Quantum Chemical Consideration of Photostability of Imidacloprid and Related Compounds

Chloronicotinyl Insecticides (Part 9)

Abstract

The electronic configurations in ground and excited states of imidacloprid and related compounds were obtained by AM1 and ab-initio methods. The evaluated energy gap from the ground state to the singly excited state (S1) is in order of cyanoimine>nitroimine>nitromethylene, which is in accord with the actual photostability order among these compounds. Judging from that the HOMOs reside mostly in the guanidino or amidino part comprising the imidazolidine nitrogens and the LUMOs stretch toward the nitro and cyano groups, the primary excitations by UV absorption of these molecules is classified as the intramolecular donor-acceptor electron transfer. To this context more electron-donating thiazolidine exhibits a bathocromic shift from the imidazolidine compounds. The nitro group is coplanar with the guanidino or amidino part in the ground states. In the triplet excited state (T1) the nitro group becomes twist from the plane in the nitroamidine, on the other hand in the nitroimine the distortion is to such a small degree as the resonance stability can be still expected. The energy gap between HOMO and LUMO, and the bonding nature in the excited states are the determining factor for the photolytical behavior of chloronicotinyl molecules on field.