Structure and isomeric studies of 1,3-diaryl-H-benzo[f]chromene, catalyst effect or thermodynamic stability? An ab initio study

Two possible isomers of some 1,3-diaryl-H-benzo[f]chromene have been studied using density functional theory. Structures of E1 and E2 isomers were optimized at the B3LYP and MP2 levels with different basis sets. The total electronic energies show that E2 isomer is about 3-5 kcal/mol more stable than E1 isomer and this energy difference is attributed to the planarity of heterocyclic ring and more establishment resonance in E2 isomer, that is confirmed by second order interaction energies E(2) of NBO results. The calculated geometry for both chromene isomers were also compared with the experimental data. The X-ray data indicate the E1 isomer as the stable structure for 1,3-diaryl-1H-benzo[f]chromene, while the E2 isomer is fixed for 3-phenyl-1-p-tolyl-1H-benzo[f]chromene. The compared dihedral angles of both isomers show that phenyl (I) group in E2 isomer has more contribution in resonance with the heterocyclic and naphthalene rings than that in E1, while in E1 isomer phenyl (II) group is more engaged in resonance than that in E2.