Quantifying the Substituent Effects on O–H···O Hydrogen Bonding Dominance in Hydroxylic Dimers: A Rotational Spectroscopic and Topological Study of the 2-Butyn Alcohol Dimer

Hydrogen bonds are fundamental interactions that govern molecular aggregation. Although the O-H···O hydrogen bond is dominant in simple hydroxylic dimers, how this dominance is perturbed by substituents remains a key question. This study investigates the 2-butyn alcohol dimer using pulsed-jet Fourier transform microwave spectroscopy and theoretical calculations to quantitatively assess the impact of an alkynyl substituent on the dominant O-H···O hydrogen bond. The observed isomer is stabilized by a primary -H···O hydrogen bond and secondary O-H···π and C-H···π interactions. Quantum theory of atoms in molecules analysis revealed that the -H···O hydrogen bond energy contributes merely 55.0% to the total dimerization energy─the lowest value observed among a series of hydroxylic dimers, including water, alcohols, and phenols. This systematic comparative analysis reveals that introducing substituents, particularly those with π-systems or elongated alkyl chains, consistently reduces the energetic contribution ratio of the O-H···O hydrogen bond. The 2-butyn alcohol dimer, with its significantly weakened relative energy contribution ratio of the dominant hydrogen bond, approaches a threshold where the dominance of the O-H···O hydrogen bond could be challenged, providing a crucial perspective for understanding hierarchical molecular self-assembly.