Toward a Better Determination of Infrared Molar Absorptivities and Dimer Formation Constants in Self-association Through Hydrogen Bonding: 3-Ethyl-2-methyl-3-pentanol in Tetrachloroethylene as an Example

Abstract

The monomer-dimer self-association of the dilute 3-ethyl-2-methyl-3-pentanol in tetrachloroethylene in the very dilute state was studied by infrared spectroscopy at several temperatures. The solute was deliberately chosen so that higher oligomers were suppressed by the steric hindrance arising from bulky groups on both sides of hydroxyl group. Two linear utility equations were derived to treat, respectively, the integrated absorbance of the monomer band, A(m), and of the dimer band, A(d), as functions of the initially prepared solute concentration, [B](0). The respective molar absorptivities were obtained by fitting these equations to the data. Unlike previous methods, the dimerization constant (K) can be obtained from either A(m) or A(d). Any discrepancy between these two values of K serves as a measure of the quality of the data. The values of K at different temperatures were employed to calculate the standard enthalpy and entropy of dimerization by using a van't Hoff plot. The dimer is predominantly in the cyclic form where both hydroxyl protons are hydrogen-bonded. This is inferred from the following observations: (1) the spectrum displays only two bands between 3300 and 3750 cm(-1); (2) the constancy of A(d)/A(m)(2) as a function of [B](0); and (3) the linearity of both plots [B](0)/A(m) vs. A(m), and [B](0)/A(d) vs. A(d)(2).