Proton lifetimes

Chromium(III) has a ground state in pseudo-octahedral symmetry. The absence of low-lying excited states excludes fast electron relaxation, which is in fact of the order of 10 -10 ° s. The main electron relaxation mechanism is ascribed to the modulation of transient ZFS. Figure 18 shows the NMRD profiles of hexaaqua chromium(III) at different temperatures (62). The position of the first dispersion, in the 333 K profile, indicates a correlation time of 5 X 10 ° s. Since it is too long to be the reorientational time and too fast to be the water proton lifetime, it must correspond to the electron relaxation time, and such a dispersion must be due to contact relaxation. The high field dispersion is the oos dispersion due to dipolar relaxation, modulated by the reorientational correlation time = 3 x 10 s. According to the Stokes-Einstein law, increases with decreasing temperature, and... 

The contribution of reaction (a) to the H1 line width may be estimated from the line width at very low pH where [OH-] and [NH3] are very small and accordingly kb, kc, and kd are relatively unimportant. This sets an upper limit to ka of <0.6 X 10-2 sec-1 M at 21°. Since the analysis of exchange was studied in the pH range 1.5-2.5, reaction (b) is relatively unimportant because [OH-] is very small and the upper limit for kb was estimated to be 10-12 sec-1 Thus, under these experimental conditions, only kc and kd contribute significantly to the overall exchange broadening, and these are related to t, the proton lifetime in a given NH4+ ion by Eq. (35). 

The conditions required for a non-symmetric Universe were first put forward by Sakharov [16] they include non-conservation of the baryon number, C and CP symmetry violation, and the existence of a period of thermal non-equilibrium during the evolution however, the present limits on the proton lifetime (1033 years) are inconsistent with the first condition, and the small degree of CP symmetry violation displayed by kaons is not compatible with the second condition. 

The rate coefficients are numbered as in the literature on these reactions. Rate coefficients are found from proton lifetimes, r, by measurement of r for varying pH and amine concentration, and fitting the results to an equation of the type... 

As a second case of high complexity, let us discuss how protons are exchanged in pure protic liquids. This problem has been studied by Gerritzen et al. [93] who studied the inverse proton lifetimes of CH3OH = AH in the pure liquid and... 

The values of k + k for water are very close to those of methanol. Using the known dependence of as a function of temperature, the proton lifetimes in pure water were estimated [93]... 

Lagodzinskaya GV, Yunda NG, ManeUs GB (2002) H + -catalyzed symmetric proton exchange in neat liquids with a network of N-H-N and O-H-O hydrogen bonds and molecular mechanism of Grotthus proton migration. Chem Phys 282 51-61 Lamb WJ, Brown DR, Jonas JJ (1981) Temperature and density dependence of the proton lifetime min liquid water. Phys Chem 85 2883-1887... 

In sunmary, a model for G-C destabilization has been derived from nucleotide hydrogen exchange studies that is consistent with thermodynamic considerations of H-bond strength and DN4 stability. This model contains the point of view that DN4 stability may depend on the weakness of the interbase oxygen-to-amino H-bonds If the nucleobase aminos were strong H-bond donors a stable double helix could not be formed, due to the competition of water for the donor sites. Sequence specific interactions of protein cationic sidechains, or non-specific interactions of small cations, both with the G(N-7) site accessible to solvent in the large helical groove, would decrease the interbase proton lifetime and allow a drastic increase in solvent (water) interaction of G-C-rich sequences exclusively. A central feature of the model s extension, the requirement for simultaneous interaction of... 

Fig. 5.9 Effect of hydrogen-deuterium substitution on the quantum yield (A). The quantum yields of the protonated solutions have been rescaled so as to be superimposed at the lowest wavelengths. The data for the deuterated compounds have been kept in scale with the corresponding protonated ones. At A = 532 nm, the deuterated-protonated lifetime ratio is also shown for HK271 in 5CB and paraffin...

Gamma radiation produces free carriers much as does visible light (36). High energy protons and electrons produce defects that reduce minority carrier lifetime according to equation 8 ... 

The enol can be observed by NMR spectroscopy and at —20°C has a half-life of several hours. At -1-20°C the half-life is only 10 minutes. The presence of bases causes very r id isomerization to acetaldehyde via the enolate. Solvents have a significant effect on the lifetime of such unstable enols. Solvents such as DMF and DMSO, which are known to slow rates of proton exchange by hydrogen bonding, increase the lifetime of unstable enols. ... 

We have seen that 10" M s is about the fastest second-order rate constant that we might expect to measure this corresponds to a lifetime of about 10 " s at unit reactant concentration. Yet there is evidence, discussed by Grunwald, that certain proton transfers have lifetimes of the order 10 s. These ultrafast reactions are believed to take place via quantum mechanical tunneling through the energy barrier. This phenomenon will only be significant for very small particles, such as protons and electrons. 

The amide group of coelenteramide is an extremely weak acid thus, it will be rapidly protonated in a neutral protic environment, changing into its neutral (unionized) form. If the rate of the protonation of the excited amide anion is sufficiently fast in comparison with the rate of its de-excitation, a part or most of the excited amide anion will be converted into the excited neutral species within the lifetime of the excited state of the amide anion, resulting in a light emission from the excited neutral coelenteramide (kmax about 400 nm). 

If the carbanion has even a short lifetime, 6 and 7 will assume the most favorable conformation before the attack of W. This is of course the same for both, and when W attacks, the same product will result from each. This will be one of two possible diastereomers, so the reaction will be stereoselective but since the cis and trans isomers do not give rise to different isomers, it will not be stereospecific. Unfortunately, this prediction has not been tested on open-chain alkenes. Except for Michael-type substrates, the stereochemistry of nucleophilic addition to double bonds has been studied only in cyclic systems, where only the cis isomer exists. In these cases, the reaction has been shown to be stereoselective with syn addition reported in some cases and anti addition in others." When the reaction is performed on a Michael-type substrate, C=C—Z, the hydrogen does not arrive at the carbon directly but only through a tautomeric equilibrium. The product naturally assumes the most thermodynamically stable configuration, without relation to the direction of original attack of Y. In one such case (the addition of EtOD and of Me3CSD to tra -MeCH=CHCOOEt) predominant anti addition was found there is evidence that the stereoselectivity here results from the final protonation of the enolate, and not from the initial attack. For obvious reasons, additions to triple bonds cannot be stereospecific. As with electrophilic additions, nucleophilic additions to triple bonds are usually stereoselective and anti, though syn addition and nonstereoselective addition have also been reported. 

When water undergoes self-ionization, a range of cationic species are formed, the simplest of which is the hydronium ion, HjO (Clever, 1963). This ion has been detected experimentally by a range of techniques including mass spectrometry (Cunningham, Payzant Kebarle, 1972), as have ions of the type H+ (HaO) with values of n up to 8. Monte-Carlo calculations show that HjO ions exist in hydrated clusters surrounded by three or four water molecules in the hydration shell (Kochanski, 1985). These ions have only a short lifetime, since the proton is highly mobile and may be readily transferred from one water molecule to another. The time taken for such a transfer is typically of the order of 10 s provided that the receiving molecule of water is correctly oriented. 

---