Spectra and Dissociation Constants

Table 1. Ultraviolet spectra and dissociation constants RRjN-C-NH-C-NHR4... 

Herington, E.F.G., Kynaston, W. (1957) The ultraviolet absorption spectra and dissociation constants of certain phenols in aqueous solution. Trans. Farad. Soc. 53, 138-142. 

Wolfbeis O S, KoUer E, Hochmuth P. The unusually strong effect of a 4-cyano group upon electronic spectra and dissociation constants of 3-substituted 7-hydroxycoumarins. Bull Chem Soc Jpn 1985 58 731-4. 

Jellinek HHG and Wayne MG, Nicotinamide Ultraviolet spectra and dissociation constants, /. Phys. Chem., 55, 173-179 (1951). NB Used glass electrode to measure pH values. 

Spectro H2O t = 25.0 1=0.005-0.025 to fully deprotonated D, neutral to fully deprotonated. Also reported the corresponding data for picolinic and isonicotmic acids. Jellinek HHG and Urwin JR, Ultraviolet absorption spectra and dissociation constants of picolinic, isonicotmic acids and Uieir amides, /. Phys. Chem., 58,548-550 (1954). Used glass electrode to measure pH values and recorded spectra with a Hilger Uvispek spectrophotometer. Ionic strengtti was extrapolated to zero wiBi Bie Debye-Huckel equation. Fischer A, Galloway WJ and Vau an J, Structure and reactivity in the pyridine series. I. Acid dissociation constant of pyridinium ions, /. Chem. Soc. B, 3591-3596 (1964). All data were cited in Perrin Bases no. 1076-77 Perrin Bases suppl. no. 5081. 

Hayon, E., Ibata, T., Lichtin, N. N., Simic, M. (1972). Electron and hydrogen atom attachment to aromatic carbonyl compounds in aqueous solution. Absorption spectra and dissociation constants of ketyl radicals. The Journal of Physical Chemistry, 76(15), 2072-2078. 

The n.m.r. spectra and dissociation constants of a series of monoximes, dioximes, and mono-(9-methyldioximes of 2,l,3-benzothiazole-4,5- and -4,7-diones (188), (189) have been measured. The 4,5- and 4,7-di-O-methyldiox-iminobenzothiadiazoles were also prepared and investigated. Four and two geometrical isomers of the 4,5- and 4,7- derivatives, respectively, have been separated and described. ... 

Determination of the dissociation constants of acids and bases from the change of absorption spectra with pH. The spectrochemical method is particularly valuable for very weak bases, such as aromatic hydrocarbons and carbonyl compounds which require high concentrations of strong mineral acid in order to be converted into the conjugate acid to a measurable extent. 

The course of alkylations of 6-azauracil is in good agreement with the results of determination of the dissociation constants of 6-azauracil and of its two monomethyl derivatives. On the assumption that a methyl group does not much affect the dissociation constant, and on the basis of the lactam structure, it may be concluded from the values of the dissociation constants iKa of 6-azauracil = 7.00, of l-methyl-6-azauracil = 6.99, and of 3-methyl-6-azauracil = 9.52) that dissociation takes first place at the NH group in position 3. The same results are obtained independently by comparing the pH dependence of the XJV spectra of these compounds. These results represent an exact confirmation of the older observation by Cattelain that the monoalkyl derivatives of 6-substituted dioxotriazines possess different acidity. 

It should be mentioned that a similar comparison of the dissociation constant values of uracil monoalkyl derivatives does not permit the determination of the sequence of dissociation on account of the small differences between the pEo values. However, the pH dependence of the XJV spectra showed that the first dissociation of uracil occurs at the NH group in position 1 and thus differently than in 6-azauracil. This, together with different acidity, represents the main differences between the properties of uracil and its 6-aza analogs. 

By comparing the dissociation constant of 6-azauracil and 6-aza-uridine with those or uracil and uridine, 6-azauridine is now considered to be 1-ribofuranosyl derivative (2-ribofuranosyl-3,5-dioxo-2,3,4,5-tetrahydro-l,2,4-triazine), The same was shown more exactly by comparing the UV and IR spectra and the dissociation constants of 6-azauridine with the two monomethyl derivatives of 6-aza-uracil," Enzymatic synthesis thus, proceeds, in the same way in natural bases and in their aza analogs. 

The physical properties of the pyridopyrimidines closely resemble those of their nearest A-heteroeyclie neighbors the quinazolines and the pteridines. Thus, in common with the pteridines, the presence of groups capable of hydrogen-bonding markedly raises the melting point and lowers the solubility. - The acid dissociation constants (pif a values) and ultraviolet absorption spectra of all four parent pyridopyrimidines have been determined by Armarego in a comprehensive study of covalent hydration in these heterocyclic systems. The importance of these techniques in the study of covalent hydration, and... 

Proton dissociation in the excited states commonly occurs much easier than in the ground states, and the great difference in proton dissociation constants by several orders of magnitude is characteristic for photoacids [47]. These dyes exist as neutral molecules and their excited-state deprotonation with the rate faster than the emission results in new red-shifted bands in emission spectra [48]. Such properties can be explored in the same manner as the ground-state deprotonation with the shift of observed spectral effect to more acidic pH values. 

The line-broadening data as a function of pH, typically shown for the W(IV) in Figs. 13 and 14, incorporating the known pKa values (Table II), were fitted in 5 X 5 Kubo-Sack matrices describing the exchange based on the above schemes (6, 57). The experimentally determined chemical shift and linewidth data in the absence of exchange for the aqua oxo, hydroxo oxo, and dioxo species and the pH-dependent species distribution as calculated from the acid dissociation constants for the four systems were all introduced in the different matrices and the spectra were computer simulated. For each set of chosen rate con-... 

Conventional absorptiometric and fluorimetric pH indicators show a shift of band positions in absorption and emission spectra between the protonated and deprotonated forms. This feature allows the spectroscopic measurement of the acid dissociation constant in the ground state, Ka, and also the evaluation of the dissociation constant in the excited state, Ka (Eq. (5.5)), from the Forster cycle under the assumption of equivalent entropies of reaction in the two states.<109 112)... 

If the probe display shifts in absorption and/or emission spectra, the apparent dissociation constant will depend not only on the measured parameter but also on the excitation and emission wavelengths. For probes that display a shift in the absorption spectrum on analyte binding, the value of f/ bdepends on excitation wavelength, which affects the value of Koa (see Eq. (10.24)). For probes which display shift in emission spectrum the value of may depend on observation wavelength. The... 

Neither Fj nor F2 alone gave the characteristic fluorescence of fa and nicked fa in the presence of L-serine and pyridoxal phosphate. However, titration of a fixed amount of F2 with F2 gave rise to a fluorescence intensity 80-90% that of nicked fa at a stoichiometric ratio of Ft to F2. Moreover, both the excitation and emission spectra of the stoichiometric mixture were the same as for nicked fa. In addition, the same specific quenching of this fluorescence was shown in recombined Fj and F2 as in nicked fa. Further, the dissociation constants for L-serine and for indole were determined to be the same within experimental error for recombined Fj and F2, as for nicked fa. No significant differences were found between nicked fa and reconstituted Fj F2 in the intrinsic fluorescence of the aromatic residues, or in the sedimentation coefficients or the 200-250 nm CD spectra. From the foregoing independent lines of evidence, F2 and F2 combine to produce a structure very similar to that of nicked fa. 

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