Substituent-Controlled Quantum Dynamics in 2-Butynyl Alcohol-Water Dimer: Insights from Rotational Spectroscopy.

The binary 2-butynyl alcohol-HO complex was explored using pulsed-jet Fourier transform microwave spectroscopy with complementary quantum chemical calculations. Distinct tunneling-induced splittings in the rotational spectra reveal two large-amplitude motions: internal rotation of water = 4.33 kJ mol and concerted tunneling of water and the hydroxyl group = 6.61 kJ mol. The observed isomer is stabilized by dual OH···O and O-H···π hydrogen bonds. Electronic structure analyses indicate that methyl substitution strengthens the O-H···π interaction, oppositely modulating these two tunneling pathways─restricting water rotation while facilitating skeletal torsion. These findings demonstrate how substituent effects control tunneling cooperativity in hydrogen-bonded systems, offering mechanistic insight into substituent-controlled quantum hydrogen dynamics in weakly bound clusters.