Quantum
mechanics calculations on the diastereomeric salts of cyclic phosphoric
acids with ephedrine
Author(s): Schaftenaar G, de Wijs GA, Sanchez-Portal D, Vlieg E
Abstract:
Using three different density functional packages. DMol(3), VASP, and
SIESTA, we have calculated the relaxed structures and relative energies
of the diastereomers of cyclic phosphoric acid and its chlorine
derivative with ephedrine. The structures predicted by the three codes
are in very good agreement with experiment. Also in accordance with
experiment, very small energy differences (< 3 kcal/mol) are found
between diastereomers. However all the three packages fail to correctly
reproduce the stability order of all diastereomers. This failure is
probably related to the inability of the currently available
approximations for the exchange-correlation energy functional to
reproduce dispersive long-range interactions (i.e. van der Waals). In
spite of the large unit cell, the relative stability of the different
structures is quite sensitive to the k-space sampling. On the one hand,
VASP and SIESTA codes predict the correct stability order when only
Gamma point is used for the k-space integration. A better k-space
sampling however, destroys the agreement between theory and experiment,
increasing the energy of the experimentally most stable
chlorine-containing diastoreomer. On the other hand, DMol(3), with
which only Gamma point calculations have been performed; predicts the
stability order of one pair of diastereomers correctly, but the
stability order of a third polymorph is predicted wrongly. An
experimentally unknown chlorine-free analogue of the most stable
chlorine-containing diastereomer, was calculated to be the least stable.