Observation of the Hydrogen Migration in the Cation-Induced

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Observation of the Hydrogen Migration in the CationInduced Fragmentation of the Pyridine Molecules Tomasz Jaroslaw Wasowicz, and Boguslaw Pranszke J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b11298 • Publication Date (Web): 02 Feb 2016 Downloaded from http://pubs.acs.org on February 6, 2016

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The Journal of Physical Chemistry

Observation of the Hydrogen Migration in the Cation-Induced Fragmentation of the Pyridine Molecules Tomasz J. Wasowicz†*, Bogusław Pranszke‡,§ †

Department of Physics of Electronic Phenomena, Gdańsk University of Technology,

ul. G. Narutowicza 11/12, 80-233 Gdańsk, Poland ‡

Institute of Experimental Physics, University of Gdańsk, ul. Wita Stwosza 59, 80-952 Gdańsk,

Poland

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ABSTRACT The ability to selectively control of chemical reactions related to biology, combustion, and catalysis has recently attracted much attention. In particular, the hydrogen atom relocation may be used to manipulate bond-breaking and new bond-forming processes and may hold promise for far-reaching applications. Thus, the hydrogen atom migration preceding fragmentation of the gas-phase pyridine molecules by the H+, H2+, He+, He++ and O+ impact has been studied experimentally in the energy range of 5-2000 eV using collision-induced luminescence spectroscopy. Formation of the excited NH(A3Π) radicals was observed among the atomic and diatomic fragments. The structure of the pyridine molecule is lacking of the NH group, therefore observation of its A3Π→X3Σ‾ emission bands is an evidence of the hydrogen atom relocation prior to the cation-induced fragmentation. The NH(A3Π) emission yields indicate that formation of the NH radicals depends on the type of selected projectile and can be controlled by tuning its velocity. The plausible collisional mechanisms as well as fragmentation channels for NH formation in pyridine are discussed.


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1. INTRODUCTION Many investigations have been devoted to the understanding of the alterations induced by ion collisions with biological systems, particularly within living cells and the DNA/RNA molecules.1 It is known that the primary ionizing beams and the secondary charged particles such as electrons, radicals and ions can induce important damages to the DNA helix, including singleand double-strand breaks2. At the microscopic scale these lesions are related to the bond cleavage of the DNA building units, which before dissociation may be a subject of molecular isomerization and the migration of the hydrogen atoms. In the hydrogen migration the hydrogen atom moves from one location to another within a molecule.3 This movement can participate in the chemical bond rearrangement leading to deformation of molecular skeletal structure. The hydrogen migration may thus open new reaction pathways that could not be achieved when reactions start from the initial molecular geometries323

. It usually occurs on a femtosecond time scale4,14-16,20 and is faster than the molecular bond

breaking in dissociation. It may therefore be relevant for controlling chemical bond scission or new bond formation in biological radiation damage processes8,21,22 or in industry relevant reactions such as combustion20,24 and catalysis.25 The six-membered ring of pyridine molecule, C5H5N (Figure 1), which contains one nitrogen atom that replaces methine group (=CH-) in benzene, has many astrochemical, biological and industrial applications and for that reason is of great relevance to studies of fragmentation processes. In particular, the investigations of the fragmentation patterns and resistance of the pyridine molecules to ionizing agents can elucidate the origin of these molecules in space. For example, in the astrobiology studies it is expected that N-heterocycles such as pyridine could


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play an important role in the formation of the DNA components and thus have been searched for on extraterrestrial media.26,27

Figure 1. The pyridine molecule, C5H5N. The labeling of the atoms is shown. The color code: carbon atom is gray, nitrogen atom is blue, and hydrogen atom is white. Furthermore, pyridine molecules occur within molecular structure of many compounds that play important role in a wide range of biologically related substances. The pyridine ring was found in the B3 and B6 vitamins,28 the coenzymes such as nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP),29 as well as in alkaloids that are produced by a large variety of organisms, e.g. bacteria, plants and animals29. It is also utilized in many pharmaceutical drugs28 and more commonly used as precursor to agrochemical products such as bactericides, insecticides and herbicides.30 There is a very limited amount of work performed on the ion-induced processes in pyridine in the gas phase so far. Besides the very low energy (10 to 22 eV) studies of rate coefficients for several ionic projectiles performed by Fondren et al.,31 measurements of fragmentation of pyridine by ion impact are essentially nonexistent.


