Ab inito study and NBO analysis of the enantiomerization energy profile of tetra- peri-substituted overcrowded naphthalenes

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 2 h symmetry). MP2/3-21G*//HF/3-21G* results show that the plane-symmetrical ( C 2 h symmetry) geometries of compounds 1– 4 are less stable (about 8.40, 5.18, 8.01, and 6.37 kcal mol −1, respectively) than the axial-symmetrical geometries ( D 2 symmetry). Also, in compounds 1– 4, the plane symmetrical intermediate ( C 2 h symmetry) is less stable than the axial-symmetrical ( D 2 symmetry) geometry, about 10.07, 6.80, 9.49 and 7.33 kcal mol −1, respectively, as calculated by B3LYP/3-21G*//HF/3-21G* level of theory. The barrier heights for interconversion of the D 2 to C 2 h geometries for compounds 1– 4 are 29.32, 14.89, 12.28 and 9.24 kcal mol −1, 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 2 ′ ) for these compounds is 28.41, 13.48, 11.52, 8.33 kcal mol −1. In both axial and plane symmetrical ( D 2 and C 2 h ) 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/6-311+G**//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/6-311+G**//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 σ C aryl – M to σ C 9 – C 10 * delocalization is 1.43, 3.53, 3.96 and 4.62 kcal mol −1, 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.