Pfeiffer rotation

Because the Pfeiffer effect is exhibited by tris(l,10-phenanthroline)-nickel(II) ion and d-a-bromocamphor-7r-sulfonate and, because the complex has an absorption band in the visible region, this system was studied using optical rotatory dispersion techniques. The study revealed that the optical rotatory dispersion curves showing Pfeiffer rotation vs. wavelength were very similar to that of the resolved complex (Figures 3 and... 

Figure 3. Comparison of the optical rotatory dispersion and the Pfeiffer rotation of [Ni o-phen)Jp ...

Table III. Observed Pfeiffer Rotation (Degrees) for Systems with Different Ligand Metal Ratios at 546 Millimicronso...

Figure 5 it can be seen that changing the metal ion of the complex from Zn(II) to Cd(II) to Hg(II) enhances the observed Pfeiffer rotation. 

It was found that the Pfeiffer rotation is nearly linear with regard to the concentration of either of two of the major constituents of the system (the complex or the environment) if the other is held constant. This linearity holds best at low concentrations of the solutes. 

Since the Pfeiffer rotation is linear with respect to the concentration of each of the major constituents, it follows that it would be linear with respect to the product of the concentrations of these constituents. Figure 6 shows a plot of the Pfeiffer rotation vs. the product of the molar concentration of tris(l,10-phenanthroline)zinc(II) ion and the molar concentration of ammonium d-a-bromocamphor-7r-sulfonate. As can be seen, the deviation from linearity is very slight. It should be also noted that at that high end of the curve, the relationship between the concentration of the two constituents is not so favorable as at other parts of the curve. The linearity of this plot suggests that an equation of the type utilized for the calculation of molar rotation might apply to this phenomenon also. Rewriting the specific rotation equation in standard form results in the following ... 

Table IV. Variation of Pfeiffer Rotation (Fobs.) with Concentration...

S Pfeiffer rotation is also a linear function of concentration. a similar equa-... 

Since the magnitude of the Effect is proportional to the concentrations of both the environment substance and the complex, a series of equations have been developed for observed Pfeiffer rotation, specific Pfeiffer rotation, and molar Pfeiffer rotation which are analagous to those for observed optical rotation, specific optical rotation, and molar optical rotation (3.it,6,10). These are... 

In support of the equilibrium displacement mechanism, a study of the Pfeiffer rotation of the system D, L-[ Ni ( o-phen) 3] 2+ with levo-malic acid as an environment substance in water produces some interesting results (Figure 2). The Pfeiffer Effect reaches its maximum in just over k days, and if, at that time, exactly as much dextro-malic acid is added to the system as the levo enantiomer already present, then this has the effect of removing (deactivating) the environment substance, so that the excess of the complex enatiomer has no alternative but to undergo racemization. It should be noted (Figure 3) that the rate of this racemization is identical to the racemization rate of the optically pure complex which has been resolved by conventional means. 

Enantiomer Concentrations Complex Envir. Observed Pfeiffer Rotation Equilibrium Constant... 

In 1931, Pfeiffer and Quehl observed that the optical rotation of a solution of an optically active compound (e.g., ammonium ti-a-bromo-camphor-TT-sulfonate, hereinafter called an environment compound) changes significantly upon the addition of solutions of racemic mixtures of certain coordination compounds (e.g., D,L-[Ni(o-phen)3]Cl2). Brasted and his students, and Dwyer and his co-workers have studied this effect in some detail with particular reference to the source and nature of the effect. Kirschner and his co-workers have also studied the effect and have found considerable evidence in support of the mechanism for this effect described by Dwyer and his co-workers. Kirschner and Magnell" have developed quantitative expressions for the rotation change due to the Pfeiffer effect, and have defined a positive Pfeiffer rotation as an enhancement of optical... 

Molar Pfeiffer Rotation, Absorption, and Circular Dicbroism of Metal Oxalate Complexes... 

Initial racemic complex Environment compound Pfeiffer rotation (Pobs.) (deg.) Cone, of complex, M/liter Observed rotation, obs. [M] °/ /o resolution... 

Even in the earliest work of Pfeiffer it was noted that the observed rotation was a function of the wavelength of the plane polarized light. For solutions 0.08 M in NH4BCS and 0.04 M in Zn, the rotation varied from 3.69 at 656.3 mju to 6.05 at 546 m/i. When the solutions were 0.12 M (a three to one ratio) in phen, the a obs. values were 6.65 at 656.3 m/z and 10.62 at 546 mja. Properly treated data using the Drude equations have been found useful by the present authors in investigating the source or sources of the Pfeiffer rotation. 

The Pfeiffer rotation as a function of the concentration of the ligand (e.g., phenanthroline and dipyridyl) was originally observed by Pfeiffer and... 