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In the present study, the fragmentation of the gas-phase pyridine molecules in collisions with the H+, H2+, He+, He++ and O+ cations was investigated for the first time exploiting the collisioninduced luminescence spectroscopy. During heavy ion irradiation of biological material secondary atomic cations such as H+, O+, C+, and N+ with energies from hyperthermal to hundreds of eV can be produced from DNA/RNA bases and can additionally interact with the individual DNA components, enhancing fragmentation, thus, in our experiment, the incident impact energies in the laboratory frame were varied between 5 and 2000 eV and covered a velocity range from 30 to 440 km/s. In particular, our attention was directed to formation of the free NH(A3Π) radicals, observed among other fragments. Because they are not structural components of pyridine molecules, observation of their A3Π→X3Σ‾ bands gives a clear evidence of the hydrogen migration prior to the cation-induced dissociation. The present results show that the creation of the excited NH(A3Π) molecules depends on the type of selected projectile and its velocity. The measurements of the NH(A3Π) emission yields by detecting the intensities of the A3Π→X3Σ‾ emission bands at different projectile energies (velocities) allowed investigating the evolution of the underlying hydrogen transfer mechanism and to propose possible mechanisms for the NH formation.

2. EXPERIMENTAL SECTION The experiment was carried out at the University of Gdańsk using an apparatus that was described in detail previously,32 which employs the collision-induced luminescence technique.33 The applied technique allows identification of the fragmentation products by detecting their emission.32


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Briefly, the set up consisted of a cation source, magnetic mass selector, collision cell, and an optical spectrometer equipped with sensitive multichannel photon-counting detector. Cations were created in a colutron-type source, working for the hydrogen cations at pressures of 50-100 Pa of H2, while helium and oxygen cations were produced from He and O2 gases under 400 and 100 Pa pressures, respectively. The discharge current was maintained at 500 mA for fixed anodeto-cathode voltage of 100 or 120 V. An electrode set to 1000 V was used to extract cations from the discharge region. Then cations were directed through a hole in the anode into the 60° magnetic mass selector. At this stage cation beam current was of the order of several µA. Three immersion lenses were next used to slow down the cations to a desired energy. After reaching the collision chamber, the cation beam current was measured on the rear slit of the collision cell. Typical cation beam current in the collision region was about 0.4 nA at 5 eV and 1 nA at 1000 eV for protons and 1.5 nA and 2 nA at 1000 eV for H2+ and O+, respectively. The current for helium cations was approximately 100 nA at 1000 eV and reduced by a factor of 100 as the energy was decreased to 50 eV. Next, the cation beam traversed pyridine vapours, which were directed into the collision cell via a gas inlet system. During the entire experiment the pressure of the target gas was maintained at 15 mTr, as determined with the Barocel capacitance manometer. Luminescence emitted by the dissociation fragments from the interaction region was reflected by a concave Al mirror and focused onto the entrance slit of a McPherson 218 spectrograph equipped with two gratings: the 300 l/mm blazed at 500 nm or 1200 l/mm blazed at 250 nm. The dispersed light was then detected by a 1024 channel “Mepsicron” detector, sensitive in the 180600 nm wavelength range. The luminescence signal integrated over all 1024 detector channels was between 3 and 300 counts/s corresponding to the cation beam current. Similarly, typical detector dark counts in operation were of the order of 2 counts/s. The luminescence signal was


