ARTICLE pubs.acs.org/JPCA
Matrix Infrared Spectra and Density Functional Calculations of the H2CCN and H2CNC Radicals Produced from CH3CN Han-Gook Cho† and Lester Andrews*,‡ † ‡
Department of Chemistry, University of Incheon, 119 Academy-ro, Songdo-dong, Yonsu-gu, Incheon, 406-772, South Korea Department of Chemistry, University of Virginia, P.O. Box 400319, Charlottesville, Virginia 22904-4319, United States
bS Supporting Information ABSTRACT: The H2CCN and H2CNC radicals are observed in matrix IR spectra from acetonitrile exposed to radiation from laser ablation of transition metals, whereas cyc-H2CCN, another plausible isomer, is not. Density functional frequency calculations and D and 13C isotopic substitutions substantiate the vibrational assignments. The cyano methyl radical converts to the 95 kJ/mol higher energy isocyano counterpart on uv photolysis. Computations show that the cyclic isomer is a shallow energy minimum between two transition states. Intrinsic reaction coordinate calculations indicate that conversion between the two products is feasible via the cyclic configuration.
’ INTRODUCTION Acetonitrile is prone to photoisomerization and fragmentation, leading to the generation of more unstable species including smaller radicals and ions, and they have been the subjects of numerous spectroscopic and molecular dynamics studies.16 In particular, interconversions between the cyano and isocyano isomers, formation of the cyclic derivatives, electron-trapping, and CH bond dissociation of the precursor and its derivatives have drawn much attention. Whereas the spectroscopic properties of many acetonitrile fragments are well-known, it is surprising that only the CH2 wagging (ν5) frequencies have been reported for the cyano and isocyano methyl radicals (H2CCN and H2CNC), and no spectroscopic information is available for the cyclic isomer (cyc-H2CCN).35 Recently, the reactions of laser-ablated transition-metal atoms and acetonitrile in excess Ar have been studied, and small transitionmetal insertion, methylidene, and methylidyne products containing an isocyanide group have been observed in the matrix IR spectra along with the π- and N-coordination complexes.7 In addition, the absorptions of many acetonitrile isomers and fragments, such as H2CCNH, HCNCH2, HCCN, and HCNC, are also observed in the product spectra,1,2,6 due to plume irradiation from laser ablation of transition metals, and they show intensity variation on photolysis as well. In this Article, we report observation of the H2CCN and H2CNC radicals in the matrix IR spectra. Whereas the radicals show evidence of photoconversion, the intermediate cyclic isomer is not observed. DFT and intrinsic reaction coordinate (IRC) computations show that cyc-H2CCN is in fact a shallow energy minimum between two transition states, making it difficult to trap in the matrix. ’ EXPERIMENTAL AND COMPUTATIONAL METHODS The H2CCN and H2CNC spectra shown in this report were recorded from samples prepared by codeposition of laser-ablated r 2011 American Chemical Society
Zr with acetonitrile isotopomers (CH3CN, CD3CN, and 13 CH313CN) in excess argon at 10 K using a closed-cycle refrigerator (Air Products, Displex). However, group 5 and 6 metals also yield the same H2CCN and H2CNC absorptions, whereas the relative intensities between H2CCN and H2CNC vary owing to different laser ablation plume radiation from specific metal surfaces.7 Hence, these metal-independent absorptions do not involve a metal-containing species. In our experiments, Zr atoms and intense radiation from the laser ablation plume impinge on the depositing argon matrix sample. (See Figure 1 in ref 8c.) These methods have been described in detail in previous publications.8 Reagent gas mixtures are typically 0.25 to 0.50% in argon. The Nd/YAG laser fundamental (1064 nm, 10 Hz repetition rate, 10 ns pulse width) was focused on the rotating zirconium metal target (Johnson-Matthey) using 510 mJ/pulse. After codeposition, infrared spectra were recorded at 0.5 cm1 resolution using a Nicolet 550 spectrometer with a HgCdTe range B detector. Then, samples were irradiated for 20 min periods by a mercury arc street lamp (175 W) with the globe removed using a combination of optical filters or annealed to 28 K to allow further reagent diffusion. Complementary density functional theory (DFT) calculations were carried out using the Gaussian 09 package,9 the B3LYP density functional,10 and 6-311++G(3df,3pd) basis sets for C, H, and N to provide a consistent set of vibrational frequencies and energies for the reaction products and their analogues. Geometries were fully relaxed during optimization, and the optimized geometry was confirmed by vibrational analysis. Additional BPW9111 calculations were done to confirm the B3LYP results. The vibrational frequencies were calculated analytically, and the zero-point Received: May 25, 2011 Revised: July 6, 2011 Published: July 07, 2011 8638
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Figure 1. IR spectra in the H2CCN and H2CNC absorptions regions for laser-ablated Zr atoms codeposited with CH3CN in excess argon at 10 K and their variation. (a) Zr + 0.25% CH3CN in Ar codeposited for 1 h. (b) As (a) after photolysis (λ > 420 nm). (c) As (b) after photolysis (240 < λ < 380 nm). (d) As (c) after annealing to 28 K. m and π stand for the Zr + CH3CN reaction product absorptions, ref 7a, and p and c indicate precursor and absorptions common to CH3CN samples.
