Beyond the Vibrational Stark Effect: Unraveling the Large Redshifts of Alkyne C-H Bond in Solvation Environments

C-H···O hydrogen bonds are formed in systems ranging from biomolecular complexes to small-molecule structures. Previous work has focused on the blueshifts in the C-H stretching frequency induced by these hydrogen bonds and their chemical and biological roles. Here, we show that, in contrast, terminal alkyne C-H hydrogen bonds exhibit large redshifts (50 - 100 cm^-1) upon hydrogen bonding with oxygen-containing solvents. Using spectroscopic and computational approaches, we elucidate and compare the roles of the vibrational Stark effect, bond polarization, and charge transfer in driving the C-H redshift. We show that the redshifts of alkyne’s terminal C-H upon the formation of hydrogen bonds correlate with the Lewis basicity of the solvent and are significantly larger than those arising solely from solvent electric fields (vibrational Stark effect), differing from the well-studied redshift of carbonyl vibrations induced by hydrogen bonds. Through a decomposition of vibrational frequency shifts based on DFT calculations using absolutely localized molecular orbitals, we demonstrate that including the effects of bond polarization and charge transfer, in addition to the vibrational Stark effect, results in quantitative agreement between experimentally observed C-H frequency shifts and the theoretically predicted values in various oxygen-containing solvents. Our results highlight the significance of effects beyond pure electrostatics in accounting for the large redshifts in C-H···O hydrogen bonds and exemplify our approach to quantifying the contributions from different physical effects.