Reactivity of the Ethenium Cation (C2H5+) with Ethyne (C2H2): A Combined Experimental and Theoretical Study.

The gas-phase reaction between the ethyl cation (C2H5+) and ethyne (C2H2) is re-investigated by measuring absolute reactive cross sections (CSs) and branching ratios (BRs) as a function of collision energy, in the thermal and hyperthermal energy range, via tandem-guided ion beam mass spectrometry under single collision conditions. Dissociative photoionization of C2H5Br using tuneable VUV radiation in the range 10.5-14.0 eV is employed to generate C2H5+, which has also allowed us to explore the impact of increasing (vibrational) excitation on the reactivity. Reactivity experiments are complemented by theoretical calculations, at the G4 level of theory, of the relative energies and structures of the most relevant stationary points on the reactive potential energy hypersurface (PES) and by mass-analyzed ion kinetic energy (MIKE) spectrometry experiments to probe the metastable decomposition from the [C4H7]+ PES and elucidate the underlying reaction mechanisms. Two main product channels have been identified at a centre-of-mass collision energy of ∼0.1 eV: (a) C3H3++CH4, with BR = 0.76±0.05 and (b) C4H5++H2, with BR = 0.22±0.02. A third channel giving C2H3+ in association with C2H4 is shown to emerge at both high internal excitation of C2H5+ and high collision energies. From CS measurements, energy-dependent total rate constants in the range 4.3×10-11-5.2×10-10 cm3·molecule-1·s-1 have been obtained. Theoretical calculations indicate that both channels stem from a common covalently bound intermediate, CH3CH2CHCH+, from which barrierless and exothermic pathways exist for the production of both cyclic c-C3H3+ and linear H2CCCH+ isomers of the main product channel. For the minor C4H5+ product, two isomers are energetically accessible: the three-member cyclic isomer c-C3H2(CH3)+ and the higher energy linear structure CH2CHCCH2+, but their formation requires multiple isomerization steps and passages via transition states lying only 0.11 eV below the reagents' energy, thus explaining the smaller BR. Results have implications for the modeling of hydrocarbon chemistry in the interstellar medium and the atmospheres of planets and satellites as well as in laboratory plasmas (e.g., plasma-enhanced chemical vapor deposition of carbon nanotubes and diamond-like carbon films).

Intracardiac hematoma treated conservatively by ECMO support.

Clinical pathway of an intracardiac hematoma treated by ECMO support.

Radial versus femoral access and bivalirudin versus unfractionated heparin in invasively managed patients with acute coronary syndrome (MATRIX): final 1-year results of a multicentre, randomised controlled trial.

