Chains of hydrogen bonds such as those found in water and proteins are often presumed to be more stable than the sum of the individual H bonds. However, the energetics of cooperativity are complicated by solvent effects and the dynamics of intermolecular interactions, meaning that information on cooperativity typically is derived from theory or indirect structural data. Herein, we present direct measurements of energetic cooperativity in an experimental system in which the geometry and the number of H bonds in a chain were systematically controlled. Strikingly, we found that adding a second H‐bond donor to form a chain can almost double the strength of the terminal H bond, while further extensions have little effect. The experimental observations add weight to computations which have suggested that strong, but short‐range cooperative effects may occur in H‐bond chains.

Hydrogen bonds are ubiquitous interactions in molecular recognition. The energetics of such processes are governed by the competing influences of pre-organization and flexibility that are often hard to predict. Here we have measured the strength of intramolecular interactions between H-bond donor and acceptor sites separated by a variable linker. A striking distance-dependent threshold was observed in the intramolecular interaction energies. H-bonds were worth less than -1 kJ mol when the interacting groups were separated by ≥6 rotating bonds, but ranged between -5 and -9 kJ mol for ≤5 rotors. Thus, only very strong external H-bond acceptors were able to compete with the stronger internal H-bonds. In addition, a constant energetic penalty per rotor of ∼5-6 kJ mol was observed in less strained situations where the molecule contained ≥4 rotatable bonds.

The applications of commercially available Diamond Like Carbon (DLC) coatings have seen a rapid increase whilst a lot of tribochemistry problems are not fully understood. This study looks into a tungsten doped hydrogenated DLC coating reciprocating against cast iron in model oils. Friction and wear of such system are then compared with steel. Raman spectroscopy is used extensively to understand the coating interactions with the lubricant/additives. Our results have demonstrated the way to characterize carbon coating structure in lubrication. It clarified the chemical decomposition of MoDTC to MoS 2 is the dominant process rather than the possible formation of WS 2 in such system. It also enbles application of in-situ tribochemical aspects using Raman spectroscopy in ambient conditions.

Improvements of coating deposition technology enable the mass production of high

Chains of hydrogen bonds such as those found in water and proteins are often presumed to be more stable than the sum of the individual Hbonds.However,the energetics of cooperativity are complicated by solvent effects and the dynamics of intermolecular interactions,m eaning that information on cooperativity typically is derived from theory or indirect structural data. Herein, we present direct measurements of energetic cooperativity in an experimental system in which the geometry and the number of Hbonds in achain were systematically controlled. Strikingly,w ef ound that adding asecond H-bond donor to form achain can almost double the strength of the terminal Hbond, while further extensions have little effect. The experimental observations add weight to computations whichh ave suggested that strong, but shortrange cooperative effects mayo ccur in H-bond chains.Chainsofhydrogenbondsareprevalentstructural motifs in supramolecular and biological systems. Hbonds are widely proposed to exhibit positive cooperativity, [1] which may be manifested by acombination of conformational [1,2] and electronic effects that may make ac hain more stable than the sum of its parts.[3] Such cooperative effects have been shown to influence reactivity, [4] to contribute to the structure,i nteractions,a nd properties of biomolecules and materials, [5] and to facilitate the communication of chemical information.[6] Hbonded water clusters and chains have been isolated in the solid state [7] and studied experimentally in both liquid and gas phases.[8] Although many nanoscale and bulk properties may be influenced by the cooperativity of H-bond networks,itisnot possible to directly quantify interaction energies from structural or vibrational characteristics.I na ddition, discussion of the relative contributions of electrostatics,p olarization, and covalencyi nHbond cooperativity [5b, 9] is further exacerbated by the challenge of considering the influence of the surrounding solvent.Herein, we have employed synthetic molecular balances [10] to directly measure the effect of H-bond-chain length on the strength of H-bonding interactions in solution. At the outset of our investigation we identified the series of phenol, catechol, and pyrogallol ( Figure 1B)a sapertinent model system for examining cooperativity in H-bond chains.Indeed, H-bond chains have previously been proposed to contribute to the supramolecular properties of catechol and pyrogallol derivatives. [3b, 11] We reasoned that the pre-organization and proximity of the intramolecular H-bond donors and acceptors in this series of compounds would minimize conformational entropic effects to allow examination of cooperative electronic influences.I nitially we measured the experimental complexation Gibbs energies of phenol, catechol, and pyrogallol with the strong H-bond acceptor tri-n-butylphosphine oxide using 31 PNMR spectroscopy.T he binding energies became more favorable as the number of OH groups was increased ( Figure 1A). Such atrend could be rationalized by cooperative e...