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Helium Tagging Infrared Photodissociation Spectroscopy of Reactive Ions Jana Roithová,* Andrew Gray, Erik Andris, Juraj Jašík, and Dieter Gerlich Department of Organic Chemistry, Faculty of Science, Charles University in Prague, Hlavova 2030, Prague 2, 128 43, Czech Republic

CONSPECTUS: The interrogation of reaction intermediates is key for understanding chemical reactions; however their direct observation and study remains a considerable challenge. Mass spectrometry is one of the most sensitive analytical techniques, and its use to study reaction mixtures is now an established practice. However, the information that can be obtained is limited to elemental analysis and possibly to fragmentation behavior, which is often challenging to analyze. In order to extend the available experimental information, different types of spectroscopy in the infrared and visible region have been combined with mass spectrometry. Spectroscopy of mass selected ions usually utilizes the powerful sensitivity of mass spectrometers, and the absorption of photons is not detected as such but rather translated to mass changes. One approach to accomplish such spectroscopy involves loosely binding a tag to an ion that will be removed by absorption of one photon. We have constructed an ion trapping instrument capable of reaching temperatures that are sufficiently low to enable tagging by helium atoms in situ, thus permitting infrared photodissociation spectroscopy (IRPD) to be carried out. While tagging by larger rare gas atoms, such as neon or argon is also possible, these may cause significant structural changes to small and reactive species, making the use of helium highly beneficial. We discuss the “innocence” of helium as a tag in ion spectroscopy using several case studies. It is shown that helium tagging is effectively innocent when used with benzene dications, not interfering with their structure or IRPD spectrum. We have also provided a case study where we can see that despite its minimal size there are systems where He has a huge effect. A strong influence of the He tagging was shown in the IRPD spectra of HCCl2+ where large spectral shifts were observed. While the presented systems are rather small, they involve the formation of mixtures of isomers. We have therefore implemented two-color experiments where one laser is employed to selectively deplete a mixture by one (or more) isomer allowing helium tagging IRPD spectra of the remaining isomer(s) to be recorded via the second laser. Our experimental setup, based on a linear wire quadrupole ion trap, allows us to deplete almost 100% of all helium tagged ions in the trap. Using this special feature, we have developed attenuation experiments for determination of absolute photofragmentation cross sections. At the same time, this approach can be used to estimate the representation of isomers in a mixture. The ultimate aim is the routine use of this instrument and technique to study a wide range of reaction intermediates in catalysis. To this end, we present a study of hypervalent iron(IV)−oxo complexes ([(L)Fe(O)(NO3)]+). We show that we can spectroscopically differentiate iron complexes with S = 1 and S = 2 according to the stretching vibrations of a nitrate counterion.

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of short-lived reaction intermediates.1−5 It has been particularly helpful and subsequently widely used to investigate reactions with elusive reactive intermediates that cannot be characterized by other spectroscopic techniques such as NMR or X-ray spectroscopy.6−8 While the exact mass obtained from traditional mass spectrometry experiments can provide detailed information

INTRODUCTION

Mass spectrometry (MS) has growing importance in almost all fields of chemical and biochemical research. New and innovative developments continue to expand the dynamic range of applications where its use can be beneficial. Within synthetic chemistry, mass spectrometry is applied as one of the prime analytical tools. Following the advent of electrospray ionization (ESI), mass spectrometry also developed into a technique capable of playing an important role in the investigation of reaction mechanisms and the characterization © XXXX American Chemical Society

Received: November 4, 2015

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DOI: 10.1021/acs.accounts.5b00489 Acc. Chem. Res. XXXX, XXX, XXX−XXX

Article

Accounts of Chemical Research

Figure 1. Geometry of the ISORI instrument. (a) Design of the wire quadrupole ion trap mounted into a copper box. (b) Simulation of the potential of the w4PT. Contours show the difference between the numerically calculated potential and the ideal quadrupole potential in the depicted color code. The surrounding cylinder (diameter of 27 mm) has been set to 0 V.33

