Reactions in 1,1,1-Trifluoroacetone Triggered by Low Energy Electrons

Article pubs.acs.org/JPCA

Reactions in 1,1,1-Trifluoroacetone Triggered by Low Energy Electrons (0−10 eV): From Simple Bond Cleavages to Complex Unimolecular Reactions Eugen Illenberger* Institut für ChemiePhysikalische und Theoretische Chemie, Freie Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany

Martina C. Meinke* CharitéUniversitätsmedizin Berlin, Klinik für Dermatologie, Venerologie und Allergologie Charitéplatz 1, D-10117 Berlin, Germany ABSTRACT: The impact of low energy electrons (0−10 eV) to 1,1,1-trifluoroacetone yields a variety of fragment anions which are formed via dissociative electron attachment (DEA) through three pronounced resonances located at 0.8 eV, near 4 eV, and in the energy range 8−9 eV. The fragment ions arise from different reactions ranging from the direct cleavage of one single or double bond (formation of F−, CF3−, O−, (M−H)−, and M−F)−) to remarkably complex unimolecular reactions associated with substantial geometric and electronic rearrangement in the transitory intermediate (formation of OH−, FHF−, (M−HF)−, CCH−, and HCCO−. The ion CCH−, for example, is formed by an excision of unit from the target molecule through the concerted cleavage of four bonds and recombination to H2O within the neutral component of the reaction.

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INTRODUCTION We present an experimental study on dissociative electron attachment (DEA) to 1,1,1-trifluoroacetone (CF3C(O)CH3) by means of a crossed electron-molecular beams experiment and mass spectrometric detection of the negatively charged fragments. The present study is an extension of the recently published results on DEA to acetone (CH3C(O)CH3) and perfluoroacetone CF3C(O)CH3).1 Low energy electrons can trigger remarkably complex unimolecular reactions via DEA. In a recent study by our laboratory it has been demonstrated that one single electron at an energy near 2 eV can induce the excision of CN− from acetamide which proceeds via the concerted cleavage of several bonds and the formation of new molecules.2 In a theoretical study it has been predicted that one electron of 1.5 eV energy can break two bonds by entering the lowest unoccupied molecular orbital (LUMO) of a cyclic compound (quadricyclanone).3 After completing the cycloelimination reaction, the excess electron is liberated and hence the authors call it bond breaking by a catalytic electron (BBCE). In a further cyclic compound (norbornadienone ketal) a four-bond breaking BBCE reaction is predicted.4 DEA to single acetone, perfluoroacetone1, and also to the homogeneous clusters of acetone, trifluoroacetone, and perfuoroacetone5 was recently studied by our laboratory. Acetone is a weak electron scavenger and DEA is mainly operative via a resonance near 8.5 eV leading to a variety of fragment anions. Perfuoroacetone is a more effective electron scavenger with the nondecomposed parent anion (M−) as the dominant signal appearing within a narrow resonance near threshold (0 eV). The electron capture properties of acetone © 2014 American Chemical Society

significantly change when going to homogeneous clusters [5] as the DEA reactions near 8.5 eV are completely suppressed in favor of a strong low energy resonance (near 0.5 eV) leading to nondecomposed ionic complexes (Mn)− also subjected to the loss of two neutral H2 molecules ((Mn − 2H2)−). In clusters of trifluoroacetone, ionic complexes consisting of nondecomposed ions (Mn−) and ionic complexes subjected to the loss of one and two HF molecules are observed right at the threshold (0 eV).

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EXPERIMENTAL SECTION The experiments were performed by means of a crossed electron/molecular beams apparatus as described previously.6,7 The apparatus consists of an electron monochromator, an effusive beam system with a collision chamber, and a quadrupole mass analyzer that are housed in an UHV chamber at a base pressure of 10−8 mbar. In brief, an electron beam of well-defined energy (fwhm ≈ 0.1−0.2 eV, electron current ≈ 20 nA) generated from a trochoidal electron monochromator8 orthogonally intersects with an effusive molecular beam of gas phase trifluoroacetone. The experiments were carried out at a sample pressure of 10−5 mbar (reading at the ion gauge) corresponding to a target gas pressure in the collision chamber by 1 to 2 orders of magnitude lower. This safely ensures single Special Issue: Franco Gianturco Festschrift Received: March 29, 2014 Revised: May 13, 2014 Published: May 14, 2014 6542

