Contributions of Nearly-Degenerate States to the Photoelectron

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Contributions of Nearly-Degenerate States to the Photoelectron Spectra of the Vanadium Dicarbide Anion Le Nhan Pham, and Marc F.A. Hendrickx J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b09498 • Publication Date (Web): 07 Nov 2016 Downloaded from http://pubs.acs.org on November 10, 2016

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The Journal of Physical Chemistry

Contributions of Nearly-Degenerate States to the Photoelectron Spectra of the Vanadium Dicarbide Anion

Le Nhan Pham1,2, Marc F.A. Hendrickx1* 1. Afdeling Kwantumchemie en Fysicochemie, Departement Chemie, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Heverlee-Leuven, Belgium 2. Department of Chemistry, the University of Dalat, 01 Phu Dong Thien Vuong, Ward 8, 670000 Dalat City, Lam Dong, Vietnam

Abstract A theoretical study by using a wide variety of quantum chemical methods has been carried out to investigate the nature of the ionization processes that are responsible for the experimental observed photoelectron spectra of the anionic VC2- stoichiometry. In agreement with previous studies, the most stable structures for the anionic and neutral vanadium dicarbide species were unambiguously found to be cyclic isomers. However, concerning the nature of the ground state of the anionic cluster there appear to be two candidates which are nearly degenerate. Only by considering both these anionic states as initial states a substantial novel and complete assignment for the observed anion photoelectron spectra could be proposed. A thorough analysis of the electronic structures not only allows to distinguish the one-electron processes, but also enables to disclose their natures. All the lower binding energy bands involve ionizations out of a dominant V+ orbital. Opposed, the higher positioned bands are the outcome of an electron detachment out of the C22- ligand 3σg orbital. Finally, the experimentally observed vibrational progressions in the photoelectron spectra of VC2- were simulated on the basis of harmonic frequency analyses at the B3LYP level and the derived Franck-Condon factors.

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Introduction Vanadium is known as a transition metal to combine with other elements such as oxygen, sulfur, carbon, halogens, or to bind to organic ligands, to form clusters and complexes that play versatile roles as catalysts for several reactions in industrial applications and academic researches. 1–8

Vanadium-carbide compounds, in general, are not exceptional in terms of effective catalysts9–11

and additionally possess excellent hardness when coated on the surface of metallic materials. 12–14 Because of this diversity of applications, the electronic structures and chemical properties of vanadium-carbide compounds have been examined both experimentally and theoretically, with most of the studies focused on the simplest representative which is vanadium carbide (VC).15–18 In 1999, a systematic study on dicarbide compounds of the first-row transition metals including vanadium was conducted.19 In this work, the authors used anion photoelectron spectroscopy to investigate the electronic structures of VC2-/0. At two energy levels of laser photons (532 nm and 355nm), several low-lying states of the neutral VC2 were detected (as bands X, A, B and C), for which vibrational frequencies and detachment energies were reported. Very recently, the photoelectron spectra of VC2- have been measured again at a higher level of photon energy (266 mm),20 which revealed two new bands with higher ionization energies. Hence, these bands can be noted as D and E according to increasing ionization energy (Figure 1). In total, only 5 bands were completely observed in the two experimental studies, as for the E band only the beginning was reordered. To be more specific, in the former publication, there are four bands of which the one with the lowest binding energy, the X band, is believed to be an electronic transition between the ground states of the anionic cluster and the neutral cluster, while the higher energy bands (A, B and C) were ascribed to the excited states of the neutral cluster formed by one electron detachments from the anionic ground state. No assignments for the D band at 3.72 eV and E band above 3.90 eV are available to date. From the above experimental data, it is possible to extract some physical and chemical data about vanadium dicarbides in the gas phase, but it turns out to be impossible to attain a complete insight into its electronic structure. Therefore, the experimental data need to be supplemented with additional information in order to be entirely understood. For instance, which specific initial state of the anionic and final states of the neutral clusters underlie the ionization processes observed, or in other words from which orbitals are electrons removed? To obtain these insights, computational data are definitely potential candidates. Indeed, numerous computational studies were able to provide assignments for several analogous clusters. In these works, multi2 ACS Paragon Plus Environment

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reference wave function theories are frequently utilized as reliable methods for the prediction of the leading configurations. For obtaining more accurate relative energies between the lowest states, coupled cluster methods have been proven to be useful. Typically, photoelectron spectra of several anionic iron oxides21–24 and iron sulfides25–28 were assigned by combining multi-reference, density functional and couple cluster calculations. The same computational strategy was likewise successfully applied to anionic clusters of other transition metal oxide29, sulfide30 and carbides.31– 33

As for VC2-, a previous computational study34 proposed an assignment for the observed bands in the anion photoelectron spectra by using complete active space self-consistent field (CASSCF), multi-reference singles and doubles configuration interaction (MRSDCI) and density functional (DFT) computational methods. In doing so, the authors assumed that the 5B1, which was proposed as an isolated lowest electronic state of the cyclic VC2- isomer, as the unique anionic ground state and consequently all assignments for the experimental bands were based on this state as the initial state. These proposed assignments were additionally supported by computed DFT harmonic vibrational frequencies of the final states. Accordingly, the X band was ascribed to the 4B1 neutral ground state, whereas the ionization to the 6B1 was held responsible for the C band in the spectra. Further, the B band was assigned to 4A2 and/or 2B2, whereas the A band, which as a weak band is positioned slightly at higher energies than the X band, was ascribed to a 4B2. This mainly on the basis of the computed B3LYP vibrational frequencies. A later TDDFT study only confirmed the assignation for the X band, but assigned the A and B band to a 4A1 and 6B1, respectively.35 With the motivation for a complete understanding of these spectra, this work is dedicated to a detailed investigation into the electronic structures of all low-lying states of both cyclic and linear isomers of VC2-/0. From our point of view, the explanation for the B band in the literature33 is still unclear in the sense that the 5B1 → 2B2 ionization was invoked to assign this band. Unambiguously, this is a violation of the spin-selection rule. Further, the D and E bands, as reported very recently, were not identified. Therefore, this work will examine in detail all low-lying states via their leading configurations as withdrawn from CASSCF, while their relative energies will be improved via CASPT2 calculations by recovering an increasing amount of dynamic electron correlation energy. Moreover, B3LYP and BP86 density functional theory methods, and the RCCSD(T) couple cluster method will be utilized for the purpose to verify the relative energies of the ground state and the low-lying states of VC2-/0. From the low-lying states of VC2- and their 3 ACS Paragon Plus Environment


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leading configurations, all possible one-electron detachment transitions will be considered with the purpose to find out the true nature of the ionizations underlying the experimental bands. In particular, it will be demonstrated that this will lead to new assignments for all bands except the X band. Harmonic vibrational frequencies and Franck-Condon factors at the DFT level for some possible transitions will be presented to provide additional proof for the proposed assignments.

