Fragmentation of Tetrahydrofuran Molecules by H+, C+, and O+

Article pubs.acs.org/JPCA

Fragmentation of Tetrahydrofuran Molecules by H+, C+, and O+ Collisions at the Incident Energy Range of 25−1000 eV Tomasz J. Wasowicz*,† and Bogusław Pranszke‡,§ †

Department of Physics of Electronic Phenomena, Gdańsk University of Technology, ul. G. Narutowicza 11/12, 80-233 Gdańsk, Poland ‡ Gdynia Maritime University, ul. Morska 81-87, 81-225 Gdynia, Poland § Institute of Experimental Physics, University of Gdańsk, ul. Wita Stwosza 59, 80-952 Gdańsk, Poland ABSTRACT: We have studied fragmentation processes of the gas-phase tetrahydrofuran (THF) molecules in collisions with the H+, C+, and O+ cations. The collision energies have been varied between 25 and 1000 eV and thus covered a velocity range from 10 to 440 km/s. The following excited neutral fragments of THF have been observed: the atomic hydrogen H(n), n = 4−9, carbon atoms in the 2p3s 1P1, 2p4p 1D2, and 2p4p 3P states and vibrationally and rotationally excited diatomic CH fragments in the A2Δ and B2Σ− states. Fragmentation yields of these excited fragments have been measured as functions of the projectile energy (velocity). Our results show that the fragmentation mechanism depends on the projectile cations and is dominated by electron transfer from tetrahydrofuran molecules to cations. It has been additionally hypothesized that in the C++THF collisions a [C−C4H8O]+ complex is formed prior to dissociation. The possible reaction channels involved in fragmentation of THF under the H+, C+, and O+ cations impact are also discussed.

1. INTRODUCTION The fragmentation of DNA and its constituents is a one of the most important processes in radiation damage of the living cells. It may be produced by absorption of radiation or by collisions with charged particles. The interaction of charged particles with the biological cells, for example cations in hadrontherapy, may produce structural and chemical modifications by bond cleavages in the DNA helix.1 These modifications originate from the interaction of the primary beam and the secondary particles such as low-energy electrons, radicals, and ions which are formed within the track.2,3 Unlike electrons and radicals, the interactions of ions (primary and secondary) with biomolecules are poorly understood despite their importance in interactions with living cells.4 To determine the most sensitive part of the DNA or RNA molecular chains to the cation-induced bond rupture, it is important to explore fragmentation processes of components of the DNA helix taking place in collisions with the charged particles. In this context, ionization and ionic fragmentation of 2-deoxy-D-ribose (dR), uracil, thymine, adenine, thymidine, and amino acids under cation (e.g., H+, He+, He2+, Cq+, q = 1−5) collisions have been investigated in the gas phase and condensed on the surface.4−10 It was shown that studied molecules underwent radical disintegration, but the deoxyribose was particularly vulnerable to cation impact.7,8 Thymidine damage, on the other hand, involved base or sugar losses and damage of either moiety.7 In the amino acids, the C−Cα bond scission is the major fragmentation channel.10 During hadrontherapy, abundant secondary atomic cations such as O+, C+, and N+ with energies from hyperthermal to hundreds of © XXXX American Chemical Society

eV can be also produced from DNA/RNA bases and thus can additionally interact with the individual DNA components, enhancing fragmentation.5,6 Furthermore, fragmentation and 3D mapping of the living cells adsorbed on the surfaces using the focused ion beam etching has recently become a subject of increased experimental investigations.11 In view of the collision mechanism, we mention that fragmentation of biomolecules is usually weak for projectiles with a major electron capture cross section, because electron transfer leaves most of the energy on the projectile cation.4 Nevertheless, in the case where electron capture is possible the projectiles may still induce collisional excitation of the electronic system of the target molecule, leading to the process which is usually called electronic stopping of the projectile.12 The density functional theory calculations of electronic friction coefficients at different values of the density parameter show13,14 that the electronic excitation of a molecular target by an atomic projectile depends on the projectile atomic number Z. For velocities lower than 7600 km/s, this dependence oscillates for cationic projectiles with different atomic numbers (having low values for protons and reaching a first maximum for projectiles with Z = 6−7). Recent studies on energy transfer, fragmentation, and ionization in collisions between protons and fullerenes15 showed that electronic stopping of H+ is in fact of minor importance. Furthermore, investigations of fullerene collisions with cations with Z = 2−18 Received: October 21, 2014 Revised: December 30, 2014

