Experimental and theoretical characterization of a

JOURNAL OF CHEMICAL PHYSICS

VOLUME 119, NUMBER 5

1 AUGUST 2003

Experimental and theoretical characterization of a C2 H2 O2¿ cation in solid argon Jian Dong, Lei Miao, and Mingfei Zhoua) Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Department of Chemistry, Fudan University, Shanghai 200433, People’s Republic of China

共Received 29 April 2003; accepted 12 May 2003兲 Laser ablation of transition metals with concurrent codeposition of C2 H2 /O2 /Ar mixtures at 11 K produced metal independent absorptions at 1493.1 cm⫺1. On the basis of isotopic shifts and splittings, enhancement in doping with electron trapping gas, and quantum chemical frequency calculations, the band is assigned to the O–O stretching vibration of the C2 H2 O⫹ 2 cation, which was predicted to have a 2 A ⬙ ground state with a nonplanar C s symmetry. © 2003 American Institute of Physics. 关DOI: 10.1063/1.1588633兴

INTRODUCTION

sition metal targets with C2 H2 and oxygen mixtures in excess argon.

Acetylene plays an important role in the chemistry of combustion and atmosphere. It is one of the most important intermediate species in the oxidation of hydrocarbons, having a significant influence on the heat release, overall fuel destruction, and molecular mass growth.1 In the field of atmospheric chemistry, it also represents a potential source in studying air traffic. As such, understanding acetylene oxidation not only is an important step toward the development of combustion models for realistic fuels, but it also provides insights on soot nucleation and growth in diffusion flames. There are a variety of atmospheric ion–molecule reactions at the focus of current research interests. The mechanisms of the reactions of acetylene cation with CH4 , 2 NH3 , 3 and CH3 OH 共Ref. 4兲 have been recently studied both experimentally and theoretically. These studies have provided some of the most detailed information yet obtained regarding the dynamics of acetylene cation reactions. The formation of covalent complex is found to be an important pathway in the case of C2 H⫹ 2 ⫹CH4 , whereas, the mechanism could not be observed for C2 H⫹ 2 ⫹NH3 . For the latter system only proton transfer and charge transfer seem to occur. Apparently, the observation and characterization of the possible reaction intermediates are very important in interpreting the different reaction mechanisms. Here we report an experimental and theoretical characterization of the C2 H2 O⫹ 2 cation, a poten⫹ tial intermediate in the C2 H⫹ 2 ⫹O2 or C2 H2 ⫹O2 reactions. Laser ablation has proven to be a powerful technique to produce reactive intermediates and free radicals for gas phase as well as matrix isolation spectroscopic studies.5 Laser ablation of transition metal target produces electrons and cations, and as a result, charged species can be formed and trapped in solid matrices. Using this technique, we have reported a variety of charged species.6 – 8 The C2 H2 O⫹ 2 cation was produced in the same method by co-condensation of the reactive species generated by laser ablation of different tran-

EXPERIMENTAL AND COMPUTATIONAL METHODS

The experimental setup for pulsed laser ablation and matrix infrared spectroscopic investigation is similar to that described previously.9 The 1064 nm Nd:YAG laser fundamental 共Spectra Physics, DCR 150, 20 Hz repetition rate and 8 ns pulsewidth兲 was focused onto a rotating metal target through a hole in a CsI window, and the ablated metal atoms were codeposited with C2 H2 and O2 mixtures in excess argon onto a 11 K CsI window, which was mounted on a cold tip of a closed-cycle helium refrigerator 共Air Products, Model CSW202兲 for 1 h at a rate of 2– 4 mmol/h. Typically, 5–10 mJ/pulse laser power was used. C2 H2 was subjected to several freeze–pump–thaw cycles before used. The isotopic substituted 13C2 H2 , C2 D2 , and 18O2 共99%, Cambridge Isotope Laboratories兲 and selected mixtures were used in different experiments without further purification. Infrared spectra were recorded on a Bruker IFS 113V spectrometer at 0.5 cm⫺1 resolution using a DTGS detector. High pressure mercury arc photolysis, matrix annealing and CCl4 doping experiments were performed to aid assignments of the observed IR bands. Quantum chemical calculations were performed to predict the structure and vibrational frequencies of the 10 CalcC2 H2 O⫹ 2 cation using the GAUSSIAN 98 program. ulations were performed using post-Hartree–Fock ab initio as well as hybrid density functional methods. The coupled cluster approach with inclusion of single, double and iterative inclusion of triple excitations 关CCSD共T兲兴 from the Hartree–Fock determinant was applied. For the density functional approach, the three-parameter hybrid functional according to Becke with additional correlation corrections due to Lee, Yang, and Parr were utilized 共B3LYP兲.11,12 The 6-311⫹⫹G(d,p) basis sets were used for C, O, and H atoms.13,14 The vibrational frequencies were calculated at the B3LYP/6-311⫹⫹G(d,p) level of theory with analytic second derivatives.