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found to be linear as a function of the pyridine gas pressure up to 30 mTr, indicating that the observed emission is the result of primary processes, not affected by secondary collision mechanisms such as collisional quenching of excited fragments or trapping of emitted radiation. In the first step of the measurements, high resolution spectra were obtained in the 326–346 nm wavelength range for a fixed cation energies with a 1200-l/mm grating. The optical resolution ∆λ was about 0.4 nm (FWHM) and these studies allowed an accurate identification of the spectral components. In the second step, the emission yields were measured as functions of the incident cation energies. Measurements were performed using a 300-l/mm grating, which at each collision energy allowed the CCD to record the luminescence spectrum in the 200 nm wavelength range window with an optical resolution ∆λ of 2.5 nm (FWHM). The background signal due to contaminations of the residual gases in the vacuum chamber was measured by cutting off the pyridine beam flow. It was then subtracted from the emission spectra. The spectra were next corrected for the wavelength dependence of the sensitivity of the optical detection channel. The total intensities of the NH(A3Π) emission observed in the luminescence spectra were obtained by integrating over the band area containing the vibrational and rotational lines. The background in the original spectra was taken to be the average of that below and above the studied band and was subtracted from the spectra. The resulting emission yields were next normalized to the cation beam current (nA) and recording time (1 min). The pyridine sample was purchased from Sigma Aldrich, Poland, with a declared purity of 99.9%. Pyridine is liquid at the room temperature, but it is volatile enough to be measured without heating the sample. The pyridine sample was outgassed through freeze-pump-thaw cycles, until no release of contaminating gases from the melting ices was observed.


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3. RESULTS AND DISCUSSION 3.1. LUMINESCENCE SPECTRA Luminescence spectra of the different fragments formed in collisions of the pyridine molecules with several cations have been measured and a typical spectra for collisions between pyridine and the O+, He+, H2+ and H+ cations are depicted in Figure 2. The obtained fragmentation patterns point mostly at production of the excited CH and CN diatomic molecules. These fragments were identified by their decay with emission of the molecular systems of CH(B2Σ+→X2Πr, C2Σ–→X2Πr) and CN(B2Σ+→X2Σ+). However, the most remarkable feature displayed in Figure 2 is the occurrence of the A3Π→X3Σ‾ band from the excited diatomic NH fragment, that emerges from 326 nm up to 345 nm. This is unique because the NH radical is not a structural unit of pyridine molecule and observation of its emission clearly indicates the hydrogen migration alongside the ring. Therefore, in this work detailed analysis of the production of the excited NH radical in collisions of the pyridine molecules with different cations was performed. For an accurate identification of the NH(A3Π→X3Σ‾) spectral components, high-resolution spectra of pyridine were measured for a fixed cation energy.


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Figure 2. Luminescence spectra measured for the O+, He+, H2+ and H+ cations with an optical resolution ∆λ of 2.5 nm (FWHM). The spectra were corrected for the wavelength dependence of the sensitivity of the detection system.

In Figure 3 we show high resolution luminescence spectra measured in the 326-346 nm range for the He+, He++ and H+ cations impact. The A3Π→X3Σ‾ emission bands of the excited NH radical are clearly seen in all spectra. A high resolution spectroscopic analysis of these bands has been performed by Brazier et al.34 and the positions of the rotational lines of the P, Q, R branches of the ∆ν = 0 vibrational transitions given therein are also shown in Figure 3a. It is visible that the shape of the NH spectrum is formed by the overlapping rotational lines of the (0,0) and (1,1) vibrational bands. The peak maxima at 336.2 and 337.4 nm arise due to the


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transitions of the Q branches of the (0,0) and (1,1) bands, respectively. The rotational lines of other branches appear as wings below and above the maxima.

Figure 3. Luminescence spectra measured with an optical resolution ∆λ of 0.4 nm (FWHM) for collisions of the He+, He++, and H+ cations. The spectra were not corrected for the wavelength dependence of the sensitivity of the detection system. The rotational lines of the P, Q, R branches of the ∆ν = 0 vibrational transitions given by Brazier et al.34 are marked with vertical lines.

These observations are supported by calculations of the NH emission spectrum carried out by us, utilizing the molecular spectra simulation program.35,36 Recently similar computer technique, used by us in the analysis of spectra of diatomic molecules, occurring in photon- and cationinduced dissociation of five- and six-membered heterocyclic compounds, has been proven to be efficient and reliable.22,32,37,38 Thus, theoretical spectra at different vibrational and rotational


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temperatures have been calculated with the use of improved set of the vibrational and rotational constants of the A3Π and X3Σ‾ electronic states of NH39 and fitted to the experimental spectrum. In these calculations the rotational and vibrational levels populations were assumed to follow the Boltzmann distributions. Moreover, in the simulation of the NH(A3Π→X3Σ‾) band, known HönlLondon factors40 and the spectral resolution of the measured spectrum were used. Figure 4 shows an exemplary comparison of the simulated and the measured spectra. Satisfactory agreement between the two spectra was achieved for the populations of the vibrational and rotational levels, described by the characteristic temperatures of 7000 and 11000 K, respectively. The calculations show additionally that a low-intensity peak at 338.9 nm may be produced by the Q branch of the (2,2) vibrational transition.