BACKGROUND: The Minimizing Adverse Haemorrhagic Events by Transradial Access Site and Systemic Implementation of Angiox (MATRIX) programme was designed to assess the comparative safety and effectiveness of radial versus femoral access and of bivalirudin versus unfractionated heparin with optional glycoprotein IIb/IIIa inhibitors in patients with the whole spectrum of acute coronary syndrome undergoing invasive management. Here we describe the prespecified final 1-year outcomes of the entire programme. METHODS: MATRIX was a programme of three nested, randomised, multicentre, open-label, superiority trials in patients with acute coronary syndrome in 78 hospitals in Italy, the Netherlands, Spain, and Sweden. Patients with ST-elevation myocardial infarction were simultaneously randomly assigned (1:1) before coronary angiography to radial or femoral access and to bivalirudin, with or without post-percutaneous coronary intervention infusion or unfractionated heparin (one-step inclusion). Patients with non-ST-elevation acute coronary syndrome were randomly assigned (1:1) before coronary angiography to radial or femoral access and, only if deemed eligible to percutaneous coronary intervention after angiography (two-step inclusion), entered the antithrombin type and treatment duration programmes. Randomisation sequences were computer generated, blocked, and stratified by intended new or current use of P2Y12 inhibitor (clopidogrel vs ticagrelor or prasugrel), and acute coronary syndrome type (ST-elevation myocardial infarction, troponin-positive, or troponin-negative non-ST-elevation acute coronary syndrome). Bivalirudin was given as a bolus of 0·75 mg/kg, followed immediately by an infusion of 1·75 mg/kg per h until completion of percutaneous coronary intervention. Heparin was given at 70-100 units per kg in patients not receiving glycoprotein IIb/IIIa inhibitors, and at 50-70 units per kg in patients receiving glycoprotein IIb/IIIa inhibitors. Clinical follow-up was done at 30 days and 1 year. Co-primary outcomes for MATRIX access and MATRIX antithrombin type were major adverse cardiovascular events, defined as the composite of all-cause mortality, myocardial infarction, or stroke up to 30 days; and net adverse clinical events, defined as the composite of non-coronary artery bypass graft-related major bleeding, or major adverse cardiovascular events up to 30 days. The primary outcome for MATRIX treatment duration was the composite of urgent target vessel revascularisation, definite stent thrombosis, or net adverse clinical events up to 30 days. Analyses were done according to the intention-to-treat principle. This trial is registered with ClinicalTrials.gov, number NCT01433627. FINDINGS: Between Oct 11, 2011, and Nov 7, 2014, we randomly assigned 8404 patients to receive radial (4197 patients) or femoral (4207 patients) access. Of these 8404 patients, 7213 were included in the MATRIX antithrombin type study and were randomly assigned to bivalirudin (3610 patients) or heparin (3603 patients). Patients assigned to bivalirudin were included in the MATRIX treatment duration study, and were randomly assigned to post-procedure infusion (1799 patients) or no post-procedure infusion (1811 patients). At 1 year, major adverse cardiovascular events did not differ between patients assigned to radial access compared with those assigned to femoral access (14·2% vs 15·7%; rate ratio 0·89, 95% CI 0·80-1·00; p=0·0526), but net adverse clinical events were fewer with radial than with femoral access (15·2% vs 17·2%; 0·87, 0·78-0·97; p=0·0128). Compared with heparin, bivalirudin was not associated with fewer major adverse cardiovascular (15·8% vs 16·8%; 0·94, 0·83-1·05; p=0·28) or net adverse clinical events (17·0% vs 18·4%; 0·91, 0·81-1·02; p=0·10). The composite of urgent target vessel revascularisation, stent thrombosis, or net adverse clinical events did not differ with or without post-procedure bivalirudin infusion (17·4% vs 17·4%; 0·99, 0·84-1·16; p=0·90). INTERPRETATION: In patients with acute coronary syndrome, radial access was associated with lower rates of net adverse clinical events compared with femoral access, but not major adverse cardiovascular events at 1 year. Bivalirudin with or without post-procedure infusion was not associated with lower rates of major adverse cardiovascular events or net adverse clinical events. Radial access should become the default approach in acute coronary syndrome patients undergoing invasive management. FUNDING: Italian Society of Invasive Cardiology, The Medicines Company, Terumo, amd Canada Research Chairs Programme.

Bivalirudin or Unfractionated Heparin in Acute Coronary Syndromes.

BACKGROUND: Conflicting evidence exists on the efficacy and safety of bivalirudin administered as part of percutaneous coronary intervention (PCI) in patients with an acute coronary syndrome. METHODS: We randomly assigned 7213 patients with an acute coronary syndrome for whom PCI was anticipated to receive either bivalirudin or unfractionated heparin. Patients in the bivalirudin group were subsequently randomly assigned to receive or not to receive a post-PCI bivalirudin infusion. Primary outcomes for the comparison between bivalirudin and heparin were the occurrence of major adverse cardiovascular events (a composite of death, myocardial infarction, or stroke) and net adverse clinical events (a composite of major bleeding or a major adverse cardiovascular event). The primary outcome for the comparison of a post-PCI bivalirudin infusion with no post-PCI infusion was a composite of urgent target-vessel revascularization, definite stent thrombosis, or net adverse clinical events. RESULTS: The rate of major adverse cardiovascular events was not significantly lower with bivalirudin than with heparin (10.3% and 10.9%, respectively; relative risk, 0.94; 95% confidence interval [CI], 0.81 to 1.09; P=0.44), nor was the rate of net adverse clinical events (11.2% and 12.4%, respectively; relative risk, 0.89; 95% CI, 0.78 to 1.03; P=0.12). Post-PCI bivalirudin infusion, as compared with no infusion, did not significantly decrease the rate of urgent target-vessel revascularization, definite stent thrombosis, or net adverse clinical events (11.0% and 11.9%, respectively; relative risk, 0.91; 95% CI, 0.74 to 1.11; P=0.34). CONCLUSIONS: In patients with an acute coronary syndrome, the rates of major adverse cardiovascular events and net adverse clinical events were not significantly lower with bivalirudin than with unfractionated heparin. The rate of the composite of urgent target-vessel revascularization, definite stent thrombosis, or net adverse clinical events was not significantly lower with a post-PCI bivalirudin infusion than with no post-PCI infusion. (Funded by the Medicines Company and Terumo Medical; MATRIX ClinicalTrials.gov number, NCT01433627.).

