DETERMINATION OF REACTION VOLUMES OF HYDROGEN-BONDING EQUILIBRIA BY HIGH-PRESSURE NEAR-INFRARED SPECTROSCOPY .1. SELF-ASSOCIATION OF EPSILON-CAPROLACTAM IN CCL4 UP TO 2-KBAR

The self-association of ∈-caprolactam in CCl4 is studied at 293.2 and 333.2 K up to pressures of 2 kbar and concentrations c ≤ 0.15 mol dm-3 by near-infrared spectroscopy. The high-pressure vessel developed and the measuring technique are described. The molar absorption coefficients were found to be pressure dependent. The dimerization equilibrium constants Kc and Kx are calculated from monomer concentrations determined spectroscopically. The reaction enthalpy was ΔH° = (-30 ± 1.5) kJ mol-1 and the reaction volume was ΔV° = (-1.3 ± 0.3) cm3 mol-1, independent of pressure. The small negative value of the reaction volume is explained by a negative contribution of the H bonds and a positive contribution of the cyclization reaction. © 1979 American Chemical Society.