Dissociative ionisation of adamantane: a combined theoretical and experimental study

Photoelectron spectra of functionalized adamantanes

Diamondoids are a class of aliphatic molecules with cage-like structures and serve as a bridge between small hydrocarbons and large nanodiamond macromolecules. Because their optical properties are highly dependent on the size, shape, and functionalization of the carbon network, they have many applications in the fields of nanotechnology and spectroscopy. Still, much remains to understand the geometric and electronic effects induced by functionalization of diamondoids. To this end, we perform gas phase photoelectron spectroscopy (PES) of functionalized adamantanes using a helium discharge lamp as a photon source and a hemispherical electron analyzer. We present the photoelectron spectra of 1-cyanoadamantane, 1-amantadine, and 2-adamantanol and compare these to the known PES spectra of adamantane, 1-adamantanol, and urotropine, remeasured herein with improved resolution and a more thorough assignment of vibronic features, with the aid of time-dependent density functional theory calculations.

Fingerprinting fragments of fragile interstellar molecules: dissociation chemistry of pyridine and benzonitrile revealed by infrared spectroscopy and theory

The cationic fragmentation products in the dissociative ionization of pyridine and benzonitrile have been studied by infrared action spectroscopy in a cryogenic ion trap instrument at the Free-Electron Lasers for Infrared eXperiments (FELIX) Laboratory. A comparison of the experimental vibrational fingerprints of the dominant cationic fragments with those from quantum chemical calculations revealed a diversity of molecular fragment structures. The loss of HCN/HNC is shown to be the major fragmentation channel for both pyridine and benzonitrile. Using the determined structures of the cationic fragments, potential energy surfaces have been calculated to elucidate the nature of the neutral fragment partner. In the fragmentation chemistry of pyridine, multiple non-cyclic structures are formed, whereas the fragmentation of benzonitrile dominantly leads to the formation of cyclic structures. Among the fragments are linear cyano-(di)acetylene˙+, methylene-cyclopropene˙+ and o- and m-benzyne˙+ structures, the latter possible building blocks in interstellar polycyclic aromatic hydrocarbon (PAH) formation chemistry. Molecular dynamics simulations using density functional based tight binding (MD/DFTB) were performed and used to benchmark and elucidate the different fragmentation pathways based on the experimentally determined structures. The implications of the difference in fragments observed for pyridine and benzonitrile are discussed in an astrochemical context.

Infrared spectrum of the 1-cyanoadamantane cation: evidence of hydrogen transfer and cage-opening upon ionization

The radical cations of diamondoids are important intermediates in their functionalization reactions and are also candidates as carriers for astronomical absorption and emission features. Although neutral diamondoids have been studied extensively, information regarding their radical cations is largely lacking, particularly for functionalized diamondoid derivatives. Herein, we characterize the structure of the 1-cyanoadamantane radical cation (C10H15CN+, AdCN+) using infrared photodissociation (IRPD) spectroscopy of mass selected AdCN+N2 clusters in the XH stretch range (2400-3500 cm-1) and dispersion-corrected density functional theory calculations (B3LYP-D3BJ/cc-pVTZ). A group of three distinct CH stretch bands are observed in the 2800-3000 cm-1 range, in addition to a highly redshifted absorption at 2580 cm-1 attributed to the acidic CH proton predicted by calculations. An unexpected broad absorption peaking at 3320 cm-1 is also detected and assigned to an NH stretch mode based on its width and frequency. Calculations indicate that hydrogen atom transfer (HAT) from the adamantyl cage (C10H15, Ady) to the N atom of the CN group yields lower energy structures, with an open-cage isomer exhibiting such hydrogen transfer being the global minimum on the potential energy surface. The energy barriers involved in the formation of this open-cage isomer are also lower than those calculated for generation of the analogous open-cage 1-amantadine cation isomer which has previously been identified by IRPD. The combined consideration of IRPD spectra and calculations indicates a major population of the nascent canonical closed-cage isomer and a smaller population of the global minimum isomer featuring both cage-opening and hydrogen transfer.