Figure 2. IR spectra in the D2CCN and D2CNC absorptions regions for laser-ablated Zr atoms codeposited with CD3CN in excess argon at 10 K and their variation. (a) Zr + 0.50% CD3CN in Ar codeposited for 1 h. (b) As (a) after photolysis (λ > 420 nm). (c) As (b) after photolysis (240 < λ < 380 nm). (d) As (c) after annealing to 28 K. m and π stand for the Zr + CD3CN reaction product absorptions: p and c indicate the precursor and common absorptions.
energy is included in the calculation of binding energy of a metal complex.
’ RESULTS AND DISCUSSION Figures 1 and 2 and Figure S1 (Supporting Information) show the matrix IR spectra in the cyano and isocyano methyl radical absorption regions from codeposition of Zr with CH3CN, CD3CN, and 13CH313CN and their variation in the subsequent process of photolysis and annealing. The CH2CNH, CH2NCH, and Zr + acetonitrile reaction product absorptions are also shown in the spectra.2,7a The cyano and isocyano methyl radical absorptions
marked “H2CCN” and “H2CNC” remain essentially unchanged on the first visible (λ > 420 nm) irradiation, but the stronger H2CCN absorptions decrease ∼15% on uv (240 < λ < 380 nm) irradiation, whereas the weaker H2CNC absorptions double their intensities (Figures 1 and 2 and Figure S1ac of the Supporting Information). There is, however, a small reversal on the final visible irradiation (not shown), where the H2CNC absorptions decrease 3 to 4%, and the H2CCN absorptions increase slightly. These product absorptions decease on subsequent annealing. The observed H2CCN and H2CNC frequencies are compared with the B3LYP and BPW91 computed values in Tables 1 and 2. The strong H2CCN absorption at 664.6 cm1 is accompanied by D and 13C counterparts at 544.4 and 657.0 cm1 (H/D and 12/13 ratios of 1.221 and 1.012). This argon matrix frequency is compared with 680 cm1 (538 cm1 for the D counterpart) deduced for the ν5 (CH2 wagging) mode of H2CCN by Moran et al. from photoelectron spectroscopy, and a later value of 663.79398(85) cm1 determined by Sumiyoshi et al. with diode laser spectroscopy.3 It also agrees well with the B3LYP value of 682.5 cm1 and its D and 13C shifts of 118.7 and 8.0 cm1, as shown in Table 1. Other H2CCN absorptions also support the formation of H2CCN. The absorption at 1026.0 cm1 shifts to 910.6 and 1001.8 cm1 on deuteration and 13C substitution, and its frequency, large 13C shift, and good correlation with the B3LYP value and D and 13C shifts of 1059.5 and 131.4 and 24.7 cm1 lead to an assignment to the A1 CC stretching (ν4) mode. A weaker H2CCN absorption at 1432.7 cm1 is designated to the A1 CH2 scissoring (ν3) mode with its 13C counterpart at 1423.9 cm1 without observation of the D counterpart. The H2CCN radical has been prepared by uv photolysis of H2CCN, ClH2CCN, and CH3CN at 185 or 193 nm and observed by means of photoelectron, IR-diode, and ESR spectroscopies.3,4 However, only the CH2 wagging (ν5) mode, which is the strongest in the IR spectrum, has been observed in the previous studies. The present results show that the combination of transition-metal laser ablation and matrix IR spectroscopy is an effective method to produce and observe the cyano methyl radical. The focused, pulsed laser creates a bright plume on the metal target surface, and this radiation causes photodissociation of CH3CN in the sample during deposition, reaction 1,14,7,8c leading to the strong 8639
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Table 1. Observed and Calculated Fundamental Frequencies of H2CCN Isotopomers in the Ground 2B1 Statea H2CCN approximate description
obsb
B3LYPc intc BPW91d intd
B3LYPc intc BPW91d intd
obsb
B3LYPc intc BPW91d intd
B2 CH2 as str.
