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

The binary 2-butynyl alcohol-H₂O 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 (<i>V</i>₂ = 4.33 kJ mol^-1) and concerted tunneling of water and the hydroxyl group (<i>B</i>₂ = 6.61 kJ mol^-1). The observed isomer is stabilized by dual OH···O_w and O_w-H···π_C≡C hydrogen bonds. Electronic structure analyses indicate that methyl substitution strengthens the O_w-H···π_C≡C 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.