A tandem mass spectrometry study of [C,H

₂,X₂,Si]^+" ions (X = H, D, Cl) and the generation of their neutral counterparts

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Eur. J. Mass Spectrom. 6, 89 - 96 (2000)

A tandem mass spectrometry study of [C,H₂,X₂,Si]^+" ions (X = H, D, Cl) and the generation of their neutral counterparts

Dmitri V. Zagorevski
Department of Chemistry, University of Missouri-Columbia, Columbia, Missouri 65211, USA
Christiane Aubry and John L. Holmes^*
Chemistry Department, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5

ABSTRACT:

Two [Si,C,H₄]^+" isomers, having similar stabilities and a relatively low barrier for their interconversion, have been separately generated in mass spectrometric experiments. CH₂SiD₂^+" and D₂-CH₃SiH^+" ions were produced from the same ionic precursor, namely ionized 1,1-dideutero-silacyclobutane (D₂-I). Dissociation of the latter in the ion source resulted in the silaethene structure, whereas metastable dissociation gave rise to the dideuterated methylsilylene. Appearance energy measurements confirmed the formation of the two [Si,C,H₂,D₂]^+" isomers, whose heats of formation were estimated to be 1017 and 1038 kJ mol¹ for CH₃SiH^+" and CH₂SiH₂^+", respectively. Both ion-source and metastable dissociations of ionized D₂-I resulted in [Si,C,H₃,D]^+" and [Si,C,H₄]^+" ions possessing the methylsilylene structure. The collision-induced dissociation (CID) and neutralizationreionization (NR) mass spectra of CH₂SiD₂^+" and D₂-CH₃SiH^+" ions were distinctively different. The characteristic dissociation channel for the CH₂SiD₂ structure was the loss of CH₂. Some dissociations of the ionized silaethene involved the formation of the methylsilylene intermediate. However, CH₃SiH^+" ions produced by this isomerization appeared to be unstable and did not survive neutralizationreionization. As a result, the CID mass spectra of CH₂SiD₂^+" ions prior to neutralization and surviving the NR event were almost identical. Strong recovery signals in the NR mass spectra were consistent with the intrinsic stability of neutral silaethene and methylsilylene. The interconversion of these structures generated by neutralization of their positively-charged counterparts seems to be very unlikely. The CID, NR and NR/CID mass spectra of [Si,C,H₂,Cl₂]^+" ions produced from various precursors and in different time frames were indistinguishable. They corresponded to the CH₂SiCl₂ connectivity of the atoms. The heat of formation for CH₂SiCl₂^+" ions was estimated to be ~732 kJ mol¹, based on appearance energy measurements. The presence of a strong recovery signal in the NR mass spectra was consistent with the formation of stable neutral 1,1-dichlorosilaethene in the gas phase on the microsecond time frame.

Keywords: tandem mass spectrometry, metastable ions, collision-induced dissociation, neutralizationreionization, ion structures, ion thermochemistry, elusive neutrals, silicon-containing compounds

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