0 Clusters from

Article pubs.acs.org/JPCA

Geometrical and Electronic Structures of MnS3−/0 Clusters from Computational Chemistry and Photoelectron Spectroscopy Van Tan Tran*,† and Quoc Tri Tran† †

Theoretical and Physical Chemistry Division, Dong Thap University, 783-Pham Huu Lau, Ward 6, Cao Lanh City, Dong Thap Vietnam ABSTRACT: The B3LYP functional and CASPT2 method have been applied to investigate the geometrical and electronic structures of η2(S2)MnS−/0, η2-(S3)Mn−/0, and MnS3−/0 isomers of MnS3−/0 clusters. The ground state of the anionic cluster is computed to be the 5B2 of η2-(S2)MnS− isomer, whereas that of the anionic cluster is calculated to be the 4B1 of the same isomer. The photoelectron spectrum of MnS3− cluster is interpreted by electron detachment processes from the most stable η2-(S2)MnS− and from the metastable η2-(S3)Mn− and MnS3− isomers. The first and second bands with low intensities are, respectively, attributed to the 7A′ → 6A′ and 7A′ → 8 A′ transitions within the η2-(S3)Mn−/0 isomers. The third band with the highest intensity in the spectrum can be assigned to the 5B2 → 4B1, 5B2 → 6 B1, and 5B2 → 4A2 transitions within the most stable η2-(S2)MnS−/0 isomers, the 3B1 → 2A1 transition within the metastable MnS3−/0 isomers, and the 7A′ → 36A′ transition within the metastable η2-(S3)Mn−/0 isomers. Because the η2-(S2)MnS− is computed to be the most stable isomer of the MnS3− cluster, we believe that the highest intensity third band in the spectrum is mainly the result of electron detachments from this isomer.

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detachment energies, the first band in the spectrum was assigned to the detachment of one electron from the metastable η2-(S3)Mn−, the third band was assigned to the removal of one electron from the most stable D3h MnS3−, and the second band was not explained. In order to understand which electron detachment happens under the second band, more quantum chemical calculations need to be done. From the experience on the geometrical and electronic structural of FeS3−/0 clusters, we feel that the previous calculations7 do not cover all the important isomers of MnS3−/0 clusters. For the FeS3− cluster, the computational results show that the most stable isomer is not the D3h FeS3− isomer but the η2-(S2)FeS− isomer which contains a side-on S2 and an atomic S ligand bound to Fe.4 Although the D3h MnS3− was calculated to be the most stable isomer of MnS3− cluster by the BLYP functional in the previous work, the relative stability of η2-(S2)MnS− isomer as compared to D3h MnS3− was not reported. Therefore, quantum chemical calculations should be performed for η2-(S2)MnS− isomer in order to obtain the relative stability of this isomer. In this work, the geometrical and electronic structures of η2(S2)MnS−/0, MnS3−/0, and η2-(S3)Mn−/0 isomers of MnS3−/0 clusters are investigated by quantum chemical calculations. Because of the complicated electronic structures of the studied clusters, density functional and CASPT2 (complete active space

INTRODUCTION The geometrical and electronic structures of transition metal− sulfur clusters have been known to be complicated due to the near degeneracy of the low-lying electronic states and the nearly equal stability of different isomers. These properties can be seen in many clusters such as FeSn−/0 (n = 1−4), Fe2S2−/0/+/2+, and MnnSm−/0 (n = 1−10, m = 1−10).1−8 In this work, we are interested in the MnS3−/0 clusters because the geometrical and electronic structures of these clusters have been probed for a long time by anion photoelectron spectroscopy, but the observed spectrum is not fully understood.7,8 The photoelectron spectrum of MnS3− was measured with a photon energy of 355 nm, and three bands were reported.8 Accordingly, the first band with low intensity starts at 1.56 eV, whereas the second band begins at around 2.60 eV. The first and the second bands of the spectrum are very broad and may contain several vibrational progressions. The third band begins at around 3.30 eV and has the highest intensity in the photoelectron spectrum of MnS3−. Because the photon energy used to detach electron is not high enough, only part of the third band is observed. The photoelectron spectrum of MnS3− cluster was explained on the basis of the BLYP functional calculation results.7 Two isomers of MnS3−cluster were predicted to contribute to the spectrum. The first isomer is η2-(S3)Mn−, which contains a S3 ligand side-on bound to Mn. Furthermore, the second isomer is D3h MnS3−, which comprises three atomic S moieties bound to Mn. The BLYP calculation results showed that D3h MnS3− is the most stable isomer. On the basis of the BLYP adiabatic © XXXX American Chemical Society

