Selective Generation of the Radical Cation Isomers [CH3CN]â¢+ and

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Selective Generation of the Radical Cation Isomers [CHCN] and [CHCNH] via VUV Photoionization of Different Neutral Precursors and Their Reactivity with CH 3

•+

2

•+

2

4

Miroslav Polášek, Emilie-Laure Zins, Christian Alcaraz, Jan Žabka, V#ra K#ížová, Linda Giacomozzi, Paolo Tosi, and Daniela Ascenzi J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b12757 • Publication Date (Web): 18 Feb 2016 Downloaded from http://pubs.acs.org on March 2, 2016

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The Journal of Physical Chemistry

Selective Generation of the Radical Cation Isomers [CH3CN] •+ and [CH2CNH] •+ Via VUV Photoionization of Different Neutral Precursors and Their Reactivity with C2H4 Miroslav Polášek,a Emilie-Laure Zins,b Christian Alcaraz,c,d Jan Žabka,a Věra Křížová,a Linda Giacomozzie, Paolo Tosie and Daniela Ascenzie * a

J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejškova 2155/3, 18223 Prague 8, Czech Republic b Sorbonne Universités, UPMC Univ. Paris 06, MONARIS, UMR 8233, Université Pierre et Marie Curie, 4 Place Jussieu, case courrier 49, F-75252 Paris Cedex 05, France c Laboratoire de Chimie Physique, Bât. 350, UMR 8000, CNRS-Univ. Paris-Sud &Paris-Saclay, Centre Universitaire Paris-Sud, 91405 Orsay Cedex, France d Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin – BP 48, 91192 Gif-sur-Yvette, France e Department of Physics, University of Trento, Via Sommarive 14, 38123 Povo, Italy

* Corresponding author’s contact information: e-mail: [email protected] Phone: +39 0461 281693. Fax: +39 0461 281696

Abstract

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, while 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 0K), 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.

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Introduction It is well known that upon ionization, the acetonitrile molecule can isomerize from the [CH3CN]•+ radical cation into the ketenimine ion [CH2CNH]•+ or even into the isonitrile isomers [CH2NCH]•+ and [CH3NC]•+:1,2,3 [CH3CN]•+ → [CH2CNH]•+

(1)

Hence, when working with standard ionization sources using 70 eV energy electrons it is difficult to estimate the fraction of the various isomers produced. The experimental characterization of the isomers generated upon ionization of CH3CN has created significant disputes over the years and various experimental techniques have been used for this purpose, ranging from the study of the ratio of the CH3+/ CH2+ fragments by collision induced dissociation experiments3,4,5,6 to selected ion-molecule reactions3,7. Theoretical calculations have shown that the isomerization from [CH3CN]•+ into [CH2CNH]•+ is exothermic by about 2 eV (more specifically 2.13 eV according to Mair et al.1, 2.15 according to Choe et al.2 and 2.40 eV for DePetris et al.3) but it is hampered by a barrier smaller than 0.95 eV1 (0.85 eV according to Choe et al.2 and 0.73 eV according to DePetris et al.3). Despite the small differences in the predicted energetics, the important message is that upon removal of one electron from acetonitrile a substantial rearrangement in the structure occurs and the [CH3CN]•+ is not the most stable of all the possible [C2H3N]•+ isomers. In DePetris et al.,3 an alternative way to generate the ketenimine ion [CH2CNH]•+ is suggested, that starts from electron impact dissociative ionization of butanenitrile CH3CH2CH2CN. The exclusive formation of the [CH2CNH]•+ isomer is proposed, on the basis of results of ion-molecule reactions with CO2: in fact, due to high endothermicity both charge exchange and proton transfer reactions are closed for [CH2CNH]•+ while proton transfer is energetically allowed for the [CH3CN]•+ isomer8. So, while it seems that the ketenimine isomer can be produced relatively pure, the same is not true for the unrearranged acetonitrile isomer, for which according to DePetris et al.3 “any ionization method other than perhaps threshold coincidence spectroscopy of cold neutral acetonitrile molecules will inevitably generate ionized ketenimine as a co-product”. The potential energy diagram for the interconversion of the various CH3CN isomers upon ionization is reported in Fig. 1, where the energies of the various structures are taken from NIST9, calculated values2 and our G4 calculations for butanenitrile fragmentation (to be discussed later). When the [CH3CN]•+ isomer has an internal energy higher than about 0.85 eV, it can isomerize into [CH2CNH]•+. With only a slightly higher threshold of 0.89 eV, the isomerization can proceed from [CH2CNH]•+ to the isonitrile isomer [CH2NCH]•+. In addition to isomerization, a similar excess energy of about 0.9 eV allows the population of the first electronic excited state 2A1 of the [CH3CN]•+ isomer (for which Tο=0.94±0.04 eV10,11,12). On such basis, when the ionization of ACS Paragon Plus 2 Environment

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CH3CN is carried out with energetic electrons and de-excitation mechanisms are not operative a mixture of [CH3CN]•+, [CH2CNH]•+ and [CH3CN]•+* (2A1) can be produced. The situation should change when starting from butanenitrile as precursor: its dissociative ionization into [CH2CNH]•+ plus C2H4 (red lines in Fig. 1) is overall exothermic by ~0.5 eV although hampered by a small barrier (see the next section for details). However, once [CH2CNH]•+ is produced, it requires to overcome a barrier of ~3 eV (i.e. the isomerization endothermicity plus the activation barrier) to isomerize into [CH3CN]•+. Hence it should be expected that the isomerization is less likely and an almost pure ketenimine isomer should be produced.

