Three polymorphic
structures of mefenamic acid, which is a very
popular drug, have been studied using quantum chemical methods. It
has been shown that the centrosymmetric dimer formed due to two O–H···O
hydrogen bonds is a complex building unit in all of the polymorphic
structures under study. On the basis of an analysis of the pairwise
interaction energies between molecules, the polymorphic forms
I
and
II
are classified as columnar-layered while
the polymorphic form
III
has a columnar structure. The
stabilities of the three polymorphic forms of mefenamic acid under
ambient conditions (
I
>
II
>
III
) correlate with the degree of anisotropy of the interaction
energies
between columns (primary basic structural motifs) formed due to stacking
interactions. The shear deformation modeling of strongly bound layers
in all of the polymorphic structures has not revealed any possibility
for deformation of the crystal structure. The construction of the
shift energy profiles and calculation of the energy barriers for the
displacement along the (100) crystallographic plane in the [100],
[010], and [011] crystallographic directions make it possible to explain
the experimental data obtained for commercially available polymorphic
structure
I
in a diamond anvil cell. The absence of any
local minimum near the starting point on the shift energy profile
and the extremely high energy barrier can be considered as criteria
for the impossibility of a crystal structure deformation under pressure.
Electrochemical behavior of vinpocetine at a plain carbon paste electrode and paste electrodes modified with different calixarenes and ammonium metavanadate was studied by means of cyclic and differential pulse voltammetry. The effect of pH and scan rate on the response at both plain and modified carbon paste electrodes has been revealed. Modification of an electrode with a mixture of ammonium metavanadate and some calixarenes led to considerable decrease of background current along with sensitivity increase as compared to an unmodified carbon paste electrode or an electrode modified with calixarenes only. For the most effective electrode composition, the linear working range was shown to be 2 . 10 −8 -6 . 10 −6 M with a detection limit of 5 . 10 −9 M.
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