A Tandem Mass Spectrometry Study of [C,H2,X2,Si]+• Ions (X = H, D, Cl) and the Generation of Their Neutral Counterparts

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 mol−1 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 neutralization–reionization (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 neutralization–reionization. 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 mol−1, 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.