Reactions of Yttrium and Scandium Atoms with Acetylene: A Matrix

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J. Phys. Chem. A 2010, 114, 9069–9073

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Reactions of Yttrium and Scandium Atoms with Acetylene: A Matrix Isolation Infrared Spectroscopic and Theoretical Study Yun-Lei Teng and Qiang Xu* National Institute of AdVanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577 and Graduate School of Engineering, Kobe UniVersity, Nada Ku, Kobe, Hyogo 657-8501, Japan ReceiVed: April 19, 2010; ReVised Manuscript ReceiVed: July 30, 2010

Laser-ablated yttrium and scandium metal atoms have been codeposited at 4 K with acetylene in excess argon. Products, Y(C2H2), HYCCH, HScCCH-, and HScScCCH-, have been formed in the present experiments and characterized using infrared spectroscopy on the basis of the results of the isotopic shifts, mixed isotopic splitting patterns, stepwise annealing, the change of reagent concentration and laser energy, and the comparison with theoretical predictions. Density functional theory calculations have been performed on these molecules. The agreement between the experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts supports the identification of these molecules from the matrix infrared spectra. Plausible reaction mechanisms have been proposed to account for the formation of these molecules. Introduction Considerable attention has been paid to the interactions of metal atoms with acetylene due to the industrial importance of hydrocarbon hydrogenation and dehydrogenation processes.1-3 The interactions of metal atoms with acetylene have been investigated using the matrix isolation technique, and four bonding models, side-on M-η2-(C2H2) (Cu, Ag, Au, Cr, Ni, Al, Pt, Pd, Li, U, Th, La, Ti, Zr, Hf, Ge, Sn), vinyl form M-C2H2 (Al, Au), inserted HMCCH (Be, B, Cr, Pt, Al, Fe, Cr, La, Ti, Zr, Hf, Ge, Sn, Pb), and vinylidene MdCdCH2 (Ni, Au, Pt, Na, La) have been identified and characterized by infrared and electron spin resonance spectroscopies.4-17 In addition, kinds of products, M(C2H2)2 (Pd, U, Th), MCCH (Mg, U, Th), and Pd2(C2H2), have also been identified and characterized through the matrix isolation technique.11,17,18 The interactions of metal with acetylene have also been extensively investigated by quantum chemical computations.18-22 The bonding of Y and Sc metal ions with C2H2 has been investigated using the theoretical method that includes electron correlation.23 However, as far as we know, the reactions of Y and Sc metal atoms with acetylene have not been studied using the matrix isolation techniques. Recent studies have shown that, with the aid of isotopic substitution techniques, matrix isolation infrared spectroscopy combined with quantum chemical calculations is very powerful for investigating structure and bonding of novel species.14-17,24,25 To understand the interaction of acetylene and Y and Sc metal atoms, the reactions of laser-ablated Y and Sc atoms with acetylene in a solid-argon matrix have been performed. IR spectroscopy and theoretical calculations provide evidence for the formation of products, Y(C2H2), HYCCH, HScCCH-, and HScScCCH-. Experimental and Theoretical Methods The experiment for laser ablation and matrix-isolation infrared spectroscopy is similar to those previously reported.25 Briefly, the Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate * To whom correspondence should be addressed. E-mail: [email protected].

with 10 ns pulse width) was focused on the rotating Y and Sc metal targets. The laser-ablated Y and Sc metal atoms were codeposited with C2H2 (99.96%, Cambridge Isotopic Laboratories) in excess argon onto a CsI window cooled normally to 4 K by means of a closed-cycle helium refrigerator. Typically, a 3-20 mJ/pulse laser power was used. The isotopic 13C2H2 and C2D2 (99%, Cambridge Isotopic Laboratories) were used to prepare the C2H2/Ar mixtures. In general, matrix samples were deposited for 30-60 min with a typical rate of 2-4 mmol per hour. After sample deposition, IR spectra were recorded on a BRUKER IFS 66 v/s spectrometer at 0.5 cm-1 resolution using a liquid nitrogen cooled HgCdTe (MCT) detector for the spectral range of 5000-600 cm-1. Samples were annealed at different temperatures and subjected to broad-band irradiation (λ > 250 nm) using a high-pressure mercury arc lamp (Ushio, 100 W). Quantum chemical calculations were performed to predict the structures and vibrational frequencies of the observed reaction products using the Gaussian 03 program.26 The B3LYP density functional method was utilized.27 The 6-311++G(d, p) basis set was used for Sc, H, and C atoms and SDD for Y atom.28,29 Geometries were fully optimized and vibrational frequencies were calculated with analytical second derivatives. Results and Discussion Experiments have been done with C2H2 concentrations ranging from 0.05 to 0.5% in excess argon. Typical infrared spectra for the reactions of laser-ablated Y and Sc metal atoms with C2H2 in excess argon in the selected regions are illustrated in Figures 1-5, and the absorption bands are listed in Table 1. The stepwise annealing and irradiation behaviors of these product absorptions are also shown in the figures and will be discussed below. Quantum chemical calculations have been carried out for the possible isomers and electronic states of the potential product molecules. Figure 6 shows the optimized structures and electronic ground states. Tables 2-5 report the calculated IR frequencies and isotopic frequencies of the reaction products. Y(C2H2). The absorption at 1100.6 cm-1 appears on sample deposition, visibly increases on sample annealing to 25 and 30

