Intramolecular proton transfer reaction

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... 

Next, Ch. 11 by Lochbrunner, Schriever and Riedle deals with excited electronic state intramolecular tautomerization proton transfers in nonpolar, rather than polar, solvents. But there is a connection to the previous chapter the ultrafast optical experiments discussed here emphasize evidence that the proton is not the reaction coordinate. The proton transfer is controlled by low vibrational modes of the photo-acids, rather than by the proton motion itself, an interpretation supported by separate vibrational spectroscopic studies and theoretical calculations The key role of modes reducing the donor-acceptor distance for proton transfer is highlighted, and for the featured molecule of this chapter, the proton adiabatically follows the low frequency modes, and no tunneling or barrier for the proton occurs. (See also Ch. 15 by Elsaesser for direct ultrafast vibrational studies on these issues). 

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. 

As discussed below, studies of the multiple kinetic hydrogen/deuterium isotope effects show that the reactions of 16-19 exhibit only a single barrier, involving a concerted hydrogen bond compression followed by a concerted multiple proton transfer. The different behavior of these complexes as compared to the stepwise intramolecular multiple proton transfers will be discussed later. 

The situation presented in fig. 29 corresponds to the sudden limit, as we have already explained in the previous subsection. Having reached a bend point at the expense of the low-frequency vibration, the particle then cuts straight across the angle between the reactant and product valley, tunneling along the Q-direction. The sudden approximation holds when the vibration frequency (2 is less than the characteristic instanton frequency, which is of the order of In particular, the reactions of proton transfer (see fig. 2), characterised by high intramolecular vibration frequency, are being usually studied in this approximation [Ovchinnikova 1979 Babamov and Marcus 1981]. 

A large red shift observed in polar solvents was indicative of the intramolecular charge transfer character of the triplet state. The change of dipole moment accompanying the transition Tj - Tn, as well as rate constants for electron and proton transfer reactions involving the T state of a-nitronaphthalene, were determined. The lower reactivity in polar solvents was attributed to a reduced n-n and increased charge transfer character of the triplet state... 

The ionization of (E)-diazo methyl ethers is catalyzed by the general acid mechanism, as shown by Broxton and Stray (1980, 1982) using acetic acid and six other aliphatic and aromatic carboxylic acids. The observation of general acid catalysis is evidence that proton transfer occurs in the rate-determining part of the reaction (Scheme 6-5). The Bronsted a value is 0.32, which indicates that in the transition state the proton is still closer to the carboxylic acid than to the oxygen atom of the methanol to be formed. If the benzene ring of the diazo ether (Ar in Scheme 6-5) contains a carboxy group in the 2-position, intramolecular acid catalysis is observed (Broxton and McLeish, 1983). 

A true intramolecular proton transfer in the second step of an azo coupling reaction was found by Snyckers and Zollinger (1970a, 1970b) in the reaction of the 8-(2 -pyridyl)-2-naphthoxide ion (with the transition state 12.151). This compound shows neither a kinetic deuterium isotope effect nor general base catalysis, in contrast to the sterically similar 8-phenyl-2-naphthoxide ion. Obviously the heterocyclic nitrogen atom is the proton acceptor. 

The minimum-energy TSs are planar and the O—H and C—H bond orders were usually less than 0.4 and less than 0.5, respectively, and the S—C bond order was less than 0.5. The C-C bond order was around 1.3. The reaction can be described as a concerted intramolecular proton transfer, with the sulfoxide oxygen acting as a base and the sulfur as a leaving group. 

Formosinho, S. J. Amaut, L. G. Excited-state proton transfer reactions. II. Intramolecular reactions. J. Photochem. Photobiol. A Chem. 1993, 75, 21—48. 

The rate maximum at pH 4 is assigned to a specific reaction of the monoester anion 104 which exists exclusively under these conditions. Westheimer 57) first advanced a metaphosphate ion mechanism in which 102 is formed via a six-membered monoester-anion/water complex (103). An intramolecular proton transfer via a four-membered ring according to 105 m is also conceivable, as is the formation of a zwitterion 106 in a prior protonation equilibrium. 

As for the acetyl phosphate monoanion, a metaphosphate mechanism has also been proposed 78) for the carbamoyl phosphate monoanion 119. Once again, an intramolecular proton transfer to the carbonyl group is feasible. The dianion likewise decomposes in a unimolecular reaction but not with spontaneous formation of POf as does the acetyl phosphate dianion, but to HPOj and cyanic acid. Support for this mechanism comes from isotopic labeling proof of C—O bond cleavage and from the formation of carbamoyl azide in the presence of azide ions. 

Figure 11. CID mass spectrum of precursor ion Ni(H20) +, which shows that Ni(H20) + undergoes intramolecular proton transfer (see equation 23), which leads to NiOH (H2O2 and H30+. This reaction is competitive with simple H2O loss leading to Ni(H20) +. From Blades, A. T. Jayaweera, P. Ikonomou, M. G. Kebarle, P Int. J. Mass Spectrom. Ion Proc. 1990, 101, 325 with permission.

Excited-state intramolecular proton transfer (ESIPT) exhibits different regularities [49, 50]. Commonly, this is a very fast and practically irreversible reaction proceeding along the H-bonds preexisting in the ground state. Therefore, only the reaction product band is seen in fluorescence spectra. Such cases are not interesting for designing the fluorescence reporters. The more attractive dual emission is... 

Oncul S, Demchenko AP (2006) The effects of thermal quenching on the excited-state intramolecular proton transfer reaction in 3-hydroxyflavones. Spectrochim Acta A Mol Biomol Spectrosc 65 179-183... 

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