Observation of metastable structures of the ethylene glycol–water dimer in helium nanodroplets

Ethylene glycol (EG) is the simplest organic diol. Here we measure infrared spectra of the EG monomer and its dimer with water, the complex, EG(H2O), embedded in superfluid helium nanodroplets. For the monomer, only a single, gauche, conformation is observed. For EG(H2O), no trace of the global energy minimum is seen, a structure that would maximize the hydrogen bonding contacts. Instead, only metastable structures are formed, suggesting that dimerization in a superfluid environment leads to kinetic trapping in local energy minima. In addition, we obtain evidence for a dimer where the conformation of EG switches from gauche to trans on account of dimerization with a water molecule. This observation is assumed to be driven over an energy barrier by utilizing the energy released as hydrogen bonding occurs.