10.1021/jp103486f  2010 American Chemical Society Published on Web 08/09/2010

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Figure 1. Infrared spectra in the 1550-1400 and 1210-1000 cm-1 regions from codeposition of laser-ablated Y atoms with 0.4% C2H2 in argon: (a) 60 min sample deposition at 4 K; (b) after annealing to 25 K; (c) after annealing to 30 K; (d) after 15 min of broad-band irradiation; (e) after annealing to 35 K.

TABLE 1: Infrared Absorptions (cm-1) from Codeposition of Laser-Ablated Y and Sc Metal Atoms with Acetylene in Excess Argon 12

C2 H 2

1415.3 1100.6 1387.1 1365.3

13

C2 H 2

1415.6 1083.4 1387.0 1365.4

12

C2 D2

1014.4 997.2 981.9

assignment HYCCH, Y- H stretching Y(C2H2), in-plane CCH deformation HScCCH-, Sc-H stretching HScScCCH-, Sc-H stretching

TABLE 2: Calculated (B3LYP/6-311++G(d,p)-SDD) Vibrational Frequencies and Intensities of Y(C2H2) (2A1)a Y(12C2H2) 3087.3 (68) 3057.4 (34) 1422.2 (2) 1109.4 (32) 983.7 (0) 852.1 (0.5) 629.5 (54) 534.9 (42) 519.0 (24)

Y(13C2H2)

Y(12C2D2)

mode

3076.3 (70) 2303.8 (25) symm C-H stretching 3048.5 (34) 2252.9 (15) antisymm C-H stretching 1369.4 (2) 1382.9 (2) C-C stretching 1093.1 (32) 927.1 (13) in-plane CCH deformation 974.4 (0) 778.0 (0) C2H2 torsion 851.3 (1) 612.5 (1) in-plane CCH deformation 626.0 (53) 521.2 (40) C2H2 tilt 519.5 (40) 478.2 (37) symm Y-C stretching 505.0 (22) 457.5 (24) antisymm Y-C stretching

a Vibrational frequencies are given in reciprocal centimeters; intensities (in parentheses) are given in kilometers per mole.

K, and disappears after broad-band irradiation (Table 1 and Figure 1). The 1100.6 cm-1 band shifts to 1083.4 cm-1 with 13 C2H2, exhibiting isotopic frequency ratio of 1.0159 (12C2H2/ 13 C2H2). As shown in Figure 2, the mixed 12C2H2 + 13C2H2 isotopic spectra only provide the sum of pure isotopic bands, which indicates that only one C2H2 subunit is involved in this mode. Therefore, the 1100.6 cm-1 band is assigned to the CCH deforming vibration of the Y(C2H2) molecule. The assignment is strongly supported by DFT calculations. As shown in Figure 6 and Table 2, the Y(C2H2) molecule is predicted to have an 2A1 ground state with C2V symmetry (Figure 6) and is the most stable structural isomer of Y-C2H2. The CCH deformation mode is calculated at 1109.4 cm-1 (Table 2), which is in good agreement with the experimental observation (1100.6 cm-1). The isotopic frequency ratio calculated for the CCH deformation mode (12C2H2/13C2H2, 1.0150) is also consistent with the experimental observation (12C2H2/13C2H2, 1.0159). Scale factors of 0.992 and 1.001 for this deforming vibration of Y(C2H2) molecule are needed to fit the B3LYP frequency and isotopic ratio to the argon matrix observations, respectively. These agreements between the experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts confirm the identification of the Y(C2H2) molecule from the matrix IR spectra.

Figure 2. Infrared spectra in the 1150-1050 cm-1 region from codeposition of laser-ablated Y atoms with C2H2 in argon: (a) 0.4% 12 C2H2; (b) 0.3% 12C2H2 + 0.3% 13C2H2; (c) 0.4% 13C2H2.

