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Trifluoroacetic Acid and Trifluoroacetic Anhydride Radical Cations Dissociate near the Ionization Limit Lukas Lesniak,† Juana Salas,‡ Jake Burner,† Malick Diedhiou,† Maxi A Burgos Paci,‡ Andras Bodi,§ and Paul M Mayer*,† †
Department of Chemistry and Biomolecular Sciences, University of Ottawa, Ottawa K1N 6N5, Canada INFIQC − CONICET, Departamento Fisicoquimica, Universidad Nacional de Córdoba, Cordoba 5000, Argentina § Paul Scherrer Institute, Villigen, 5232 Switzerland Downloaded via BUFFALO STATE on July 28, 2019 at 22:06:42 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: The threshold photoelectron spectra (TPES) and ion dissociation breakdown curves for trifluoroacetic acid (TFA) and trifluoroacetic anhydride (TFAN) were measured by imaging photoelectron photoion coincidence spectroscopy employing both effusive room-temperature samples and samples introduced in a seeded molecular beam. The fine structure in the breakdown diagram of TFA mirroring the vibrational progression in the TPES suggests that direct ionization to the X̃ + state leads to parent ions with a lower “effective temperature” than nonresonant ionization in between the vibrational progression. Composite W1U, CBS-QB3, CBS-APNO, G3, and G4 calculations yielded an average ionization energy (IE) of 11.69 ± 0.06 eV, consistent with the experimental value of 11.64 ± 0.01 eV, based on Franck−Condon modeling of the TPES. The measured 0 K appearance energies (AE0K) for the reaction forming CO2H+ + CF3 from TFA were 11.92 for effusive data and 11.94 ± 0.01 eV for molecular beam data, consistent with the calculated composite method 0 K reaction energy of 11.95 ± 0.08 eV. Together with the 0 K heats of formation (ΔfH0K) of CO2H+ and CF3, this yields a ΔfH0K of neutral TFA of −1016.6 ± 1.5 kJ mol−1 (−1028.3 ± 1.5 kJ mol−1 at 298 K). TFAN did not exhibit a molecular ion at room-temperature conditions, but a small signal was observed when rovibrationally cold species were probed in a molecular beam. The two observed dissociation channels were CF3C(O)OC(O)+ + CF3 and the dominant, sequential reaction CF3CO+ + CF3 + CO2. Calculations revealed a low-energy isomer of ionized TFAN, incorporating the three moieties CF3CO+, CF3, and CO2 joined in a noncovalent complex, mediating its unimolecular dissociation.
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INTRODUCTION Ionic species play an important role in reaction mechanisms in combustion, atmospheric, and astrochemistry. Plasma-assisted combustion processes are of great importance in advanced propulsion systems,1−5 while gas-phase ions play a central role in interstellar chemistry.6−8 In the atmosphere, the primary cations N2+, O2+, and NO+ eventually react with H2O to form hydrates [H3O(H2O)n]+ by charge transfer.9 Numerous studies indicate that these cations react quickly with organic molecules in further electron transfer reactions producing cationic species.10−12 Information about the stability of cationic species that could be formed from the reactions of fluorinated compounds is scarce. Some years ago, de Petris et al.10,13 studied the cationic species formed by reactions of O3 with hydrogenated halocarbons in ionized mixtures, observing the formation of CHXYO3+ (X = H, Cl, F; Y = Cl, F), which are metastable and lose CO. More recently, Oomens and Morton revisited the reactions of CF3+ with aldehydes and ketones using infrared multiple photon dissociation to investigate the adducts and branching ratios of further decomposition.14 Perfluorinated © 2019 American Chemical Society
free radicals, such as CF3On and FCOn (n = 1 and 2), are known to be involved in the formation of tropospheric pollutants under environmental conditions.15 They can be generated easily from fluorinated acids and their derivatives. Of particular interest in the present study are trifluoroacetic acid (TFA) and trifluoroacetic anhydride (TFAN). In the upper atmosphere, under dilute conditions, they could exist as neutral molecules and be photoionized. However, there has been no previous study of the ion chemistry of either species, with the exception of protonated TFA, which was shown to undergo metastable loss of HF and CO.16 The use of conventional mass spectrometry to detect and study the chemistry of these species is hindered by the low stability of their molecular ions, resulting in mass spectra containing only fragment ions.17 Electron ionization with 70 eV electrons, commonly used in analytical mass spectrometers, generates ions with a broad internal energy distribution, exacerbating the problem. By Received: May 23, 2019 Revised: June 27, 2019 Published: June 28, 2019 6313
