Hydrogen bond competition in the ethanol–methanol dimer

Abstract

Previous theoretical work on the ethanol-methanol dimer has been inconclusive in predicting the preferred hydrogen bond donor/acceptor configuration. Here, we report the microwave spectrum of the dimer using a chirped pulse Fourier transform microwave spectrometer from 8-18 GHz. In an argon-backed expansion, 50 transitions have been assigned to a trans-ethanol-acceptor/methanol-donor structure that is likely stabilized by a secondary weak C-HO hydrogen bond. A higher energy conformer was observed in a helium-backed expansion and tentatively assigned to a gauche-ethanol-acceptor/methanol-donor structure. No ethanol-donor/methanol-acceptor dimers have been found, suggesting such interactions are energetically disfavored. A preliminary analysis of the A-E splitting due to the internal rotation of the methanol methyl group in the ground state species is also presented. We find evidence of the Ubbelohde effect in the measured A-E splittings of three deuterated isotopologues and the normal species of this conformer.

Ethanol dimer: Observation of three new conformers by broadband rotational spectroscopy

“…The rotational spectrum recorded using Ne as carrier gas (Figure 2, middle) showed a much better signal-to-noise ratio than the one with He but only three conformers, g+g+, g−t, and tt, could be detected. In the Ar jet spectrum only one conformer, homochiral g+g+, was present (Figure 2, bottom), confirming that is the lowest energy form in agreement with results reported by Finneran et al 16 Considering the results above, the conformers g−t and tt detected in a Ne expansion are the next conformers in energy ordering above the global minimum, and the three conformers detected only in He, the higher energy ones. In addition to these observations, we have carried out careful relative intensity measurements of the measured a-type transitions common to all conformations, and corrected them considering that transition intensity is proportional to the square of the corresponding dipole moment component µ a .…”

“…However, this is not a widespread occurrence. In the ethanol-methanol complex 16 ethanol remains in the trans configuration in the lowest energy conformation, and in complexes of ethanol with other partners (including NH 3 36 , dimethylether 37 , and oxirane derivatives 38,39 ) both gauche and trans ethanol have been found to participate in the lowest-energy form. All ethanol complexes above show several configurations very close in energy, which arise from the ease of ethanol to switch conformations and therefore makes it difficult to predict which conformation will be adopted by ethanol in the global minimum.…”

“…There have been several theoretical and experimental investigations of ethanol dimer [5][6][7][8][9][10][11][12][13][14][15][16] . Initial vibrational studies using molecular beam depletion 5 , FTIR 6 and cavity ring-down spectroscopy 7 provided evidence for up to four conformations, although it was not possible to assign spectral features to individual conformers.…”

“…Therefore, by changing the carrier gas it is possible to determine the identity of the global minimum and energy ordering of the different conformations. This effect has been explored in two different experiments in supersonic jets using FTIR 12 and Raman spectroscopy 15 , where the vibrational band corresponding to the most stable species of ethanol dimer was assigned, and later in a recent microwave experiment which confirmed the g+g+ species as the global minimum 16 .…”

Interactions between Ketones and Alcohols: Rotational Spectrum and Internal Dynamics of the Acetone–Ethanol Complex

“…When alcohols form dimers, one moiety acts as proton donor, the other as proton acceptor . In heterodimers, it seems that there is preference for the role of proton donor/acceptor: Only two isomers with ethanol‐acceptor/methanol‐donor structures have been observed, which suggests ethanol‐donor–methanol‐acceptor interactions are energetically disfavored …”