HYCCH. A new band at 1415.3 cm-1 (Table 1 and Figure 1) appears on sample deposition, slightly increases on sample annealing to 25 and 30 K, visibly increases after broad-band irradiation, and sharply increases on sample annealing to 35 K (Table 1 and Figure 1). The band is assigned to the HYCCH molecule on the basis of isotopic substitution and DFT calculations. The 1415.3 cm-1 band shows no carbon-13 shift but shifts to 1014.4 cm-1 with 12C2D2, exhibiting isotopic frequency ratio of 1.3952 (12C2H2/12C2D2). The band position as well as the isotopic 12C2H2/12C2D2 ratio is close to the values of the Y-H stretching vibration of various previously characterized Y hydrides in excess argon,30 implying that the 1415.3 cm-1 band is due to a Y-H stretching vibration. The mixed 12C2H2 + 12 C2D2 isotopic spectra only provide the sum of pure isotopic bands, which indicates that only one Y-H subunit is involved in this mode (Figure 3). Therefore, the 1415.3 cm-1 band is assigned to the Y-H stretching vibration of the HYCCH molecule. The present calculations lend support for the assignment of HYCCH. Our calculations predict that HYCCH has an 2A′ ground state with Cs symmetry (Figure 6). The Y-H stretching vibrational frequency is calculated at 1488.1 cm-1 (Table 3), which is in agreement with the experimental observation (1415.3 cm-1). The calculated 12C2H2/12C2D2 isotopic frequency ratio (1.4010) of Y-H stretching mode is also consistent with the

Reactions of Yttrium and Scandium Atoms with Acetylene

Figure 3. Infrared spectra in the 1450-1000 cm-1 region from codeposition of laser-ablated Y atoms with C2H2 in argon: (a) 0.4% 12 C2H2; (b) 0.3% 12C2H2 + 0.3% C2D2; (c) 0.4% C2D2.

TABLE 3: Calculated (B3LYP/6-311++G(d,p)-SDD) Vibrational Frequencies and Intensities of HYCCH (2A′)a HYCCH

HY13C13CH

DYCCD

mode

3439.9 (26) 2025.7 (65) 1488.1 (458) 716.3 (40) 707.3 (44)

3422.3 (45) 1953.0 (59) 1488.1 (458) 709.6 (40) 700.7 (44)

2658.3 (2) 1900.4 (77) 1062.2 (237) 566.5 (17) 559.6 (19)

C-H stretching C-C stretching Y-H stretching out-of-plane CCH bending HYC bending

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Figure 4. Infrared spectra in the 1450-1350 cm-1 region from codeposition of laser-ablated Sc atoms with 0.4% C2H2 in argon: (a) 60 min sample deposition at 4 K; (b) after annealing to 25 K; (c) after annealing to 30 K; (d) after 15 min of broad-band irradiation; (e) after annealing to 35 K; (f) 0.4% C2H2 + 0.08% CCl4 after annealing to 35 K.

a Vibrational frequencies are given in reciprocal centimeters; intensities (in parentheses) are given in kilometers per mole.

experimental observation (1.3952). To fit the B3LYP frequency and isotopic ratio to the argon matrix observations, scale factors of 0.951 and 0.996 for this stretching mode of HYCCH molecule are needed, respectively. These agreements between the experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts confirm the identification of the HYCCH molecule from the matrix IR spectra. Two bands at 1448.6 and 1434.8 cm-1 can be observed after broadband irradiation, which show no shift after isotopic 12C2H2 substitution but shifts to 1038.3 and 1033.6 cm-1 after isotopic C2D2 substitution. These two bands may be due to the Y-H stretching modes of some kinds of molecules. HScCCH-. Upon deposition of Sc atoms in the presence of acetylene, a new band at 1387.1 cm-1 (Table 1 and Figure 4) appears on annealing to 25 K, slightly changes on annealing to 30 K and after broad-band irradiation, and sharply increases on further annealing to 35 K. The band is assigned to the HScCCHmolecule on the basis of isotopic substitution and DFT calculations. The 1387.1 cm-1 band shows no carbon-13 shift but shifts to 997.2 cm-1 with 12C2D2, exhibiting isotopic frequency ratio of 1.3910 (12C2H2/12C2D2). The band position as well as the isotopic 12C2H2/12C2D2 ratio is close to the values of the Sc-H stretching vibration of various previously characterized Sc hydrides in excess argon,30 implying that the 1387.1 cm-1 band is due to a Sc-H stretching vibration. The mixed 12C2H2 + C2D2 isotopic spectra only provide the sum of pure isotopic bands, which indicates that only one Sc-H subunit is involved in this mode (Figure 5). Doping with CCl4 decreases the band (Figure 4, trace f), suggesting that the product is anionic.31 Our experimental results show that broad-band irradiation can also decrease this band. Therefore, the 1387.1 cm-1 band is assigned to the Sc-H stretching vibration of the HScCCH- molecule.