Figure 4. Comparison of the simulated (red curve) and experimental (black dots) NH(A3Π→X3Σ‾) spectra. The line at the bottom of the picture presents difference between calculated and experimental profiles.

3.2. EMISSION YIELDS The emission yields of the NH(A3Π) measured in collisions of pyridine with the O+, He+, H2+ and H+ cations plotted as functions of the velocity (bottom axis) and energy (top axis) of the


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projectiles are shown in Figure 5. The experimental uncertainties in the emission yields are the mean standard deviations obtained from several independent measurements performed at fixed cation velocity. As is seen in Figure 5, the NH production depends significantly on both the type and velocity of the incident projectile.

Figure 5. Emission yields of the excited NH(A3Π) fragments obtained in collisions of the O+, He+, H2+ and H+ cations with pyridine molecules.


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The measurements for the collisions with oxygen cations (Figure 5a) were performed in the 350−650 eV energy range, which corresponds to velocities of 65−88 km/s. In this collision system the maximum for the NH radical formation is observed for oxygen cations of an incident velocity of approximately 82 km/s. Figure 5b shows the NH(A3Π) emission yield measured in pyridine under He+ projectiles impact. The collision energies of the incident helium cations were varied between 50 and 1000 eV, which corresponded to velocity range from 49 to 220 km/s. This curve rises rapidly above 50 km/s and shows a resonance at 120 km/s. The emission yield for the NH(A3Π) radicals in the H2+ collisions is depicted in Figure 5c. These measurements were carried out in the 100-1000 eV energy range (velocity range 98-311 km/s). The production of the NH(A3Π) fragments occurs above 140 km/s and displays a maximum at 200 km/s. In contrast to the He+ collisions the emission yield curve for H2+ above the maximum seems to increase. The measurements for the H++pyridine collisions were performed for the 5-1000 eV projectiles energies, which covered the velocity range of 30-440 km/s. The emission yield curve measured in this collision system is presented in Figure 5d. This velocity dependence is considerably different from other emission yield curves (Figure 5). It has well defined threshold for the production of the NH(A3Π) fragment, which was found to be 275 km/s. In addition, the curve obtained in this collision system increases gradually above the threshold and shows no resonances in the presented velocity range.


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3.3. COLLISIONAL PROCESSES Reaction channels producing the NH(A3Π) fragments may be activated by the following collision processes: a) electron capture (EC) from the pyridine molecule to the projectile followed by fragmentation of the pyridine cation, b) direct ionization of the pyridine molecule and further fragmentation of the pyridine cation (dissociative ionization (DI)), c) excitation of pyridine molecule and further fragmentation of the excited molecule (dissociative excitation (DE)), d) the transient cation-molecule complex (TC) formation prior to fragmentation. In general the electron transfer occurs at relatively large projectile−target distances where it is a dominant mechanism, but fragmentation of target molecule requires closer collisions involving dissociative ionization and excitation processes.41 Since the formation of the ion−molecule complexes may be the result of an ion−dipole interaction,42 it may occur at rather large projectile−target distances. It is clearly seen from Figures 5a, b and c that the emission yield curves measured for the O+, He+ and H2+ projectiles reveal maxima. An appearance of the pronounced peaks in the cross section curves corroborates the formation of the ion-molecule transient complexes, which were found to be ubiquitous intermediates in the collisional systems.32,42-46 The corresponding ion−molecule components remain bound by ion−dipole interaction. This long-range attractive force between the reactants is described by a ~r-2 electrostatic potential47 and is often encountered in combination with subsequent H-transfer steps42. The charged and neutral components, on one hand align so that the positive and negative groups are next to one another