The Selective Role of Long-Range Forces in the Stereodynamics of Ion-Molecule Reactions: The He+ +Methyl Formate Case From Guided-Ion-Beam Experiments.

Long-range intermolecular forces play a crucial role in controlling the outcome of ion-molecule chemical reactions, such as those determining the disappearance of organic or inorganic "complex" molecules recently detected in various regions of the interstellar medium due to collisions with abundant interstellar atomic ions (e.g. H+ and He+ ). Theoretical treatments, for example, based on simple capture models, are nowadays often adopted to evaluate the collision-energy dependence of reactive cross sections and the temperature dependent rate coefficients of many ion-molecule reactions. The obtained results are widely used for the modelling of phenomena occurring in different natural environments or technological applications such as astrophysical and laboratory plasmas. Herein it is demonstrated, through a combined experimental and theoretical investigation on a prototype ion-molecule reaction (He+ +methyl formate), that the dynamics, investigated in detail, shows some intriguing features that can lead to rate coefficients at odds with the expectations (e.g. Arrhenius versus anti-Arrhenius behaviour). Therefore, this study casts light on some new and general guidelines to be properly taken into account for a suitable evaluation of rate coefficients of ion-molecule reactions.

Radial versus femoral access in patients with acute coronary syndromes with or without ST-segment elevation.

Aims: To assess whether radial compared with femoral access is associated with consistent outcomes in patients with ST-segment elevation myocardial infarction (STEMI) and non-ST-segment elevation acute coronary syndrome (NSTE-ACS). Methods and results: In the Minimizing Adverse Haemorrhagic Events by TRansradial Access Site and Systemic Implementation of angioX (MATRIX) programme patients were randomized to radial or femoral access, stratified by STEMI (2001 radial, 2009 femoral) and NSTE-ACS (2196 radial, 2198 femoral). The 30-day co-primary outcomes were major adverse cardiovascular events (MACE), defined as death, myocardial infarction, or stroke, and net adverse clinical events (NACE), defined as MACE or major bleeding In the overall study population, radial access reduced the NACE but not MACE endpoint at the prespecified 0.025 alpha. MACE occurred in 121 (6.1%) STEMI patients with radial access vs. 126 (6.3%) patients with femoral access [rate ratio (RR) = 0.96, 95% CI = 0.75-1.24; P = 0.76] and in 248 (11.3%) NSTE-ACS patients with radial access vs. 303 (13.9%) with femoral access (RR = 0.80, 95% CI = 0.67-0.96; P = 0.016) (Pint = 0.25). NACE occurred in 142 (7.2%) STEMI patients with radial access and in 165 (8.3%) patients with femoral access (RR = 0.86, 95% CI = 0.68-1.08; P = 0.18) and in 268 (12.2%) NSTE-ACS patients with radial access compared with 321 (14.7%) with femoral access (RR = 0.82, 95% CI = 0.69-0.97; P = 0.023) (Pint = 0.76). All-cause mortality and access site-actionable bleeding favoured radial access irrespective of ACS type (Pint = 0.11 and Pint = 0.36, respectively). Conclusion: Radial as compared with femoral access provided consistent benefit across the whole spectrum of patients with ACS, without evidence that type of presenting syndrome affected the results of the random access allocation.

Experimental investigation of the reaction of helium ions with dimethyl ether: stereodynamics of the dissociative charge exchange process.

