Formation of neutrals of interstellar significance in the gas phase by one-electron oxidation of precursor anions of known bond connectivity

Abstract

In principle, Franck-Condon one-electron oxidation of an anion of known bond connectivity in a collision cell of a mass spectrometer may give the corresponding neutral. This neutral may be transmitted to a second collision cell in which it may be ionised. In order to be detected in the second cell, a neutral must be stable for the 10-6 sec between formation in one collision cell and ionisation in the second. If the neutrals are converted to cations in the second cell, the positive ion fragmentations may, in principle, be used to probe the structure of the neutral(s) formed in the first collision cell, particularly if there is a peak corresponding to the ionised neutral. This technique is called neutralisation reionisation mass spectrometry, or specifically for the stepwise two-electron oxidation of an anion through a neutral to a cation, -NR+. The analogous -NR- technique may also be used to study the neutral(s) formed in the first collision cell; in this case negative ion fragmentations are used to provide information concerning the structure(s) of the neutral(s). The -NR methods are techniques whereby transient and often reactive neutrals of unusual structures already detected (by rotational or infrared spectroscopy) in interstellar dust clouds or circumstellar clouds surrounding carbon-rich suns may be formed and investigated in the laboratory. Examples are described whereby -NR methods produce stable neutrals, e.g. the cumulenes C5H (this system is under further investigation), C7K2, and various oxo-cumulenes like CnO (n = 3, 5 and 7) and NCnO (n = 3 and 4). Some of these neutrals are known stellar molecules, others are awaiting experiments to test for their presence in either interstellar dust clouds or circumstellar envelopes. Some neutrals formed by -NR experiments have sufficient excess energy to undergo decomposition during the microsecond timeframe of the neutralisation reionisation experiment, e.g. energised oxo-cumulenes lose CO, and transient O2C-CO decomposes to CO and CO2. Other energised neutrals may rearrange to another isomer. The most interesting examples given in this article fall into this category. These include (i) the linear-C4 to rhombic-C4 rearrangement, and the analogous rearrangement of linear-C5, both of which effectively randomise all carbons in these molecules, (ii) the complex atom scrambling of NCCCN, and (iii) those rearrangements where a neutral isomer rearranges to one which is more negative in energy, including the processes CCCHO to HCCCO, and C2COC2 to CCCCCO.