Bimodal Evans-Polanyi Relationships in Hydrogen Atom Transfer from C(sp3)-H Bonds to the Cumyloxyl Radical. A Combined Time-Resolved Kinetic and Computational Study.

The applicability of the Evans-Polanyi (EP) relationship to HAT reactions from C(sp3)-H bonds to the cumyloxyl radical (CumO•) has been investigated. A consistent set of rate constants, kH, 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 kH' 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 ΔG⧧HAT vs ΔG°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(sp3)-H donors.

Computational Study of Hydrogen Bond Interactions in Water Cluster-Organic Molecule Complexes.

We analyzed the interactions present in complexes that acetone, azomethane, dimethylamine, dimethyl ether, methyl acetate, and oxirane form with 39 different (H2O)n clusters (n = 1-10). A random generation of configurations and a subsequent screening procedure were employed to sample representative interactions. Using quantum chemical computations, we calculated the associated binding energies, which range from -0.19 to -10.76 kcal/mol at the DLPNO-CCSD(T)/CBS level. It was found that the binding energies can be understood in terms of various factors, including the water cluster size, the nature of the organic molecule, and the type of hydrogen bond donor. We find that the most stable complexes often arise from a combination of a strong hydrogen bond plus a secondary interaction between the organic molecule and the water cluster.

Directing the Crystal Packing in Triphenylphosphine Gold(I) Thiolates by Ligand Fluorination.

We explore herein the supramolecular interactions that control the crystalline packing in a series of fluorothiolate triphenylphosphine gold(I) compounds with the general formula [Au(SRF)(Ph3P)] in which Ph3P = triphenylphosphine and SRF = SC6F5, SC6HF4-4, SC6F4(CF3)-4, SC6H3F2-2,4, SC6H3F2-3,4, SC6H3F2-3,5, SC6H4(CF3)-2, SC6H4F-2, SC6H4F-3, SC6H4F-4, SCF3, and SCH2CF3. We use for this purpose (i) DFT electronic structure calculations and (ii) the quantum theory of atoms in molecules and the non-covalent interactions index methods of wave function analyses. Our combined experimental and computational approach yields a general understanding of the effects of ligand fluorination in the crystalline self-assembly of the examined systems, in particular, about the relative force of aurophilic contacts compared with other supramolecular interactions. We expect this information to be useful in the design of materials based on gold coordination compounds.

Evaluation of Polar Effects in Hydrogen Atom Transfer Reactions from Activated Phenols.

Evaluation of polar effects in hydrogen atom transfer (HAT) processes is made difficult by the fact that in most cases substrates characterized by lower bond dissociation energies (BDEs), activated from an enthalpic point of view, are also more activated by polar effects. In search of an exception to this general rule, we found that the introduction of a methoxy substituent in the 3-position of 2,6-dimethylphenol results in a small increase in the O-H BDE and a decrease of the ionization potential of the phenol. These findings suggest that the enthalpic effect associated with the addition of the m-methoxy group to 2,6-dimethylphenol will decrease reaction rates, while the polar effects will increase reaction rates. Our model analysis of polar effects has been experimentally validated by comparing the reactivity of 2,6-dimethylphenol with that of 2,6-dimethyl-3-methoxyphenol in HAT promoted by a series of radicals (cumyloxyl, galvinoxyl, 2,2-diphenylpycrylhydrazyl, phthalimide- N-oxyl, and benzotriazole- N-oxyl radicals). In line with our predictions, the ratio of HAT rate constants ( kH mOMe/ kHH) is larger in cases where there is a greater contribution of polar effects in the HAT reaction, i.e., in HAT promoted by N-oxyl radicals containing electron-withdrawing groups or when more polar solvents are employed.

Extremely Fast Hydrogen Atom Transfer between Nitroxides and HOO· Radicals and Implication for Catalytic Coantioxidant Systems.

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 × 109 M-1 s-1 for the reductive reaction TEMPO + HOO· → TEMPOH + O2 and 5.0 × 107 M-1 s-1 for the oxidative reaction TEMPOH + HOO· → TEMPO + H2O2. 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 × 106 M-1 s-1). 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.

Acidity and basicity interplay in amide and imide self-association.

Amides dimerise more strongly than imides despite their lower acidity. Such an unexpected result has been rationalised in terms of the Jorgensen Secondary Interactions Hypothesis (JSIH) that involves the spectator (C[double bond, length as m-dash]OS) and H-bonded (C[double bond, length as m-dash]OHB) carbonyl groups in imides. Notwithstanding the considerable body of experimental and theoretical evidence supporting the JSIH, there are some computational studies which suggest that there might be other relevant intermolecular interactions than those considered in this model. We conjectured that the spectator carbonyl moieties could disrupt the resonance-assisted hydrogen bonds in imide dimers, but our results showed that this was not the case. Intrigued by this phenomenon, we studied the self-association of a set of amides and imides via 1H-NMR, 1H-DOSY experiments, DFT calculations, QTAIM topological analyses of the electron density and IQA partitions of the electronic energy. These analyses revealed that there are indeed repulsions of the type OS···OHB in accordance with the JSIH but our data also indicate that the C[double bond, length as m-dash]OS group has an overall attraction with the interacting molecule. Instead, we found correlations between self-association strength and simple Brønsted-Lowry acid/base properties, namely, N-H acidities and C[double bond, length as m-dash]O basicities. The results in CDCl3 and CCl4 indicate that imides dimerise less strongly than structurally related amides because of the lower basicity of their carbonyl fragments, a frequently overlooked aspect in the study of H-bonding. Overall, the model proposed herein could provide important insights in diverse areas of supramolecular chemistry such as the study of multiple hydrogen-bonded adducts which involve amide or imide functional groups.

