Article Cite This: J. Phys. Chem. A 2018, 122, 2353−2360
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Spin−Orbit Effects in the Spectroscopy of the X2Π and a4Σ− Electronic States of Carbon Iodide, CI D. Khiri,†,‡ M. Hochlaf,† G. Maroulis,¶ and G. Chambaud*,† †
Laboratoire Modélisation et Simulation Multi Echelle, Université Paris-Est, MSME UMR 8208 CNRS, 5 bd Descartes, 77454 Marne-la-Vallée, France ¶ Department of Chemistry, University of Patras, GR-26500 Patras, Greece ABSTRACT: Highly correlated ab initio calculations have been performed to describe the potential energy curves (PECs) and the spectroscopic properties of the X2Π state and of the first excited state of the CI radical. Multi Reference configuration interaction calculations with Davidson correction (MRCI+Q) and relativistic effective core potential for the iodine atom have been performed. It is found that the two lowest electronic states, the X2Π and the a4Σ− states, are stable against dissociation and well separated from the other electronic states. Spectroscopic constants of these two states have been evaluated using their calculated PECs. Because of the presence of the iodine atom in this molecular system, spin−orbit (SO) interactions are playing an important role in the molecular and in the dissociation regions. The excitation energy of the a4Σ− state is calculated 1.67 eV (MRCI) above the X2Π ground state and 1.70/1.62 eV (MRCI with SO correction) for the Ω = 1/2 and 3/2 transitions, respectively. The dissociation energy D0 of the X2Π ground state is evaluated 2.66 eV (MRCI calculation) without SO correction and 2.46/2.36 eV with SO correction for the Ω = 1/2 and 3/2 components, respectively. The dissociation energy D0 of the a4Σ− state is evaluated 0.99 eV (MRCI calculation) without SO correction and 0.83/0.72 eV with SO correction for the Ω = 1/2 and 3/2 components, respectively. This work should help for the identification of this radical in laboratory and in atmospheric media.
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INTRODUCTION Small halogenated molecules are of great interest for the understanding of the atmospheric chemistry where they can result from decomposition of larger halogenated compounds. Specifically, the concentration of atmospheric iodine is closely connected and influenced by sea surface emissions of organic1−8 and inorganic iodine9 compounds. Because of their rather high reactivity in the destruction of stratospheric ozone, the thermochemical properties of series of chlorine10 and bromine compounds11 have been studied using highly correlated wave functions (e.g., coupled cluster theory). Several experimental and theoretical studies have focused on halogen oxides and their spectroscopic and thermochemical properties because of their importance for stratospheric chemistry. An accurate prediction of the spectroscopic constants of these species was provided by Peterson et al.12 Moreover halocarbynes (CF, CCl, CBr, and CI) play an important role as intermediates in flame inhibition with molecules like CF3Br or CF3I and in the incineration of chlorine-containing organic molecules, therefore their enthalpies of formation and some structural properties have been studied by Marshall et al.13 to understand the chemical mechanisms and by Bacskay.14 The relative stability of different carbon iodides CIn (n = 1−4) has been computationally investigated by Hargittai et al.15 to help the analysis of electron diffraction experiments and a theoretical analysis of low lying © 2018 American Chemical Society
electronic states of the CI radical, including spin−orbit contribution, has been realized by Alves and Ornellas.16 The study of diatomic molecules containing iodine17−19 with ab initio electronic structure calculations is challenging since it requires a high level of electron correlation and inclusion of relativistic effects indirectly via relativistic effective core potential and directly through the computations of the spin− orbit corrections. It is shown that calculations using core pseudopotential on heavy atoms provide results which are generally in good agreement with experimental ones.12 In the present contribution, we report calculated spectroscopic data and transition energies between the two first electronic states of the CI radical, taking into account spin− orbit interaction. In a first step, the potential energy curves (PECs) are calculated at the multireference configuration interaction including Davidson correction (MRCI+Q) level of theory using a relativistic atomic pseudo potential on the iodine atom. In a further step, the spin−orbit interaction is introduced and the PECs are corrected: the vibrational spectroscopy is reported here with and without the spin−orbit interactions. Even in the molecular region where the two lowest electronic states are well separated from the other ones, and where their Received: September 16, 2017 Revised: February 20, 2018 Published: February 20, 2018 2353
DOI: 10.1021/acs.jpca.7b09240 J. Phys. Chem. A 2018, 122, 2353−2360
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The Journal of Physical Chemistry A Table 1. Electronic States of the CI Radical Correlated to the Three Lowest Dissociation Limits ΔE (cm−1) dissociation limits C(3Pg) + I (2Pu) C(1Dg) + I (2Pu) C(1Sg) + I (2Pu) a
electronic states
experimental
(Σ+, Σ−(2), Π(2), Δ) (Σ+(2), Σ−, Π(3), Δ(2), Φ) 2 (Σ+, Π)
0.0 10193.7 21648.4
2,4 2
a
this workb 0.0 10380.7 21853.4
Atomic energy levels of C from NIST Atomic Spectra Database.29 bMRCI+Q method.
