Cooperativity in alkane‐1,2‐ and 1,3‐polyols: NMR, QTAIM, and IQA study of O─H^…OH and C─H^…OH bonding interactions

Abstract

Proton nuclear magnetic resonance chemical shifts and atom–atom interaction energies for alkanepolyols with 1,2‐diol and 1,3‐diol repeat units, and for their 1:1 pyridine complexes, are computed by density functional theory calculations. In the 1,3‐polyols, based on a tG'Gg' repeat unit, the only important intramolecular hydrogen bonding interactions are O─H…OH. By quantum theory of atoms in molecules analysis of the electron density, unstable bond and ring critical points are found for such interactions in 1,2‐polyols with tG'g repeat units, from butane‐1,2,3,4‐tetrol onwards and in their pyridine complexes from propane‐1,2,3‐triol onwards. Several features (OH proton shifts and charges, and interaction energies computed by the interacting quantum atoms approach) are used to monitor the dependence of cooperativity on chain length: This is much less regular in 1,2‐polyols than in 1,3‐polyols and by most criteria has a higher damping factor. Well defined C─H…OH interactions are found in butane‐1,2,3,4‐tetrol and higher members of the 1,2‐polyol series, as well as in their pyridine complexes: There is no evidence for cooperativity with O─H…OH bonding. For the 1,2‐polyols, there is a tenuous empirical relationship between the existence of a bond critical point for O─H…OH hydrogen bonding and the interaction energies of competing exchange channels, but the primary/secondary ratio is always less than unity.

Cooperativity in a cycloalkane‐1,2/1,3‐polyol corona: Topological hydrogen bonding in 1,2‐diol motifs

“…In a recent paper on alkane-1,2-and 1,3-polyols, we have discussed the relevant literature on intermolecular and intramolecular hydrogen-bond cooperativities and their main features. [27] An important point in the present context is that the hydrogen bond strengths or other relevant metrics are mutually enhanced. [33] This is most clearly appreciated in the simplest examples, structures 3 and 4, where 1,2-and 1,3-diol motifs are combined in two different ways.…”

“…In structure 5, where the 1,3-diol motif is bracketed by two 1,2-diols, the NMR shift of the central hydrogen-bonded proton is enhanced by 1.37 ppm relative to the value in propane-1,3-diol, 2, as against 1.61 ppm when the situation is reversed, in structure 6. When two tGg 0 motifs bracket a tGg 0 1,2-diol unit in an appropriate butane-1,-2,3,4-tetrol conformer, [27] the shift of the central OH proton is enhanced by 0.92 ppm and the corresponding interaction energy by 11.1 kcal mol −1 . The effects of the 1,2-and 1,3-diol motifs are, therefore, not so different as it would appear at first sight.…”

“…Similar shift/charge correlations were found for hydrogen-bonded NHÁ Á ÁN protons in a series of congested polycyclic diaza-alkanes, [47] and for OHÁ Á ÁO protons in single-conformer alkane-1,3-polyols. [27] The hydrogen-bonded OH groups lying on the periphery of structures 1-10 are relatively free of influences from the alkane skeleton, and the deshielding of the protons is very largely determined by their charges. It is not surprising that the head protons, which are not hydrogenbonded, do not follow the same correlation as the hydrogen-bonded ones, but why the average values correlate is not clear, and may be to some extent fortuitous.…”

“…The 0.73-ppm enhancement of the shift of the pro-S proton in 11 is clearly related to the presence of a hydrogen bond (interaction energy, −10.9 kcal mol −1 ), somewhat stronger than those found in alkane-1,-2-polyols based on a tG 0 g repeat unit. [27] 3 | DISCUSSION…”

“…[9,11,21] In recent work, BCPs have been found for alkane-1,2-polyols with tG 0 g repeat units, from butane-1,2,3,4-tetrol onwards, despite the fact that β is never greater than 0.9, even for the 1:1 pyridine complexes of these polyols, where the values are systematically higher. [27] However, even quite short alkane-1,-2-polyols have a screw-like structure with C-HÁ Á ÁO-H interactions between the head and the tail, which could affect the O-HÁ Á ÁO-H interactions. In contrast, alkane-1,3-polyols with a tG 0 Gg 0 repeat unit are quasi-linear.…”