J. Phys. Chem. A 2010, 114, 5035–5040
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A Theoretical Study of the Low-Lying Electronic States of the AlCCH Radical and Its Ions Yue-Jie Liu, Zeng-Xia Zhao, Ming-Xing Song, Hong-Xing Zhang,* and Chia-Chung Sun State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin UniVersity, 130023 Changchun, People’s Republic of China ReceiVed: December 31, 2009; ReVised Manuscript ReceiVed: March 8, 2010
The AlCCH radical is a photolysis product of the aluminum-acetylene adducts and has been considered as a molecule with potential interest in astrophysics. In this study, the low-lying electronic states of the AlCCH radical, cation, and anion have been studied by using complete active space self-consistent field and multiconfigurational second-order perturbation theory. The geometrical parameters, electron configurations, excitation energies, oscillator strengths, and harmonic vibrational frequencies are calculated in CS symmetry. For the X1Σ+ state of AlCCH, the calculated C-C and C-Al stretching modes are in good agreement with experimental reports. Moreover, the vertical excitation energy (TV) of 11Π is 3.68 eV, which is close to the experimental value of 3.57 eV. The electron transitions of AlCCH+, X2Σ+ f 12Π, X2Σ+ f 22Σ+, and X2Σ+ f 12Σ-, are predicted at 2.57, 4.51, and 4.61 eV, respectively. For AlCCH-, the transition X2Π f 12Σoccurs at 3.02 eV. The ionization potentials of AlCCH are computed in order to provide a theoretical guidance to the photoelectron spectrum of the AlCCH radical. 1. Introduction The metal-bearing halides, cyanides, and isocyanides including NaCl, KCl, AlCl, AlF, MgCN, NaCN, MgNC, and AlNC, have been detected astronomically in the circumstellar envelope of carbon-rich stars such as IRC+10216 and CRL 2688.1–7 One of the most common metallic elements in IRC+10216 is aluminum, and it could react with the C2H, C4H, and C6H radicals to form the AlCCH radical,8,9 which is considered as a molecule with potential interest in astrophysics.10 Moreover, the reaction products of metal atoms and hydrocarbon molecules are significant for chemisorption and catalytic action.11 Via electron spin resonance (ESR) technique, the aluminum-acetylene adduct was found in neon matrices by Kasai and co-workers.12 Andrews et al. put forward the photolysis mechanism and identified the photolysis product, AlCCH, in the Al + C2H2 system11,13
Al + C2H2 f (HAlCCH)* f HAlCCH f H + AlCCH They assigned the bands at 1977, 684, and 513 cm-1 to the CtC stretching, HCC bending and CsAl stretching modes of AlCCH, respectively. In 2007, Apetrei et al. detected the gas phase electronic spectrum of AlCCH in the region 315-355 nm and assigned the spectrum to the electron transition X1Σ+ f A1Π.10 The aluminum-acetylene adducts and their stable Al-C2H2 isomers were theoretically studied in a number of reports.11,13–16 However, there are few reports on the low-lying electronic states of AlCCH and its ions. For example, Chertihin et al. and Apetrei et al. calculated the equilibrium geometries and harmonic vibrational frequencies of the ground state of AlCCH at the CASSCF/VDZ13 and DFT/aug-cc-pVTZ10 levels, respectively. Apetrei et al. also computed the vertical excitation energies (Tv) and oscillator strengths for the singlet excited states of linear AlCCH.10 In addition, for the AlCCH ions, no information has * To whom correspondence should be addressed.
been supplied except the geometry of the ground state of AlCCH+ at the MP2 (full)/6-31G* level.17 Considering the lack of theoretical studies on the excited states of AlCCH and its ions, we make a comprehensive investigation into the AlCCH radical and its ions by using the complete active space self-consistent field (CASSCF) and multiconfigurational second-order perturbation theory (CASPT2) methods, which have been shown to provide accurate interpretation and prediction for the excited states in numerous applications.18–29 On the basis of our calculations, we locate seven, eight, and three lowlying electronic states for the AlCCH radical, cation, and anion, respectively. Among these electronic states, the X1Σ+ and 11Π states of AlCCH and the X2Σ+ state of AlCCH+ have been studied by the previous works.10,13,17 Five triplet excited states (13Π, 13Σ-, 23Σ-, cis-23A′ and trans-23A′′) of AlCCH, seven excited states (12Π, 14Σ+, 24Σ+, 14Σ-, 24Σ-, 22Σ+, 12Σ-) of AlCCH+, and three electronic states (X2Π, 14Σ-, 12Σ-) of AlCCH- have been reported for the first time. In the present work, all the electronic states are linear except the cis-23A′ and trans-23A′′ states of the AlCCH radical. 2. Calculation Details On the basis of the reference functions constituted by the CASSCF26,27 wave functions, the CASPT228,29 calculations were performed to compute the first-order wave function and the second-order energy in the full configuration interaction space, which were carried out by using the MOLCAS 6.2 program. The large atomic natural orbital (ANO-L) basis sets30,31 were employed in all the calculations. The equilibrium geometries of the low-lying electronic states of the AlCCH radical and its ions were calculated in CS symmetry by using the CASSCF optimization method. On the basis of the corresponding optimized geometries, the CASPT2 program was adopted to compute the dynamic correlation energies. The weight values of the CASSCF reference functions in the first-order wave functions were larger than 80%, unless otherwise noted. The vertical excitation energies, ionization potentials, and detachment energies were obtained at the
10.1021/jp912291u 2010 American Chemical Society Published on Web 03/25/2010
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J. Phys. Chem. A, Vol. 114, No. 15, 2010
Liu et al.