fine structure states can be easily identified, a full spin−orbit treatment is necessary because of the very large relativistic effects of the iodine atom. This effect is particularly evidenced in the dissociation region where the lowest asymptotic limits are found separated by the I[2P3/2/2P1/2] splitting of 7603 cm−1 (experimental value).20
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COMPUTATIONAL DETAILS To determine the PECs of the ground and low lying excited electronic states of the CI radical, electronic structure calculations have been performed at the MRCI level of theory on top of complete active space self consistent field (CASSCF)21,22 computations in the C2v point group as implemented in the MOLPRO program package.23 For better accuracy, the Davidson correction (MRCI+Q) has been applied when necessary. Calculations have been performed employing the ECP28MDF relativistic core pseudopotentials for the iodine atom and the corresponding aug-cc-pVTZ-PP basis set.12 For the carbon atom, the aug-cc-pVTZ basis set of Dunning24,25 has been used. In the CASSCF step we considered all the configurations obtained with 11 valence electrons distributed in the eight valence orbitals. Such an ansatz leads to approximately 250 configuration states per C2v symmetry. In MRCI calculations, we considered all configurations of the CI expansion of the CASSCF wave functions as a reference, with a frozen core of 20 electrons. Additional calculations including the correlation of these 20 internal electrons have been performed. The spin−orbit (SO) interaction has been investigated using the effective Breit− Pauli SO operator HSO as implemented in the MOLPRO package.26 The spectroscopic constants for the two first electronic states have been deduced at the MRCI and MRCI +Q levels of theory with and without SO corrections using the Numerov algorithm package27 where the nuclear motion problem was solved using the method of Cooley.28 These constants were evaluated using the derivatives at the minimum energy distances and standard perturbation theory.
Figure 1. Potential energy curves of the low lying electronic states of CI calculated at the MRCI+Q level without SO interactions. The reference energy is the energy of the X2Π state at equilibrium (E = −332.684641 au).
At this level of theory (MRCI+Q without SO correction) the two lowest electronic states, the X2Π and the a4Σ− states, are bound and well separated energetically from the other states. In the internuclear range RCI = [3−10] bohr, the X2Π state remains the lowest electronic state, even at large internuclear distances when all the states correlated to the lowest asymptote, including these two, are coming close to each other. In the molecular region, the a4Σ− state is the lowest excited state. It is crossed by the b4Π state for RCI = 5.5 bohr and then by the first excited 2Π state at RCI = 6.2 bohr. Note that for long internuclear distances and particularly in the dissociation region (typically for RCI larger than 6 bohr) the electronic states are lying very close together, except the X2Π which remains well separated from the other ones, and only the description of the states including the SO interaction can be correct. The wave functions of the X2Π and a4Σ− states have multiconfigurational character, however with a dominant configuration in the molecular region which is (···σ2π4π*) for the X2Π ground state and (···σ1π4π*2) for the a4Σ− excited state. These outermost σ and π molecular orbitals (MOs) have a large iodine character whereas the antibounding π* MO is mainly located on the carbon atom. The σ − π* electron excitation, associated with the transition between the X2Π and
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RESULTS AND DISCUSSION Potential Energy Curves. The low lying electronic states correlated to the first three dissociation limits of CI, without considering the spin−orbit fine structure of the atoms, are given in Table 1. In the first step of the study, all states correlated to these three lowest dissociation limits have been included in the averaged-CASSCF step preceding the MRCI calculation. This ansatz includes all components of 17 electronic states in doublet and 6 states in quartet spin multiplicities. Since the density of states is rather high, only the potential energy curves correlated to the two lowest dissociation limits calculated at the MRCI+Q level of theory without spin−orbit correction are displayed in Figure 1 for internuclear separations in the range RCI = [3−8] bohr (1 bohr =0.5291 Å). 2354
DOI: 10.1021/acs.jpca.7b09240 J. Phys. Chem. A 2018, 122, 2353−2360
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The Journal of Physical Chemistry A the a4Σ− states, thus results in a reduction of the negative charge on iodine. This is illustrated in the dipole moment curves of these two states calculated at the MRCI level of theory as presented in Figure 2. Indeed, this figure shows that
Table 2. Calculated Vertical Energies, ΔE, of the Lowest Excited States of the CI Radical in Doublet and Quartet Symmetriesa ΔE (eV)
Figure 2. Evolution of the dipole moments of the X2Π and a4Σ− states along the internuclear separations (RCI) as computed at the MRCI level of theory.
