We report quantum chemical calculations performed on three popular deep eutectic solvents (DESs) in order to elucidate the molecular interactions, charge transfer interactions, and thermodynamics associated with these systems. The DESs studied comprise 1:2 choline chloride/urea (reline), 1:2 choline chloride/ethylene glycol (ethaline), and 1:1 choline chloride/malonic acid (maloline). The excellent correlation between calculated and experimental vibrational spectra allowed for identification of dominant interactions in the DES systems. The DESs were found to be stabilized by both conventional hydrogen bonds and C-H···O/C-H···π interactions between the components. The hydrogen-bonding network established in the DES is clearly distinct from that which exists within the neat hydrogen-bond donor dimer. Charge decomposition analysis indicates significant charge transfer from choline and chloride to the hydrogen-bond donor with a higher contribution from the cation, and a density of states analysis confirms the direction of the charge transfer. Consequently, the sum of the bond orders of the choline-Cl(-) interactions in the DESs correlates directly with the melting temperatures of the DESs, a correlation that offers insight into the effect of the tuning of the choline-Cl(-) interactions by the hydrogen-bond donors on the physical properties of the DESs. Finally, the differences in the vibrational entropy changes upon DES formation are consistent with the trend in the overall entropy changes upon DES formation.

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Experimental and ab initio dissociation energies of the (H2O)n(CH3CN)mH+ ions are reported. The experimental energies range from 10–35 kcal/mol. The proton is best stabilized by placing the maximum number of acetonitrile molecules close to the protonated center in such a way that the formation of a network of strong hydrogen bonds is still possible. Other results from this work are: (1) Distinct solvent shells can be distinguished in these complex ions. (2) Mixtures of several isomeric structures are unlikely for n≤4. (3) When a water or an acetonitrile molecule clusters with (H2O)(CH3CN)H+, the proton is transferred from the acetonitrile to the water. (4) Although electrostatic interactions make the dominant contribution to the bonding in these systems, polarization and charge-transfer effects contribute also. (5) There is a cooperativity effect among the hydrogen bonds that leads to extensive changes in geometry and charge distribution as successive hydrogen bonds are formed. (6) The relative complexation energies along a series of reactions correlate with many properties of the electron donor and with several properties of the proton donor.

Thermodynamic versus kinetic control: Small‐angle neutron scattering (SANS) and single‐crystal X‐ray diffraction data have been used to elucidate the effect of temperature, solvent, and metal identity on the formation of dimeric or hexameric metal‐seamed pyrogallol[4]arene capsules. Higher temperatures, methanol solution, and the use of nickel metal favor dimer formation (see scheme).