Dissociative ionisation of adamantane: a combined theoretical and experimental study

Abstract

aDiamond nanoparticles, or nanodiamonds, are intriguing carbon-based materials which, maybe surprisingly, are the most abundant constituent of presolar grains. While the spectroscopic properties of even quite large diamondoids have already been explored, little is known about their unimolecular fragmentation processes. In this paper we characterise the dissociative ionisation of adamantane (C 10 H 16 ) -the smallest member of the diamondoid family -utilising imaging Photoelectron Photoion Coincidence (iPEPICO) spectroscopy and Density Functional Theory (DFT) calculations. We have found adamantane to dissociatively photoionise via several parallel channels of which H, C 3 H 7 and C 4 H 8 losses are the most important ones. Calculations confirm the existence of a rate-limiting transition state for the multiple C-loss channels, which is located at 10.55 eV with respect to neutral adamantane. In addition, we found dissociation channels leading to small cationic hydrocarbons, which may be relevant in the interstellar medium.

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

“…IR induced dissociation of this fragment ion yields only a single product at m/z 77, presumably by H 2 loss. [46][47][48] The finding that the arenium ion is formed can also be compared with results for VUV induced dissociation reported recently by Candian et al [35] VUV dissociation was found to mostly result in a radical-cation C 6 H 8 * + fragment with the evenelectron C 6 H 7 + ion being a weaker contributor. Other isomers such as 1and 2-protonated fulvene (at + 39.3 and + 108.7 kJ/mol relative to protonated benzene, respectively) and the methyl-substituted cyclopentadienyl cation (+ 137.1 kJ/mol) do not match the recorded spectrum (see SI Figure S1).…”

“…Structures of the product ions from electron ionization of adamantane as suggested by mid-IR action spectroscopy (left) compared to the main ionic fragments found in VUV dissociative ionization as characterized by DFT potential energy surface computations. [35] Identification of the m/z 107 ion is perhaps most challenging in the present study; it is difficult to make a unique assignment based on the IR spectra, but two ethylbenzene and two meta-xylene structures are likely contributors to the ion population, while two protonated methylcycloheptatrienes isomers cannot be ruled out. [33] Furthermore, this finding is also in agreement with extensive C 10 H 16 * + PES computations that suggest that the formation of toluenium is kinetically favored.…”

“…One notices that the main products from EI ionization are in all cases evenelectron species. [35] The two processes therefore appear to be fundamentally different. [63] In contrast, the main product ions from VUV-induced dissociation comprise both even-and odd-electron ions.…”

“…[49,[54][55][56] Dissociation of protonated ortho-and meta-xylenium ions was found to yield mostly CH 4 -loss with a smaller fraction of H 2 - loss, while that of the ethylbenzenium ions yields mostly loss of ethene [49] and this was found to be in agreement with the C 8 H 11 + potential energy surface. [35] Instead, VUV ionization produces an ion of C 8 H 12 * + composition. These dissociation products are largely in line with the suggestion of protonated ethylbenzenes and protonated xylenes being formed from EI induced dissociation of adamantane.…”

“…[29] In an interstellar environment, nanodiamonds are subject to strong energetic processing, such as irradiation by strong interstellar radiation fields, and chemical transformations including unimolecular dissociations may occur. [35] Based on potential energy surface (PES) calculations, structures were suggested for the dissociation products and accurate appearance energies were derived from a statistical model. The closed-shell product formed by hydrogen atom loss from the adamantane radical cation, the adamantyl cation (C 10 H 15 + ), was spectroscopically characterized in the gas phase.…”