Volume 33 Issue 5
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Xiao-na Lia, Li-xue Jiang, Qing-yu Liu, Yi Ren, Gong-ping Wei. Hydrogen-Assisted C-C Coupling on Reaction of CuC3H- Cluster Anion with CO†[J]. Chinese Journal of Chemical Physics , 2020, 33(5): 628-634. doi: 10.1063/1674-0068/cjcp2006094
Citation: Xiao-na Lia, Li-xue Jiang, Qing-yu Liu, Yi Ren, Gong-ping Wei. Hydrogen-Assisted C-C Coupling on Reaction of CuC3H- Cluster Anion with CO[J]. Chinese Journal of Chemical Physics , 2020, 33(5): 628-634. doi: 10.1063/1674-0068/cjcp2006094

Hydrogen-Assisted C-C Coupling on Reaction of CuC3H- Cluster Anion with CO

doi: 10.1063/1674-0068/cjcp2006094
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  • Corresponding author: Xiao-na Li, E-mail:lxn@iccas.ac.cn
  • Part of the special issue on "the Chinese Chemical Society's 16th National Chemical Dynamics Symposium".
  • Received Date: 2020-06-13
  • Accepted Date: 2020-07-13
  • Publish Date: 2020-10-27
  • A fundamental study on C-C coupling, that is the crucial step in the Fischer-Tropsch synthesis (FTS) process to obtain multi-carbon products, is of great importance to tailor catalysts and then guide a more promising pathway. It has been demonstrated that the coupling of CO with the metal carbide can represent the early stage in the FTS process, while the related mechanism is elusive. Herein, the reactions of the CuC$ _3 $H$ ^- $ and CuC$ _3 $$ ^- $ cluster anions with CO have been studied by using mass spectrometry and theoretical calculations. The experimental results showed that the coupling of CO with the C$ _3 $H$ ^- $ moiety of CuC$ _3 $H$ ^- $ can generate the exclusive ion product COC$ _3 $H$ ^- $. The reactivity and selectivity of this reaction of CuC$ _3 $H$ ^- $ with CO are greatly higher than that of the reaction of CuC$ _3 $$ ^- $ with CO, and this H-assisted C-C coupling process was rationalized by theoretical calculations.
  • Part of the special issue on "the Chinese Chemical Society's 16th National Chemical Dynamics Symposium".
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Hydrogen-Assisted C-C Coupling on Reaction of CuC3H- Cluster Anion with CO

doi: 10.1063/1674-0068/cjcp2006094

Abstract: A fundamental study on C-C coupling, that is the crucial step in the Fischer-Tropsch synthesis (FTS) process to obtain multi-carbon products, is of great importance to tailor catalysts and then guide a more promising pathway. It has been demonstrated that the coupling of CO with the metal carbide can represent the early stage in the FTS process, while the related mechanism is elusive. Herein, the reactions of the CuC$ _3 $H$ ^- $ and CuC$ _3 $$ ^- $ cluster anions with CO have been studied by using mass spectrometry and theoretical calculations. The experimental results showed that the coupling of CO with the C$ _3 $H$ ^- $ moiety of CuC$ _3 $H$ ^- $ can generate the exclusive ion product COC$ _3 $H$ ^- $. The reactivity and selectivity of this reaction of CuC$ _3 $H$ ^- $ with CO are greatly higher than that of the reaction of CuC$ _3 $$ ^- $ with CO, and this H-assisted C-C coupling process was rationalized by theoretical calculations.