Resonant Inner-Shell Photofragmentation of Adamantane (C10H16)

Adamantane, the smallest diamondoid molecule with a symmetrical cage, contains two distinct carbon sites, CH and CH2. The ionization/excitation of the molecule leads to the cage opening and strong structural reorganization. While theoretical predictions suggest that the carbon site CH primarily causes the cage opening, the role of the other CH2 site remains unclear. In this study, we used advanced experimental Auger electron-ion coincidence techniques and theoretical calculations to investigate the fragmentation dynamics of adamantane after resonant inner-shell photoexcitation. Our results demonstrate that some fragmentation channels exhibit site-sensitivity of the initial core-hole location, indicating that different carbon site excitations could lead to unique cage opening mechanisms.

Microhydration of the adamantane cation: intracluster proton transfer to solvent in [Ad(H2O)n=1-5]+ for n ≥ 3

Radical cations of diamondoids are important intermediates in their functionalization reactions in polar solvents. To explore the role of the solvent at the molecular level, we characterize herein microhydrated radical cation clusters of the parent molecule of the diamondoid family, adamantane (C₁₀H₁₆, Ad), by infrared photodissociation (IRPD) spectroscopy of mass-selected [Ad(H₂O)_n=1-5]⁺ clusters. IRPD spectra of the cation ground electronic state recorded in the CH/OH stretch and fingerprint ranges reveal the first steps of this fundamental H-substitution reaction at the molecular level. Analysis of size-dependent frequency shifts with dispersion-corrected density functional theory calculations (B3LYP-D3/cc-pVTZ) provides detailed information about the acidity of the proton of Ad⁺ as a function of the degree of hydration, the structure of the hydration shell, and the strengths of the CH⋯O and OH⋯O hydrogen bonds (H-bonds) of the hydration network. For n = 1, H₂O strongly activates the acidic C-H bond of Ad⁺ by acting as a proton acceptor in a strong CH⋯O ionic H-bond with cation-dipole configuration. For n = 2, the proton is almost equally shared between the adamantyl radical (C₁₀H₁₅, Ady) and the (H₂O)₂ dimer in a strong C⋯H⋯O ionic H-bond. For n ≥ 3, the proton is completely transferred to the H-bonded hydration network. The threshold for this size-dependent intracluster proton transfer to solvent is consistent with the proton affinities of Ady and (H₂O)_n and confirmed by collision-induced dissociation experiments. Comparison with other related microhydrated cations reveals that the acidity of the CH proton of Ad⁺ is in the range of strongly acidic phenol⁺ but lower than for cationic linear alkanes such as pentane⁺. Significantly, the presented IRPD spectra of microhydrated Ad⁺ provide the first spectroscopic molecular-level insight of the chemical reactivity and reaction mechanism of the important class of transient diamondoid radical cations in aqueous solution.

Electronic spectroscopy of cationic adamantane clusters and dehydrogenated adamantane in helium droplets

We report the first helium-tagged electronic spectra of cationic adamantane clusters, along with its singly, doubly, and triply dehydrogenated analogues embedded in helium droplets. Absorption spectra were measured by recording the evaporation of helium atoms as a function of laser wavelength in the range of 300-2150 nm. Experimental spectra are coupled with simulated spectra obtained from quantum chemical calculations. The spectrum of cationic adamantane agrees with the electronic photodissociation spectrum measured previously, with an additional low-energy absorption at around 1000 nm. The spectra of the dehydrogenated molecules present broad absorptions exclusively in the high-energy region (300-600 nm). For the higher order adamantane dimer and trimer ions, strong absorptions are observed in the low-energy region (900-2150 nm), rationalised by transitions delocalised over two adamantane units.

Opening of the Diamondoid Cage upon Ionization Probed by Infrared Spectra of the Amantadine Cation Solvated by Ar, N2 , and H2 O