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DOI: 10.1021/jp5105856 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A gave evidence14 that electronic stopping of C+ and O+ projectiles should be more extensive than that for H+. The studies of fragmentation of gas-phase deoxyribose in H+, He+, and He2+ collisions have lately indicated16 more intense fragmentation for He+ than for H+ and He2+, which was explained in the framework of charge exchange and electronic stopping. The ab initio quantum chemistry molecular calculations of the collisions of Cq+ (q = 2−4) carbon cations and protons with deoxyribose17,18 and the RNA bases19 enabled a deeper insight into the collision mechanism. These studies focused on the electron capture dynamics, and in the case of deoxyribose they show sharp avoided crossings between the entrance channel H++dR and the different H+dR+ electron transfer levels at the potential energy curves at the distance of R = 2.5 Å.17,18 As the charge transfer process is mostly driven by the nonadiabatic interactions between the adiabatic potential curves at their points of closest approach, it was pointed out that these avoided crossings should induce efficient electron charge transfer. The corresponding electron capture cross sections were significantly higher for the collisions with H+ than those with the C4+ projectiles,17 indicating that collisions with carbon cations would lead to a more efficient dissociation of deoxyribose than collisions with protons. Furthermore, the charge transfer mechanism in ion−biomolecule collisions was highly anisotropic.19 Deoxyribose in the furanose form has a central position in the chemical structure of the RNA or DNA. It links the phosphate groups to form the backbone of the RNA or DNA chains and provides linkage between backbones and nucleobases. However, in the gas-phase deoxyribose is in the pyranose form20 and to study interaction of the ionizing radiation with its furanose structure, several analogues, namely tetrahydrofuran, α-tetrahydrofurfuryl alcohol, and 3-hydroxytetrahydrofuran molecules, are used as corresponding targets. Among them, the tetrahydrofuran molecule, C4H8O, (Figure 1), is often considered as the simplest analogue to deoxyribose,21−23 and thus its fragmentation became recently a subject of intensive experimental and theoretical studies.

context, studies of infrared laser-induced decomposition of tetrahydrofuran26 showed that the photon-induced degradation of THF results in the formation of the transient 1,5-diradical •CH2−CH2−CH2−CH2−O• following cleavage of the C−O bond. This open-ring THF isomer may further decompose into aldehydes. Scala and Rourke27 pointed out that in the photolysis of THF and its methyl-substituted derivatives at 147.0 nm, fragmentation of these molecules is also initiated by C−O bond cleavage. More recently, Scala et al.,28 using a femtosecond-resolved mass spectrometry technique combined with density-functional-theory calculations of the reaction pathways, studied the β-cleavage dynamics of THF. They found that photofragmentation of the diradical intermediate •CH2−CH2−CH2−CH2−O• occurs via β-elimination involving the C−C and C−H σ-bonds with simultaneous C−C and C−O π bond formation resulting from the electron on oxygen and from the electron on carbon atoms.28 Photon-induced ionic fragmentation of the THF in the gas phase was later studied by Mayer et al.29 who used a tunable synchrotron radiation technique. They applied threshold photoelectron photoion coincidence spectroscopy to measure breakdown curves to explore the loss of a hydrogen atom from ionized THF over the photon energy range of 9.9−10.4 eV. A variety of ring-opening reactions of THF were also probed at the B3LYP/ 6-31+G(d) and G3 levels of theory to make a comparison with the experiment.29 Mayer et al.29 pointed out that the isomer •CH2CH2CH2OCH2+ of THF cation and all of its H-loss products have significantly higher energies than the closed ring parent cation, practically eliminating the possibility that ionized THF dissociates via a ring-opening reaction similar to that occurring for the neutral molecule. Lee30 who used photofragment translational spectroscopy and direct VUV photoionization later found that the photodissociation of THF at 193.3 nm proceeds mainly on the ground-state potential energy surface following ring opening and efficient internal conversions from the excited to the ground state. He suggested that dissociation of the •CH2−CH2−CH2−CH2−O• diradical proceeds through cleavage of the C(1)−C(2), C(2)−C(3), and C(1)−H bonds, yielding aldehydes. Moreover, it was shown that hydrogen migration between the C(3) and C(1) atoms competes with direct fragmentation of the diradical. More recently, in the vacuum-ultraviolet photofragmentation of tetrahydrofuran over the energy range 14−68 eV31 small neutral fragments were detected. Here, fragmentation proceeds via excitation to higher lying superexcited states, which then dissociate to produce the hydrogen atoms H(n), n = 3−11, and the diatomic CH(A2Δ), CH(B2Σ−), and C2(d3Πg) fragments, which are vibrationally and rotationally excited. We have also noted the most recent studies of fragmentation of THF molecules induced by collisions with the low-energy electrons32 and neutral potassium atoms.23 The electron impact ionization and ionic fragmentation studies of THF molecules performed by Dampc et al.32 in the energy range from ionization threshold up to 150 eV established that the most abundant cation in the mass spectra occurred at the mass of m/ q = 42. In agreement with the previous thermal decomposition studies it has been assigned to the C3H 6+ fragment. Furthermore, several dissociation channels leading to polyatomic ionic fragments were also discussed. Almeida et al.23 reported on negative ion formation from electron transfer in collisions of neutral potassium particles with THF in the 20− 100 eV collision energy range. The most abundant fragments in negative ion TOF mass spectra were identified as O− (m/q =

Figure 1. The THF molecule, C4H8O, in the C2 (twisted) conformation, showing the labeling of the atoms. Color code: carbon atom is gray, oxygen atom is red, and hydrogen atom is blue.