Computational details From previous computational studies, it appears that there are two low-lying geometric structures for the VC2-/0 stoichiometries, which are the cyclic V-C2 isomer and the linear V-C-C isomer. The former exhibits C2v symmetry, while the latter possesses C∞v symmetry. Both are depicted in Figure 2, which also includes the employed coordinate systems. As can be seen for the cyclic isomer, the vanadium nucleus is positioned at the origin and both carbon nuclei are located in the yz-plane, with the CC bond perpendicular to the z axis. Conventionally, all nuclei are situated on the z axis for the linear isomer. In order to determine the most stable isomers of the VC2-/0 clusters in the gas phase, all relevant spin-multiplicity states for the cyclic and linear VC2-/0 isomers were considered, by performing geometry optimizations without any symmetry constraints (structural and electronically). This process allows to preliminarily identify all low-lying spin-multiplicities of the two different isomers. To ensure that all of the obtained structures are stationary points on the potential energy surfaces, frequency analyses for each optimized geometry were performed as well. All calculations in this step were carried out at the B3LYP/def2-TZVP36 level of computation as coded in the TURBOMOLE 6.6 package37. Thereafter, during a second step, these optimized geometries are used in calculations with the intention to determine the lowest energies in regard to the C2v spatial symmetry of the electronic wave function. Although, C∞v is the point group of linear VCC-/0, however as a nonAbelian point group it is not implemented in the employed software packages. The four irreducible representations of the C2v point group for both linear and cyclic isomers are therefore considered at the CASSCF level in combination with the somewhat larger aug-cc-pwCVTZ-DK38,39 basis sets. Because vanadium has an open 3d shell and a close-lying 4s level, these valence orbitals must be included in the active space of the CASSCF calculations, besides the 2p orbitals of the two carbon atoms. Hence, the total orbitals in the active space amounts to 12, which are occupied by a total 4 ACS Paragon Plus Environment

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of 9 or 10 electrons depending on the charge of the isomers. In this way the CASSCF results provided reliable leading configurations of the various spatial symmetric states which allowed to optimize their geometries further with the B3LYP40–42 hybrid functional and the pure exchangecorrelation GGA (generalized gradient approximation) functional BP8641,43 by employing the augcc-pwCVTZ-DK basis sets. At the B3LYP geometries and for each state considered, single point RCCSD(T) calculations were performed with the same basis sets. The relative energies, as collected in Table 1, were obtained by using the MOLPRO 2012 package44 and including scalar relativistic effects by means of the Douglas-Kroll-Hess transformation to second –order.45 For the purpose of identifying the one-electron detachment processes, vertical detachment energies (VDEs) were calculated by using the previously obtained B3LYP geometry of the 5B1 anionic ground state. State average CASSCF over four roots and employing the same active space as in the previous paragraph were performed. In order to obtain more reliable VDEs, CASPT246 (complete active space second-order perturbation theory) energies for all roots were subsequently derived. The results can be found in Table 2 together with the leading configurations for each state. By performing population analyses at the CASSCF level the corresponding charge distributions were collected in Table 3. Additionally, Table 2 included the VDEs as calculated at the RCCSD(T)47,48 (restricted couple cluster with full treatment of singles, doubles and many-body perturbation treatment for triples excitations by using the restricted open-shell Hartree-Fock wave functions as a reference) level by the same procedure as used to acquire the CASPT2 results. Adiabatic detachment energies (ADEs) at the B3LYP, RCCSD(T) and CASPT2 levels are collected in Table 4. The ADEs were obtained by performing single point calculations at the B3LYP geometries of Table 1. For all these CASPT2 and RCCSD(T) calculations (Tables 2 and 4), the 3s and 3p electrons of vanadium and the 2s electrons of carbon were correlated in addition to the valence electrons already included in the active space of CASSCF. With the purpose to further enhance the recovery of the dynamic correlation energies, all B3LYP and RCCSD(T) results in these tables were calculated with the larger quintuple-ζ basis sets (aug-cc-pwCV5Z-DK38,39) for vanadium and carbon. For the CASPT2 single point calculations the ANO-RCC basis sets [7s,6p,4d,3f,2g] for vanadium49 and [5s,4p,3d,2f] for carbon50 were used. As scalar relativistic effects are of importance in computations on transition metal compounds, these effects were included by the use of the Douglas-Kroll-Hess transformation to second order.45 All B3LYP, BP86 and RCCSD(T) results of Tables 2 and 4 were obtained with the MOLPRO 2012 package44, while the CASSCF and CASPT2 energies were calculated with MOLCAS 8.0.51 5 ACS Paragon Plus Environment


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Finally, harmonic vibrational frequencies of the ground states and all accessible low-lying states of the cyclic VC2-/0 clusters were computed at the B3LYP/def2-TZVP level. These calculations provided not merely the vital evidence that the considered states are true stationary points on the potential energy surface, but also afforded the necessary equilibrium structures, harmonic vibrational normal modes and frequencies for the calculation of the multi-dimensional Franck−Condon factors by means of the MOLFC package.52 The comparison of these computed vibronic progressions with the experimental spectrum turns out to be useful for proposing a final assignment for some observed bands.

Results and discussion Ground states and low-lying states of the VC2-/0 clusters Equilibrium geometries for all low-lying spin multiplicities of both cyclic and linear VCC-/0 isomers were firstly calculated at the fast B3LYP/def2-TZVP level, starting from various geometries without implying any symmetry, neither on the geometric structures nor on the wavefunctions. By using these small basis sets in combination with a DFT method, it was possible to efficiently examine the potential hypersurfaces in great detail. The results are graphically depicted in Figure 3. All geometry optimizations for various low-lying spin multiplicities were found to convert to either cyclic C2v or linear structures, which were identified as stationary states by performing harmonic vibrational frequency analyses. For the anionic cyclic cluster, a quintet state is predicted as the lowest state as it is separated clearly from the other cyclic spin multiplicities by as much as 0.5 eV. Furthermore, all different spin multiplicity states of the linear VCC- structure are less stable. Again a quintet state is found as the most stable anionic linear state. By comparing the relative stabilities of these two anionic isomers, a cyclic quintet state is already proposed as the anionic ground state. However, since its energy difference with the linear VCC- quintet amounts to about 0.27 eV at this low level of computation, a further investigation by using more sophisticated computational techniques is clearly needed in order to reach a final conclusion about the true anionic ground state. For the neutral clusters the cyclic geometry appears considerably more stable than the linear isomer. The lowest energy was found for a cyclic quartet state, which turns out to be 0.36 eV lower than that of the linear quartet, the most stable spin multiplicity of the neutral linear isomer. As a temporary conclusion, the cyclic equilibrium structures are more stable than the 6 ACS Paragon Plus Environment

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linear isomers for both the anionic and neutral clusters.

