NEGATIVE-ION MASS-SPECTROMETRY OF SOME C(6)H(10) ISOMERS

The O- chemical ionization (CI) mass spectra of thirteen acyclic C6H10 isomers have been determined. The base peak in all mass spectra is [M - H]-, arising by proton abstraction from the hydrocarbon. Other prod uct ions observed, depending on the isomer are [M - 2H]-, [M - H + O]- , [M - CH3 + O]- and [M - C2H5 +O]-, with the [M - 2H]- ion signal bei ng particularly pronounced for the four alkynes studied. In contrast t o the positive ion mass spectra, some isomer distinction is possible f rom the O- CI mass spectra; in particular, the alkynes can be identifi ed. The O- CI mass spectra of 1,3-cyclohexadiene and 1,4-cyclohexadien e are very different, the former showing [M - H + O]- as the base peak while the latter exhibits only a low intensity [M - H + O]- ion signa l with [M - H]- being the base peak. The collisional charge inversion mass spectra of the [M -H]- ions derived from the C6H10 isomers also h ave been determined. These spectra readily identify the alkynes but pr ovide only limited distinction among the hexadiene and methylpentadien e isomers because, for isomers differing in double bond position, the [M - H]- ions formed are, in fact, resonance structures of the same io n.