the dipole moment of the X2Π state decreases linearly with RCI in the molecular region until RCI = 5 bohr, corresponding to a C+qI−q polarization with a negative (almost) constant charge of −0.80 e on iodine. For the a4Σ− state the situation is completely different: the dipole moment has a small positive slope, almost constant in the molecular region, corresponding to a small positive charge of 0.08 e on iodine. The MRCI and MRCI+Q vertical energies of the lowest excited electronic states of CI calculated at the equilibrium geometry of the X2Π ground state are given in Table 2. It is shown that the first excited state is located at 1.67 eV (MRCI) and 1.70 eV (MRCI+Q) above the ground state, which is in good agreement with the previous calculated value of 1.75 eV by Alves and Ornellas.16 The dissociation energy De of the X2Π state, calculated as the difference between the energy at the equilibrium geometry (see Table 3) and the extrapolated energy at infinite separation, is computed as 2.70 (MRCI) and 2.80 eV (MRCI+Q), which are slightly smaller (by 0.24−25 eV) than the value determined by Alves and Ornellas.16 The next group of five excited states is located between 3.13 and 3.65 eV (MRCI+Q) above the minimum of the ground state, that is above the first dissociation limit. The pattern of these 5 excited states is comparable to that described by Alves et al.16 with however some differences particularly for the 2Δ state which is found only slightly bound in our calculations. Interestingly these excited states are mostly repulsive, this is relevant for the photodissociation of CI since it should decompose (and hence not be stable) in the upper atmosphere. Spin−Orbit Corrections. The iodine atom has a large spin−orbit constant, thus spin−orbit effects are expected to be important in the CI radical. For the iodine atom in its 2Pu state the spin−orbit interaction has been evaluated at the MRCI level with an active space containing 7 active electrons distributed in 4 active orbitals corresponding to the valence shell of iodine: the calculated value of the HSO integral is equal
state
MRCI
MRCI+Q
X2Π a4Σ− b4Π 2 + Σ 2 Δ 2 − Σ 2 Π 2 + Σ 2 Φ 2 Π 4 Π 2 Π 4 + Σ 4 Δ 4 − Σ 2 Δ 2 − Σ 2 Π 2 + Σ 2 Δ 2 − Σ 2 + Σ 2 Π
0.0 1.67 3.20 3.30 3.60 3.66 3.75 4.58 4.78 4.95 5.13 5.34 5.65 5.94 6.11 6.36 6.52 6.71 6.72 7.11 7.26 8.39 8.47
0.00 1.70 3.13 3.21 3.52 3.61 3.65 4.50 4.68 4.85 5.06 5.25 5.55 5.84 6.03 6.28 6.41 6.62 6.64 7.03 7.19 8.29 8.39
a ΔE values are calculated at the equilibrium geometry of the electronic ground state X2Π.
