C―H···O hydrogen bonding interactions for sterically hindered phenols and their phenoxyl radicals

Abstract

Sterically hindered phenols have been widely used as chain-breaking antioxidants in chemical and polymeric industries, and the remarkable stabilities of their phenoxyl radicals are generally attributed to the steric effects and resonance stabilization of the unpaired electron into the aromatic ring. Noncovalent interaction (NCI) analysis shows that there are four intramolecular C-H···O hydrogen bonds between oxygen atom and the ortho t-butyl C-H bonds for the sterically hindered phenoxyl radicals and their phenolic precursors. The C-H···O hydrogen bonds can be directly visualized and quantified from the reduced density gradient NCI isosurfaces and atoms-in-molecules analysis, respectively. Furthermore, the phenolic O-H bond strength decreases gradually with increasing volume of the ortho alkyl substituents in three different phenol model systems ( Table 5). The nice linear correlation of the O-H bond dissociation enthalpies with the Taft's steric parameters (Figure 8) provides additional evidences to support formation of the intramolecular C-H···O hydrogen bonds between oxygen atom and the ortho t-alkyl C-H bonds. KEYWORDS atoms-in-molecules (AIM), non-covalent interaction (NCI), sterically hindered phenols/phenoxyls, hydrogen bonding interactions

Frustrated and Realized Hydrogen Bonding in 4-Hydroxy-3,5-ditertbutylphenylphosphine Derivatives

“…Classic hydrogen bonds are “realized” in all these phosphine oxide structures ( 2c , 2c′ , 4c , 4c′ , and 4c″ ), while 4c , 4c′ , and 4c″ also display partial frustration of H-bonding (details about the intermolecular interactions are described in the following section). Noteworthy in the refinement model of 4c″ is that the single OH group that is not involved in any intermolecular short contacts is disordered 50:50 into H5 and H5A, both involved in internal short contacts to the tert butyl CH groups (as described previously) …”

“…Another feature of 2 and all the other structures of phenols with flanking tert butyl groups is the presence of several significantly short intramolecular phenol OH-to-tertbutyl C–H contacts. A thorough computational investigation of these effects has recently been reported, which are inevitable consequences of the steric crowding. The presence of these internal contacts has been implicated as contributing to the frustration of intermolecular H-bonding in such phenols …”

“…Noteworthy in the refinement model of 4c″ is that the single OH group that is not involved in any intermolecular short contacts is disordered 50:50 into H5 and H5A, both involved in internal short contacts to the tertbutyl CH groups (as described previously). 56 Supramolecular Structures and Interaction Contacts. The 17 crystallographic structures described above can be classified according to intermolecular interactions into 4 different categories: those with only dispersion-induced short contacts (1, 3, 3a, and 3b), which are not further discussed; those with frustration of H-bonding (2, 2a, 2a′, 2b, 4, 4a, 4a′, and 4b); those with only classic H-bonding (2c and 2c′); and those with combinations of classic and frustration of Hbonding (4c, 4c′, and 4c″).…”

“…By contrast, the O2H2 and O3H3 phenols lack any intermolecular interactions, participating only in intramolecular contacts to methyl CH units of the flanking tertbutyl groups. 56 This represents a second type of frustration of H-bonding: the failure of such sterically shielded phenols to locate any suitable intermolecular acceptors. Because there is no structural distinction among them, a reasonable assumption is that both phenomenacomplete frustration of contacts for O2H2 and O3H3 and unusually weak R 2 2 (4) contacts for O1H1have the same origin.…”

First hydrogen-bonded adduct of sterically hindered 2-tert-butyl-4-methylphenol (TBMP) with 1,3,6,8-tetraazatricyclo[4.4.1.13,8]dodecane (TATD) via coupling of classical hydrogen bonds and C—H...π non-covalent interactions

“…The resultant crystal structure reported here also exhibits C-HÁ Á ÁO hydrogen-bonding interactions, which constitute a fundamental force in maintaining crystal and three-dimensional chemical structures in chemistry and biology (Wang et al, 2019).…”