Carbon isotope effects in Cu(I)–CO complex systems

Abstract

The ¹³C isotope fractionations in the exchange reaction

were experimentally determined for a chemical system consisting of H₂O and CH₃OH solvents and NH₄Cl, NH₄OH and HCl solutes at ambient temperature under a pressure of 9.5×10⁴ Pa. The heavy isotope component, ¹³C, was found to be enriched in the CO–Cu(I) complex in the solution phase. Both CO solubility and the isotope separation factor α were found to increase with decreasing temperature in all Cu(I)Cl solutions. An isotope separation factor of 1.023 and concentration of CO absorption of 1.2 M were obtained for an aqueous solution composition of 1.5 M Cu(I)Cl, 0.75 M NH₄Cl, 7.0 M NH₄OH at a temperature of 8.5°C. The enthalpy and entropy changes in isotope exchange reactions were calculated from the temperature dependence of the isotope separation factor. In addition, the experimentally determined isotope separation coefficients (ε=ln α) were found to be well fitted by the equation ε=a/T+b/T², and the values of a and b were obtained for both experimental systems of aqueous solutions and methanol–water mixed solutions. The isotope exchange reaction was seen to reach an equilibrium state within 1 min at a stirring speed of 4000 rpm.