to −2292 cm−1, resulting in a splitting of 6877 cm−1 between the two 2P3/2 and 2P1/2 components of atomic iodine. This is in agreement with the experimental splitting of 7603.15 cm−1 reported by Moore.20 When the active space is enlarged to 9 orbitals, the HSO integral equals −2554 cm−1 giving a splitting of 7662 cm−1 in better agreement with the experimental splitting. The calculations of the molecular system involving 8 active orbitals (the valence shells of C and I), result at large internuclear separation into an intermediate value between these two and thus close to the experimental splitting. For the carbon atom the spin−orbit effect is much smaller than for the iodine atom: the splitting of the fine structure states is calculated 0.0, 13.1, and 39.1 cm−1 for the C(3P0), C(3P1), and C(3P2) respectively which compares well with the experimental values of 0.0, 16.4, and 43.5 cm−1.20 In order to have a proper description of the spin−orbit correction in the lowest electronic states correlated to the first dissociation limit, [C(3Pg) + I (2Pu)], it is necessary to include in the SO calculations all the 18 electronic components of the 12 electronic states given in Table 1. This results into 54 spin− orbit components, organized in 27 degenerated Kramers doublets in the present case, corresponding to the different possible (j−j) combinations. In this step of the calculations all these states correlated to the first asymptote have been averaged in the CASSCF step using the MOLPRO averaging procedure and included in the SO treatment. In the region close to equilibrium geometry of the X2Π and a4Σ− states, their fine structure components can be clearly identified. It is interesting to note that in this region, the Ω = 1/2 components are the lowest ones for both states as expected from an 2355
DOI: 10.1021/acs.jpca.7b09240 J. Phys. Chem. A 2018, 122, 2353−2360
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The Journal of Physical Chemistry A Table 3. Spectroscopic Constants for the X2Π and a4Σ− states of CI radical method
Re (Å)
ωe (cm−1)
ωeχe (cm−1)
Be (cm−1)
αe (cm−1)
μ (au)
5.03 4.83 3.0 4.0 3.7 3.42
0.3637 0.3655 0.3702
0.0028 0.0027 0.003
0.698 0.698
0.3694
0.0028
0.65
5
0.35
0.0032
5.25 3.50
0.3636 0.3675
0.0026 0.0028
5.06 3.45
0.3618 0.3692
0.0028 0.0028
6.97 6.25 4.38
0.3653 0.3697 0.3719
0.0033 0.0038 0.0024
8.69 4.09
0.3619 0.3735
0.0038 0.0038
8.53 4.13
0.3624 0.3729
0.0038 0.0035
XΠ 2
MRCI MRCI+Q CCSD(T)a QCISD(T)[AE]/b QCISD(T)[ECP]b MRCI+Qc LDA/NLd expte
2.056 2.051 2.036 2.048 2.023 2.040 2.027 2.10
621.7 626.5 623.1 627 636 632 630 X2Π1/2
MRCI+SO MRCI+SOc
2.056 2.044
621.4 623 X2Π3/2
MRCI+SO MRCI+SOc
2.061 2.040
613.4 632 a4Σ−
MRCI MRCI+Q MRCI+Qc LDA/NLd
2.051 2.039 2.030 2.004
601.3 606.2 611
MRCI+SO MRCI+SOc
2.061 2.036
577.8 635
0.295
a4Σ−1/2
a4Σ−3/2 MRCI+SO MRCI+SOc a
2.059 2.036
580.3 638
Reference 15. bReference 13. cReference 16. dReference 30. eReference 31.
equal to 793 cm−1 at the MRCI/SO level, comparable with the 833 cm−1 calculated in ref 16. One can see in Figure 3 that the displacement of the lowest component of the X2Π state is almost constant in the molecular region and that of the highest component is steadily decreasing with RCI, starting from a positive value it goes to a negative one and finally joins the lowest component at large distance (RCI ≈ 10 bohr) with a displacement of ≈ −2300 cm−1. For the a4Σ− state, the displacements of its fine structure components are always negative and decreasing with RCI. At the equilibrium geometry of the a4Σ− state the displacements are equal to −307 and −192 cm−1 for the Ω = 1/2 and 3/2 respectively. They decrease then to a value which is close to −2500 cm−1 for RCI = 5 bohr and for larger internuclear separations it is impossible to follow them because they strongly mix with the components of other states. The analysis of the vectors of the four lowest Ω states close to equilibrium geometry shows that these states are almost pure (the weight of the dominant configuration is larger than 0.98) in full agreement with the analysis presented in ref 16. The negative sign of the energy displacements results from the strong spin−orbit and spin−spin interactions of the iodine atom and the fine structure states are energetically below their parent electronic state, not only at dissociation but already in the molecular region for the a4Σ− state. For instance we present in Figure 4 the potential energy curves of the X2Π and a4Σ− states together with their fine structure components in the molecular region to illustrate the deformations of their shapes induced by the SO interaction. In the upper part the energy scale has been magnified to distinguish more easily the deformations in the a4Σ− state. At large interatomic distances, the 27 fine structure states corresponding to the 12 electronic states correlated to the
electronic state with a wave function having only one electron (less than half-full) in a π degenerate shell contrary to the 2Pu state of the iodine atom where the j = 3/2 component is the lowest one (more than half-full shell). The variations with the internuclear distance of the displacement of the fine structure components of the X2Π and of the a4Σ− states resulting from the SO interaction are displayed in Figure 3. At the equilibrium geometry of the X2Π state, the splitting between its two fine structure components is
Figure 3. Displacements of the fine structure components of the two lowest electronic states X2Π and a4Σ− of the CI radical. 2356
DOI: 10.1021/acs.jpca.7b09240 J. Phys. Chem. A 2018, 122, 2353−2360
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The Journal of Physical Chemistry A
Figure 4. Potential energy curves of the X2Π and a4Σ− states and of their fine structure components in the molecular region. The a4Σ− part has been enlarged for clarity.
