Article pubs.acs.org/JPCA
CRASY: Correlated Rotational Alignment Spectroscopy Reveals Atomic Scrambling in Ionic States of Butadiene Christian Schröter, Chang Min Choi, and Thomas Schultz* Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan 689-798, Republic of Korea S Supporting Information *
ABSTRACT: We use correlated rotational alignment spectroscopy to reveal atomic scrambling in transient ionic states of butadiene. Investigating a natural sample, we assign signals of two trans-1,3-butadiene isotopologues that contain a single 13C isotope and quantify the fragment abundance for the 13CH3 loss channel for each isotopologue.
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INTRODUCTION Isotopic substitution can be used to mark individual atoms in molecules without significant perturbation of the molecular electronic properties. This is particularly useful for the investigation of dissociative reactions because the isotopically labeled atoms can be identified in the fragments, e.g., by mass spectrometric detection1 or vibrational resolved fluorescence spectroscopy.2 Unfortunately, isotopically labeled compounds are rarely at hand and their synthesis is expensive and timeconsuming. Mass-correlated rotational alignment spectroscopy (massCRASY) allows us to distinguish signals from naturally occurring isotopologues3 with identical mass-to-charge ratio. This removes the requirement for isotope labeling via chemical synthesis and allows the investigation of selected isotopologues in a natural sample. In this work, we investigate the fragmentation of trans-1,3butadiene after photoionization. We use mass-CRASY to assign reaction products to parent isotopologue species, namely trans1,3-butadiene (BD), 1-13C-trans-1,3-butadiene (1-13C-BD), and 2-13C-trans-1,3-butadiene (2-13C-BD). We address the question whether photoinduced fragmentation leads to preferential loss of a terminal carbon atom or if atomic scrambling in transient ionic states facilitates the loss of an inner carbon atom. BD is an inversion symmetric polyene with two π-conjugated double bonds. The molecular structure and labeling of atomic positions is shown in Figure 1. The molecule has a high polarization anisotropy,4 with Δα = 50.4 e/a02 at 800 nm according to density functional theory calculations and with the largest polarizability along the molecular (z-) axis. Details about the calculations can be found in the Supporting Information. Because of the high polarization anisotropy, BD is well-suited for rotational Raman excitation and the experimental investigation with CRASY. Ionization of BD can lead to isomerization via several intermediate structures.5,6 Depending on the excess energy in the ionic state, a number of different cationic fragmentation © XXXX American Chemical Society
Figure 1. Symmetry and labeling of carbon atoms in BD. The molecule belongs to the C2h point group, and we labeled carbon atoms according to their distance from the inversion symmetry center.
products can be observed. In 2011, Fang et al. determined the appearance energies for several ionic fragments of BD.7 Accompanying density functional theory (DFT) and ab initio molecular orbital calculations were used to assign the observed fragmentation channels as depicted in Figure 2. The BD ionization energy (and the appearance energy of the intact cation) is 9.09(1) eV (numbers in parentheses give the confidence range of the last digit). The lowest ionic fragmentation pathway with an appearance energy of 11.50(2) eV creates a cyclopropenyl cation (C3H3+) by loss of a CH3 radical. The theoretically predicted cationic fragmentation pathway is depicted in Figure 2a and involves trans−cis isomerization followed by tautomerization, cyclization, and loss of a methyl group. The second-lowest fragmentation channel, with an appearance energy of 11.72(2) eV, forms a butadienyl cation (C4H5+) through the loss of an H atom. The theoretically predicted fragmentation pathway is drawn in Figure 2b and occurs through a cyclobutene radical cation. The appearance energies Received: November 19, 2014 Revised: January 30, 2015
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to generate the fourth harmonic wavelength (200 nm) with 1− 2 μJ pulse energy. Pump pulses were compressed to a nonFourier-limited 2 ps temporal width. An opto-mechanical delay stage (Physik Instrumente, MD-531) allowed the shortening of the optical path of the pump pulses for an adjustable time delay between pump and probe pulses within a 2 ns range. The pulse energy of the pump pulses was adjusted by an attenuator to approximately 200 μJ. Both beams were recombined and focused with a spherical mirror with a focal length of 37.5 cm into the spectrometer. Pump and probe pulses were linearly polarized with identical polarization. Pump pulses excited a rotational wave packet in the molecular ground state of the sample molecules by means of impulsive rotational Raman excitation (rotational alignment).11 The resulting transient alignment of the molecular ensemble along the axis defined by the pump laser polarization was probed after