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allowing for maximum attraction and on the other hand are sufficiently separated that they show reactivities similar to those expected for the isolated species.42 In consequence the target molecule may be treated as excited to quasi cationic states.42,47 Because pyridine molecule offers π-electron system as an attractive site for molecular interaction and also provides a site for the weak interaction at the non-bonding electron pair of heteroatom,48 it readily forms complexes with, for example, alkali metal ions49 as well as neutral halogen molecules.48,50,51 Recent theoretical calculations showed that the low electrostatic potential on the nitrogen atom in pyridine is sufficient to produce stable van der Waals σN-type complexes at the site of the heteroatom rather than the π-electron complexes.48-50 Moreover, the studies of the nature of interactions of pyridine with various hydrides revealed that interactions in the σN-type complexes are predominantly electrostatic in nature, while the dispersion and electrostatic interactions dominate in the case of the π-type complexes.51 Thus, the appearances of strong resonances in the NH(A3Π) emission yields may be a signatures of a mechanisms proceeding through intermediate [O-C4H8O]+, [He-C4H8O]+ and [H2-C4H8O]+ complexes formation prior to fragmentation. In collisions of pyridine with the O+, He+ and H2+ cations the constituent units may strongly interact electrostatically due to the attractive force between the charge of the O+, He+ and H2+ cations and the pyridine permanent dipole moment (2.21 D48) to promote rearrangement and further dissociation into the NH(A3Π) fragments. This however would require the complex to exist for a sufficient amount of time. At the resonance velocities, which are in fact quite low, the time of interaction of particular cation and pyridine molecule will be long enough to form the complex and to transfer the hydrogen to nitrogen atom. After a breakup of the complex, the NH(A3Π) fragment may appear. At higher velocities of the cations the time of interaction between cations and pyridine molecules becomes too short for an internal


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rearrangement. In consequence, production of the NH(A3Π) fragments starts being ineffective and the corresponding emission yield decreases. It is of note that analogous mechanism was observed in similar experiment concerning collisions of C+ with NH3 molecules,45 where rapid increase of the excited CN fragments cross sections at lower velocities occurred. On the basis of earlier ab initio calculations46 the occurrence of a transient ionic complex [C-NH3]+ in which the constituent units interact electrostatically, rearrange and dissociate into the CN fragments was suggested.45 Present observations are also in agreement with our previous studies of the C+ with tetrahydrofuran (THF) collisions. In particular, we observed an enhancement of the emission yield of the CH(A2∆) fragment, which could not be explained as a simple abstraction or substitution reaction.32 The occurrence of a transient ionic complex [C-C4H8O]+ was suggested due to the attractive force between the C+ and the THF permanent dipole moment.32 Moreover, Figures 5a, 5b and 5c show that the maxima of the resonances in the emission yield curves measured for O+, He+ and H2+ cations shift toward lower velocities with increasing mass of the projectile. The resonance for O+ impact occurs around the 82 km/s, the emission yield for the He++pyridine peaks at the 120 km/s velocity and for the H2++pyridine system it arises at 200 km/s. These results indicate that in the presented velocity range the complex formation may depend on the mass of the projectile. It is thus possible that the heavier the projectile the lower the velocity that may be required to form the complex. It is of note that in the H++pyridine collisions, no luminescence of the NH was observed at lower velocities where for other impact systems the complexes occurred. It is thus possible that in this collision system the [H-C4H8O]+ transient ionic complex does not appear. Indeed, recent experimental32,41,43,52 and theoretical53 studies on fragmentation and ionization of biomolecular


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targets induced by collisions with protons showed that electron capture from the target molecule to the H+ is the major collision mechanism, especially at higher velocities, where the time of interaction with target molecule is too short for the complex formation, but still it is sufficient to ensue the charge transfer.53 Most of the energy would be then deposited to the neutralized H+ projectiles, yielding a high abundance of hydrogen atoms excited to several different states. In fact, very high yields for production of excited hydrogen atoms in relation to minor fragmentation of the target molecule have been recently observed in the H++THF collisions.32 This observation was explained as an effect of the distant single electron transfer. According to this, it could be expected that electron capture without major fragmentation of pyridine would also dominate the H++pyridine interaction. Actually, the charge transfer seems to be the only important collisional process taking place during proton irradiation and for this reason the emission yield values for the H++pyridine collisions are small (see Figure 5d). In contrast, the interactions with O+, He+, H2+ cations lead to more extensive production of the NH radicals, because apart from the charge transfer reaction, other collisional processes may simultaneously occur. In particular, the complex formation would govern the H2++pyridine interaction at lower velocities, while at higher velocities the electron capture would start occurring. Therefore, the emission yield curve for H2+ increases above the resonance (Figure 5c). For He+ resonant electron capture from the pyridine HOMO is energetically precluded, reducing the easiest electron removal mechanism. Also the DI and DE processes seem to be not so efficient. Therefore, the corresponding emission yield decreases above the resonance (Figure 5b). It is seen in Figure 5a that impact with oxygen cations prompts the most effective production of the NH fragments. This observation may be explained as an effect of all collisional processes. In particular, relatively high intensity of NH(A3Π) for the O+