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INSTRUMENT Our home-built instrument, ISORI (ion spectroscopy of reaction intermediates), for helium tagging photodissociation experiments is based on a commercial Thermo Finnigan TSQ 7000 platform (Figure 1).33 The ion-source chamber is used in its original form; therefore we can use all available ionization modes (electron ionization, chemical ionization, electrospray ionization, etc.). The generated ions can be mass selected by a quadrupole (4P1) and guided (QPB and 8P) toward the wire quadrupole ion trap (w4PT). The trap is based on a linear quadrupole geometry where each of the rods is approximated by six copper wires and temperatures below 3 K can be reached. The effective formation of helium complexes requires not only the low temperature but also a high number density of helium (1015 cm−3), which is reached after a helium pulse in the beginning of each trapping period. For reactive ions, such as C6H62+, we detected up to 11 attached helium atoms.36 However, the experiments were performed with ions that have been tagged by single atoms, because they are available in relatively high abundances and are likely to provide the lowest degree of structural perturbation.31,37 The instrument is fitted with two infrared optical parametric oscillators (OPO) to supply variable wavelength photon beams to irradiate the trapped ions. The effect of irradiation is monitored in two alternating cycles. After the helium gas is pumped down, the ions are irradiated by photons from the OPO/OPA system and extracted toward the second quadrupole (4P2), which is set to transmit only the ions with m/z of the helium complexes. After the mass selection, the number of helium complexes is counted (Ni). The subsequent cycle is identical, but the photon beam is blocked. This means that we acquire the total number of generated helium complexes N0. The helium tagging IRPD spectra are then obtained in the form of a relative attenuation (1 − Ni(ν̃)/N0).

about the elemental composition of the detected ions, it says nothing about the structure. Many textbooks and databases have been devoted to the determination of structure from the fragmentation patterns of open-shell organic ions such as those obtained from electron impact ionization.9 However, the inherent variations associated with the technique have meant that no general schemes can be developed for closed-shell reaction intermediates detected by ESI-MS.10 This difficulty has necessitated the development of alternative approaches to the empirical analysis of fragmentation patterns, and as such, methodologies for applying IR, UV−vis, or fluorescence spectroscopy to the mass selected ions have been under development.11−15 In particular infrared spectroscopy of mass-selected gaseous ions has enjoyed a significant boom in recent years. It has been used for a number of different purposes such as the investigation of solvent effects,16−18 in addressing the structures of various organic cations,19 for the investigation of reaction intermediates,20 and also for the investigation of biorelevant protonated molecules.21,22 Such spectroscopy can be performed by application of multiple photon packets to trapped ions in order to induce their fragmentation (infrared multiphoton dissociation spectroscopy).23,24 In order to perform favorable single-photon infrared photodissociation (IRPD) spectroscopy, the required dissociation energy has to be below the energy of one photon (on the order of tens of electronvolts). The simplest trick to overcome this constraint involves a loosely bound tag that detaches upon absorption of a single IR photon and has minimal impact on the studied ion such that its infrared spectrum is not perturbed. The most popular tags to date have been atoms of rare-gases (usually argon)25−29 and the hydrogen molecule.17,30 Our ultimate goal has been to investigate the structure of reactive intermediates present in solution; therefore the genuine “innocence” of the tag was of prime interest.31,32 We have constructed a unique instrument that can perform infrared photodissociation spectroscopy using helium tagging.33,34 In order to attach helium, the ions have to be internally relaxed to a sufficiently low temperature.25,35 While the formation of such complexes has been for a long time restricted to supersonic expansions, our instrument enables the ion cooling and the subsequent formation of the helium complexes inside an ion trap. The advantages of helium tagging were initially demonstrated by the measurement of infrared photodissociation spectra of hydrocarbon dications.

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HELIUM AS AN INNOCENT TAG Ion spectroscopy of small molecular dications has to be performed using either helium or neon tagging because argon and heavier rare gases (and molecular tags such as H2 or H2O) usually provide electron transfer and thus cause destruction of the dications.38,39 Alternatively, heavier rare gas atoms can form tightly bound species with the investigated ions, which leads to significant changes in their infrared spectra.40−42 Figure 2 shows an example of a helium tagging IRPD spectrum of the C7H62+ dication generated from toluene. The theoretical B