Radical cations of diamondoids, a fundamental class of very stable cyclic hydrocarbon molecules, play an important role in their functionalization reactions and the chemistry of the interstellar medium. Herein, we characterize the structure, energy, and intermolecular interaction of clusters of the amantadine radical cation (Ama+ , 1-aminoadamantane) with solvent molecules of different interaction strength by infrared photodissociation (IRPD) spectroscopy of mass-selected Ama+ Ln clusters, with L=Ar (n≤3) and L=N2 and H2 O (n=1), and dispersion-corrected density functional theory calculations (B3LYP-D3/cc-pVTZ). Three isomers of Ama+ generated by electron ionization are identified by the vibrational properties of their rather different NH2 groups. The ligands bind preferentially to the acidic NH2 protons, and the strength of the NH…L ionic H-bonds are probed by the solvation-induced red-shifts in the NH stretch modes. The three Ama+ isomers include the most abundant canonical cage isomer (I) produced by vertical ionization, which is separated by appreciable barriers from two bicyclic distonic iminium ions obtained from cage-opening (primary radical II) and subsequent 1,2 H-shift (tertiary radical III), the latter of which is the global minimum on the Ama+ potential energy surface. The effect of solvation on the energetics of the potential energy profile revealed by the calculations is consistent with the observed relative abundance of the three isomers. Comparison to the adamantane cation indicates that substitution of H by the electron-donating NH2 group substantially lowers the barriers for the isomerization reaction.

Continuous Pyrolysis Microreactors: Hot Sources with Little Cooling? New Insights Utilizing Cation Velocity Map Imaging and Threshold Photoelectron Spectroscopy

Resistively heated silicon carbide microreactors are widely applied as continuous sources to selectively prepare elusive and reactive intermediates with astrochemical, catalytic, or combustion relevance to measure their photoelectron spectrum. These reactors also provide deep mechanistic insights into uni- and bimolecular chemistry. However, the sampling conditions and effects have not been fully characterized. We use cation velocity map imaging to measure the velocity distribution of the molecular beam signal and to quantify the scattered, rethermalized background sample. Although translational cooling is efficient in the adiabatic expansion from the reactor, the breakdown diagrams of methane and chlorobenzene confirm that the molecular beam component exhibits a rovibrational temperature comparable with that of the reactor. Thus, rovibrational cooling is practically absent in the expansion from the microreactor. The high rovibrational temperature also affects the threshold photoelectron spectrum of both benzene and the allyl radical in the molecular beam, but to different degrees. While the extreme broadening of the benzene TPES suggests a complex ionization mechanism, the allyl TPES is in fact consistent with an internal temperature close to that of the reactor. The background, room-temperature spectra of both are superbly reproduced by Franck-Condon simulations at 300 K. On the one hand, this leads us to suggest that room-temperature reference spectra should be used in species identification. On the other hand, analysis of the allyl iodide pyrolysis data shows that iodine atoms often recombine to form molecular iodine on the chamber surfaces. Such sampling effects may distort the chemical composition of the scattered background with respect to the molecular beam signal emanating directly from the reactor. This must be considered in quantitative analyses and kinetic modeling.

Infrared Spectrum of the Amantadine Cation: Opening of the Diamondoid Cage upon Ionization

Radical cations of diamondoids, a fundamental class of highly stable cycloalkanes, are intermediates in functionalization reactions and possibly present in the interstellar medium. Herein, we characterize the structure of the radical cation of 1-amantadine (1-C₁₀H₁₅NH₂⁺, Ama⁺), the amino derivative of the parent adamantane (C₁₀H₁₆⁺, Ada⁺), by infrared spectroscopy and density functional theory calculations. The structural isomers of Ama⁺ produced by electron ionization are probed by infrared photodissociation of cold Ar-tagged ions. In addition to the canonical nascent Ama⁺ isomer with an intact C₁₀H₁₅ cage, we identify two distonic bicyclic iminium isomers in which the adamantyl cage opens upon ionization, one of which is lower in energy than the cage isomer. The reaction profile with barriers and intermediates for this cage-opening reaction are determined. Comparison with Ada⁺ suggests that this type of ionization-induced cage-opening may be a common feature for diamondoids and important for their reactivity.

Time-resolved relaxation and cage opening in diamondoids following XUV ultrafast ionization

Unraveling ultrafast processes induced by energetic radiation is compulsory to understand the evolution of molecules under extreme excitation conditions. To describe these photo-induced processes, one needs to perform time-resolved experiments to follow in real time the dynamics induced by the absorption of light. Recent experiments have demonstrated that ultrafast dynamics on few tens of femtoseconds are expected in such situations and a very challenging task is to identify the role played by electronic and nuclear degrees of freedom, charge, energy flows and structural rearrangements. Here, we performed time-resolved XUV-IR experiments on diamondoids carbon cages, in order to decipher the processes following XUV ionization. We show that the dynamics is driven by two timescales, the first one is associated to electronic relaxation and the second one is identified as the redistribution of vibrational energy along the accessible modes, prior to the cage opening that is involved in all fragmentation mechanisms in this family of molecules.

