Hydrogen bond competition in the ethanol–methanol dimer

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–H⋯O 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