M. Zieliński1, A. Zielińska1, H. Paul2, H. Papiernik-Zielińska1
1 Isotope Laboratory (Laboratory of Nuclear Chemistry), Faculty of Chemistry, Jagiellonian University, 3 Ingardena Str., 30-060 Krakow, Poland,
2 Laboratory of Stable Isotopes, Geological Institute, Swiss Federal Institute of Technology, ETH Zentrum, CH-8092, Zurich, Switzerland
Carbon-13 isotope fractionation in the decomposition of liquid formic acid of natural isotopic composition in the presence of pyrophosphoric acid has been studied in the temperature range 20-70°C. The initial fractionation of 13C in the decarbonylation of formic acid catalyzed by dehydrating species depends on the initial ratio of the substrate-formic acid to the dehydrating pyrophosphoric acid introduced into the reaction vessel. The 13CO fractionation increases with reaction time from the lowest value, which is close to the 13C KIE expected for the 13C-1H bond rupture in the rate determining step, to the highest boundary values observed in the thermal decomposition of pure liquid formic acid and approaching gradually the values of 13C KIE expected for the 13C-16OH bond splitting in the rate determining step. The observed time and temperature dependences of the 13C fractionation are explained by the dependence of the 13C fractionation in catalysed dehydration of F.A. on the degree of solvation of dehydrating species with water molecules derived from the decomposing formic acid.