Do Nonbonded H-H Interactions in Phenanthrene Stabilize It Relative to Anthracene? A Possible Resolution to this Question and Its Implications for Ligands such as 2,2′-Bipyridyl

The problem of whether interactions between the hydrogen atoms at the 1,10 positions in the cleft of the bent phenanthrene stabilize the latter molecule thermodynamically relative to linear anthracene, or whether the higher stability of phenanthrene is due to a more energetically favorable pi-system, is considered. DFT calculations at the X3LYP/cc-pVTZ(-f)++ level of the ground state energies (E) of anthracene, phenanthrene, and the set of five benzoquinolines are reported. In the gas phase, bent phenanthrene was computed to be thermodynamically more stable than linear anthracene by -28.5 kJ mol(-1). This fact was attributed predominantly to the phenomenon of higher aromatic stabilization of the pi-system of phenanthrene relative to anthracene, and not to the stabilizing influence of the nonbonding H-H interactions in its cleft. In fact, these interactions in pherianthrene were shown to be destabilizing. Similar calculations for five benzoquinolines (bzq) indicate that Delta E values vary as 6,7-bzq (linear) <= 2,3-bzq (linear) < 5,6-bzq (bent) <= 3,4-bzq, < 7,8-bzq (bent, no H-H nonbonding interactions in cleft), supporting the idea that it is a more stable pi-system that favors 7,8-bzq over 2,3-bzq and 6,7-bzq, and that the H-H interactions in the clefts of 3,4-bzq and 5,6-bzq are destabilizing. Intramolecular hydrogen bonding in the cleft of 7,8-bzq plays a secondary role in its stabilization relative 6,7-bzq. The question of whether H-H nonbondecl interactions between H atoms at the 3 and 3' positions of 2,2'-bipyridyl (bpy) coordinated to metal ions are stabilizing or destabilizing is then considered. The energy of bpy is scanned as a function of N-C-C-N torsion angle. (chi) in the gas-phase, and it is found that the trans form is 32.8 kJ mol(-1) more stable than the cis conformer. A relaxed coordinate scan of energy of bpy in aqueous solution as a function of chi is modeled using the PBF approach, and it is found that the trans conformer is still more stable than the cis, but now only by 5.34 kJ mol(-1). The effect that the latter energy has on the thermodynamic stability of complexes of metal ions with bpy in aqueous solution is discussed.