Ion chemistry of phosphorus in hydrocarbon flames: Part II. Positive Ion Formation

Abstract

Premixed methane/oxygen flames at atmospheric pressure of both fuel-rich (FR) and fuel-lean (FL; i.e. oxygen-rich) composition were doped with small amounts (< 0.1 mol%) of volatile phosphorus alkyl triesters. The phosphorus positive ions (cations) produced by chemical ionization reactions were observed by sampling the flame through a nozzle into a mass spectrometer. This study follows on from Part I in which the phosphorus negative ions (anions) were similarly investigated. About a dozen phosphorus cations were observed in each flame below 160 u, many of them common to both flames. The cations can be represented by five series. The first involves ions of the type (RO) 4- n P(OH) + n with n = 0–4 (FR, R = CH 3) or n = 2–4(FL, R = CH 2H 5). Evidence was obtained for a methyl cation transfer reaction. The remaining four series amount to the hydrates PO + n · xH 2O with n = 1–4; x has various values for each series in each flame in the range 0–3. However, many of these “hydrates” have the general form PO a (OH) b (O 2) c (H 2O) + d involving coordinate bond formation of O 2 and H 2O with the central P atom. Virtually all the high-mass ions appear to have four-coordinate tetrahedral structures. The phosphorus cation formation chemistry is discussed in detail involving three-body association, nucleophilic displacement (S N2) and proton transfer reactions. Finally, some points of comparison are made between the cations and the phosphorus anions presented in Part I.