Data derived by Dwyer on the ratio of the rate constants (kjki) has been criticized by Harris since the values used by Dwyer for the rotation at the end of the racemization of d-[Ni(phen)3] were those obtained immediately after adding the optically active ion to the racemic mixture of the complex in solution. The ratio thus was not the equilibrium rotation. It was not reasonable, according to Harris, to use the data in support of an equilibrium theory for the Pfeiffer effect. Contradictory results are reported by Craddock and Jones in that no difference was found for the racemization rates for either isomer (e.g., d- or /-[Ni(phen)3] ) if the complex is in the presence of an optically active species. These authors point out that another environmental factor, temperature, could have accounted for unusual or anomalous rotations previously found. It is evident that something more than an equilibrium shift or a configurational activity is needed to explain Pfeiffer rotations. 

Fig. 2. Pfeiffer rotation as a function of concentration of the product of [Zn(phen)3] and [BCS"]. Measurements made at 25 C, 405 m i and using a...

Fig. 3. Pfeiffer rotation as a function of [Zn(phen)3] (BCS)2 concentration at constant ionic strength, 0.30, using (NH4)2S04. Temperature 25°C, 405 mfi and 2 dm tube.

Much of the Pfeiffer data is reported on solutions prepared by mixing stoichiometric proportions of metal salt and ligand with the active species. The optimum concentration of the latter for maximum Pfeiffer rotation is not clear. The original work of one of the author s (RCB) using strychnine infers that a 5 2 ratio of complex to alkaloid is ideal. The optical measurements on the solutions may be suspect due to the presence of so-called inert species (ionic and molecular). In the Introduction a number of investigations were reported that strongly suggested that inert ions may play a role in rotatory measurements. " ... 

Fig. 5. Pfeiffer rotation of the system [Zn(phen)3] (BCS)2(0.01 M) as a function of sodium chloride concentration. 25°C, 2 dm tube at 365 mfi.

Since a i is-complex in the cis configuration is a potentially resolvable species it could contribute to a Pfeiffer rotation either by itself or by supplementing the residual tris-complex. Kuhajek" performed a series of determinations on the effect of the feis-complex. 

Crystalline Zn(BCS)2 was used to prepare a 0.023 M solution. Two 100 ml aliquots of this solution were used, one diluted to 200 ml and the second, mixed with 0.0100 mole of phen was diluted to 200 ml. These two solutions were then mixed in various ratios totaling 20 ml volume, and then all solutions were diluted to 25 ml. Each solution had a constant (0.00982 M) zinc concentration (as Zn(BCS)2). Any change in the observed rotation of these solutions is actually a Pfeiffer rotation. The observed a values for 12 such solutions were plotted in Fig. 8 with the a obs. plotted against the ratio of phen Zn. The calculated values are incorporated in the plot as the curve C on the assumption that all of the rotation is due only to the tris-complex, [Zn(phen)3] The successive k dissociations for the complex" were used to estimate the concentration of the trfs-complex in each of the solutions. 

As the plots are examined, it is evident that Pfeiffer rotations appear before the 3 1 complex can be present and that the discrepancy between the measured and the calculated curves is greatest at a concentration ratio of two moles of phen to one of Zn defining a complex [Zn(phen)2]. That this... 

BCS" as the active species and [Zn(phen)3] in methanol as solvent. The loss of Pfeiffer activity in this medium has been proven. Kirschner has reported data using ethanol, dimethyl formamide (DMF) and acetic acid as solvents. For all solvents (or mixtures of solvents) with lower dielectric constants than water there is a trend toward lowering the Pfeiffer rotation as the mole percent of water decreases. An exception is found with DMF where Kirschner reports an increase in a-Pfeiffer above about 40vol.% of DMF. Nordquist reports data on the [Zn(phen)3] -cinchonine system in isopropanol at a number of wavelengths. The data are found in Table 2. When the active species carries a negative ionic charge (e.g., CS" or BCS") one can muster considerable support for a simple ion pair explanation for a... 

Although not a solvent, urea is an uncharged species that has long been known to have profound effect upon systems hydrophobically bound. If such bonds are operative in generating new asymmetric or electronic centers giving rise to Pfeiffer rotations, it is reasonable that urea would alter their rotations just as it alters the nature of and rotation of certain biologically optically active systems (e.g., proteins hydrophobically bound by solvent). Table 3 gives data for two Pfeiffer active systems in which BCS" and strychnine are the optically active species and [Zn(phen)3] the resolvable complex." ... 

It is evident that urea brings about a sharp decrease in the a-Pfeiffer. In 3.3 M urea the BCS-activated system, the Pfeiffer rotation is but 33 % of the activity in absence of urea. With a strychnine-activated system the rotation in presence of urea is 55 % of that in its absence. 

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