Unpicking the determinants of amide NH•••O=C hydrogen bond strength with diphenylacetylene molecular balances

Hydrogen bonding plays an essential part in dictating the properties of natural and synthetic materials. Secondary amides are well suited to cross-strand interactions through the display of both hydrogen bond donors and acceptors and are prevalent in polymers such as proteins, nylon, and Kevlar (TM). In attempting to measure hydrogen bond strength and to delineate the stereoelectronic components of the interaction, context frequently becomes vitally important. This makes molecular balances - systems in which direct comparison of two groups is possible - an appealing bottom up approach that allows the complexity of larger systems to be stripped away. We have previously reported a family of single molecule conformational switches that are responsive to diverse stimuli including Bronsted and Lewis acids, anions, and redox gradients. In this work we assess the ability of the scaffold, based on a 2,6-disubstituted diphenylacetylene, to measure accurately the difference in hydrogen bond strength between variously functionalised amides. In all of the examples investigated hydrogen bond strength closely correlate to measures of Bronstead acidity suggesting that the scaffold is well-suited as a platform for the accurate determination of bond strength in variously substituted systems.