Pyrogallol complex, caffeine

Thus, if van der Waals forces are responsible for the stability of the caffeine-pyrogallol complex we would expect the molecules to be oriented as in V. Bearing in mind, however, the uncertainty as to the exact orientation of caffeine s dipole vector, the superposition of molecular planes might in the final analysis not be so different from I. In that case one would have to look to other evidence to establish whether or not charge transfer forces were involved. 

The first step in the Fourier synthesis of the caffeine-pyrogallol complex was to choose an initial model so an electron density projection could be calculated. From the previous results it was determined that the unit cell was tetragonal measuring 23.26 X 23.26 X 6.99 A., and that eight caffeine-pyrogallol complex moieties reside in the unit cell. This information alone does not give any indication of the way in which the molecules are packed inside the unit cell, but the symmetry operations of the space group can be used to eliminate many models. For example, one of the symmetry operators in the unit cell is a fourfold axis. 

From other planar structures which have been studied, specifically planar complexes such as the tetramethyluric acid-pyrene complex, it is known that the molecules tend to stack plane to plane at a distance of about 3.4 A. (6). One unit cell dimension in this case is 6.99 A. Thus, it would be reasonable to expect the caffeine and pyrogallol molecules to be stacked with their planes roughly perpendicular to this short axis, and it would be likely that a projection of the caffeine-pyrogallol complex down this axis to the x,y plane would result in a two-dimensional picture of the complex with a small amount of distortion owing to the tilt of the planes of the molecules with respect to the x,y plane. 

Figure 7. Flotation density of crystalline caffeine-pyrogallol complex measured after sample was equilibrated at various relative humidities...

Figure I. Formation of caffeine-pyrogallol crystalline complex by cooling a saturated aqueous solution from above room temperature. Left, homogeneous solution, 0.1M in each component, above room temperature center, preliminary crystallization near room temperature right, gel-like structure of complex crystals after standing at room temperature for one day...

Table I. Percent Weight Loss as a Function of Relative Humidity for 1 1 Caffeine-Pyrogallol Crystalline Complex...

Figure 9. Electron density sections of the caffeine-pyrogallol crystalline complex. The sections show 1/8 of unit cell (the other 7/8 is generated by the symmetry of the space group) at the present stage of refinement. Each section corresponds to a slice through the unit cell at a constant z value with the x coordinate running from 0.00 to 0.50 and y from 0.00 to 0.25 (units are in fractions of unit cell dimensions)...

The results indicate that caffeine and pyrogallol form a 1 1 crystalline complex in which five water molecules associate with each caffeine-pyrogallol pair. 

If 0.01 mole of caffeine and 0.01 mole of pyrogallol are dissolved in 100 ml. of water at 60°C., a 1 1 crystalline complex will appear as the solution cools. Long needle-shaped crystals growing radially from central nuclei cause the entire solution to solidify into a gel-like structure with the fibrous appearance of mold (Figure 1). 

To determine the ratio of caffeine to pyrogallol in the complex, the following spectrophotometric technique was employed. 

The ultraviolet spectra of the precipitate can be seen in Figure 2 along with the spectra of pure caffeine and pure pyrogallol solutions. With such low concentrations it is assumed that the caffeine and pyrogallol are completely dissociated and that their ultraviolet spectra will be additive. Hence, if we let the ratio of pyrogallol with respect to caffeine be 1 P, then the extinction coefficient of the complex is ... 

Thus, we have a 1 1 complex. (The same result is observed when the complex is precipitated from a non-1 1 mixture of caffeine and pyrogallol (e.g., 1 2 mixture). 

---