DOI: 10.1021/acs.jpca.9b04883 J. Phys. Chem. A 2019, 123, 6313−6318
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RESULTS AND DISCUSSION The threshold photoelectron spectrum (TPES) of TFA introduced by the effusive inlet is shown in Figure 1 along
electron kinetic energy analysis, imaging photoelectron photoion coincidence spectroscopy (iPEPICO) allows for internal energy selection in the parent ion.18 By scanning the photon energy, unimolecular fragmentation pathways, energetics, and trends can be revealed in unprecedented detail for small organics to large polycyclic aromatic hydrocarbons.19,20 The objective of the current study is to employ iPEPICO to examine the unimolecular reactions of low-internal-energy ions to learn the energetics of the ion dissociation processes. The difference between room-temperature (effusive) and internally cold starting molecules (formed in a seeded molecular beam) was probed to gain understanding of the role of the initial internal energy distribution of the neutral in the dissociative ionization process.
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EXPERIMENTAL AND COMPUTATIONAL PROCEDURES Trifluoroacetic acid (TFA) and trifluoroacetic anhydride (TFAN) were purchased from Sigma-Aldrich (Oakville, ON, Canada, both >99%) and used without further purification. iPEPICO spectroscopy experiments were conducted on the VUV beamline at the Swiss Light Source (SLS, Paul Scherrer Institute, Villigen, Switzerland) and have been described in detail elsewhere.21,22 Sample vapor was introduced into the experimental chamber in two different ways. An effusive inlet at 298 K was used to introduce and hence measure the roomtemperature TPES and breakdown curves, while a seeded molecular beam (750 torr Ar backing pressure, 100 μm orifice) was used to prepare and measure rovibrationally cold samples. Synchrotron radiation was collimated, dispersed using a 600 grooves/mm grating, and focused into a differentially pumped gas filter filled with a Ne/Ar mixture at 9 mbar over 10 cm optical length to filter out higher harmonic radiation.23 Finally, the monochromatic VUV light photoionized the molecules in a 2 × 2 mm2 interaction region at 3 meV energy resolution. Both photoions and photoelectrons were velocity-mapped on imaging multichannel plate (MCP) detectors. The electrons are time- and position-stamped at the detector, and the corresponding photoions are detected in delayed coincidence. Threshold electrons account for the majority of the signal at the center of the MCP, whereas kinetic-energy electrons are detected according to their off-axis momentum. Kinetic energy (hot) electrons without an initial lateral momentum component also have a trajectory to hit the center spot. The mass spectrum based on electrons detected in a ring around the center spot is used to subtract the hot electron contamination from the center signal to obtain the threshold photoionization mass spectrum.24 For TFA, the photon energy range used was 11.6−12.05 eV, with data points taken every 0.001 eV. For TFAN, the range was 11.0−12.2 eV, with data points taken every 0.002 eV. Ab initio and density functional calculations were performed with the Gaussian 16 suite of programs.25 The W1U,26 CBSQB3,27 CBS-APNO,28 G4,29 and G330 formalisms were employed to calculate product relative energies and ionization energies. Franck−Condon simulations utilized the B3-LYP/6311++G(d,p) level of theory in the double harmonic approximation including the Duschinsky rotation.31 The same level of theory was used to explore the potential energy surface of the TFAN cation in constrained optimizations to locate transition states and intermediate minima and account for TFAN’s exclusively dissociative photoionization behavior at room temperature.
