Dicyclobuta[<i>d</i><i>e</i>,<i>i</i><i>j</i>]naphthalene and Dicyclopenta[<i>c</i><i>d</i>,<i>gh</i>]pentalene: \nA Theoretical Study

Abstract

The structures, energetics, and aromatic character of dicyclobuta[<i>de</i>,<i>ij</i>]naphthalene, <b>1</b>, dicyclopenta[<i>cd</i>,<i>gh</i>]pentalene, <b>2</b>, dihydrodicyclobuta[<i>de</i>,<i>ij</i>]naphthalene, <b>3</b>, and dihydrocyclopenta[<i>cd</i>,<i>gh</i>]pentalene,\n<b>4</b>, have been examined at the B3LYP/6-311++G**//B3LYP/6-31G* level of theory. All molecules\nare bowl-shaped, and the pentalene isomers, <b>2</b> and <b>4</b>, are most stable. A comparison with other\nC₁₂H₆ and C₁₂H₈ isomers indicates that <b>2</b> is ∼25 kcal/mol less stable than 1,5,9-tridehydro[12]annulene and <b>4</b> is ∼100 kcal/mol higher in energy than acenaphthylene, both of which are\nsynthetically accessible. The transition state structure for bowl-to-bowl inversion of <b>1</b> is planar\n(<i>D</i>₂<i>_h</i>) and lies 30.9 kcal/mol higher in energy than the ground state; the transition state for inversion\nof <b>2</b> is <i>C</i>₂<i>_h</i> and lies 46.6 kcal/mol higher in energy. Symmetry considerations, bond length\nalternations, and NICS values (a magnetic criterion) all indicate that the ground states of <b>1</b>, <b>3</b>,\nand <b>4</b> are very aromatic; however, HOMA values (a measure of bond delocalization) indicate that\n<b>3S</b> and <b>4S</b> are aromatic but that <b>1S</b> is less so. NICS values for the ground state of <b>2</b> strongly\nindicate aromaticity; however, bond localization, symmetry, and HOMA values argue otherwise.