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collision system may suggest noticeable fragmentation of pyridine triggered by the DI and DE processes. Indeed, in the similar study of collisions of O+ with tetrahydrofuran major fragmentation of the THF molecules was observed.32 Since close collisions regime was there relevant, it was pointed out that the DI and DE processes could occur simultaneously leading to fragmentation.

3.4. THE NH FORMATION MECHANISM As seen in Figure 1 the pyridine molecule consists of five CH units and one N heteroatom and is lacking the NH group. However, in all impact systems the NH(A3Π→X3Σ‾) bands have been observed. Thus, now we consider the possible fragmentation mechanisms leading to the NH formation. As was shown above the collisional processes may leave the excited pyridine molecule (DE) or its excited parent cation (DI, EC, TC). Recent ab initio calculations have shown that both pyridine54 and its radical cation55 may isomerize via the H atom migration. The calculated isomerization barriers were in the range of 2.5–6.1 eV, depending on the relative positions of the nitrogen and the roaming hydrogen54,55 and in fact were lower than for other competing mechanisms. Furthermore, the process that had the lowest energy barrier of 2.5 eV55 and may directly lead to production of the NH fragments arises for radical cation of pyridine. It is initiated by a 1,2 shift of the H atom from C(2) to N(1) to form an α-distonic isomers of pyridine.55 This new isomer was more stable than the neutral pyridine molecule and its potential energy was found to be 0.08 eV below that of the ring structure.55 The cyclic structure of αdistonic isomer of pyridine implies a fragmentation reaction, which proceeds through ring opening by scission of one of the two C–N bonds. Finally, the second C–N bond cleavage in sequence generates NH. On the other hand, the production of NH fragments through


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isomerization of neutral structure of pyridine required the 3.64 eV to overcome the energy barrier.54 The potential energy of this new isomer was found to be 1.91 eV54 above the energy of pyridine. Taking that into account and keeping in mind that only DE process may lead to excited neutral molecule this fragmentation pathway seems to be of minor importance.

4. CONCLUSIONS We have shown that collisions of low energy cations with pyridine can induce the isomerization of the target molecule. This in turn can influence its dissociation mechanism, leading to some unexpected fragmentation pathways. In particular, the formation of the excited NH(A3Π) fragments was observed in the collisions of pyridine molecules with the H+, H2+, He+, He++ and O+ cations applying the collision-induced luminescence spectroscopy. Since pyridine molecules have not got any NH structural components at all, the occurrence of the luminescence from the excited NH(A3Π) free radicals can be regarded as the straightforward indicator of chemical bond rearrangement process associated with the migration of one of the hydrogen atoms prior to the cation-induced dissociation. The plausible mechanism for this unusual reaction channel was suggested. This reaction most likely follows the intramolecular rearrangement that involves a 1,2 shift of the H atom from C(2) to N(1) to form an α-distonic isomer of pyridine cation, and finally, cleavage of two C–N bonds to detach the NH radical. In order to recognize the evolution of the underlying hydrogen transfer mechanism in the presence of low energy cations, the NH(A3Π) emission yields were measured as functions of the projectile energy (velocity). The obtained curves show that production of the NH(A3Π) fragments depends on the selected cation and can be selectively activated by tuning the collision velocity. Each projectile may induce different collisional process that may stimulate the rearrangement of the target


