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The enhanced stability of bent or kinked polycyclic benzenoids over linear ones is well established,phenanthrene and anthracene being archetypal representatives. The question why kinked is more stablethan linear is, however, still a matter of discussion. Recently, it has been proposed that H-H bondinginteractions between the two hydrogen atoms in the bay region of phenanthrene are responsible for thelarger stability of this molecule as compared to anthracene. This conclusion conflicts with the vast bodyof evidence for nonbonded steric repulsion between these hydrogen atoms. In this work, we providenew, complementary evidence for the repulsive character of the H-H interactions in phenanthrene's bayregion. We have traced the origin of phenanthrene's enhanced stability to the more efficient bonding inthe
-electron system using, among others, a quantitative energy decomposition analysis of the bondingbetween the two constituting 2-methtriyl-phenyl fragments in both phenanthrene and anthracene (i.e.,C
14H
10 = C
6H
4
-CH
+ C
6H
4-CH
). The scope of our study is extended to polycyclic benzenoids byanalyzing also hexacene and various bent isomers of the latter. Our results once more falsify one of thecore concepts of the
theory of atoms-in-molecules (AIM), namely, that the presence of bond paths andthe presence of bond critical points (they exist indeed between the two bay H atoms in phenanthrene) aresufficient indicators for a stabilizing interaction. Instead, our results confirm that these AIM parametersmerely diagnose the proximity or contact between charge distributions, be this contact stabilizing ordestabilizing.