Received: March 14, 2016 Revised: April 28, 2016

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DOI: 10.1021/acs.jpca.6b02631 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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Figure 1. Geometrical structures and coordinate systems for the η2(S2)MnS−/0, MnS3−/0, and η2-(S3)Mn−/0 isomers.

use C2v symmetry for η2-(S2)MnS−/0 and MnS3−/0, and Cs symmetry for η2-(S3)Mn−/0 isomers. Geometrical structures of the low-lying states of MnS3−/0 clusters were optimized with density functional theory (DFT). The DFT calculations utilized B3LYP15−17 functional and 6311++G(3df,3pd)18,19 basis sets for both Mn and S in an unrestricted way. Harmonic vibrational frequency calculations were performed to make sure the optimized structures correspond to minima on potential energy surfaces. The calculated vibrational frequencies and normal modes were used in Franck−Condon factor simulations. The density functional theory calculations were carried out with ORCA 3.01 package,20 whereas Franck−Condon factor simulations were performed with MOLFC 2.3.7 code.21 The electronic structures of low-lying states of MnS3−/0 clusters were investigated with CASPT2 method. Single-point CASPT2 calculations were performed on the basis of the B3LYP optimized geometries. The ANO-RCC basis sets with a contraction of 7s6p4d3f2g for Mn22 and of 6s5p3d2f for S23 are employed. The scalar-relativistic effects were included in CASPT2 calculations through the second-order Douglas− Kroll Hamiltonian.24−26 In order to eliminate intruder states, we applied an imaginary shift of 0.1.27 Also, an IPEA shift of 0.25 was used in the CASPT2 calculations. Otherwise, Cholesky decomposition with a threshold of 10−5 au was implemented to reduce the memory needed for storing twoelectron integrals.28−30 The orbitals for CASPT2 were obtained from CASSCF calculations with an active space of 19 or 20 electrons in 15 orbitals. Such active space has been tested to perform well for FeO3−/0 and FeS3−/0 clusters. All the CASSCF/CASPT2 calculations were done with MOLCAS@ UU 8.0.31

(S2)MnS−/0 isomers contain a side-on S2 moiety and an atomic S ligand bound to Mn. The MnS3−/0 isomers have three atomic S ligands bound to Mn. Additionally, the η2-(S3)Mn−/0 isomers comprise of a S3 moiety side-on bound to Mn. For convenience, all the computations carried out in this work

RESULTS AND DISCUSSION Most Stable Isomer of MnS3−/0 Clusters. The relative energies (RE’s) of the low-lying states of η2-(S2)MnS−/0, η2(S3)Mn−/0, and MnS3−/0 isomers as calculated with B3LYP

second-order perturbation theory) methods are employed. The B3LYP functional has been tested to perform well for many transition metal−sulfur clusters,2−5,9 and therefore, this functional is used for MnS3−/0 clusters. Also, the CASPT2 method has been widely known to be suitable to investigate the complicated electronic structures of transition-metal-containing clusters.2−5,10−14 On the basis of the computational results, we propose assignments for all features in the photoelectron spectrum of MnS3− cluster. The Franck−Condon factor simulations have been known to be sufficient to produce the vibrational progressions which belong to one band in photoelectron spectra of transition-metal-containing clusters.4,5,10−13 In order to explain the shapes of the first two bands in the photoelectron spectrum of MnS3− cluster, Franck−Condon simulations will be performed on the basis of B3LYP geometries, harmonic vibrational frequencies, and normal coordinates.