lowest dissociation limits split into two sets. The first set consists of 18 fine structure states: two states with Ω = 1/2 and 3 /2 correlated to the [C(3P0) + I (2P3/2)] limit, six states with Ω = 1/2(3), 3/2(2), and 5/2 correlated to the [C(3P1) + I (2P3/2)] limit, and 10 states with Ω = 1/2(4), 3/2(3), 5/2(2), and 7/2 correlated to the [C(3P2) + I (2P3/2)] limit. The second set consists of nine fine structure states: one state with Ω = 1/2 correlated to the [C(3P0) + I (2P1/2)] limit, three states with Ω = 1/2(2) and 3/2 correlated to the [C(3P1) + I (2P1/2)] limit, and five states with Ω = 1/2(2), 3/2(2), and 5/2 correlated to the [C(3P2) + I (2P1/2)] limit. The potential energy curves of these 27 states are given in Figure 5. For RCI = 9 bohr, the asymptotic limit is not yet completely reached but the energies of the first set of 18 states spread over only 120 cm−1 with the two lowest ones separated by 6 cm−1 adiabatically correlated to the two fine structure components of the X2Π, the next ones with Ω = 1 /2 and 3/2 are lying between 30 and 70 cm−1, the lowest ones being adiabatically correlated to the a4Σ− state. The second set of states is calculated around 7000 cm−1. Note that the asymptotic behavior of the X2Π and a4Σ− states is governed by the SO structure of the carbon atom. The lowest fine structure component of the X2Π state is lowered by 2280 cm−1 at RCI = 9 bohr. Because of the
Figure 5. Potential energy curves of the 27 lowest Ω states of CI calculated at the MRCI+Q level with SO interactions.
calculated SO displacements at long and at equilibrium distances, the dissociation energy De of the X2Π1/2 fine structure state is reduced to 2.50 eV and that of the X2Π3/2 2357
DOI: 10.1021/acs.jpca.7b09240 J. Phys. Chem. A 2018, 122, 2353−2360
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The Journal of Physical Chemistry A
length by 0.01 Å. For the a4Σ− state the harmonic wavenumber is reduced by 4 cm−1 and the equilibrium bond length is reduced by less than 0.01 Å. The SO correction of the PECs does not induce large modifications in the vibrational spectroscopy of the X2Π1/2 fine structure component since the displacement in energy is rather small and almost constant in the bonding region. On the contrary, the SO energy displacement of the X2Π3/2 fine structure state varies with the internuclear distance and induces a decrease of the harmonic wavenumber and a subsequent lowering of the vibrational levels. For the a4Σ− state the SO energy displacements are steadily decreasing with RCI, resulting in a flattening of the PECs for both SO components. Since the modification of the PEC is larger for the Ω = 1/2 component its harmonic wavenumber decreases more than that of the Ω = 3/2 component. The vibrational levels of the fine structure states are given in Table 4 where they can be directly compared with the uncorrected ones. These data including SO corrections may help for the characterization of the atmospheric relevant CI radical in laboratory and in the atmosphere. In Table 5, the variationally determined transition energies between some vibrational states (v = 0−3) of X2Π1/2,3/2 and
fine structure state to 2.40 eV compared with the value of 2.70 eV obtained without SO correction. These values compare well with the estimated experimental dissociation energy (De = 2.21 ± 0.2 eV for X2Π32) and previous theoretical value (De = 2.74 eV derived from CCSD(T) calculation of X2Π,15 and also De = 2.73 eV and De = 2.63 eV for X2Π1/2 and X2Π3/2 respectively16). It could also be important to consider in the SO treatment the second dissociation limit of CI corresponding to [C(1D2) + I (2P1/2,3/2)] which involves nine electronic states. Since the fine structure of the C(1D2) has only one component, one can expect that the asymptotic behavior will be governed by the SO character of the iodine atom and will present two sets of SO states separated by ≈7600 cm−1 as for the first dissociation limit. This means that the lowest set of this second dissociation limit will be approximately located at only 2600 cm−1 above the upper set of the first dissociation limit (10193−2530−5060) and that interactions can take place between these two sets. This has not been considered here. Spectroscopy. For the two first electronic states X2Π and 4 − a Σ of CI we give in Table 3 the equilibrium distances (Re), the harmonic wavenumbers (ωe), anharmonic constants (ωeχe), rotational terms (Be and αe), and dipole moments (μe) at equilibrium geometry. These data are calculated at different levels of theory and compared with previous data. The equilibrium distance of the X2Π state is equal to 2.056 Å at the MRCI level, which is 0.044 Å shorter than the experimental value31 (2.10 Å) but very close to previous theoretical values. The equilibrium distance of the a4Σ− state, 2.051 Å (MRCI), is slightly shorter but very close to that of the X2Π state. The harmonic wavenumbers of both states are close to 600 cm−1 in good agreement with known values of previous theoretical and experimental determinations. For both states, the anharmonic constants are relatively small. The vibrational levels of the X2Π and a4Σ− states are calculated using Numerov package27 and they are reported in Table 4.