Fig. 1: Potential energy diagram for the interconversion of various [C2H3N]•+ isomers and alternative generation via dissociative ionization of butanenitrile CH3(CH2)2CN (in red). The zero of the energy scale is taken at the energy of the [CH2CNH]•+ isomer as calculated in Choe et al.2

In this work we have investigated the possibility to generate pure [CH3CN]•+ and [CH2CNH]•+ isomers using VUV photoionization via synchrotron radiation starting from either CH3CN or CH3CH2CH2CN neutral precursors and using the “reactive monitoring technique” with C2H4 to demonstrate that a selective generation of the different isomers is indeed possible. The choice of C2H4, the simplest unsaturated hydrocarbon, for the reactive monitoring technique relies also on some possible astrochemical implications of its reactivity with nitriles and ketenimine ions. Molecules containing C-N or C≡N bonds such as nitriles have been detected, both as stable neutrals as well as radicals or cations, in various regions of the interstellar medium (ISM), in comets and in the atmosphere of Saturn’s satellite Titan.13 The presence of nitriles in various celestial bodies deserves the special attention of astrobiologists, since they are prebiotic species (i.e. molecules that are key intermediates for the synthesis of biologically relevant substances): nitriles, as well as amines, are considered to be precursors in the extraterrestrial synthesis of aminoacids and nitrogen-containing nucleobases14. Of the various neutral isomers with empirical formula C2H3N, both CH3CN and CH2CNH, as well as CH3NC have been detected in the ISM. The most stable CH3CN isomer was first detected using radioastronomy more than 40 years ago, toward the massive star-forming regions Sagittarius A and B molecular clouds15, where its presence was finally ACS Paragon Plus 3 Environment


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confirmed more than 20 years later16. It has also been detected in dark clouds such as TMC-117, in hot cores18,19, in the photon dominated region of the Horsehead nebula20, around low-mass protostars21 and in the circumstellar shell of carbon-rich stars22. CH3CN is among the few complex molecules detected in external galaxies23,24 as well as in comets25. The ketenimine CH2CNH isomer has been detected in absorption toward the star-forming region Sagittarius B226,27. In the Solar System, nitrile chemistry is very important in the atmosphere of Titan, the largest moon of Saturn. Titan has attracted the interest of astrochemists and astrobiologists because of its dense and chemically rich atmosphere, that is believed to be similar to the primordial atmosphere of Earth.28,29 The Cassini spacecraft equipped with an ion and neutral mass spectrometer revealed that its atmosphere is one of the most complex in the solar system, containing heavy hydrocarbons and nitrogen-bearing compounds.30,31,32,33,34,35 In particular, CH3CN is one of the most abundant nitrogen-containing species, with a mixing ratio of 1.51x10-6 at 1025 km above Titan’s surface where

the •+

total

density

is

about

4.8x109

cm-3,36

hence

the

mixture

of

isomers

•+

[CH3CN] /[CH2CNH] can be formed by ionization of acetonitrile through UV photons or highly energetic particles. On the other hand, C2H4 is the second most abundant hydrocarbon after CH4, with a mixing ratio of 3.91x10-4 at 1050 km34,36, therefore the reaction of [CH3CN]•+/[CH2CNH]•+ isomers with C2H4 may be of relevance to shape the chemical composition of Titan’s upper atmosphere.

Experimental and theoretical methodologies Experimental set-up: The experiments were performed with the CERISES apparatus37,38, which was installed on the DESIRS beamline39 of the synchrotron radiation source SOLEIL in St. Aubin (France). The undulator-based beamline provides tunable radiation in the vacuum ultraviolet (VUV) range from about 5 to 40 eV. Photons at the desired wavelength are selected and scanned simultaneously with the undulator peak energy by a normal incidence monochromator equipped with a low dispersion uncoated SiC grating (200 grooves⋅mm-1) optimized to provide photon flux in the 1012-1013 photon⋅s-1 range with a moderate energy resolution in the 5–20 eV range. In the present experiments, the monochromator slits were set to deliver a photon energy bandwith, at a photon energy of 12 eV, of 17 meV (when the exit slits were set to 200 µm during measurements on the primary and fragment ions [C2H3N]•+ from acetonitrile and butanenitrile) and between 35 and 61 meV (when the exit slits were opened to 400 µm or 700 µm, during signal acquisition on the products of the reactions of [C2H3N]•+ with C2H4). Second order light from the undulator was removed with an upstream gas filter filled with Ar at about 0.2 mbar.40 The photon energy was calibrated with an accuracy better than 5 meV by measuring ns and ns’ resonance lines of Ar starting at 11.624 eV and 11.828 eV for n=4.41 ACS Paragon Plus 4 Environment

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The CERISES apparatus37,38 is a guided ion beam tandem mass spectrometer composed of two octopoles located between two quadrupole mass spectrometers in a Q1-O1-O2-Q2 configuration (where Q stands for quadrupole and O for octopole) that permits investigation of bi-molecular reactions of mass-selected ions, by measuring parent and product photoion yields from which absolute reaction cross sections and branching ratios as a function of collision energies and photon energy are derived. Neutral precursors (acetonitrile, acetonitrile-d3, butanenitrile) were introduced via a gas inlet in the source region where ions are generated via VUV photoionization and extracted by a field of 1 V/cm toward Q1. For the determination of photoionization thresholds and fragment appearance energies of butanenitrile the photon energy was scanned in steps of 10 meV, Q2 was used to mass-select the ions of interest, whereas Q1, O1, and O2 just served as ion guides. For the ion-molecule reaction experiments, the [C2H3N]•+ reagent ions were mass-selected using Q1 and focused into the O1 radio frequency guide towards a 4-cm long scattering cell filled with the target gas (C2H4 in our case) at room temperature. The absolute value of the neutral gas pressure was measured by a Baratron capacitance manometer and adjusted to a value about 1-2x10−4 mbar to ensure single-collision conditions. The reactant ion kinetic energy is defined by the dc potential difference between O1 and the center of the ion source. It can be varied between practically zero and 20 eV in the laboratory energy frame, with a typical distribution width of 0.5 eV full width at half maximum (FWHM), leading to a collision energy in the center of mass (CM) frame up to 8 eV with a width of about 0.2 eV (FWHM). Reactant and product ions were confined by the radio frequency guiding field of O1, guided by O2, mass selected in the Q2 mass filter, and finally detected by a multi-channel plate operating in the counting mode. For the reactive-monitoring experiments, Q1 was set to mass-select the [C2H3N]•+ parent, Q2 was set to the mass-to-charge ratio of the parent or product ions and the photon energy was scanned in steps of 20 meV, while keeping all the other experimental parameters (neutral gas pressure, collision energy, focusing ion optics potentials, etc…) fixed. During the measurements of the ion yields, also photon fluxes were simultaneously recorded by photoemission currents from a gold grid and the raw data for the measured ion yields were accordingly corrected for the photon flux of the beamline as a function of the photon energy.42 From the ratio of product to parent ion yields and the C2H4 target pressure measurement, the absolute reaction cross sections is derived following standard procedures.37,38 Finally, the precision on the appearance energy determination (A.E.) is related to the precision on the calibrated photon energies mentioned above and the various resolutions used to record the ion yields. Some experiments were performed outside SOLEIL: in particular, electron ionization mass spectra of butanenitrile and (4,4,4-2H3)butanenitrile were measured on the ZAB2-SEQ mass spectrometer with the following ion source conditions: temperature 200°C, electron energy 70 eV; ACS Paragon Plus 5 Environment


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electron current 50µA. (4,4,4-2H3)Butanenitrile was prepared according to a procedure described previously.

