Fluorescence hydration number

Hydroxide, aqua, and hydrates. From the similar absorption spectra of Am in aqueous solution, AmCls, and in Lads, and the linear relationship between the decay rate of the americium fluorescence and the number of inner-sphere water molecules, it has been concluded that Am " is coordinated by nine inner-sphere water molecules. Similarly, the hydration number for the Cm ion has been estimated to be nine on the basis of fluorescence lifetimes. EXAFS studies of aqueous Am and Cm, however, have suggested coordination numbers closer to 10. " EXAFS investigation of Cf " " in aqueous solution indicates a coordination number of 8.5 ( 1.5), with a Cf—O distances of 2.41 0.02 A. This coordination number was confirmed for Am in the solid state by isolation of single crystals of the triflate salt of nonaqua complex, which contains a tricapped, trigonal prismatic cation that is isostructural with the analogous Pu" compound. ... 

Fluorescence kinetic measurements were applied to the determination of hydration numbers in lanthanide-polyaminocarboxylate complexes the results are given in table 5 (Brittain et al. 1991, Brittain and Jasinski 1988). Measurements of hydration numbers as a function of the solution pH for Eu " and Tb " complexes showed the presence of three buffer regions the first one at low pH in which the hydration number of the cations is equivalent to that of the free ions the second one in the pH range of 4-8 in which complexation decreases the hydration numbers and the third one at high pH (ca. 9-12) being associated with the formation of ternary hydroxo complexes. 

Whether or not the coordination number of the lanthanide ion in aqueous complexes remains constant as ML, MLj, etc., the form of the species is an open question. Using the fluorescence technique, Albin et al. (1984) reported the hydration number of Eu(EDA) to be seven and of Eu(EDA)2" to be three. Since EDA is a tetradentate ligand, these values lead to CN = 11 in both complexes. Lifetime measurements on the crystalline hydrate Na[Eu(EDA)2(H20), ] are consistent with the presence of two water molecules in the primary sphere (CN = 10), reflecting that the solid compounds may not be valid structural models for similar solution species. 

The hydration number q-. this parameter influences strongly the IS contribution (equations (3) and (4)). If this number increases from 1 to 2, the relaxivity increases by about 30%, but most of the Gd-complexes have a q number equal to one. This number can be measured by fluorescence study of europium or terbium complexes or by NMR measuring the lanthanide induced shift (LIS). 

E = fluorescence enhancement AG = free energy AH = free enthalpy Ka = association constant Kd = dissociation constant n = hydration number Pm = fractional population at site M R = the gas constant r = internuclear distance AS" = free entropy Ti = longitudinal relaxation time Tim = longitudinal relaxation time in a paramagnetic complex... 

Various experimental probes on the hydration-shell structure of Cm (aq) reported in literature have yielded a wide range of coordination numbers. To mention a few, EXAFS experiments measured primary hydration numbers of 9 or 7 (based on the truncation of the EXAFS fitting data) in 1M HCIO4 acid [114] and 10 in 0.25 M HCl acid [115]. High energy X-ray scattering (HEXS) experiments yielded a hydration number of 8.8 [114]. Time-resolved laser fluorescence spectroscopy (TREES) found coordination numbers between... 

Principal component analysis has been used in combination with spectroscopy in other types of multicomponent analyses. For example, compatible and incompatible blends of polyphenzlene oxides and polystyrene were distinguished using Fourier-transform-infrared spectra (59). Raman spectra of sulfuric acid/water mixtures were used in conjunction with principal component analysis to identify different ions, compositions, and hydrates (60). The identity and number of species present in binary and tertiary mixtures of polycycHc aromatic hydrocarbons were deterrnined using fluorescence spectra (61). 

A number of studies on the fluorescence decay of tyrosine, tyrosine derivatives, and small tyrosyl peptides have been carried out. 36-38 Whereas the tyrosine zwitterion and tyrosine derivatives with an ionized a-carboxy group exhibited monoexponential fluorescence decay (x = 3.26-3.76 ns), double- or triple-exponential decay was observed in most other cases. As in the case of the tryptophan model compounds, the complex decay kinetics were again interpreted in terms of rotamer populations resulting from rotation around the C —Cp bond. There is evidence to indicate that the shorter fluorescence lifetimes may arise from rotamers in which the phenol ring is in close contact with a hydrated carbonyl group 36 37 and that a charge-transfer mechanism may be implicated in this quenching process. 39 ... 

The hydrophilic groups on mucoadhesive polymers and the large amount of water associated with mucin present two possible adhesion mechanisms (i) hydrogen bonding and (ii) interpenetration of a swollen gel network with hydrated mucin. Many methods have been used for the assessment of bioadhesive properties, including fluorescent techniques and tensile tests. By using these methods, a number of natural and synthetic polymers have been discovered possessing mucoadhesive properties. 

The correlation between fluorescence intensity and salt concentration can be explained as follows the salt concentration in the organic phase increases as more hydrated trioctylammonium carboxylate is formed [Eq. (13)] and consequently micelle formation increases. The chromophore is distributed between the micellar phase and the solution, and with the rise in micelle concentration the number of fluorescent chromophores also rises. 

With these association colloids the fluorescence is strongly dependent on the pH. A shift of the pH from 7 to 8 makes the fluorescence (and the association) rise considerably. Katheder s explanation that in this case the hydration of the OH ions would extract water from the aggregate seems extremely improbable. Each OH ion (at pH = 8 ) would have to bind a very large number of water molecules. It does in fact seem more probable that just as in the soaps a suppression of the dissociation will promote the association. The charge of the dye stuff ions will indeed be of... 

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