Spectroscopic and thermodynamic studies of liquid n-butanol + n-hexane and + cyclohexane mixtures based on quantum mechanical ab initio calculations of n-butanol clusters

Infrared OH stretching spectra of n-butanol in n-hexane mixtures have been measured at three temperatures as a function of mixture composition up to a mole fraction 0.3 of n-butanol. The spectra were analyzed using DFT (B3LYP) and ab initio (MP2) calculations. Hydrogen-bonded n-butanol clusters up to four members have been calculated using DFT and high-level ab initio methods, including cyclic trimers and tetramers. A distinct cooperative effect of the hydrogen bonding energy has been observed. On the basis of these theoretical results, a new modified ERAS (extended real associated solution) model incorporating tetramer cyclic species in addition to the linearly associated species was applied for describing simultaneously the measured monomer concentration of n-butanol obtained from the IR spectra, the chemical shift of the hydroxyl proton of n-butanol (δOH) obtained from 1H NMR measurements, and experimental data of the molar excess enthalpy (HE). 1H NMR- and HE-data covering the whole range of concentration in the mixture have been used.