Theoretical Studies on the Structure and Spectrum of Imidazole-Chloranil Charge Transfer Complex

UV-Vis absorption spectra of the molecular complex formed by imidazole (Im) and chloranil (CA) were measured in chloroform. The stoichiometry of the imidazole-chloranil (Im-CA) complex was determined as 1:1 by applying Benesi-Hildebrand's equation and Job's continuous variation method. Density function theory (DFT) and MP2 calculations were performed to study the structures and the binding energies of the Im-CA complex. The calculations located four conformations (denoted as S1-S4) for the Im-CA complex, two edge(Im)-to-face(CA) linked and two edge(Im)-to-edge(CA) linked. It was found that the edge-to-face conformers are more stable than the edge-to-edge ones. The bonding characteristics of these conformers were investigated with natural population analysis (NPA), topological analysis of electron density, and natural bond orbital (NBO) analysis. It was revealed that the edge-to-face conformers are charge-transfer (CT) complexes whereas the edge-to-edge conformers are the hydrogen bond complexes. For the most stable conformation of the Im-CA complex (S1), the charge transfer interaction of the imidazole n(N15) lone pair orbital with the chloranil ∏*(C1=O7) orbital plays a crucial role in the Im-CA binding, and the binding is further strengthened by the O7… H20 hydrogen bond. The electronic excitation energies of the complex (S1) were calculated with time-dependent DFT (TDDFT), and the observed UV-Visible spectrum of the complex was analyzed based on the computed results.