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0.69 g (14 mmol) of NaCN (Aldrich) was dried in vacuum and mixed with 2.6 ml of

ethylene glycol (Aldrich). Subsequently, 1 g (7.9 mmol) of 1-bromo(3,3,3-2H3)propane (Aldrich) was added to the suspension which was then warmed, while being stirred continually, up to 75 °C for approx. 24 hours. Product of the reaction, (4,4,4-2H3)butanenitrile, was distilled from the reaction mixture at the pressure of 6 mbar, at laboratory temperature (approx. 23 °C) and condensed in a flask immersed in liquid nitrogen.

Theoretical methods: Electronic structure calculations to interpret the experimental results were performed with Gaussian 09 suite of programs.44 Potential energy surfaces (PES) were mapped at B3LYP/cc-pVTZ level of theory.45,46,47 A composite method denoted as G448 was used to obtain more accurate energies. This method uses B3LYP/6-31G(2df,p) optimized geometries in combination with a set of single point correlation energy calculations, Hartree-Fock (HF) energy limit and several empirical corrections to get total energies that are effectively at the CCSD(T,full)/G3LargeXP + HF limit level of theory. The average absolute deviation from experiment is 0.83 kcal⋅mol-1 (3.5 kJ⋅mol-1) as assessed on the G3/05 test set of 454 experimental energies.48

Results and discussion Ionization of butanenitrile and generation of the ketenimine [CH2CNH]•+ radical cation The VUV photoionization of butanenitrile CH3CH2CH2CN was not previously reported in the literature. Hence, prior to the ion-molecule reaction experiments, we have measured the ion yields for the production of the radical cation [C4H7N]•+ and of the primary dissociation channel to give [CH2CNH]•+ as a function of the photon energy. It is known that the ionization of alkylnitriles produces a dramatic change in their geometry involving the lengthening of the RH2C−CH2CN bond (in butanenitrile it is about 20%) which lowers the barrier for H shift to give the [RHC−CH2CNH]+ isomer.49 The latter can subsequently easily fragment to yield [CH2CNH]•+ plus a neutral alkene, thus making the generation of molecular ions of alkyl nitriles in a mass spectrometer quite a challenge: in the NIST database, the most abundant peak in the MS spectrum of CH3CH2CH2CN is at m/z 41 and the intensity of the molecular ion peak is about 0.1%. For a detailed experimental and theoretical study on the photoionization of various C5 alkylnitrile isomers see Mayer et al.49 Photoionization and photodissociation yields for the parent ion at m/z 69 and for the fragment at m/z 41 are shown in Fig. 2 as a function of the photon energy. The spectra represent the average of two or three individual scans with the vertical error bars representing the one standard deviation in the averaging. In Fig.2 an expanded inset is shown to illustrate how the ionization threshold is ACS Paragon Plus 6 Environment

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derived. A linear fit just before the onset of signal is used to determine the average baseline, a similar linear fit is employed for the rising ion signal in the linear regime and the experimental ionization threshold is defined as the intersection of these lines. The appearance energy (A.E.) determined in this way is 11.29±0.05 eV, however deriving a value for the adiabatic ionization energy IEa of butanenitrile from the measured photoionization threshold requires some attention, since the experimental data are convoluted by the distribution of internal energies of the neutral at the room temperature (≈ 300 K) of the source region of the CERISES set-up. Hence the ion signal may be present even below the adiabatic ionization threshold due to ionization of rotationally or vibrationally excited neutrals, as shown by the slow rise of the ion yield at the onset of signal. A mean thermal energy of about 0.12 eV can be estimated for neutral CH3CH2CH2CN at 300K, which is in agreement with the appearance of non zero ion signal about 0.1 eV lower than the A.E. value estimated as detailed above. We note that the uncertainty of ±0.05 eV in the determination of the A.E. is an estimate that takes into account the inherent noise level in the experimental data and the photon energy bandwith, i.e. about 17 meV at 12 eV in this case. Fig. 2: Photoionization and dissociative photoionization efficiency curves in the threshold

region

for

butanenitrile

CH3CH2CH2CN. Black data refer to [CH3CH2CH2CN]•+ parent ions while blue data refer to [CH2CNH]•+ fragments. The red lines show the linear fits employed to evaluate the appearance energies (A.E.) for parent and fragment ions.

Our experimental ionization energy can be compared with experimental values found in the literature of 11.67±0.0550 and 11.2-11.3 eV51: while the former value is stated to be an adiabatic IE measured using a photoionization method, the latter one is not directly reported in the paper but it can be estimated from the onset of the HeI Photoelectron spectrum reported in Fig. 4 of Ref.51 and therefore it should also correspond to the adiabatic value. In addition, Watanabe et al.50 gives a theoretical value of 11.34 eV for IEa at the G3//B3-LYP level of theory. Our calculations at the G4 level of theory are in perfect agreement with such previous theoretical results as well as with our experimental results, giving a value of 11.33 eV for IEa. We note that calculations reported in Watanabe et al.50 give a difference between vertical and adiabatic ionization energies IEv - IEa =0.74 eV that is explained by the mentioned change in geometry between the neutral and the cation



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and by the sudden nature of the photoionization event. In fact, due to the above mentioned lengthening of the RH2C−CH2CN bond, a vertical transition according to the Franck-Condon principle will most likely populate a certain vibro-rotationally excited state of the ion, thus making difficult the determination of adiabatic ionization energies via photoionization or photoelectron spectroscopy experiments (see refs.52,53 for models that include calculations of Franck-Condon factors in the determination of appearance energy thresholds). According to our experimental results, the A.E. for the dissociative photoionization channel to give [CH2CNH]•+ is at 11.52±0.05 eV, i.e. about 0.2 eV higher than the A.E. of the molecular ion, thus demonstrating that dissociation is hampered by a small barrier. We have explored theoretically, at the G4 level of theory, the fragmentation mechanism of ionized butanenitrile with special reference to process (2) leading to the ketenimine ion, and results are reported in Fig. 3: [CH3CH2CH2CN]•+ →[CH2CNH]•+ + C2H4

