Letter pubs.acs.org/JPCL
Photoisomerization and Infrared Spectra of Allene and Propyne Cations in Solid Argon Meng-Chen Liu,† Sian-Cong Chen,† Chih-Hao Chin,† Tzu-Ping Huang,† Hui-Fen Chen,‡ and Yu-Jong Wu*,† †
National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu Science Park, Hsinchu 30076, Taiwan Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, 100, Shih-Chuan first Road, Kaohsiung 80708, Taiwan
‡
S Supporting Information *
ABSTRACT: Allene and propyne as well as their cationic forms play important roles in combustion and interstellar chemistry and serve as a model system for molecular spectroscopic studies. Both cations show Jahn−Teller (J−T) distortions in their ground states. These J−T distortions make the theoretical and experimental studies of their electronic structures difficult. We produced allene cations upon electron bombardment during matrix deposition of Ar containing a small proportion of allene. The intensities of the absorption features of the allene cation decreased after irradiation with UV light, whereas new bands attributed to propyne cations increased. The observed line wavenumbers, relative intensities, and deuterium-substituted isotopic ratios of the isomers of C3H4+ agree satisfactorily with those predicted by density functional theory at the B3PW91/ aug-cc-pVTZ level of theory. This method produced the hydrocarbon cations of interest with few other fragments that enabled the clear identification of the IR spectra of allene and propyne cations.
T
or propyne, followed by codeposition at 5 K.12 The infrared spectra of the matrix sample and predominant IR features of H2CCCH2+ were observed and assigned in these allene experiments. In contrast, only one band at 3226.7 cm−1 was tentatively assigned to the C−H stretching mode of H3CCCH+.12 In these propyne experiments, the propyne cations produced were concluded to predominantly isomerize to the more stable allene cations.12 Recently, Xing et al. recorded rotationally resolved photoelectron spectra of H3CCCH+ using the pulsed-field ionization photoelectron (PFI−PE) technique.13,14 The vibrational frequency of the C−H stretching mode (ν1+) was determined to be 3217.1 cm−1, which is close to the value observed in solid Ne.12 Subsequently, Schulenburg and Merkt used a similar technique to record the photoelectron spectra of H2CCCH2; the rovibronic progression of H2CCCH2+ was observed mainly by the torsional mode. In addition, the rotational constants, J− T coupling constants, and the lowest two torsional levels were also determined.15,16 Few theoretical calculations of the potential energy surfaces (PESs) of C3H4+ have been reported. Theoretical calculations have predicted that the allene cation is more stable than the propyne cation by 15.4 kcal mol−1 and that there are substantial energy barriers for isomerization between H2CCCH2+ and
he chemical formula C3H4 has two commonly stable structural isomers: (i) allene (H2CCCH2), the smallest cumulated diene, and (ii) propyne (H 3 CCCH). Both compounds are important intermediates in combustion and astrochemistry.1−5 The removal of an electron from the highest occupied molecular orbital of C3H4 results in C3H4+, which exists in the doubly degenerate ground state (X 2E) and exhibits the Jahn−Teller (J−T) effect. In addition, the loss of an electron from allene decreases its symmetry from D2d to D2.6,7 Similarly, propyne decreases its symmetry from C3v to Cs in the cationic form.6,7 The J−T distortion of these cations results in the splitting of energy levels in the electronic structures, and thus it is challenging to determine the accurate energy levels and structure experimentally as well as theoretically. Furthermore, these species are important in astrochemical models for the growth of large hydrocarbons in interstellar mediums and carbon films through plasma-assisted chemical vapor deposition processes. To our knowledge, few spectroscopic studies have been conducted to elucidate the electronic structure and energy levels of C3H4+. Early photoelectron spectra had insufficient resolution and could only resolve one vibrational mode of allene cation and two vibrational modes of propyne cation.8−10 Ho et al.,11 measured the photoabsorption and photoionization cross sections of propyne with synchrotron radiation and determined several vibrational modes of H3CCCH+ in the ground state. In addition, Jacox and coworkers used excess Ne atoms excited by a microwave discharge to collide with allene © 2015 American Chemical Society