Therefore, in the next step our

investigation, we can focus on the quintet and quartet cyclic states for the purpose to identify the detailed electronic structures of the ground states of VC2-/0 at a higher level of computation. However, for insuring a reliable assignment of the anion photoelectron spectra, linear quintets of the anionic cluster were also investigated as the lowest of them is just 0.27 eV less stable at the B3LYP/def2-TZVP level. The correct identification of the ground state of anionic cluster is a crucial factor in determining the nature of the observed ionizations in the photoelectron spectra. Table 1 contains the results of the more detailed B3LYP and BP86 geometry optimizations and single-point RCCSD(T) calculations at the B3LYP geometries by using the larger aug-cc-pwCVTZ-DK basis sets. They reveal that the cyclic 5B1 state is the ground state of the anionic cluster VC2- although its energy is not significantly lower than the cyclic 5A1. In fact, the difference in energies predicted by B3LYP and RCCSD(T) are 0.04 eV and 0.09 eV, respectively. The only exception being BP86, which, therefore should be viewed as less reliable for carbon-containing clusters such as VC2-/0. As a consequence of this conclusion, 5B1 should be considered as the initial state which is at the origin of the intense bands in the anion photoelectron spectra, although when considering the spectra in detail at a higher computational level, the anionic ground state issue will be addressed further. In any case, because of its comparable stability, the 5A1 is likely to be populated to some extent for the experimental conditions under which the spectra were measured. These findings are only to some degree in accordance with the previous computational study,34 which also predicted the 5B1 as the ground state of VC2- at the MRSDCI+Q level. However, at the same level of computation the lowest 5A1 state was positioned suspiciously much higher at 3.34 eV, while a 5A2 was put forward as the lowest excited stated at 0.13 eV. The latter state is predicted by our RCCSD(T) calculations at 0.34 eV above the ground state (Table 1). Concerning the neutral clusters, the ground state could unambiguously be predicted without any interference of low-lying excited states. All methods are in agreement that 14B1 is the ground state. Indeed, its energy is much lower than the lowest excited state which appears as a 14A1. According to Table 1, B3LYP, BP86 and RCCSD(T) place this state adiabatically at 0.469 eV, 0.690 eV and 0.430 eV higher than the ground state 14B1, values that will appear as important when discussing the photoelectron spectra in detail. CASPT2 vertical relative energies, as included in Table 2, support this conclusion. From these results, the 14A1 first excited state is positioned 0.40 eV above the anionic ground state. 7 ACS Paragon Plus Environment


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Electronic structures of VC2-/0 A thorough understanding of the electronic structures of the 5B1 anionic ground state and the nearly degenerate 5A1 state, together with those of the low-lying states of the neutral cluster is of paramount importance for applying the one-electron detachment rule when assigning the photoelectron spectra. In the following paragraphs, the CASSCF leading configurations for several important states, as given in Table 2, will be analyzed in detail. As mentioned above, there are 12 orbitals included in the active space. Commonly for 5B1 and 5A1, there appear to be three doubly occupied active space orbitals in their leading configurations, namely 8a1, 9a1, and 3b1. As can be deduced from Figure 4 (5B1) and Figure 5 (5A1) the 2p orbitals of the two carbon atoms dominantly contribute to the shape of these closed shell orbitals. More specifically they are predominantly either 1πu (8a1, 3b1) or 3σg (9a1) of the C2 moiety, which consequently constitutes a C22- ligand. This observation allows to formally summarize the charge distribution in these near-degenerate states as V+(C22-), implying that the remaining four electrons each occupy a different dominantly vanadium valence orbital, all with parallel spins. According to Figure 4, for 5B1 the specific metallic nature of these singly occupied orbitals is 10a1 (4s,4pz), 11a1 (3dx2- y2,3dz2), 5b2 (3dyz) and 1a2 (3dxy), and its overall electronic configuration corresponds to: (8a1)2 (9a1)2 (10a1)1 (11a1)1 (3b1)2 (5b2)1 (1a2)1. Table 2 and Figure 5 show that the CASSCF leading configuration of 5A1 distinguishes itself from that of the 5B1 ground state by a transfer of an electron from 11a1 of 5B1 to 4b1(dxz) of 5A1, giving rise to the following electronic configuration for the latter state: (8a1)2 (9a1)2 (10a1)1 (3b1)2 (4b1)1 (5b2)1 (1a2)1. Concerning the neutral cluster, the leading configurations of its ground state and the lowlying quartet and the sextet states are discussed, in this and the two following paragraphs, in relation to possible one-electron detachments. An analysis of the doubly and singly occupied active space orbitals of the 14B1 ground state showed that they are all very similar in shape to the corresponding orbitals of the anionic ground state 5B1. If an electron is removed from the singly occupied orbital 10a1 (4s,4p) of 5B1, the resulting configuration is that of 14B1 (Table 2). Quite similarly, the lowest excited state 14A1 can be obtained from the anionic 5A1 state by removal of a single electron out of the same 10a1 orbital, which will turn out to be a significant finding in the context of proposing our assignment of the photoelectron spectra. In the same context, 24A1 and 34A1 emerge by removal of one electron from the doubly occupied 9a1 orbital of the 5A1 state. On


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the other hand, 44A1 can be the corresponding state of a one-electron detachment from the 5A1 by removing an electron from the C22- ligand orbital 8a1. Further according to Table 2, three leading configurations of other quartet states can be identified as final states of one-electron detachments from the ground state 5B1. They are as follows: 34B1, 34A2 and 44B2. Indeed, by removing an electron from the singly occupied orbital 11a1 of 5B1, the configuration of 34B1 is obtained. In this configuration, the singly orbital 10a1 is dominantly composed from 4s and 4p of vanadium. In case of 34A2, the 5b2 orbital is unoccupied, and there are three remaining singly occupied orbitals 10a1 (4s,4pz), 11a1 (3dx2-y2) and 1a2 (3dxy). Concerning the 44B2 state, the three singly occupied orbitals are 10a1 (4s,4pz), 11a1 (3dx2-y2) and 5b2 (3dyz). Apparently, the dominant contributions to 11a1 of 34A2 and 44B2 are somewhat different from the 11a1 orbital of 5B1. While both 3dx2- y2 and 3dz2 nearly equally contribute to the 11a1 orbital of 5B1, 11a1 of 34A2 and 44B2 is predominantly 3dx2-y2. These pronounced orbital relaxation effects between 34A2 and 44B2 on the one hand and 5B1 on the other hand, lead to the conclusion that we must rule out the former two states as final states of pure one-electron ionizations from the anionic ground state 5B1. A visualization of this is presented in Figure 6. Since sextet states satisfy the spin selection rule of photoelectron spectroscopy in case of a quintet anionic ground state, they need also to be considered. The underlying ionization processes must involve the removal of an electron from the highest C22- ligand orbitals. From their leading configurations in Table 2, it is possible to identify the 6B1 as a prospect for a final state observed in the photoelectron spectra. Its electronic configuration is the result of a detachment of an electron from the doubly occupied 9a1 orbital of 5B1, which is mainly the 3σg of the C22- ligand. Indeed, this is the HOMO of the free ligand and therefore the removal of an electron from this orbital can be expected to give rise to the lowest sextet state reachable from the 5B1 anionic ground state as a one-electron detachment. The other sextets correspond definitely not to one-electron detachments from the 5B1 states. They all have the 4b1 orbital singly occupied, which is a virtual orbital of 5B1. It should however be mentioned that 6A1 can be obtained from the low-lying anionic 5A1 by removal of a single electron from 9a1. From the discussion above it has become clear that the ground states of the anionic and neutral clusters are composed of a closed shell C22- ligand bound to the open-shell V+ or V2+ metal systems, respectively. As is well known, these cations are formed from the neutral vanadium atom by ionizations out of the 4s orbital, which in the clusters has acquired some 4p character to become a 4s4p hybrid lone pair (10a1 orbital in Figures 4 and 5). Mulliken and Bader charge 9 ACS Paragon Plus Environment