(2)

The dissociation proceeds from the ionized nitrile (structure A1) via rotation around the elongated C-C bond and subsequent H shift from the methyl group to the nitrogen atom to yield structure B (with a ZPVE corrected relative energy of -1.07 eV, see Fig. 3) in a process that is exothermic by 1.08 eV but goes via a TSA2/B about 0.29 eV higher in energy than A1. Structure B is a weakly bound complex in which an ethylene molecule is proton bound to a CH2CN radical and dissociation from such structure into [CH2CNH]•+ plus C2H4 requires about 0.59 eV, thus making the overall process (2) exothermic by 0.49 eV. We also note that several isomers of ionized butane nitrile, namely the CH3•CHCH2CNH+ (C), CH3CH2CHCNH+• (D) and CH3CH2CHCHN+• (E) have already been calculated49 together with the transition states for their formation from A1 (data not shown in Fig. 3). Our G4 relative energies (relative to A1) of these transition states, i.e. TSA1/C, TSA1/E and TSE/D, are 0.64, 0.70 and 0.85 eV, respectively, which indicates that a possible role of isomers

C, D and E in reaction (2) is marginal. The calculated energetics of the fragmentation process (2) are in good agreement with the experimental results: in fact the experimental A.E.([CH2CNH]•+)= 11.52±0.05 eV is consistent with an adiabatic IEa=11.29±0.05 eV for CH3CH2CH2CN plus an energy barrier for isomerization of 0.29 eV. However some questions may arise on the amount of energy retained by the ketenimine ion. In fact, even at the photon energy threshold for dissociative photoionization, the [CH2CNH]•+ fragment is formed with a certain amount of internal energy that can be as high as 0.78 eV (i.e. the difference in energy between transition state TSA2/B and fragmentation products CH2CNH•+ plus C2H4, see Fig. 3) if we assume that all of the available energy is retained as internal energy of the ion. However the amount of internal excitation is well below the energy required for isomerization into [CH3CN]•+ or [CH2NCH]•+ that amounts to about 3 eV. ACS Paragon Plus 8 Environment

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Fig. 3: Fragmentation pathway for CH3CH2CH2CN•+ calculated at G4 level of theory. The ZPVE corrected relative energies at 0 K are given in eV.

In order to look at fragmentation of the butanenitrile radical cation in more detail, the electron ionization mass spectra of isotopically labeled butanenitrile were measured. In the mass spectrum of CD3CH2CH2CN (Fig. 4), an H/D scrambling is observed upon the fragmentation reaction (2) as not only the [CH2CND]•+ ion at m/z 42 corresponding to the regiospecific elimination of C2H2D2, but also its non-deuterated analogue, [CH2CNH]•+, at m/z 41 is present. A possible explanation for this scrambling can be found in the reaction scheme in Fig. 3. Before a molecule of ethylene is eliminated, complex B can rearrange back to A1. Due to a symmetry of B, the C-C bond can be reformed by connecting the C(2) to any of the two C atoms of the ethylene molecule in B. By repeating the back and forth transitions through TSA1/B several times, the hydrogen atoms in positions 3- and 4- can be completely scrambled. The ratio of [m/z 42]/[m/z 41] corresponding to such scrambling in CD3CH2CH2CN would be 1.5 (one atom of the ensemble of two H and three D atoms is transferred to N atom). The experimentally observed ratio is 1.45, not too far from this statistical value.

Fig.4: EI (70 eV) mass spectra of butanenitrile and its 4-d3 analogue.



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It is worth noting that a competitive loss of CH2CN• radical leading to ethyl cation proceeds without any H/D scrambling as only the CH2D3+ ion at m/z 32 was observed in the mass spectrum of CD3CH2CH2CN (Fig. 4). It indicates that the CH2CN• radical is eliminated from A1 or A2 by a barrier-less direct cleavage of the elongated central C-C bond. Nevertheless, corroboration of this hypothesis would require a thorough study of the reaction dynamics, which is beyond the scope of this paper.

Generation of the acetonitrile [CH3CN]•+ radical cation VUV photoionization of CH3CN has been amply studied:54,55,56 the A.E. to generate the ground electronic state 2E of the [CH3CN]•+ cation is at 12.20±0.02 eV, while the first excited state 2A1 opens up at 13.1 eV. As already mentioned, several experimental evidences exist that the [CH3CN]•+ cation is not the lowest energy isomer but can isomerize into the more stable ketenimine [CH2CNH]•+ ion provided enough energy is given to overcome the isomerization barrier.1,2,3,7 Literature data on the energetics of the isomerization process (1) is sparse. Mair et al.1 performed preliminary DFT calculations at the B3LYP/6-31G∗∗ level of theory to explain relative abundances of reaction products resulting from the impact of acetonitrile molecular ions on a hydrocarboncovered stainless steel surface as a function of the collision energy. They found the ketenimine cation to be more stable than [CH3CN]•+ by 2.13 eV and they gave an upper limit for the height of the isomerization barrier of 0.95 eV. In a subsequent paper, Choe et al.2 used UB3LYP density functional level of theory with the 6-311++G(3df,3pd) basis set to get a relative energy of 2.15 eV, inclusive of zero-point vibrational energies, of [CH2CNH]•+ with respect to [CH3CN]•+. The isomerization occurs by a 1,3-H shift and the isomerization barrier is 0.85 eV. The process has been reinvestigated by de Petris et al.3 at the G3 quantum chemistry composite method with optimized MP2/6-31+G(d) geometries to get results that are in agreement with previous works: the ketenimine cation is 2.40 eV lower in energy than [CH3CN]•+ and the isomerization requires a 1,3-hydrogen shift with a TS lying 0.73 eV above reagents. In order to obtain accurate relative energies and enthalpies, G4 method48 was used in this paper. The G4 exothermicity (in terms of ZPVE corrected energies at 0K) for the isomerization reaction (1) is 2.28 eV and the respective isomerization barrier is 0.84 eV. Generally, the G4 energy values are to be considered as more accurate than those obtained using DFT methods. A comparison with G3 data published by de Petris et al.3 cannot be made as it is not clear from this paper what their exothermicity and barrier height values refer to (ZPVE corrected energies at 0K ?). The isomerization process can in principle lead also to the formation of isonitrile isomers: with a higher threshold of 0.89 eV (according to UB3LYP density functional calculations2) from [CH2CNH]•+ the isomerization can proceed to the isonitrile isomer [CH2NCH]•+ and even to ACS Paragon Plus 10 Environment

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[CH3NC]•+, but the formation of the latter requires to overcome a TS lying 2.05 eV above the energy of [CH3CN]•+ and therefore it is very unlikely that it will be populated in our experiments.

