Pyrolysis of 2-methoxy-2,3,3-trimethylbutane (MTMB) monitored by 118Â nm photoionization mass spectrometry

Pyrolysis/supersonic jet expansion/118.2 nm photoionization TOF mass spectrometry is used to examine competitive thermal dissociations of a deuterated analogue of the title compound, a potential antiknock additive for motor fuel. At high temperatures neutral (CH3)3CC(CD3)2OCH3 (MTMB-d6) undergoes homolytic cleavage of its 2,3-bond in competition with molecular elimination of methanol. G3X and CCSD calculations predict the dissociation energy of this bond to differ only slightly from that of the 2,3-bond in hexamethylethane (HME). Theory also predicts that the 2,3-bond is the weakest bond of MTMB. Pyrolysis of MTMB-d6 below 1000 K shows no production of CD3 radicals, but only methyl and tert-butyl radicals, along with acetone-d6 and the products from molecular elimination of methanol. While MTMB-d6 exhibits desirable solubility characteristics in water (relative to the presently used fuel oxygenates MTBE and TAME), the predominance of bond homolysis at temperatures >900 K suggests (according to one current model for how fuel oxygenates inhibit autoignition) that MTMB ought to exhibit poor antiknock characteristics.