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distributions obtained at the CASSCF level for the spectroscopically relevant states, as given in Table 3, confirm this rudimentary description of the electronic structure. As usual for the kind of clusters studied presently, the actual charges of the metal center in the ground states are much smaller than the just mentioned formal charges of +1 and +2. In particularly for the neutral 14B1 ground state both the Mulliken and AIM analyses propose a metal charge of +0.68 e and +1.26 e, respectively, which is about half of the oxidation state of +2. For the two nearly degenerated anionic quintet states, the vanadium charge is about 0.5 e-. This implies that although the initial ionization process underlying the experimental X band is the removal of an electron from the predominant 4s4p lone pair orbital of vanadium (vide infra), the accompanying relaxation effects in the other occupied orbitals counteract this primary ionization process. Indeed, electron density in the order of 0.5 e is transferred from the C22- ligand to the metal cation. Smaller relaxation effects are observed for the ionization from 5B1 to 6B1. The initial removal of an electron from the 9a1 orbital reduces the charge of C22- ligand by approximately 0.8 e. The corresponding relaxations in the other orbitals induce an increase of the positive charge on vanadium by 0.2 e. Photoelectron Spectra The low energy part of the spectrum as recorded with 532 nm photons and presented in Figure 1(a),19 reveals two ionization processes. The lowest band, labeled as the X band by the authors, is considerably more intense and exhibits a well-resolved vibrational progression between 1.4 eV and 1.85 eV. Its lowest experimentally detectable peak was thought to correspond to the vibrational 0-0 transition, allowing to propose an ADE of 1.42 eV. The A band, which appears as a second progression around 2 eV, is much less intense and broad by comparison to the former and possesses an ADE of 1.9 eV. As a first step in the assignment for these low energy bands an unambiguous determination of the ground state of the anionic cluster is of vital importance. For this purpose, the adiabatic relative energies of Table 4 are the most appropriate. B3LYP, RCCSD(T) and CASPT2 agree that 5B1 is the anionic ground state, albeit only slightly more stable than the 5A1. Rather suspiciously, a considerably larger energy difference of 0.16 eV between these two states was found with CASPT2 by using the small active space of 12 orbitals. Thus, additional and more elaborated CASPT2 calculations on the 5A1 and 5B1 states were performed by utilizing a larger active space (14 electrons in 14 orbitals). At this level an energy difference of 0.098 eV was obtained, which nearly equals the RCCSD(T) value. The above relative energies of the two quintet states are predicted by all methods employed to be smaller than 0.1 eV. Although this is within the 10 ACS Paragon Plus Environment

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error bars of the computational methods, the ground state of the anionic cluster is most likely the 5

B1. Indeed, it is predicted by all methods employed, except BP86, as the lowest state.

Correspondingly, under the reported experimental conditions,19 the nearly-degenerate state 5A1 is likely to be populated, implying that, this nearly-degenerate state could underlie bands in the photoelectron spectra of VC2-. Based upon the found 5B1 ground state of the anionic cluster VC2and the 14B1 neutral ground state, which electronically differ only by the singly occupied 4s4p vanadium lone pair (10a1), the assignment of the X band is straight forward: 5B1 → 14B1. According to Table 4, the computationally estimated ADE for this ionization at the B3LYP level is 1.37 eV, a slight underestimate compared with the experimental value of 1.42 eV. The RCCSD(T) ADE of 1.45 eV, which overestimates only by 0.03 eV the experimental result, is our best result. Also, the BP86 functional performs rather well (1.35 eV), while CASPT2 appears as the least accurate method in this particular case. The calculated ADE value of 1.29 eV by this method is too low compared to the experimental value and illustrates that as a second-order perturbation computational technique, it is less suited for retrieving the differential correlation energy between the anionic and neutral clusters. Due to the different number of electrons in these two molecular systems, a too small CASPT2 ionization energy is predictable. All these findings corroborate the earlier made assignment for this band, reached by using MRSDCI34 and TDDFT.35 Our assignment for the A band turned out to be much more difficult and will be found to be at variance with this previous MRSDCI study34 but in agreement with the TDDFT results.35 Indeed, based on the leading configurations as presented in Table 2, the previously proposed 14B2 state for this band does not have the proper configuration for being a one-electronic detachment from the 5B1 anionic ground state. Within the a1 representation it has two predominantly 3d orbitals singly occupied, i.e. 11a1 and 12a1, which is not the case in 5B1. Table 2 allows to deduce similar conclusions for the other low-lying quartet states with detachment energies in the region of 2 eV, besides the 14B1 which is already assigned to the X Band. Sextets are excluded because they are all positioned at more than 3 eV. Therefore, we have to invoke the 5A1 as the initial state being responsible for the A band. According to the B3LYP and RCCSD(T) results of Table 4, the neutral 14A1 is positioned at about 0.4 eV above 14B1, while Table 2 demonstrates that the transition 5A1 → 14A1 represents a one-electron detachment from the 4s4p lone pair orbital on vanadium (10a1). For this transition, B3LYP, RCCSD(T) and CASPT2 appear to reproduce the experimental ADE quite well. From Table 4 the values are 1.79 eV, 1.80 eV and 1.67 eV, respectively, which therefore are in reasonable agreement with the experimental ADE for the A 11 ACS Paragon Plus Environment


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band of 1.90 eV. These data already permit indubitably to attribute the A band to the transition 5

A1 → 14A1. As already mentioned, the intensity of the A band in the 532 nm spectrum (Figure

1(a)) is distinctly lower than that of the X band. This can be explained by the fact that all computational methods mentioned in Table 4 place the 5A1 at just a slightly higher energy than the ground state 5B1, which means that 5A1 as a very low lying excited state is to some extent populated in the experimental conditions under which the photoelectron spectra were recorded. In addition to these two already assigned bands in the previous paragraphs, two higher energy bands, labeled as B and C bands, in the energy range from 2.9 eV to 3.3 eV, are recognizable in the 355 nm spectrum of Figure 1(b).19 While the band B is more intense than the A band and exhibit comparable intensity as the X band, the C band is of lower intensity. This means that most likely the anionic 5B1 ground state must be the underlying initial state of the B band, and the C band originates from the 5A1. The experimental ADEs are 2.91 eV for the B band and 3.13 eV for the C band. The previous MRSDCI study34 attributed the C band to the 5B1 → 6B1 transition. However, based on our calculated ADEs and electronic wave functions for this transition, two potential candidates can be the final state underlying the C band. Indeed, the CASSCF leading configuration in Table 2 identifies the 6A1 and 6B1 as the results of a one-electron detachment from the 5A1 and from the 5B1, respectively. The RCCSD(T) and CASPT2 ADEs place the 6B1 higher than the 5B1 by a value of 3.38 eV and of 3.23 eV, while those of 6A1 are 2.99 eV and 3.00 eV (Table 4). Although these ionization energies are in reasonable agreement with the experimental ADE of 3.13 eV, the estimated intensities of these transitions should be different. As expected, if a transition originates from the anionic ground state 5B1, the intensity would be comparable to that of the X band (Figure 1). This, apparently, is not the case as the experimental intensity of the C band is substantially lower, and therefore the 6A1 is believed to be the most appropriate final state for the C band. Thus, the new assignment for this band is the 5A1 → 6A1 transition rather than the one from the previous study.34 For the more intense B band the 5B1 is beyond any doubt the initial state. With regard to the leading configurations listed in Table 2, there are three candidates for the final state. These are 34B1, 44B2 and 34A2, which at first glance correspond to ionization processes from the 11a1, 1a2 and 5b2, respectively. The CASPT2 calculations predict VDEs of respectively, 2.86 eV, 2.90 eV and 2.94 eV, all corresponding well with the experimental value of 2.97 eV. In support, RCCSD(T) calculates ionization energies for 34B1 and 34A2 as 3.12 eV and 3.02 eV, respectively. However, when thoroughly analyzing the electronic structures of these candidates, 34B1 appears to possess 12 ACS Paragon Plus Environment