Reaction monitoring with synchrotron radiation: the reactivity of [CH2CNH]•+ and [CH3CN]•+ with C2H4 The “reaction monitoring technique” has been developed and applied by D. Schroeder and coworkers and the idea behind the method is to sample the changes in ion reactivity induced upon ionization of a precursor species with photons of variable energies.57,58,59,60,61,62,63 This technique may allow users to identify electronic states or isomers of the parent species and can also allow the determination of activation barriers of chemical reactions. Using single-ion monitoring and a suitable reagent, the amount of a specific reaction product being formed from the rearranged ion is recorded as a function of the energy of the ionizing photons. It is important to realize that, in such an experiment, all parameters (mass-selection, collision energy, pressure of the reagent etc.) are kept constant, with the exception of the photon energy. In the case of [CH3CN]•+/[CH2CNH]•+ isomers, when energy selected photons are used to ionize acetonitrile and the photon energy is kept below the threshold for isomerization, only the [CH3CN]•+ isomer will be generated. Conversely, when energy selected photons are used to generate [C2H3N]•+ by dissociative photoionization of butanenitrile and the energy is smaller than that required for isomerization, only the [CH2CNH]•+ will be formed. To demonstrate the idea we have chosen C2H4 as a reagent because the reaction energetics is different for the two nitrile isomers. In particular, the reaction between ketenimine [CH2CNH]•+ and C2H4 is endothermic both for proton transfer (PT) and charge transfer (CT), while the reaction between [CH3CN]•+ and C2H4 is exothermic both for CT and PT. A comparison between the reaction enthalpies (∆rH°) estimated using experimental values for the standard heat of formations of reagents and products (∆fH°) and calculated ∆rH° at the G4 level of theory for the CT and PT channels from the two isomers is reported in Table 1. To our best knowledge, there are no experimental ∆fH° values for [CH2CNH]•+ and CH2CNH available in the literature, hence the entries relative to the experimental ∆rH° for the ketenimine isomer are missing in the Table. In the case of PT, two different experimental ∆rH° are reported, to account for the two different values existing for ∆fH°(CH2CN), as detailed in the Table caption. Hence, in the presence of a pure [CH2CNH]•+ ion beam and at a collision energy smaller than the endothermicity, no products should be observed, while in the case of a pure [CH3CN]•+ both CT and PT will be energetically allowed.



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experimental ∆rH° (eV)

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calculated ∆rH° (eV)

[CH2CNH]•+ + C2H4 reactants → CH2CN + C2H5+

PT

not available

+0.83

→ CH2CNH + C2H4+ CT

not available

+1.79

[CH3CN]•+ + C2H4 reactants → CH2CN + C2H5+

PT

-1.54a / -1.64b

-1.45

→ CH3CN + C2H4+

CT

-1.69

-1.68

Table 1: Comparison between experimental and calculated reaction enthalpies ∆rH° at 298 K for CT and PT reactions of [CH2CNH]•+ and [CH3CN]•+ isomers with C2H4. Experimental experimentally determined heat of formations

∆rH° are obtained from

∆fH° of reagents and products, while calculated ones refer

to our G4 calculations. a

Value calculated using ∆fH°(CH2CN)= 2.62±0.04 eV64, in agreement with the value of 2.62±0.09 eV65

b

Value calculated using ∆fH°(CH2CN)= 2.52±0.13 eV66

We note that other potential reactive channels are adduct formation (stabilized by secondary collisions) and bond-forming reactions with elimination of atomic or molecular hydrogen from the adduct: [CH2CNH]•+ + C2H4 → [C3H7CN] •+ •+

(3)

“

“

“

→ [C3H6CN]

+H

“

“

“

→ [C3H5CN] •+ +H2

(4) (5)

Using butanenitrile as a precursor for [CH2CNH]•+ ions via dissociative photoionization, the absolute value of the cross section for the PT channel was measured at a fixed collision energy Ecoll= 0.11 eV in the CM frame as a function of the photon energy (Ephoton), and results are shown in Fig. 5. We note in passing that the absolute value of the cross section was measured at Ephoton=12.49 and 15.49 eV (blue points in Fig. 5) and the values at the other photon energies were rescaled accordingly. As already mentioned, due to the exothermicity of process (2), the photo-generated ion at m/z 41 is formed with a certain amount of internal energy, that is anyway well below the energy required for isomerization into [CH3CN]•+ or [CH2NCH]•+, thus ensuring isomer-selected reactivity. In Fig. 5, the vertical black dashed line indicates the threshold region for isomerization into [CH3CN]•+ and the green line the region at which channel (3) becomes exothermic (according to our G4 value in Table 1). Such values have been estimated by combining our experimental A.E. of ACS Paragon Plus 12 Environment

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[CH2CNH]•+ from butanenitrile (11.52±0.05eV) with our theoretical G4 values for isomerization and reaction enthalpies. The horizontal black double-headed arrow indicates the min-max amount of internal energy available for [CH2CNH]•+. With the exception of the region around threshold, where signal is anyway small, the PT cross section is non zero even below threshold (vertical green bar) due to internal excitation of [CH2CNH]•+ and the cross section increases with the photon energy due to the increasingly larger amount of internal energy available to the reagent ion. Fig. 5: Absolute cross section for the PT reaction of [CH2CNH]•+ with C2H4 as a function of the photon energy at fixed Ecoll=0.11 in the CM. The blue points are the photon energies at which the absolute value of the cross section was measured. The vertical green bar defines the energy at which PT reaction becomes energetically accessible, the black dashed line is the threshold region for reagent isomerization and the horizontal black double-headed arrow indicates the min-max amount of internal energy available for [CH2CNH]•+.