The earliest investigations into the fragmentation of THF concentrated on thermal decomposition.24,25 These studies showed two major fragmentation channels, both leading to production of aldehydes: CH2CH2 + CH2−CH2−O, and CH 2 −CH 2 −CH 2 + H 2 CO. Several minor products, including H2, CH4, C2H2, two isomers of C3H4 (allene and methylacetylene), C 4 H 8 , C 4 H 6 , C 4 H 4 , C 4 H 2 , and C 6 H 5 molecules, have also been identified.25 Investigations of photon-induced fragmentation of THF molecules provided deeper insight into its dynamics. In this B


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The Journal of Physical Chemistry A 16) and C2H3O− (m/q = 43), followed closely by CH− (m/q = 13), CH2− (m/q = 14), C2− (m/q = 24), and C2H− (m/q = 25) anions. They implied23 that, depending on the site of bond cleavage in THF, different fragments would be formed. For example, a bond rupture of the remaining C−O bond in the diradical would most likely lead to O− formation, whereas scissions of the various C−C bonds entail the formation of other observed fragments. Apart from these studies, to the best of our knowledge, no data were reported previously on cationic interactions with either gas- or condensed-phase THF or similar deoxyribose analogues. The present work concentrates on detailed identification of neutral fragments produced from the gasphase THF under H+, C+, and O+ cation collisions. As such, these results are complementary to the previous mass spectrometric experimental data. The incident collision energies in the laboratory frame varied between 25 and 1000 eV and thus covered the velocity range from 10 to 440 km/s. The following neutral products were observed: the excited atomic hydrogen H(n), n = 4−9, carbon atoms in the 2p3s 1P1, 2p4p 1 D2, and 2p4p 3P states, and vibrationally and rotationally excited diatomic CH fragments in the A2Δ and B2Σ− states. Fragments were identified by comparing the measured luminescence spectra with the existing spectroscopic data. Spectra of molecular fragments were also compared with their simulated contours obtained using the LIFBASE program.33 The fragmentation yields of the excited fragments were measured as functions of the projectile cations energy (velocity). Our results demonstrate that the fragmentation mechanism strongly depends on the projectile cations and is dominated by electron transfer from tetrahydrofuran molecules to the cations. The rapid enhancement of the fragmentation yield of the CH(A2Δ) occurring at lower velocities in the C++THF collisions may be somewhat regarded as an indicator of the [C−C4H8O]+ complex formation prior to dissociation. The possible reaction channels involved in fragmentation of THF under the H+, C+, and O+ cations impact are also discussed.

Figure 2. Experimental setup.

measured on the rear slit of the collision cell, strongly depends on the energy. The current usually dropped by a factor of 100 as the energy was lowered from the 1000 eV to 25−50 eV. The typical cation beam current in the collision region was about 0.3 nA at 50 eV and 15 nA at 1000 eV for protons and 15 nA and 3.5 nA at 1000 eV for carbon and oxygen cations, respectively. Light produced in the collision region was reflected by a concave Al mirror and focused onto the entrance slit of a McPherson 218 spectrograph. It was next dispersed by 300 l/ mm blazed at 500 nm or 1200 l/mm blazed at 250 nm gratings and detected by a 1024 channel “Mepsicron” detector, sensitive in the 180−600 nm wavelength range. The luminescence signal integrated over all 1024 detector channels was between 3 and 500 c/s corresponding to the cation beam current. The detector dark count rate was 2 c/s. During the experiment, care was taken to maintain conditions of linear dependences between the luminescence signal and the target gas pressure and intensity of the incident cation beam to avoid collisional quenching of excited fragments, trapping of emitted radiation, or double cation collisions. The THF sample was purchased from Aldrich with a declared purity of 99.9%. Tetrahydrofuran is liquid at room temperature, but the vapor pressure of THF is high enough so that the gas-phase studies could be performed without heating the sample. Before each measurement, the sample was additionally purified through freezing−pumping−thawing cycles, until no release of other gases from the melting ices was observed. The THF vapor was then directed through a gas inlet system into the interaction region. The pressure of the target gas was kept constant at 15 mTr, as determined with the Barocel capacitance manometer. For an accurate identification of the spectral components, high-resolution spectra were measured for a fixed cation energy with a 1200-l/mm grating. In a single data collection run, the detector recorded a luminescence emission spectrum over a 40 nm wavelength range with the optical resolution Δλ of 0.3 nm (fwhm). The complete emission spectrum obtained in the 170−515 nm range consisted of several 40 nm scans measured in consecutive wavelength regions. The wavelength scale was calibrated against the positions of the Hβ to Hθ lines of the Balmer series to within ±0.1 nm. To find contributions of residual gases in the vacuum chamber, the background signal was measured by cutting off the THF flow. It was subsequently

