Hydrogen bond competition in the ethanol-methanol dimer

Area of Science:

• Physical Chemistry
• Molecular Spectroscopy
• Supramolecular Chemistry

Background:

• Theoretical studies on the ethanol-methanol dimer have yielded inconclusive results regarding preferred hydrogen bonding configurations.
• Understanding intermolecular interactions is crucial for predicting the behavior of molecular systems.

Purpose of the Study:

• To experimentally determine the preferred hydrogen bond donor/acceptor configuration of the ethanol-methanol dimer.
• To investigate the structures and relative energies of different conformers.
• To analyze the internal rotation of the methanol methyl group and its effect on spectral properties.

Main Methods:

• Chirped pulse Fourier transform microwave spectroscopy was employed to record the dimer's spectrum across the 8-18 GHz range.
• Argon- and helium-backed supersonic expansions were used to generate and stabilize different conformers.
• Analysis of spectral transitions and internal rotation splittings provided structural information.

Main Results:

• Fifty microwave transitions were assigned to a trans-ethanol-acceptor/methanol-donor structure, stabilized by a C-HO hydrogen bond.
• A higher energy gauche-ethanol-acceptor/methanol-donor conformer was observed in helium expansion.
• Ethanol-donor/methanol-acceptor configurations were not detected, indicating they are energetically unfavorable.
• Preliminary analysis of A-E splitting revealed evidence of the Ubbelohde effect in deuterated and normal isotopologues.

Conclusions:

• The trans-ethanol-acceptor/methanol-donor structure is the most stable configuration for the ethanol-methanol dimer.
• The experimental findings provide crucial data for refining theoretical models of hydrogen bonding.
• The study demonstrates the utility of microwave spectroscopy in elucidating the structures of weakly bound molecular complexes.