dx.doi.org/10.1021/jp503129z | J. Phys. Chem. A 2014, 118, 6542−6546

The Journal of Physical Chemistry A

Article

collision conditions and hence the anions should arise from unimolecular processes (although contributions from ion− molecule reactions can never completely be ruled out). Negative ions formed in electron−molecule collisions are extracted from the reaction zone toward a quadrupole mass analyzer and detected by a single pulse counting technique. The intensity of negative ions is recorded as a function of the incident electron energy. The ion intensity can be considered as a rough image of the relative cross section for DEA into that particular ionic fragment. It has to be noted, however, that the ion count rate depends on the (mass dependent) transmission of the quadrupole, the kinetic energy release of the respective ion in the DEA reaction, and further experimental parameters like ion draw out conditions, etc. The electron energy scale is calibrated using the well-known SF6− signal which exhibits a sharp peak near 0 eV.

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RESULTS AND DISCUSSION In Figures 1−4 the yields due to the various anionic products arising from electron impact to trifluoroacetone are plotted.

Figure 2. Ion yields for the fragments CF3− and O− arising from DEA to trifluoroacetone.

Figure 3. Ion yields for the fragments (M−HF)− (loss of a neutral HF molecule) and the bifluoride ion (FHF)− arising from DEA to trifluoroacetone.

e− + RX → RX−#

(1a)

which is characterized by a particular electronic structure. In the second step, the TNI (RX−#) decomposes into a stable fragment ion and a neutral molecule or radical according to

Figure 1. Ion yields for the fragments (M−H)− (hydrogen loss), (M− F)− (loss of neutral F atom), and F− arising from DEA to trifluoroacetone.

RX−# → R + X−

(1b)

At sufficiently high energies, the neutral component may consist of more than one neutral fragment. In competition to dissociation, the transient anion may loose the attached electron to recover the neutral molecule, eventually in an excited state (resonant scattering). The terms “transient negative ion” (TNI) and “resonance” are used synonymously and describe a metastable (or quasi-bound) quantum state embedded in the continuum (represented by the neutral molecule and the extra electron at infinity) with the extra electron temporarily trapped in the vicinity of the molecule.9 The TNI is characterized by either the accommodation of the extra electron directly into one of the normally unoccupied molecular orbitals (MOs) or by electronic excitation induced by the incoming electron which is concomitantly trapped in the field of the excited target. In the first case a so-called “shape

These fragment ions are exclusively due to DEA as obvious from the pronounced resonant features. The ion yields for the DEA reactions involving simple bond cleavages are plotted in Figures 1 and 2, and those for the more complex reactions are plotted in Figures 3 and 4. From the ion yields it is obvious that three resonance regions are present, namely a low energy feature peaking at 0.8 eV, a second resonance peaking near 4.3 eV, and a third feature peaking in the range 8−9 eV. To characterize the electronic structure of the these resonances, one has to remember that DEA can be considered as a two-step process in which in the first step an electron is resonantly captured by a neutral molecule (RX) to form a so-called transient negative ion (TNI): 6543



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higher energies, unimolecular reactions decomposing into more than one neutral fragment may become energetically accessible. Formation of (M−H)−. The loss of a neutral hydrogen atom via DEA in the subexcitation domain (below the level of electronic excitation) is a reaction common to many organic molecules including organic acids11 or the DNA nucleobases.12 The reaction creates the closed shell anion (M−H)− according to e− + M → (M − H)− + H

(2)

with M being the target molecule. The appreciable electron affinity of most radicals (M−H) (often in the range 3−4 eV12) is in fact the energetic driving force for hydrogen loss to take place at energies considerably below the (O−H) or (C−O) bond dissociation energy. The mechanism of hydrogen loss from organic acids has recently been a matter of controversy (electron capture into a π*(CO) MO and coupling to a σ*(OH) state13 versus direct attachment into a σ*(OH) state14). Recent experiments from the Fribourg group on formic acid using isotope labeling15 clearly indicate that the a σ*CH mechanism is operative in formic acid. In the present molecule, H-loss necessarily proceeds from the methyl group. If it is formed via the low energy resonance around 0.8 eV one can propose electron capture into the π*CO MO following predissociation, that is, coupling of the π*CO MO with a σ*CH MO. An alternative description would be direct electron capture into a σCH * MO creating a TNI with an exceptionally short autodetachment lifetime (like that proposed in ref 14) finally resulting in a DEA resonance at low energy. Interestingly, the (M−H)− ion is observed at increasing energy along all three resonance regions. While the reaction within the first resonance (0.8 eV) creates the two fragments in their electronic ground states, the reactions via the two higher lying resonances must create significantly excited (M−H)− ions, extending far into the energy range above the continuum for both detachment and dissocication continuum of the extra electron, that is, the fragment ion (M−H)− must be in a metastable state with respect to both dissociation and electron loss. The Complementary Ion Pair F− and (M−F)−. These two fragment ions arise from a single C−F bond cleavage with the excess electron finally localized on either of the two fragments according to the processes