2. EXPERIMENTAL SECTION The experiment was performed using cation-induced luminescence spectroscopy with the apparatus that was described in detail previously.34 It consists of an cation source, magnetic mass selector, reaction cell, and an optical spectrometer equipped with a multichannel photon-counting system to detect luminescence from the excited fragments (Figure 2). Cations were produced in a colutron-type source, operating for the hydrogen cations at a pressure of 70−100 Pa of H2, while the carbon and oxygen cations were produced from CO under 100 Pa pressure. The distance between cathode and anode in the cation source was set at 5 mm. The anode-to-cathode voltage was between 100 to 120 V and the discharge current was maintained at 500 mA. It is of note that the contribution of cations in the metastable states in the cation beam depends on the anode-to-cathode voltage. Low voltage corresponds to a low percentage of metastable fraction. The produced cations were extracted from the discharge region by an electrode at 1000 V potential and directed through a hole in the anode into a 60° mass selector mounted in a vacuum chamber. At this point, the cation beam current was of the order of a few μA. Before reaching the collision cell, the cations are decelerated to a given laboratory energy by three simple immersion lenses. The cation beam current, which is C


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The Journal of Physical Chemistry A

vibrationally and rotationally excited diatomic CH. The A2Δ → X2Πr system of CH spreads from 415 to 445 nm. The shape of its spectrum, seen clearly in Figure 3b,c, is due to overlapping rotational lines of the (0,0), (1,1), and (2,2) vibrational bands.35 The asymmetric peak at 431 nm arises from overlapping Q branches of the (0,0) and (1,1) vibrational transitions, while its low-wavelength shoulder arises from overlapping R branches. A low-intensity peak at 432.5 nm is produced by lines of the Q branch of the (2,2) vibrational transition. Moreover, the structure above 433 to 440 nm is built from lines of the P branches of all vibrational transitions, which are partly obscured by the Hγ line. These observations are supported by calculations of the CH emission spectrum utilizing the LIFBASE molecular spectra simulation program33 with the use of the vibrational and rotational constants of the A2Δ and X2Πr electronic states of CH from refs 36−38. Recently, such a computer technique used by us in the analysis of spectra of CN(B2Σ+) free radicals occurring in photodissociation of pyridine and pyrimidine molecules has proved to be efficient and reliable.39 In the case of the THF collisions with the C+ (Figure 3b) the best agreement between simulated (not shown in Figure 3) and measured spectra was achieved for the populations of the vibrational and rotational levels described by the characteristic temperatures of 5200 and 8900 K, respectively. Similarly, for the O+ impact we have obtained 5000 and 9000 K for rotational and vibrational temperatures, respectively. In both cases, populations were assumed to follow the Boltzmann distributions. The second observed system of CH emission, B2Σ−→ X2Πr, occurs between 387 and 409 nm.35 It reveals high rotational excitation of CH excited fragment. The shape of its spectrum is partly obscured by the hydrogen lines of the Balmer series. The band head of the R branch at 387 nm overlaps with the Hη line (not marked in Figure 3b,c), and the rotational lines produced by overlapping P and Q branches are affected by the Hε and Hζ hydrogen lines. Furthermore, the Hζ line interferes with the band heads of the P and Q branches. The structure of this band has been also simulated using the LIFBASE program.33 In the analysis we have used the vibrational and rotational constants of the B2Σ− and X2Πr electronic states of CH from refs 40−42. In the case of the THF collisions with C+ the best agreement between simulated (not shown in Figure 3) and measured spectra was obtained for the vibrational and rotational temperatures of 3000 and 3700 K, respectively. For O++THF, the best fit was obtained using 4000 and 3000 K temperatures, respectively. As shown in Figure 3, the intensity of the CH luminescence as compared to H(n) emission changes with the projectile. For the C+ and O+ collisions, we observe very strong emission of the A2Δ → X2Πr bands, while for H+ this band is barely distinguishable. The most remarkable feature displayed in Figure 3b and 3c for the C++THF and O++THF collisions, respectively, is the occurrence of emission from carbon atoms excited to the 2p3s 1P1, 2p4p 1D2, and 2p4p 3P states, which were not detected for collisions with the H+ projectiles. Because of lack of the ion−molecule reactions involving THF molecules that have been studied, no direct comparison could be made to literature data. However, our luminescence spectra may be compared with the emission spectra measured for photodissociation of THF.31 These emission spectra displayed production of hydrogen lines of the Balmer series, Hα to Hθ, and the A2Δ → X2Πr and B2Σ−→ X2Πr emission bands of CH followed by weak d3Πg → a3Πu bands of the C2 molecule.31

