A Comparative Theoretical Study on DMABN: Significance of Excited State Optimized Geometries and Direct Comparison of Methodologies

Abstract

Theoretical studies are presented for 4,-N,N-dimethylaminobenzonitrile (DMABN) by using the semiempirical Austin model 1 (AM1) and ab initio Hartree−Fock (HF) methodology for optimization of the electronic ground and AM1/configuration interaction with both single and double excitations (CISD) and HF/configuration interaction with single excitation (CIS) for the lowest excited states. For a correct description of the ground-state structure, additional polarization functions and at least a split-valence double-ζ basis set have to be used. For both the ground and excited states of DMABN, the relative orientation of the two methyl groups is important: AM1/CISD predicts both the first (1Lb character) and second excited state (1La character) to be of untwisted and slightly pyramidalized structure with the methyl groups oriented in a staggered conformation. HF/CIS computes the La state at lower energy than the Lb state in contrast to experimental data. This incorrect state ordering represents a serious problem ...