Figure 4. Ion yields for the fragments OH−, CCH−, and HCCO− arising from DEA to trifluoroacetone. CCH− and HCCO− are formed via a concerted cleavage of several bonds and recombination (see the text).

resonance” is formed and in the second case a “core excited resonance”. Shape resonances involving the lowest unoccupied MO are usually present in the energy range below 4 eV. Core excited resonances are present at higher energies, that is, in the range of electronic excitation. If the energy of the TNI is located below that of the associated electronically excited neutral, it is commonly referred to as a Feshbach resonance. Since Feshbach resonances cannot decay into the neutral molecule via a one electron transition, they are comparatively long-lived and hence play a significant role in DEA in the energy range above 4 eV. The energy range near the threshold (0 eV) plays a particular role as weak polarization forces or permanent dipoles may weakly bind an extra electron, and the TNI may subsequently undergo DEA.10 The formation of TNIs near the threshold is beyond the Born−Oppenheimer approximation and the picture of localized potential energy surfaces. In light of that, one can safely assume that the first feature can be assigned as shape resonance, most likely involving the π*CO MO. The second feature peaking near 4.3 eV may consist of shape resonances involving antibonding σ*CF and σ*CH MOs with possible contributions of Feshbach resonances. The third feature most likely consists of Feshbach resonances with possible contributions of higher energy shape resonances. In the following we shall consider in more detail the mechanism of the corresponding DEA reactions. Figures 1 and 2 present the ion yields of fragment ions arising from the cleavage of a single bond while Figures 3 and 4 display the DEA product ions arising from more complex reactions. A. Reactions Arising from Direct Bond Cleavages. The fragments ions plotted in Figures 1 and 2 formally arise from the cleavage of one bond. This is true for energies not far above the threshold for the corresponding bond cleavage process. At

e− + M → F− + (M − F)

(3a)

e− + M → F + (M − F)−

(3b)

Apparently, formation of F− is not observed from the low energy resonance (Figure 1) which is most likely due to energy constraints. The energy threshold (or reaction enthalpy, ΔH0) for a DEA reaction yielding two fragments (process 1) can be expressed as ΔH0 = D(R − X) − EA(X)

(4)

where D is the bond dissociation energy and EA is the electron affinity of the neutral fragment on which the excess electron becomes localized. While the electron affinity of F is very well established (3.4 eV),16 typical C−F binding energies are in the range 5.0−5.5 eV,17−19 and hence the energy threshold for F− formation is located above 1.6 eV and energetically not accessible within the lowest resonance. The fact that (M−F)− is formed already from the first resonance, in turn indicates that 6544



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and CO bond in the TNI, accompanied by hydrogen transfer and recombination to OH−. Figure 4 shows that it is only generated from the third resonance. The cleavage of the two bonds requires an energy of about 12 eV which is compensated by the O−H binding energy (4.1 eV) and the electron affinity of the OH radical (1.83 eV)16 which directly explains why OH− formation from the first and second resonance feature is energetically inaccessible. Like the hydroxyl ion, the ion CCH− is also only formed within the third resonance region with an additional structure at 12 eV not present in the other DEA products. Its formation represents an excision of the HCC unit from the TNI. The energetically most favorable reaction is