Received: July 13, 2015 Accepted: August 3, 2015 Published: August 3, 2015 3185
DOI: 10.1021/acs.jpclett.5b01495 J. Phys. Chem. Lett. 2015, 6, 3185−3189
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The Journal of Physical Chemistry Letters H3CCCH+ via the intermediate H2CCHCH+ (19.9 kcal mol−1 relative to allene cation).6,7 The intermediate H2CCHCH+ can also rearrange to another high-energy isomer cyc-C3H4+ (77.2 kcal mol−1);7 however, early calculations with the MRCI// ROHF/6-31G** level of theory may not be sufficiently accurate. Recently, Mebel and Bandrauk performed more detailed calculations on the isomerization and decomposition of the PES of C3H4+ with the CCSD(T)/cc-pVTZ//B3LYP/6311G** level of a theory. As a result, a reaction pathway involving a transition state for the direct isomerization of allene and propyne cations was determined with an energy barrier of approximately 37.5 kcal mol−1 relative to the allene cation.17 Later, other calculations focused on the simulations of photoelectron spectra with spin−orbit interactions were reported. For instance, Marquez et al.18 simulated the photoelectron spectrum of the propyne cation (X 2E) with spin−orbit interaction using an intensity borrowing approach and compared their results with the experimental results obtained by Xing et al.13 In addition, Schulenburg and Merkt used Fourier series instead of Taylor series to treat the J−T effect and the hindered internal rotations of the PES for the allene cation, which were compared with their previous PFI− PE data.16 Although the IR spectra of allene cations in solid matrices are reported,12 the IR features of propyne cations and their photochemistry remain unclear. Herein, we measured the IR spectra of the isomers of C3H4+ in solid Ar and observed the UV light-induced isomerization of allene cations into propyne cations. The C3H4+ cation was produced by electron bombardment of a gaseous sample of Ar containing a small proportion of allene during deposition onto a cold Cu substrate at 8 K to form a solid Ar matrix. IR absorptions of C3H4+ decreased in intensity when the matrix sample was kept in the dark for a prolonged period of time and its corresponding neutral form increased in intensity. Therefore, a slow recombination reaction with trapped electrons is taking place in the dark, and the IR features corresponding to cations were unambiguously distinguished. A partial IR spectrum of a H2CCCH2/Ar (1/500) matrix bombarded with electrons during deposition is shown in Figure 1A, and a spectrum covering a C−H stretching region is available in Figure 2A. Absorption features of allene and propyne are labeled as A and P, respectively. The IR spectrum of a H2CCCH2/Ar sample without electron bombardment is available in the Supporting Information (Figure S1), and these absorptions of allene and propyne isolated in solid Ar were previously reported.19 Additional weak bands were also observed and are associated with the products generated by electron bombardment of the matrix sample. These products were readily characterized as C2H2 (HCCH, 737.3/741.0 and 3284.9/3309.0 cm−1),20 C3H3 (H2CCCH, 686.8 cm−1),12,19 and C3H2 (HCCCH, 547.8 and 3263.0/3266.7 cm−1).12,19 In addition, five lines observed at 874.4, 791.4, 1307.3, 2929.0, and 3020.8 cm−1, indicated by A+ in Figures 1A and 2A, might be attributed to the H2CCCH2+ cation that were reported in solid Ne12 (Table 1). A full list of all vibrational modes of the allene cation is available in Table S1. Next, the matrix was maintained in darkness for 12 h and the difference spectrum shown in Figure 3A was obtained by subtraction of the spectrum recorded before from that recorded after the matrix was kept in darkness. Lines pointing upward indicate the production of species, whereas those pointing downward indicate the destruction of species. When the matrix
Figure 1. Partial IR spectra of (A) electron-bombarded H2CCCH2/Ar (1/500) and (B) difference spectrum of the matrix sample upon irradiation at 385 nm for 1 h. A: allene, P: propyne, A+: allene+, P+: propyne+.