The fate of dimethyl ether (DME, CH3OCH3) in collisions with He+ ions is of high relevance for astrochemical models aimed at reproducing the abundances of complex organic molecules in the interstellar medium. Here we report an investigation on the reaction of He+ ions with DME carried out using a Guided Ion Beam Mass Spectrometer (GIB-MS), which allows the measurement of reactive cross-sections and branching ratios (BRs) as a function of the collision energy. We obtain insights into the dissociative charge (electron) exchange mechanism by investigating the nature of the non-adiabatic transitions between the relevant potential energy surfaces (PESs) in an improved Landau-Zener approach. We find that the large interaction anisotropy could induce a pronounced orientation of the polar DME molecule in the electric field generated by He+ so that at short distances the collision complex is confined within pendular states, a particular case of bending motion, which gives rise to intriguing stereodynamic effects. The positions of the intermolecular potential energy curve crossings indicate that He+ captures an electron from an inner valence orbital of DME, thus causing its dissociation. In addition to the crossing positions, the symmetry of the electron density distribution of the involved DME orbitals turns out to be a further major point affecting the probability of electron transfer. Thus, the anisotropy of the intermolecular interaction and the electron densities of the orbitals involved in the reaction are the key "ingredients" for describing the dynamics of this dissociative charge transfer.

Effects of collision energy and vibrational excitation of CH3+ cations on its reactivity with hydrocarbons: But-2-yne CH3CCCH3 as reagent partner.

The methyl carbocation is ubiquitous in gaseous environments, such as planetary ionospheres, cometary comae, and the interstellar medium, as well as combustion systems and plasma setups for technological applications. Here we report on a joint experimental and theoretical study on the mechanism of the reaction CH3+ + CH3CCCH3 (but-2-yne, also known as dimethylacetylene), by combining guided ion beam mass spectrometry experiments with ab initio calculations of the potential energy hypersurface. Such a reaction is relevant in understanding the chemical evolution of Saturn's largest satellite, Titan. Two complementary setups have been used: in one case, methyl cations are generated via electron ionization, while in the other case, direct vacuum ultraviolet photoionization with synchrotron radiation of methyl radicals is used to study internal energy effects on the reactivity. Absolute reactive cross sections have been measured as a function of collision energy, and product branching ratios have been derived. The two most abundant products result from electron and hydride transfer, occurring via direct and barrierless mechanisms, while other channels are initiated by the electrophilic addition of the methyl cation to the triple bond of but-2-yne. Among the minor channels, special relevance is placed on the formation of C5H7+, stemming from H2 loss from the addition complex. This is the only observed condensation product with the formation of new C-C bonds, and it might represent a viable pathway for the synthesis of complex organic species in astronomical environments and laboratory plasmas.

Radial versus femoral access in patients with acute coronary syndromes undergoing invasive management: a randomised multicentre trial.

BACKGROUND: It is unclear whether radial compared with femoral access improves outcomes in unselected patients with acute coronary syndromes undergoing invasive management. METHODS: We did a randomised, multicentre, superiority trial comparing transradial against transfemoral access in patients with acute coronary syndrome with or without ST-segment elevation myocardial infarction who were about to undergo coronary angiography and percutaneous coronary intervention. Patients were randomly allocated (1:1) to radial or femoral access with a web-based system. The randomisation sequence was computer generated, blocked, and stratified by use of ticagrelor or prasugrel, type of acute coronary syndrome (ST-segment elevation myocardial infarction, troponin positive or negative, non-ST-segment elevation acute coronary syndrome), and anticipated use of immediate percutaneous coronary intervention. Outcome assessors were masked to treatment allocation. The 30-day coprimary outcomes were major adverse cardiovascular events, defined as death, myocardial infarction, or stroke, and net adverse clinical events, defined as major adverse cardiovascular events or Bleeding Academic Research Consortium (BARC) major bleeding unrelated to coronary artery bypass graft surgery. The analysis was by intention to treat. The two-sided α was prespecified at 0·025. The trial is registered at ClinicalTrials.gov, number NCT01433627. FINDINGS: We randomly assigned 8404 patients with acute coronary syndrome, with or without ST-segment elevation, to radial (4197) or femoral (4207) access for coronary angiography and percutaneous coronary intervention. 369 (8·8%) patients with radial access had major adverse cardiovascular events, compared with 429 (10·3%) patients with femoral access (rate ratio [RR] 0·85, 95% CI 0·74-0·99; p=0·0307), non-significant at α of 0·025. 410 (9·8%) patients with radial access had net adverse clinical events compared with 486 (11·7%) patients with femoral access (0·83, 95% CI 0·73-0·96; p=0·0092). The difference was driven by BARC major bleeding unrelated to coronary artery bypass graft surgery (1·6% vs 2·3%, RR 0·67, 95% CI 0·49-0·92; p=0·013) and all-cause mortality (1·6% vs 2·2%, RR 0·72, 95% CI 0·53-0·99; p=0·045). INTERPRETATION: In patients with acute coronary syndrome undergoing invasive management, radial as compared with femoral access reduces net adverse clinical events, through a reduction in major bleeding and all-cause mortality. FUNDING: The Medicines Company and Terumo.