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COMPUTATIONAL METHODS The coordinate systems of η2-(S2)MnS−/0, η2-(S3)Mn−/0, and MnS3−/0 isomers are showed in Figure 1. As can be seen, the η2-

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Table 1. Geometrical Structures, Vibrational Frequencies, and Relative Energies (RE’s) of the Low-Lying States of MnS3−/0 Clusters As Computed with B3LYP Functional isomer η2-(S2)MnS−

state 5

B2 B1 5 A2 4 B1 4 B2 6 B1 4 A2 6 B2 3 B1 3 A2 5 A1 1 A1 2 A1 4 A″, Cs (4A2, C2v) 4 B1 7 A′ 5 A′ 6 A′ 8 A′ 5

η2-(S2)MnS

MnS3−

MnS3

η2-(S3)Mn− η2-(S3)Mn

structure parameters (r1 (Å), r2 (Å), θ (deg), φ (deg)) 2.134, 2.267, 2.150, 2.124, 2.225, 2.132, 2.169, 2.226, 2.074, 2.110, 2.092, 2.003, 2.038, 2.024, 2.024, 2.431, 2.382, 2.294, 2.570,

2.306, 2.312, 2.540, 2.379, 2.407, 2.404, 2.249, 2.434, 2.087, 2.074, 2.092, 2.003, 2.036, 2.036, 2.050, 2.114, 2.127, 2.124, 2.019,

54.66 56.83 47.02 49.87 49.93 49.40 54.95 49.32 123.20 117.59 120.00 120.00 210.00 2.069, 107.29, 125.67 107.22 85.18, 179.79 89.44, 163.70 94.23, 126.82 80.09, 179.87 B

RE (eV)

frequencies (cm−1)

0.00 0.72 0.71 3.05 3.10 3.12 3.35 3.44 1.06 1.06 1.18 1.48 4.43 4.82 4.82 1.50 1.76 3.11 3.97

73, 96, 267, 300, 437, 512 72, 72, 235, 333, 354, 451 62, 72, 193, 228, 459, 565 60, 72, 241, 242, 434, 586 91, 98, 218, 227, 362, 553 58, 69, 246, 260, 432, 583 48, 63, 249, 279, 435, 533 50, 56, 209, 244, 398, 564 103, 142, 172, 339, 396, 500 100, 134, 155, 315, 472, 506 70, 98, 99, 361, 362, 386 121, 173, 174, 458, 589, 590 84, 151, 152, 359, 462, 464 38, 66, 111, 146, 378, 542 48, 82, 115, 117, 376, 537 62, 191, 256, 308, 456, 457 54, 175, 263, 319, 442, 446 103, 190, 325, 336, 452, 502 103, 176, 180, 292, 523, 527 DOI: 10.1021/acs.jpca.6b02631 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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Table 2. Relative Energies (RE) of the Low-Lying States of MnS3−/0 Clusters and Adiabatic and Vertical Detachment Energies (ADE’s and VDE’s) of the Anionic Cluster As Calculated at CASPT2 Levela isomer η -(S2)MnS 2

state −

5

B2 B1 5 A2 4 B1 4 B2 6 B1 4 A2 6 B2 3 B1 3 A2 5 A1 3 a A1 1 A1 2 A1 2 a A2 2 a B1 4 a A2 4 B1 7 A′ 5 A′ 16A′ 26A′b 36A′b 6 A″b 8 A′ 5

η2-(S2)MnS

MnS3−

MnS3

η2-(S3)Mn− η2-(S3)Mn

leading configuration

weight (%)

RE (eV)

18a1119a1120a107b1111b202a223a21 18a1119a1120a107b1011b212a223a21 18a1119a1120a107b1111b212a223a20 18a1119a1120a107b1111b202a223a20 18a1119a1120a107b1011b212a223a20 18a1119a1120a107b1111b202a213a21 18a1119a1120a107b1011b202a12 3a21 18a1119a1120a107b1011b212a21 3a21 18a1119a1020a107b1111b2212b202a223a20 18a1119a1020a107b1011b2212b202a223a21 18a1119a1020a107b1111b2112b202a223a21 18a1119a1020a107b1111b2112b202a223a21 18a1219a1020a107b1011b2212b202a223a20 18a1119a1020a107b1011b2212b202a223a20 18a1119a1020a107b1111b2112b202a223a20 18a1119a1020a107b1011b2112b202a223a21 18a1119a1020a107b1111b2112b202a223a20 18a1119a1020a107b1011b2112b202a223a21 2 1 1 1 1 1 1