TOLUENIUM AND OTHER GASEOUS METHYLBENZENIUM IONS: COMPLEX INTERPLAY OF PROTONATED ARENES AND CYCLO-OLEFINS

The development of the current knowledge of the gas-phase chemistry of protonated methylbenzenes, such as toluenium, xylenium and mesitylenium ions, their higher congeners as well as of their mostly cyclo-olefinic isomers by mass spectrometric methodology is presented. Starting from the observation of the characteristic expulsion of dihydrogen from metastable C₇ H₉ ⁺ ions, which is associated with the release of large amounts of kinetic energy, and the composite C- and H-scrambling prior to the loss of methane, in particular, insights into the isomerization scenario of various isomeric C₇ H₉ ⁺ , C₈ H₁₁ ⁺ , and C₉ H₁₃ ⁺ ions, based on a large variety of independent techniques, are discussed. Besides isotope labeling and metastable ion methodology, these include flowing afterglow mass spectrometry, gas-phase titration and infrared spectroscopy of mass-selected ions. The particularly complex energy hypersurface of isomerizing and fragmenting toluenium ions, which has been elaborated in various reports over the years, is presented in a combined way to assess the role of protonated cycloheptatriene, norbornadiene, and 6-methylfulvene as well as a number of further C₇ H₉ ⁺ isomers. The formation and nature of C₇ H₉ ⁺ ions generated by fragmentation of various hydrocarbon precursors, such as monoterpenes and adamantane, is also addressed. The contribution of infrared multiphoton dissociation spectroscopy (IRMPD) and tagged-ion infrared photodissociation (IRPD) of the gaseous C₇ H₉ ⁺ ions as compared to the wealth of previous understanding of their chemistry is commented on as well. Finally, remarkable parallels of the gas-phase chemistry of methylbenzenium ions and the role of such species within the cavities of acidic zeolite catalysts in the course of the industrially important methanol-to-hydrocarbon reaction are discussed. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

One-Stage Catalytic Oxidation of Adamantane to Tri-, Tetra-, and Penta-Ols

Tertiary tetraols of adamantane (C10H16, Tricyclo[3.3.1.1(3,7)]decan) have been widely used for the synthesis of highly symmetric compounds with unique physical and chemical properties. The methods for one-stage simultaneously selective, deep, and cheap oxidation of adamantane to tetraols of different structures have not yet been developed. In this research, chemically simple, cheap, and environmentally friendly reagents are used and that is the first step in this direction. The conditions, under which the impact of a hydrogen peroxide water solution on adamantane dissolved in acetonitrile results in full conversion of adamantane and formation of a total 72% mixture of its tri-, tetra-, and penta-oxygenated products, predominantly poliols, have been found. Conversion and adamantane oxidation depth are shown to depend on the ratio of components of the water-acetonitrile solution and the method of oxidizer solution introduction when using the dimer form of 1:1 dimethylglyoxime and copper dichloride complex as a catalyst. Under the conditions of mass-spectrometry ionization by electrons (70 eV), fragmentation across three C–C bonds of the molecular ions framework of adamantane tertiary alcohols Ad(OH)n in the range n = 0–4 increases linearly with the rise of n.

Quantum Chemistry Calculations for Metabolomics

A primary goal of metabolomics studies is to fully characterize the small-molecule composition of complex biological and environmental samples. However, despite advances in analytical technologies over the past two decades, the majority of small molecules in complex samples are not readily identifiable due to the immense structural and chemical diversity present within the metabolome. Current gold-standard identification methods rely on reference libraries built using authentic chemical materials ("standards"), which are not available for most molecules. Computational quantum chemistry methods, which can be used to calculate chemical properties that are then measured by analytical platforms, offer an alternative route for building reference libraries, i.e., in silico libraries for "standards-free" identification. In this review, we cover the major roadblocks currently facing metabolomics and discuss applications where quantum chemistry calculations offer a solution. Several successful examples for nuclear magnetic resonance spectroscopy, ion mobility spectrometry, infrared spectroscopy, and mass spectrometry methods are reviewed. Finally, we consider current best practices, sources of error, and provide an outlook for quantum chemistry calculations in metabolomics studies. We expect this review will inspire researchers in the field of small-molecule identification to accelerate adoption of in silico methods for generation of reference libraries and to add quantum chemistry calculations as another tool at their disposal to characterize complex samples.