Table 5. Calculated Transition Energies (in cm−1) between Some Vibrational States of the X2Π (v) and the a4Σ− (v′) States at the MRCI and MRCI+SO Levels of Theory E MRCI+SO
Table 4. Calculated Vibrational Levels G(v) (in cm−1) for the X2Π and a4Σ− States at the MRCI and MRCI+SO Level of Theorya X2 Π
X2Π1/2
X2Π3/2
a4Σ−
a4Σ−1/2
a4Σ−3/2
0 1 2 3 4 5 6 7 8
312.2 924.8 1529.9 2129.3 2721.3 3306.1 3884.7 4455.8 5020.0
311.9 923.9 1528.2 2126.9 2718.4 3302.6 3880.5 4450.8 5013.8
308.2 912.4 1508.9 2099.6 2683.1 3258.9 3828.2 4389.9 4944.2
301.3 889.3 1465.1 2030.0 2580.8 3116.2 3636.0 4138.6 4623.4
289.2 850.1 1394.7 1923.9 2434.7 2923.6 3389.7 3831.3 4245.2
290.5 854.2 1401.9 1934.6 2449.0 2941.5 3410.9 3854.8 4269.1
v′
MRCI
Ω = /2
Ω = 3/2
0 0 0 1 2 3
0 1 2 0 0 0
13447 14035 14611 12834 12229 11630
13698 14259 14803 13086 12482 11883
13040 13604 14151 12436 11839 11249
some vibrational states (v′ = 0−2 of the associated a4Σ−1/2,3/2 components) are given. As expected from the larger SO splittings and energetic displacements in the molecular region for the X2Π1/2,3/2 states than for the a4Σ−1/2,3/2 states, the transition energies are smaller for the Ω = 3/2 than for the Ω = 1 /2 components. The intensities of these transitions, resulting from the SO interactions are small, nevertheless the transitions can be observed in the 13000−14000 cm−1 domain.
G(v) v
v
1
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CONCLUSIONS The potential energy curves of the CI radical have been calculated using MRCI and MRCI+Q levels of theory with a relativistic pseudopotential on the iodine atom. It was found that the X2Π and a4Σ− electronic states are bound and well separated energetically from the other low lying excited electronic states. The spin−orbit effects have a significant influence on the structural and spectroscopic properties of CI. Interestingly, it is the SO fine structure of carbon atom, C(3P0,1,2), which induces that Ω = 1/2 and 3/2 components of the two lowest electronic states correlate to the lowest asymptotic fine structure limits corresponding to I(2P3/2). The calculated dissociation energy of the X2Π ground state into the lowest spin component of the [C(3Pg) + I(2Pu)] is De = 2.40 eV and D0 = 2.36 eV (for the X2Π3/2 component) in very good agreement with experimental estimated values D0 = [2.0−
a
For each series of levels, the origin is taken at the minimum of the energy of the corresponding PEC. Taking as reference the minimum of energy of the X2Π state (−332.660801 au, MRCI), these origins are 0.00, −591, +192, 13458, 13130, and 13250 cm−1 for X2Π, X2Π1/2, X2Π3/2, a4Σ−, a4Σ−1/2, and a4Σ−3/2 respectively.
