Proton transfers, rates effects

The ESPT of naphthylammonium [190] and phenanthrylammonium [191] ions in their 18-crown-6 ether complexes in MeOH-H20 (9 1) solvent shows that the excited-state proton-transfer rate decreases markedly on complexing. The back-protonation rate in the excited state is negligibly small compared with those of the other decay processes, which essentially means that there is no excited-state protropic equilibrium in the crown complexes. The one-way proton-transfer reaction is elucidated by the presence of the excited neutral amine-crown complex (RNH2-crown) produced by deprotonation of (RN+H3-crown). There is a large steric effect on protonation to the amino group of the excited neutral complex. 

Cui Q, M Karplus (2002) Quantum mechanics/molecular mechanics studies of triosephosphate isomerase-catalyzed reactions Effect of geometry and tunneling on proton-transfer rate constants. J. Am. Chem. Soc. 124 (12) 3093-3124... 

Those EGBs for which proton-transfer rates are easily measured are radical anions derived by one-electron electrochemical reduction from azobenzenes (Sec. III.A.l), aromatic (Sec. III.C.3), and heteroaromatic hydrocarbons (Sec. III.A.3), and dioxygen (Sec. III.B.l). In those cases the protonated EGB is removed in a fast disproportionation reaction (cf. Sec. II.B, Eq. 2-4), and the proton-transfer step therefore is made effectively irreversible. In CV experiments with addition of an acidic substrate, protonation of the already mentioned radical anions is observed as an increase in the cathodic peak current (change from a one-electron to a two-electron process) and a decrease in the anodic peak current. Where the proton transfer reaction is fast compared to the time scale of the CV experiment, the cathodic peak current is doubled and the anodic peak completely vanishes. If the CV at low scan rates is unchanged after addition of (an excess of) acidic substrate, the EGB is too weak a base to deprotonate the substrate at a reasonable rate. 

In Fig. 1 (right panel), we plot the time-dependent rate coefEcient obtained from an average over 16000 trajectories. We see that kAB t) falls quickly from its initial transition state theory value in a few tenths of a picosecond to a plateau from which the rate constant can be extracted. The decrease in the rate coefficient from its transition state theory value is due to recrossing by the trajectory of the barrier top before the system reaches a metastable state. The value of kAs obtained from the plateau is kAB = 0.013 ps F The adiabatic rate constant is k g = 0.019 ps, indicating that nonadiabatic effects influence the proton transfer rate. 

Apparently neither electrostatic interactions nor reduced diffusibility of protons is the major cause for the decrease in the proton transfer rate. As these effects are dominating in ion-pair recombination and ion-pair separation, we have to focus our attention to the primary event in proton dissociation ion-pair formation. In this reaction, the hydrogen of the OH bond of the excited parent molecule forms a hydrogen bond with the nearest H2O molecule, which itself is hydrogen bonded to other water... 

In 1966 Wehry and Rogers [464] examined the pK s of 2-naphthol and several substituted phenols and found ApK, = pKj,(D20) - pK (H20 ranged from 0.48 to 0.70. The same general trend held true for pK s in the lowest excited singlet states of the phenols and for the pK, s in a small set of benzoic acids and naphthoic acids. Stryer [463] found that excited-state proton transfer rates in a set of aromatics were greater in HjO than in D2O As a result, the fluorescence quantum yields were higher in DjO than in H2O in most cases. Jencks and Salvesen [486] found similar effects in a small group of thiol acids, with ApK = 2.0-2.5. 

Secondary isotope effects (15) and nmr evidence (16) clearly show that a C—O bond scission occurs during the acid hydrolysis of oxathiolanes. Proton-transfer rates for acidic alcohols are several orders of magnitude higher than those for the corresponding thiols (17). These species-specific interactions are in good agreement with the HSAB dictum. 

AI-water complexes with more than three waters have received less attention because it is believed that such large complexes cannot be directly involved in the tautomerization. Moreover, these complexes are difficult to be spectroscopically assigned due to the complexity of their electronic [11] and vibrational [10] structures. 7AI with four waters was studied by Fohner et al. [27] using ultrafast pump-probe spectroscopy combined with theoretical calculations. Their results revealed that the proton-transfer rate increases compared to that of 7AI with two and three waters. Their deuteration studies provided proof for the occurrence of proton transfer (PT), although it was not conclusively confirmed that the proton transfer resulted in a complete tautomerization of the 7AI monomer. For even bigger clusters of 7AI with five waters, there are no experimental investigations available only a theoretical study was reported on the second hydration shell effect [45]. 

