Polycyclic Aromatic Hydrocarbon Cations Research Articles

The Adverse Effect of Air Pollution with Polycyclic Aromatic hydrocarbon (PAH) on 8-OXO-DG and gene expression (HOGG1) in Midland Refineries Company-Daura Refinery Workers

Introduction: Polycyclic aromatic hydrocarbons (PAHs) are a group of different kind of hazardous organic chemicals which are considered to be the top of pollutants that released by petroleum industries exploration activities to the environment. PAH Metabolites that affect initiation of cancer by reaction with DNA are practically modified chemically by enzymes. PAH Mutagenic metabolites include radical PAH cations, diol epoxides, and quinones. Reactive oxygen species (ROS) mostly produce by routine oxidation processes in mitochondria is responsible for many types of DNA damage. The aim of study identify the DNA alteration due to air pollution by modulation of gene expression and regulatory gene Human 8-oxoguanine DNA Glycosylase (HOOG1) and association with 7,8-dihydro-8-oxoguanine (8-oxo-dg). Methods: there were 168 participants included in this study divided in three groups (control, office workers and in field workers) .PAH, 8-OXO-DG and gene expression (HOGG1) detected for each participant by GC/MS, competitive ELISA and quantitative-competitive reverse transcription-PCR respectively. Results: PAH were not detected in the blood of control group, and there was significant difference in the concentration of PAH between office and field workers. The concentration of (8-oxo DG) and (hOGG1) was significantly higher in field worker than in office worker and control group, significant difference was found between office workers and control. Discussion: It is observed that increased levels of PAH and its metabolites when exposed to polluted air. PAHs metabolism is associated with production of ROS as well as oxidative damaged. Among this damage, the most common lesion is (8-OHdG) .Increase in (8-OHdG) levels associated with increase oxidative stress and (HOGG1) gene. Conclusion: (a) increased PAH and its metabolites levels in serum causing high 8-OHdG and (HOGG1) expression levels in refineries workers (b) there is a correlation between 8-OHdG and hOGG1 expression with PAH.

Mid-infrared spectroscopic signatures of dibenzopyrene cations – The effect of symmetry on PAH IR spectroscopy

In this work, we characterize – for the first time – in the gas phase infrared spectra of three isomeric Polycyclic Aromatic Hydrocarbon (PAH) cations of C24H14 composition that belong to distinctly different symmetry groups (C2h, Cs and C1). Mid-infrared (Mid-IR) spectra are recorded by means of infrared multiple photon dissociation (IRMPD) spectroscopy at the free electron laser for infrared experiments (FELIX) laboratory. The measured infrared (IR) band positions compare reasonably well with density functional theory (DFT) calculated values. The number of IR active bands increases as the symmetry of the molecule lowers. The IRMPD spectra of irregular PAHs are found to be dense and do not resemble the sharp signatures typical of astronomical IR bands, but rather look like the broad plateau on which these are perched. This lends credit to the GrandPAH hypothesis that suggests that small and irregular PAHs are weeded out by the strong interstellar radiation field and only large regular PAHs remain.

Astrochemical relevance of VUV ionization of large PAH cations.

As a part of interstellar dust, polycyclic aromatic hydrocarbons (PAHs) are processed by the interaction with vacuum ultra-violet (VUV) photons that are emitted by hot young stars. This interaction leads to the emission of the well-known aromatic infrared bands but also of electrons, which can significantly contribute to the heating of the interstellar gas. Our aim is to investigate the impact of molecular size on the photoionization properties of cationic PAHs. Trapped PAH cations of sizes between 30 and 48 carbon atoms were submitted to VUV photons in the range of 9 to 20 eV from the DESIRS beamline at the synchrotron SOLEIL. All resulting photoproducts including dications and fragment cations were mass-analyzed and recorded as a function of photon energy. Photoionization is found to be predominant over dissociation at all energies, which differs from an earlier study on smaller PAHs. The photoionization branching ratio reaches 0.98 at 20 eV for the largest studied PAH. The photoionization threshold is observed to be between 9.1 and 10.2 eV, in agreement with the evolution of the ionization potential with size. Ionization cross sections were indirectly obtained and photoionization yields extracted from their ratio with theoretical photoabsorption cross sections, which were calculated using time-dependent density functional theory. An analytical function was derived to calculate this yield for a given molecular size. Large PAH cations could be efficiently ionized in H i regions and provide a contribution to the heating of the gas by photoelectric effect. Also, at the border of or in H ii regions, PAHs could be exposed to photons of energy higher than 13.6 eV. Our work provides recipes to be used in astronomical models to quantify these points.

