetramer (similar to pyrazole tetramers) showing ISSPT (intramolelecular solid state proton transfer). This prediction was only partly true because 258 crystallizes in two cyclic dimers, both presenting proton disorder [204] (Fig. 8). 

Herbstein FH, Kapon M, Reisner GM, Lehmann MS, Kress RB, Wilson RB, Shiau W-I, Duesler EN, Paul IC, Curtin DY (1985) Polymorphism of naphthazarin and its relation to solid-state proton transfer. Neutron and X-ray diffraction studies on naphthazarin C. Proc R Soc Lond A 399 295-319... 

The solid state and N CPMAS NMR have been applied to study 3,5-di-/er -butyl-l,2,4-diazaphosphole and 3,5-diphenyl-l,2,4-diazaphosphole. The X-ray structure of the first compound was already known (a cyclic dimer with localized N-H protons) while the structure of the second cannot be determined due to the difficulty of growing suitable single crystals. NMR results pointed out that 3,5-di-/err-butyl-l,2,4-diazaphosphole is a classical compound while 3,5-diphenyl-1,2,4-diazaphosphole is probably a tetramer showing intermolecular solid-state proton transfer. GlAOlab initio calculations have been carried out to estimate the absolute H, C and N shieldings and to assign the signals of carbons C-3 and C-5. 

Solution and solid-state proton transfer from phenols to triphenylphosphine oxide has been studied by H, and P NMR spectroscopy, with several complexes between substituted phenols and triphenylphosphine oxide having been examined. The degree of proton transfer in solution was studied by the 8ih value of the phenolic OH proton and by the 813c values of the phenol C-O (C-1) and para (C-4) carbons. The solid phase was studied through the changes in the P shielding tensor of the triphenylphosphine oxide residue and by the C-1 and... 

The tautomerism is not always detectable by X-ray diffraction performed at a single temperature. Some N-unsubstituted pyrazoles exist as tautomers that can interconvert in the solid state by intermolecular proton transfer. From 34 structures of NH pyrazoles reported by 1999, for instance, 8 structures displayed intermolecular solid-state proton transfer [11]. A typical example of a system that displays... 

Solid state proton transfer (SSPT) occurs between tautomers and, even if the initial and the final are the same (degenerate tautomerism, K = 1), it constitutes one ofthe best known kinetic processes. The loss of freedom due to the crystal structure allows for accurate kinetic models to be used, including the Car-Parrinello [51] and Bell-Limbach tunneling model [52]. This field owes much to the works of Limbach et /. [53, 54] and of Claramunt ef /. [55, 56]. 

The most extensive studies of solid-state proton transfer have been undertaken on two types of systems - complexes (salts and cocrystals) of carboxylic acid with nitrogen bases, and carboxylic acid dimers. In the first case, the proton transfer occurs along the unsymmetrical H N, and in the other along a (usually) symmetrical O- -H- -O bond. As both these types of proton transfer can (in principle) also lead to tautomerization, we shall briefly expound the main finds of these studies. 

The solid-state proton transfer and keto - enol tautomeric transformations were accomplished by heating or by mechanochemical treatment (Scheme 13.10). The thermal transition was monitored by in situ time-resolved PXRD and IR methods. The in situ monitoring revealed that the keto-enol transition proceeded through the formation of metastable enol polymorph, which finally converted to the thermodynamically stable form of enol polymorph. 

One may consider the relaxation process to proceed in a similar manner to other reactions in electronic excited states (proton transfer, formation of exciplexes), and it may be described as a reaction between two discrete species initial and relaxed.1-7 90 1 In this case two processes proceeding simultaneously should be considered fluorescence emission with the rate constant kF= l/xF, and transition into the relaxed state with the rate constant kR=l/xR (Figure 2.5). The spectrum of the unrelaxed form can be recorded from solid solutions using steady-state methods, but it may be also observed in the presence of the relaxed form if time-resolved spectra are recorded at very short times. The spectrum of the relaxed form can be recorded using steady-state methods in liquid media (where the relaxation is complete) or using time-resolved methods at very long observation times, even as the relaxation proceeds. 

Very important experiments with the labeled system LiBILj N(CH2CH20D)3 have shown that the proton transfer is slow and a rate-determining step of the transformation, as demonstrated in Scheme 10.2. Finally, it should be noted that the activation energy parameters found for solid-state proton nansfer are comparable with those measured in solution. 

Ionization reactions can occur under vacuum conditions at any time during this process but the origin of ions produced in MALDI is still not fully understood [27,28], Among the chemical and physical ionization pathways suggested for MALDI are gas-phase photoionization, excited state proton transfer, ion-molecule reactions, desorption of preformed ions, and so on. The most widely accepted ion formation mechanism involves proton transfer in the solid phase before desorption or gas-phase proton transfer in the expanding plume from photoionized matrix molecules. The ions in the gas phase are then accelerated by an electrostatic field towards the analyser. Figure 1.15 shows a diagram of the MALDI desorption ionization process. 

Kaufman V.R., Avnir D., Pines-Rojanski D., Huppert D. Water consumption during the early stages of the sol-gel tetramethylorthosilicate polymerization as probed by excited state proton transfer. J. Non-Cryst. Solids 1988 99 379-386... 

The chemical and crystallographic literature contains a copious amount of structure determinations where X-ray diffraction analysis has been invoked to provide direct evidence of the tautomeric form of a particular compound in the solid state. Many systematic studies of both ground-state and excited-state proton transfer processes of a larger number of tautomeric compounds or of the same compound under varying conditions have also been reported. Rather than attempting to provide a comprehensive overview of all known examples of tautomeric systems characterized by X-rays, we have selected here a handful of cases that illustrate how the method can be applied to unravel details that are relevant to the identity... 

The imbalance between and NMR studies in the solid state (Section VI,F) partly reflects the fact that it is easier to introduce N than into heterocyclic compounds, particularly azoles (DNMR in the solid state usually requires isotopic enrichment). Compared to solution studies, solid-state intermolecular proton transfer between tautomers has the enormous advantage that the structure of the species involved is precisely defined. 

The first observation of the proton transfer in pyrazoles in the solid state was made for the intermolecular tautomerism in 3,5-dimethylpyrazole 10b (85JA5290). The degenerate rearrangement was recorded using the... 

No proton transfers were observed in linear oligomers (catemers) of pyrazoles 8 in the solid, a fact which was understandable because such rearrangements would require a very high activation energy [97JCS(P2)101]. A possible exception to this rule is a catemer 8f, for which slow proton transfer was observed in the solid state [97JCS(P2)1867]. 

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. 

Different solid-state NMR techniques CPMAS NMR, the second moment of the signal, the spin-lattice relaxation time in the rotating frame T p) were combined to reach the conclusion that in the case of por-phine H2P the double-proton transfer is followed by a 90° rotation within the crystal (see Scheme 2). 

---