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.