Part of the special issue on "the Chinese Chemical Society's 16th National Chemical Dynamics Symposium".
Xiao-na Lia, Li-xue Jiang, Qing-yu Liu, Yi Ren, Gong-ping Wei. Hydrogen-Assisted C-C Coupling on Reaction of CuC3H- Cluster Anion with CO†[J]. Chinese Journal of Chemical Physics , 2020, 33(5): 628-634. doi: 10.1063/1674-0068/cjcp2006094
Citation: Xiao-na Lia, Li-xue Jiang, Qing-yu Liu, Yi Ren, Gong-ping Wei. Hydrogen-Assisted C-C Coupling on Reaction of CuC3H- Cluster Anion with CO[J]. Chinese Journal of Chemical Physics , 2020, 33(5): 628-634. doi: 10.1063/1674-0068/cjcp2006094
  • To face the growing concerns about the shortage of fossil fuels, the importance of CO serving as C1 building block to produce compounds has attracted increasing attentions. The Fischer-Tropsch synthesis (FTS) is a well-known industrial-scale process to produce liquid products using syngas, a mixture of CO and H2 [1-5]. Transition metal catalysts play vital roles in the FTS process to drive the crucial steps, such as the reductive cleavage of CO and C-C coupling, to proceed under relatively milder conditions [6]. The C-C coupling step is crucial to obtain multi-carbon products [6-8], while it is difficult to control the length of carbon chain and a wide distribution of hydrocarbons is generally produced in the final outputs. A fundamental understanding on the factors that can affect the C-C coupling reaction is of great importance to guide the design of advanced catalysts and then shift a specific reaction toward a desirable product. However, the details on related elementary steps are obscure because the information on chemical bonding is difficult to obtain in real-life catalysis.

    A specific chemical reaction is closely related to a limited number of atoms. In this case, atomic clusters can be the ideal active sites on heterogeneous catalysts [9-21]. The fascinating feature of gas-phase atomic clusters is that cluster reactions can be performed under isolated conditions to eliminate the effects from solvent, defects, impurity, and so on, and then the nature of bond formation and cleavage behaviors can be obtained at a strictly molecular level. The polymerization of surface CH$ _x $ units is the generally accepted process to obtain multi-carbon compounds [5]. It has also been demonstrated that carbon can induce the reconstruction of catalysts [22-27] in the FTS process and the surface with terminated metal carbide is more reactive and can function as precursors to couple with CO [28, 29]. Thus, metal carbide can represent the active sites on the surface of practice catalysts to investigate the C-C coupling mechanism. Moreover, transition metal carbides have attracted growing attention as important catalytic materials for a variety of energy-related reactions in the field of heterogeneous catalysis [30]. In the field of gas-phase studies, the geometrical and electronic structures of metal carbide species have been extensively studied [31, 32], in contrast, the reactivity was relatively rarely observed. The reactions of FeC$ _n $$ ^- $ ($ n $ = 3, 6), Ta2C4$ ^- $, MoC$ ^+ $, and Mo2C2$ ^- $ species with CH4 [33-37], Ta2C4$ ^- $ and TaC$ _n $$ ^+ $ ($ n $ = 1-4) with N2 [38, 39], MoC3$ ^- $ with C2H$ _6 $ [40], and TaC$ _n $$ ^- $ ($ n $ = 1-14) with D2 [41] have been reported, while the related reaction of metal carbide species with CO is not available, to our knowledge. In this study, the reactions of the CuC3H$ ^- $ and CuC3$ ^- $ cluster anions with CO have been investigated by using mass spectrometry, photoelectron imaging spectroscopy, and quantum chemical calculations. The experimental results showed that the CuC3H$ ^- $ cluster can induce more effective and selective coupling of the C3H$ ^- $ moiety with CO to generate ion product COC3H$ ^- $ than the reaction of CuC3$ ^- $ with CO. The crucial roles of the attached H atom on CuC3H$ ^- $ to improve C-C coupling have been rationalized by using theoretical calculations.

  • The CuC3H$ ^- $, CuC3D$ ^- $, and CuC3$ ^- $ cluster anions were generated by laser ablation of a $ ^{63} $Cu disk in the presence of 0.5%CH4/CD4 seeded in a He carrier gas with the backing pressure of 5.0 standard atmospheres. The cluster ions of interest (CuC3H$ ^- $, CuC3D$ ^- $, and CuC3$ ^- $) were mass-selected using a quadrupole mass filter (QMF) and then entered into a linear ion trap (LIT) reactor, where they were cooled by collisions with a pulse of He gas and then interacted with a pulse of CO for around 1.3 ms and 2.3 ms for CuC3H$ ^- $ (CuC3D$ ^- $)+CO and CuC3$ ^- $+CO, respectively. The temperature of cooling gas (He), reactant gases (CO), and the LIT reactor was around 298 K. The cluster ions ejected from the LIT were detected by a reflector time-of-flight mass spectrometer (TOF-MS). The details of running the TOF-MS [42], the QMF [43], and the LIT [44] can be found in our previous work. To calculate the reaction efficiencies (the possibilities of reaction upon each collision), the collision rate constants were calculated on the basis of the surface charge capture model developed in Ref.[45].