a variable time delay via resonant two-photon ionization. Photoions were detected in a time-of-flight mass spectrometer. Ionization with 200 nm photons occurred via vibrational states of the 1Bu state with a state origin at 216 nm.12 The amount of population transferred to the exited (and subsequently to the ionic) states was modulated by the transient molecular alignment and the associated orientation of electronic transition dipole moments within the probing laser field. The molecular axis with highest polarizability nearly coincides with the orientation of the transition dipole for the ππ* excitation of BD.12,13 Values for the transition dipole orientation and oscillator strength, as calculated with timedependent density functional theory, can be found in the Supporting Information. The modulation frequencies (rotational Raman transition frequencies) were recovered by a Fourier transformation of the time-dependent mass signals after baseline subtraction. The resulting mass-CRASY signals therefore correlated the mass of every photoionized species with the rotational Raman transition frequencies of their neutral parent molecules. Thermal instabilities in the experimental laboratory degraded the signal-to-noise ratio by several orders of magnitude as compared to previously published data.3 To obtain significant experimental data for carbon disulfide and butadiene, we implemented an active, computer-controlled stabilization of the pump−probe overlap via motorized mirror mounts and of the probe pulse duration via a motorized stage in the laser compressor. Carbon disulfide was used to ensure the correct calibration of the mass and frequency axes.
Figure 2. Calculated fragmentation pathways for butadiene cation according to Fang et al.7 (a) The lowest-energy fragmentation pathway involves trans−cis isomerization and migration of a hydrogen atom (tautomerization), cyclization, and the formation of a cyclopropenyl cation after methyl-loss. (b) The second-lowest-energy fragmentation pathway occurs through a cyclobutene intermediate. The loss of a hydrogen atom then leads to formation of cyclobutenyl cation.
of other fragments are above 12.4 eV and, except for the C2H4+ fragment with 12.44(3) eV, are beyond the energy range accessible in our experiment. For butadiene, several isotopologues were commercially synthesized with isotopic purities up to 90%. Mass-analyzed ion kinetic energy spectrometry of these isotopologues revealed carbon atom randomization on the time scale of several microseconds upon decomposition of ionized 1,3-butadiene.8
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EXPERIMENTAL SECTION A mixture of butadiene and helium (mixing ratio 1:100) was used as sample gas. The sample gas, with 50 bar pressure, was expanded through a pulsed valve9 into high vacuum. The resulting molecular beam was skimmed to obtain a cold supersonic beam in the interaction region of a mass spectrometer. Short valve opening times were used to minimize the amount of molecular clusters in the beam. A Wiley− McLaren type mass spectrometer10 was used to analyze cation masses after photoionization of molecules in the molecular beam. In the following text, masses are given in atomic units (u). Femtosecond laser pulses with a central wavelength of 800 nm were generated by a femtosecond Ti:Sa oscillator (Spectra Physics, Tsunami). The pulse duration was stretched before amplification in a regenerative amplifier (Spectra-Physics, Spitfire). After amplification, the laser pulses were split into two parts (“pump” and “probe”), which were compressed by separate compression stages. Probe pulses were compressed to 60 fs temporal width (full width at half-maximum) and nonlinear BBO crystals were used
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RESULTS Figure 3 shows a mass-CRASY correlation map for butadiene and carbon disulfide. Cations and cationic fragments were identified in the integrated mass spectrum (top panel). The cation for BD was observed at mass 54 u. The main fragmentation pathways of BD cation lead to formation of cyclopropenyl cation at mass 39 u and butadienyl at mass 53 u. A minor quantity of BD fragments was observed at mass 27−28 u (C2H3/4 cations). Cations formed from the two 13C-BD isotopologues were identified in the mass channels 40 and 55 u. The signal at mass 76 u was due to a small amount of carbon disulfide, which remained in the sample line from an earlier experiment and served as calibration compound. A signal at mass 80 u may be due to fragmentation of 4-vinylcyclohexene, a common impurity of butadiene,14 and molecular clusters. B
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Figure 4. Comparison of rotational-Raman-spectra in ion mass channels 54 and 55 u. Signals in mass channel 54 u stem from the photoionization of the main isotopologue of BD, whereas those in mass channel 55 u are formed by photoionization of the 1-13C-BD and 2-13C-BD isotopologues. Transition frequencies (J ± 2) for the main rotational axis are marked for the main BD isotopologue (blue), 1-13CBD (green), and 2-13C-BD (red).
