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Do Non-bonded 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.
MedLine Citation:
PMID:  22823493     Owner:  NLM     Status:  Publisher    
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 π-system, is considered. DFT calculations of the ground state energies (E) of the set of five benzoquinolines at the X3LYP/6-311G**++ level are reported. The five benzoquinolines (bzq) fall into three groups: 2,3-bzq and 6,7-bzq are linear bases resembling anthracene; 3,4-bzq and 5,6-bzq are bent like phenanthrene, and like phenanthrene have H--H interactions in the cleft; 7,8-bzq is bent like phenanthrene, but has a nitrogen atom in its cleft, so no H--H non-bonded interactions are present. If H--H non-bonding interactions are the major factor stabilizing phenanthrene relative to anthracene, then 7,8-bzq should have a particularly unfavorable E value. The relative -ΔE values rise in the order 6,7-bzq ≾ 2,3-bzq < 5,6-bzq ≾3,4-bzq < 7,8-bzq, supporting the idea that it is a more stable π-system that stabilizes 7,8-bzq relative to 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. By extension, one then concludes that phenanthrene is not stabilized relative to anthracene by favorable H--H non-bonded interactions in its cleft. The question of whether H--H non-bonded 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 E of bpy is scanned as a function of N-C-C-N torsion angle (χ) 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 E of bpy in aqueous solution as a function of χ is modeled by 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.
Robert Douglas Hancock; Igor V Nikolayenko
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Publication Detail:
Type:  JOURNAL ARTICLE     Date:  2012-7-23
Journal Detail:
Title:  The journal of physical chemistry. A     Volume:  -     ISSN:  1520-5215     ISO Abbreviation:  -     Publication Date:  2012 Jul 
Date Detail:
Created Date:  2012-7-24     Completed Date:  -     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  9890903     Medline TA:  J Phys Chem A     Country:  -    
Other Details:
Languages:  ENG     Pagination:  -     Citation Subset:  -    
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