Spectroscopic Characterization of the Product Ions Formed by Electron Ionization of Adamantane

Abstract A structural characterization of the products formed in the dissociative electron ionization of adamantane (C10H16) is presented. Molecular structures of product ions are suggested based on multiple‐photon dissociation spectroscopy using the Free Electron Laser for Infrared eXperiments (FELIX) in combination with quantum‐chemical calculations. Product ions are individually isolated in an ion trap tandem mass spectrometer and their action IR spectra are recorded. Atomic hydrogen loss from adamantane yields the 1‐adamantyl isomer. The IR spectrum of the C8H11 + product ion is best reproduced by computed spectra of 2‐ and 4‐protonated meta‐xylene and ortho‐ and para‐protonated ethylbenzenes. The spectrum of the product ion at m/z 93 suggests that it is composed of a mixture of ortho‐protonated toluene, para‐protonated toluene and 1,2‐dihydrotropylium, while the spectrum of the m/z 79 ion is consistent with the benzenium ion. This study thus suggests that adamantane is efficiently converted into aromatic species and astrophysical implications for the interstellar medium are highlighted.

In the main text of the manuscript, only the computed spectra of the best matching isomers are shown. Here, for completeness, a detailed overview showing comparison between measured IRMPD spectra and different isomers is presented.

m/z 79 fragment
The loss of a C4H9 fragment from the adamantane parent species results in an ion of C6H7 composition. The IRMPD spectrum recorded of the fragment is compared to the DFT computed spectra of various isomeric structures in Figure S1. A clear identification of protonated benzene can be made based on this comparison. Figure S1 Measured IRMPD spectrum of the C6H7 cation compared to four isomers that possibly contribute.
The net loss of C3H6 from the adamantane precursor ion results in an ion of C7H9 composition. The IRMPD spectrum recorded for this fragment is compared to the DFT computed spectra of various isomeric structures in Figures S2-S4. Figure S2: Measured IRMPD spectrum of the m/z=93 fragment generated by EI of adamantane compared to computed spectra of 1-, 2-, 3-, and 4-protonated toluene. Figure S3: Measured IRMPD spectrum of the m/z=93 fragment generated by EI of adamantane compared to computed spectra of the various protonated isotoluenes. Figure S4: Measured IRMPD spectrum of the m/z=93 fragment generated by EI of adamantane compared to computed spectra of the various protonated cycloheptatrienes.
A cation of C8H11 composition is formed upon electron ionization of adamantane. The IRMPD spectrum recorded for this fragment is compared with the DFT computed spectra of various isomeric structures in Figures S5-S10. Figure S5: Experimental IRMPD spectrum of the m/z 107 (C8H11 + ) fragment compared to computed spectra of the various isomers of protonated ethylbenzene. Figure S6: Experimental IRMPD spectrum of the m/z 107 (C8H11 + ) fragment compared to computed spectra of various isomers of protonated vinylcyclohexadiene. Figure S7: Experimental IRMPD spectrum of the m/z 107 (C8H11 + ) fragment compared to computed spectra of the various isomers of protonated meta-xylene isomers. Figure S8: Experimental IRMPD spectrum of the m/z 107 (C8H11 + ) fragment compared to computed spectra of the various isomers of protonated ortho-and para-xylene isomers.  Despite the very weak signal at m/z 121 in the mass spectrum, we have recorded a IRMPD spectrum of the [M-CH3] + fragment. Many isomers can be responsible for this signal. The IRMPD spectrum recorded of the fragment is compared to the DFT computed spectra of various isomeric structures in Figures S11-S16. No unique isomer identification can be made based on these comparisons.

Spectrum of the m/z 135 fragment
The IRMPD spectrum of the m/z 135 fragment is displayed in Figure S14. The best match is found between the measured spectrum and the DFT computed spectrum of 1-adamantyl and the largest mismatch between the two spectra is found in the range around 1000 cm -1 . 1-Adamantyl is also the thermodynamically preferred isomer. Figure S17: IRMPD spectrum (black) of the m/z=135 (C10H15 + ) ion formed from the dissociative ionization of adamantane plotted together with the DFT computed spectrum (red) of the 1-adamantyl (top) and 2adamantyl cation (bottom