The molecular orbital calculations (MOCs) were performed
using semi-empirical molecular orbital calculation. The method
used in these computations is the parametric method (PM3)
described by Stewart [20]. The default criteria for terminating all
optimizations were increased by a factor of 100 (keyword PRECISE).
Vibrational frequencies were computed for the studied structures
(keyword FORCE) so as to check whether the newly designed
geometries are local minima. All the molecular orbital calculations were carried out at the unrestricted Hartree–Fock level (UHF)
for there positively charged ions using PM-3 method followed by
full optimization of all geometrical variables (bond lengths, bond
angles, and dihedral angles), without any symmetry constraint. All
structures were optimized to a gradient normalization of 0.01–0.05,
using the eigenvector following (EF) routine [21]. All the semi
empirical MO calculations were performed with the MOPAC2000
software package [22] implemented on an Intel Pentium IV 3.0 G
Hz computer.