the electron affinity of (M−H) exceeds that of F, i.e., EA(CF2C(O)CH3) ≥ 3.4 eV. As for the fragment (M−H)− considered above, the ion (M− F)− formed via the higher energy resonances must be in a metastable state with respect to both loss of the extra electron and dissociation. Formation of CF3− and O−. These two fragments are also generated by the cleavage of a single bond and their ion yields are plotted in Figure 2. CF3− is formed by cleavage of the C− CF3 bond, it is not present from the resonance of low energy (0.8 eV). The electron affinity values of CF3 reported in the literature scatter between 1.8 and 3.2 eV due to the often indirect means of determination.16 Photodetachment is the most direct way to obtain electron affinities and a recent study on CF3 gives a value of 1.82 eV for the (adiabatic) electron affinity of CF3.20 Since the C−CF3 bond dissociation energy is in the range of 4 eV it is clear the formation of CF3− is not accessible from the lowest resonance at 0.8 eV O− is formed by a direct cleavage of the CO double bond, the strength of which is in the range of 7.6 eV for ketones.21,22 With EA (O) = 1.8 eV16 a value of 5.8 eV is obtained for the energy threshold of O− formation which explains that this ion is energetically not accessible via the first two resonances. B. Complex DEA Reactions. Figures 3 and 4 display the ion yields of fragments arising from more complex DEA reactions. This concerns the cleavage of more than one bond and formation of new bonds such as in the generation of OH− or (M−HF)−, but also reactions associated with substantial electronic and geometrical rearrangement in the TNI and formation of new ionic and neutral species. The Loss of a Neutral HF Molecule and Formation of the Bifluoride Anion FHF−. The loss of a neutral HF molecule generating the ion (M−HF)− exclusively occurs from the lowest resonance located near 0.5 eV (Figure 3). This reaction requires the cleavage of two bonds, hydrogen transfer and recombination to HF. From energetics, the two bond cleavages require an energy in the range of 9.0−9.5 eV which is compensated by the HF binding energy (5.8 eV)16 and the electron affinity of C3H2F2O (M−HF) for which no data is available so far. The loss of a neutral HF unit is a comparatively weak DEA reaction. In the previous study on electron attachment to clusters of trifluoroacetone,5 the dimer anion (M2−) was found to be a particularly strong signal at energies near threshold followed by the series (M2−nHF) . These reactions were interpreted by the formation of a cyclic structure (1,4cyclohecanedione) subjected to the loss of HF units. The bifluoride ion (FHF)− is characterized by an unusually strong hydrogen bond (D (FH−F−) ≥ 1.5 eV)23 and hence a remarkable stability (ΔHf° (FHF−) = −713 kJ mol−1).24 It has previously been observed in DEA to difluoroethylene within a resonance around 2 eV.25 In the present compound, it is already formed right at the threshold (0 eV), and one can suppose that the precursor represents an ion in which the excess charge is weakly bound by polar forces. Formation of (FHF)− requires the cleavage of three bonds and recombination to (FHF)−. It must be emphasized that an excess electron with virtually no extra energy triggers a reaction of such complexity. The energetic situation, however, cannot be evaluated so far since there is no information on the neutral counterpart(s) formed in the respective DEA reaction. Formation of OH−, HCCO−, and CCH−. The formation of a hydroxyl anion (OH−) proceeds through cleavage of the C−H

e− + CF3C(O)CH3 → HCC− + H 2O + CF3

(5)

which requires the cleavage of four bonds and recombination to H2O. To obtain the reaction enthalpy of reaction 5, one needs the heat of formation of trifluoroacetone which is not available from standard thermochemical sources.16,17 But taking the medium between acetone (−218 kJ mol−1) and perfluoroacetone (−1433 kJ mol−1) should give a reasonable estimate yielding ΔHf° (CF3C(O)CH3) = −825 kJ mol−1. With ΔHf° (CCH) = 477 kJ mol−1, EA (CCH) = 2.97 eV, ΔHf° (H2O) = −242 kJ mol−1 and ΔHf° (CF3) = −470 kJ mol−1,16 one obtains ΔHf° = 303 kJ mol−1 that is, the reaction is energetically accessible for electron energies above 3.1 eV. The experimental appearance energy of CCH− is significantly higher, and the peak maximum of the ion yield is located at 8.8 eV. At that energy, the neutral component of the reaction products in eq 5 may consist of H2 + O + CF3 or OH + CHF3. An ion with the stoichiometric composition C2HO− is observed from the second and third resonance. It is likely that it represents the HCCO− anion, the radical of which is an important intermediate in combustion chemistry in the oxidation of small hydrocarbons such as ethylene.26 It is formed from the TNI by the excision of a HCCO unit via the following possible low energy reactions e− + CF3C(O)CH3 → HCCO− + H 2 + CF3