Figure 5. Infrared spectra in the 1450-900 cm-1 region from codeposition of laser-ablated Sc atoms with C2H2 in argon: (a) 0.4% 12 C2H2; (b) 0.2% 12C2H2 + 0.2% C2D2; (c) 0.4% C2D2.

The present calculations lend support for the assignment of HScCCH-. Our calculations predict that HScCCH- has an 1A′ ground state with Cs symmetry (Figure 6). The Sc-H stretching vibrational frequency is calculated at 1373.4 cm-1 (Table 4), which is in agreement with the experimental observations (1387.1 cm-1). The calculated 12C2H2/12C2D2 isotopic frequency ratio (1.3943) of Sc-H stretching mode is also consistent with the experimental observation (1.3910). Scale factors of 1.001 and 0.998 for this stretching mode are needed to fit the B3LYP frequency and isotopic ratio to the argon matrix observations, respectively. The reason that the C-C stretching mode is not observed in the present experiment is due to its weak intensity as predicted by our DFT calculations. These agreements between the experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts confirm the identification of the HScCCH- molecule from the matrix IR spectra. HScScCCH-. The absorption at 1365.3 cm-1 (Table 1 and Figure 4) appears on annealing to 25 K, slightly changes on annealing to 30 K and after broad-band irradiation, and sharply increases on further annealing to 35 K. The band is assigned to

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Figure 6. Optimized structures (bond length in angstrom, bond angle in degree) of the possible products calculated at the B3LYP level.

TABLE 4: Calculated (B3LYP/6-311++G(d,p)) Vibrational Frequencies and Intensities of HScCCH Anion (1A′)a

a

HScCCH

HSc13C13CH

DScCCD

mode

3439.7 (9) 1999.2 (4) 1373.4 (263) 661.4 (27) 648.2 (33)

3422.5 (8) 1927.3 (4) 1373.4 (263) 655.0 (27) 641.9 (33)

2651.0 (12) 1880.1 (2) 985.0 (128) 525.1 (13) 515.7 (18)

C-H stretching C-C stretching Sc-H stretching out-of-plane CCH bending in-plane CCH bending

Vibrational frequencies are given in reciprocal centimeters; intensities (in parentheses) are given in kilometers per mole.

TABLE 5: Calculated (B3LYP/6-311++G(d,p)) Vibrational Frequencies and Intensities of HScScCCH Anion (2A)a

a

HScScCCH

HScSc13C13CH

DScScCCD

mode

3438.1 (20) 2002.5 (0) 1328.0 (647) 664.6 (41) 651.1 (34)

3420.8 (19) 1930.5 (0) 1328.0 (647) 658.2 (41) 644.8 (34)

2651.0 (19) 1882.5 (2) 953.3 (325) 527.8 (22) 516.7 (19)

C-H stretching C-C stretching Sc-H stretching out-of-plane CCH bending in-plane CCH bending

Vibrational frequencies are given in reciprocal centimeters; intensities (in parentheses) are given in kilometers per mole.

the HScScCCH- molecule on the basis of isotopic substitution and DFT calculations. The 1365.3 cm-1 band shows no carbon13 shift but shifts to 981.9 cm-1 with 12C2D2, exhibiting isotopic frequency ratio of 1.3905 (12C2H2/12C2D2). The band position as well as the isotopic 12C2H2/12C2D2 ratio is close to the values of the Sc-H stretching vibration of various previously characterized Sc hydrides in excess argon,30 implying that the 1365.3 cm-1 band is due to a Sc-H stretching vibration. The mixed 12 C2H2 + 12C2D2 isotopic spectra only provide the sum of pure isotopic bands, which indicates that only one Sc-H subunit is involved in this mode (Figure 5). Doping with CCl4 decreases the band (Figure 4, trace f), suggesting that the product is anionic.31 Therefore, the 1365.3 cm-1 band is tentatively assigned to the Sc-H stretching vibration of the HScScCCHmolecule. The present calculations lend support for the tentative assignment of HScScCCH-. Our calculations predict that HScScCCH- has an 2A ground state with C1 symmetry (Figure 6). The Sc-H stretching vibrational frequency is calculated at 1328.0 cm-1 (Table 5), which is in agreement with the experimental observation (1365.3 cm-1). The calculated 12C2H2/ 12 C2D2 isotopic frequency ratio (1.4053) of Sc-H stretching mode is also consistent with the experimental observation (1.3930). The reason that the C-C stretching mode is not observed in the present experiment is due to its weak intensity as predicted by our DFT calculations.