Figure 1. Atom labels for the AlCCH radical and its ions used in the present work (the molecule lies in the YZ plane).
CASPT2 level. The oscillator strengths were computed with the CASSCF state interaction (CASSI) program using the CASPT2 energy differences.32 Multiconfigurational linear response (MCLR) was used to calculate the harmonic vibrational frequencies.33 It is important to select the active space for obtaining accurate calculation results. We performed a HF calculation for the AlCCH radical, and the electron configuration of the ground state can be presented as (core)(6σ)2(7σ)2(8σ)2(2π)4(9σ)2, in which the (core) denotes (1σ)2(2σ)2(3σ)2(4σ)2(1π)4(5σ)2. We chose the 12 valence electrons as the active electrons. Besides the six occupied orbitals by the 12 valence electrons, we added five vacant orbitals (one 3p orbital of Al atom, one π* orbital, and three σ* orbitals) of a1 symmetry and two vacant orbitals (one 3p orbital of Al atom and one π* orbital) of a2 symmetry into the active orbitals. These gave an active space including the 12 electrons and 13 orbitals (12, 13) for the AlCCH radical. The active orbitals remained unchanged, and the active electrons decreased (or increased) by one for the cation (or anion). 3. Results and Discussion 3.1. The AlCCH Radical. 3.1.1. Geometries. Figure 1 shows the atom labels of the AlCCH radical. The geometrical parameters, leading configurations and their CI coefficients for the low-lying electronic states of the AlCCH radical are listed in Table 1. At the CASSCF and CASPT2 levels, the X1Σ+ state is predicted to be the ground state. For the X1Σ+ state, the obtained C-H bond length (1.077 Å) is slightly longer than that (1.064 Å) at the DFT/aug-cc-pvtz level,10 the C-C and C-Al bond lengths (re(C-C) ) 1.233 Å, re(C-Al) ) 1.980 Å) are in agreement with those at the CASSCF/VDZ13 and DFT/augcc-pvtz10 levels. As shown in Table 1, the X1Σ+ state has a dominant electronic configuration of (core)(6σ)2(7σ)2(8σ)2(2π)4(9σ)2 with the coefficient of 0.920. The plots of the occupied orbitals for the X1Σ+ state are shown in Figure 2. The 6σ, 7σ, and 8σ orbitals are of
Figure 2. The plots of the occupied orbitals at the CASSCF/ANO-L level for the ground state of the AlCCH radical.
σ-type bonding character. The 2πx and 2πy orbitals have π bonding character between the C1 and C2 atoms. It is obvious that the 9σ orbital is mostly composed of the 3s orbital of Al atom. In addition, Figure 3 displays some orbitals in relation to electron transitions. The 3πx (or 3πy) orbital is constituted by the 3px (or 3py) orbital of Al atom. Besides the 3px (or 3py) component of Al atom, the 3πx′ (or 3πy′) orbital includes the 2px (or 2py) component of C1 atom. The 4πx and 4πy orbitals present π antibonding character between the C1 and C2 atoms, while the effects of π* (C1-C2) and π (C2-Al) are simultaneously involved in 4πx′ and 4πy′ orbitals. In Table 1, it can be found that the 13Π and 11Π states have single reference character. The electron is promoted from the 9σ orbital to the 3π (or 3π′) orbital with R (or β) spin, producing the 13Π (or 11Π) state. As shown in Figure 2 and Table S1 in Supporting Information, the 9σ orbital of X1Σ+ is mainly dominated by the 3s component of the Al atom, which agrees with the report of Apetrei et al.10 Because the transitions from 9σ to 3π (or 3π′) can be simply described as 3s (Al) f 3p (Al), the C-H and C-C bond lengths of 13Π and 11Π have changed little. The C-H bond lengths of 13Π and 11Π are still
TABLE 1: Bond Lengths (re), Leading Configurations (the weights exceed 5%) and CI Coefficients, Adiabatic and Vertical Excitation Energies (Ta and Tv), and Oscillator Strengths for the Ground and Low-Lying Excited States of AlCCHa state 1 +
1
X Σ (1 A′)
re(C-H)
re(C-C)
re(C-Al)
13Π(13A′,13A′′) 11Π(11A′′,21A′)
1.077 1.068c 1.064d 1.077 1.077
1.233 1.245c 1.215d 1.228 1.237
1.980 1.987c 1.977d 1.916 1.885 1.775d
expt. 13Σ-(23A′′) 23Σ-(33A′′)
1.080 1.082
1.306 1.293
2.120 2.351
cis-23A′f trans-23A′′f
1.103 1.107
1.359 1.360
1.970 2.001
configurationb
coef
Ta(CASSCF)
Ta(CASPT2)
Tv
(2π) (9σ)2
0.00
0.00
0.00
-0.948 -0.933
(2π)4(9σ)R(3π)R (2π)4(9σ)R(3π′)β
2.03 3.61
2.32 3.56
0.930 -0.664 0.664 0.880 0.925
(2πx)R(2πy)2(9σ)2(4πy′)R (2πx)R(2πy)2(9σ)2(3πy)R (2πx)2(2πy)R(9σ)2(3πx)R (2a′′)2(10a′)R(11a′)2(12a′)R (2a′′)R(10a′)2(11a′)2(12a′)R
5.01 5.33
4.93 5.15
2.39 3.68 3.64d 3.57e 5.13 5.23
4.05 4.76
3.83 4.60
0.920
4
f