    The photoelecron imaging spectroscopy (PEIS) experiments were carried out with a separate TOF-MS equipped with a laser-ablation source, a cryogenic ion trap, and a photoelectron imaging spectromter. The CuC3H$ ^- $ clusters were generated according to the procedure described in the reactivity experiment. After generation, the cluster anions were directed into a linear octopole ion trap. The ion trap was held at 10 K and filled with pulsed bursts of precooled buffer gas of He. The ions were traped in the ion trap for around 80 ms and then extracted from the trap into a TOF-MS. After mass selection by a mass gate, the clusters reached the interaction region, where they were photo-detached with a wavelenth-tunable laser beam delivered from an optical parametric oscillator laser source. In the room temperature PEIS, the ion trap ran at the ion-guided mode. The kinetic energies or velocities of the photo-detached electrons were measured by the photoelectron imaging spectromerter. The details about performing the experiment can be found in a previous study [46].

  • Density functional theory (DFT) calculations using the Gaussian 09 [47] program were carried out to study the mechanisms on the reactions of CuC3H$ ^- $ and CuC3$ ^- $ cluster anions with CO. The TPSS functional has been demonstrated to perform well for the Cu-related system [48], thus the results by TPSS [49] were given throughout this work. The 6-311+G(d) basis sets were used for all the atoms. The relaxed potential energy surface scans were used to obtain good guess structures for the intermediates (Is) and the transition states (TSs) along the pathways. The TSs were optimized by using the Berny algorithm method [50]. Vibrational frequency calculations were performed to check that each I or TS has zero or only one imaginary frequency, respectively. Intrinsic reaction coordinate [51, 52] calculations were performed so that each TS connects two appropriate local minima.

  • The TOF mass spectra for the interactions of laser-ablation generated and mass-selected CuC3H$ ^- $, CuC3D$ ^- $, and CuC3$ ^- $ cluster anions with CO are shown in FIG. 1. For the interaction of CuC3H$ ^- $ with CO (FIG. 1(B, C)), the single product COC3H$ ^- $ can be clearly identified, indicating that CuC3H$ ^- $ can react with CO to evaporate a neutral Cu atom (reaction (1)). This channel was further confirmed by isotope labeling experiment (FIG. 1(E)). For the reaction of CuC3$ ^- $ with CO (FIG. 1(G)), the evaporation of Cu atom can also take place to generate product COC3$ ^- $ (reaction (2)), whereas CO adsorption products CuC3(CO)$ ^- $ and CuC3(CO)2$ ^- $ also appeared. This experiment revealed that the attachment of a H atom on CuC3H$ ^- $ can obviously lead to a higher product selectivity.

    Figure 1.  Time-of-flight mass spectra for the reactions of mass-selected CuC3H$ ^- $, CuC3D$ ^- $, and CuC3$ ^- $ anions with He (A, D, and F) and CO (B, C, E, and G). The reaction time is about 1.3 ms for CuC3H$ ^- $+CO and CuC3D$ ^- $+CO, and 2.3 ms for CuC3$ ^- $+CO.

    The pseudo-first-order rate constants ($ k_1 $) on the reactions of CuC3H$ ^- $, CuC3D$ ^- $, and CuC3$ ^- $ with CO have been well fitted and the results are shown in FIG. S1 in supplementary materials. The fitted results indicate that about 31% of experimentally generated CuC3H$ ^- $ is inert toward CO and the inert component for CuC3D$ ^- $ is about 37%. The generation of cluster isomers with very different reactivity has frequently been observed in laser-ablation experiments [53-58]. The determined rate constants for the reactive component of CuC3H$ ^- $ and CuC3D$ ^- $ are about (3.4$ \pm $1.1)$ \times $10$ ^{-12} $ cm$ ^3\cdot $molecule$ ^{-1}\cdot $s$ ^{-1} $ and (4.8$ \pm $1.4)$ \times $10$ ^{-12} $ cm$ ^3\cdot $molecule$ ^{-1}\cdot $s$ ^{-1} $, corresponding to the reaction efficiencies of about (0.5$ \pm $0.1)% and (0.7$ \pm $0.2)% by using the surface charge capture model [45], respectively. The rate constant for reaction CuC3$ ^- $+CO is about (0.8$ \pm $0.2)$ \times $10$ ^{-12} $ cm$ ^3\cdot $molecule$ ^{-1}\cdot $s$ ^{-1} $ and the reaction efficiency is about (0.10$ \pm $0.03)%. The comparable rate constants for reactions CuC3H$ ^- $+CO and CuC3D$ ^- $+CO indicate that the hydrogen atom transfer could not be the rate-determining step. The much lower rate constant for reaction CuC3$ ^- $+CO emphasizes that the attached H atom on CuC3H$ ^- $ can significantly improve the coupling of CO with the C3H moiety in CuC3H$ ^- $.

