Relevance of weak hydrogen bonds in the conformation of organic compounds and bioconjugates: evidence from recent experimental data and high-level ab initio MO calculations.

Organic chemists tend to consider the conformation of compounds as a consequence of repulsive steric interactions. In other words, “the steric effect” means “repulsive” in many cases. Thus, folded conformations observed in organic molecules have been often regarded as unusual, while the reason remained undecided. However, accurate determinations by modern spectroscopic methods, recent crystallographic data, and high-level ab initio MO calculations have demonstrated that the folded conformation is by no means exceptional. We will show that the gauche or folded conformation prevails in organic compounds bearing at least an electronegative or π-group in the molecule. We consider that the above phenomenon finds its origin, in most cases, in nonconventional hydrogen bonds such as the CH/X (X ) O, halogen, etc.) and the XH/π (X ) O, N, C, etc.) hydrogen bonds. * To whom correspondence should be addressed. E-mail: shu@ hiroshima-u.ac.jp. ‡ Hiroshima University. § Yokohama National University. ⊥ The CHPI Institute. Chem. Rev. 2010, 110, 6049–6076 6049