Proving and Probing the Presence of the Elusive C‐H∙∙∙O Hydrogen Bond in Liquid Solutions at Room Temperature

Hydrogen bonds (H-bonds) play a major role in defining the structure and properties of many substances, as well as phenomena and processes. Traditional H-bonds are ubiquitous in nature and have received considerable attention, yet the demonstration of weak H-bonds, as occur between a highly polarized C-H group and an electron-rich oxygen atom, have proven elusive. Herein, we detail linear and non-linear IR spectroscopy experiments that reveal the presence of H-bonds between the chloroform C-H group and an amide carbonyl oxygen, in solution at room temperature. Evidence is provided for an amide solvation shell featuring two clearly distinguishable chloroform arrangements that undergo chemical exchange with a time scale of ~2 ps. Furthermore, the enthalpy of breaking the hydrogen bond is found to be 6-20 kJ/mol.  Ab-initio computations support our findings of two distinct solvation shells formed by three chloroform molecules, where one thermally undergoes hydrogen bond making and breaking. Our experiments also illustrate extension of the phenomenon to other carbonyl-containing molecules.