The High Performance of Choline Arginate for Biomass Pretreatment Is Due to Remarkably Strong Hydrogen Bonding by the Anion

The ionic liquid choline arginate [Ch][Arg] is more effective for biomass pretreatment than other choline based amino acid ILs, but the underlying mechanism has been unclear. In the present work we use the high-level CCSD(T)/CBS(MP2) and G4(MP2) thermochemical protocols to probe the H-bonding interactions of [Ch][Arg] with water, and organic functional groups commonly found in biomass. We show that the [Ch][Arg] IL forms unusually strong H-bonding interactions with prototypical H-bond donors. For example, we obtain H-bonding interactions of 76.6, 80.0, and 103.6 kJ mol^-1 with water, methanol, and phenol, respectively. Our theoretical results shed light on the capacity of [Ch][Arg] to dissolve biomass, and they demonstrate the importance of ion conformation, in addition to speciation, for IL performance more generally. As a point of reference, we compare the H-bonding interactions of [Ch][Arg] with those of a related IL, choline glycinate ([Ch][Gly]), which does not dissolve biomass as effectively as [Ch][Arg].