Laboratory formation and photochemistry of covalently bonded polycyclic aromatic nitrogen heterocycle (PANH) clusters in the gas phase

ABSTRACT To examine the evolution processes of the nitrogen-containing polycyclic aromatic hydrocarbon (PAH) molecules occurring in interstellar environments, in this work we focus on the formation of large covalently bonded N-substituted polyaromatic species and their photochemistry behaviour in the gas phase. The experimental results show large PANH (e.g. DC/acridine and DC/phenazine) cluster cations formed in a chemical reaction between large PAH (e.g. dicoronylene, DC, C48H20) cations and small PANHs (e.g. acridine, C13H9N, or phenazine, C12H8N2) by gas-phase condensation through ion–molecule reactions. With laser irradiation, PANH cluster cations are involved in a complex photofragmentation process (e.g. dehydrogenation, HCN/CN, C2 or N2 units lost) and then form large PANH/PAH or multiple dehydrogenated molecules; in particular, the dehydrogenation of PANH clusters provides a possible way to synthesize large nitrogen-containing graphene species (e.g. C59N+ and C61N+). Also, we perform quantum-theoretical calculations on the formation and photochemistry of DC/acridine and DC/phenazine cluster cations: two types of molecular cluster are considered (C–C and C–N bond type) and the formation pathway and dissociation energy for each isomer are determined. The experimental and theoretical findings obtained give a general molecular growth pathway toward all-benzenoid aromatic species with size (> 60 C atoms) in the astrophysically relevant range, during a ground-up formation process, and offer understanding of the nitrogen element effect on their chemical-evolutionary behaviour. Also, studies of DC/acridine and DC/phenazine clusters (89–112 atoms, ∼2 nm in size) offer a feasible means of explanation for the formation of nanoscale dust grains (nitrogen element included) in space.

Atomic hydrogen interactions with small polycyclic aromatic hydrocarbons cations

When exposed to a thermal beam of hydrogen atoms, gas-phase coronene cations C24H12+ can be sequentially hydrogenated. This process is accompanied by a gradual transition of the electronic structure from aromatic to aliphatic. The planar very stable coronene structure transforms into the significantly weaker corrugated structure, typical for aliphatic molecules. In this study, we have investigated the hydrogenation of 5 smaller polycyclic aromatic hydrocarbon cations using a combination of radiofrequency ion trapping with time-of-flight mass spectrometry. Anthracene (C14H10+), pyrene (C16H10+), triphenylene (C18H12+), tetracene (C18H12+) and 8-9-benzofluoranthene (C20H12+) only cover a small mass range, but differ in carbon/hydrogen ratio, number of outer-edge sites and overall structure. We have observed qualitatively similar initial hydrogenation patterns for all 5 molecular ions, with odd hydrogenation states being dominant. Strong quantitative differences in hydrogenation and in attachment-induced fragmentation were found. For the case of pyrene cations, we have also investigated exposure to atomic D. Clear lines of evidence for HD/D2 abstraction reactions of Eley–Rideal type were found, as previously observed for coronene cations.Graphical abstract

Probing the size and charge of polycyclic aromatic hydrocarbons

ABSTRACT We present a new method to accurately describe the ionization fraction and the size distribution of polycyclic aromatic hydrocarbons (PAHs) within astrophysical sources. To this purpose, we have computed the mid-infrared emission spectra of 308 PAH molecules of varying sizes, symmetries, and compactness, generated in a range of radiation fields. We show that the intensity ratio of the solo CH out-of-plane bending mode in PAH cations and anions (referred to as the ‘11.0’ μm band, falling in the 11.0–11.3 μm region for cations and anions) to their 3.3 μm emission scales with PAH size, similarly to the scaling of the 11.2/3.3 ratio with the number of carbon atoms (NC) for neutral molecules. Among the different PAH emission bands, it is the 3.3 μm band intensity that has the strongest correlation with NC, and drives the reported PAH intensity ratio correlations with NC for both neutral and ionized PAHs. The 6.2/7.7 intensity ratio, previously adopted to track PAH size, shows no evident scaling with NC in our large sample. We define a new diagnostic grid space to probe PAH charge and size, using the (11.2 + 11.0)/7.7 and (11.2 + 11.0)/3.3 PAH intensity ratios, respectively. We demonstrate the application of the (11.2 + 11.0)/7.7–(11.2 + 11.0)/3.3 diagnostic grid for galaxies M82 and NGC 253, for the planetary nebula NGC 7027, and the reflection nebulae NGC 2023 and NGC 7023. Finally, we provide quantitative relations for PAH size determination depending on the ionization fraction of the PAHs and the radiation field they are exposed to.

