Double proton transfers

Pyridone also catalyzes epimerization of the anomeric position of the tetramethyl ether of glucose. The mechanism involves two double proton transfers. The first leads to a ring-opened intermediate, and the second results in ring closure to the isomerized product ... 

New IR techniques introduced for the study of prototropic tautomerism include IR dicroism for the photoinduced double proton transfer in por-phine 71 (Scheme 24) [89CPH(136)165], and IR spectroscopy in a supersonic jet (less than 50 K) for demonstrating the presence, in these conditions, of 2//-benzotriazole (57b) [96CPL(262)689]. 

Isotope effect between the HH, HD, DH, and DD isotopomers was used as an important tool to determine the mechanism of the double-proton transfer. For concerted degenerate double-proton transfers in the absence of tunneling, the rule of the geometrical mean (RGM) should hold in good approximation, which states that /chh/ hd = /cdh/ dd-Tunneling may lead to a breakdown of this rule but the relation /chh > hd = dh > dd should remain valid. In the absence of secondary isotope effects the relation /chh HD = DH = 2 /cdd sliould liold for a stepwise pathway, even if tunneling is involved. 

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). 

FIGURE 7.2. Two alternative mechanisms for the catalytic reaction of serine proteases. Route a corresponds to the electrostatic catalysis mechanism while route b corresponds to the double proton transfer (or the charge relay mechanism), gs ts and ti denote ground state, transition state and tetrahedral intermediate, respectively. 

Double proton transfer mechanism, see Serine proteases, charge-relay mechanism... 

As a result, similar to 7AI, 3CNAI, and 3,5CNAI undergo methanol-catalyzed excited-state double proton transfer (ESDPT), revealing dual (normal and proton transfer) emission. However, proton transfer is prohibited for 5CNAI and DiCNAI... 

Chou PT, Yu WS, Wei CY et al (2001) Water-catalyzed excited-state double proton transfer in 3-cyano-7-azaindole the resolution of the proton-transfer mechanism for 7-azaindoles in pure water. J Am Chem Soc 123 3599-3600... 

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... 

J. M. Lopez, F. Mannle, I. Wawer, G. Buntkowsky, and H. H. Limbach, NMR studies of double proton transfer in hydrogen bonded cyclic N,N diarylformamidine dimers Conformational control, kinetic HH/HD/DD isotope effects and tunneling. Phys. Chem. Chem. Phys. 9, 4498 4513 (2007). 

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

The double proton transfer of [2,2 -Bipyridyl]-3,3 -diol is investigated by UV-visible pump-probe spectroscopy with 30 fs time resolution. We find characteristic wavepacket motions for both the concerted double proton transfer and the sequential proton transfer that occur in parallel. The coherent excitation of an optically inactive, antisymmetric bending vibration is observed demonstrating that the reactive process itself and not only the optical excitation drives the vibrational motions. We show by the absence of a deuterium isotope effect that the ESIPT dynamics is entirely determined by the skeletal modes and that it should not be described by tunneling of the proton. 

After the ESIPT the molecule exhibits a pronounced ringing in this mode. The 295 cm-1 mode is a symmetric in-plane stretching vibration (see Fig. 3). The corresponding contraction of the molecule reduces the donor acceptor distances in both chelate rings simultaneously and initiates the electronic configuration change of the concerted ESIPT. The concerted double proton transfer leads therefore to a ringing of the molecule in this second mode. 

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 .

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. 

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). 

DFT calculations were performed for the double proton transfer in bicyclic 2,2 -bis(4,5,6,7-tetrahydro-l,3-diazepine) (Figure 8) <2001CPL591>. Both a concerted and a stepwise mechanism for proton transfer are considered. Though the concerted transition state has two imaginary eigenfrequencies, dynamical calculations have demonstrated that it has to be taken into account in the mechanism of the proton transfer even if it is not a true reaction path. 

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. 

---