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Ab initio studies on excited state intramolecular electron transfer in 4-amino-N-methylphthalimide and 3-amino-N-methylphthalimide

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Abstract

The molecules 4-amino-N-methylphthalimide (4AMP) and 3-amino-N-methylphthalimide (3AMP) are bichromophoric compounds composed of both electron donors and acceptor groups. They undergo intramolecular electron transfer (ET), and these types of compounds are frequently used to probe microenvironments in proteins, micelles, membranes, polymer, surfaces, etc. Ab initio calculations using restricted Hartree–Fock (RHF), Møller–Plesset second-order perturbation theory (MP2) and the multi-configurational self-consistent field (MCSCF) methods with the 6-31G** basis set have been performed to characterize the intermediates of the ET process. Analysis of geometrical and electronic parameters, e.g., changes in bond lengths, angles, dihedrals, and charge density, generally used to predict the ET pathway, is not always sufficient to understand the through-bond intramolecular charge-transfer (ICT) process. Hence we have attempted to support the predicted ET pathway in a very unique way by analyzing normal modes of vibrations for molecules in both the ground and excited states. Our results predict a planar ICT model that exhibits a conformational change of the amino nitrogen from nearly sp3 in the ground state to approximately sp2 type in the ICT state. A through-bond ET occurs from the amino group of both molecules to the proximal carbonyl group in 4AMP and to the distant carbonyl group in 3AMP. Besides ET, the proximal carbonyl group in 3AMP also takes part in hydrogen bonding with the same amino group especially in excited state, which may also contribute toward ET. Such a hydrogen bond, which demands a planar amino group, is not observed in ground-state 3AMP. These results indicate that nonplanar amino group geometry in the vicinity of a conjugated system is favored, rather than the conventional planar geometry, even in the presence of a suitable hydrogen bond acceptor. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

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