  • The DFT calculations were carried out to investigate the structures of the CuC3H$ ^- $ and CuC3$ ^- $ clusters and the mechanisms on the reactions of CuC3H$ ^- $ and CuC3$ ^- $ with CO. As shown in FIG. 2(A) and FIG. S2 (in the supplementary materials), several low-lying CuC3H$ ^- $ isomers have been predicted. The DFT calculations show that the linear CuC3H$ ^- $ isomer (IS1) in the doublet electronic state is the lowest-lying structure, while single-point energy calculations using the high level [CCSD(T)] [59] method suggest that IS4 is the most stable CuC3H$ ^- $ isomer. The internal conversion among different isomers has been performed (FIG. 2(B) and FIG. S3 in the supplementary materials) and the results reveal that the high barriers may prevent their intra-conversion. Thus, isomers IS1-IS5 may all be generated in the cluster source. In contrast, the linear structure in the triplet electronic state with a terminal Cu atom was unambiguously identified as the lowest-lying isomer for CuC3$ ^- $.

    Figure 2.  (A) The DFT-calculated low-lying isomers for CuC3H$ ^- $. Single point energies calculated at the CCSD(T) level are listed in the square brackets. The relative energies are given in eV. M2 and M4 represent the doublet and the quartet electronic states, respectively. (B) Potential energy profile for internal conversion among different CuC3H$ ^- $ isomers, and details can be found in FIG. S3 in the supplementary materials.

    To further characterize the structure of CuC3H$ ^- $, the anion photoelectron imaging spectroscopy experiments were performed to gain more information, and the results are shown in FIG. 3. For the spectra collected at 520 nm at 298 K (FIG. 3(A)) and 10 K (FIG. 3(B)), a well-resolved vibrational progression with a peak spacing of about 403 cm-1 can be observed. These spectra are in agreement with the Franck-Condon (FC) simulated profile of isomer IS4 (FIG. 3(F), $ ^1 $A$ \leftarrow $$ ^2 $A) at 298 K. Moreover, the relatively weak bonds with binding energy of 1.48 eV to 2.28 eV in the 520 nm spectrum collected at 298 K are generally consistent with the simulation of isomer IS3 (FIG. 3(E)), which may also be generated in the experiment. The FC simulated profile of isomer IS2 (FIG. 3(D)) can reproduce the experimental spectrum collected at 700 nm at 298 K (FIG. 3(A)) very well, suggesting the possible generation of IS2 in the cluster source. The 380 nm spectrum collected at 298 K has a broad band centered at 2.82 eV (FIG. 3(A)). This spectrum can assign the population of IS1, which has a calculated vertical electron detachment energy (VDE) of 2.80 eV ($ ^1 $A$ \leftarrow $$ ^2 $A) (FIG. 3(C)) and matches the experimental positions well. In our experiment, it is difficult to estimate the possible presence of isomer IS5 because of its large VDE value of 3.50 eV.

    Figure 3.  Photoelectron spectra of CuC3H$ ^- $ measured with (A) 520, 700, and 380 nm photons at 298 K and (B) 520 and 880 nm photons at 10 K, respectively. The simulated Franck-Condon (FC) spectra for the low-lying CuC3H$ ^- $ isomers are shown in panels (C-F). The calculated vertical electron detachment energies for each isomer are listed in the square brackets.

