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.