Predicting Hydrogen Bond Donating Ability between Nitroaniline Dyes and Type III DES Using Tzvp Based COSMO Models

Type III deep eutectic solvents (DES) are formed by hydrogen bond donors and acceptors (HBD and HBA respectively). These HBA’s and HBD’s form a liquid that has a lower melting point then either of the individual compounds. The main interaction that drives this depression in melting point is the hydrogen bonding between the HBD and the halogen cation of the HBA. In addition to this, the HBA itself may have hydrogen’s able to participate in bonding; such as the primary alcohol in the common HBA choline chloride. These complex hydrogen bonding systems have been qualitatively described with methods such as pulse-field gradient nuclear magnetic resonance spectroscopy. This method does not necessarily describe the changes in hydrogen bonding potential difference between the individual HBA and HBD’s and the resulting DES. Solvatochromic measurements are a quick experimental tool that can utilize dyes with UV-Vis spectra that change based on their environment. Dye pairs such as 4-nitroaniline and N,N-diethyl 4-nitroaniline can be used with Type III DES to measure hydrogen bonding acceptance capacity; so how saturated the DES is with hydrogen bonds.

In addition to experimental methods, computational modeling can attempt to predict the ability of a DES to form additional hydrogen bonds. COSMO (Conductor-like Screening Model) uses electron distribution calculations to determine thermophysical properties and phase properties such as liquid-liquid equilibrium of binary and ternary systems. One output of these electron density calculations for Type III DES is the surface potential of the molecules. This potential is directly related to the molecules ability to interact with potential bonding sources, such as hydrogens available for bonding. Knowing how different HBA’s and HBD’s interact will be useful for designing DES for target specific applications (such as liquid-liquid extraction or gas absorption) where hydrogen bonding may be desirable.

This study evaluates Type III DES’s ability to form hydrogen bonds through both experimental measurement with solvatochromic dyes and computational prediction through COSMO calculations with a TZVP basis set. DES made from HBA (quaternary ammonium and polyphenol phosphonium salts) and HBD (glycol, glycerol, and carboxylic acids) were used with dyes of 4-nitroaniline, N,N-dimethyl 4-nitroaniline, and N,N-diethyl 4-nitroaniline to examine hydrogen bonding ability of the DES with dyes of different polarity and basicity. Electron distribution based modeling in COSMO was used to predict the relative effect of each component used to form DES in the solvent’s ability to form hydrogen bonds with the selected dyes.