68 45 34 24 18 56 42 43 53 52 60 55 69 54 48 38 49 55 96 40 95 76 66 54 94

0.00 0.87 0.93 3.10 3.57 3.23 3.24 3.66 0.43 0.48 0.84 0.66 0.82 3.58 4.12 4.14 4.30 4.26 1.44 2.11 3.30 4.26 4.87 5.25 3.97

22a′ 23a′ 24a′ 25a′ 26a′ 13a″ 14a″ 22a′223a′224a′125a′126a′013a″114a″1 22a′223a′124a′125a′126a′013a″114a″1 22a′123a′124a′125a′126a′113a″114a″1 22a′123a′124a′125a′126a′113a″114a″1 22a′223a′124a′125a′126a′013a″114a″0 22a′123a′124a′125a′126a′113a″114a″1

orb.c

ADE (eV)

VDE (eV)

expt. (eV)

3a2

3.10

3.11

3.30

2a2 7b1

3.23 3.24

3.26 3.28

3.30 3.30

7b1 11b2

3.15

3.24 3.69

3.30

26a′ 22a′ 22a′ 14a″ 22a′

1.86

2.30 2.82 3.43 3.81 2.75

1.56, 2.18

2.53

3.30 2.60

Single-point energies as computed with the geometry of 3A2 of MnS3− isomer. bSingle-point energies as computed with the geometry of 7A′ of η2(S3)Mn− isomer. cThe orbital in which one electron is detached.

a

η2-(S3)Mn isomer and is much more stable than the MnS3 isomer. The relative stability of η2-(S2)MnS−/0, η2-(S3)Mn−/0, and MnS3−/0 isomers as calculated at B3LYP and CASPT2 levels are completely new as compared to the previous results. The previous BLYP calculations do not investigate the η2-(S2)MnS−/0 isomers,7 whereas in this work, these isomers are computed to be the most stable isomers of MnS3−/0 clusters. This result brings us to an opportunity to explain the main features in the photoelectron spectrum of MnS3− cluster by using the 5B2 of η2-(S2)MnS− isomer as initial state for electron detachments. Electronic Structures of MnS3−/0 Clusters. Figure 2 presents the CASSCF molecular orbitals and electron occupation numbers of the 5B2 ground state of η2-(S2)MnS− isomer. This figure shows that for the predominantly Mn orbitals, the 18a1(3dx2), 19a1(3dy2−z2), 7b1(3dxz), and 3a2(3dxy) orbitals are all singly occupied, while the 20a1 (4s) and 11b2 (3dyz) are unoccupied. Because the Mn atom has an electron configuration of [Ar]3d54s2, this atom loses three electrons to the ligands in the 5B2 of η2-(S2)MnS−. Therefore, an oxidation state of +3 is proposed for Mn. On the other hand, the electron configuration of S atom is [Ne]3s23p4, and all three 3p orbitals of atomic S ligand in the 5B2 of η2-(S2)MnS−, namely, the 16a1, 6b1, and 10b2 are doubly occupied, so we have in this case an S2− ligand. Because the molecular S2 ligand has five fully occupied orbitals (15a1, 17a1, 5b1, 9b2, and 2a2) and one unoccupied orbital (12b2), this ligand has an oxidation state of −2. Overall, in the 5B2 of η2-(S2)MnS−, the oxidation state of Mn is +3, while that of S and S2 ligands are −2.

