Pyronin cation

The task of finding systems of electrophiles and nucleophiles for which rates and equilibria of the simple combination reactions can be measured is not an easy one, and data have accumulated slowly. The efforts in my laboratory have focused primarily on carbocations, with pKR values in the range measurable in dilute aqueous solution, reacting with common anionic and neutral nucleophiles. The pyronin cation [3,6-bis(dimeth-ylamino)xanthylium cation] is particularly well suited for such studies (1). This cation has an unusually high pKR of 11.5 and gives measurable rates and equilibria with a wide range of nucleophiles. Pyronin is also stable enough in dimethyl sulfoxide [(CH3)2SO] solution to allow studies in that solvent (2) for... 

Our most recent report (3) on reactions of pyronin cation showed a correlation between the rates of reactions of a variety of nucleophiles and the one-electron oxidation potentials of the nucleophiles in aqueous solution. The correlation included anionic, neutral, and a-effect nucleophiles only thiolate ion nucleophiles showed significant deviations from the correlation. 

Table I shows the standard oxidation potentials and other related thermodynamic quantities along with the free energies of activation for reactions with pyronin cation in aqueous solution for all nucleophiles for which we have data. Most of the data were reported previously (3), but several errors in...

Table I. Oxidation Potentials and Reactivities with Pyronin Cation in Water...

From the value of the CEC, the interlamellar surface area per cation equivalent could be calculated to be ca 91.7 A. With the assumption that the cross section of the dye molecule was a square, the approximate areas were estimated to be ca 90 A for pyronine and ca 180 A for rhodamine. In this... 

The extent of this failure is evident from comparisons of experimental measurements of rate and equilibrium constants. One comparison in the literature is provided by Ritchie and coworkers study of the relatively stable cation, pyronin (the 3,6-bis(dimethylamino)xanthylium cation 71) with a series of nucleophiles.252 Another example is McClelland s measurements of rate and equilibrium constants for the reactions of halide and acetate ions with the trityl cation.19 As already mentioned fluoride and acetate are less reactive than bromide and chloride despite their equilibrium affinities being much greater. This is reflected indeed in the much lower rates of solvolysis of the fluoride and acetate than bromide or chloride as leaving groups... 

In addition to the methods described above a battery of other staining procedures are available. These include use of alcian blue (22) to stain glycoproteins, ethidium bromide (23) to stain DNA, and methylene blue (14) and pyronine (16) to stain RNA. A relatively new stain has been nicknamed stains-all, because of its ability to stain most macromolecules. This dye is a cationic carbocyanine and stains RNA bluish purple, DNA blue, protein red, acid mucopolysaccharides various shades of blue to purple, and phosphoproteins blue (24). It is presently the most widely used stain for RNA. 

Figure 9,11 Framework structure of zeolite L. (a) Projection along the c axis (b) projection perpendicular to the main channel direction, with A-E representing the positions of the cations (c) 12-membered-ring main channel (d) scanning electron microscopoe image of zeolite L crystals (e) structures of pyronine (left) and oxonine (right). Reproduced with permisson from [66], Copyright (2003) Wiley-VCH...

The recently reported correlation of reactivities, and one-electron oxidation potentials of nucleophiles is examined with new data for hydrazine in aqueous solution and several nucleophiles in (CH3)2SO solution. The correlation fails to apply to these reactions. A thermodynamic cycle is utilized to estimate the free energies of ionization of pyronin-nucleophile adducts both in solution and in the solid state. A satisfying rationalization of the dichotomy of ionic and covalent crystals of these and similar compounds is obtained. The equilibrium constants for reactions of nucleophiles with several types of cations are examined as indicators of specific bonding effects such as steric and gem interactions. 

In this chapter, newly available data for hydrazine (4) and data for reactions of pyronin in (CH3)2SO solution (2) will be used to further test this correlation, and the last-mentioned thermodynamic cycle will be applied to the estimation of equilibrium constants for cation-nucleophile combination reactions that cannot be measured directly. The equilibrium constants for reactions of nucleophiles with several types of cations will be compared. 

If differential steric effects for reactions of different cations are not too serious, we may expect the differences in AG°ion for various Xs to be the same as those in Table III. We (1) reported, for example, that the relative equilibrium constants for reactions of various Xs are the same for pyronin and [(dimethylamino)phenyl]tropylium cations. From the known pKR, 11.5 (1), for pyronin and the estimates given in Table III, the maximum pKR that a cation could have and still give measurable equilibria with the various X s in Table III can be estimated. 

Calculated values for pyronin derivatives and the maximum pKR of the cation allowing formation of covalent solid are given in Table IV. 

In addition to ruthenium(II) polypridine complexes and pyrene, other cationic dyes have been examined using absorption and luminescence characteristics upon intCTcalation. Adsorption of cationic dyes such as methylene blue (129-136), pyronine Y, (137), rhodamines (138-145), and coumarines (146) on smectites in colloidal suspension has been documented. Adsorption behavior of the dye molecules has been investigated from changes in visible absorption spectra. The concentrations of dyes and hosts, the relative amounts of dye to hosts, and the nature of the hosts all affect the states of the adsorbed dyes. 

ABSTRACT. Equilibrium and temperature-jump spectrophotometrie studies show that the mono-cation of pyronine B (PB) is included by beta- and gamma-cyclodextrins (3CD and yCD) to form the labile complexes PB. CD, PB.yCD and (PB)2.YCD in water. The equilibrium, kinetic and structural aspects of these complexes and those formed by pyronine Y are discussed in conjunction with data characterizing the inclusion of other dyes by cyclodextrins. 

The inclusion of organic dyes by cyclodextrins is a well known phenomenon, but the kinetics and mechanisms of such inclusion processes, particularly for the larger cyclodextrins, have not been much investigated. " In order to determine the effect of annular size on the dynamics of the inclusion process w e have studied the inclusion of the mono-cations of pyronine B and pyronine Y (PB and PY) ... 

Cationic dyes acridine orange, crystal violet, Janus green B, methyl violet, neutral red, Pyronin B, Pyronin Y (G), Safranin, Victoria blue B, and Victoria blue 4R, commonly used in histology, were studied by TLC on the Marshall and Lewis system (115). Marshall also separated some Sudan dyes, used for the histological staining of fats, on silica gel TLC sheets using benzene-CHCl3 (10 1) as the mobile phase (115). 

---