The effect of the core−valence correlation has been partially checked by introducing the correlation of the subvalence shell in the electronic calculation of the X2Π and a4Σ− states. The effect is rather small and can be summarized as follows: it consists in an increase of the harmonic wavenumber of the X2Π state by 11 cm−1 and a shortening of the equilibrium bond 2358
DOI: 10.1021/acs.jpca.7b09240 J. Phys. Chem. A 2018, 122, 2353−2360
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The Journal of Physical Chemistry A 2.37] eV. The first excited state a4Σ− is located at 1.67 eV (MRCI) above the ground state without SO interaction. The excitation energy of the a4Σ−3/2 is equal to 1.62 eV when the SO interaction is taken into account, that of the a4Σ−1/2 is equal to 1.70 eV with SO interaction. Because of spin−orbit interaction the (0,0) X2Π - a4Σ− transitions can be observed as weak bands at 13698 cm−1 (Ω = 1/2) and 13040 cm−1 (Ω = 3/2). Higher excited electronic states are mostly repulsive; this is relevant for the photodissociation of CI since it should decompose (and hence not be stable) in the upper atmosphere.
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of small halogenated compounds. J. Chem. Phys. 2003, 118, 3510− 3522. (12) Peterson, K. A.; Shepler, B. C.; Figgen, D.; Stoll, H. On the spectroscopic and thermochemical properties of ClO, BrO, IO, and their anions. J. Phys. Chem. A 2006, 110, 13877. (13) Marshall, P.; Misra, A.; Schwartz, M. A Computational Study of the Enthalpies of Formation of Halomethylidynes. J. Chem. Phys. 1999, 110, 2069−2073. (14) Bacskay, G. B. Quantum Chemical Characterization of the X(1A′), a(3A″) and A(1A″) States of CHBr and CHI and Computed Heats of Formation for CHI and CI. J. Phys. Chem. A 2010, 114, 8625−8630. (15) Hargittai, M.; Schultz, G.; Schwerdtfeger, P.; Seth, M. The Structure of Gaseous Carbon Tetraiodide from Electron Diffraction and All Carbon Iodides, CIn(n = 1−4), and Their Dimers, C2I2n from High-Level Computation. Any Other Carbon-Iodide Species in the Vapor? Struct. Chem. 2001, 12, 377−391. (16) Alves, T. V.; Ornellas, F. R. Exploring the electronic states of iodocarbyne: a theoretical contribution. Phys. Chem. Chem. Phys. 2014, 16, 9530−9537. (17) Ramirez-Solis, A.; Daudey, J. P.; Teichteil, C. Multiconfigurational self-consistent field-multireference Configuration Interaction Study of the Spectroscopy of CuI Including Spin-orbit Couplings. J. Chem. Phys. 1990, 93, 7277−7283. (18) Roszak, S.; Krauss, M.; Alekseyev, A. B.; Liebermann, H.-P.; Buenker, R. J. Spin-Orbit Configuration Interaction Calculation of the Potential Energy Curves of Iodine Oxide. J. Phys. Chem. A 2000, 104, 2999−3003. (19) Kurosaki, Y.; Matsuoka, L.; Yokoyama, K.; Yokoyama, A. Ab initio study on the ground and low-lying excited states of cesium iodide (CsI). J. Chem. Phys. 2008, 128, 024301−024307. (20) Moore, C. E. Atomic Energy Levels; NSRDS-NBS 35; USGPO: Washington, DC, 1971; Vols. II and III. (21) Knowles, P. J.; Werner, H.-J. An efficient 2nd-order MC SCF method for long configuration expansions. Chem. Phys. Lett. 1985, 115, 259. (22) Werner, H.-J.; Knowles, P. J. A 2nd-order multiconfiguration SCF procedure with optimum convergence. J. Chem. Phys. 1985, 82, 5053. (23) Werner, H.-J., Knowles, P. J., Knizia, G., Manby, F. R., Schütz, M. et al. MOLPRO, version 2012.1, a package of ab initio programs; see: http://www.molpro.net. Werner, H.-J.; Knowles, P. J.; Knizia, G.; Manby, F. R.; Schütz, M. Molpro: a general-purpose quantum chemistry program package. WIREs Comput. Mol. Sci. 2012, 2, 242− 253. (24) Dunning, T. H., Jr. Gaussian Basis Sets for Use in Correlated Molecular Calculations. The Atoms Boron through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007−1023. (25) Kendall, R. A.; Dunning, T. H.; Harrison, R. J. Electron-affinities of the 1rst row atoms revisited - systematic basis-sets ans wavefunctions. J. Chem. Phys. 1992, 96, 6796. (26) Berning, A.; Schweizer, M.; Werner, H.-J.; Knowles, P. J.; Palmieri, P. Spin-orbit matrix elements for internally contracted multireference configuration interaction wavefunctions. Mol. Phys. 2000, 98, 1823. (27) Senekowitsch, J. Spectroscopic properties of triatomic molecules; Ph.D. Thesis, Johann Wolfang Goethe Universität: Frankfurt am Main, Germany, 1988. (28) Cooley, J. W. An improved eigenvalue corrector formula for solving the Schrödinger equation for central fields. Math. Comput. 1961, 15, 363. (29) Kramida, A.; Ralchenko, Yu; Reader, J.; NIST ASD Team 2015 NIST Atomic Spectra Database, ver. 5.3; 2015. (30) Gutsev, G. L.; Ziegler, T. Theoretical Study on Neutral and Anionic Halocarbynes and Halocarbenes. J. Phys. Chem. 1991, 95, 7220−7228. (31) Gurvich, L.; Veyts, I. V.; Alcock, C. B. Thermodynamic Properties of Individual Substances; Hemisphere: New York, 1992; Vol 2.
