A density functional study on interaction of first-row transition-metal dicarbides, C2X (X = Sc-Zn) with O2

Chemical Physics Letters 626 (2015) 1–5

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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett

A density functional study on interaction of first-row transition-metal dicarbides, C2 X (X = Sc-Zn) with O2 Saroj K. Parida a , Sridhar Sahu a,∗ , Sagar Sharma b,1 a b

Department of Applied Physics, Indian School of Mines, Dhanbad, Jharkhand 826004, India Department of Organic Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel

a r t i c l e

i n f o

Article history: Received 18 December 2014 In final form 2 March 2015 Available online 12 March 2015

a b s t r a c t We present our calculations based on density functional theory to explore the molecular adsorption of O2 on the first-row transition-metal dicarbides C2 X (X = Sc-Zn). Degree of activation of O2 is observed to be less in C2 XO2 (X = Sc-V) as compared with C2 XO2 (X = Cr-Zn) except for X = Cu, and is marked by the amount of total charge on O2 . Topological analysis shows that the interaction in the cases of C2 XO2 (X = ScV) is prominent between X and O and mostly being intermediate type whereas in C2 XO2 (X = Cr-Zn), both C O and X O bondings are observed with shared-kind and intermediate characteristics respectively. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Since last many years, hetero-atom doped carbon clusters involving first-row, second-row and transition metal (TM) elements have acquired meticulous attention due to their enormous prospective applications such as hydrogen storage, chemical catalysts, electronic devices, superconductors and so on [1–9]. Since the discovery of carbide chains doped with first- and second-row elements in interstellar media, a lot of theoretical and experimental works have been reported exploring the adsorption of other elements specifically TMs on these clusters. For example, Roszak et al. computed the structural and electronic properties of ScC2 using correlation consistent ab initio methodologies and obtained a C2v structure which closely matched with the experimental results provided by Haque et al. [10,11]. Similarly, Sumathi et al. calculated the structural and electronic properties of TiC2 using ab initio many-body methods and found a C2v ground state geometry with 3 B state [12]. Largo et al. reported their extensive theoretical works 1 on the structural and electronic properties of VCn and ZnCn clusters [13,14]. Moreover, Barrientos et al. investigated structures of TiCn using density functional theory (DFT) and found fan isomers to be most favored for small-sized clusters [15,16]. Similarly, Hendrickx et al., using ab inito approach, explored the structure and bonding of FeC2 and FeC3 clusters and noted that for FeC2 , a C2v structure with 5 A state was most stable [17,18]. Similarly, electronic structures 1

∗ Corresponding author. E-mail address: [email protected] (S. Sahu). 1 Present Address: Physical Sciences Division,Institute of Advanced Study in Science and Technology,Paschim Boragaon, Guwahati 781035, Assam, India. http://dx.doi.org/10.1016/j.cplett.2015.03.005 0009-2614/© 2015 Elsevier B.V. All rights reserved.

of neutral and ionic MnC2 was studied by Tran et al. using both DFT and CCSDT (T) methodologies, who obtained high-spin states (7 A1 and 6 A1 ) cyclic isomers to be stable structures [19]. Moreover, the bonding mechanisms in dicarbides doped with first-row transition metals have also been discussed in detail by Largo et al. [20]. However, except a few reports, reactivity of carbon chains towards small molecules such as O2 , CO, CO2 etc. are yet to be investigated thoroughly [21,22]. H2 adsorption on alkali metal (Li, Na) terminated carbon chains has been investigated theoretically by Liu et al. [23]. Similarly, theoretical work of Moras et al. showed that the chemisorption of O2 molecules on carbon chains resulted in cleavage and shortening of the carbynoid structures [24]. Dibben et al. performed both theoretical and experimental work to investigate the interaction of linear carbon clusters with water molecule and revealed the photoproduction of Cn O compound from the Cn .H2 O complexes [25]. Furthermore, Eichelberger et al. performed experimental investigation on the reactivity of ionic carbon chains with H, N, and O atoms and found that the ionic chains are more reactive towards O than H and N [26]. However, only a handful of investigation focusing on the reactivity profile of heteroatom-doped carbon chains have been performed [27,28]. In this letter we present our theoretical study of interaction of first-row transition-metal dicarbides with O2 molecule. 2. Computation For the present study, all the structures were optimized within the framework of density-functional theory (DFT). In particular, we used Becke’s three-parameter hybrid exchange functional and Lee–Yang–Parr correlation functional employing triple split-valence basis set 6-311+G(3df) which includes diffuse and

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polarization functions and Wachters-Hay basis set with scaling factor of Raghavachari and Trucks designed for the first-row transition metal elements [30–33,29]. As effective core potential basis sets of double-zeta (DZ) quality such as Los Alamos National Laboratory (LANL2DZ) basis sets have been widely used in the study of clusters containing heavy elements, we also implemented it to track the variation in the results. In addition, we have also employed 6-311++G(d,p) basis set for a comparative purpose. As the success of different hybrid functionals are reported to be system specific, we, therefore, used Becke’s three-parameter hybrid exchange functional and the Perdew and Wang’s 1991 gradientcorrected correlation functional (B3PW91) as well as the 1996 gradient-corrected correlation functional of Perdew, Burke and Ernzerhof (PBE1PBE) for the investigation of our systems, and for both the cases we used LANL2DZ basis sets [34,35]. All the host clusters were optimized with O2 molecule placed at different possible sites of C2 X (X = Sc-Zn) and the optimizations were achieved without any imaginary harmonic frequencies. Moreover, considering the fact that some of the TM dicarbides are more likely to show high-spin ground states, we also optimized all the structures (without and with O2 molecules) at different spin multiplicities. In this work, only the molecular adsorptive mechanism of O2 has been discussed. Gaussian 09 and the graphical user interface Gaussview and Chemcraft softwares have been used for the detailed calculations [36,37], and moreover, reactivity of the clusters towards O2 molecule has also been explored through electron localization function (ELF) analysis and Bader’s theory of atoms in molecules using Top-Mod and AIMALL computational packages respectively [38,39].