subtracted from the emission spectra. The obtained spectra were not corrected for the wavelength dependence of the sensitivity of the optical detection channel. The intensity scale in the luminescence spectra is given in arbitrary units, which corresponds to the detector output signal normalized to the cation beam current and acquisition time. Fragmentation yields were measured as functions of the incident cation energy. Measurements were performed using a 300-l/mm grating, which allowed us to record the luminescence spectrum in the 200 nm wavelength range with an optical resolution Δλ of 2.5 nm (fwhm). Thus, a complete spectrum covered the 180−570 nm wavelength range. As in the case of the high-resolution experiment, the background measurements enabled residual gas contaminations to be subtracted from the original spectra. After subtraction, the spectra were corrected for the wavelength dependence of the sensitivity of the optical detection channel. The intensities of the emission lines observed in the luminescence spectra were obtained by integrating over the peak areas (for molecules also containing vibrational and rotational bands). The background in the original spectra was taken to be the average of that below and above the studied lines and then was subtracted from the spectra. Afterward, the intensities were normalized to the cation beam current and recording time.

3. RESULTS AND DISCUSSION 3.1. Luminescence Spectra. In Figure 3 we compare luminescence spectra measured for collisions between THF and the H+, C+, and O+ cations at 1000 eV laboratory energy. All spectra contain the hydrogen lines of the Balmer series, Hβ to Hθ, due to the excited hydrogen H(n) atoms, n = 4−9, and the A2Δ → X2Πr and B2Σ−→ X2Πr emission bands of the

Figure 3. Luminescence spectra measured for H+, C+, and O+ at an energy of 1000 eV. The spectra were not corrected for the wavelength dependence of the sensitivity of the detection system. D


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The Journal of Physical Chemistry A Present results of fragmentation of the THF molecule may be also compared to the fragmentation of other five- and sixmembered heterocyclic molecules. The investigations of the photon43 and electron44 impact fragmentation of isoxazole molecules showed production of the same excited fragments, H(n), n = 3−7, CH(A2Δ, B2Σ−) and C2(d3Πg) apart from CN(B2Σ+). The vacuum-ultraviolet photofragmentation of pyridine and pyrimidine molecules,39 the six-membered heterocyclic compounds, also demonstrated fragmentation into alike atomic and diatomic fragments. The emission spectra of both molecules revealed39 production of hydrogen lines of the Balmer series, the A2Δ → X2Πr and B2Σ−→ X2Πr emission bands of the CH molecule and the B2Σ+ → X2Σ+ emission band of CN. Moreover, our analysis showed that the CH products are excited vibrationally and highly rotationally. In view of this ro-vibrational analysis we mention that fluorescence spectra also showed pronounced ro-vibrational emission bands of the CH43 and CN39,43 radicals. In those studies ro-vibrational temperatures were deduced from the rotational and vibrational populations representing a Boltzmann distribution and, for example, for the CH(A2Δ) state were equal 3500 and 3200 K, respectively.43 The above comparison may suggest that a similar mechanism may be responsible for neutral fragmentation of the five- and six-membered molecules. It is of note that similar fragmentation was also observed by Kong et al.,45 who, for example, used an intense laser field to excite and dissociate CH4. Their fluorescence spectra also showed pronounced rovibrational emission of the CH radical.45 3.2. Fragmentation Yields. Fragmentation yields, σrel, for production of the observed fragments in collisions with the H+, C+, and O+ cations plotted as functions of the projectiles velocity are shown in Figures 4, 5, and 6, respectively. The experimental uncertainties in σrel are the mean standard deviations obtained from 3 up to 15 independent measurements performed at fixed energies. H++THF Collisions. Fragmentation yields for the H(n), n = 4−7, and the CH(A2Δ) and CH(B2Σ−) fragments are shown in Figure 4. The measurements with H+ were performed in the 50−1000 eV energy range. These energies correspond to velocities of 100−440 km/s. The σrel obtained for the above fragments rise gradually in the presented velocity range. However, fragmentation yields for production of the H(n=4) atoms are higher than that observed for the CH(A2Δ) by a factor of about 10. In addition, the σrel of H(n), n = 4−7, decrease with increasing principal quantum number n. C++THF Collisions. Figure 5 shows fragmentation yields for the H(n), n = 4−6, and CH(A2Δ, B2Σ−) fragments plotted as a function of the C+ velocity. It also shows fragmentation yield for the excited carbon C(2p3s1P1) atoms which most likely occur due to transfer of an electron from the THF molecule to the projectile. The collision energies of the incident carbon cations were varied between 25 and 1000 eV correspond to velocity range from 20 to 127 km/s. It is seen that the functions have different shapes. Namely, the H(n) fragmentation yields increase with velocity. They show two weak maxima, first at the velocity of 63 km/s and second at 120 km/s. The σrel of the CH(A2Δ) and CH(B2Σ−) rise rapidly above 20 km/s to a maximum at 63 km/s and then decrease to become almost constant above 90 km/s. Moreover, in the entire velocity range the CH(A2Δ) fragmentation yield is higher than that of H(n). The fragmentation yield curve of the C(2p3s 1P1) increases gradually above 28 km/s up to a maximum at 110 km/s. This dependence is in contrast to the other two fragmentation yield