Figure 2. Partial IR spectra (C−H bond stretching region) of (A) electron-bombarded H2CCCH2/Ar (1/500) and (B) difference spectrum of the matrix sample upon irradiation at 385 nm for 1 h. A: allene, P: propyne, A+: allene+, P+: propyne+.
Table 1. Comparison of Vibrational Wavenumbers (in cm−1) and Relative IR Intensities of the Observed Lines of H2CCCH2+ mode
sym
mode descriptiona
ν4 ν5 ν7 ν8 ν10 ν13 ν14
A B1 B1 B2 B2 B3 B3
torsion CH2 stretch CH2 deform CH2 stretch CH2 oph wag CH2 iph wag CH2 twist
predictionb 799.5 3065.0 1339.6 3162.0 901.3 923.4 766.5
(0)c (100) (46) (60) (11) (25) (3)
literature 745.7d 2956.0f 1314.4f 3047.3f 880.4f 738.4f
Ar matrix 2929.0 1307.3 3020.8 791.4 874.4
a
iph: in phase; oph: out of phase. bHarmonic vibrational wavenumbers calculated with B3PW91/aug-cc-pVTZ. cRelative IR intensities normalized to the 212 km mol−1 value predicted for the most intense line for H2CCCH2+ at 3065.0 cm−1 with the B3PW91 method. dData taken from ref 15. fData taken from ref 12.
sample was maintained in darkness for a prolonged period, the trapped electrons diffused slowly to recombine with the cations 3186
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Table 2. Comparison of Vibrational Wavenumbers (in cm−1) and Relative IR Intensities of the Observed Lines of H3CCCH+ mode
sym
mode descriptiona
ν1 ν3 ν4 ν6 ν7 ν10 ν13
A′ A′ A′ A′ A′ A′ A″
CH stretch CH stretch CC stretch CH3 umbrella CC stretch deform CCH op bend
predictionb 3354.0 2868.6 2085.4 1267.1 988.5 343.5 637.0
(100)c (90) (3) (49) (8) (5) (20)
literatured 3217.1e 2767f ? 1996 ± 38f
Ar matrix 3214.5 2780.2 1244.0
940 ± 40g 289 ± 65f 586 ± 74f
583.3
a
ip: in plane; op: out of plane. bHarmonic vibrational wavenumbers calculated with B3PW91/aug-cc-pVTZ. cRelative IR intensities normalized to the 237 km mol−1 value predicted for the most intense line for H3CCCH+ at 3354.0 cm−1 with the B3PW91 method. d“?” denotes tentative assignments. eData taken from ref 13. fData taken from ref 11. gData taken from ref 10.
Figure 3. (A) Difference spectrum of electron-bombarded H2CCCH2/ Ar (1/500) matrix after maintaining the sample in darkness for 12 h and (B) difference spectrum of another electron-bombarded H2CCCH2/Ar (1/500) matrix upon irradiation with 385 nm for 2 h followed by maintaining the sample in darkness for 12 h. A: allene, P: propyne, A+: allene+, P+: propyne+.