Figure 1. Effusive (●,○) and molecular beam (■,□) breakdown curves for trifluoroacetic acid. The room-temperature threshold photoelectron spectrum is superimposed on the data (+). Solid lines represent the fitted breakdown diagram to the molecular beam data yielding an AE0K of 11.94 eV and a neutral molecule temperature T of 145 K.
with the breakdown curves from both effusive and molecular beam data. For the effusive data, dissociation to m/z 45 (CO2H+) takes place already at the onset of ionization, meaning that some of the cation internal energy distribution always reaches into the dissociation continuum. The molecular beam data, however, displays an intact molecular ion below 11.75 eV. The first band in the TPES has a maximum at 11.64 ± 0.01 eV. The TPES exhibits a rising threshold electron signal as a function of photon energy with a pronounced vibrational progression of approximately 340 cm−1. The TPES derived from the molecular beam data (Figure S1) exhibits similar behavior, but it does not significantly affect the appearance of the breakdown curve in Figure 1 because the crossover region is much narrower in the molecular beam due to rovibrational cooling. The room-temperature breakdown diagram exhibits a structure that correlates with the vibrational fine structure seen in the TPES. This is surprising because the breakdown diagram of the overwhelming majority of room-temperature samples has the shape of the integral of the thermal energy distribution,32 which would correspond to a monotonically and quite smoothly rising CO2H+ breakdown curve in TFA. As explained in a previous study on iodomethane,33 the prompt product of photoabsorption can be a neutral, for example, a Rydberg state. Neutral dissociation and autoionization processes will then compete, and their rates may depend on the rovibrational state of the Rydberg intermediate. If this mechanism applies only in TPES resonances and rotationally excited intermediates preferentially dissociate to neutral products, the internal energy distribution of TFA cations will be distinctly “colder” and the parent ion signal will be enhanced in the breakdown diagram. In iodomethane, continuum nonresonant autoionization was assumed to yield X̃ + state molecular ions in between resonances and autoionization of preferentially rovibrationally cold Rydberg intermediates à + state molecular ions in resonances.33 However, time-dependent DFT calculations at the B3-LYP/ 6314
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the O=C−O bond angle decreases as the C−C bond length increases, is the second most intense contributor to the photoelectron spectrum at 184 cm−1. The CF3 group also becomes more planar upon ionization, and the corresponding modes at 666 and 718 cm−1 are also active, although less so than the modes with a C−C stretch character. The overall band profile is the result of these vibrations and does not correspond to the progression of a single vibrational mode. This also explains why the fine structure is not measurably sharper in the molecular beam TPES (Figure S1) since peak widths are predominantly determined by the interplay of closely lying final vibrational states instead of the rotational envelope at the measurement temperature. In the threshold photoionization mass spectra, all fragment ion TOF peak shapes were symmetric, which is indicative of a fast dissociation and the absence of a kinetic shift.32 The 0 K appearance energy (AE0K) corresponds to the photon energy at which even the neutral molecules originally in the rovibrational ground state gain enough energy upon threshold photoionization to fragment. In molecules with a sharp onset of their internal energy distribution,36 this corresponds to the disappearance of the molecular ion signal from the breakdown diagram. Using the statistical modeling program to compute the internal energy distribution of TFA and model the breakdown diagram,32 the AE0K’s were determined to be 11.92 for effusive data and 11.94 ± 0.01 eV for molecular beam data. The fit is based on a neutral sample temperature of 145 K (solid lines, Figure 1), while the shift in the cross-over point for the two curves suggests a cooling of 122 meV,37 and a similar final temperature of 130 K (based on B3-LYP/6-31G(d) harmonic frequencies, neutral TFA has an average thermal energy of 171 meV at 298 K). The threshold was calculated at the W1U (11.946 eV), CBS-QB3 (12.006 eV), CBS-APNO (11.928 eV), G4 (11.885 eV), and G3 (11.974 eV) levels of theory yielding an average calculated IE of 11.95 ± 0.05 eV. Together with the 0 K heats of formation (ΔfH0K) of CO2H+ (600 ± 1 kJ mol−1)38 and CF3 (−465.0 ± 0.5 kJ mol−1),39 a 0 K AE0K of 11.94 ± 0.01 eV yields a ΔfH0K of neutral gaseous TFA of −1016.6 ± 1.5 kJ mol−1 (−1028.3 ± 1.5 kJ mol−1 at 298 K), consistent with a G3(MP2)//B3LYP calculated 298 K value of −1028 kJ mol−1 40 and 3 kJ mol−1 higher than quoted by Pedley et al.41 Trifluoroacetic anhydride (TFAN) mass spectra never exhibit a molecular ion peak when an effusive sample is ionized at room temperature. Observed mass spectral peaks at m/z 69 (CF3+) and 141 (CF3C(O)OC(O)+) have a small and constant absolute abundance over the photon energy range explored (data not shown). A probable source for these