The population of rotational states with even and odd symmetry therefore differs because of Pauli’s exclusion principle, and even rotational states are occupied with a ratio of 28:36 relative to odd rotational levels. The 13C-BD isotopologues have no inversion symmetry, and even and odd states are occupied with a ratio of 1:1. The rotational Raman transition frequencies observed at mass 55 u were wellreproduced by simulated spectra for 1-13C-BD and 2-13C-BD isotopologues (PGOPHER simulations15). Rotational constants for the simulations were taken from a publication of Craig et al. and references cited therein.17 Ionic fragments in the mass channel 40 u were due to cyclopropenyl cations that contained a 13C isotope. These fragments were formed from 13C-BD molecules by methyl loss. If the fragmentation process selectively removes an outer carbon, then the signals from 1-13C-BD should show a suppressed signal amplitude as compared to the signals from 2-13CBD. The latter can lose only a 12C isotope, while the former would have a 50% probability of losing the heavy carbon isotope (and the resulting cyclopropenyl fragment would appear in mass channel 39 u). A comparison of rotational Raman spectra of parent (55 u) and fragment (40 u) is shown in Figure 5 and shows a modest decrease in the 1-13C-BD isotopologue signal as compared to the 2-13C-BD isotopologue signal. Comparing the integrated signal amplitudes for the five dominant transition bands (marked with * in Figure 5) in mass channel 55 u, we find a signal ratio of 1-13C-BD: 2-13C-BD = 1.1:1.0. A ratio of 1.0:1.0 is expected, and we must conclude that, because of experimental noise, we have a significant error of about 10% in the determination of the isotopologue ratio. While this error is undesirably large, it is still small enough to perform a meaningful analysis of the fragmentation mechanism. In mass channel 40 u, the signal ratio of integrated signal amplitudes for the same five transition bands is 1-13CBD:2-13C-BD = 0.6:1.0. We therefore estimate the ratio of 1-13C-BD:2-13C-BD to be 37%:63%. Unfortunately, a fragmentation product of the main BD isotopologue also contributes to the signal at mass 40 u. Because of a slow fragmentation process (on the microsecond time-scale of ion acceleration in the mass spectrometer), the cyclopropenyl fragment with a nominal mass of 39 u shows an extended tail toward higher masses, as seen in Figure 6a. The 40 u signal therefore contains signal from heavy and light
Figure 3. Mass-CRASY data set for butadiene and carbon disulfide. The integrated mass spectrum (top) shows the dominant ion signals from BD (54 u), carbon disulfide (76 u), and their fragments. The two-dimensional intensity plot (bottom) shows ion masses correlated to rotational Raman transition frequencies with frequency spectra scaled to the noise amplitude. Insets show vertical slices (i.e., rotational Raman spectra) at the mass of carbon disulfide (76 u), BD (54 u), and cyclopropenyl (39 u). The signal of carbon disulfide (yellow) reproduced previously reported data3 and was used for calibration. BD and cyclopropyl signals (green) show identical rotational Raman spectra, and both are therefore photoionization and photofragmentation products from the same neutral precursor.