Photolysis-induced scrambling of PAHs as a mechanism for deuterium storage

Aims. We investigate the possible role of polycyclic aromatic hydrocarbons (PAHs) as a sink for deuterium in the interstellar medium (ISM) and study UV photolysis as a potential underlying chemical process in the variations of the deuterium fractionation in the ISM. Methods. The UV photo-induced fragmentation of various isotopologs of deuterium-enriched, protonated anthracene and phenanthrene ions (both C14H10 isomers) was recorded in a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer. Infrared multiple photon dissociation spectroscopy using the Free-Electron Laser for Infrared eXperiments was applied to provide IR spectra. Infrared spectra calculated using density functional theory were compared to the experimental data to identify the isomers present in the experiment. Transition-state energies and reaction rates were also calculated and related to the experimentally observed fragmentation product abundances. Results. The photofragmentation mass spectra for both UV and IRMPD photolysis only show the loss of atomic hydrogen from [D − C14H10]+, whereas [H − C14D10]+ shows a strong preference for the elimination of deuterium. Transition state calculations reveal facile 1,2-H and -D shift reactions, with associated energy barriers lower than the energy supplied by the photo-excitation process. Together with confirmation of the ground-state structures via the IR spectra, we determined that the photolytic processes of the two different PAHs are largely governed by scrambling where the H and the D atoms relocate between different peripheral C atoms. The ∼0.1 eV difference in zero-point energy between C–H and C–D bonds ultimately leads to faster H scrambling than D scrambling, and increased H atom loss compared to D atom loss. Conclusions. We conclude that scrambling is common in PAH cations under UV radiation. Upon photoexcitation of deuterium-enriched PAHs, the scrambling results in a higher probability for the aliphatic D atom to migrate to a strongly bound aromatic site, protecting it from elimination. We speculate that this could lead to increased deuteration as a PAH moves towards more exposed interstellar environments. Also, large, compact PAHs with an aliphatic C–HD group on solo sites might be responsible for the majority of aliphatic C–D stretching bands seen in astronomical spectra. An accurate photochemical model of PAHs that considers deuterium scrambling is needed to study this further.

Learning mid-IR emission spectra of polycyclic aromatic hydrocarbon populations from observations.

The James Webb Space Telescope (JWST) will deliver an unprecedented quantity of high-quality spectral data over the 0.6-28 μm range. It will combine sensitivity, spectral resolution, and spatial resolution. Specific tools are required to provide efficient scientific analysis of such large data sets. Our aim is to illustrate the potential of unsupervised learning methods to get insights into chemical variations in the populations that carry the aromatic infrared bands (AIBs), more specifically polycyclic aromatic hydrocarbon (PAH) species and carbonaceous very small grains (VSGs). We present a method based on linear fitting and blind signal separation for extracting representative spectra for a spectral data set. The method is fast and robust, which ensures its applicability to JWST spectral cubes. We tested this method on a sample of ISO-SWS data, which resemble most closely the JWST spectra in terms of spectral resolution and coverage. Four representative spectra were extracted. Their main characteristics appear consistent with previous studies with populations dominated by cationic PAHs, neutral PAHs, evaporating VSGs, and large ionized PAHs, known as the PAH x population. In addition, the 3 μm range, which is considered here for the first time in a blind signal separation (BSS) method, reveals the presence of aliphatics connected to neutral PAHs. Each representative spectrum is found to carry second-order spectral signatures (e.g., small bands), which are connected with the underlying chemical diversity of populations. However, the precise attribution of theses signatures remains limited by the combined small size and heterogeneity of the sample of astronomical spectra available in this study. The upcoming JWST data will allow us to overcome this limitation. The large data sets of hyperspectral images provided by JWST analysed with the proposed method, which is fast and robust, will open promising perspectives for our understanding of the chemical evolution of the AIB carriers.