A kinetic study of the acid-catalysed loss of alkoxide and thiolate ions from alkoxide and thiolate ion adducts, respectively, of benzylidene Meldrum s acid, methoxy-benzylidene Meldrum s acid, and thiomethoxybenzylidene Meldrum s acid has been reported. The reactions appear to be subject to general acid catalysis, although the catalytic effect of buffers is weak and the bulk of the reported data refers to H+ catalysis. a-Carbon protonation and, in some cases, protonation of one of the carbonyl oxygens to form an enol compete with alkoxide or thiolate ion expulsion. This scenario rendered the kinetic analysis more complex but allowed the determination of p/fa values and of proton-transfer rate constants at the a-carbon. In conjunction with the previously reported data on the nucleophilic addition of RO and RS ions to the same Meldmm s acid derivatives, rate constants for nucleophilic addition by the respective neutral alcohols and thiols could also be calculated. ... 

Recent developments in ultrafast spectroscopy have enabled us to investigate directly the ultrafast proton-transfer reactions in the excited state of aromatic compounds. The effect of electronic sfiucture on proton transfer rate is of great interest not only from fundamental aspects in reaction dynamics, but also from the viewpoint of developing new photoacids. Among a number of photoacids investigated so far, 1- and 2-naphthols (1-NL and 2-NL) are representative compounds for investigating... 

Substituent effects on proton transfer to water of protonated aniline derivatives have been investigated by picosecond time-resolved fluorescence measurements [92-94]. Protonated aniline in the Sj state releases proton to water with a rate constant of 1.3 x 10 °s in aqueous solution. The proton transfer rate is significantly increased by substitution of cyano group at the meta-position k = 3.7 x 10"s ). In contrast, the methoxy substitution at the meta-position decreases the rate remarkably... 

X lO s )-Either substituent at thepara-position shows only slightinfluences on the rate. The results of a kinetic study [94] are summarized in Table 2.5 together with the and p values. The less prominent effect of the para substituent is qualitatively explained by the direction of <- A transition moment, which is perpendicular to the direction of the two substituents. A quantitative explanation for the remarkable substituent effects on proton transfer rate can be made in terms of the free energy change (AG ) for the proton transfer reactions in the excited state. In Figure 2.5, the values of log(fcjj ) are plotted as a function of AG [94]. 

There are a large number of pathways from the external solvent that converge on a few key residues near Qb- The residues at the points of convergence of the pathways play a particularly important role in proton transfer. Consequently, site-directed mutagenesis of these residues to non-protonatable amino acids is expected to have a dramatic effect on the measured proton transfer rates. In contrast, site-directed mutation of residues that are farther removed from Qb should have a relatively small effect, because other parallel proton pathways can compensate for the blockage introduced by the mutation. 

Equation 4.2 gives the VMR for compound M in ppbv if Na is given in cm . As an illustration, suppose we measure 7(MH + )/7(H30 +) = 10 ". If the reaction time is 100 [xs and the proton transfer rate coefficient is 2 x 10 cm s, both of which are fairly typical values, then we calculate a mixing ratio of 10.7 ppbv for compound M. It should be noted that the above calculation assumes that there is a negligible contribution to the drift tube pressure from ingress of gas from the ion source. If this is not the case then this dilution effect must be allowed for in the calculation. 

Figure A3.8.3 Quantum activation free energy curves calculated for the model A-H-A proton transfer reaction described 45. The frill line is for the classical limit of the proton transfer solute in isolation, while the other curves are for different fully quantized cases. The rigid curves were calculated by keeping the A-A distance fixed. An important feature here is the direct effect of the solvent activation process on both the solvated rigid and flexible solute curves. Another feature is the effect of a fluctuating A-A distance which both lowers the activation free energy and reduces the influence of the solvent. The latter feature enliances the rate by a factor of 20 over the rigid case.

The details of proton-transfer processes can also be probed by examination of solvent isotope effects, for example, by comparing the rates of a reaction in H2O versus D2O. The solvent isotope effect can be either normal or inverse, depending on the nature of the proton-transfer process in the reaction mechanism. D3O+ is a stronger acid than H3O+. As a result, reactants in D2O solution are somewhat more extensively protonated than in H2O at identical acid concentration. A reaction that involves a rapid equilibrium protonation will proceed faster in D2O than in H2O because of the higher concentration of the protonated reactant. On the other hand, if proton transfer is part of the rate-determining step, the reaction will be faster in H2O than in D2O because of the normal primary kinetic isotope effect of the type considered in Section 4.5. 

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