Intramolecular hydrogen bonds in a single macromolecule:Strength in high vacuum versus liquid environments

As a weak non-covalent interaction,hydrogen bond(H-bond)is highly susceptible to the environmental interference.However,the direct quantification of a single H-bond under an interference-free condition is still a challenge.Herein,the intramolecular H-bond in a model system,poly(N-isopropylacrylamide),is studied in high vacuum by single-molecule atomic force microscopy and steered molecular dynamics simulations,which allows the precise quantification of H-bond strength in an interference-free state.Control experiments show that the H-bond is significantly weakened in nonpolar solvent,even if the dielectric constant is very close to vacuum.If a polar solvent is used as the environment,the H-bond will be further weaker or even broken.These results imply that for experiments in any liquid environment,the H-bond strength(△G)will be only -50% or even less of that measured in vacuum.Further analysis shows that in liquid environments,AG decays in a quasi-linear way with the increase of the dielectric constant(ε).For H-bond studies in future,the result measured in vacuum can be set as the standard value,namely,the inherent strength.This approach will provide fundamental insights into the H-bond participated nano-structures and materials in different environments.