Figure 4. Selected frontier orbitals distributions of the both
compounds.
Nitro groups affected the HOMO, LUMO, HOMO+1, HOMO+2, HOMO+3, LUMO-1,
LUMO-2 and LUMO-3 molecular orbitals because of the discrepancy between
molecular and crystal structures of the studied molecules. HOMO and
HOMO-1 orbital of AB1 were delocalized over the donor and acceptor
moieties of the molecule while they were distributed over the pyrazole
rings of the AB2 as the electron-donating character of
substituents increases. For the AB1 , LUMO and LUMO+1
orbitals were localized over the donor and acceptor sides of the
molecule whereas LUMO and LUMO+1 orbitals of the AB2 mainly
located electron acceptor groups. AB2 displayed a major
charge-transfer transition from a pyrazole ring delocalized HOMO and
HOMO-1 to nitrophenyl ring delocalized LUMO and LUMO+1, due to efficient
HOMO/LUMO overlap and the strong electron-withdrawing ability of the
nitro groups.