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an electronic configuration that is the most in agreement with the one-electron detachment rule of photoelectron spectroscopy. Indeed, as depicted in Figure 6 the 11a1 orbitals of both 44B2 and 34A2 are quite different from 11a1 of 5B1.

These pronounced orbital relaxation effects in

combination with the ionization of an electron from either the 1a2 (44B2) and 5b2 (34A2) orbitals, makes that the corresponding transitions are definitely not pure one-electron detachments. In conclusion, the B band should be ascribed as a 5B1 → 34B1 ionization. It should be noted that this assignment is in contradiction with the previous MRSDCI study34, which on the basis of the calculated relative energies and B3LYP frequencies proposed a 4A2 and a 2B2 as possible states underlying the B band. From our CASSCF leading configurations for the four lowest 4A2 states in Table 2, not a single one has an appropriate configuration. Indeed, either the 12a1 (14A2 and 44A2) or 4b1 (24A2) orbitals are occupied (not occupied in 5B1) or there are pronounced relaxation effects in 11a1 (34A2, see previous paragraph), which prevents them of being one-electron detachment processes from 5B1. An ionization to 2B2 obviously violates the spin-selection rule and cannot be at the origin of an intense vibronic progression such as the B band in the 355 nm photoelectron spectrum of VC2-. It would most certainly be overshadowed by the allowed the 5B1 → 34B1 ionization. Our assignment of the B band to the 5B1 → 34B1 ionization is also in disagreement with the TDDFT study,35 which attributes this band to the 6B1.

Along the same line of reasoning, the D band of the 266 nm spectrum (Figure 1(c)) exhibits a comparable intensity as the X and B bands. This observation allows to ascribe this band to the 5

B1 → 6B1 transition, which is indeed fully allowed. The corresponding VDE is predicted to be 3.40

at the RCCSD(T) and CASPT2 levels, both underestimating the VDE by almost 0.3 eV. Because the E band was only partially observed and consequently no VDE is available to date, it is considered premature to attempt an assignment for this band.

Franck-Condon simulations In the 532 nm and 355 nm photoelectron spectra of VC2- (Figures 1(a) and (b)), there are two well-resolved vibrational progressions: the X band with 6 and the B band with 4 distinguishable peaks. In addition, three lesser defined peaks appear in the A band, and a broad range of irregular peaks are observable in the C band. The measured experimental vibrational frequencies of these bands are presented in Table 5. With the intension of substantiating our 13 ACS Paragon Plus Environment


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proposed assignments made in the previous paragraphs, the B3LYP accessible spectroscopic relevant states, i.e. 5B1, 5A1, 14B1, 14A1, 6B1 and 6A1, were subjected to a harmonic vibrational analysis with the purpose not only to calculate their frequencies but also the Franck-Condon factors. The latter are depicted in Figure 7 for the two electronic allowed ionizations. All three normal vibrational modes were incorporated in these simulations. Since the C2v symmetry was conserved during the ionization processes the asymmetric vibration mode was found not to give rise to an observable progression, as well as the symmetric C-C stretching mode due to the small changes calculated for this bond. The corresponding small FCF’s were omitted for clarity. The obtained intensity profile for the X band (5B1 → 14B1) in Figure 7(a) shows a nearly perfect match with the experimental profile in Figure 1(a) and is due to the totally symmetric V-(C2) stretching normal mode. Also the corresponding B3LYP harmonic frequency of 574 cm-1 (Table 5) falls within the experimental error bar of 550 (40) cm-1. The pronounced vibrational progression is the outcome of a substantial reduction of the V-C bond distance from 2.05 Å (5B1) to 1.93 Å (14B1). For the

5

A1→14A1 transition the calculated relative intensities are in disagreement with the

experimental silhouettes of the band A. In particular, the theoretical progression for 5A1→14A1 (Figure 7(b)) exhibits monotonically decreasing intensities with increasing energies, whereas in the 532 nm spectra this appears not to be the case for the A band. The smaller reduction of the V-C bond distances from 2.06 Å (5A1) to 1.98 Å (14A1) explains the calculated different profiles between the X and the A bands (Figures 7(a) versus Figure 7(b)). In any case, Table 5 shows that the B3LYP harmonic frequencies are within the experimental error bars for the A and C bands. Overall, it is possible to conclude that the presented vibrational analyses largely qualitatively (shapes) and quantitatively (frequencies) substantiate our proposed assignments.

Conclusions Electronic structures and geometries of VC2-/0 have been investigated at the B3LYP, BP86, RCCSD(T), CASSCF and CASPT2 levels of computation. The calculations proved that the cyclic isomer is the most stable form of both anionic and neutral vanadium dicarbide. Concerning the determination of the anionic ground state, the 5B1 and 5A1 states are nearly degenerate and are therefore competitive candidates for the initial states of the one-electron ionization observed in the photoelectron spectra of VC2-. However, the 5B1 is consistently predicted as the ground state. Beyond any doubt the 14B1 is the ground state of the neutral cluster. The electronic structures of these ground states and several spectroscopic relevant low-lying quartet states revealed that 14 ACS Paragon Plus Environment

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vanadium has an oxidation state of +1 in the anionic and +2 in the neutral cluster, whereas the C2 moiety always bears a charge of -2. A novel interpretation of the VC2- photoelectron spectra is proposed, which is based on the two nearly-degenerate initial states of the cyclic cluster VC2-. According to the calculated ionization energies and analyses of the wave functions, the lowest binding energy bands (X, A and B) are the results of one-electron photodetachment from a dominant vanadium orbital. In particular, the X band is ascribed to the 5B1 → 14B1 ionization process. The experimental A band, exhibiting a weak intensity, is assigned to the 5A1 → 14A1 transition. The assignment of the second well-resolved progression, the B band, is ascribed to the 5

B1 → 34B1 ionization. For the higher binding energy bands in the spectra, i.e. the C and D bands,

all correspond to a removal of an electron from the highest doubly occupied C22- ligand 3σg orbital, resulting in the 5A1 → 6A1 and 5B1 → 6B1, respectively. For the well-resolved X band, the assignment is reconfirmed by a nearly perfect match between relative Franck-Condon factors and experimental intensities, as well as the number of significant peaks in this vibrational progression. The calculated harmonic frequencies provided corroborating evidence for the proposed assignments of the vibrational resolved bands.