The PT cross section was also measured as a function of the collision energy (Fig. 6) at two different photon energy values (12.49 and 15.49 eV, respectively below and above the threshold for isomerization to [CH3CN]•+) and in both cases it is found to increase with the collision energy.

Fig. 6: Absolute cross section for the PT reaction of [CH2CNH]•+ with C2H4 as a function of the collision energy at two different photon energies.

We highlight the fact that the CT channel is never observed, even at the highest photon energies, where the reactive system has enough internal energy to make this channel energetically accessible.



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Using acetonitrile as a precursor we have measured the absolute value of the cross section for ions at m/z 41 reacting with C2H4 at fixed collision energy of 0.11 eV in the CM frame and as a function of the photon energy (Ephoton), and results are reported in Fig. 7. Red points correspond to CT reactions (indicated as σCT hereafter) while blue points are from PT. We note that deuterated acetonitrile CD3CN has been used to avoid measuring at m/z 41 where contamination from previously used CH3CH2CH2CN would appear, hence deuteron transfer (DT) rather than PT cross sections have been measured, indicated as σDT hereafter. However, with the photon energy resolution used throughout the experiments the photoion yield curve for CH3CN and CD3CN are identical, as shown in Fig. 8. In Fig. 7, the vertical green dashed line indicates the threshold for isomerization into [CD2CND]•+ (according to our G4 values for the fully hydrogenated system and assuming no H/D isotope effect in the isomerization energies), while the blue dashed line represents the opening of the first excited state (2A1) of CH3CN•+ 10,11,12. At photon energies below the isomerization threshold, σCT and σDT should correspond to products from the reaction of [CD3CN]•+ isomers. The absolute value of the cross sections σDT and

σCT were measured at Ephoton=12.39 eV (point with the error bar in Fig. 7) and the values at the other photon energies were rescaled accordingly. As expected from the exothermicity values, both DT and CT channels are observed starting from the A.E. threshold of [CD3CN]•+ but reactivity is dominated by CT: at Ephoton=12.39 eV σCT is about 60(±30%) Å2 while σDT is 0.8(±30%) Å2. The CT cross section (open red points) is rather independent of the photon energy up to about 0.7-0.8 eV above threshold, where it starts decreasing. This corresponds quite well with the opening of the isomerization of [CD3CN]•+ into [CD2CND]•+ that is non-reactive for CT. Hence the cross section behavior of the CT channel is an indirect evidence that photoionization of CH3CN below 12.8 eV gives only the [CH3CN]•+ isomer. Fig. 7: Absolute cross sections for CT (red) and DT reactions (blue) of [CD3CN]•+ with C2H4 as a function of the photon energy at fixed Ecoll=0.11 eV in the CM. The vertical green dashed line indicates the calculated threshold

for

isomerization

into

•+

[CH2CNH] and the blue dashed line the opening of the first excited state (2A1) of [CH3CN]•+.


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The PT channel (blue filled points) has a small and constant yield only up to about 0.3 eV above threshold and then increases linearly with the photon energy, showing a discontinuity in the energy region corresponding to the opening of isomerization into [CD2CND]•+ and of the first excited state 2

A1 of [CD3CN]•+. The increase of σDT with the photon energy is an indication that, despite being

exothermic, DT from non-excited [CD3CN]•+ is disfavoured and it becomes more probable either when the [CD3CN]•+ ion has some internal excitation (both vibrational and electronic) or when it isomerizes into internally excited [CD2CND]•+

Fig. 8: Ion yields for photoionization of CH3CN (red points) and CD3CN (black points). The vertical green dashed line indicates the calculated threshold for isomerization into [CH2CNH]•+ and the blue dashed line the opening of the first excited state (2A1) of [CH3CN]•+ while the black arrow is the A.E. of its ground state.

Formation of new C-C bond bearing species: When working with acetonitrile as neutral precursor, the formation of products at m/z 70 and 68 is observed and attributed to species corresponding to the loss of one or two D atoms from an adduct ion of molecular formula [CD3CNC2H4]•+, as shown in the mass spectrum of Fig. 9 recorded at a pressure of C2H4 ≈1.1x10-4 mbar in the scattering cell and at a photon energy of 12.39 eV. We note that the very small amount of signal detected at m/z 72 is due to adduct stabilization by secondary collisions. The relative cross sections for product channels [CD2CNC2H4]•+ and [CDCNC2H4]•+ have been measured as a function of the photon energy at a fixed collision energy Ecoll = 0.16 eV and results are shown in Fig. 10.



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Fig. 9: MS spectra in the region of D and D2

loss

[CD3CNC2H4]

products •+

adduct,

from recorded

the at

Ecoll=0.16 eV in the CM and at a Ephoton=12.39 eV. Pressure of C2H4 in the scattering cell was ≈1.1x10-4 mbar.

The two products have similar behaviors with Ephoton: they have small but constant cross sections up to about 0.5 eV above threshold, and then the yields decrease, in coincidence with the energy threshold for reagent isomerization (vertical green line in Fig. 10). Interestingly, products [CH2CNC2H4]•+ and [CHCNC2H4]•+ are not observed when the ketenimine isomer [CH2CNH]•+ is used as reactant in analogous reaction monitoring experiments with butanenitrile as neutral precursor. This finding is consistent with the behavior of cross sections of Fig. 10 at high photon energies: at Ephoton higher than the isomerization threshold, a fraction of the ion beam is composed of the non-reactive isomer, hence the overall reaction probability is decreased. Fig. 10: Relative cross section for product channels [CD2CNC2H4]•+ and [CDCNC2H4]•+ from the reaction of [CH3CN]•+ with C2H4 recorded as a function of the photon energy at a fixed Ecoll=0.16 eV in the CM. The green and blue dashed lines are the same as in Figs. 7 and 8.

