Abstract
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.
Highlights
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
The free energies for compounds 2H and 3H are vertically displaced from the 0X correlation in Figure 2D by similar amounts (ΔG2xHB and ΔG3xHB), confirming the minimal energetic effect of extending a H-bond chain beyond two H-bonds, even when background substituent effects are taken into account
The dependence of the energies on the number of H-bonds in the chain, rather than the number of OH groups confirmed that the observed cooperative effects originated from the formation of an intramolecular H-bond network, while ruling out significant contributions from through-bond substituent effects
Summary
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. 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.
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