Acknowledgements: Technical support of Mrs Tran Dieu Hang is greatly appreciated.

Authors Information: * Emails: [email protected] Telephone: +32-16327362



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Table 1. Determination of the anionic and neutral ground state by geometry optimizations at the B3LYP and BP86 computational levels and RCCSD(T) single point calculations at the B3LYP geometries. Aug-cc-pwCVTZ-DK basis sets used and scalar relativistic effects included.

Clusters

States 5

A1 B1 5 B2 5 A2 5 A1 5 B1 5 B2 5 A2 4 A1 4 B1 4 B2 4 A2 5

Cyclic VC2-

Linear VC2-

Cyclic VC2

B3LYP Geometry (Å) V-C C-C 2.060 1.272 2.042 1.273 2.126 1.259 2.118 1.260 2.025 1.259 1.944 1.264 1.944 1.264 1.972 1.268 1.981 1.276 1.930 1.287 2.013 1.258 2.011 1.266

Relative energies (eV) B3LYP BP86 RCCSD(T) 0.04 -0.01 0.09 0.00 0.00 0.00 0.31 0.46 0.34 0.44 0.68 0.34 1.35 1.34 0.59 0.29 0.30 0.59 0.29 0.30 0.59 0.69 0.79 0.81 0.47 0.69 0.43 0.00 0.00 0.00 0.68 0.84 0.76 0.61 0.84 0.59


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Table 2. CASSCF leading configurations and RCCSD(T)/aug-cc-pwCV5Z-DK and CASPT2/ANO-RCC vertical detachment energies (VDE) from the 5B1 initial state at its B3LYP geometry (Table 1). Cluster

State 5

VC2-

VC2

8a12 9a12 10a11 11a10 12a10 1a21 5 B1 8a12 9a12 10a11 11a11 12a10 1a21 4 1 A1 8a12 9a12 10a10 11a10 12a10 1a21 4 2 A1 8a12 9a11 10a11 11a10 12a10 1a21 4 3 A1 8a12 9a11 10a11 11a10 12a10 1a21 4 4 A1 8a11 9a12 10a11 11a10 12a10 1a21 4 1 B1 8a12 9a12 10a10 11a11 12a10 1a21 24B1 8a12 9a12 10a10 11a10 12a11 1a21 4 3 B1 8a12 9a12 10a11 11a10 12a10 1a21 4 4 B1 8a12 9a12 10a10 11a11 12a11 1a20 4 1 B2 8a12 9a12 10a10 11a11 12a11 1a20 4 2 B2 8a12 9a12 10a10 11a10 12a11 1a21 34B2 8a12 9a12 10a11 11a10 12a10 1a21 4 4 B2 8a12 9a12 10a11 11a11 12a10 1a20 1 4 A2 8a12 9a12 10a10 11a11 12a11 1a21 4 2 A2 8a12 9a12 10a10 11a11 12a10 1a20 4 3 A2 8a12 9a12 10a11 11a11 12a10 1a21 4 4 A2 8a12 9a12 10a11 11a10 12a11 1a21 6 A1 8a12 9a11 10a11 11a10 12a10 1a21 6 B1 8a12 9a11 10a11 11a11 12a10 1a21 6 B2 8a12 9a11 10a11 11a11 12a10 1a21 6 A2 8a12 9a11 10a11 11a11 12a10 1a20 (*) Vertical energies with respect to 5A1. A1

3b12 4b11 5b21

RCCSD(T) 0.09

VDE (eV) CASPT2 0.16

3b12 4b10 5b21

0.00

0.00

3b12 4b11 5b21

1.86

2.05(*)

1.96 (A)

5

A1 → 14A1

3b12 4b11 5b21

-

4.19(*)

>3.90 (E)

5

A1 → 24A1

3b12 4b11 5b21

-

4.36

3b12 4b11 5b21

-

5.93

3b12 4b10 5b21

1.62

1.65

1.56 (X)

5

B1 → 1 4 B1

3b12 4b10 5b21

-

2.36

3b12 4b10 5b21

3.12

2.86

2.97 (B)

5

B1 → 3 4 B1

3b12 4b11 5b20

-

2.90

3b12 4b10 5b21

-

2.10

3b12 4b11 5b20

-

2.59

3b12 4b11 5b20

-

2.76

3b12 4b10 5b21

-

2.90

3b12 4b10 5b20

-

2.23

3b12 4b11 5b21

-

2.31

3b12 4b10 5b20

3.02

2.94

3b12 4b10 5b20

-

4.02

3b12 4b11 5b21

3.15

3.15(*)

3.13 (C)

5

A1 → 6 A1

3b12 4b10 5b21

3.40

3.40

3.72 (D)

5

B1 → 6 B1

3b12 4b11 5b20

3.63

3.66

3b12 4b11 5b21

3.96

3.81

Leading configuration

Ionization Exp.19,20



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Table 3. Atomic charges of vanadium and carbon analyzed at the CASSCF level States

Mulliken charge (e) V C C +0.50 -0.75 -0.75 +0.30 -0.65 -0.65 +0.78 -0.39 -0.39 +0.68 -0.34 -0.34 +0.46 -0.23 -0.23 +0.44 -0.22 -0.22

5

A1 B1 14A1 14B1 6 A1 6 B1 5

AIM charge (e) V C C +0.62 -0.81 -0.81 +0.60 -0.80 -0.80 +1.38 -0.69 -0.69 +1.26 -0.63 -0.63 +0.80 -0.40 -0.40 +0.80 -0.40 -0.40

Table 4. Adiabatic detachment energies (ADE) calculated at the B3LYP, BP86 and RCCSD(T) levels by employing aug-cc-pwCV5Z-DK basis sets, and at the CASPT2 level employing ANO-RCC basis sets. B3LYP geometries of Table 1 used. Cluster

State 5

VC2-

VC2

A1 B1 1 4 A1 14B1 6 A1 6 B1 6 B2 6 A2 5

B3LYP 0.04 0.00 1.79 1.37 3.24 2.92 3.08 2.93

BP86 -0.01 0.00 2.05 1.35 3.16 3.07 3.45 3.30

ADE (eV) RCCSD(T) 0.09 0.00 1.80 1.45 2.99 3.38 3.26 3.15

CASPT2 0.16 0.00 1.67 1.29 3.00 3.23 3.29 3.16

Exp.19,20

1.90 (A) 1.42 (X) 3.13 (C) -

Table 5. The B3LYP/def2-TZVP equilibrium geometries and harmonic vibrational frequencies for the initial states 5B1 and 5A1 and the final states, employed to calculate Frank-Condon factors. Comparison with experimental frequencies. State 5

A1 B1 4 1 A1 14B1 6 A1 6 B1 5

B3LYP Geometry (Å) V-C C-C 2.06 1.27 2.05 1.27 1.98 1.28 1.93 1.29 2.21 1.27 2.18 1.27

B3LYP Frequencies (cm-1) 210, 479, 1767 373, 489, 1767 319, 553,1743 375, 574,1691 265, 396, 1797 365, 394, 1770

Band

A X C D

Experimental Frequencies19 (cm-1)

520(50) 550(40) 370 (40)

Ionization

5

A1 → 14A1 5 B1 → 1 4 B1 5 A1 → 6 A1 5 B1 → 6 B1


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 1. Experimental photoelectron spectra of VC2- as recorded with 532, 355 and 266 nm detachment photons according to Reference 19 (a) and (b) (reproduced from Reference 19. Copyright 1999, AIP Publisher, License Number 3971890628646), and (c) (reproduced from reference 20, copyright 2016, ACS). Underlying neutral electronic states according to the novel proposed assignment. 20 ACS Paragon Plus Environment

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Figure 2. Calculated cyclic and linear VC2-/0 isomers and coordinate systems used.