ions is the thermolysis of the neutral sample into CF3 and CF3C(O)OC(O) already at room temperature, both of which have low ionization energies (G4 values of 9.12 and 7.41 eV, respectively) and can be ionized at the photon energies employed. Only m/z 97 (CF3CO+) had an absolute increase after 11.45 eV. The TFAN molecular ion is observed in the seeded molecular beam as a minor peak (Figure 3). The G4 value for the vertical ionization energy is 11.97 eV and that for the adiabatic IE is 11.25 eV. G3 and CBS-QB3 values for the latter are 11.51 and 11.37 eV, yielding an average adiabatic IE of 11.38 ± 0.13 eV. The small (note the scale change for this ion) band shown for the molecular ion in Figure 3 is really not a band, as dissociation starts competing with ionization almost immediately and the molecular ion is completely dissociated by 11.63 eV. Again, m/z 97 is the predominant
6-311++G(d,p) level of theory suggest that, in the case of TFA, the vertical transition to A ′ X̃ + is followed by a 1.2 eV Franck− Condon gap (FC) until the FC point of the A ″ Ã + state. This is also confirmed by the earlier HeII photoelectron spectrum.34 Therefore, in the energy range of the cationic ground X̃ + state, initial transitions will end in the X̃ + state directly or in the Rydberg state manifold associated with it. Apparently, threshold ionization is nevertheless favored for rovibrationally cold initial neutrals near the cationic resonances. Presumably, initial rovibrational excitation may shorten the Rydberg-state lifetime and lead to predissociation instead of ionization. Alternatively, intermediate high-N Rydberg states in between vibrational resonances and with non-negligible oscillator strength are highly excited rotationally and may be preferentially formed from rotationally hot neutrals because of rotational selection rules. The FC simulations of the TPES at 300 and 50 K (employing the B3-LYP/6-311++G(d,p) level of theory), shown in Figure 2, are consistent with, albeit not
Figure 2. Franck−Condon simulations of the TPES for TFA at the B3-LYP/6-311++G(d,p) level of theory at two temperatures, namely, T = 50 K (resonances convoluted with a 10 meV Gaussian) and T = 300 K (resonances convoluted with a 22 meV Gaussian), superimposed on the experimentally recorded TPES at room temperature. FC resonances shown as vertical lines. Based on the FC simulations, the IE is determined as 11.64 ± 0.01 eV.
quantitatively, the observed features and were optimized to an IE of 11.64 eV. NIST quotes 11.5 eV as an evaluated IE,17 mostly based on early ultraviolet photoelectron spectroscopy results of Sweigart and Turner35 as well as Åsbrink et al.,34 who reported 11.46 and 11.6 eV as adiabatic ionization energies, respectively. The corresponding vertical ionization energies were quoted as 11.77 and 12.6 eV, respectively. We calculated the IE at the W1U (11.685 eV), CBS-QB3 (11.705 eV), CBSAPNO (11.696 eV), G4 (11.605 eV), and G3 (11.763 eV) levels of theory yielding an average calculated IE of 11.69 ± 0.06 eV, in excellent agreement with the experimental and FC simulation values. Analyzing the most intense transitions also reveals that the approximately 340 cm−1 progression in the TPES is in fact the result of several active normal modes and their combination bands. The most significant geometry change upon ionization is the elongation of the C−C bond from 1.552 to 1.869 Å. The O=C−O bond angle also increases from 126.7 to 141.0°. The most intense transition corresponds to a normal mode of synchronous C−C stretch and O=C−O bend motions at 443 cm−1. The asynchronous mode, in which 6315
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of CF3 to form m/z 141 and then loss of CO2, consistent with the experimental data. The appearance of a stable intermediate CF3C(O)OC(O)+ could indicate a stabilization with respect to the neutral because decarboxylation lifetimes for aroyloxy radicals have been reported to be on the scale of about 50 ps in the condensed phase.42,43 The dissociation onset in the molecular beam data is approximately 11.5 eV, essentially the same as the product energies in Figure 4. According to the potential energy surface, the TFAN ions that lose CF3 should almost always be energetic enough to form a CO2 neutral as well. However, kinetic energy is released in the CF3-loss step, and the internal energy of the leaving neutral is also lost to the system. This energy loss results in a fraction of the reactive flux being trapped in the m/z 141 potential energy well up to a photon energy of approximately 11.9 eV. The onset of the TFAN cation signal lies significantly higher than the adiabatic ionization energy. This is quite often the case in systems with large geometry relaxation upon ionization. For example, the onset of the adipic acid photoelectron spectrum lies at 10.1 eV, while its adiabatic ionization energy is 1.5 eV lower, at approximately 8.6 eV.44 In TFAN, the lack of the molecular ion signal between 11.25 and 11.5 eV is due to unfavorable FC factors upon ionization. The consequence of such a scenario is two-fold: the adiabatic ionization energy remains hidden from sight, and the produced parent ions are always highly energetic, which leads to an often immediate fragmentation onset.