Selective Generation of the Radical Cation Isomers [CH3CN](•+) and [CH2CNH](•+) via VUV Photoionization of Different Neutral Precursors and Their Reactivity with C2H4.

Experimental and theoretical studies have been carried out to demonstrate the selective generation of two different C2H3N(+) isomers, namely, the acetonitrile [CH3CN](•+) and the ketenimine [CH2CNH](•+) radical cations. Photoionization and dissociative photoionization experiments from different neutral precursors (acetonitrile and butanenitrile) have been performed using vacuum ultraviolet (VUV) synchrotron radiation in the 10-15 eV energy range, delivered by the DESIRS beamline at the SOLEIL storage ring. For butanenitrile (CH3CH2CH2CN) an experimental ionization threshold of 11.29 ± 0.05 eV is obtained, whereas the appearance energy for the formation of [CH2CNH](•+) fragments is 11.52 ± 0.05 eV. Experimental findings are fully supported by theoretical calculations at the G4 level of theory (ZPVE corrected energies at 0 K), giving a value of 11.33 eV for the adiabatic ionization energy of butanenitrile and an exothermicity of 0.49 for fragmentation into [CH2CNH](•+) plus C2H4, hampered by an energy barrier of 0.29 eV. The energy difference between [CH3CN](•+) and [CH2CNH](•+) is 2.28 eV (with the latter being the lowest energy isomer), and the isomerization barrier is 0.84 eV. Reactive monitoring experiments of the [CH3CN](•+) and [CH2CNH](•+) isomers with C2H4 have been performed using the CERISES guided ion beam tandem mass spectrometer and exploiting the selectivity of ethylene that gives exothermic charge exchange and proton transfer reactions with [CH3CN](•+) but not with [CH2CNH](•+) isomers. In addition, minor reactive channels are observed leading to the formation of new C-C bonds upon reaction of [CH3CN](•+) with C2H4, and their astrochemical implications are briefly discussed.

Growth of polyphenyls via ion-molecule reactions: an experimental and theoretical mechanistic study.

The reactivity of biphenylium cations C12H9(+) with benzene C6H6 is investigated in a joint experimental and theoretical approach. Experiments are performed by using a triple quadruple mass spectrometer equipped with an atmospheric pressure chemical ion source to generate C12H9(+) via dissociative ionization of various isomers of the neutral precursor hydroxybiphenyl (C12H10O). C-C coupling reactions leading to hydrocarbon growth are observed. The most abundant ionic products are C18H15(+), C18H13(+), C17H12(+), and C8H7(+). The dependence of product ion yields on the kinetic energy of reagent ions, as well as further experiments performed using partial isotopic labelling of reagents, support the idea that the reaction proceeds via a long lived association product, presumably the covalently bound protonated terphenyl C18H15(+). Its formation is found to be exothermic and barrierless and, therefore, might occur under the low pressure and temperature conditions typical of planetary atmospheres and the interstellar medium. Theoretical calculations have focussed on the channel leading to C8H7(+) plus C10H8, identifying, as the most probable fragments, the phenylethen-1-ylium cation and naphthalene, thus suggesting that the pathway leading to them might be of particular interest for the synthesis of polycyclic aromatic hydrocarbons. Both experiments and theory agree in finding this channel exoergic but hampered by small barriers of 2.7 and 3.7 kcal mol(-1) on the singlet potential energy surface.