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Figure 3. Determination of the spin-multiplicities and structures of the ground states of the VC2-/0 clusters. Values in parentheses are B3LYP/def2-TZVP level relative energies in eV.


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Figure 4. The CASSCF pseudonatural orbitals of the cyclic ground state 5B1 and their occupation numbers in parentheses.



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Figure 5. The CASSCF pseudonatural orbitals of the cyclic ground state 5A1 and their occupation numbers in parentheses.


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Figure 6. Plots of the singly occupied orbitals of 5B1, 44B2, 34A2 and 34B1 states. Large relaxation effects between 11a1 of the 5B1 anionic ground state and the 11a1 orbitals of the 34A2 and 44B2 neutral states ruling out the corresponding ionizations as pure one-electron detachment processes.



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Figure 7. Franck-Condon factor simulations for the X band (a) and the A band (b). Abscissa vibrational transition in wavenumbers and ordinate intensities in arbitrary units.


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References. (1)

Simard, G. L.; Steger, J. F.; Arnott, R. J.; Siegel, L. A. Vanadium Oxides as Oxidation Catalysts. Ind. Eng. Chem. 1955, 47, 1424–1430.

(2)

Nomura, K.; Zhang, S. Design of Vanadium Complex Catalysts for Precise Olefin Polymerization. Chem. Rev. 2010, 111, 2342–2362.

(3)

Hlatky, G. G. Metallocene Catalysts for Olefin Polymerization. Annual Review for 1996. Coord. Chem. Rev. 1999, 181, 243–296.

(4)

Nomura, K.; Sagara, A.; Imanishi, Y. Olefin Polymerization and Ring-Opening Metathesis Polymerization of Norbornene by (Arylimido)(aryloxo)vanadium(V) Complexes of the Type VX2(NAr)(OAr‘). Remarkable Effect of Aluminum Cocatalyst for the Coordination and Insertion and Ring-Opening Metathesis. Macromolecules 2002, 35, 1583–1590.

(5)

Xu, D.; Zhu, W.; Li, H.; Zhang, J.; Zou, F.; Shi, H.; Yan, Y. Oxidative Desulfurization of Fuels Catalyzed by V2O5 in Ionic Liquids at Room Temperature. Energy and Fuels 2009, 23, 5929– 5933.

(6)

Mizuno, N.; Kamata, K. Catalytic Oxidation of Hydrocarbons with Hydrogen Peroxide by Vanadium-Based Polyoxometalates. Coordination Chemistry Reviews. 2011, 255, 2358– 2370.

(7)

Hirao, T. Vanadium in Modern Organic Synthesis. Chemical Reviews 1997, 97, 2707–2724.

(8)

Licini, G.; Conte, V.; Coletti, A.; Mba, M.; Zonta, C. Recent Advances in Vanadium Catalyzed Oxygen Transfer Reactions. Coordination Chemistry Reviews. 2011, 255, 2345–2357.

(9)

Choi, J. G. Ammonia Decomposition over Vanadium Carbide Catalysts. J. Catal. 1999, 182, 104–116.

(10)

Kwon, H.; Thompson, L. T.; Eng Jr, J.; Chen, J. G. N-Butane Dehydrogenation over Vanadium Carbides: Correlating Catalytic and Electronic Properties. J. Catal. 2000, 190, 60–68.

(11)

Rodríguez, P.; Brito, J. L.; Albornoz, A.; Labadí, M.; Pfaff, C.; Marrero, S.; Moronta, D.; Betancourt, P. Comparison of Vanadium Carbide and Nitride Catalysts for Hydrotreating. Catal. Commun. 2004, 5, 79–82.

(12)

Aghaie-Khafri, M.; Fazlalipour, F. Vanadium Carbide Coatings on Die Steel Deposited by the Thermo-Reactive Diffusion Technique. J. Phys. Chem. Solids 2008, 69, 2465–2470.

(13)

Chicco, B.; Borbidge, W. E.; Summerville, E. Experimental Study of Vanadium Carbide and Carbonitride Coatings. Mater. Sci. Eng. A 1999, 266, 62–72.

(14)

Ferro, D.; Rau, J. V.; Generosi, A.; Rossi Albertini, V.; Latini, A.; Barinov, S. M. Electron Beam Deposited VC and NbC Thin Films on Titanium: Hardness and Energy-Dispersive X-Ray Diffraction Study. Surf. Coatings Technol. 2008, 202, 2162–2168.

(15)

Kalemos, A.; Dunning, T. H.; Mavridis, A. The Electronic Structure of Vanadium Carbide, VC. J. Chem. Phys. 2005, 123, 014301.

(16)

Kerkines, I.; Mavridis. Electronic Structure of Vanadium and Chromium Carbide Cations, VC+ and CrC+. Ground and Low-Lying States. Mol. Phys. 2004, 102, 2451–2466. 27 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

(17)

Zhukov, V. P.; Gubanov, V. A.; Jepsen, O.; Christensen, N. E.; Andersen, O. K. Calculated Energy-Band Structures and Chemical Bonding in Titanium and Vanadium Carbides, Nitrides and Oxides. Journal of Physics and Chemistry of Solids. 1988, 49, 841–849.

(18)

Chen, J. G.; Kirn, C. M.; Frühberger, B.; DeVries, B. D.; Touvelle, M. S. A NEXAFS Determination of the Oxidation State of Vanadium Carbide on V(110): Observation of Charge Transfer from Vanadium to Carbon. Surface Science. 1994, 321, 145–155.

(19)

Li, X.; Wang, L. S. Electronic Structure and Chemical Bonding between the First Row Transition Metals and C2-: A Photoelectron Spectroscopy Study of MC2- (M=Sc, V, Cr, Mn, Fe, and Co). J. Chem. Physics. 1999, 111, 8389–8395.

(20)

Yuan, J.; Wang, P.; Hou, G.-L.; Feng, G.; Zhang, W.-J.; Xu, X.-L.; Xu, H.-G.; Yang, J.; Zheng, W.J. Structural Evolution and Electronic Properties of VnC20/- and VnC40/- (n= 1-6) Clusters: Insights from Photoelectron Spectroscopy and Theoretical Calculations. J. Phys. Chem. A 2016, 120, 1520–1528.

(21)

Tran, V. T.; Hendrickx, M. F. a. A CASPT2 Description of the Electronic Structures of FeO3-/0 in Relevance to the Anion Photoelectron Spectrum. J. Chem. Theory Comput. 2011, 7, 310– 319.

(22)

Tran, V. T.; Hendrickx, M. F. A. Description of the Geometric and Electronic Structures Responsible for the Photoelectron Spectrum of FeO4−. J. Chem. Phys. 2011, 135, 094505.

