Strong Proper C-H Hydrogen Bonds: Experimental Evidence across Spectral Ranges

C-H bonds are known as weak hydrogen bond (H-bond) donors, and the H-bonds that they typically form are improper: C-H bond shortens, the frequency of its vibrational transition blueshifts, and intensity decreases. All of these characteristics are opposite to familiar proper H-bonds. Here, we demonstrate that an sp-hybridized terminal alkyne (C-H) is a potent proper hydrogen bond donor. Tuning across >50 solvation environments, we demonstrate the propensity of this moiety to form H-bonds with heteroatoms in polar solvents, aromatic π-systems, and, most surprisingly, even with single π-bonds in nonpolar molecules. We directly observe the H-bond formation and comprehensively characterize it using a combination of infrared (IR) absorption, ¹H and ¹³C NMR spectroscopies, and broadband time-domain terahertz spectroscopies. Experimental results are interpreted using ab initio molecular dynamics simulations. Both electric field effects and nonelectrostatic short-range interactions determine the frequency of the hydrogen-bonded C-H stretch. Based on purely experimental observables, we propose a method to disentangle and quantify these contributions. We also show that a non-hydrogen-bonded C-H is a powerful sensor of dispersion interactions via its IR shift even in the presence of much stronger competing interactions. Overall, the alkynyl C-H mimics an O-H group rather than resembling a typical aliphatic C-H. Its distinct local character and spectral isolation from other C-H stretches, high electric field sensitivity, strong propensity toward H-bond interactions, compactness, and ease of incorporation into molecular scaffolds make this overlooked vibrational marker a strong contender for sensing.