3267.5
2
3205.1
1
2436.2
0
2389.0
0
3258.8
2
3191.7
3163.1
0
3102.3
0
2292.9
0
2249.5
0
3157.8
0
3097.1
0
A1 CN str.
2130.4
2
2075.2
2
2128.4
2
2072.6
2
2078.8
2
2024.0
2
1170.3
0
1144.1
0
1423.9f
1438.9
3
1391.0
4
928.1
6
907.1
5
1001.8
1034.8
6
1023.4
5
852.0
0
823.4
0
1021.1
0
984.8
0
563.8
25
547.5
24
674.5
46
653.7
45
404.8 346.7
0 1
390.7 335.4
0 1
422.0 376.8
0 1
408.1 365.2
0 0
A1 CH2 scis.
1432.3f
1447.7
3
1399.8
4
A1 CC str.
1026.0
1059.5
7
1047.0
5
1035.3
0
998.7
0
682.5
46
661.6
45
433.9 385.3
0 1
419.6 373.4
0 0
B1 CH2 wag
664.6, [663.79]e
B1 CCN oop bend B2 CCN ip bend
d
obsb
A1 CH2 s str.
B2 CH2 rock
a
H213C13CN
D2CCN
910.6 544.4, 538e
657.0
2
Frequencies and intensities are in cm1 and km/mol and calculated with 6-311++G(3df,3pd). b Observed in an argon matrix. c Computed with B3LYP. Computed with BPW91. e From gas phase, ref 3. f Tentative assignment. CH2CN has a C2v structure.
Table 2. Observed and Calculated Fundamental Frequencies of H2CNC Isotopomers in the Ground 2B1 Statea H2CNC approximate description
obsb
H213CN13C
D2CNC
B3LYPc intc BPW91d intd
obsb
B3LYPc intc BPW91d intd
obsb
B3LYPc intc BPW91d intd
B2 CH2 as str.
3277.8
0
3211.5
0
2449.7
1
2399.5
1
3263.4
0
3197.4
A1 CH2 s str.
3155.4
0
3092.4
0
2280.0
1
2235.0
1
3150.8
0
3087.9
0
2036.7 1476.6
46 0
1956.9 1430.9
38 0
1982.1
2034.7 1191.2
45 0
1954.7 1171.5
37 0
1946.8
2001.3 1468.9
45 0
1923.8 1422.8
37 0
852.1
1063.0
A1 NC str. A1 CH2 scis.
1981.7
A1 CH2 rock
1071.6
1135.7
6
1098.7
5
1111.3
1
1101.4
0
586.7
50
585.5
49
B1 CNC oop bend
372.7
0
360.7
0
B2 CNC ip bend
288.3
0
284.7
0
B2 CN str. B1 CH2 wag
587.5, [615]e
465.2, 486e
897.5
6
870.1
6
978.7
1
955.6
0
474.0
23
472.9
23
355.2
1
343.9
1
269.8
0
265.9
0
582.0
0
1128.0
6
1091.1
5
1076.7
1
1067.1
0
581.2
50
579.9
49
368.4
0
356.6
0
285.1
0
281.5
0
Frequencies and intensities are in cm1 and km/mol and calculated with 6-311++G(3df,3pd). b Observed in an argon matrix. c Computed with B3LYP. d Computed with BPW91. e From gas phase, ref 5. CH2NC has a C2v structure. a
H2CCN absorptions. This process involves CH bond dissociation by vacuum-uv radiation in the ablation plume, as concluded for the CH3 radical formed in similar methane experiments, reaction 2.12 CH3 CN þ hν f 3 H þ 3 H2 CCN
ð1Þ
CH4 þ hν f 3 H þ 3 CH3
ð2Þ
Figures 1 and 2 show that whereas the H2CCN absorptions decrease on uv irradiation, the H2CNC absorptions increase, suggesting that H2CCN is photoisomerized to another species responsible for the H2CNC absorptions. The strongest H2CNC absorption is observed at 1981.7 cm1 along with its D and 13C counterparts at 1982.1 and 1946.8 cm1. The large 13C shift and the frequency suggest an isocyano molecule. The H2CNC absorption in the low-frequency region at 587.5 cm1 and its D counterpart at 465.2 cm1 are below the 615 and 486 cm1 photoelectron measurements for the CH2 wagging (ν5) modes of H2CNC and D2CNC,5 and the H/D frequency ratios, 1.263 and 1.265, are appropriate for this mode. The 13C counterpart is observed at 582.0 cm1 in solid argon. We assign the absorptions observed at 1981.7 and 587.5 cm1 to the NC stretching (ν2) and CH2 wagging modes of the isocyano methyl radical (H2CNC).