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benzene dication may be considered as a prototypical member of the nonclassical carbocation family.45−47 The permethylated analog of C6H62+, C6(CH3)62+, can be prepared in Magic acid solution from various precursors, and it has been shown to possess a pyramidal structure with a five-membered base and one carbon at the apex.48 It was found that vertical double ionization of benzene primarily yields dications with sixmembered rings (312+ and 112+, Scheme 1),49 but theoretically, it was predicted that C6H62+ should also adopt the pyramidal structure (122+).50−52 The helium tagging IRPD spectrum (Figure 3a) shows several bands that do not correspond with any of the theoretical IR spectra calculated for the most feasible C6H62+ structures. Different theoretical models gave a consistent picture. We have also calculated theoretical spectra of optimized helium complexes of the C6H62+ isomers. Helium can coordinate via a hydrogen atom or to π-electrons of carbon−carbon bonds with binding energies on the order of 1−3 kJ mol−1. The effect of the coordination on the IR spectra is negligible (details can be found in the Supporting Information of ref 44). The assumption that several isomers of C6H62+ contribute to the IRPD spectrum was confirmed by two-color IRPD experiments (Figure 3b,c). To perform two-color IRPD spectroscopy, we use photons from one OPO to selectively deplete the population of a certain isomer or isomers leaving a simplified population to be interrogated by photons from another OPO. Fixing the photon energy of the first OPO to correspond with the 3040 cm−1 band (blue arrow in Figure 3a) led to the depletion of complexes with this absorption band. The second OPO was then scanned, and the resulting spectrum is shown in Figure 3b. This spectrum could be associated with the benzene dication with a distorted six-membered ring in the singlet state (112+). When the first OPO was set to the 2940 cm−1 band (red arrow in Figure 3a), the helium tagged 112+ isomers were depleted, and the second OPO provided an IRPD spectrum that could be associated with the pyramidal isomer 1 2+ 2 (Figure 3c). Due to the optimized overlap of the ion cloud, confined by the effective potential of the quadrupole, with the wellcollimated photon beam, we are able to deplete almost 100% of the trapped helium complexes (cf. Figure 3). In addition, counting the ions with and without laser, as well as accurately determining the relative photon fluence, allows us to measure absolute photofragmentation cross sections (Figure 4).36 The depletion of helium tagged complexes by irradiation at a certain band is followed as a function of photon beam pulse energy (Figure 4b,c). The resulting dependence can be fitted with eq 1 and the characteristic energy, Ej, can be transformed to the absorption cross section (red bars in Figure 4a).

Figure 2. (a) Helium tagging IRPD spectrum of C7H62+ generated by electron ionization from toluene and its comparison with the B3LYP/ cc-pVTZ calculated IR spectra of (b) bare, (c) helium tagged and (d) argon tagged dications.33

spectrum of the bare dication (Figure 2b) as well as the helium tagged dication (Figure 2c) agree well with the experiment. The binding energy of helium was calculated to be 1 kJ mol−1 (method B3LYP/cc-pVTZ). In comparison, the spectrum of an argon tagged dication is completely different due to geometry distortions imposed by binding argon (binding energy is 37 kJ mol−1). Argon thus cannot be considered as an “innocent” tag for reactive ions such as those that we will present here. We mention in passing a nice example of helium “innocence” from the laboratory of John Maier.43 They employed helium tagging to study the electronic transitions in C60+. It was shown that tagging with one or two He atoms produced consistent line positions in the NIR region. They were able to estimate that the perturbation from helium tagging was less than 0.1 cm−1 for their system. This study led to the verification of the first two diffuse interstellar bands.43 The original case study for our instrument was the investigation of the benzene dication structure.36,44 The

Scheme 1. Isomers of C6H62+ and Their Relative Energies at 0 K Obtained with the CCSD(T)/aug-cc-pVTZ// MP2/aug-ccpVTZ Level of Theory

Reproduced with permission from ref 36. Copyright 2015 American Chemical Society. C


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Figure 3. (a) IRPD spectrum of the helium tagged C6H62+ dications generated from benzene. Two-color IRPD spectra obtained with one OPO fixed at (b) 3040 cm−1 or (c) 2940 cm−1. The bar spectra indicate theoretical (PBEhPBE/aug-cc-pVTZ) IR transitions of the C6H62+ isomers (the spectra are normalized to 1). Adapted from ref 36. Copyright 2015 American Chemical Society

energy with helium of about 7−9 kJ mol−1 was estimated based on CCSD(T)/aug-cc-pVTZ calculations. The most stable complex is formed when the helium atom attaches to the hydrogen atom of H−C−Cl2+. The next most stable possibility involves binding between the helium and carbon atoms (see the structures in Figure 5). If the vibrational modes in the fingerprint region are compared with experiment, the effect of the helium tagging is not very pronounced (see, δ(H−C−Cl) and ν(C−Cl)). On the other hand, a strong influence of the He tagging can be observed in the higher wavenumber range. Here, the C−H stretch of the complex with helium bound via the hydrogen atom is red-shifted by 200 cm−1 in the theoretical spectrum (red bar at 2510 cm−1) and almost 300 cm−1 in the experimental spectrum.32 For comparison, Figure 5a shows results of neon tagging. The mass effect of neon can be discerned not only from a massive shift of the C−H vibration, by more than 500 cm−1, but also by the red-shift of the C−Cl vibration to 1280 cm−1. The large shifts that were observed with both helium and neon show that binding these tags significantly changes structures and IR spectra. This means that any vibrational assignment based on comparison with theory would be very ambiguous; therefore it had to be completed by isotopic labeling.32