a兲

Author to whom correspondence should be addressed. Electronic mail: [email protected]

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© 2003 American Institute of Physics

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J. Chem. Phys., Vol. 119, No. 5, 1 August 2003

FIG. 1. Infrared spectra in the 1510–1470 cm⫺1 region from co-deposition of laser-ablated iron atoms and 0.3% C2 H2 ⫹0.8% O2 in argon. 共a兲 1 h sample deposition at 11 K; 共b兲 after 25 K annealing; 共c兲 after 20 min broadband photolysis; and 共d兲 0.3% C2 H2 ⫹0.8% O2 ⫹0.05% CCl4 , 1 h sample deposition.

RESULTS AND DISCUSSIONS

Experiments were performed with different transition metal targets 共Ti, V, Mn, Fe, and Cu兲. Metal independent absorptions as well as metal dependent absorptions were observed, and the metal dependent absorptions have been discussed elsewhere,15 here we report only the metal independent absorptions. After sample deposition at 11 K, the absorptions due to CCH 共1845.8 cm⫺1兲, CCH⫺ 共1770.5 ⫹ ⫺1 ⫺1 cm⫺1兲, O⫺ 4 共953.8 cm 兲, and O4 共1118.6 cm 兲 were 16 –18 observed. In addition, a new absorption at 1493.1 cm⫺1 was also produced. Figure 1 shows the spectra in the 1510– 1470 cm⫺1 region for iron after deposition, annealing and photolysis. 25 K annealing had little effect on the 1493.1 cm⫺1 band, but broadband photolysis using a high pressure mercury lamp destroyed the 1493.1 cm⫺1 band. One experiment was done with CCl4 doped to serve as

Theoretical characterization of C2 H2 O2⫹ argon

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an electron trap.19 Comparison is made to an experiment with 0.3% C2 H2 ⫹0.8% O2 and the same laser power as the experiment used for Fig. 1共a兲, and 0.05% CCl4 added to the argon matrix gas. A spectrum with CCl4 doping is shown in Fig. 1共d兲. All of the metal dependent absorptions and the CCH absorption of the deposited sample were much the same as the spectrum without CCl4 doping. The O⫺ 4 absorption was eliminated, whereas the 1493.1 cm⫺1 band and the O⫹ 4 absorption highly increased. Similar experiments were also done using 18O2 /C2 H2 , 16 O2 ⫹ 18O2 /C2 H2 , O2 / 13C2 H2 , O2 /C2 D2 , and O2 /C2 H2 ⫹C2 D2 samples, and the spectra in the 1510–1400 cm⫺1 region are shown in Fig. 2. The 1493.1 cm⫺1 band was only produced on sample deposition in the transition metal⫹O2 /C2 H2 experiments, suggesting that both O2 and C2 H2 are involved in this species. This band shifted to 1409.4 cm⫺1 with 18O2 /C2 H2 . The 16 O/ 18O ratio of 1.0594 is characteristic of a terminal O–O stretching vibration. In the mixed 16O2 ⫹ 18O2 /C2 H2 experiment, only pure isotopic counterparts were observed, indicating that only one O2 subunit is involved in this species. This band also showed small carbon-13 and deuterium isotopic shifts. It shifted to 1491.7 cm⫺1 with O2 / 13C2 H2 and to 1487.8 cm⫺1 with O2 /C2 D2 . In the mixed O2 /C2 H2 ⫹C2 D2 experiment, no obvious intermediate absorption was produced, indicating that only one C2 H2 subunit is involved in this molecule. The isotopic shifts and splittings suggest that the species involves one O2 subunit and one C2 H2 subunit, and has the C2 H2 O2 formula. The 1493.1 cm⫺1 band is photosensitive, it was destroyed on broadband photolysis, and did not recover on higher temperature annealing. Furthermore, this band was enhanced in CCl4 doping experiment, which aids the survival of cations by trapping electrons. The photosensitive behavior and its enhancement in CCl4 doping experiment strongly suggest a cation assignment. Therefore, we assign the 1493.1 cm⫺1 band to the O–O stretching vibration of the C2 H2 O⫹ cation. 2

FIG. 2. Infrared spectra in the 1510–1400 cm⫺1 region from co-deposition of laser-ablated iron atoms with different isotopic samples in excess argon. 共a兲 0.8% O2 ⫹0.3% C2 H2 ; 共b兲 0.8% O2 ⫹0.3% 13C2 H2 ; 共c兲 0.8% O2 ⫹0.3% C2 D2 ; 共d兲 0.8% O2 ⫹0.2% C2 H2 ⫹0.2% C2 D2 ; 共e兲 0.8% 18O2 ⫹0.3% C2 H2 ; and 共f兲 0.3% 16O2 ⫹0.5% 18O2 ⫹0.3% C2 H2 .