Calculations of adsorption, coadsorption, diffusion, and reaction barriers of H atoms in the H2 formation on a positively charged coronene

DFT-D calculations of adsorption, diffusion and reaction barriers of atomic hydrogen radicals (H) to form H2 on positively charged coronene (Cor+) were studied for cis dimers (ortho, meta, and para) of H on central and edge rings. Results show, by comparison with the neutral system, that cationic polycyclic aromatic hydrocarbons (PAH) are more efficient for hydrogen recombination because these systems have a lower diffusion and reaction barriers. In some cases, hydrogen recombination does not take place because H desorption occurs. However, the adsorption of a third H leads to lower reaction barriers than in the dimer systems.

Threshold Dissociation of the 1-ethynylpyrene Cation at Internal Energies Relevant to H i Regions

Abstract Photoelectron–photoion coincidence spectroscopy has been used to measure the threshold photoelectron (TPE) spectrum of 1-ethynylpyrene and to obtain the breakdown graph describing the dissociation of the 1-ethynylpyrene cation. The TPE measurement has allowed us to improve the determination of the ionization energy of 1-ethynylpyrene at 7.391 ± 0.005 eV. Concerning the main dissociation channels, the analysis of the breakdown graph has given 3.70 ± 0.60 eV as the activation energy for the loss of one H atom and 2.98 ± 1.80 eV for the loss of a second independent H atom. The corresponding entropies of activation are affected by large errors as observed in similar studies of other polycyclic aromatic hydrocarbon (PAH) cations. Minor dissociation channels were also detected and identified as the loss of the C2H group and the loss of a C2H2 unit and/or that of an H atom plus the C2H group. The activation energies and the entropies of activation of these minor pathways could not be derived from the measurements. It is found that the cation of 1-ethynylpyrene behaves like that of pyrene and is consequently more photostable than that of 1-methylpyrene. We conclude that photodissociation is not the leading cause of the low abundance, if not the absence, of ethynyl-substituted PAH species in the interstellar medium.

High resolution energy-time of flight spectrometer: Dehydrogenation of fluorene cation as case study

Testing and performance validation of a new multipurpose time-of-flight mass spectrometer followed by an energy analyzer is presented. The instrument with high mass and energy resolution is primarily designed to study cations of polycyclic aromatic hydrocarbons (PAHs) and their dehydrogenation process. The energy correlated time-of-flight measurement is supplemented by Monte Carlo simulation to probe the dehydrogenation process in a relatively small PAH cation. The experiment is carried out on fluorene+ on a timescale of several microseconds. Fluorene cations with high internal energies were produced using UV multiphoton ionization. Specific n-photon processes leading to ionization as well as H-loss reaction were identified. The average value of dehydrogenation rate is estimated by fitting the measured data to the outcome of simulations. The quantification of H loss decay rate is in agreement with previously reported decay rate measurement. This corresponds to the internal energy available by inner valence electron emission caused by three photon process. The effectiveness of the instrument to access a range of decay rates (103–107 s−1) in a single measurement is demonstrated.

Understanding and Optimizing the Ionization of Polycyclic Aromatic Hydrocarbons in Dielectric Barrier Discharge Sources.

Fundamental aspects of undoped and dopant-assisted dielectric barrier discharge ionization (DBDI) were here addressed to help overcome the suboptimal ionization efficiency of nonpolar species. In the absence of dopants, polycyclic aromatic hydrocarbons (PAHs) mostly form [M + H]+ rather than [M]+• ions. Humidity was shown to further increase the relative amount of protonated PAHs, while the introduction of other dopants, such as fluorobenzene and chlorobenzene, shifted the ionization toward radical cations. The source of the protons was clarified through the use of deuterated species and confirmed both water as well as dopants as potential sources. The type of plasma gas also influenced the ionization, with the relative amount of radical cations increasing from CO2, to N2, to air. The addition of SF6 gas and the relative amount of low-energy (0-11 eV) electrons in the source suggests that Penning ionization leads to the relatively high amount of radical cations in air plasma. Penning ionization of excess dopant to form radical dopant species, and subsequent charge exchange with the PAHs, is proposed as the principal ionization mechanism for the formation of radical cationic PAHs in dopant-assisted DBDI. Benzylamine "thermometer molecules" were used to evaluate the energy involved in the ionization process and showed that dopants can affect the internal energy (-39.4 kJ mol-1 with humidity in a nitrogen plasma and +15.7 kJ mol-1 with fluorobenzene in an air plasma). A deeper understanding of fundamental aspects, especially of dopant-assisted ionization in a dielectric barrier discharge source, will be vital for future method optimization, particularly with regards to difficult to ionize (e.g., nonpolar) species.

