Proton transfer timing

Table 5-7. Comparison of Barrier Penetration Model Calculations with Measured Proton Transfer Time Constants"...

The proton transfer time of formic acid dimer in the electronic ground state is of the order of ns and can be compared with recent theoretical predictions which covered more than four orders of magnitude. 

The observed proton transfer times of the order of 50 fs have already been discussed in earlier work with respect to the importance of skeletal vibrations [16-18, 46[. It was proposed that a reduction in the distance between the proton donor and the acceptor results in a decrease in the energetic barrier between the enol-and the keto-form. At times when the barrier is suppressed the proton can tunnel or jump from its enol position to the keto site. In the case of HBO it was suggested that, in particular, the in-plane bending vibration modulates the donor-acceptor distance and thereby enables the proton movement [17]. This model was then applied to MS and to 2-(2 -hydroxyphenyl)-5-pheny]oxazole [18, 46]. However, due to insufficient time resolution of these experiments it was not possible to give experimental evidence for this model. 

Average distances at the proton-transfer time are given in parenthesis (A)... 

Moiisco F, Aprea E, Lembo V, et al. Rapid breath prinf of liver cirrhosis by proton transfer time-of-flight mass spectrometry. A pilot study. PLoS One. 2013 8 e59658. 

The equilibrium shown m the equation lies to the right =10 for proton transfer from the conjugate acid of aniline to cyclohexylamine making cyclohexylamine 1 000 000 times more basic than aniline... 

Hydrogen transfer in excited electronic states is being intensively studied with time-resolved spectroscopy. A typical scheme of electronic terms is shown in fig. 46. A vertical optical transition, induced by a picosecond laser pulse, populates the initial well of the excited Si state. The reverse optical transition, observed as the fluorescence band Fj, is accompanied by proton transfer to the second well with lower energy. This transfer is registered as the appearance of another fluorescence band, F2, with a large anti-Stokes shift. The rate constant is inferred from the time dependence of the relative intensities of these bands in dual fluorescence. The experimental data obtained by this method have been reviewed by Barbara et al. [1989]. We only quote the example of hydrogen transfer in the excited state of... 

Certain molecules that can permit concerted proton transfers are efficient catalysts for reactions at carbonyl centers. An example is the catalytic effect that 2-pyridone has on the aminolysis of esters. Although neither a strong base (pA aH+ = 0.75) nor a strong acid (pJsfa = 11.6), 2-pyridone is an effective catalyst of the reaction of -butylamine with 4-nitrophenyl acetate. The overall rate is more than 500 times greater when 2-pyridone acts... 

Another type of bifunctional catalysis has been noted with a,cn-diamines in which one of the amino groups is primary and the other tertiary. These substituted diamines are from several times to as much as 100 times more reactive toward imine formation than similar monofunctional amines. This is attributed to a catalytic intramolecular proton transfer. 

Scheme VIII has the form of Scheme II, so the relaxation time is given by Eq. (4-15)—appjirently. However, there is a difference between these two schemes in that L in Scheme VIII is also a participant in an acid-base equilibrium. The proton transfer is much more rapid than is the complex formation, so the acid-base system is considered to be at equilibrium throughout the complex formation. The experiment can be carried out by setting the total ligand concentration comparable to the total metal ion concentration, so that the solution is not buffered. As the base form L of the ligand undergoes coordination, the acid-base equilibrium shifts, thus changing the pH. This pH shift is detected by incorporating an acid-base indicator in the solution.

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). 

The proton transfer in these clusters via the water bridge was found to be about three times as fast as a nonassisted transfer, underscoring the importance of the solvent for the reaction mechanism [98IJQ855]. In addition to the relative stabilities of the cytosine tautomers, the structures and properties of some cytosine derivatives have been investigated, mainly those of 5-hydroxycytosine 111 and 5,6-dihydroxycytosine 112 (Scheme 73) [99JST1, 99JST49]. 

Electrical Conduction by Proton Jumps. As mentioned in Sec. 24, a hydroxyl ion may be regarded as a doubly charged oxygen ion 0 , containing a proton inside the electronic cloud of the ion, which has the same number of electrons as a fluoride ion. The radius of the hydroxyl ion cannot be very different from that of the fluoride ion. But it will be seen from Table 2 that the mobility of the hydroxyl ion is about four times as great. This arises from the fact that a large part of the mobility is undoubtedly due to proton transfers.1 Consider a water molecule in contact with a hydroxyl ion. If a proton jumps from the molecule to the ion,... 

