Tests for aromaticity applied to the pentalenoquinones - A computational study

Criteria for aromaticity and antiaromaticity were applied to the four penta lenoquinones, 1,2-, 1,5-, 1,4-, and 1,6-pentalenoquinone, i.e., bicyclo[3.3 .0]octa-4,6,8-triene-2,3-dione (7a), bicyclo[3.3.0]octa-3,5,8-triene-2,7-di one (7b), bicyclo[3.3.0]octa-1(5),3,7-triene-2,6-dione (7c), and bicyclo[3. 3.0]octa-1(5),3,6-triene-2,8-dione (7d). Geometry optimizations and frequen cy calculations were done with the pBP/DN* DFT method as implemented in Spa rtan, and single-point HF/3-21G calculations to obtain Lowdin bond orders ( Spartan), as well as HF/6-31G* NICS calculations (Gaussian 98) were also ca rried out. Geometries and bond orders, chemical hardness, and NICS values g ave no definite indication of aromatic or antiaromatic character. However, homodesmotic ring-opening reactions to give acyclic analogues indicated tha t 7a and 7b are nonaromatic (resonance energies -11 and 5 kJ mol(-1)) while 7c and 7d are antiaromatic (resonance energies -83 and -54 kJ mol(-1)). Th e resonance energies were obtained with the aid of an estimate of the strai n energy of the molecules 7 (86 kJ mol(-1)) by a novel extrapolation proced ure on hydropentalenes. Calculated pBP/DN* activation energies for Diels-Al der reactions with ethyne and ethene placed 7a and 7b in an "unreactive" cl ass similar to 1,3-butadiene and fulvene, and 7c and 7d in a "reactive" cla ss, similar to cyclopentadienone.