Photoprocessing of large PAH cations.

In cosmic environments, polycyclic aromatic hydrocarbons (PAHs) strongly interact with vacuum ultraviolet (VUV) photons emitted by young stars. Trapped PAH cations ranging in size from 30 to 48 carbon atoms were irradiated by tunable synchrotron light (DESIRS beamline at SOLEIL). Their ionization and dissociation cross sections were determined and compared with TD-DFT computed photoabsorption cross sections. Evidence for radiative cooling is reported.

Interstellar polycyclic aromatic hydrocarbons: Spectroscopy, photofragmentation and photoproducts

AbstractUbiquitous strong mid-infrared emission bands are observed towards many objects and are attributed to interstellar Polycyclic Aromatic Hydrocarbons (PAHs). PAHs are ionized, or even dissociate, when exposed to strong interstellar radiation fields. By means of ion trap mass spectrometry, light-induced dissociation patterns of PAH cations are measured and the mid-infrared spectroscopic signatures of the parent ion and its dissociation products are characterized. These results are then combined with density functional theory (DFT) calculations to obtain insight into the dissociation characteristics of interstellar PAHs at a molecular level.

PAH under XUV excitation: an ultrafast XUV-photochemistry experiment for astrophysics

Understanding processes induced by XUV excitation of Polycyclic Aromatic Hydrocarbons (PAHs) is at the heart of molecular astrophysics, which aims at understanding molecular evolution in interstellar media. We used ultrashort XUV pulses to produce highly excited PAHs cations. The photo-induced dynamics is probed using a pump-probe XUV-IR spectroscopy. By studying PAH from small (naphthalene) to large (hexabenzocoronene) PAHs, we show that the dynamic is governed by the large density of states, in which many-body quantum effects are dominant.

Isotope and Temperature Effects on the Electronic Spectra of Large Carbonaceous Molecular Ions of Interstellar Relevance

The electronic spectra of isotopologues of protonated coronene in the gas phase were measured at a vibrational and rotational temperature of ~10K in a 22-pole ion trap. The (1) 1A′←X 1A′ transition of these polycyclic aromatic hydrocarbon cations with one to three carbon-13 have origin band maxima that blue-shift successively by 0.03nm. All isotopologues show distinct vibrational structure in the (1) 1A′ state. These results are compared with the effect of 13C substitution on the near infrared electronic absorptions of C60+. The (1) 1A←X 1A electronic transition of monodeuterated coronene was also recorded and its origin band is red-shifted to that of protonated coronene by 0.8nm. The implications for astronomical observations are considered.

Photoinduced polycyclic aromatic hydrocarbon dehydrogenation

The physical and chemical conditions in photodissociation regions (PDRs) are largely determined by the influence of far ultraviolet radiation. Far-UV photons can efficiently dissociate molecular hydrogen, a process that must be balanced at the HI/H2 interface of the PDR. Given that reactions involving hydrogen atoms in the gas phase are highly inefficient under interstellar conditions, H2 formation models mostly rely on catalytic reactions on the surface of dust grains. Additionally, molecular hydrogen formation in polycyclic aromatic hydrocarbons (PAHs) through the Eley–Rideal mechanism has been considered as well, although it has been found to have low efficiency in PDR fronts. In a previous work, we have described the possibility of efficient H2 release from medium to large sized PAHs upon photodissociation, with the exact branching between H-/H2-loss reactions being molecule dependent. Here, we investigate the astrophysical relevance of this process, by using a model for the photofragmentation of PAHs under interstellar conditions. We focus on three PAHs cations (coronene, ovalene, and circumcoronene), which represent three possibilities in the branching of atomic and molecular hydrogen losses. We find that, for ovalene (H2-loss dominated) the rate coefficient for H2 formation reaches values of the same order as H2 formation in dust grains. This result suggests that this hitherto disregarded mechanism can account, at least partly, for the high level of molecular hydrogen formation in dense PDRs.