Theoretical calculations at the G4 level of theory were performed to shed some lights on the different C-C bond-forming reactivity of the acetonitrile and ketenimine radical cations. The energies and structures of some relevant points on the potential energy surfaces are summarized in Figs. 11, 12 and 13. For both tautomers, the addition of an ethylene molecule to the radical cation leads to the formation of covalently bound adducts. In the acetonitrile radical cation (Fig.11) the attacks can occur between the nitrogen atom and one of the two olefinic carbons, leading to structure E. Our electronic structure calculations (at B3LYP/cc-pVTZ level of theory) showed that 80% of the Mulliken spin density in the acetonitrile radical cation is located at CN group and 62% ACS Paragon Plus 16 Environment

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is on N atom whereas the charge density is distributed much more uniformly all over the ion. From this point of view, the formation of E is the most probable reaction channel. Due to a large exothermicity of the formation of E, this ion can easily rearrange to structures F, G, H and I as the relative energies of all the relevant transition states are significantly lower than the energy of reactants. In the case of the ketenimine radical cation (Fig. 12) the radical site in the cation (about 70% of spin density is located at CH2 group) can attack olefinic carbons to generate ion N that can form subsequently the four- or five-membered ring structures O, and M. Alternatively, the long range attraction between the ion and the neutral drives the complex in a weakly “proton bound” structure B in which the H atom of the NH+ group of the ketenimine ion interacts with the π cloud of the ethylene molecule. Complex B can rearrange via complex J to covalently bound structures K, L and M. However, transition states along this pathway lie close to or above the energy of reactants.

Fig. 11: PES scheme for formation of adducts of C2H4 with the [CH3CN]•+ radical cation and fragmentation by H/H2 losses (in red). ZPVE corrected relative energies, calculated at G4 level of theory, are in kJ⋅mol-1.

17

ACS Paragon Plus Environment


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Fig. 12: PES scheme for formation of adducts of C2H4 with the [CH2CNH]•+ radical cation. ZPVE corrected relative energies, calculated at G4 level of theory, are given in kJ⋅mol-1.

Our calculations thus show that both tautomers can form stable, covalently bound complexes via overall exothermic pathways. Subsequently, we have explored some of the possible fragmentation channels leading to H and H2 losses (see red lines in Figs. 11 and 13). It would be beyond the scope of the paper to present a full potential energy surface including H/H2 losses from all the possible geometries of adducts reported in Fig. 11 and 12, therefore we chose to compare fragmentations from the most stable covalently bound structures that are directly formed upon C2H4 attack on both tautomers, i.e. geometry E in the case of [CH3CN]•+ and N for the other isomer. It turns out that, while fragmentations from structure E are overall exothermic (with respect to the entrance channel [CH3CN]•+ plus C2H4), in the case of structure N all the fragmentations channels but one are endothermic (with respect to the entrance channel [CH2CNH]•+ plus C2H4). In fact, the channel corresponding to 1,2-H2 elimination from N leading to CH2=CH-CHCNH•+ is overall exothermic by 70 kJ⋅mol-1 (Fig. 13). However, we were unable to locate a transition state: despite several attempts, including two-dimensional PES scans, the system did not evolve to the requested channel. Although our failed attempts are not a definite proof for the non existence of the TS, we can argue that if it exists, it has to be a tight, conformationally demanding TS, most probably lying above the 18


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energy of reactants. It can be judged from the fact that the only pathway found that leads from N to CH2=CH-CHCNH•+ goes via two consecutive 1,2-H migration steps (structures P and Q and corresponding transition states at 62 and 2 kJ mol-1), followed by the concerted 1,2-H2 elimination from Q that requires passing through a tight transition state lying 138 kJ mol-1 above the reactants. The important finding from our calculations is that the examined pathways leading to H or H2 elimination from adducts are endothermic (relative to reactants) when starting from [CH2CNH]•+ whereas they are exothermic from the [CH3CN]•+ tautomer. As a consequence, the probability of formation of products [CH2CNC2H4]•+ and [CHCNC2H4]•+ will be higher for acetonitrile than for ketenimine radical cations. In fact, in the latter case, due to the presence of high energy barriers for H and H2 loss from the adduct structure N, the complex will have a higher chance to decompose back to the reactants rather than to cleave off H or H2. In conclusion, although our calculations for the PESs of H and H2 losses from the reaction of both tautomers with ethylene are far from being exhaustive, they provide an explanation for the experimental finding that only the [CH3CN]•+ isomer leads to C-C bond forming products [CH2CNC2H4]•+ and [CHCNC2H4]•+ upon reaction with ethylene.

19



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Fig. 13: PES scheme for formation of adducts of C2H4 to [CH2CNH]•+ and fragmentation by H/H2 losses (in red). ZPVE corrected relative energies, calculated at G4 level of theory, are in kJ⋅mol-1.

Conclusions Experimental and theoretical studies have demonstrated the possibility to selectively generate the different tautomer radical cations acetonitrile [CH3CN]•+ and ketenimine [CH2CNH]•+ by photoionization experiments from different neutral precursors (acetonitrile and butanenitrile) at selected photon energies in the 10-15 eV energy range. Theoretical calculations at the G4 level of theory have confirmed previous findings1,2,3 that isomerization from [CH3CN]•+ to [CH2CNH]•+ is exothermic by 2.28 eV, but it is hampered by an energy barrier of 0.84 eV. In addition, a slightly higher excess energy of about 0.9 eV allows the population of the first electronic excited state 2A1 of the [CH3CN]•+ isomer. By exploiting the different reactivity of the two tautomers with ethylene (that it is known to be unreactive with [CH2CNH]•+ but to give exothermic charge exchange and 20


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proton transfer reactions with [CH3CN]•+) using the reaction monitoring technique in a guided ion beam tandem mass spectrometer, we have demonstrated that photoionization of CH3CN below 12.8 eV produces solely the [CH3CN]•+ isomer, and the [CH2CNH]•+ tautomer will start being populated at photon energies above the isomerization threshold. It has been proposed that, using the TESICO (Threshold Electron Secondary Ion COincidence spectroscopy) technique,67 the ground and first electronically excited states of [CH3CN]•+ ion are selectively generated at different photon energies. However, our study demonstrates that, at the energy required to populate the excited 2A1 state, the isomerization channel into [CH2CNH]•+ is also open, thus posing some uncertainty in the possibility to generate a pure beam of electronically excited [CH3CN]•+. When reacting [CD3CN]•+ with C2H4, in addition to charge transfer and proton transfer products, minor reactive channels are observed corresponding to loss of D or D2 molecule from adduct ions of molecular formula [CD3CNC2H4]•+, but similar products are not observed when the ketenimine isomer [CH2CNH]•+ is used as reactant. G4 level calculations have shown that, although both tautomers can form stable, covalently bound complexes via overall exothermic pathways, the elimination of H atoms or H2 molecules from such adducts is exothermic for [CH3CN]•+ but mostly endothermic for the other isomer, thus favoring the decomposition of the adduct back to the reactants rather than the formation of new C-C bond bearing products. In conclusion, we note that the different reactivity of [CH3CN]•+ and [CH2CNH]•+ isomers with ethylene may have some astrochemical implications, with special reference to Titan, where cyano compounds are known to play a prominent role in shaping the chemical composition of the satellite’s upper atmosphere. We note in particular that a peak at m/z 68 in the Cassini ion mass spectrum is detected with an observed density of 3 cm-3