functional are presented in Table 1. As can be seen in this table, the most stable isomer of anionic cluster is the η2-(S2)MnS− with a ground state of 5B2. This quintet has Mn−S bond distances of 2.134 and 2.306 Å and a SMnS bond angle of 54.66°. Around 1 eV above this quintet ground state, our results show the 3B1, 3A2, and 5A1 states of MnS3− isomer with relative energies of 1.06, 1.06, and 1.18 eV, respectively. The 7 A′ of η2-(S3)Mn− is 1.50 eV less stable than the anionic ground state. Overall, we can say that the stability of MnS3− cluster decreases from the η2-(S2)MnS− through MnS3− to η2-(S3)Mn− isomer. The CASPT2 results as presented in Table 2 also confirm the B3LYP stability order of η2-(S2)MnS−, MnS3−, and η2-(S3)Mn− isomers. For the neutral cluster, the B3LYP functional predicts the 4B1 of η2-(S2)MnS to be the ground state. As compared to the anionic ground state, this quartet has a relative energy of 3.05 and 3.10 eV as calculated at B3LYP and CASPT2 levels, respectively. The B3LYP geometry optimization gives Mn−S bonds of 2.124 and 2.379 Å, and SMnS bond angle of 49.87° for this quartet. Just above the neutral 4B1 ground state about 0.05−0.07 eV is the 4B2 and 6B1 of the same isomer. Moreover, the η2-(S3)Mn is calculated to be slightly less stable than the 4 B1 of η2-(S2)MnS. Particularly, the 6A′ is placed at 0.06 and 0.20 eV above the 4B1 ground state at B3LYP and CASPT2 levels, respectively. Also, the 2A1 of MnS3 isomer is computed to be as much as 1.38 eV less stable than the 4B1 of η2-(S2)MnS by B3LYP method. At the CASPT2 level, the energy difference between these two states is reduced to 0.48 eV, but the 2A1 of MnS3 isomer is still less stable. In general, for the neutral cluster we can say that the η2-(S2)MnS is slightly more stable than the C

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Figure 3. Pseudonatural CASSCF molecular orbitals and occupation numbers of the 7A′ state of η2-(S3)Mn− isomer. Figure 2. Pseudonatural CASSCF molecular orbitals and occupation numbers of the 5B2 state of η2-(S2)MnS− isomer.

The electron leading configurations of MnS3−/0 clusters are collected in Table 2. From this table, we can see that the ground and some excited states of η2-(S2)MnS isomer can be obtained by electron detachments from the 5B2 of η2(S2)MnS−. Particularly, the 4B1 ground state can be obtained by the removal of one electron from the 3a2 (3dxy) orbital of 5 B2 of η2-(S2)MnS−. As a result, the oxidation state of Mn increases to +4 while that of ligands remains unchanged. The 6 A1 and 4A2 excited states of η2-(S2)MnS isomer are formed by the detachments of one electron from the 2a2 and 7b1 orbital, respectively. It should be noted that the reference weight of the neutral 4B1 ground state is only 24%, which implies a very strong multireference character in the wave function of this state. The molecular orbitals of 7A′ η2-(S3)Mn− isomer is presented in Figure 3 in which all the predominantly 3d and 4s orbitals of Mn are singly occupied. These orbitals are labeled as the 23a′ (4s), 24a′ (3dx2−y2), 25a′ (3dyz), 26a′ (3dz2), 13a″ (3dxz), and 14a″ (3dxy) in Figure 3. In nine 3p orbitals of S3 ligand, seven orbitals (19a′, 20a′, 21a′, 22a′, 10a″, 11a″, and 12a″) are fully occupied while two orbital (27a′ (σ*) and 15a″ (σ*)) are unoccupied. It means that Mn has an oxidation state of +1 and the S3 ligand has an oxidation state of −2. A detachment of one electron from the 26a′ (3dz2) will result in the 6A′ state of η2-(S3)Mn isomer, while a removal of one electron from the 22a′ orbital will form the 8A′. In Figure 4, we present the CASSCF molecular orbitals of the 3B1 of MnS3− isomer. It can be seen that all the 3p orbitals of the atomic S ligand are fully occupied, and therefore, all three atomic S ligands are doubly charged (S2−). On the other hand, the Mn atom has one electron in the 18a1 (3dx2) and one electron in the 7b1 (3dxz) orbital, while there is no electron in the 19a1 (3dy2−z2), 20a1 (4s), and 3a2 (3dxy) orbital. This implies that the oxidation state of Mn in the 3B1 of MnS3− isomer is +5. Starting from the 3B1, the transfer of one electron

Figure 4. Pseudonatural CASSCF molecular orbitals and occupation numbers of the 3B1 state of MnS3− isomer.

from 7b1 to 3a2 orbital will form the 3A2. Because the 7b1 and 3a 2 orbitals are two elements of the e″ irreducible representation in D3h point group, the 3B1 and 3A2 state are two components of the 3E″ state in D3h symmetry. The degenerate 3E″ states in D3h symmetry are not stable and split D

