Carbon-13 isotope fractionation in the decarboxylation of phenylpropiolic acid in orthophosphoric acid and in diphosphoric acid

C-13 isotope fractionation in the decarboxylation of phenylpropiolic acid ( PPA) in the 10% solution of diphosphoric acid (H4P2O7) in orthophosphoric a cid (H3PO4) and in the solution of diphosphoric acid in metaphosphoric acid (HPO3) has been studied between 20-140 degrees C (293.16-413.16). The init ial C-13 fractionation observed in the decarboxylation of PPA in 10% soluti on of H4P2O7 in H3PO4 in the temperature interval 20-30 degrees C coincides with (k(12)/k(13)) KIE = 1.045, calculated assuming the "full" C-C bond ru pture in the decarboxylation transition state. The subsequent carbon dioxid e probes collected between 40-70 degrees C are depleted in carbon-13 by abo ut 1% more than the carbon-13 isotope effects calculated for this higher te mperature interval. The total yield of carbon dioxide was 96% only. The fas t polymerization of phenylpropiolic acid accompanying the decarboxylation o f this acid in the solution of pyrophosphoric acid in metaphosphoric acid d ecreased the total yield of carbon dioxide probes, collected between 40-140 degrees C (so far investigated) to 33%. The mass spectrometric determinati ons of the isotopic compositions of these probes (R-pf) showed that they ar e enriched in C-13 With respect to the "initial C-13/C-12 carbon isotope ra tio R-so" for carboxylic carbon of the starting PPA used in the decarboxyla tion experiments. The last small fractions of CO2, collected at higher temp eratures, were produced with the apparent reversed C-13 KIE, (k(12)/k(13)) < 1.0