Figure 4. (a) Comparison of the two-color helium tagging IRPD spectrum for the C6H62+ dications with one OPO fixed at ν̃2 = 3040 cm−1 with the absorption cross sections determined from the attenuation experiments. (b, c) Attenuation measurements at the indicated wavelengths. Data fitting reveals that 40% of the ions are 122+ isomers. The exponential fits provide characteristic energies, Ej, from which the absorption cross sections are calculated. Adapted from ref 36. Copyright 2015 American Chemical Society

N (E)/N0 = f j + (1 − f j ) exp( −E /Ej)

(1)

The parameter f j in eq 1 denotes the fraction of the complexes that do not interact with the given photons. It can be advantageously used to determine the isomeric composition in the mixture of trapped ions. As shown in Figure 4b, we could estimate that the 1[2·He]2+ isomers represent 40% of the trapped [C6H6·He]2+ complexes.36

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APPLICATION FOR CATALYSIS Having a powerful and effective tool in hand, the foremost aim is to apply it to the investigation of reactive intermediates in catalysis.53 One of the most prominent challenges of current chemistry is to find sufficiently reactive catalysts to achieve C− H activation of nonactivated hydrocarbons with the ultimate goal of methane activation. As has been amply demonstrated, catalytic systems based on metal−oxo species with an unpaired electron localized at the oxygen atom are suitable for C−H activation.54 These species are highly reactive; therefore the least reactive tag is required. In 2013, the groups of Asmis and Schwarz both showed that the helium tagging IRPD spectrum of the [VPO4]•+ complex can be nicely interpreted by

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HELIUM AS A NONINNOCENT TAG While we have shown for medium sized hydrocarbon dications that helium tagging has a minimal effect on their structure and the resulting spectra can be readily interpreted based on comparison with the theoretical calculations for the naked ions, there are of course limits. As an example of where helium tagging has a significant impact on the structure of the ion of interest and the subsequent IR spectra, the case of three-atom HCCl2+ dications is presented.32 For these ions, an interaction D


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Figure 5. (a) Neon tagging IRPD and (b) helium tagging IRPD spectra of HCCl2+ dications. The assignment was made based on isotope labeling. The asterisk denotes a band originating from the C−Cl−H2+ isomers. (c) CCSD(T)/aug-cc-pVTZ calculated IR bands for the naked (gray) and helium-tagged HCCl2+ dications. The attachment of helium via the hydrogen atom (in the red frame) leads to the IR spectrum shown in red, and the side-on attachment of helium to the carbon atom (in the blue frame) leads to the spectrum shown in blue.32

Figure 6. (a) Generation of [(PyTACN)Fe(O)(NO3)]+ by oxidation of the iron(II) precursor in solution or by in-source CID during electrospray ionization. (b) IRPD spectra of [(PyTACN)Fe(O)(NO3)]+ (m/z 382) generated by oxidation in solution (blue trace, shifted on the y-scale by 0.4) or by in-source collisional activation during ESI (red trace). (c) Theoretical IR spectra of 3A, 5A, 3B, and 5B calculated with B3LYP-D3/6-311+ +G**. Adapted from ref 56. Copyright 2016 Wiley-VCH.

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electron densities provided by a number of DFT functionals, we have concluded that the determined electronic ground state is highly dependent on the applied DFT functional and cannot be considered as reliable. Second, the stretching vibration of the nitrate counterion is dependent on the spin state of the complex whereas the effect of isomerism between A and B is only minor. For the quintet state, the νas(NO2) vibration is always blue-shifted with respect to the triplet state. The opposite effect is observed for the ν(NO) vibration (cf. Figure 6). Third, the band shift of the quintet state complexes 5A and 5 B is caused by spin delocalization from the iron center to the nitrate counterion. The characteristic stretching vibrations might thus be a useful marker for spin state determination of the iron(IV)−oxo complexes.56