Furthermore, most of the observed intense IR-active modes are consistent with the photoelectron data, except for the ν6 mode, presumably because the mode assignments in photoelectron spectra were made based on the comparison of observed spacing from the transition origin and predicted wavenumbers without information on intensity. In addition, the ν4, ν7, and ν10 modes reported by photoelectron measurements, were not observed because weak IR intensities were predicted for these modes by theory. For further confirmation of the spectral assignments of H2CCCH2+ and H3CCCH+ in solid Ar, experiments on the electron bombardment of a mixture of D2CCCD2/Ar (1/500) were performed. Similar experimental procedures were followed, and a representative difference IR spectrum in the range of 600−1000 and 2000−2500 cm−1 upon irradiation at 385 nm is shown in Figure 4. A list of observed lines from D2CCCD2+ and D3CCCD+ is compared in Tables S3 and S4 with vibrational wavenumbers and isotopic ratios predicted with the B3PW91/aug-cc-pVTZ level of theory. In both cases, the observed deuterium isotopic ratios for D2CCCD2+ and D3CCCD+ agree with the theoretically predicted values with deviations less than 1%. The deuterium isotopic experiments clearly support the previous assignments of A+ to the allene cation and P+ to the propyne cation. On the basis of the potential energy surface for the isomerization of C3H4+ reported by Mebel and Bandrauk,17 it is clear that irradiation with 385 nm (74.3 kcal mol−1) can be used to overcome the energy barrier (37.5 kcal mol−1) for isomerization from the allene cation to the propyne cation. Another pathway leading to the formation of the vinylmethylene cation (H2CCHCH+), with a predicted energy barrier of 24.2 kcal mol−1, is also accessible energetically; however, no features for the vinylmethylene cation were found. A feature at 775.7 cm−1, shown in Figure 1B, might be due to its neutral counterpart H2CCHCH instead; this value agrees with an early observation in a Xe matrix.22 Because the energy of the selected UV light is greater than the calculated barrier for the isomerizations of C3H4+, photoisomerization is energetically feasible. Further spectroscopic measurements in the UV region and theoretical calculations of the excited electronic states of C3H4+ are in progress to determine which process takes place. Electron bombardment during the deposition of a mixture of allene and Ar at 8 K was employed to generate allene cations. Irradiation of the matrix with UV light at 385 nm diminished
forming neutral species.20,21 Figure 3A shows that bands of A+ decreased and bands of A increased, indicating that A+ and A are counterparts. Therefore, this result confirms that the A+ bands are correctly assigned as absorption features of allene cations. The matrix samples were further irradiated at 385 nm, and the resulting difference spectra are shown in Figures 1B and 2B. Irradiation of the matrix with UV light is expected to release electrons trapped in the matrix,21 resulting in the neutralization of cations. As expected, a decrease in A+ bands is observed after irradiation at 385 nm; however, both the bands of A and the bands of propyne (denoted as P) showed increased intensity. Moreover, a set of unknown bands (denoted as P+) also increased in intensity, and these P+ bands showed similar behavior upon further irradiation with UV light at 210, 250, and 280 nm from synchrotron radiation. The P+ bands decreased rapidly in intensity upon irradiation at 210 and 280 nm but decreased relatively slowly when irradiated at 250 nm. In a separate experiment, the electron-bombarded sample irradiated at 385 nm was kept in darkness for 12 h, and the difference spectrum was plotted (Figure 3B). As shown in Figure 3B, one band of P+ decreased and bands of P increased in intensity, while another band of P+ at 583.3 cm−1 decreased uncertainly due to interference fringes of the difference spectrum that arose from the slight change in thickness of the matrix sample. This observation, shown in Figure 3B, suggests that the bands of P+ might be due to propyne cations. Therefore, deuterium-isotopic experiments and quantum-chemical calculations were performed to obtain further information for assigning these lines. As shown in Table 2, representative lines of P+ are compared with theoretical calculations and photoelectron data. A full list of all vibrational modes of the propyne cation is available in Table S2. Almost all features predicted with an observable IR intensity in the probed spectral region were observed. Intense lines predicted near 3354.0 and 2868.6 cm−1 for the C−H stretching modes are observed at 3214.5 and 2780.2 cm−1, respectively. The CH3 umbrella mode predicted at 1267.1 cm−1 is observed at 1244.0 cm−1, and the CCH bending mode predicted at 637.0 cm −1 is observed at 583.3 cm −1 . 3187
DOI: 10.1021/acs.jpclett.5b01495 J. Phys. Chem. Lett. 2015, 6, 3185−3189
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Figure 4. Difference spectrum of electron-bombarded D2CCCD2/Ar (1/500) matrix upon irradiation with 385 nm for 1 h. A: allene, P: propyne, A+: allene+, P+: propyne+. “?” denotes unassigned spectral features.