Figure 3. Intrinsic reaction coordinate (IRC) calculation between H2CCN and H2CNC on the doublet potential energy surface. Apparently, cyc-H2CCN, which is not observed in this study, is a shallow energy minimum between the two transition states.
They also correlate well with the B3LYP values of 2036.7 and 586.7 cm1 and the isotopic shifts, as shown in Table 2. A weaker H2CNC absorption at 1071.6 cm1 shifts to 852.1 and 1063.0 cm1 on D and 13C substitution and is designated to the CH2 rocking (ν4) mode on the basis of its frequency, large D 8640
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Figure 4. B3LYP structures of H2CCN, cyc-H2CCN, H2CNC, and the two transition states. The cyano and isocyano methyl radicals have planar C2v structures, whereas cyc-H2CCN and the transition states have Cs structures. The bond lengths and angles are in angstroms and degrees.
shift, and good correlation with the DFT values (Table 2). The observed three (ν2, ν4, and ν5) bands are the strongest bands, and all other absorptions are too weak to observe here. They substantiate generation of H2CNC from CH3CN during laserablation and codeposition of transition-metal atoms. Observation of the isocyano methyl radical and the two-fold increase in the H2CNC absorptions on subsequent uv irradiation (with the decrease in the H2CCN absorptions) are indicative of the uv photoconversion from the cyano methyl radical to the isocyano counterpart, reaction 3. There is evidence of a slight reversal on the final visible irradiation, where the H2CNC absorptions decrease 3 to 4% and the H2CCN absorptions exhibit detectable increases. uv
H2 CCN sf H2 CNC
ð3Þ
Cyclic-H2CCN, another plausible isomer, would show strong absorptions at ∼1820 and 870 cm1, but no such absorptions were observed. The cyano methyl radical is the most stable; H2CNC and cyc-H2CCN are 95 and 222 kJ/mol higher than H2CCN, consistent with the stronger initial H2CCN absorptions relative to the H2CNC absorptions. Interconversions between H2CCN and H2CNC most probably occur via cyc-H2CNC. The linear CCN moiety of H2CCN bends to form a triangular cyclic configuration (cyc-H2CCN), followed by CC bond elongation with N slipping in between the C atoms to generate the linear CNC moiety (H2CNC), and vice versa. Our failure to observe cyc-H2CCN might be due to its photolysis to H2CCN or H2CNC by the laser-ablation plume. The transition states between H2CCN and cyc-H2CCN (TS1) and between cyc-H2CCN and H2CNC (TS2) are 7 and 6 kJ/mol higher than cyc-H2CCN (Figure 3 and Figure S2 of the Supporting Information). The low-energy barriers to the cyano and isocyano radicals are consistent with the absence of cycH2CCN in matrix IR spectra. To investigate further the conversion possibilities between the isomers on the doublet potential energy surface, IRC computations were carried out.9 Figure 3 clearly shows that photochemical conversion of the cyano methyl radical to the isocyano counterpart via cyc-H2CCN and vice versa is feasible. The nonbonded CN distance in TS1 is 1.962 Å, and the nonbonded CC distance in TS2 is 1.766 Å. Although the
activation energy (229 kJ/mol from H2CCN) is substantial, it is well within the reach of near-uv radiation in the laser ablation plume. The cyano and isocyano methyl radicals have C2v structures with a linear molecular axis, whereas the cyclic radical has a planar Cs structure, as shown in Figure 4. The CN atomic distance of 1.166 Å in H2CCtN is slightly shorter than the NC atomic distance of 1.182 Å in H2CNtC, in line with the computed natural bond orders of 2.861 and 2.846.13 The spin density is concentrated on the methylene carbon in both the cyano and isocyano methyl radicals (Mulliken spin densities of 0.88 and 0.85), whereas the spin density is distributed between the methylene and cyano carbon atoms in cyc-H2CCN (Mulliken spin densities of 0.47 and 0.54).