29,33

and it is assigned to an ion of

molecular formula C4H5NH+, i.e. the same one that is observed in this paper upon H loss reaction of [CH3CN]•+ with C2H4. Two reactions have been proposed for the synthesis of such ion in Titan’s ionosphere33 : either radiative association reaction of C3H5+ with HCN or proton transfer reactions from C2H5+ and HCNH+ ions to C4H5N neutrals. Our findings implicate that an additional mechanism should be considered for the formation of the C4H5NH+ species: the reaction of [C2H3N]•+ ions with C2H4, a reaction that is isomer-selective since it occurs only from [CH3CN]•+ and not from [CH2CNH]•+.

21



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Acknowledgements. This work was supported by the Department of Physics (UniTN). The synchrotron measurements at SOLEIL were supported by the FP7 CALIPSO Transnational Access program and we thank the DESIRS beamline manager Dr. L. Nahon and his team for assistance during the measurements and the technical staff of SOLEIL for running the facility. C.A. acknowledges the synchrotron SOLEIL for the support to the associated CERISES setup since 2008. D.A., J.Z. and M.P. acknowledge support from the Czech Academy of Science and the Italian Consiglio Nazionale delle Ricerche via the bilateral scientific cooperation agreement AVCR-CNR 2010-2012. M.P., J.Z. and V.K. acknowledge support from the Ministry of Education Youth and Sports of the Czech Republic (grant No. LD14024) and the Czech Science Foundation (grant No.14-19693S).

TOC Graphic

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Fig. 1: Potential energy diagram for the interconversion of various [C2H3N]•+ isomers and alternative generation via dissociative ionization of butanenitrile CH3(CH2)2CN (in red). The zero of the energy scale is taken at the energy of the [CH2CNH]•+ isomer as calculated in Choe et al.2 54x36mm (300 x 300 DPI)



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Fig. 2: Photoionization and dissociative photoionization efficiency curves in the threshold region for butanenitrile CH3CH2CH2CN. Black data refer to [CH3CH2CH2CN]•+ parent ions while blue data refer to [CH2CNH]•+ fragments. The red lines show the linear fits employed to evaluate the appearance energies (A.E.) for parent and fragment ions. 55x37mm (300 x 300 DPI)


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Fig. 3: Fragmentation pathway for CH3CH2CH2CN•+ calculated at G4 level of theory. The ZPVE corrected relative energies at 0 K are given in eV. 46x25mm (300 x 300 DPI)



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Fig. 4: EI (70 eV) mass spectra of butanenitrile and its 4-d3 analogue 62x46mm (300 x 300 DPI)


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Fig. 5: Absolute cross section for the PT reaction of [CH2CNH]•+ with C2H4 as a function of the photon energy at fixed Ecoll=0.11 in the CM. The blue points are the photon energies at which the absolute value of the cross section was measured. The vertical green bar defines the energy at which PT reaction becomes energetically accessible, the black dashed line is the threshold region for reagent isomerization and the horizontal black double-headed arrow indicates the min-max amount of internal energy available for [CH2CNH]•+ 57x39mm (300 x 300 DPI)



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Fig. 6: Absolute cross section for the PT reaction of [CH2CNH]•+ with C2H4 as a function of the collision energy at two different photon energies. 60x42mm (300 x 300 DPI)


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Fig. 7: Absolute cross sections for CT (red) and DT reactions (blue) of [CD3CN]•+ with C2H4 as a function of the photon energy at fixed Ecoll=0.11 eV in the CM. The vertical green dashed line indicates the calculated threshold for isomerization into [CH2CNH]•+ and the blue dashed line the opening of the first excited state (2A1) of [CH3CN]•+ 58x40mm (300 x 300 DPI)



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Fig. 8: Ion yields for photoionization of CH3CN (red points) and CD3CN (black points). The vertical green dashed line indicates the calculated threshold for isomerization into [CH2CNH]•+ and the blue dashed line the opening of the first excited state (2A1) of [CH3CN]•+ while the black arrow is the A.E. of its ground state. 58x40mm (300 x 300 DPI)


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Fig. 9: MS spectra in the region of D and D2 loss products from the [CD3CNC2H4]•+ adduct, recorded at Ecoll=0.16 eV in the CM and at a Ephoton=12.39 eV. Pressure of C2H4 in the scattering cell was ∽1.1x10-4 mbar. 85x58mm (300 x 300 DPI)



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Fig. 10: Relative cross section for product channels [CD2CNC2H4]•+ and [CDCNC2H4]•+ from the reaction of [CH3CN]•+ with C2H4 recorded as a function of the photon energy at a fixed Ecoll=0.16 eV in the CM. The green and blue dashed lines are the same as in Figs. 7 and 8 55x36mm (300 x 300 DPI)


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Fig. 11: PES scheme for formation of adducts of C2H4 with the [CH3CN]•+ radical cation and fragmentation by H/H2 losses (in red). ZPVE corrected relative energies, calculated at G4 level of theory, are in kJ/mol-1. 124x86mm (300 x 300 DPI)



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Fig. 12: PES scheme for formation of adducts of C2H4 with the [CH2CNH]•+ radical cation. ZPVE corrected relative energies, calculated at G4 level of theory, are given in kJ mol-1 103x60mm (300 x 300 DPI)


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Fig. 13: PES scheme for formation of adducts of C2H4 to [CH2CNH]•+ and fragmentation by H/H2 losses (in red). ZPVE corrected relative energies, calculated at G4 level of theory, are in kJ mol-1 135x103mm (300 x 300 DPI)



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Table of Content Graphic 57x39mm (300 x 300 DPI)


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