e− + CF3C(O)CH3 → HCCO− + H + CHF3

(6a) (6b) −27,28

From the photoelectron detachment spectra of HCCO the (adiabatic) electron affinity of HCOO was derived as EA(HCCO) = 2.34 eV, and its heat of formation was ΔHf° (HCCO−) = −56 kJ mol−1. By taking the well established data (ΔHf°(H) = 218 kJ mol−1 ΔHf° (CHF3) = −697 kJ mol−1) and the other numbers from our discussion one arrives at a reaction enthalpy of 299 kJ mol−1 for reaction 6a and 290 kJ mol−1 for reaction 6b. These numbers clearly indicate that HCOO− is energetically not accessible from the low energy resonance. Although reaction 6b is energetically more favorable by a small amount, it additionally requires hydrogen transfer. It is hence likely that both reactions 6a and 6b contribute to the ion yield in Figure 4.

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CONCLUSION From the experiments presented here it can be seen that in trifluoroacetone low energy electrons drive a variety of unimolecular reactions via DEA involving three pronounced resonances peaking at 0.8 eV, near 4 eV, and in the energy range 8−9 eV. These reactions range from simple bond cleavages creating a negative and a neutral fragment ion to 6545



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(14) Gallup, G. A.; Burrow, P. D.; Fabrikant, I. I. Electron-induced bond breaking at low energies in HCOOH and glycine: The role of very short-lived σ* anion states. Phys. Rev. A 2009, 79, 042701. (15) Janeckova, R.; Kubala, D.; May, O.; Fedor, J.; Allan, M. Experimental evidence on the mechanism of dissociative electron attachment to formic acid. Phys. Rev. Lett. 2013, 111, 213203. (16) The NIST Chemistry WebBook: http://webbook.nist.gov/ (accessed March 2014). (17) CRC Handbook of Chemistry and Physics, 78th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1997. (18) McMillen, D. F.; Golden, D. M. Hydrocarbon bond dissociation energies. Annu. Rev. Phys. Chem. 1982, 33, 493−491. (19) Radzig, A.; Smirnov, B. M. Reference Data on Atoms, Molecules and Ions; Springer-Verlag: Berlin, 1985. (20) Deyerl, H. J.; Alconcel, L. S.; Continetti, R. E. Photodetachment imaging studies of the electrom affinity of CF3. J. Phys. Chem. A 2001, 105, 552−557. (21) Christen, H. Grundlagen der Organischen Chemie; Verlag Sauerländer: Aarau, Germany, 1985. (22) Orchin, M.; Kaplan, F.; Macomber, R. S.; Wilson, R. M.; Zimmer, H. The Vocabulary of Organic Chemistry; John Wiley & Sons: New York, 1980. (23) Heni, M.; Illenberger, E. The stability of the bifluoride ion (FHF)− in the gas phase. J. Chem. Phys. 1985, 83, 6056−6057. (24) Wenthold, P. G.; Squires, R. R. Bond dissociation energies of F2− and HF2−. A gas phase and G2 theoretical study. J. Phys. Chem. 1995, 99, 2002−2005. (25) Süzer, S.; Heni, M.; Illenberger, E.; Baumgärtel, H. The dissociation of the 2∏ fluoroethylene anions. Chem. Phys. Lett. 1982, 87, 244−248. (26) Gaydon, A. G. The Spectroscopy of Flames, 2nd ed.; Wiley: New York, 1974. (27) Oakes, J. M.; Jones, M. E.; Bierbaum, V. M.; Ellison, G. B. Photoelectron spectroscopy of CCO− and HCCO−. J. Chem. Phys. 1983, 87, 4810−4815. (28) Schäfer-Bung, B.; Engels, B.; Taylor, T. R.; Neumark, D. M.; Botschwina, P.; Peric, M. Measurement and theoretical simulation of the HCCO- anion photoelectron spectrum. J. Chem. Phys. 2001, 215, 1777−1788.

complex reactions involving multiple bond cleavages and formation of new molecules. The closed shell fragment ion due to the loss of a neutral hydrogen atom, (M−H)−, and that due to the loss of a neutral fluorine atom, (M−F)− necessarily arise from DEA reactions generating two fragments which in turn indicates that within the third resonance feature (8−9 eV) both, (M−H)− and (M−F)− must be metastable with respect to dissociation and autodetachment.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from Deutsche Forschungsgemeinschaft (DFG), Fonds der Chemischen Industrie (VCI), and Freie Universität Berlin is gratefully acknowledged.

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REFERENCES

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Reactions in 1,1,1-Trifluoroacetone Triggered by Low Energy Electrons

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