CO2 Reduction by Nanosecond-Plasma Discharges: Revealing the Dissociation's Time Scale and the Importance of Pulse Sequence.

Power-to-chemical technologies with CO2 as feedstock recycle CO2 and store energy into value-added compounds. Plasma discharges fed by renewable electricity are a promising approach to CO2 conversion. However, controlling the mechanisms of plasma dissociation is crucial to improving the efficiency of the technology. We have investigated pulsed nanosecond discharges, showing that while most of the energy is deposited in the breakdown phase, CO2 dissociation only occurs after an order of microsecond delay, leaving the system in a quasi-metastable condition in the intervening time. These findings indicate the presence of delayed dissociation mechanisms mediated by CO2 excited states rather than direct electron impact. This "metastable" condition, favorable for an efficient CO2 dissociation, can be prolonged by depositing more energy in the form of additional pulses and critically depends on a sufficiently short interpulse time.

Coronary calcification in patients presenting with acute coronary syndromes: insights from the MATRIX trial.

AIMS: The role of coronary calcification on clinical outcomes among different revascularization strategies in patients presenting with acute coronary syndromes (ACSs) has been rarely investigated. The aim of this investigation is to evaluate the role of coronary calcification, detected by coronary angiography, in the whole spectrum of patients presenting with acute ACS. METHODS AND RESULTS: The present study was a post hoc analysis of the MATRIX programme. The primary endpoint was major adverse cardiovascular events (MACE), defined as the composite of all-cause mortality, myocardial infarction (MI), or stroke up to 365 days. Among the 8404 patients randomized in the MATRIX trial, data about coronary calcification were available in 7446 (88.6%) and therefore were included in this post hoc analysis. Overall, 875 patients (11.7%) presented with severe coronary calcification, while 6571 patients (88.3%) did not present severe coronary calcification on coronary angiography. Fewer patients with severe coronary calcification underwent percutaneous coronary intervention whereas coronary artery bypass grafting or medical therapy-only was more frequent compared with patients without severe calcification. At 1-year follow-up, MACE occurred in 237 (27.1%) patients with severe calcified coronary lesions and 985 (15%) patients without severe coronary calcified lesions [hazard ratio (HR) 1.91; 95% confidence interval (CI) 1.66-2.20, P < 0.001]. All-cause mortality was 8.6% in patients presenting with and 3.7% in those without severe coronary calcification (HR 2.38, 1.84-3.09, P < 0.001). Patients with severe coronary calcification incurred higher rate of MI (20.1% vs. 11.5%, HR 1.81; 95% CI 1.53-2.1, P < 0.001) and similar rate of stroke (0.8% vs. 0.6%, HR 1.35; 95% CI 0.61-3.02, P = 0.46). CONCLUSION: Patients with ACS and severe coronary calcification, as compared to those without, are associated with worse clinical outcomes irrespective of the management strategy.

6 months of "temporary" support by Levitronix left ventricular assist device.

An otherwise healthy 47-year-old man presented to the emergency department in cardiogenic shock after suffering a massive myocardial infarction due to left main occlusion. He was initially supported by extracorporeal membrane oxygenation and subsequently was converted to paracorporeal support with a Levitronix left ventricular assist device. He experienced multiple postoperative complications including renal failure, respiratory failure, retroperitoneal hematoma requiring suspension of anticoagulation, and fungal bloodstream infection precluding transition to an implantable device. He was reconditioned and successfully underwent orthotopic heart transplant 183 days after presentation. A discussion of the relevant issues is included.

A study of resveratrol-copper complexes by electrospray ionization mass spectrometry and density functional theory calculations.