Because of the small amount of signal in this mass channel, no meaningful spectrum could be extracted. Slices through the correlation map for a selected cation mass show the rotational Raman spectra of the respective neutral precursor molecule. Signals at masses 27, 28, 39, 53, and 54 u showed identical rotational Raman spectra and are photoionization products of the main isotopologue of butadiene. The computer program PGOPHER15 was used to fit the rotational constants at mass 39 and 54 u and gave the values A = 41(2) GHz, B = 4.438(2) GHz, and C = 4.005(2) GHz, in good agreement with values reported in the literature.16 The signal at mass 76 u reproduces the rotational Raman spectra of carbon disulfide with a rotational constant of B = 3.2714(2) GHz.3 The rotational Raman spectra obtained in the mass channel of 13C-BD at 55 u differed from the spectra obtained for the main BD isotopologue, as shown in Figure 4. Because of the natural abundance of 13C and 2H, the ratio of 13C-BD isotopologues to 2H-BD isotopologues is 64:1 and signal contributions from deuterium-containing BD could be neglected. The different moments of inertia for the 1-13C-BD and 2-13C-BD isotopologues lead to transition frequencies that are shifted as compared to those of the main isotopologue. The band amplitudes are also different because of the breaking of the molecular inversion symmetry in the heavy isotopologues. The main BD isotopologue has an inversion symmetry with respect to the center of mass (point group C2h). C
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isotopologues. The frequency resolution is insufficient to separate the relevant rotational bands of the main isotopologue from those of 2-13C-BD. The relevant section of the massCRASY data set is enlarged in Figure 6b and shows that the rotational progressions of BD and 2-13C-BD overlap. We therefore overestimated the signal contribution of 2-13C-BD in the preliminary analysis above. To account for the slow fragmentation process described above, we fitted the resulting tail for the 39 u mass signal by an exponential function. This allowed us to analyze the relative signal contributions of light and heavy isotopologues to the 40 u mass channel; 23% of the total signal in mass channel 40 u must be assigned to the lighter isotopologue. To correct the preliminary analysis of 13C-BD isotope ratios, we must subtract this value from the 2-13C-BD species signals and remain with a ratio of 37%:40%:23% for signals from 1-13C-BD:2-13C-BD:BD. The corrected ratio of 1-13C-BD:2-13C-BD is therefore 48%:52%. Direct fragmentation of BD to cyclopropenyl (without scrambling) would result in the loss of an outer carbon atom. The 13C-cyclopropenyl signal would therefore be twice as likely to stem from 2-13C-BD (no loss of the inner 13C atom) than from 1-13C-BD (50% loss of the outer 13C atom). This does not agree with our observation that both 13C-BD parent molecules contribute almost evenly to the signal in this mass channel. Accounting for our estimated error boundaries, we find that scrambling occurs with 95% ± 15% probability. Our data therefore indicates complete or nearly complete scrambling in the ionic states of butadiene before fragmentation to cyclopropenyl occurs.
Figure 5. Comparison of rotational-Raman-spectra obtained in ion mass channel 40 and 55 u. Cations in both mass channels contain a single 13C isotope and are photoionization products of 1-13C-BD and 2-13C-BD. The relative band amplitudes from the two isotopologues differ in the considered mass channels. Transition frequencies (J ± 2) are marked for the main rotational axis of 1-13C-BD (green) and 2-13CBD (red). Bands used for determination of signal ratios between both isotopologues are indicated by an asterisk (*).
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DISCUSSION Resonant two-photon ionization with 200 nm photons corresponds to an activation with 12.4 eV energy. This energy is well above the BD ionization potential of 9.09(1) eV and is sufficient to induce cationic fragmentation. We observed the ionic products of the lowest two fragmentation pathways described by Fang et al.:7 the formation of cyclopropenyl cation with an appearance energy of 11.50(2) eV and the formation of butadienyl cation with an appearance energy of 11.72(2) eV. We also observed minor signals at mass at mass 27 and 28 u. The cation at mass 28 u (C2H+4) has a reported appearance energy of 12.44(3) eV.7 This value is on the edge of the energy range accessible with two 200 nm photons. The fragment signal observed at mass 27 u has a reported appearance energy of 15.15(3) eV (single photon ionization),7 beyond the energy accessible by two-photon ionization with 200 nm. This fragment was also observed in previous resonant two-photon ionization experiments12 and photodissociation studies,18 and we can speculate that it may be accessible via a pathway not identified by Fang et al. The rotational Raman spectra observed at mass 54 u agreed well with the BD rotational constants reported in the literature.16,19 The rotational spectra at mass 53 u (butadienyl) and 39 u (cyclopropenyl) were identical to that at 54 u and confirmed that these fragments were formed from BD upon photoionization. The signals at mass 27 and 28 u were too small for a detailed investigation but showed the main rotational transition frequencies of BD. To address the question whether atomic carbon scrambling occurred in ionic states, we focused the investigation on BD molecules and fragments containing one 13C carbon atom at the 1-C or 2-C position. These heavier isotopologues appeared
Figure 6. Enlarged section of the integrated mass spectrum (a) and of the correlation map (b) for cyclopropenyl cation at 39−40 u. The gray area in (a) shows the integration boundaries used in the signal analysis, and numbers denote the integrated signal contributions from cyclopropenyl cations containing none or one 13C isotope (see text). The enlarged section in (b) shows that the main-axis transition frequency of the light isotopologue (BD) overlaps with that of 2-13CBD but is well-separated from that of 1-13C-BD.
