Conformations of phthalan and 1,3-benzodioxole in their S0 and S1(π,π) electronic states:: theoretical study

Theoretical ab initio calculations using the HF, B3LYP, MP2, CIS and td-B3LYP methods have been performed in order to investigate the structural features of phthalan and 1,3-benzodioxole in their S-0 and S-1(pi,pi*) electronic states. The calculated results predict that S-0 --> S-1 electronic excitation leads to considerable changes of their structural properties due to the decrease of pi electron character in the S-1 state. Extensive comparisons are made with the previously reported far-infrared, Raman and laser-induced fluorescence excitation spectra, and a reasonable agreement has been achieved. Natural bond orbital analysis has also been carried out in order to explain why the five-membered ring of 1,3-benzodioxole should have more puckered configuration in the S-1 state. Our ab initio calculations, performed for the S-0 and S-1 states, not only strongly support the recently reported experimental results but also explain the theoretical basis for the conformational preferences of phthalan and 1,3-benzodioxole. (C) 2001 Elsevier Science B.V. All rights reserved.