Figure 4. Fragmentation yields of the excited fragments obtained in collisions of H+ with THF.

curves. The C(2p3s 1P1) fragmentation yield at its maximum is 2 and about 20 times higher from the maximum values of the σrel of the CH(A2Δ) and H(n = 4), respectively. O++THF Collisions. The studies of tetrahydrofuran and oxygen cations collisions were carried out at the 100, 500, and 1000 eV energies corresponding to 35, 78, and 110 km/s velocities, respectively. The fragmentation yields obtained for the H(n), n = 4−6, CH(A2Δ, B2Σ−), and C(2p3s 1P1) fragments are presented in Figure 6. It is seen that all yields increase with increasing O+ velocity. 3.3. Collision Mechanisms. According to experimental7,8,16 and theoretical17,18 studies, it could be expected that single electron capture without major fragmentation of THF would dominate the H++THF interaction, while in C+ and O+ collisions apart from the electron capture processes more extensive fragmentation of THF should occur. For the H++THF collisions from the luminescence spectrum measured (Figure 3a) and the obtained values of the fragmentation yields shown in Figure 4, it is seen that the H(n) are the most abundant fragments in the entire studied energy range, while the CH radicals have negligible intensities. On the contrary, in two other systems the CH emission is much stronger than that of H(n) fragments (see Figures 3, 5, and 6). This would be clearly seen if we consider the intensity ratios of obtained fragments calculated as quotients of their corresponding fragmentation yields. Thus, in Figure 7a an evolution of the ratio of the H(n = 4) to CH(A2Δ) fragmentation yields in the studied collision systems has been illustrated. It is visible that for fast protons, values of the σH(n=4)/σCH(A2Δ) ratio are higher than that for massive and slow C+ and O+ cations. The enhancement of hydrogen peaks in the present experiment cannot be explained by fragmentation of THF itself, because on one hand the formation of the CH fragments is insignificant as compared to other impact systems (Figure 7a) and on the other hand the production of the E


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Figure 6. Fragmentation yields of the excited fragments obtained in collisions of O+ with THF. Figure 5. Fragmentation yields of the excited fragments obtained in collisions of C+ with THF.

excited carbon atoms from C+ due to the electron capture mechanism is energetically the most favored channel, it is not possible to ascertain unambiguously which route plays a major role in yielding this fragment in the present experiment. However, it is interesting to look once again at the graph with the σC(2p3s1P1)/σCH(A2Δ) ratio. At low C+ velocities below 63 km/ s, this relation mostly resembles those determined for O+ cations. Hence, it is possible that at low velocities the close collisions of carbon cations with the THF molecule dominate, and electron capture, dissociative ionization, and dissociative excitation occur simultaneously.16 The ratio, despite this, increases considerably from 63 to 110 km/s. At those velocities, the cations are fast and time of interaction with the target molecule may be very short. However, the projectile still may feel the energy potential of the target molecule, leading to charge transfer.17,18 This may suggest that at higher C+ velocities electron capture may prevail. Most of the energy would be then deposited to the neutralized C+ projectiles, yielding a high abundance of carbon atoms excited to several different states (see Figure 3b). It is of note that at higher collision velocities the fragmentation yields of other fragments have still significant values, indicating high electronic stopping of the C+ cations. Production of excited CH fragments in the C+ + THF collisions poses one intriguing problem that needs to be discussed. We have mentioned that the fragmentation yield of