Intramolecular CH3-migration-controlled cation reactions in the VUV photochemistry of 2-methyl-3-buten-2-ol investigated by synchrotron photoionization mass spectrometry and theoretical calculations

2-Methyl-3-buten-2-ol (MBO232) is a biogenic volatile organic compound (BVOC), and has a large percentage of emission into the atmosphere. The vacuum ultraviolet (VUV) photochemistry of BVOCs is of great importance for atmospheric chemistry. Studies have been carried out on several BVOCs but have not extended to MBO232. In the present report, the photoionization and dissociation processes of MBO232 in the energy range of 8.0-15.0 eV have been studied by tunable VUV synchrotron radiation coupled with a time-of-flight mass spectrometer. By measuring the photoionization spectra, the adiabatic ionization energy (AIE) of MBO232 and the appearance energies (AEs) of the eight identified fragment ions (i.e., C4H7O+, C3H7O+, C5H9+, C3H6O+, CH3CO+, CH3O+, C4H5+, and C3H5+) were determined. High-level quantum chemistry calculations suggest that there are 3 direct channels and 5 indirect channels via transition states and intermediates accountable for these fragments. Among the reaction channels, the direct elimination of CH3 is the most dominant channel and produces the resonance-stabilized radical cation. Most interestingly, our results show that the CH3 selectively migrates towards the cation, which leads to the different indirect channels. The CH3 migration is a rare process in the dissociative photoionization of metal-free organic molecules. We explain the process by molecular orbital calculations and electron localization function analysis and explore the non-conventional dissociation channels via the CH3 roaming mechanism. We further perform kinetics analysis using RRKM theory for the channels of interest. The activation barrier, and rate constants are analyzed for the branching fractions of the products. These results provide important implications for the VUV photochemistry of BVOCs in the atmosphere.

Valence Photoionization and Energetics of Vanillin, a Sustainable Feedstock Candidate

We studied the valence photoionization of vanillin by photoelectron photoion coincidence spectroscopy in the 8.20-19.80 eV photon energy range. Vertical ionization energies by EOM-IP-CCSD calculations reproduce the photoelectron spectral features. Composite method calculations and Franck-Condon simulation of the weak, ground-state band yield the adiabatic ionization energy of the most stable vanillin conformer as 8.306(20) eV. The lowest energy dissociative photoionization channels correspond to hydrogen atom, carbon monoxide, and methyl losses, which form the dominant C₈H₇O₃⁺ (m/z 151) and the less intense C₇H₈O₂⁺ (m/z 124) and C₇H₅O₃⁺ (m/z 137) fragment ions in parallel dissociation channels at modeled 0 K appearance energies of 10.13(1), 10.40(3), and 10.58(10) eV, respectively. On the basis of the breakdown diagram, we explore the energetics of sequential methyl and carbon monoxide loss channels, which dominate the fragmentation mechanism at higher photon energies. The 0 K appearance energy for sequential CO loss from the m/z 151 fragment to C₇H₇O₂⁺ (m/z 123) is 12.99(10) eV, and for sequential CH₃ loss from the m/z 123 fragment to C₆H₄O₂⁺ (m/z 108), it is 15.40(20) eV based on the model. Finally, we review the thermochemistry of the bi- and trifunctionalized benzene derivatives guaiacol, hydroxybenzaldehyde, anisaldehyde, and vanillin. On the basis of isodesmic functional group exchange reactions, we propose new enthalpies of formations, among them Δ_fH°_298K(vanillin, g) = -383.5 ± 2.9 kJ mol^-1. These mechanistic insights and ab initio thermochemistry results will support analytical works to study lignin conversion involving vanillin.