Detailed characterization of glycosylated sensory-active volatile phenols in smoke-exposed grapes and wine.

The exposure of Vitis vinifera L. vines to smoke from wildland fires can alter the chemical composition of the berries, such that the resulting wine can possess a defect known as smoke-taint. This work constitutes a complete method for the analysis of simple volatile phenol glycosides (VP-glycosides) that can be elevated in berries and wine following smoke exposure. We synthesized 16 model VP-glycosides, four of which are not reported previously, to facilitate method development. Fragmentation analysis using high-resolution accurate-mass spectrometry demonstrated that the glycone and aglycone influenced the fragmentation pattern of VP-glycosides. Diagnostic fragmentation patterns for the synthesized VP-glycosides were applied to identify several VP-glycosides in smoke-exposed berries and wine. The fragmentation pattern of VP-disaccharides should facilitate the characterization of modified glycones. Putative non-VP glycosides elevated in smoke-exposed berries are demonstrated for the first time. In tandem with VP-glycosides, such compounds may contribute to the expression of smoke taint.

Hydrogen-Bond Weakening through π Systems: Resonance-Impaired Hydrogen Bonds (RIHB).

Redefining interactions: The concept of the resonance-impaired hydrogen bond (RIHB) as an interaction in which a conjugated π system strongly impairs the formation of a hydrogen bond (HB) is introduced. A typical HB involving charged species can have a formation energy of tens of kcal mol-1 , whereas the corresponding value for the examined RIHB is only 2.6 kcal mol-1 . Quantum chemical topology tools are used to analyse the low formation energy of the studied RIHBs.

The bifunctional catalytic role of water clusters in the formation of acid rain.

State-of-the-art chemical bonding analyses show that water clusters have a bifunctional catalytic role in the formation of H2SO4 in acid rain. The embedded H2O monomers mitigate the change in the chemical bonding scenario of the rate-limiting step, reducing thereby the corresponding activation energy in accordance with Hammond's postulate. We expect that the insights given herein will prove useful in the elucidation of the catalytic mechanisms of water in inorganic and organic aqueous chemistry.

Cooperative and anticooperative effects in resonance assisted hydrogen bonds in merged structures of malondialdehyde.

We analyzed non-additive effects in resonance assisted hydrogen bonds (RAHBs) in different ß-enolones, which are archetypal compounds of these types of interactions. For this purpose, we used (i) potential energy curves to compute the formation energy, ΔE, of the RAHBs of interest in different circumstances along with (ii) tools offered by quantum chemical topology, namely, the Quantum Theory of Atoms In Molecules (QTAIM) and the Interacting Quantum Atoms (IQA) electronic energy partition. We established the effect that a given H-bond exerts over ΔE associated with another RAHB, determining in this way the cooperativity or the anticooperativity of these interactions. The mesomeric structures and the QTAIM delocalisation indices are consistent with the determined cooperative or anticooperative character of two given RAHBs. The HB cooperativity and anticooperativity studied herein are directly reflected in the IQA interaction energy E, but they are modulated by the surrounding hydrocarbon chain. The IQA decomposition of ΔEcoop, a measure of the cooperativity between a pair of interacting RAHBs, indicates that the analyzed H-bond cooperative/anticooperative effects are associated with greater/smaller (i) strengthening of the pseudo-bicyclic structure of the compounds of interest and (ii) electron localisations with its corresponding changes in the intra and intermolecular exchange-correlation contributions to ΔE. Overall, we expect that this investigation will provide valuable insights into the interplay among hydrogen bonded atoms and the π system in RAHBs contributing in this way to the understanding of the general features of H-bonds.

The nature of resonance-assisted hydrogen bonds: a quantum chemical topology perspective.

Resonance Assisted Hydrogen Bonds (RAHBs) are particularly strong H-Bonds (HBs) which are relevant in several fields of chemistry. The traditional explanation for the occurrence of these HBs is built on mesomeric structures evocative of electron delocalisation in the system. Nonetheless, there are several theoretical studies which have found no evidence of such electron delocalisation. We considered the origin of RAHBs by employing Quantum Chemical Topology tools, more specifically, the Quantum Theory of Atoms in Molecules (QTAIM) and the Interacting Quantum Atoms energy partition. Our results indicate that the π-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.

Hydrogen bond cooperativity and anticooperativity within the water hexamer.

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.