Figure 3. Absolute peak abundances plotted as a function of photon energy for TFAN. Note the scale change for the molecular ion m/z 210.
fragment ion, rising steadily from 11.5 to a maximum at 11.9 eV, around the vertical IE, and then dropping at higher energies. A small competing process forms m/z 141, but this channel is outcompeted by m/z 97 by 11.67 eV. This is also evidence that the m/z 69 observed in the effusive sample was from ionized neutral CF3 as it is clear that CF3+ is not a competitive dissociation of low-internal-energy molecular ions. The fact that m/z 69 is the base peak in the electron ionization mass spectrum of TFAN is in turn explained by the sequential fragmentation process of m/z 97 yielding CO and CF3+, a process not allowed in the energy range studied herein. The decrease in absolute abundance of m/z 97 above the vertical IE is due to the poor FC factors above this energy, decreasing the threshold ionization cross section of TFAN at these photon energies. The G4 potential energy surface for the dissociation of the TFAN molecular ion can be found in Figure 4. The global minimum for ionized TFAN corresponds to a loosely bound adduct consisting of a core CF3CO+ ion electrostatically bound to CF3 and CO2. At 10.77 eV, this ion lies very low in energy relative to neutral TFAN. Once formed, it undergoes facile loss
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CONCLUSIONS Both trifluoroacetic acid (TFA) and trifluoroacetic anhydride (TFAN) dissociate close to their ionization limit. The appearance energy for the formation of CO2H+ from the former yielded a slightly revised heats of formation for gaseous TFA, that is, −1016.6 ± 1.5 kJ mol−1 at 0 K and −1028.3 ± 1.5 kJ mol−1 at 298 K. The TPES exhibited a vibrational fine structure superimposed on a broad continuum. Based on the breakdown diagram, cold neutrals are preferentially ionized in vibrational resonances, and the internal energy distribution is broader and indicative of a higher temperature of the neutrals in between resonances. This is proposed to be due to different ionization mechanisms in and in between resonances. Ionized TFAN was found to undergo a rapid, sequential dissociation yielding CF3 and CO2. The reaction was mediated by a lowenergy isomer consisting of a CF3CO+ ion electrostatically bound to CF3 and CO2.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.9b04883. Complete citation for ref 25. Comparison of TPES from effusive and molecular beam sources (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Maxi A Burgos Paci: 0000-0003-2002-7481 Andras Bodi: 0000-0003-2742-1051 Paul M Mayer: 0000-0001-6112-4398 Notes
Figure 4. G4 reaction pathway for the dissociation of ionized TFAN. FC refers to the vertical IE.
The authors declare no competing financial interest. 6316
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ACKNOWLEDGMENTS P.M.M. thanks the Natural Sciences and Engineering Research Council of Canada for continued financial support. The iPEPICO experiments were carried out at the VUV beamline of the Swiss Light Source at the Paul Scherrer Institute with the support of Dr. Patrick Hemberger.
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