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  1. Previous microwave studies of naphthol monomers were supplemented by measuring spectra of all 13 C mono-substituted isotopologues of the cis - and trans -conformers of 1-naphthol and 2-naphthol in their natural abundances. The resulting data were utilized to determine substitution structures and so-called semi-experimental effective structures. Results from electronic structure calculations show that the OH group of cis -1-naphthol points ≈6° out of plane, which is consistent with the inertial defect data of cis - and trans -1-naphthol. The non-planarity of cis -1-naphthol is a result of a close-contact H-atom–H-atom interaction. This type of H–H interaction has been the subject of much controversy in the past and we provide here an in-depth theoretical analysis of it. The naphthol system is particularly well-suited for such analysis as it provides internal standards with its four different isomers. The methods used include quantum theory of atoms in molecules, non-covalent interactions, independent gradient model, local vibrational mode, charge model 5, and natural bond orbital analyses. We demonstrate that the close-contact H–H interaction is neither a purely attractive nor repulsive interaction, but rather a mixture of the two. 
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  2. Abstract

    We evaluate the correlation between binding energy (BE) and electron densityρ(r)at the bond critical point for 28 neutral hydrogen bonds, recently reported by Emamian and co‐workers (J. Comput. Chem.,2019,40, 2868). As an efficient tool, we use local stretching force constantderived from the local vibrational mode theory of Konkoli and Cremer. We compare the physical nature of BE versus, and provide an important explanation for cases with significant deviation in the BE–relation as well as in the BE–ρ(r)correlation. We also show that care has to be taken when different hydrogen bond strength measures are compared. The BE is a cumulative hydrogen bond strength measure whileis a local measure of hydrogen bond strength covering different aspects of bonding. A simplified and unified description of hydrogen bonding is not always possible and needs an in‐depth understanding of the systems involved.

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