Intramolecular Hydrogen Bonds in ortho-Substituted Hydroxybenzenes and in 8-Susbtituted 1-Hydroxynaphthalenes: Can a Methyl Group Be an Acceptor of Hydrogen Bonds?
Considering the findings of Fujii et al. showing that the cis isomer of the o-cresol radical cation shows a low-frequency shift of the OH stretching attributed to an intramolecular hydrogen bond with the CH3 group and considering the studies of Knak Jensen et al. concluding that such an O−H···C interaction was not possible, the work presented in this article tries to understand if this is a consequence of the nature of the hydrogen bond acceptor (a CH3group) or of the five-member ring that would be formed as a result of the intramolecular interaction. Thus, we have studied o-cresol, 8-methyl-1-hydroxynaphthalene, 1-hydroxy-1-propene, 1-hydroxy-3-methyl-1,3-butadiene, and their derivatives in which the −CH3 group has been substituted by a −F atom or by an −OH group. Taking into account interaction distances and angles, interaction energies (from isodesmic reactions), and electron density characteristics, we can conclude that, in general, a methyl group cannot behave as a hydrogen bond acceptor. In addition, we found that the formation of intramolecular hydrogen bonds driving to the formation of five-member rings is not favored even in the presence of a good acceptor. Moreover, different methods of evaluating intramolecular interaction energies have been analyzed.