Ab inito study and NBO analysis of the enantiomerization energy profile of tetra-peri-substituted overcrowded naphthalenes Davood Nori-Shargh a,b, * , Maryam Malekhosseini b , Farzad Deyhimi c,a a Chemistry Department, Graduate Faculty, Arak branch, Islamic Azad University, Arak, Iran b Chemistry Department, Science and Research Campus, Islamic Azad University, Hesarak, Poonak, Tehran, Iran c Chemistry Department, Shahid Beheshti University, Evin-Tehran 19839, Iran Received 13 August 2005; received in revised form 12 January 2006; accepted 22 January 2006 Abstract Ab initio molecular orbital (MO) and density functional theory methods were used to investigate the structural and conformational properties of 1,4,5,8-tetra-tert-butyl naphthalene (1), 1,4,5,8-tetrakis(trimethylsilyl)naphthalene (2), 1,4,5,8-tetrakis(trimethylgermyl)naphthalene (3) and 1,4,5,8-tetrakis(trimethylstannyl)naphthalene (4). HF/3-21G*//HF/3-21G*, MP2/3-21G*//HF/3-21G*, B3LYP/3-21G*//HF/3-21G*, HF/LANL2DZ*//HF/LANL2DZ*, MP2/LANL2DZ*//HF/LANL2DZ* and B3LYP/LANL2DZ*//HF/LANL2DZ* results revealed that the most stable form of these compounds has D 2 symmetry, and, therefore, they should present optical activity. The enantiomerization processes for compounds 1–4 could take place via plane symmetrical intermediates (with C 2h symmetry). MP2/3-21G*//HF/3-21G* results show that the plane- symmetrical (C 2h symmetry) geometries of compounds 1–4 are less stable (about 8.40, 5.18, 8.01, and 6.37 kcal mol K1 , respectively) than the axial-symmetrical geometries (D 2 symmetry). Also, in compounds 1–4, the plane symmetrical intermediate (C 2h symmetry) is less stable than the axial-symmetrical (D 2 symmetry) geometry, about 10.07, 6.80, 9.49 and 7.33 kcal mol K1 , respectively, as calculated by B3LYP/3-21G*//HF/3- 21G* level of theory. The barrier heights for interconversion of the D 2 to C 2h geometries for compounds 1–4 are 29.32, 14.89, 12.28 and 9.24 kcal mol K1 , as calculated by MP2/3-21G*//HF/3-21G* level of theory. Also, B3LYP/3-21G*//HF/3-21G* results show that the required energy for ring flipping (D 2 to D 0 2 ) for these compounds is 28.41, 13.48, 11.52, 8.33 kcal mol K1 . In both axial and plane symmetrical (D 2 and C 2h ) geometries of compounds 1–4, the naphthalene rings are puckered and their aromatic characters are relatively perturbed. For compounds 1–3, the calculations were also performed at B3LYP/6-311CG**//HF/6-31G*, MP2/6-31G*//HF/6-31G* and HF/6- 31G*//HF/6-31G* levels of theory. However, the comparison showed that the results at HF/3-21G*//HF/3-21G*, B3LYP/3-21G*//HF/3-21G* and MP2/3-21G*//HF/3-21G* methods correlated well with those obtained at HF/6-31G*//HF/6-31G*, B3LYP/6-311CG**//HF/6-31G* and MP2/6-31G*//HF/6-31G* levels of theory. Further, NBO analysis, based on the HF/3-21G* optimized ground state geometries, revealed that in compounds 1–4, the resonance energy associated with s C aryl –M to s  C9–C10 delocalization is 1.43, 3.53, 3.96 and 4.62 kcal mol K1 , respectively. These resonance energy values could explain the easiness of ring flipping processes from compound 4 to 1. The NBO results are in good agreement with the calculated energy barriers for ring flipping in compounds 1–4, as calculated by MP2, B3LYP and HF methods, using all electron (3-21G*) and pseudopotential (LANL2DZ*) basis sets. q 2006 Elsevier B.V. All rights reserved. Keywords: Tetra-peri-substituted naphthalene; Molecular modeling; Chiral stability; Ab initio 1. Introduction The steric strain, associated with 1,8-disubstiuted naphthalene, has been the subject of much work [1]. The substituents in the 1 and 8 positions in naphthalene ring may be accommodated by distortion of the peri bonds in in-plane or out-of-plane deflections of these substituents. There is no reported experimental or theoretical data about the structures and dynamic behaviour of 1,4,5,8-tetra-tert-butyl naphthalene (1), 1,4,5,8-tetrakis(trimethylsilyl)naphthalene (2), 1,4,5,8-tetrakis(trimethylgermyl)naphthalene (3) and 1,4,5,8- tetrakis(trimethylstannyl)naphthalene (4). However, the reported experimental results for 1,8-di-tert-butylnaphthalene (5) have revealed that in this compound, the non-bonded repulsions result in a wraped naphthalene ring [2–7] (Scheme 1). Journal of Molecular Structure: THEOCHEM 763 (2006) 187–198 www.elsevier.com/locate/theochem 0166-1280/$ - see front matter q 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2006.01.032 * Corresponding author. Address: Chemistry Department, science and Research Campus, Islamic Azad University, Hesarak, Poonak, Tehran, Iran. Tel.: C98 21 4817172; fax: C98 21 4817175. E-mail address: nori_ir@yahoo.com (D. Nori-Shargh).