DOI: 10.1021/acs.jpca.6b02631 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A into 3B1 and 3A2 in C2v symmetry due to the Jane−Teller distortion. However, such splitting has been known to be very small in energy for transition metal-containing compounds.4,13 This is actually the case here where the energy difference between these two states is only 0.00 and 0.05 eV as calculated at B3LYP and CASPT2 level, respectively. Otherwise, the 2A1 state of MnS3 isomer is obtained by a detachment of one electron from the 7b1 orbital of 3B1. Assignments of the Photoelectron Spectrum of MnS3− Cluster. The photoelectron spectrum of MnS3− was measured with photon energy of 355 nm.8 As can be seen in Figure 5, the spectrum shows three bands with very different

calculated at BLYP level are 1.32 and 3.31 eV, respectively, and no assignment was made for the second band in the spectrum. Although the ADE of 3.31 eV of the electron detachment from the MnS3− isomer agrees very well with the position of the third band, our B3LYP and CASPT2 results in this work show that the MnS3− isomer is less stable than the η2-(S2)MnS− isomer. Therefore, the contribution of MnS3− and η2-(S2)MnS− isomers to the third band in the spectrum is still in question. By using CASPT2 results as shown in Table 2, new assignment of the photoelectron spectrum of MnS3− cluster is proposed. Similar to the previous BLYP assignment, the first band with the lowest intensity is assigned to the 7A′ → 16A′ transition within the metastable η2-(S3)Mn−/0 isomers in which one electron is removed from the 26a′ orbital. The ADE and VDE of 7A′ → 16A′ transition as calculated at CASPT2 level are 1.86 and 2.30 eV, respectively. These calculated ADE and VDE values are in agreement with the experimental values of 1.56 and 2.18 eV of the first band. The second band in the spectrum is attributed to the 7A′ → 8A′ transition of the metastable η2(S3)Mn−/0 isomers in which one electron is detached from the 22a′ orbital. The ADE of 7A′ → 8A′ transition calculated at CASPT2 level is 2.53 eV, and this ADE value is well-compared to the experimental value of around 2.60 eV. Franck−Condon factor simulations for the 7A′ → 16A′ and 7 A′ → 8A′ transitions within η2-(S3)Mn−/0 isomers confirm our assignments of the first and second bands in the photoelectron spectrum of MnS3− cluster. The simulated results as presented in Figure 6a,b show that both of the7A′ → 16A′ and 7A′ → 8A′ transitions within η2-(S3)Mn−/0 isomers are composed of several vibrational progressions. These features can be explained by the geometrical difference between the anionic 7 A′ state and the neutral 16A′ and 8A′ state as presented in Table 1. Because the first and the second bands in the photoelectron spectrum of MnS3− are also very broad, we can say that there is a good agreement between simulated and the experimental results. In general, it can be said that the first and second bands in the photoelectron spectrum of MnS3− cluster originate from the electron detachment processes within the metastable η2-(S3)Mn−/0 isomers. The strong vibrational progessions of the 7A′ → 16A′ and 7A′ → 8A′ transitions within η2-(S3)Mn−/0 isomers are the results of the geometrical difference between the initial and the final states. Particularly in the 7A′ → 16A′ transition, one electron is detached from the 26a′ orbital. As a result, the Mn−S bond (r1) is reduced from 2.431 to2.294 Å, and the MnSSS angle (φ)

Figure 5. Photoelectron spectrum (355 nm) of MnS3− cluster as reported in ref 8 and the assignments as derived from computational results of the present work. Reproduced with permission from ref 8. Copyright 1996 American Institute of Physics.

intensities. The first band has the lowest intensity and is a very broad band. The adiabatic and vertical detachment energy (ADE and VDE) of this band is 1.56 and 2.18 eV, respectively. Additionally, the spectrum shows the second band at around 2.60 eV, which is also very broad. Moreover, because the photon energy used to detach an electron is not high enough, only one peak of the third band at around 3.30 eV is observed. It should be noted that the peak at 3.30 eV has the highest intensity in all three bands of the spectrum. On the basis of the BLYP calculations, assignment of the photoelectron spectrum of MnS3− was proposed.7 Particularly, the first band at 1.56 eV was ascribed to the detachment of one electron from the less stable η2-(S3)Mn− isomer, while the third band was attributed to the removal of one electron from the more stable MnS3− isomer. The ADE’s of these two bands