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pathways predicted by Fang et al.7 An equilibrium between the BD cation and cyclic intermediates,6 separated by barriers well below the appearance energy of low-energy fragmentation products, leads to the transient formation of cyclic structures. Tautomerization in these cyclic structures can explain the observed atomic scrambling. This work illustrates the capabilities of CRASY to analyze fragmentation channels using naturally occurring rare isotopologues. Unfortunately, the experiments described here were performed under unstable laboratory conditions and therefore show a signal-to-noise ratio that is orders of magnitude below that of earlier published data.3 Future CRASY experiments under enhanced experimental conditions should allow the quantitative analysis of fragmentation pathways in BD and other molecules. CRASY therefore offers the potential for the quantitative mass-spectrometric investigation of distinct isotopologues using natural samples without isotope enrichment.
at mass 55 u and showed a more congested spectrum. The transition frequencies in the main progression were reduced (Figure 4) because of the exchange of an atom with its heavier isotope and the corresponding increase of the molecular moment of inertia. The rotational Raman transition frequencies observed at mass 55 u were well-reproduced by simulated spectra for 1-13C-BD and 2-13C-BD isotopologues. The main cationic fragments from the 13C-BD isotopologues appear at masses 54 u (butadienyl cation) and 40 u (cyclopropenyl cation). The former could not be resolved because of the much larger BD cation signal at the same mass, but the latter was observed and showed rotational spectra resembling those at mass 55 u (see Figure 5). The band amplitudes for the two 13C-BD isotopologue fragments did not resemble those of their parent isotopologue species in mass channel 55 u. This was predominantly due to the contamination of the mass 40 u signal with cyclopropenyl signal from the main BD isotopologue: fragmentation of BD on the time scale of ion acceleration in the time-of-flight mass spectrometer created a tail of the mass 39 u signal toward higher masses (see Figure 6a). This tail contributed approximately 23% to the total signal in mass 40 u. The remaining signal stems in even parts from 1-13C-BD and 2-13CBD. We must point out that the error bars in this analysis are large and may exceed 10%. According to Fang et al.,7 the lowest-energy fragmentation channel of butadiene cation involves tautomerization and a methyl-cyclopropenyl intermediate (see Figure 2a). The second-lowest fragmentation channel involves a cyclobutene cation intermediate (see Figure 2b). The barrier to fragmentation is in both cases higher than the reverse barrier to the BD cation species, and we can expect an equilibration between cyclic structures and BD before fragmentation occurs. Tautomerization in either cyclic intermediate can therefore lead to a loss of information about the initial carbon positions (“scrambling”). Our observations agree with an earlier discussion of atomic scrambling in butadiene cations.6 The tail of the mass 39 u signal indicates that fragmentation takes place on the time scale of a microsecond. This is in agreement with the observation of extensive atom scrambling observed in mass-analyzed ion kinetic energy spectrometry with isotopically enriched samples.8 The error boundaries in the analysis of the mass 40 u signals are quite large, and we cannot precisely quantify the amount of scrambling in the ionic state. The dominant error resulted from the noise in the rotational Raman signal amplitudes for this small isotopologue fragment signal. The amount of scrambling (accounting for the estimated error) is 95% ± 15%. The formation of cyclopropenyl corresponds to the lowest fragmentation pathway, and we expect similar scrambling propensities for all other fragmentation pathways.
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ASSOCIATED CONTENT
S Supporting Information *
Density functional theory and time-dependent DFT calculations for the BD ground and excited states, including the molecular structure, rotational constants, static and dynamic polarizability, vertical excited-state properties, and transition dipole moments. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This paper is dedicated to Professor Kwan Kim on the occasion of his honorable retirement. We thank Dr. Frank Noack for his support by providing the laser system in the Max-Born-Institute Berlin. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2014-R1A2A1A11 053055) and the 2013 Research Fund (1.130018.01) of UNIST (Ulsan National Institute of Science and Technology).
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REFERENCES
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CONCLUSION We used CRASY to correlate the isotopic composition of butadiene photoionization fragments to the isotopologue structure of its neutral precursors. Naturally occurring isotopologue signals for 1-13C-BD and 2-13C-BD were identified by their respective rotational Raman spectra. Their relative contribution in the fragment mass channel of 13C-cyclopropenyl showed that significant carbon atom scrambling, on the order of 95% ± 15%, occurred before fragmentation. This agrees with previous observations of atomic scrambling in butadiene cations8 and with ionic structures and fragmentation E
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