comparison with the theoretical IR spectrum of the naked radical ion.27 We have concentrated inter alia on hypervalent iron(IV)− oxo complexes with a tetradentate ligand denoted as PyTACN (PyTACN = 1-[2′-(pyridyl)methyl]-4,7-dimethyl-1,4,7-triazacyclononane, Figure 6) from the group of Costas.55 In order to develop a tool for the characterization of such a catalyst, we have investigated the helium tagging IRPD spectra of [(PyTACN)Fe(O)(NO3)]+ complexes.56 These complexes can be either generated in solution by oxidation of their iron(II) precursors with peracetic acid or in the gas phase by collisional activation of [(PyTACN)Fe(NO3)2]+ complexes that lose a NO2• radical (Figure 6a). Another interesting aspect of these complexes is the possibility to exist in either their triplet or quintet spin states; the formation of each of these has been explored during our investigation. We have shown that the IRPD spectrum of the [(PyTACN)Fe(O)(NO3)]+ complexes depends on whether they originate in solution or are formed by collisional activation in the gas phase. The biggest differences in the spectra were detected in the region of the antisymmetric NO2 vibration (νas(NO2) ≈ 1580−1610 cm−1) and in the range of the FeO and N−O stretching vibrations (890−920 cm−1). The spectra of the ions generated in the gas phase have been reproducibly blue-shifted at νas(NO2) and red-shifted at ν(NO)/ν(FeO). Theoretically, two different isomers of the [(PyTACN)Fe(O)(NO3)]+ complexes can be optimized with the oxo group either in the plane of the pyridine ring (A) or perpendicular to the pyridine ring (B). While the theoretical IR spectra of the triplet states of A and B do not differ significantly, consistent band shifts are detected when the triplet states are compared with the quintet states. Because this could be a potential spectral marker of the spin state, we have further investigated the causation of these shifts. We first used DFT methods to discover the most stable isomer and spin state, because this species was expected to be generated in solution and transferred to the gas phase under soft ionization conditions. Nevertheless, it soon appeared that DFT methods almost randomly predict the energetic order of 3 A, 5A, 3B, and 5B (see Figure 7). By further systematic investigation of the relative energies, optimized geometries, and

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OUTLOOK We are witnessing a rapidly growing interest in the use of mass spectrometry for the characterization of reactive intermediates. In the past, communications have been published where the presence of highly reactive catalytic species was suggested based solely on a detected mass in a mass spectrum.57−59 To make such a characterization more convincing, it is necessary to add further dimensions to mass spectrometry experiments. Helium tagging IRPD spectroscopy is one such exciting way forward. We have demonstrated that helium tagging IRPD spectroscopy can be used for the characterization of highly reactive intermediates and that specific spectral markers for catalysts can readily be developed. We aim to assist in the establishment of IRPD spectroscopy as a practical tool in catalysis.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (420) 221951322. E-mail: [email protected]. Funding

This work was supported by the Grant Agency of the Czech Republic (14-20077S) and the European Research Council (StG ISORI). Notes

The authors declare no competing financial interest. Biographies Jana Roithová is Professor at Charles University in Prague. She received her Ph.D. from the Institute of Chemical Technology in Prague and did her postdoctoral work at the Technical University of Berlin. Her research is focused on the investigation of reaction mechanisms, and she is engaged in developing novel tools and approaches in mass spectrometry and ion spectroscopy. Recently, she was awarded the Ignaz L. Lieben Award by the Austrian Academy of Sciences (2014). Andrew Gray received his master‘s degree and Ph.D. in Chemistry from the University of Edinburgh graduating in 2012. He is currently performing postdoctoral research at Charles University in Prague. His research interests focus on the use of computational chemistry to investigate organometallic reactions.

Figure 7. Results of DFT calculations (the functionals are specified at the bottom of the data, the 6-311++G** basis set was always used, with the exception of the OPBE functional where def2-TZVP was also tested, denoted by an asterisk). The red and blue lines denote the relative energies of the isomers (left axis); the pink and violet lines show the unscaled antisymmetric NO2 stretch for the quintet and triplet states, respectively (right axis, dashed lines are for the A isomers and solid lines are for the B isomers). Adapted from ref 56. Copyright 2016 Wiley-VCH.

Erik Andris received his diploma degree in Molecular Engineering from the Institute of Chemical Techology in Prague in 2013. He is currently a graduate student at Charles University in Prague, where his research focuses on chemical and spectroscopic characterization of high-valent metal complexes. F


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Juraj Jasik received his Ph.D. in Physics from Comenius University in Bratislava in 2008. To a great extent, he was engaged in the development and building of the ISORI instrument. His research is focused on ion spectroscopy, mass spectrometry, and ion trapping. Dieter Gerlich is Prof. em. from the Technische Universität Chemnitz (TUC). He received his Diploma in Physics as well as his Ph.D. from the Albert Ludwigs-Universität Freiburg (C. Schlier). After a PostDoctoral appointment at the University of California at Berkely, USA (Y. T. Lee), and Habilitation in Freiburg, he accepted a professor position at the TUC in 1993. Since his retirement in 2009, he is actively engaged in several laboratories, especially in Prague, Basel, and Köln. In 2012 he was awarded the SASP Erwin Schrödinger Gold Medal 2012 for his outstanding contributions to ion chemistry and astrophysics. An important foundation for his scientific achievements has been the development of innovative instruments using rf ion guides and cryogenic ion traps.

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