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FIG. 3. Optimized structures 共bond length in Å, bond angle in deg兲 of the ⫹ C2 H⫹ 2 , O2 , and C2 H2 O2 cation at B3LYP and CCSD共T兲共in parentheses兲 levels.

To provide additional support on the spectroscopic assignment of the C2 H2 O⫹ 2 cation and to provide insight into its structure and bonding, we performed theoretical calculations. The hybrid DFT method was used to scan the doublet and quartet potential energy surfaces at B3LYP/ 6-311⫹⫹G(d,p). The optimized structural parameters are shown in Fig. 3. Two local minima were found on the quartet potential energy surface: a planar T-shaped form in which the O2 bond is perpendicular to the C–C bond, and a planar C 2 v form in which the O–O bond is parallel to the C–C bond. Both of these forms have quite long C–O bond distances 共2.615 Å for 4 A 2 , and 2.636 Å for 4 B 1 ). The geometric parameters of the C2 H2 subunit are very close to that of the ground state C2 H⫹ 2 cation. Therefore, both of these forms can be viewed as the C2 H⫹ 2 – O2 ion-molecular complexes. At the B3LYP/6-311⫹⫹G(d, p) level of theory, these two complexes were predicted to be bound by 3.5 and 3.0 kcal/mol,

2 3 ⫺ respectively, with respect to C2 H⫹ 2 ( ⌸ u )⫹O2 ( ⌺ g ), after zero-point energy corrections. The calculation also predicted that the lowest doublet is a 2 A ⬙ state, having a nonplanar structure with C s symmetry. This state was predicted to be the ground state of the C2 H2 O⫹ 2 cation. It is 17.6 kcal/mol ⫹ 2 lower in energy than C2 H2 ( ⌸ u )⫹O2 ( 3 ⌺ ⫺ g ). The calculated bond lengths for the 2 A ⬙ ground state C2 H2 O⫹ 2 cation are: C–O⫽2.499 Å, O–O⫽1.169 Å, C–C⫽1.226 Å, and C–H⫽1.074 Å, and the calculated C–O–O angle is 120.2°. The C–C bond length is 0.02 Å shorter than that of the C2 H⫹ 2 cation, and 0.027 Å longer than that of the C2 H2 neutral. The O–O stretch was predicted to be the most intense vibrational mode of the C2 H2 O⫹ 2 cation, which was computed at 1646.8 cm⫺1, ⬇155.7 cm⫺1 higher than the experimental value. This discrepancy is slightly out of the range of the expected accuracy of DFT calculations. However, we note that the calculated value is the harmonic frequency of the gas phase cation, whereas the experimental value is observed in solid argon matrix. As has been discussed, cations usually interact more strongly with argon matrix host than other guest species, and the vibrational frequencies of cations in solid argon are usually lower than the neon or gas phase values.5共b兲,20 As a reference point, the trans-O ⫹ 4 cation was observed at 1118.6 and 1164.4 cm⫺1 in solid argon and neon with a 45.8 cm⫺1 argon-to-neon matrix shift.17,18 We expect that the cation in the gas phase will be observed tens of wave numbers higher than that in solid argon. The DFT normal mode analysis showed that the band is predominantly the O–O stretching. The calculated isotopic frequency ratios match the observed values quite well. The calculated 16O/ 18O ratio of 1.0596, 12C/ 13C ratio of 1.0011, and H/D ratio of 1.0078 is in good agreement with the experimental values of 1.0594, 1.0009, and 1.0036, respectively. As listed in Table I, the antisymmetric C–H stretching, C–C stretching, and C–H deformation modes were predicted to have much lower intensity than the O–O stretching mode. We were not able to observe these bands in our experiments. As has been pointed out, DFT calculations do not provide very reliable IR intensity predictions in some cases. It is found that the IR intensities of the vibrations such as C–H stretching are substantially overestimated by DFT calculations.21,22

TABLE I. Calculated vibrational frequencies 共cm⫺1兲 and intensities 共km/mol兲 of the 2 A ⬙ C2 H2 O⫹ 2 cation. Frequency 共intensity, mode兲

Molecule C2 H2 O⫹ 2 C2 H2 18O⫹ 2 C2 H2 O⫹ 2

13

C2 D2 O⫹ 2

3427.7 共0, a ⬘ ) 798.3 共31, a ⬘ ) 284.7 共0, a ⬘ ) 3427.7 共0兲 798.3 共31兲 269.9 共0兲 3402.6 共1兲 795.9 共32兲 283.8 共0兲 2728.8 共32兲 619.9 共1兲 283.2 共1兲