AUTHOR INFORMATION
Corresponding Author
*(G.C.) E-mail:
[email protected]. ORCID
M. Hochlaf: 0000-0002-4737-7978 G. Chambaud: 0000-0002-8031-2746 Present Address ‡
Université Lille, CNRS,UMR 8522, Physico-Chimie des Processus de Combustion et de l’Atmosphère- PC2A, F59000 Lille, France Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS G.M. wishes to thank the UPE University for an Invited Professorship stay while preparing this work.
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REFERENCES
(1) Vogt, R. Iodine Compounds in the Atmosphere. Reactive Halogen Compounds in the Atmosphere: The Handbook of Environmental Chemistry 1999, 4E, 113−128. (2) Saiz-Lopez, A.; Plane, J. M. C.; Baker, A. R.; Carpenter, L. J.; Von Glasow, R.; Gómez Martín, J. C.; McFiggans, G.; Saunders, R. W. Atmospheric Chemistry of Iodine. Chem. Rev. 2012, 112 (3), 1773− 1804. (3) Shan, S.; Yan, P.; Zhang, X.; Yin, S.; Yuan, X.; Xu, H.; Yan, B. Exploring the structure and photodissociation mechanism of the electronic states of iodocarbene, CHI: a theoretical contribution. Phys. Chem. Chem. Phys. 2017, 19, 17735. (4) Vogt, R.; Sander, R.; von Glasow, R.; Crutzen, P. J. Iodine chemistry and its role in halogen activation and ozone loss in the marine boundary layer: A model study. J. Atmos. Chem. 1999, 32, 375− 395. (5) Law, K. S.; Sturges, W. T. Scientific Assessment of Ozone Depletion: 2006. In Global Ozone Research and Monitoring Project; Report No. 50; World Meteorological Organization: 2007; Chapter 2. (6) Liss, P. S.; Slater, P. G. Flux of gases across the air-sea interface. Nature 1974, 247, 181−184. (7) Davis, D.; Crawford, J.; Liu, S.; McKeen, S.; Bandy, A.; Thornton, D.; Rowland, F.; Blake, D. Potential impact of iodine on tropospheric levels of ozone and other critical oxidants. J. Geophys. Res. 1996, 101, 2135−2147. (8) Carpenter, L. J. Iodine in the marine boundary layer. Chem. Rev. 2003, 103, 4953−4962. (9) Carpenter, L. J.; MacDonald, S. M.; Shaw, M. D.; Kumar, R.; Saunders, R. W.; Parthipan, R.; Wilson, J.; Plane, J. M. C. Atmospheric iodine levels influenced by sea surface emissions of inorganic iodine. Nat. Geosci. 2013, 6, 108−111. (10) Dixon, D. A.; Peterson, K. A. Heats of formation of CCl and CCl2 from ab initio quantum chemistry. J. Chem. Phys. 2001, 115, 6327. (11) Feller, D.; Peterson, K. A.; de Jong, W. A.; Dixon, D. A. Performance of coupled cluster theory in thermochemical calculations 2359
DOI: 10.1021/acs.jpca.7b09240 J. Phys. Chem. A 2018, 122, 2353−2360
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The Journal of Physical Chemistry A (32) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure: IV. Constants of Diatomic Molecules; Van Nostrand Reinhold: New York, 1979.
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