The hydrogen bond (HB), arguably the most important non-covalent interaction in chemistry, is getting renewed attention particularly in materials engineering. We address herein HB non-additive features by examining different structures of the water hexamer (cage, prism, book, bag and ring). To that end, we rely on the interacting quantum atoms (IQA) topological energy partition, an approach that has been successfully used to study similar effects in smaller water clusters (see Chem. - Eur. J., 19, 14304). Our IQA interaction energies, , are used to classify the strength of HBs in terms of the single/double character of the donor and acceptor H2O molecules involved in the interaction. The strongest hydrogen bonds on this new scale entail double donors and acceptors that show larger values of than those observed in homodromic cycles, paradigms of cooperative effects. Importantly, this means that besides the traditional HB anticooperativity ascribed to double acceptors and donors, the occurrence of these species is also related to HB strengthening. Overall, we hope that the results of this research will lead to a further understanding of the HB non-additivity in intramolecular and intermolecular interactions.

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Title: The nature of resonance-assisted hydrogen bonds: a quantum chemical topology perspectiveThis work addresses the nature of Resonance Assisted Hydrogen Bonds (RAHBs) by considering malonaldehyde whose H-bonded (closed) and non H-bonded (open) conformations are represented in the bottom and top of the seesaws respectively. Although delocalization indices are more uniform in the closed form (seesaw in the left), the number of delocalized electrons diminish on RAHB formation. The interaction is characterized by an increase/decrease in the intra-atomic/inter-atomic exchange energies as schematized with sparks in the highlighted seesaw in the right. The artwork is due to Mr Víctor Duarte Alaniz. indicate that the p-conjugated bonds allow for a larger adjustment of electron density throughout the H-bonded system as compared with non-conjugated carbonyl molecules. This rearrangement of charge distribution is a response to the electric field due to the H atom involved in the hydrogen bonding of the considered compounds. As opposed to the usual description of RAHB interactions, these HBs lead to a larger electron localisation in the system, and concomitantly to larger QTAIM charges which in turn lead to stronger electrostatic, polarization and charge transfer components of the interaction. Overall, the results presented here offer a new perspective on the cause of strengthening of these important interactions.

The applicability of the Evans–Polanyi (EP) relationship to HAT reactions from C(sp 3 )–H bonds to the cumyloxyl radical (CumO • ) has been investigated. A consistent set of rate constants, k H , for HAT from the C–H bonds of 56 substrates to CumO • , spanning a range of more than 4 orders of magnitude, has been measured under identical experimental conditions. A corresponding set of consistent gas-phase C–H bond dissociation enthalpies (BDEs) spanning 27 kcal mol –1 has been calculated using the (RO)CBS-QB3 method. The log k H ′ vs C–H BDE plot shows two distinct EP relationships, one for substrates bearing benzylic and allylic C–H bonds ( unsaturated group) and the other one, with a steeper slope, for saturated hydrocarbons, alcohols, ethers, diols, amines, and carbamates ( saturated group), in line with the bimodal behavior observed previously in theoretical studies of reactions promoted by other HAT reagents. The parallel use of BDFEs instead of BDEs allows the transformation of this correlation into a linear free energy relationship, analyzed within the framework of the Marcus theory. The Δ G ⧧ HAT vs Δ G ° HAT plot shows again distinct behaviors for the two groups. A good fit to the Marcus equation is observed only for the saturated group, with λ = 58 kcal mol –1 , indicating that with the unsaturated group λ must increase with increasing driving force. Taken together these results provide a qualitative connection between Bernasconi’s principle of nonperfect synchronization and Marcus theory and suggest that the observed bimodal behavior is a general feature in the reactions of oxygen-based HAT reagents with C(sp 3 )–H donors.

We report a novel coantioxidant system based on TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical) that, in biologically relevant model systems, rapidly converts chain-carrying alkylperoxyl radicals to HOO·. Extremely efficient quenching of HOO· by TEMPO blocks the oxidative chain. Rate constants in chlorobenzene were measured to be 1.1 × 10 M s for the reductive reaction TEMPO + HOO· → TEMPOH + O and 5.0 × 10 M s for the oxidative reaction TEMPOH + HOO· → TEMPO + HO. These rate constants are significantly higher than that associated with the reaction of HOO· with α-tocopherol, Nature's best lipid soluble antioxidant ( k = 1.6 × 10 M s). These data show that in the presence of ROO·-to-HOO· chain-transfer agents, which are common in lipophilic environments, the TEMPO/TEMPOH couple protects organic molecules from oxidation by establishing an efficient reductive catalytic cycle. This catalytic cycle provides a new understanding of the efficacy of the antioxidant capability of TEMPO in nonaqueous systems and its potential to act as a chemoprotective against radical damage.