(23)

Hendrickx, M. F. A.; Anam, K. R. A New Proposal for the Ground State of the FeO- Cluster in the Gas Phase and for the Assignment of Its Photoelectron Spectra. J. Phys. Chem. A 2009, 113, 8746–8753.

(24)

Hendrickx, M. F. A.; Tran, V. T. On the Electronic and Geometric Structures of FeO2–/0 and the Assignment of the Anion Photoelectron Spectrum. J. Chem. Theory Comput. 2012, 8, 3089–3096.

(25)

Tran, V.; Hendrickx, M. Molecular Structures for FeS4(-/0) As Determined from an Ab Initio Study of the Anion Photoelectron Spectra. J Phys Chem A 2013, 117, 3227-3234.

(26)

Tran, T. Van; Hendrickx, M. F. a. Assignment of the Photoelectron Spectra of FeS3- by Density Functional Theory, CASPT2 and RCCSD(T) Calculations. J. Phys. Chem. A 2011, 115, 13956-13964.

(27)

Clima, S.; Hendrickx, M. F. A. Interpretation of the Photoelectron Spectra of FeS2- by a Multiconfiguration Computational Approach. J. Phys. Chem. A 2007, 111, 10988–10992.

(28)

Clima, S.; Hendrickx, M. F. A. Photoelectron Spectra of FeS- Explained by a CASPT2 Ab Initio Study. Chem. Phys. Lett. 2007, 436, 341–345.

(29)

Hendrickx, M. F. A.; Tran, V. T. Elucidating the Electronic Structures of the Ground States of the VO2–/0 Clusters: Synergism between Computation and Experiment. J. Chem. Theory Comput. 2014, 10, 4037–4044.

(30)

Tran, V. T.; Tran, Q. T.; Hendrickx, M. F. A. On the Multi-Reference Character of the LowLying States of the MnS−/0 Clusters by the NEVPT2 Assignment of the Anion Photoelectron Spectrum. Chem. Phys. Lett. 2015, 627, 121–125.

(31)

Tran, V. T.; Iftner, C.; Hendrickx, M. F. A. A New Interpretation of the Photoelectron Spectra of CrC2–. J. Phys. Chem. A 2013, 117, 5613–5619. 28 ACS Paragon Plus Environment

Page 29 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(32)

Tran, V. T.; Iftner, C.; Hendrickx, M. F. a. Quantum Chemical Study of the Electronic Structures of MnC2-/0 Clusters and Interpretation of the Anion Photoelectron Spectra. Chem. Phys. Lett. 2013, 575, 46–53.

(33)

Tran, V. T.; Hendrickx, M. F. A. Molecular and Electronic Structures of the NbC2−/0 Clusters through the Assignment of the Anion Photoelectron Spectra by Quantum Chemical Calculations. Chem. Phys. Lett. 2014, 609, 98–103.

(34)

Majumdar, D.; Roszak, S.; Balasubramanian, K. Electronic Structure and Spectroscopic Properties of Electronic States of VC2, VC2−, and VC2+. J. Chem. Phys. 2003, 118, 130-141.

(35)

Yong-Bo, Y.; Kai-Ming, D.; Yu-Zhen, L.; Chun-Mei, T. Assignment of Photoelectron Spectra of MC2 (M= V, Cr, Fe, and Co). Chinese Phys. Lett. 2006, 23, 1761-1764.

(36)

Weigend, F.; Ahlrichs, R. Balanced Basis Sets of Split Valence, Triple Zeta Valence and Quadruple Zeta Valence Quality for H to Rn: Design and Assessment of Accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297–3305.

(37)

TURBOMOLE V6.6 2014, a development of University of Karlsruhe and Forschungszentrum Karlsruhe GmbH, 1989-2007, TURBOMOLE GmbH, since 2007; available from http://www.turbomole.com (accessed November 3, 2016).

(38)

Balabanov, N. B.; Peterson, K. A. Systematically Convergent Basis Sets for Transition Metals. I. All-Electron Correlation Consistent Basis Sets for the 3d Elements Sc-Zn. J. Chem. Phys. 2005, 123, 064107.

(39)

Dunning Jr, T. H. Gaussian Basis Sets for Use in Correlated Molecular Calculations. I. The Atoms Boron through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007.

(40)

Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785–789.

(41)

Becke, A. D. Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior. Phys. Rev. A 1988, 38, 3098–3100.

(42)

Becke, A.; Becke, A. Density Functional Thermochemistry III The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648–5652.

(43)

Perdew, J. P. Density-Functional Approximation for the Correlation Energy of the Inhomogeneous Electron Gas. Phys. Rev. B 1986, 33, 8822–8824.

(44)

Werner, H.-J.; Knowles, P. J.; Knizia, G.; Manby, F. R.; Schütz, M.; others. MOLPRO, Version 2012.1, a Package of Ab Initio Programs. Cardiff, UK 2012.

(45)

Reiher, M.; Wolf, A. Exact Decoupling of the Dirac Hamlltonian. II. The Generalized DouglasKroll-Hess Transformation up to Arbitrary Order. J. Chem. Phys. 2004, 121, 10945–10956.

(46)

Andersson, K.; Malmqvist, P. A.; Roos, B. O.; Sadlej, A. J.; Wolinski, K. Second-Order Perturbation Theory with a CASSCF Reference Function. J. Phys. Chem. 1990, 94, 5483– 5488.

(47)

Knowles, P. J.; Hampel, C.; Werner, H.-J. Coupled Cluster Theory for High Spin Open Shell Reference Wavefunctions. J. Chem. Phys. 1993, 99, 5219–5227.

(48)

Watts, J. D.; Gauss, J.; Bartlett, R. J. Coupled-Cluster Methods with Noniterative Triple Excitations for Restricted Open-Shell Hartree–Fock and Other General Single Determinant 29 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 31

Reference Functions. Energies and Analytical Gradients. J. Chem. Phys. 1993, 98, 8718. (49)

Roos, B. O.; Lindh, R.; Malmqvist, P.-Å.; Veryazov, V.; Widmark, P.-O. New Relativistic ANO Basis Sets for Transition Metal Atoms. J. Phys. Chem. A 2005, 109, 6575–6579.

(50)

Roos, B. O.; Lindh, R.; Malmqvist, P. Å.; Veryazov, V.; Widmark, P. O. Main Group Atoms and Dimers Studied with a New Relativistic ANO Basis Set. J. Phys. Chem. A 2004, 108, 2851– 2858.

(51)

Aquilante, F.; De Vico, L.; Ferré, N.; Ghigo, G.; Malmqvist, P. Å.; Neogrády, P.; Pedersen, T. B.; Pitoňák, M.; Reiher, M.; Roos, B. O.; et al. Software News and Update MOLCAS 7: The next Generation. J. Comput. Chem. 2010, 31, 224–247.

(52)

Borrelli, R.; Capobianco, A.; Peluso, A. Franck-Condon Factors-Computational Approaches and Recent Developments. Can. J. Chem. 2013, 91, 495-504.


Page 31 of 31

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Contributions of Nearly-Degenerate States to the Photoelectron

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