3325.3 共318, a ⬙ ) 775.9 共194, a ⬘ ) 144.3 共2, a ⬘ ) 3325.3 共318兲 775.8 共104兲 140.4 共3兲 3313.5 共319兲 773.4 共105兲 141.7 共2兲 2440.4 共152兲 587.3 共12兲 141.5 共2兲

1946.6 共107, a ⬘ ) 764.8 共1, a ⬙ ) 101.8 共0, a ⬙ ) 1944.4 共137兲 764.8 共1兲 97.7 共0兲 1885.8 共77兲 755.6 共1兲 101.1 共0兲 1743.4 共0兲 571.1 共46兲 97.8 共0兲

1646.8 共767, a ⬘ ) 667.9 共0, a ⬙ ) 74.0 共2, a ⬙ ) 1554.2 共664兲 667.9 共0兲 73.8 共2兲 1645.0 共783兲 657.8 共0兲 72.0 共2兲 1634.0 共816兲 557.1 共0兲 65.9 共2兲


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FIG. 4. Depiction of the highest occupied molecular orbitals (10a ⬘ and 11a ⬘ ) of C2 H2 O⫹ 2 .

For comparison, we computed a number of related species with different structures, including the complex between CCH and O2 , the complex between CCH⫺ and O2 , and the complex between CCH⫹ and O2 . None of these were computed to have infrared spectral features that match the observed frequencies. Comparison calculation was done for the structure of the 2 A ⬙ state C2 H2 O⫹ 2 cation at CCSD共T兲/6-311⫹⫹G** level. The optimized parameters are shown in Fig. 3. The equilibrium C–O, C–C, and O–O bond lengths were predicted to be 2.317, 1.243, and 1.184 Å. The C–O bond length calculated at the CCSD共T兲 level is shorter than the B3LYP value, whereas the C–C and O–O bond lengths are slightly longer than the B3LYP values. The binding energy of C2 H2 O⫹ 2 was 2 predicted to be 10.9 kcal/mol with respect to C2 H⫹ 2 ( ⌸ u) 3 ⫺ ⫹O2 ( ⌺ g ). This value is slightly lower than that of the B3LYP calculations. For bonding analysis, the 2 A ⬙ C2 H2 O⫹ 2 cation can be fragment and a closedviewed as the interaction of a C2 H⫹ 2 shell O2 ( 1 ⌬ g ) fragment. It has an electron configuration of 共core兲 (4a ⬙ ) 2 (10a ⬘ ) 2 (11a ⬘ ) 1 . At the B3LYP level, natural bonding analysis shows that the C2 H2 O⫹ 2 cation exhibits some weak C–O covalent bonding character. Natural charge population analysis indicates that the positive charge at the 2 A ⬙ state C2 H2 O⫹ 2 cation is delocalized, with ⫹0.63 on the C2 H2 subunit, and ⫹0.37 on the O2 subunit. As shown in Fig. 4, the SOMO 11a ⬘ and the HOMO-1 10a ⬘ are largely C2 H⫹ 2 ␲ bonding orbitals, which comprise donations from the filled or half-filled orbitals of C2 H⫹ 2 into the empty antibonding ␲ * orbitals of O2 . It is suggested that the C2 H2 O⫹ 2 cations are formed via ion-molecular reactions during the concurrent codeposition ⫹ ⫹ process: C2 H⫹ 2 ⫹O2 → or C2 H2 ⫹O2 →C2 H2 O2 . It is well known that laser ablation of metal target produces metal atoms as well as metal cations, electrons and photons, therefore, charged species can be formed. A large number of charged species, including anions and cations have been produced and characterized in solid matrixes.5– 8 In present experiments, weak absorptions due to CCH⫺ and O⫺ 4 were obcation cannot be detected with served. Although the O⫹ 2 infrared absorption spectroscopy, its participation is confirmed by reactions with O2 to form O⫹ 4 during sample condensation. Very weak absorption of C2 H⫹ 2 was also observed at 3104.5 cm⫺1 in the present experiments.16

CONCLUSIONS

The C2 H2 O⫹ 2 cation was produced and characterized by matrix isolation infrared absorption spectroscopy and quantum chemical calculations. Laser ablation of transition metals with concurrent codeposition of C2 H2 /O2 /Ar mixtures at 11 K produced metal independent absorptions at 1493.1 cm⫺1. On the basis of isotopic shifts and splittings with the substitution of 13C2 H2 , C2 D2 and 18O2 and mixtures, enhancement in doping with electron trapping gas, and quantum chemical frequency calculations, the band is assigned to the O–O stretching vibration of the C2 H2 O⫹ 2 cation complex, which was predicted to have a 2 A ⬙ ground state with a nonplanar C s symmetry. ACKNOWLEDGMENT

This work is supported by NSFC 共20003003 and 20125033兲, and the NKBRSF of China. 1

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