Photoionization of methyl t-butyl ether (MTBE) and t-octyl methyl ether (TOME) and analysis of their pyrolyses by supersonic jet/photoionization mass spectrometry

• Published in: International journal of mass spectrometry 2000-06, Vol.199 (1), p.17-27
• Main Authors: Chambreau, Steven D., Zhang, Jingsong, Traeger, John C., Morton, Thomas Hellman
• Format: Article
• Language: English
• Subjects: Acetone; Antiknock; Fuel additive; Homolysis; Methyl radical; Oxygenate
• ISSN: 1387-3806; 1873-2798
• DOI: 10.1016/S1387-3806(00)00168-8

The pyrolysis products of neutral methyl- d 3 t-butyl ether (MTBE- d 3), its undeuterated analogue, and t-octyl methyl ether (TOME) have been analyzed by means of supersonic jet expansion followed by 118 nm photoionization/time-of-flight mass spectrometry. The mass spectra recorded for pyrolysis temperatures up to 1200 °C are compared with the photoionization efficiency (PIE) of room temperature samples as a function of photon energy in the domain 9–13 eV. Differences in fragment ion abundances measured by the two techniques permit the dissection of ion decomposition profiles away from thermal cracking patterns. MTBE and TOME both exhibit base peaks at m/z 73 (which shifts to m/z 76 for MTBE- d 3). For neutral MTBE at room temperature, supersonic expansion followed by 118 nm photoionization of the jet-cooled molecular beam gives a mass spectrum in which the molecular ion appears with approximately 10% the abundance of the base peak, a much higher relative intensity than is seen for the molecular ion at any wavelength when the neutral precursor is photoionized at room temperature. Pyrolysis of MTBE leads to molecular elimination of methanol as the only observed thermal decomposition (in agreement with previous studies) up to roughly 1000 °C. At temperatures ≥1050 °C, however, detectable levels of bond homolysis take place, as revealed by the production of both CH 3· and CD 3· (observed as m/z 15 and 18 in the photoionization mass spectra) from (CH 3) 3COCD 3. This result is consistent with the expectation that bond homolysis should have an Arrhenius preexponential factor >100 times greater than that for molecular elimination, which compensates for the 0.9 eV higher energy barrier difference at sufficiently elevated temperatures. TOME also displays molecular elimination, but the prevalence of homolysis in the resulting alkenes (2,4,4-trimethylpentenes) prevents assessment of direct bond homolysis in TOME.