Resveratrol is a polyphenolic compound found in plants and human foods which has shown biological activities including chemoprevention, acting through a mechanism which involves the reduction of Cu(II) species. By electrospray ionization (ESI) mass spectrometry we have produced and detected the resveratrol-copper complexes [Resv+Cu](+), [Resv+Cu+H(2)O](+) and [2Resv+Cu](+) by using a resveratrol/CuSO(4) solution in CH(3)CN/H(2)O. The most stable structures of the detected complexes have been calculated at the B3LYP/6-311G(d) level of theory. Resveratrol interacts with the copper ion through nucleophilic carbon atoms on the aromatic ring and the alkenyl group. The fact that only singly charged ions were observed implies that Cu(II) is reduced to Cu(I) in the ESI process. For investigating the structure-reactivity correlation, we have carried out a similar study on the synthetic analogue dihydroresveratrol (DHResv). For the latter only the [DHResv+Cu](+) complex has been detected.

Growth of polyaromatic molecules via ion-molecule reactions: an experimental and theoretical mechanistic study.

The reactivity of naphthyl cations with benzene is investigated in a joint experimental and theoretical approach. Experiments are performed by using guided ion beam tandem mass spectrometers equipped with electron impact or atmospheric pressure chemical ion sources to generate C(10)H(7)(+) with different amounts of internal excitation. Under single collision conditions, C-C coupling reactions leading to hydrocarbon growth are observed. The most abundant ionic products are C(16)H(13)(+), C(16)H(n)(+) (with n=10-12), and C(15)H(10)(+). From pressure-dependent measurements, absolute cross sections of 1.0±0.3 and 2±0.6 Å(2) (at a collision energy of about 0.2 eV in the center of mass frame) are derived for channels leading to the formation of C(16)H(12)(+) and C(15)H(10)(+) ions, respectively. From cross section values a phenomenological total rate constant k=(5.8±1.9)×10(-11) cm(3) s(-1) at an average collision energy of about 0.27 eV can be estimated for the process C(10)H(7)(+)+C(6)H(6)→all products. The energy behavior of the reactive cross sections, as well as further experiments performed using partial isotopic labeling of reagents, support the idea that the reaction proceeds via a long lived association product, presumably the covalently bound protonated phenylnaphthalene, from which lighter species are generated by elimination of neutral fragments (H, H(2), CH(3)). A major signal relevant to the fragmentation of the initial adduct C(16)H(13)(+) belongs to C(15)H(10)(+). Since it is not obvious how CH(3) loss from C(16)H(13)(+) can take place to form the C(15)H(10)(+) radical cation, a theoretical investigation focuses on possible unimolecular transformations apt to produce it. Naphthylium can act as an electrophile and add to the π system of benzene, leading to a barrierless formation of the ionic adduct with an exothermicity of about 53 kcal mol(-1). From this structure, an intramolecular electrophilic addition followed by H shifts and ring opening steps leads to an overall exothermic loss (-7.1 kcal mol(-1) with respect to reagents) of the methyl radical from that part of the system which comes from benzene. Methyl loss can take place also from the "naphthyl" part, though via an endoergic route. Experimental and theoretical results show that an ionic route is viable for the growth of polycyclic aromatic species by association of smaller building blocks (naphthyl and phenyl rings) and this may be of particular relevance for understanding the formation of large molecules in ionized gases.

Reactivity of C2H5+ with benzene: formation of ethylbenzenium ions and implications for Titan's ionospheric chemistry.

The reaction of ethyl cation with benzene has been investigated in a combined experimental and theoretical approach. Under single collision conditions, proton transfer affording protonated benzene concomitant with neutral ethene represents the major reaction channel. From pressure-dependent measurements, an absolute cross section of 7 +/- 2 A(2) at hyperthermal energies (about 1.0 eV in the center of mass frame) is derived for this channel, from which a phenomenological rate constant of about 2.9 x 10(-10) cm(3) s(-1) is estimated at low energies. The energy behavior of the cross section as well as several side reactions leading to C-C coupling imply that the reaction of C(2)H(5)(+) with C(6)H(6) proceeds via a long-lived association product, presumably the covalently bound protonated ethylbenzene (ethylbenzenium ion). With regard to chemical processes in the atmosphere of Titan, present results imply that termolecular association of C(2)H(5)(+) with benzene to produce protonated ethylbenzene is very likely to occur. The condensation of alkyl cations with arenes thus provides an alternative route for the growth of larger hydrocarbon molecules.

The reaction of N2O with phenylium ions C6(H,D)5(+): an integrated experimental and theoretical mechanistic study.

