Résumé:
Caffeine share his interest in biophysics creates detailed knowledge of all its properties molecular whose intermolecular potential crystal that requires knowledge of the experimental molecular conformation. The calculation of molecular conformation by the electronic functional density (DFT) was carried out by the means of program GAUSSIAN03 relating to the insulated molecule. This calculation gave a very good agreement with the experimental data obtained by the X-ray to 278K.To calculate the intermolecular potential and considering the difficulties encountered in the synthesis of a micro-crystal of caffeine, a crystal was simulated while approaching the crystallographic positions of the molecules in the elementary cell with 7% for heavy atoms. The generated crystal being formed of nineteen molecules distributed out of five plans where all the interactions were calculated compared to the molecule occupying the first crystallographic position (x, y, z).
The barrier of potential, calculated with the terms of Van der Waals and electrostatics, is of apparent ternary symmetry with three minima of energies located with 118°, 234° and 345° whose first position of balance is most probable. However, these minima of energy are shifted those given by the experiment of 118° on average. The calculated barrier is characterized by two very close heights of about 5.59 KJ/mole and 6.50 KJ/mole. These tiny heights show that; the molecule of caffeine is rather free in the crystal.
