Intramolecular Double Proton Transfer

6 and 29.7, which supports concerted transfer, is weak in porphine. 

Two sets of experimental data are available, one set measured by NMR spectroscopy in the range 200-300 K [8, 9] and another set measured by optical spectroscopy in the range 95-130 K [10]. Arrhenius plots of the data shown in Fig. 29.11 are slightly curved at low temperature the slope approaches a constant value close to the calculated cis-trans energy difference of 8.3 kcal moh. The observation that double proton transfer remains thermally activated by roughly this amount down to low temperatures, immediately suggests that the process proceeds stepwise. 

The same conclusion follows from the observation that the HD Arrhenius curve is closer to the DD than the HH curve, in agreement with Eq. (29.56). 

Using the approach of Section 29.2, we can estimate the proton-proton coupling parameter G by associating the extrema of the calculated potential with those of the model potential. However, for this we require crude-adiabatic poten- 

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Various nmr techniques have been used to investigate the intramolecular double proton transfer which occurs in the tautomerisation of meso-tetra-phenylporphyrin (40) (Limbach et al., 1982). The reaction has been studied (Storm and Teklu, 1972) by observation of the nmr signals due to the protons... 

In conclusion, a typical time of 300 fs has been found for the excited-state intramolecular double proton transfer in TAB and DAC. The proton transfer dynamics is not influenced by aggregation. In addition, a vibronic cooling time of 20 ps has been measured for the probe molecules in the molecular and stacked configurations. Finally, aggregation is found to almost completely hamper the rotational diffusion motions of the molecules during the fluorescence state lifetime of 4 ns. 

The rate constants of the intramolecular double-proton transfer of N-labelled porphine crystal have been determined by CP/MAS nmr spectroscopy, and the HH/HD/DD kinetic isotope effects have also been obtained. TTie observed kinetic isotope effects for porphine at 273 K are knu/kHD = 17, and kHn/ oo = 1-9 (Braun et al, 1994). Judging from the large difference of the isotope effect, the double-proton transfer is considered to proceed by the stepwise mechanism. The observed tendency is different from that of the tautomerization of carboxylic acids. The stepwise mechanism is also observed in solution. 

In the case of intramolecular double proton transfer a wavepacket motion is found which depends, via the excess energy, on the branching ratio between concerted double and single proton transfer. It demonstrates that the coherent wave-packet dynamics in ESIPT molecules is driven by the ESIPT itself and is specific for the reaction path. 

Tautomeric processes can be part of a more complex reaction network as is demonstrated in the case of indigodiimine 10, which exhibits an intramolecular double proton transfer [33] illustrated in Figure 14.6. This process renders the two halves of the molecule equivalent. An even faster NH2 rotation renders all NH protons equivalent. 

H. Bulska, Intramolecular cooperative double proton transfer in (2,2 -bipyridy )-3,3 -diol,Cfem. 

Pyrazolines have been used as models of intramolecular dyotropy (87T5981, 93JCS(P2)l2li). By combining primary deuterium kinetic isotope effects and X-ray crystallography, polycyclic systems, like (426), were shown to undergo a double proton transfer to (427). 

The 1977 review of Martynov et al. [12] discusses existing mechanisms of ESPT, excited-state intramolecular proton transfer (ESIPT) and excited-state double-proton transfer (ESDPT). Various models that have been proposed to account for the kinetics of proton-transfer reactions in general. They include that of association-proton-transfer-dissociation model of Eigen [13], Marcus adaptation of electron-transfer theory [14], and the intersecting state model by Varandas and Formosinho [15,16], Gutman and Nachliel s [17] review in 1990 offers a framework of general conclusions about the mechanism and dynamics of proton-transfer processes. 

Environmental effects, especially changes in solvent polarity, on the photophysical properties of dyes have been described. Likewise, solvation dynamics have been measured for dyes that undergo a substantial increase in dipole moment following excitation.Light-induced intramolecular proton transfer is an important route for non-radiative deactivation of an excited state and has been studied extensively in recent times. Double proton transfer has been reported for [2,2 -bipyridyl]-3,3 -diol ° and for 7-azaindole. ... 

Let us first discuss the intramolecular degenerate double proton transfers in azo-phenine [21] and in oxalamidines [22a[. By liquid state NMR of the labeled compounds the intramolecular pathways of the transfer processes were established and the rate constants fenn, k = and when possible k were measured. The Arrhenius diagrams are depicted in Fig. 6.40. In all cases, the reactions... 

Surprisingly, both dimethylamine and trimethylamine were able to pick up the mobile proton of the triazene at one nitrogen atom and carry it to the other nitrogen atom, resulting in an intramolecular transfer process catalyzed each time by a different base molecule. Even more surprising is that the intramolecular transfer (Fig. 6.47(a)) catalyzed by dimethylamine is faster than the superimposed inter-molecular double proton transfer (Fig. 6.47(b)). 

For compounds with parallel or branching intramolecular reaction channels, the interesting question arises whether also coherent wavepacket dynamics can be observed there and whether it can be used to analyze the different channels. This can be studied with the ESIPT compound [2,2 -bipyridyl]-3,3 -diol (BP(OH)2) which contains two H-chelate rings (Figure 4.11) and exhibits both single and concerted double proton transfer in aprotic solvents after photoexcitation. The single proton transfer occurs within 100 fs and leads to an intermediate mono-keto isomer which subsequently transforms with a time constant of 10 ps to the final diketo form [72, 73]. In addition, a second reaction channel exists that leads to the final diketo product within less than 100 fs by a simultaneous transfer of both protons. 

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