Predicting in silico electron ionization mass spectra using quantum chemistry

Compound identification by mass spectrometry needs reference mass spectra. While there are over 102 million compounds in PubChem, less than 300,000 curated electron ionization (EI) mass spectra are available from NIST or MoNA mass spectral databases. Here, we test quantum chemistry methods (QCEIMS) to generate in silico EI mass spectra (MS) by combining molecular dynamics (MD) with statistical methods. To test the accuracy of predictions, in silico mass spectra of 451 small molecules were generated and compared to experimental spectra from the NIST 17 mass spectral library. The compounds covered 43 chemical classes, ranging up to 358 Da. Organic oxygen compounds had a lower matching accuracy, while computation time exponentially increased with molecular size. The parameter space was probed to increase prediction accuracy including initial temperatures, the number of MD trajectories and impact excess energy (IEE). Conformational flexibility was not correlated to the accuracy of predictions. Overall, QCEIMS can predict 70 eV electron ionization spectra of chemicals from first principles. Improved methods to calculate potential energy surfaces (PES) are still needed before QCEIMS mass spectra of novel molecules can be generated at large scale.

Infrared Spectrum of the Adamantane+ -Water Cation: Hydration-Induced C-H Bond Activation and Free Internal Water Rotation

Diamondoid cations are reactive intermediates in their functionalization reactions in polar solution. Hydration is predicted to strongly activate their C-H bonds in initial proton abstraction reactions. To study the effects of microhydration on the properties of diamondoid cations, we characterize herein the prototypical monohydrated adamantane cation (C₁₀ H₁₆ ⁺ -H₂ O, Ad⁺ -W) in its ground electronic state by infrared photodissociation spectroscopy in the CH and OH stretch ranges and dispersion-corrected density functional theory (DFT) calculations. The water (W) ligand binds to the acidic CH group of Jahn-Teller distorted Ad⁺ via a strong CH⋅⋅⋅O ionic H-bond supported by charge-dipole forces. Although W further enhances the acidity of this CH group along with a proton shift toward the solvent, the proton remains with Ad⁺ in the monohydrate. We infer essentially free internal W rotation from rotational fine structure of the ν₃ band of W, resulting from weak angular anisotropy of the Ad⁺ -W potential.

Dissociation dynamics of the diamondoid adamantane upon photoionization by XUV femtosecond pulses

This work presents a photodissociation study of the diamondoid adamantane using extreme ultraviolet femtosecond pulses. The fragmentation dynamics of the dication is unraveled by the use of advanced ion and electron spectroscopy giving access to the dissociation channels as well as their energetics. To get insight into the fragmentation dynamics, we use a theoretical approach combining potential energy surface determination, statistical fragmentation methods and molecular dynamics simulations. We demonstrate that the dissociation dynamics of adamantane dications takes place in a two-step process: barrierless cage opening followed by Coulomb repulsion-driven fragmentation.

Valence Photoionization of Thymine: Ionization Energies, Vibrational Structure, and Fragmentation Pathways from the Slow to the Ultrafast

The photoionization of thymine has been studied by using vacuum ultraviolet radiation and imaging photoelectron photoion coincidence spectroscopy after aerosol flash vaporization and bulk evaporation. The two evaporation techniques have been evaluated by comparison of the photoelectron spectra and breakdown diagrams. The adiabatic ionization energies for the first four electronic states were determined to be 8.922±0.008, 9.851±0.008, 10.30±0.02, and 10.82±0.01 eV. Vibrational features have been assigned for the first three electronic states with the help of Franck-Condon factor calculations based on density functional theory and wave function theory vibrational analysis within the harmonic approximation. The breakdown diagram of thymine, as supported by composite method ab initio calculations, suggests that the main fragment ions are formed in sequential HNCO-, CO-, and H-loss dissociation steps from the thymine parent ion, with the first step corresponding to a retro-Diels-Alder reaction. The dissociation rate constants were extracted from the photoion time-of-flight distributions and used together with the breakdown curves to construct a statistical model to determine 0 K appearance energies of 11.15±0.16 and 11.95±0.09 eV for the m/z 83 and 55 fragment ions, respectively. These results have allowed us to revise previously proposed fragmentation mechanisms and to propose a model for the final, nonstatistical H-loss step in the breakdown diagram, yielding the m/z 54 fragment ion at an appearance energy of 13.24 eV.

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

A structural characterization of the products formed in the dissociative electron ionization of adamantane (C10 H16 ) 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 C8 H11 + 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.