Topological hydrogen-bond saturation as a design route to type V deep eutectics

Type V deep eutectic solvents (DES), formed by combination of neutral hydrogen-bonding molecules, remain difficult to predict and engineer. The recent proposal that the asymmetric nature of hydrogen bonding (HB) donor and acceptor capability of precursors might be a key approach for their formulation requires validation. Here we show that the emergence of a topologically saturated hydrogen-bond network provides a mechanistic route to formulate such systems, exemplified by the carvacrol–dimethyl sulfoxide (DMSO) mixture. COSMO-RS screening predicts a profoundly deep eutectic (about 110 K depression) arising from donor-free, strong HB acceptor DMSO when paired with asymmetric HB donor carvacrol. To our knowledge, this is the first example of a type V DES whose precursors are liquid at ambient conditions, thus enabling a detailed exploration of chemical physical properties and their excess values to be probed across the whole composition window. This approach is fundamental to describe the complex structural and dynamical organization of carvacrol–DMSO mixtures across composition and relate hydrogen-bond topology to the onset of deep eutectic behaviour, on the basis of complementary thermophysical measurements, neutron scattering, NMR and classical molecular dynamics simulations. The work addresses two main points: i) can present knowledge be used to custom engineer type V DES?; ii) how does the shifting interplay between intra- and interspecies interactions across composition translate into the deep eutectic nature of Type V mixtures?