’ CONCLUSIONS The cyano and isocyano methyl radicals (H2CCN and H2CNC) are observed in the infrared spectra of argon/acetonitrile matrix samples prepared under exposure to laser-ablation plume irradiation. The CH2 wagging frequencies are consistent with the previously reported gas-phase measurements, and the observed frequencies correlate well with the DFT computed values. Increase in the H2CNC absorptions on uv photolysis with decrease in the H2CNC absorptions is indicative of the photoconversion of the cyano radical to the isocyano counterpart. CycH2CCN, another plausible isomer, is not observed because of its high energy and the low-energy barriers to H2CCN and H2CNC. Our IRC computations show that transitions between the cyano and isocyano radicals via cyc-H2CCN are feasible. The H2CCN and H2CNC radicals have linear molecular axes with comparable CN and NC bond lengths. ’ ASSOCIATED CONTENT
bS
Supporting Information. Calculated fundamental frequencies of Cyc-H2CdNC isotopomers in the ground 2A0 state. IR spectra in the H213C13CN and H213CN13C absorption regions for laser-ablated Zr atoms codeposited with 13CH313CN in excess argon at 10 K. Relative energies of H2CCN, H2CNC, cyc-H2CCN, and the transition states between them on the doublet potential energy surface. This material is available free of charge via the Internet at http://pubs.acs.org. 8641
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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(12) (a) Andrews, L.; Cho, H.-G. J. Phys. Chem. A 2005, 109, 6796. (b) Jacox, M. E. J. Mol. Spectrosc. 1977, 66, 272. (13) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899 and references therein.
’ ACKNOWLEDGMENT We gratefully acknowledge financial support from National Science Foundation (U.S.) grant CHE 03-52487 to L.A. and support from the Korea Research Foundation (KRF) grant funded by the Korean government (MEST) (no. 2009-0075428) and KISTI supercomputing center. ’ REFERENCES (1) (a) Jacox, M. E. Chem. Phys. 1979, 43, 157. (b) Yang, X.; Maeda, S.; Ohno, K. J. Phys. Chem. A 2005, 109, 7319. (Photolysis of CH3CN) (c) Hattori, R.; Suzuki, E.; Shimizu, K. J. Mol. Struct. 2005, 738, 165. (CH3NC) (2) Maier, G.; Schmidt, C.; Reisenauer, H. P.; Endlein, E.; Becker, D; Eckwert, J.; Hess, B. A.; Schaad, L. J. Chem. Ber. 1993, 126, 2337. (b) Deng, R.; Trenary, M. J. Phys. Chem. C 2007, 111, 17088. (MeCN isomers) (3) (a) Moran, S.; Ellis, H. B., Jr.; DeFrees, D. J.; McLean, A. D.; Ellison, G. B. J. Am. Chem. Soc. 1987, 109, 5996. (b) Sumiyoshi, Y.; Tanaka, K.; Tanaka, T. J. Chem. Phys. 1996, 104, 1839. (H2CCN, ν5 band) (4) (a) Svejda, P.; Volman, D. H. J. Phys. Chem. 1970, 74, 1872. (b) Egland, R. J.; Symons, M. R. C. J. Chem. Soc. A 1970, 5, 1326. (c) Sargent, F. P. Can. J. Chem. 1970, 48, 1780. (H2CCN, ESR) (5) (a) Moran, S.; Ellis, H. B., Jr.; DeFrees, D. J.; McLean, A. D.; Paulson, S. E.; Ellison, G. B. J. Am. Chem. Soc. 1987, 109, 6004. (b) Hirao, T.; Ozeki, H.; Saito, S.; Yamamoto, S. J. Chem. Phys. 2007, 127, 134312. (H2CNC) (6) (a) Maier, G.; Reisenauser, H. P.; Rademacher, K. Chem.—Eur. J. 1998, 4, 1957. (b) Dendramis, A.; Leroi, G. E. J. Chem. Phys. 1997, 66, 4334. (c) Nimlos, M. R.; Davico, G.; Geise, C. M.; Wenthold, P. G.; Blanksby, W. C.; Lineberger, S. J.; Hadad, C. M.; Petersson, G. A.; Ellison, G. B. J. Chem. Phys. 2002, 117, 4323. (d) Jacox, M. E. J. Phys. Chem. Ref. Data 2003, 32, 1. (HCCN, HCNC, and cyc-HCNC) (7) (a) Cho, H.-G.; Andrews, L. J. Phys. Chem. A 2010, 114, 891. Zr + CH3CN(b) Cho, H.-G.; Andrews, L. J. Phys. Chem. A 2010, 114, 5997. (Group 5 + CH3CN)(c) Cho, H.-G.; Andrews, L. Dalton Trans. 2011submitted. (Group 6 + CH3CN) (8) (a) Andrews, L.; Citra, A. Chem. Rev. 2002, 102, 885 and references therein. (b) Andrews, L. Chem. Soc. Rev. 2004, 33, 123 and references therein. (c) Andrews, L.; Cho, H.-G. Organometallics 2006, 25, 4040. (9) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (10) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, Y.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (11) Burke, K.; Perdew, J. P.; Wang, Y. In Electronic Density Functional Theory: Recent Progress and New Directions; Dobson, J. F., Vignale, G., Das, M. P., Ed.; Plenum, 1998. 8642
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