emitting diode (500 mW) with emission at 385 ± 10 nm. Tunable UV light (λ < 300 nm) generated using synchrotron radiation (BL03 of NSRRC) with a bandwidth of ∼1.5 nm was applied for secondary photolysis. Ar (99.9999%, Scott Specialty Gases), allene (99.8%, Aldrich), and allene-d4 (deuterium ∼99%, Aldrich) were used without further purification, except for a freeze−pump−thaw procedure at 77 K. Energies, equilibrium structures, vibrational wavenumbers, and IR intensities were calculated with the Gaussian 09 program.24 Density functional theory calculations were performed using the B3PW91 method with Becke’s threeparameter hybrid exchange functional and an exchange functional of Perdew and Wang as the correlation functional.25,26 The basis set was used with Dunning’s correlationconsistent polarized valence triplet-zeta basis set augmented with s, p, d, and f functions (aug-cc-pVTZ).27
the lines of the allene cation, whereas those of the propyne cation increased in intensity. In separate experiments, lines from the neutral species, allene or propyne, increased in intensity, whereas those from allene and propyne cations decreased in intensity when the matrix was either maintained in the dark for a long period or irradiated with UV light. These lines were grouped according to their behaviors upon further irradiation with selected UV light and assigned according to expected chemistry, comparison with previous reports and harmonic vibrational wavenumbers, IR intensities, and deuterium-substituted isotopic ratios predicted for allene and propyne cations. IR spectra of the propyne cation are new. The proposed method is clean and results in the production of primarily C 3 H 4 + , C 3 H 2 , and C 3 H 3 without significant interference from other byproducts, suggesting that dissociation and ionization of precursors were induced by collisions with Ar atoms having excess energy deposited from emitted energetic electrons and rapidly removed excess energy from the initially formed products. Thus, cations of interest generated from their neutral counterparts are suppressed to dissociate further in the solid matrices. This method may be suitable for the investigation of the IR spectra of other alkene and alkyne cations.
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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.jpclett.5b01495. Calculated information, including vibrational wavenumbers, infrared intensities, and the IR spectrum of allene in an Ar matrix (Tables S1−S4 and Figure S1). (PDF)
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EXPERIMENTAL SECTION The experimental setup has been described previously.20,21,23 IR absorption spectra covering the spectral range 450−5000 cm−1 were recorded with an interferometric spectrometer (Bruker v80) equipped with a KBr beamsplitter and a Hg− Cd−Te detector cooled to 77 K. Typically 400 scans at a resolution of 0.5 cm−1 were recorded in each stage of an experiment. The allene and propyne cations were produced upon electron bombardment of a gaseous sample during the deposition of an Ar matrix containing a small proportion of allene. An electron beam at 200 eV with a current of 250 μA was generated with an electron gun (Kimball Physics, model EFG-7). Typically, a gaseous mixture of C3H4/Ar (1/500) was deposited over a period of 4 h with a flow rate of 5−8 mmol h−1. Photoirradiation experiments were performed with a light-
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Ministry of Science and Technology of the Republic of China (grant MOST104-2113-M-213-004) and the National Synchrotron Radiation Research Center. 3188
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(21) Bahou, M.; Wu, Y.-J.; Lee, Y.-P. Infrared spectra of protonated coronene and its neutral counterpart in solid parahydrogen: implications for unidentified interstellar infrared emission bands. Angew. Chem., Int. Ed. 2014, 53, 1021−2014. (22) Maier, G.; Lautz, C.; Senger, S. Ring opening of 1methylcyclopropene and cyclopropene: matrix infrared spectroscopic identification of 2-butene-1,3-diyl and propene-1,3-diyl. Chem. - Eur. J. 2000, 6, 1467−1473. (23) Wu, Y.-J.; Chuang, S.-C.; Chen, S.-C.; Huang, T.-P. Infrared spectra of acetylene diluted in solid nitrogen upon irradiation with vacuum ultraviolet light and electrons. Astrophys. J., Suppl. Ser. 2014, 212, 7. (24) Frisch, M. J.; et al. Gaussian 09, revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (25) Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98, 5648−5652. (26) Perdew, J. P.; Burke, K.; Wang, Y. Generalized gradient approximation for the exchange-correlation hole of a many-electron system. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 16533− 16539. (27) Woon, D. E.; Dunning, T. H., Jr. Gaussian-basis sets for use in correlated molecular calculations. 3. The atoms aluminum through argon. J. Chem. Phys. 1993, 98, 1358−1371.
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