Proton transfer in the ground state

Among the sampled paths chosen as above for the semiclassical Ehrenfest theory, we pick a couple of examples as a generic case-study of proton transfer . 

1 Over-all feature of the reactions of Water assisted transfer 

We first present the overall featme of an example of the dynamics in water-assisted proton transfer in Fig. 7.11. Panel (a) indicates that the distance of the proton Hoo from Op becomes longer than that from Ow at time about 2 fs, while panel (b) shows that the proton Hqn leaves from the site of Ow and arrives at the vicinity of N at about 5 fe. Thus the first transfer took place in the site of Op — Ow and that of Op — N followed in this particular example of proton relay. This is not always the case, though. That is, proton transfer in the site of Ow — N can precede that of Op — Ow- The tt bond-order displayed in panel (c) claims the double bond has shifted from C — N to Op — C at time about 7 fe. Since the tautomerization is completed in this way, we may judge that the present proton-relay has been achieved successfully along this path. 

We next survey the basic behavior of the electronic population on the relevant atoms in terms of the Mulliken (electronic) population in panel (d) through (f). In panel (d), we notice that the Mulliken population on both Hoo and Hqn is kept almost constant throughout the course of transfer process. This clearly indicates that these protons are covered with electrons as much as about 0.7 each. Panel (e) suggests that the electron density on Ow in the molecular plane remains almost constant, while that on Op (N) increases (decreases) to some extent. On the other hand, panel (f) indicates that TT-electron density is shifted a little from Op to N, while that on C does not change in the course of 7r-bond alternation. We note that this motion of the TT-electron density is in the reverse direction to the shift of the position of TT-bond itself. 

2 Proton transfer versus hydrogen-atom migration 

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General-acid or general-base catalysis will occur when a fherinodynainically unfavorable proton transfer in the ground state is converted to a thermodynamically favorable transfer in the transition state. 

Bond order As clearly observed in panel (c) and (f) of Fig. 7.26, only smooth interchange between the bond orders of OH and NH takes place in positive values. This directly indicates that chemical bond is smoothly shifted from OH to NH site both in the first and second excited states. This kind of smooth interchange in positive value is often observed in the typical proton transfer in the ground states. Likewise, the similar interchange is observed in Fig. 7.27 for five membered ammonia cluster. 

Provided there is a suitable proximal proton acceptor in the vicinity of the excited donor, the prohibitively high barriers for proton transfer in the ground state could... 

Itoh, M. Yoshida, N. Takashima, M. Transient absorption and two step laser-excitation fluorescence studies on the proton transfer in the ground and excited states of 3-hydroxyxanthone in alcohols. J. Am. Chem. Soc. 1985, 107, 4819-4824. 

Barone, V., Adamo, C., 1996, Proton Transfer in the Ground and Lowest Excited States of Malonaldehyde A Comparative Density Functional and Post-Hartree-Fock Study , J. Chem. Phys., 105, 11007. 

In summary, anils show fluorescence from a state (most probably TT-TT ) whfch Is similar to the cls-gulnold tautomer observed In appropriate solvents even in the ground state. This state can only be produced by Intramolecular proton transfer In the excited state. Since the energy difference between the phenolic and qulnold ground state isomers is rather small In the anils (see Section IV-B), the Increase In (-0H) acidity and/or (=N-)baslclty needs not be as large as in the case of salicylic esters. 

The H-bond strength of the third group—nonlumlnescent molecules—is similar to that of the salicylic derivatives, and no ground state tautomers are observed. Tremendous pK changes are necessary in order to facilitate proton transfer in the excited state. The deactivation path is still a matter of speculation. It is worth noting that a substance of the fluorescent class (phenyl salicylate, Table 2) has been the first ultraviolet stabilizer (9) used on a technical scale, despite its poor absorption intensity (screening effect). This substance is photo-chemlcally converted into 2,2 -dlhydroxy benzophenone (87) in a photo-Fries reaction. This can, however, not explain the total efficiency of this stabilizer (93). 

With one exception, these results are based solely on quantum-chemical calculations of the potential energy surface. Theoretical evaluation of the transfer dynamics has been attempted only for the formic acid dimer, for which two general level splittings have been observed and assigned to synchronous double proton tunneling in the ground state and a vibrational excited state, respectively. 

Scheme 15.9 Two examples of proton transfer in the excited state. The top (with -naphthol, [73]) is illustrative of an intermolecular proton transfer (to the solvent) jwocess whereas the bottom (with indigo, [22]) is illustrative of an intramolecular proton transfer. In the two cases the kinetic scheme (in the middle) applies with a single ground-state species however in the case of indigo, the back-proton transfer reaction in the excited state is unlikely...

The conclusion we can draw from all this research is that there is still no coherent picture of intramolecular ground and excited-state proton transfer reactions in tautomers. The topic is complicated from an experimental as well as a theoretical point of view, and many questions remain. Intramolecular ground-state proton transfer is hard to study directly, and although femtosecond pulsed lasers allow initiating and following proton transfers in the excited state on a very short time scale, these methods bring their own complications to the interpretation of the results. ... 

Figure 6. Jablonski diagram for the excited-state proton transfer and energy dissipation in TIN kSo s0> ks,s,-, kT,Tl- rate constants of proton-transfer processes in the ground state, first excited singlet state, and triplet state, respectively, and k,j rate constants of radiationless deactivations and k,- rate constants of intersyslem...

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