Reaction Mechanism. Laser-ablated Y and Sc metal atoms react with C2H2 to form Y(C2H2), HYCCH, HScCCH-, and HScScCCH- (Figure 1-5). According to reactions 1-4, the reactions are predicted to be exothermic by 44.2, 36.5, 49.8, and 61.2 kcal/mol, respectively. The Y(C2H2) molecule disappears, while the HYCCH molecule increases after broad-band irradiation, indicating rearrangement reactions take place during broad-band irradiation. The theoretical potential-energy surface investigation reveals that the Y(C2H2) molecule with the 2A1 state has to surmount a energy barrier of 44 kcal/mol to rearrange to the HYCCH molecule with the 2A′ state. For the reaction of laser-ablated Sc atoms with acetylene, only the HScCCH- and HScScCCH- products are observed. It is possible that irradiation in the laser ablation plume makes the adduct Sc(C2H2) and Sc2(C2H2) molecule isomerize to the HScCCHand HScScCCH- molecules completely. Therefore, the Sc(C2H2) and Sc2(C2H2) molecules are not observed in the present experiments.

Y(2D) + C2H2(1Σg+) f Y(C2H2)(2A1)

∆E ) 44.2 kcal/mol (1)

Reactions of Yttrium and Scandium Atoms with Acetylene

Y(2D) + C2H2(1Σg+) f HYCCH(2A)

∆E ) 36.5 kcal/mol (2)

Sc(2D) + C2H2(1Σg+) + e- f HScCCH-(1A) ∆E ) 49.8 kcal/mol (3) Sc2(1Σg+)+C2H2(1Σg+) + e- f HScScCCH-(2A)

∆E ) -61.2 kcal/mol (4)

Conclusions Reactions of laser-ablated Y and Sc metal atoms with acetylene in excess argon have been studied using matrixisolation infrared spectroscopy. On the basis of the isotopic shifts and splitting patterns, the Y(C2H2), HYCCH, HScCCH-, and HScScCCH- molecules have been characterized. Density functional theory calculations have been performed, which lend strong support to the experimental assignments of the infrared spectra. In addition, the plausible reaction mechanism for the formation of the products has been proposed. Acknowledgment. This work was supported by a Grant-inAid for Scientific Research (B) (Grant 17350012) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan. References and Notes (1) Rashidi, M.; Puddephatt, R. J. J. Am. Chem. Soc. 1986, 108, 7111. (2) Cremer, P. S.; Su, S. D.; Shen, Y. R.; Somorjai, G. A. J. Phys. Chem. B 1997, 101, 6474. (3) Kose, R.; Brown, W. A.; King, D. A. J. Am. Chem. Soc. 1999, 121, 4845. (4) Ozin, G. A.; McIntosh, D. F.; Power, W. J.; Messmer, R. P. Inorg. Chem. 1981, 20, 1782. (5) Kasai, P. H. J. Am. Chem. Soc. 1982, 104, 1165. (6) Kasai, P. H. J. Am. Chem. Soc. 1992, 114, 3299. (7) Kline, E. S.; Kafafi, Z. H.; Hauge, R. H.; Margrave, J. L. J. Am. Chem. Soc. 1987, 109, 2402. (8) (a) Burkholder, T. R.; Andrews, L. Inorg. Chem. 1993, 32, 2491. (b) Chertihin, G. V.; Andrews, L. J. Am. Chem. Soc. 1994, 116, 3513. (9) (a) Manceron, L.; Andrews, L. J. Am. Chem. Soc. 1985, 107, 563. (b) Lee, Y. K.; Manceron, L.; Papai, I. J. Phys. Chem. A 1997, 101, 9650. (10) Andrews, L.; Hassanzadeh, P.; Martin, J. M. L.; Taylor, P. R. J. Phys. Chem. 1993, 97, 5839. (11) (a) Teng, Y. L.; Xu, Q. J. Phys. Chem. A 2008, 112, 10274. (b) Teng, Y. L.; Xu, Q. J. Phys. Chem. A 2009, 113, 12163.

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