The viscosity of pure H2SO4 at 25°C is 27.6 times greater than that of water consequently the normal migration of ions is extremely slow. The comparatively high mobility of the (IISO4)- ion undoubtedly arises from successive proton transfers to the ion from an adjacent solvent molecule 2... 

If in a dilute solution we carry out q proton transfers according to (28), there will be a change in the cratic term, and at the same time the free energy will receive the contribution qj, that is to say, q units each equal to J Since each of the quantities qD, qL, qY, and qj consists of q equal units, we may call them unitary quantities, in contrast to the cratic term, which is a communal quantity, depending as it does on the amount of solvent as well as the amount of solute present. 

Autoprotolysis of the Solvent. While studying these proton transfers, there is another type that may be discussed at the same time, namely, the self-dissociation of the solvent itself. As is well known, highly purified solvents show at least a small electrical conductivity. In methanol, for example, it is generally recognized that this conductivity arises from the fact that, a certain number of protons havo been transferred according to the process... 

Let us first discuss the four proton transfers in class II. We sec that in the last column all four values lie near 0.016 electron-volt, indicating that the value of Jel is nearly the same in all four cases. On the other hand, we notice from Table 9 that, for both glycine and alaniue, t.he value of Kn is a hundred times smaller than I i. We must ascribe this to the presence of a larger in the basic, proton transfer. According to (143) this implies in (130) a greater value of 0. The observed values of 0 for Kb are near 90°C, while for KA the values of 0 are smaller, namely 53.9° for glycine and 44.8° for alanine. 

There are, in fact, two reasons why we should prefer to discuss proton transfers of class I. In concentrated solutions the average electrostatic forces between the ions will be intense. Only in proton transfers of class I does the number of positive and negative charges in the solution remain unaltered when the proton is transferred only here do we find the possibility that the contribution from the interionic forces will remain almost unchanged in a proton transfer. At the same time, although the number... 

In Chapter 7 we found it convenient to distinguish between proton transfers involving a solvent molecule and those involving only solute particles but this difference will lose its significance when the distinction between solvent and solute begins to break down. We recall that in Sec. 54 the mole fraction of the solvent did not differ appreciably from unity and could be omitted from (72). In investigating concentrated solutions, however, there is no question of extrapolating to infinite dilution the mole fraction of the solvent will differ from unity and will have to be retained in all formulas. At the same time each of the mole fractions will need to be multiplied by its activity coefficient. 

Steadman, J., and Syage, J. A. (1991). Time-resolved studies of phenol proton transfer in clusters. 3. solvent structure and ion-pair formation. J. Phys. Chem. 95 10326-10331. 

Jacobs et al. [59,925,926] (Fig. 17). While this scheme conveniently summarizes many features of the observed behaviour, a number of variations or modifications of the mechanisms indicated have been proposed. Maycock and Pai Vemeker [924,933] emphasize the possible role of point defects and suggest, on the evidence of conductivity measurements, that the initial step may be the transfer of either a proton or an electron. Boldyrev et al. [46] suggest that proton conduction permits rapid migration of HC104 within the reactant and this undergoes preferential decomposition in distorted regions. More recently, the ease of proton transfer and the mobilities of other species in or on AP crystals have been investigated by a.c. [360] and d.c. [934] conductivity measurements. Owen et al. [934] could detect no surface proton conductivity and concluded that electron transfer was the initial step in decomposition. At the present time, these inconsistencies remain unresolved. 

For coupling with 2-naphthol-6,8-disulphonic-l-isotope effects (kK/kD) varied with the substituent in the benzenediazonium ion as follows 4-C1 (6.55) 3-C1 (5.48) 4-N02 (4.78), i.e. the reactivity of the ion was increased so that i correspondingly decreased. Base catalysis was observed127, 129, and there was a free energy relationship between this catalytic effect and the basicity of pyridine, 3- and 4-picoline. However, for 2-picoline and 2,6-lutidine, the catalysis was 3 times and 10 times less than expected from their basicities showing that, in this particular proton transfer, steric hindrance is important. 

The rate of deuteration of (XXXIII) at the 3 position by ca. 0.1 M NaOD in D20 was 2.19 times faster than the dedeuteration of the same position by the same concentration of NaOH in H20 at 35 °C. This indicates that proton transfer takes place in a rate-determining step, but the result is not conclusive in view of the different reaction medium. 

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