Molecular hydrogen formation on interstellar PAHs through Eley-Rideal abstraction reactions

We present experimental data on H2 formation processes on gas-phase polycyclic aromatic hydrocarbon (PAH) cations. This process was studied by exposing coronene radical cations, confined in a radio-frequency ion trap, to gas-phase H atoms. Sequential attachment of up to 23 hydrogen atoms has been observed. Exposure to atomic D instead of H allows one to distinguish attachment from competing abstraction reactions, as the latter now leave a unique fingerprint in the measured mass spectra. Modelling of the experimental results using realistic cross-sections and barriers for attachment and abstraction yield a 1:2 ratio of abstraction to attachment cross-sections. The strong contribution of abstraction indicates that H2 formation on interstellar PAH cations is an order of magnitude more relevant than previously thought.

Laboratory Gas-phase Infrared Spectra of Two Astronomically Relevant PAH Cations: Diindenoperylene, and Dicoronylene,

Abstract The first gas-phase infrared spectra of two isolated astronomically relevant and large polycyclic aromatic hydrocarbon (PAH) cations—diindenoperylene (DIP) and dicoronylene (DC)—in the 530–1800 cm−1 (18.9−5.6 μm) range—are presented. Vibrational band positions are determined for comparison to the aromatic infrared bands. The spectra are obtained via infrared multiphoton dissociation spectroscopy of ions stored in a quadrupole ion trap using the intense and tunable radiation of the free electron laser for infrared experiments (FELIX). DIP+ shows its main absorption peaks at 737 (13.57), 800 (12.50), 1001 (9.99), 1070 (9.35), 1115 (8.97), 1152 (8.68), 1278 (7.83), 1420 (7.04), and 1550 (6.45) cm−1(μm), in good agreement with density functional theory (DFT) calculations that are uniformly scaled to take anharmonicities into account. DC+ has its main absorption peaks at 853 (11.72), 876 (11.42), 1032 (9.69), 1168 (8.56), 1300 (7.69), 1427 (7.01), and 1566 (6.39) cm−1(μm), which also agree well with the scaled DFT results presented here. The DIP+ and DC+ spectra are compared with the prominent infrared features observed toward NGC 7023. This results both in matches and clear deviations. Moreover, in the 11.0–14.0 μm region, specific bands can be linked to CH out-of-plane (oop) bending modes of different CH edge structures in large PAHs. The molecular origin of these findings and their astronomical relevance are discussed.

Unimolecular reaction energies for polycyclic aromatic hydrocarbon ions.

Imaging photoelectron photoion coincidence spectroscopy was employed to explore the unimolecular dissociation of the ionized polycyclic aromatic hydrocarbons (PAHs) acenaphthylene, fluorene, cyclopenta[d,e,f]phenanthrene, pyrene, perylene, fluoranthene, dibenzo[a,e]pyrene, dibenzo[a,l]pyrene, coronene and corannulene. The primary reaction is always hydrogen atom loss, with the smaller species also exhibiting loss of C2H2 to varying extents. Combined with previous work on smaller PAH ions, trends in the reaction energies (E0) for loss of H from sp2-C and sp3-C centres, along with hydrocarbon molecule loss were found as a function of the number of carbon atoms in the ionized PAHs ranging in size from naphthalene to coronene. In the case of molecules which possessed at least one sp3-C centre, the activation energy for the loss of an H atom from this site was 2.34 eV, with the exception of cyclopenta[d,e,f]phenanthrene (CPP) ions, for which the E0 was 3.44 ± 0.86 eV due to steric constraints. The hydrogen loss from PAH cations and from their H-loss fragments exhibits two trends, depending on the number of unpaired electrons. For the loss of the first hydrogen atom, the energy is consistently ca. 4.40 eV, while the threshold to lose the second hydrogen atom is much lower at ca. 3.16 eV. The only exception was for the dibenzo[a,l]pyrene cation, which has a unique structure due to steric constraints, resulting in a low H loss reaction energy of 2.85 eV. If C2H2 is lost directly from the precursor cation, the energy required for this dissociation is 4.16 eV. No other fragmentation channels were observed over a large enough sample set for trends to be extrapolated, though data on CH3 and C4H2 loss obtained in previous studies is included for completeness. The dissociation reactions were also studied by collision induced dissociation after ionization by atmospheric pressure chemical ionization. When modeled with a simple temperature-based theory for the post-collision internal energy distribution, there was reasonable agreement between the two sets of data.