excited hydrogen atoms from protons, due to the electron capture mechanism, is energetically the most favored channel. As it was pointed out in the case of studies of H+ and dR, the avoided crossings of the potential energy curves occurring even at the distance of 2.5 Å would induce efficient electron capture and therefore would decrease the probability of dissociation.17,18 Our results suggest that similar conditions may ensue for collisions between H+ and THF. Thus, distant single electron transfer occurs without major fragmentation of THF, and it dominates the H++THF interaction. Figure 7b shows the ratio of the C(2p3s 1P1) to CH(A2Δ) fragments calculated as the ratio of their fragmentation yields measured in the collisions of O+ and C+ with THF. For oxygen cations, the C(2p3s 1P1) and CH(A2Δ) fragments arise only due to disintegration of the ring of THF. We believe that production of the excited CH molecules may be enhanced in comparison to the fragmentation into carbon atoms (see Figure 1). This is exactly what we observe in Figures 3c and 6, and for this reason the values of the σC(2p3s1P1)/σCH(A2Δ) ratio for the O++THF collisions shown in Figure 7b are small. On the contrary, in the collisions of C++THF the excited carbon atoms may come from both neutralized projectiles and/or fragmentation of the molecular target, because electron capture reaction competes with other processes. Although the production of the F


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time at which fragmentation occurs varies dramatically depending on intramolecular vibrational energy relaxation as well as intramolecular proton/hydrogen motion. The hydrogen migration in biomolecules can be very fast (∼30 fs52) and can act as a trigger for specific dissociation pathways.53 Hence, at lower velocities the time of interaction of C+ and THF may be long enough to form the complex and transfer hydrogen from the THF molecule to the bound projectile. After a breakup of the complex, the CH(A2Δ) fragment may appear in increasing fragmentation yield. When the impact velocity rises, then the intermediate complex lifetime may become too short for an internal rearrangement to form CH and in consequence the intensity of CH(A2Δ) emission decreases. Finally, it is of note that an analogous situation was recently observed in a similar experiment concerning collisions of C+ with NH3 molecules.48 In particular, creation of electronically excited CN fragments and the rapid increase of the corresponding cross sections at lower velocities could not be explained as a simple abstraction or substitution reaction.48 In consequence, the occurrence of a transient ionic complex [C−NH3]+ in which the constituent units interact electrostatically, rearrange, and dissociate into the CN(B2Σ+) fragments was suggested.48 These observations are in accord with earlier ab initio calculations54 which showed the existence of a potential energy minimum at −4.77 eV below the C++NH3 level corresponding to a bound complex of the [C− NH3]+ configuration. Notwithstanding the arguments we have presented above, a definitive description for this process can only be provided in conjunction with theory and therefore the quantum chemical calculations are required. 3.4. Reaction Pathways. The excited H and CH fragments were detected in the three studied systems, but the excited carbon atoms were only observed in the collisions with the C+ and O+ cations (see Figure 3). Several different fragmentation processes may be therefore considered that lead to these products in collisions of the H+, C+, and O+ cations with the THF molecules: (a) electron capture from the THF molecule to the projectile followed by fragmentation of the parent THF cation, (b) dissociative ionization of the THF molecules (i.e., ionization of the THF molecules and further fragmentation of the parent THF cation), (c) dissociative excitation of the THF molecule (i.e., excitation of the THF molecules and further fragmentation of the excited neutral molecule). The electron transfer leading to the appearance of the parent cation usually occurs at relatively large projectile−target distances where it is a dominant mechanism.55 Fragmentation of target molecule requires closer and more violent collisions55 involving dissociative ionization and excitation processes. Each reaction may leave products both ionic and neutral, which are in their ground electronic states. In this experiment excited fragments were detected, and thus processes leading to formation of products in their ground electronic states are not discussed in the present article. As it was discussed in the previous section, the highest yields obtained for production of excited H and C atoms in the H++THF and C++THF collisions, respectively, may suggest that fragmentation processes are mostly preceded by electron transfer from tetrahydrofuran molecules to cations. Certainly, several earlier studies17,18,55,56 showed that the main process occurring in low-energy ion collisions with neutral systems is electron capture. It is thus expected that fragmentation of the

Figure 7. The ratio of fragmentation yields of the H(n = 4) to CH(A2Δ) as well as C(2p3s 1P1) to CH(A2Δ) fragments in the collisions of H+, C+, and O+ with THF.

