Single proton transfer double

Fig. 3. Excitation of vibrational modes due to different reaction channels. Concerted double proton transfer leads to a symmetric stretching vibration and symmetry breaking single proton transfer to an antisymmetric bending motion. Damping of the vibrational motion by internal vibrational redistribution is indicated by IVR .

The process entails shifting of double bonds along the polyene chain, with the formation of a "retro-retinal" structure. Peters et al. (301) interpreted their observations by identifying PBAT with an excited state of rhodopsin, where single proton transfer toward the Schiff base nitrogen leads to the formation of bathorhodopsin. This approach has been supported by the theoretical interpretation of the spectrum of rhodopsin in terms of a nonprotonated Schiff base (214-216). A mechanism involving deprotonation of the Schiff base has also been suggested (310). All these models do not require cis-trans isomerization as a primary event in the chromo-phore. 

The vibrationally adiabatic proton wave functions provide the most useful description for PCET reactions. For typical single proton transfer reactions, the lowest adiabatic vibrational state is a double well along Zp, as shown in Figure 2a. In gen-... 

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. 

Figure 29.13 Illustration of stepwise and concerted double-proton transfer in porphycene. Trans indicates the stable configuration, cis the intermediate corresponding to single proton transfer and SP(2) the saddle point of second order corresponding to concerted transfer.

The parameter controlling the concertedness is the proton-proton correlation represented by a term that is bilinear in the local proton coordinates. The model is combined with a previously developed approach to single proton transfer based on an approximate instanton method. This leads to the recognition of three coupling regimes governing the mechanism of nondassical double proton transfer. 

In contrast to the bimolecular single proton transfer reactions, large barriers may be calculated when studying double proton transfer reactions, for instance in 4- or 6-membered rings. Here, the analysis of the experimental data in gaseous or inert media... 

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. 

Macrolactonization. When a carboxylic acid is treated with 2,2 -dipyridyl disulfide in the presence of Triphenylphosphine, the corresponding 2-pyridinethiol ester is formed. Corey and Nicolaou have developed an efficient method for the synthesis of macrocyclic lactones based on these 2-pyridinethiol esters. When an m-hydroxy thiolester is heated in refluxing xylene under high dilution conditions (10 M, typically accomplished with syringe pump techniques), macrolactonization occurs, liberating triphenylphosphine oxide and pyridinethione. The reaction is quite general and is believed to proceed by a double activation mechanism in which the basic 2-pyridinethiol ester simultaneously activates both the hydroxy and the carboxylic acid moieties with a single proton transfer. It has been shown that the cyclization rate is not affected by the presence of acids, bases, or any of the possible reaction contaminants. ... 

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