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molecule. In particular, the formation of the intermediate complexes occurs in the H2+, He+ and O+ collisions, while single electron capture without major fragmentation of pyridine dominates the H++pyridine interaction. These findings may point to a new achievement in controlling chemical bond-breaking and new bond-forming processes that may have particular relevance for the investigation of early molecular processes in the nascent stages of DNA damage by the charged projectiles in the radiation therapies. We also believe that this work will help to understand the cation-induced processes occurring in biologically relevant molecules especially those with both π-electron systems and n-pairs, and may trigger future theoretical calculations on the issue.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: (+48 58) 347-10-69. Fax: (+48 58) 347-28-21. Author contributions T.J.W. conceived and designed of the study, performed the measurements, analyzed and interpreted of data, created artwork, and wrote the paper. B.P. performed the measurements. Notes §

Present Address: Gdynia Maritime University, ul. Morska 81-87, 81-225 Gdynia, Poland

The authors declare no competing financial interests.


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ACKNOWLEDGMENT This work was conducted within the framework of the COST Action CM1301 (CELINA). T.J.W. would like to thank professor M. Zubek for critical reading of the manuscript. B.P. acknowledges the financial support from University of Gdansk grant UG: 530-5200-D464-14. REFERENCES (1) Amaldi, U.; Kraft, G. Radiotherapy with Beams of Carbon Ions. Rep. Prog. Phys. 2005, 68, 1861-1882. (2) Huels, M. A.; Boudaïffa, B.; Cloutier, P.; Hunting, D.; Sanche, L. Single, Double, and Multiple Double Strand Breaks Induced in DNA by 3−100 eV Electrons. J. Am. Chem. Soc. 2003, 125, 4467-4477. (3) Schaefer III, H. F. The 1,2 Hydrogen Shift: a Common Vehicle for the Disappearance of Evanescent Molecular Species. Acc. Chem. Res. 1979, 12, 288-296. (4) Morgan, K. M.; O’Connor, M. J.; Humphrey, J. L.; Buschman, K. E. An Experimental and Computational Study of 1,2-Hydrogen Migrations in 2-Hydroxycyclopentylidene and Its Conjugate Base. J. Org. Chem. 2001, 66, 1600-1606. (5) Osipov, T.; Cocke, C. L.; Prior, M. H.; Landers, A.; Weber, Th.; Jagutzki, O.; Schmidt, L.; Schmidt-Böcking, H.; Dörner, R. Photoelectron-Photoion Momentum Spectroscopy as a Clock for Chemical Rearrangements: Isomerization of the Di-Cation of Acetylene to the Vinylidene Configuration. Phys. Rev. Lett. 2003, 90, 233002. (6) Lee, S.-H.; Lee, Y. T.; Yang, X. Dynamics of Photodissociation of 3,3,3-d3-Propene at 157 nm: Site Effect and Hydrogen Migration. J. Chem. Phys. 2004, 120, 10992.


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TOC GRAPHICS


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The pyridine molecule, C5H5N. The labeling of the atoms is shown. The color code: carbon atom is gray, nitrogen atom is blue, and hydrogen atom is white. 71x71mm (300 x 300 DPI)


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Luminescence spectra measured for the O+, He+, H2+ and H+ cations with an optical resolution ∆λ of 2.5 nm (FWHM). The spectra were corrected for the wavelength dependence of the sensitivity of the detection system. 296x528mm (150 x 150 DPI)



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Luminescence spectra measured with an optical resolution ∆λ of 0.4 nm (FWHM) for collisions of the He+, He++, and H+ cations. The spectra were not corrected for the wavelength dependence of the sensitivity of the detection system. The rotational lines of the P, Q, R branches of the ∆ν = 0 vibrational transitions given by Brazier et al.34 are marked with vertical lines. 99x135mm (300 x 300 DPI)


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Comparison of the simulated (red curve) and experimental (black dots) NH(A3Π→X3Σ‾) spectra. The line at the bottom of the picture presents difference between calculated and experimental profiles. 99x81mm (300 x 300 DPI)



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Emission yields of the excited NH(A3Π) fragments obtained in collisions of the O+, He+, H2+ and H+ cations with pyridine molecules. 98x215mm (300 x 300 DPI)


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TOC GRAPHICS 100x100mm (300 x 300 DPI)


Observation of the Hydrogen Migration in the Cation-Induced

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