The reaction of N(2)O (known to be an O atom donor under several conditions) with the phenyl cation is studied by experimental and theoretical methods. Phenyl cation (or phenylium), C(6)H(5)(+), and its perdeuterated derivative C(6)D(5)(+) are produced either by electron impact or by atmospheric pressure chemical ionization of adequate neutral precursors, and product mass spectra are measured in a guided ion beam tandem mass spectrometer. The ions C(5)(H,D)(5)(+), C(6)(H,D)(5)O(+), and C(3)(H,D)(3)(+) are experimentally detected as the most relevant reaction products. In addition, the detection of the adduct (C(6)H(5)N(2)O)(+), which is collisionally stabilized in the scattering cell of the mass spectrometer, is reported here for the first time. The reaction pathways, which could bring about the formation of the mentioned ions, are then explored extensively by density functional theory and, for the more promising pathways, by CASPT2/CASSCF calculations. The two reacting species (1) form initially a phenoxydiazonium adduct, C(6)H(5)ON(2)(+) (2a), by involving the empty in-plane hybrid C orbital of phenylium. The alternative attack to the ring pi system to produce an epoxidic adduct 2c is ruled out on the basis of the energetics. Then, 2a loses N(2) quite easily, thus affording the phenoxyl cation 3. This is only the first of several C(6)H(5)O(+) isomers (4-6 and 8-12), which can stem from 3 upon different cleavages and formations of C-C bond and/or H shifts. As regards the formation of C(5)H(5)(+), among several conceivable pathways, a direct CO extrusion from 3 is discarded, while others appear to be viable to different extents, depending on the initial energy of the system. The easiest CO loss is from 4, with formation of the cyclopentadienyl cation 7. Formation of C(3)H(3)(+) is generally hindered and its detection depends again on the availability of some extra initial energy.

Stereodynamical Effects by Anisotropic Intermolecular Forces.

Electric and magnetic field gradients, arising from sufficiently strong anisotropic intermolecular forces, tend to induce molecular polarization which can often modify substantially the results of molecular collisions, especially at low rotational temperatures and low collision energies. The knowledge of these phenomena, today still not fully understood, is of general relevance for the control of the stereo-dynamics of elementary chemical-physical processes, involving neutral and ionic species under a variety of conditions. This paper reports on results obtained by combining information from scattering, spectroscopic and reactivity experiments, within a collaboration between the research groups in Perugia and Trento. We addressed particular attention to the reactions of small atomic ions with polar neutrals for their relevance in several environments, including interstellar medium, planetary atmospheres, and laboratory plasmas. In the case of ion-molecule reactions, alignment/orientation is a general phenomenon due to the electric field generated by the charged particle. Such phenomenon originates critical stereo-dynamic effects that can either suppress (when the orientation drives the collision complex into non-reactive or less reactive configurations), or enhance the reactivity (when orientation confines reagents in the most appropriate configuration for reaction). The associated rate coefficients show the propensity to follow an Arrhenius and a non-Arrhenius behavior, respectively.

State-Selected Reactivity of Carbon Dioxide Cations ( CO 2 + ) With Methane.

The reactivity of CO 2 + with CD4 has been experimentally investigated for its relevance in the chemistry of plasmas used for the conversion of CO2 in carbon-neutral fuels. Non-equilibrium plasmas are currently explored for their capability to activate very stable molecules (such as methane and carbon dioxide) and initiate a series of reactions involving highly reactive species (e.g., radicals and ions) eventually leading to the desired products. Energy, in the form of kinetic or internal excitation of reagents, influences chemical reactions. However, putting the same amount of energy in a different form may affect the reactivity differently. In this paper, we investigate the reaction of CO 2 + with methane by changing either the kinetic energy of CO 2 + or its vibrational excitation. The experiments were performed by a guided ion beam apparatus coupled to synchrotron radiation in the VUV energy range to produce vibrationally excited ions. We find that the reactivity depends on the reagent collision energy, but not so much on the vibrational excitation of CO 2 + . Concerning the product branching ratios ( CD 4 + / CD 3 + /DOCO+) there is substantial disagreement among the values reported in the literature. We find that the dominant channel is the production of CD 4 + , followed by DOCO+ and CD 3 + , as a minor endothermic channel.