DFT-guided understanding of ester-alcohol interactions: experimental validation

A detailed theoretical investigation was conducted using Density Functional Theory (DFT) at the M05-2X/6-31G(d,p) level to explore the molecular interactions in binary mixtures of methyl heptanoate and 2-alkanols (2-propanol, 2-butanol, 2-pentanol, and 2-hexanol). The optimised geometries, topological properties, vibrational spectra, NMR characteristics, and molecular orbitals of the complexes were analyzed, revealing the critical influence of alkyl chain length on the hydrogen bonding strength and interaction nature. The computational findings were complemented by experimental data, which examined the mixtures across a temperature range of 293.15 K to 323.15 K. The experimental results indicated positive excess molar volumes, suggesting weak intermolecular forces, while a decrease in excess volume with increasing alkyl chain length was attributed to improved molecular packing. Viscosity deviations were consistently negative and diminished with temperature, aligning with the weakening of attractive forces at elevated temperatures.