The structures of gas-phase noncovalently bound clusters
have long
been studied in supersonic expansions. This method of study, while
providing a wealth of information about the nature of noncovalent
bonds, precludes observation of the formation of the cluster, as the
clusters form just after the orifice of the pulsed valve. Here, we
directly observe formation of ethanol–methanol dimers via microwave
spectroscopy in a controlled cryogenic environment. Time profiles
of the concentration of reagents in the cell yielded gas-phase reaction
rate constants ofkMe-g= (2.8 ± 1.4) × 10–13cm3molecule–1s–1andkMe-t= (1.6 ± 0.8) × 10–13cm3molecule–1s–1for the pseudo-second-order
ethanol–methanol dimerization reaction at 8 K. The relaxation
cross section between the gauche and trans conformers of ethanol was
also measured using the same technique. In addition, thermodynamic
relaxation between conformers of ethanol over time allowed for selection
of conformer stoichiometry in the ethanol–methanol dimerization
reaction, but no change in the ratio of dimer conformers was observed
with changing ethanol monomer stoichiometry.