Thermal 6 pi electrocyclization of cyclononatetraene (CNT), its hetero-substituted analogues, and its benzannelated derivatives have been investigated by using the B3LYP method employing 6-31G* and 6-311+G** basis sets. The results indicate that heterosubstitution and benzannelation influence the rate of cyclization. Nucleus independent chemical shifts (NICS), conceptual density functional theory (DFT) based reactivity descriptors, group electronegativity values, and barriers to planarity provide complementary evidence for the predicted rate of cyclization. The available experimental data are in good agreement with the computed values.