    The reactions of low-lying CuC3H$ ^- $ isomers (IS1-IS5) with CO were all calculated (FIG. 4(A) and FIGs. S4 and S5 in supplementary materials) and the results demonstrated that the positive barriers on the reactions of IS4 and IS5 with CO make both reactions kinetically hindered, thus, IS4 and IS5 may account for the inert component of CuC3H$ ^- $ generated in the experiment. The main text below focused on the pathway on reaction CuC3H$ ^- $(IS1)+CO (FIG. 4(A)) because the pathways on reactions IS2/IS3+CO are very similar to or share the same part with pathway CuC3H$ ^- $(IS1)+CO. CO can be trapped by the Cu site of CuC3H$ ^- $(IS1) with a binding energy of 0.65 eV to form intermediate 1 (I1). Then the H atom jumps onto the nearby C site to overcome a small barrier of 0.20 eV (I1$ \rightarrow $I2). In the next step, a second Cu-C bond forms favorably to generate I3 (I2$ \rightarrow $I3), from which C-C coupling is ready to take place to suffer from a higher barrier of 0.76 eV (I3$ \rightarrow $I4). Then the neutral Cu atom is evaporated to give rise to product COC3H$ ^- $. In contrast, for reaction CuC3$ ^- $+CO, the C-C coupling ($ ^3 $I6$ \rightarrow $$ ^1 $I7) and the Cu atom release steps are more difficult to proceed without the presence of such H atoms (FIG. 4(B)). Theoretical calculations strongly support the experimental results that the CuC3H$ ^- $ cluster reacts faster with CO (FIG. 1).

    Figure 4.  The DFT-calculated potential energy profiles for reactions (A) CuC3H$ ^- $(IS1)+CO on the doublet state and (B) CuC3$ ^- $+CO on both of the singlet and the triplet states. The zero-point vibration corrected energies ($ \Delta H_0 $, eV) are given. Bond lengths are given in pm.

  • CO adsorption and dissociation on heterogeneous catalysts was generally accepted as the initial steps in the FTS process, then followed by the C-C coupling. The H-assisted pathway in CO activation has been proposed theoretically and experimentally because of the very high availability of surface hydrogen [60-64]. Recent progress demonstrated that direct coupling of CO with the terminal Mo carbide serves as the initial step in FTS [28], and the sequential addition of hydride that generates from the heterolytic cleavage of H2 promotes greatly these early steps. Herein, the fundamental study on the H-assisted C-C coupling chemistry is not only scientifically interesting but also practically important to tailor catalysts. The pathway in FIG. 4(A) shows that the H atom seems to be a spectator and does not take part directly in the C-C coupling and the Cu-release steps. While further analysis evidences that the attached H atom is vital to change the geometrical and electronic structure of the CuC3H$ ^- $ cluster during the reaction. Though the existence of multiple intermediates and transition states along the pathway on reaction CuC3H$ ^- $+CO, the crucial steps, C-C coupling (I3$ \rightarrow $TS7$ \rightarrow $I4) and Cu-release (I5$ \rightarrow $P1), are energetically more favorable to proceed. In the presence of such H atoms, it is favorable to form a planar intermediate I3, which is flexible to couple with CO through the terminal C atom to surpass a barrier of 0.76 eV (FIG. 4(A)). This flexibility of the C3-chain can be reflected by the frequently and easily changed C-C bond lengths along the pathway. In sharp contrast, the linear intermediate $ ^3 $I6 is rather rigid and C-C bond is reluctant to be formed between CO and the opposite C3-chain, and a high barrier of 1.50 eV has to be crossed laboriously ($ ^3 $I6$ \rightarrow $$ ^3 $TS9$ \rightarrow $$ ^1 $I7, FIG. 4(B)). This fact emphasizes the capability of H atom to reconstruct the geometrical structure of the reaction system toward a more reactive pattern.