Figure 6. Franck−Condon factor simulations for the 7A′ → 6A′ (a) and 7A′ → 8A′ (b) transitions within the η2-(S3)Mn−/0 isomers. E

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decreases from 179.79° to 126.82°. In the 7A′ → 8A′ transition, one electron is removed from the 22a′ orbital. Subsequently, the Mn−S bond (r1) increases from 2.431 to2.570 Å, and the S−S bond (r2) decreases from 2.114 to2.019 Å. Because of the structural difference between the 7A′, 16A′, and 8A′, extensively vibrational progressions are obtained in the Franck−Condon factor simulations for the 7A′ → 16A′ and 7A′ → 8A′ transitions. The third band starting at around 3.30 eV with the highest intensity in the photoelectron spectrum of MnS3− cluster can be assigned to one-electron detachments from all three anionic isomers studied in this work. The CASPT2 results as can be seen in Table 2 show several one-electron detachment processes from the η2-(S2)MnS−, MnS3−, and η2-(S3)Mn− isomers that have VDE’s of around 3.30 eV. Particularly, the 5 B2 → 4B1, 5B2 → 6B1, and 5B2 → 4A2 transitions within η2(S2)MnS−/0 isomers have VDE’s of 3.11, 3.26, and 3.28 eV. In the transitions from 5B2 to 4B1, 2B1, and 4A2, one electron from the 3a2, 2a2, and 7b1 orbitals of η2-(S2)MnS− is respectively detached. Also, the 3B1 → 2A1 transition within MnS3−/0 isomers with a removal of one electron from the 7b1 orbital has VDE of 3.24 eV. Moreover, the 7A′ → 36A′ transition within η2-(S3)Mn−/0 isomers with a detachment of one electron from the 22a′ orbital has VDE of 3.43 eV. All these five CASPT2 VDE values correspond very well with the starting point of the third band at around 3.30 eV. It means that all these five transitions are able to contribute to the third band at around 3.30 eV. However, because the η2-(S2)MnS− is calculated to be the most stable isomer of MnS3− cluster and because the third band has the highest intensity in the spectrum, we believe that this band is mainly the result of the electron detachment processes from the η2-(S2)MnS− isomer.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] Tel: (+84)1228942399. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the Ministry of Education and Training of Vietnam under Grant No. B2016.SPD.03. The authors are grateful to Van Tran Tran for his fruitful discussion.

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CONCLUSIONS

The geometrical and electronic structures of η2-(S2)MnS−/0, η2(S3)Mn−/0, and MnS3−/0 isomers of MnS3−/0 clusters have been investigated by B3LYP functional and CASPT2 methods. The results show that for the anionic cluster, the stability decreases from η2-(S2)MnS− through MnS3− to η2-(S3)Mn− isomer. For the neutral cluster, the η2-(S2)MnS is slightly more stable than the η2-(S3)Mn and much more stable than the MnS3 isomer. Based on the B3LYP and CASPT2 computational results, the photoelectron spectrum of MnS3− cluster is explained by the one-electron detachment processes from all three isomers. The first and second bands starting at 1.56 and 2.60 eV with low intensity are ascribed to the 7A′ → 6A′ and 7A′ → 8A′ transitions within the metastable η2-(S3)Mn−/0 isomers. The ADE’s of these two transitions as calculated at CASPT2 level are 1.86 and 2.53 eV, respectively. The Franck−Condon factor simulations for these two transitions confirm the shapes of the first two bands in the spectrum. On the other hand, the third band with highest intensity at 3.30 eV can be attributed to the 5 B2 → 4B1, 5B2 → 6B1, and 5B2 → 4A2 transitions within η2(S2)MnS−/0 isomers, the 3B1 → 2A1 within MnS3−/0 isomers, and the 7A′ → 36A′ within η2-(S3)Mn−/0 isomers. The VDE’s calculated at CASPT2 level for these transitions are 3.11, 3.26, 3.28, 3.24, and 3.43 eV. Because the η2-(S2)MnS− is computed to be the most stable isomer of MnS3− cluster, electron detachments from this isomer result in the highest intensity of the third band in the spectrum. F

DOI: 10.1021/acs.jpca.6b02631 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpca.6b02631 J. Phys. Chem. A XXXX, XXX, XXX−XXX