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 (V2 = 4.33 kJ mol-1) and concerted tunneling of water and the hydroxyl group (B2 = 6.61 kJ mol-1). The observed isomer is stabilized by dual OH···Ow and Ow-H···πC≡C hydrogen bonds. Electronic structure analyses indicate that methyl substitution strengthens the Ow-H···πC≡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.
Jiang et al. (2026) studied this question.