Probing binding-site preferences in a propiolic acid complex with water at 0.4 K

Carboxylic acid⋯water complexes serve as model systems for understanding molecular interactions that are fundamental to bio- and atmospheric chemistry. Utilizing the helium nanodroplet (HND) technique, which enables the kinetic trapping of otherwise inaccessible isomeric structures, we investigated the hydrogen bonding site preferences in a 1 : 1 complex of propiolic acid (HCC-COOH, PA) with D₂O. Mass-selective infrared (IR) spectra recorded in the CO and CC stretching regions confirmed the exclusive isolation of the <i>cis</i>-PA conformer under single-molecule doping conditions of the helium droplets. Further complexation of PA with D₂O inside the droplets yielded three distinct isomers of <i>cis</i>-PA⋯D₂O dimer, with the dominant spectral features corresponding to two kinetically trapped structures stabilized by either a non-classical (C-H⋯OD₂) or a classical (CO⋯DOD) hydrogen bond. A weak IR band corresponding to the global minimum isomer, characterized by a six-membered ring involving D₂O and the COOH moiety, is also observed. The structural assignments are aided by harmonic IR spectra of the lowest-energy isomers of PA⋯D₂O complexes computed at the MP2/aug-cc-pVDZ level. These structural findings demonstrate the balance of the dipole-dipole and higher-order interactions in steering aggregation dynamics in HNDs. Notably, the polar D₂O (<i>μ</i> = 1.85 D) and PA (<i>μ</i> = 1.59 D) promote directional association leading to the formation of local minimum structures, which are lying >20 kJ mol^-1 above that of the global minimum isomer. The calculated interconversion energy barriers for <i>cis</i>-PA⋯D₂O isomers are in accordance with kinetic trapping inside HNDs at 0.4 K.