    Natural charge analysis reveals that the H atom can also drive the charge redistribution on reaction system CuC3HCO$ ^- $. In the final intermediate I5, the C atom that anchors the Cu atom is much less negatively charged (-0.68 e) with respect to that (-1.18 e) in CuC3CO$ ^- $ ($ ^1 $I7, shown in FIG. 5). The leading result is that such C atom in $ ^1 $I7 is more attractive toward the positively charged Cu atom (+0.31 e), and the higher barrier of 2.56 eV ($ ^1 $I7$ \rightarrow $P2) increases the difficulty for the Cu atom to struggle from CuC3CO$ ^- $, while the nearly neutral Cu (-0.07 e) atom in I5 can be easily evaporated. This can also be reflected by the much longer Cu-C bond (194 pm) in I5 than that (181 pm) in $ ^1 $I7. The DFT-calculated VDEs for the final products COC3H$ ^- $ and COC3$ ^- $ are 2.61 and 3.70 eV, respectively, indicating that the COC3 moiety is more attractive toward electrons. Thus, the Cu atom is positively charged in $ ^1 $I7 and gives rise to a tight COC3-Cu interaction. Moreover, the reorganization energy calculated for COC3H$ ^- $ (0.08 eV) is much smaller than that for COC3$ ^- $ (0.17 eV), indicating that the presence of such H atoms is greatly helpful to prevent the structure of the final product COC3H$ ^- $ from reorganization during the process of Cu atom evaporation and then makes the reaction energetically favorable. The capability of H atom to reshape the geometrical and electronic structure of atomic clusters has also been previously proposed [65]. This study may provide new insights into the paramount roles of surface attached hydrogen in the real-life FTS process to affect the C-C coupling reaction.

    Figure 5.  Natural charge in e distributions on I3, I5, $ ^3 $I6, and $ ^1 $I7. The blue values indicate the natural charges.

  • The reactions of the CuC3H$ ^- $ and CuC3$ ^- $ cluster anions with CO have been investigated by using mass spectrometry and density functional theory calculations. The structure of CuC3H$ ^- $ was further characterized by using photoelectron imaging spectroscopy experiment. The experimental results showed that CO can couple with the C3-moiety in CuC3H$ ^- $ and CuC3$ ^- $ to give rise to ion products COC3H$ ^- $ or COC3$ ^- $, respectively. The attached H atom can greatly increase the reactivity and product selectivity on the reaction of CuC3H$ ^- $ with CO. Theoretical studies revealed that such H atom can depress the barriers on the C-C coupling and Cu-release steps for CuC3H$ ^- $+CO, the crucial steps of both reactions. Direct coupling of CO with the terminal metal carbide on real-life catalysts has been scarcely demonstrated as the initial steps in the FTS process. This fundamental study is of great importance to permeate this process and the vital roles of surface attached hydrogen to assist C-C coupling were obtained.

    Supplementary materials: Fitted rate constants on the reactions of CuC3H$ ^- $, CuC3D$ ^- $, and CuC3H$ ^- $ with CO are available. DFT-calculated low-lying isomers for the CuC3H$ ^- $ and CuC3$ ^- $ clusters and the potential energy profiles for CuC3H$ ^- $+CO are also given.

  • This work was supported by the National Natural Science Foundation of China (No.21773254) and the Youth Innovation Promotion Association Chinese Academy of Sciences (No.2016030).

  • Figure S1.  Variation of ion intensity with respect to the pressures of CO on the reactions of CuC3H (a), CuC3D (b), and CuC3 (c) with CO. The data points were experimentally measured, and the solid lines were fitted to the experimental data points on the basis of least-square procedure. The fitted results demonstrated that about 31% of laser-ablation generated CuC3H and about 37% of such generated CuC3D were inert toward CO.

    Figure S2.  The density functional theory (DFT) calculated structures and relative energies of the CuC3H and CuC3 isomers as well as the structures of products COC3H and COC3. The relative energies are in unit of eV and bond lengths are given in pm. Single point energies calculated at the CCSD(T) level are listed in the square brackets.

    Figure S3.  The DFT-calculated profiles on the transformation of different CuC3H isomers shown in FIG. S2. The relative energies are in unit of eV and bond lengths are shown in pm. The superscripts "2" and "4" represent the doublet and the quartet electronic states, respectively.

    Figure S4.  The DFT calculated potential energy profile for reaction CuC3H + CO on the doublet state. The relative energies are in unit of eV. The structures of intermediates and transition states are plotted. Bond lengths are given in pm.

    Figure S5.  The DFT-calculated potential energy profiles on the reactions of lower-lying CuC3H isomers with CO. The relative energies are in unit of eV and the bond lengths are given in pm. The structures of intermediates and transition states are plotted.

    Figure S6.  (A) Spin crossing from the triplet state to the singlet state. The curve for the triplet state is from the intrinsic reaction coordinate (IRC) calculations. The energies of the singlet states are calculated at the IRC determined triplet state structures. (B) Spin crossing from the quartet state to the doublet state. The curve for the quartet state is from the Scan calculation. The energies of the doublet states are calculated at the Scan determined quartet state structures.

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