IONS AND CHARGED SOOT PARTICLES IN HYDROCARBON FLAMES .3. NEGATIVE-IONS IN FUEL-RICH ACETYLENE - OXYGEN FLAMES

Negative ions up to masses > 3·103 u in fuel-rich and sooting premixed low-pressure acetylene/oxygen flames have been analyzed by means of a time-of-flight mass spectrometer. This work deals with the quantitative analysis of ions up to masses of 200 u, but the larger polyhedral carbon ions, which were negative charge carriers in the soot forming zone, are not considered here. Concentration profiles of ca. 50 different ions and the dependence of maximum ion concentrations on the C/O ratio are reported. - Ions in the preheating zone were anions of carbonic acids and diacids which contained oxo, keto and peroxy groups. With increasing height in the flame these anions decomposed by decarboxylation and dissociative electron detachment. At higher temperature in the oxidation zone, polyynides (C2nH-), linear polycarbon ions and higher ketenides (CmHO-) prevailed. The polyynides were in chemical equilibrium with acetylene and higher neutral polyynes. Enthalpies of formation (referenced to that of C2H-, 274 kJ/mol) were determined for C4H- (414), C6H- (603), C8H- (810) and C10H- (1025 kJ/mol). Possible reaction mechanisms for the formation of the acid anions and the ketenides are discussed. Negative polycyclic aromatic hydrocarbon ions were completely missing in acetylene flames, in contrast to their occurrence in benzene flames.