excited CH rises rapidly above 20 km/s to a maximum at 63 km/s and then decreases fast at 90 km/s (see Figure 5). But why is it so prevalent at such low velocities? Experimentally, such an increase is an indication that transient complex formation may be taking place.46,47 In the past decades considerable experimental and theoretical investigations have demonstrated that ion-neutral molecule complexes are ubiquitous intermediates in the gas-phase reactions.46−50 The corresponding ion−molecule components remain bound together by ion−dipole attraction and may react with each other by hydrogen transfer.46 We therefore hypothesize that rapid enhancement of the fragmentation yield of the CH(A2Δ) may be a signature of a mechanism proceeding through intermediate [C−C4H8O]+ complex formation prior to dissociation. Indeed, the attraction between C+ cation and THF molecule may result from the interaction between the C+ charge and the THF molecule permanent dipole moment (1.63 D). This long-range attractive force between the reactants is described by a ∼r−2 electrostatic potential.48 Although comparable interaction may occur in other collision systems, we did not observe any signature of the complex creation. For instance, former discussion indicates that in the H++THF collisions the electron capture mechanism would rather prevail over any of the other processes. On the contrary, various processes may occur in the C++THF collisions, particularly at lower velocities. In fact, theoretical calculations of energy curves and couplings of the C2++THF molecular system have very recently shown strong delocalization of the electrons on the tetrahydrofuran ring toward the C2+ ion.51 Apart from single and double electron capture processes, in the planar approach, this led to real chemical bonding of the oxygen atom of THF and the projectile ion,51 indicating the possibility of complex formation. Similar conditions may thus arise for collisions between C+ and THF. This however requires that the complex should exist for a significant amount of time. Consequently, the G



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C4H8O+ cations formed in the excited states is the most probable route to produce observed fragments. The possible reaction channels proceeding through this pathway should therefore correlate with the ionic fragments observed in the cation mass spectra. Moreover, in fragmentation of polyatomic molecules, such as THF, a double sequential dissociation reaction is plausible, where one of the primary dissociation products undergoes further decomposition. These reactions need opening of the furanose ring of the THF molecule by scission of the weakest C−O bond and further cleavage of one of the C−C bonds (see Figure 1). Hence, the electron impact mass spectra of THF32 indicate that cyclopropane C3H6+ is the most abundant cation of THF. The corresponding neutral fragment is then the formaldehyde molecule, H2CO, which may further decompose into the C, H, and CH fragments. The second most abundant cation was observed at mass 42 amu, and it corresponds to C3H5+ in both conformations of linear and cyclopropenyl ions.32 The neutral moieties are the methoxy, CH3O radical or the formaldehyde molecule, and the H atom, and they may also undergo fragmentation, yielding the observed fragments. The last relatively abundant product ion in the mass spectrum corresponds to formation of the C4H7O+ fragment.32 It is of note that the deuterium labeling method29 established that 70% of the ions correspond to a loss of an α-H atom and the rest to a loss of a β-H atom from the THF molecule. Thus, this fragmentation channel would lead to production of neutral H(n) which would occur by detaching from the excited C4H8O+ and leaving just the C4H7O+ cation.

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (+48 58) 347-10-69. Fax: (+48 58) 347-28-21. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was conducted within the Action CM1301 (CELINA). T.J.W. Zubek for beneficial discussions. financial support from University of 5200-D464-14.

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framework of the COST thank Professor Mariusz B.P. acknowledges the Gdansk grant UG: 530-

REFERENCES

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4. CONCLUSIONS The present work provides novel data on low-energy cation collisions with tetrahydrofuran molecules, a possible model for the deoxyribose sugar unit. Fragmentation processes of THF molecules into excited neutral fragments by H+, C+, and O+ cation impact have been thus investigated. The following excited fragments have been observed: atomic hydrogen H(n), n = 4−9, carbon atoms in the 2p3s 1P1, 2p4p 1D2, and 2p4p 3P states, and vibrationally and rotationally excited diatomic CH molecules in the A2Δ and B2Σ− states. Present results demonstrate that the fragmentation mechanism strongly depends on the projectile cations. Fragmentation yields of the excited fragments have been measured as functions of the projectile energy. Rapid enhancement of the fragmentation yield of the CH(A2Δ) occurring at lower velocities in the C++THF collisions may be regarded as an indicator of the [C− C4H8O]+ complex formation prior to dissociation. Moreover, very high yields for production of excited hydrogen and carbon atoms in the H++THF and C++THF collisions, respectively, may suggest that collision processes are dominated by electron transfer from tetrahydrofuran molecules to cations. It is suggested that single electron capture without major fragmentation of THF dominates the H++THF interaction, while in C+ and O+ collisions apart from the electron capture processes more extensive fragmentation of THF occurs. These findings are qualitatively in agreement with recent experimental studies of photon-induced fragmentation of THF molecules.31 Present results are also consistent with ion-induced fragmentation of deoxyribose molecule16 and the most recent ab initio calculations of C4+ and proton collisions with deoxyribose performed by Bacchus−Montabonel.17,18 H


Article

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Fragmentation of Tetrahydrofuran Molecules by H+, C+, and O+

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