3. Results 3.1. Structural analysis In Figure 1, we present the optimized structures of C2 XO2 clusters, with X = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn (henceforth, X = Sc-Zn in general, unless otherwise specified). Other possible structure has been provided in the Supplementary material. All the structures except C2 Cu are found to be weakly modified while interacting with O2 molecule. At B3LYP/6-311+G(3df) the structure of C2 Cu is observed to be distorted from cyclic to linear. It can be noted from the figure that the interaction pattern of O2 is different for dicarbides doped with early transition-metals (Sc-V) as compared with those doped with late transition-metal elements (Cr-Zn). For the former cases, only the transition-metal elements bond with O2 molecule, whereas in the later cases, both carbon and transitionmetals take part in the bond formation with O2 . We provide various structural parameters such as the C C and O O bond lengths along with O O stretching frequencies of C2 XO2 clusters in Table 1, and their relative variations at different levels of theory have been depicted in Figure 2. It is noted that, the C C bond length, for example, in the case of C2 ScO2 increases by 0.08% at B3LYP/LANL2DZ and B3PW91/LANL2DZ levels and decreases with other computational parameters, whereas C C bond length in the case of C2 ZnO2 is increased by 0.2% at B3LYP/6-311++G(d,p) and decreases at others. C C bond lengths for other dicarbides decrease (4.85–7.58%) at all levels being maximum in the case of C2 CuO2 . Similarly, for C2 XO2 doped with early transition-metal elements (abbreviated as C2 Xearly O2 ), the increase in O O bond length is less than 23% whereas for those doped with late transitionmetal (abbreviated as C2 Xlate O2 ) (except X = Cu), the increment is between 24% and 28% at all levels of theory. Moreover, the O O stretching frequencies in C2 XO2 clusters are all found to be red-shifted as compared to that (1633 cm−1 ) in O2 molecule. This implies that maximum electron population is transferred from the

Figure 1. Optimized structures of C2 XO2 clusters with X = Sc to Zn. Other possible structures are provided in the supplementary material.

Table 1 Electronic states (C2 X/C2 XO2 ), C C (C2 X/C2 XO2 ) bond lengths, C X (C2 X/C2 XO2 ) bond lengths, O O bond lengths, O O stretching frequencies in C2 XO2 clusters, Mulliken charges (Q) on O2 , and Mayer bond order (BO) of O O in C2 XO2 clusters. Results provided here are computed at B3LYP/6-311+G(3df) level and correspond to the most stable C2 XO2 isomers. X

State

C C ˚ (A)

C X ˚ (A)

O O ˚ (A)

ωO O (cm−1 )

Q

BOO

Sc

2

A1 /2 A

1202

−0.544

1.310

3

B1 /1 A

1.459

948

−0.701

0.998

V

4

B1 /2 A

1.424

989

−0.593

0.946

Cr

5

A1 /5 A

1.529

725

−0.784

1.008

Mn

6

A1 / A

1.527

727

−0.826

0.997

Fe

5

A2 /5 A

1.523

730

−0.791

1.015

Co

4

B1 /4 A

1.529

703

−0.767

1.013

Ni

3

B1 / A

1.521

688

−0.768

1.008

Cu

2

A1 /2 A

1.352

1020

−0.563

1.246

Zn

1

A1 /1 A

2.088/ 2.066 1.973/ 1.934 1.932/ 1.926 1.985/ 2.042 1.979/ 2.026 1.915/ 1.965 1.867/ 1.982 1.871/ 1.951 1.976/ 1.835 1.976/ 1.987

1.322

Ti

1.263/ 1.260 1.286/ 1.260 1.286/ 1.268 1.273/ 1.252 1.272/ 1.251 1.294/ 1.251 1.289/ 1.263 1.295/ 1.274 1.278/ 1.216 1.276/ 1.252

1.526

685

−0.762

0.988

6



3



O

S.K. Parida et al. / Chemical Physics Letters 626 (2015) 1–5

3

% of change in C-C bond length

4 2 0 -2 -4 -6 -8 Sc

V

Ti

Cr

Mn

Fe

Co

Ni

Cu

Zn

Elements

Figure 3. Orbital composition (in %) of HOMOs of C2 X clusters.

% of change in C-X bond length

20

from the carbon atoms of the host cluster might also depend on the nature of C C and C X interactions in C2 X. Moreover, it can also be noticed that the activation of O2 is more in C2 Xlate O2 as compared with C2 Xearly O2 (vide infra).

15 10 5

3.2. Electronic properties 0 -5 -10 Sc

Ti

V

Cr

Fe Mn Elements

Co

Ni

Cu

Zn

% of change in O-O bond length

30

25

20

15

10 Sc

Ti

V

Cr

Mn Fe Elements

Co

Ni

Cu

Zn

Figure 2. Variation of bond lengths in C2 XO2 clusters at different levels of theory.

carbon atoms of the host C2 X clusters to O2 adsorbate. It can be noted that the degree of activation (in terms O O bond length or ωO O ) is marked by the amount of total charge (Q) on O2 molecule. For example, less Mulliken charges on O2 in the cases of C2 ScO2 and C2 CuO2 are accompanied by weak activation of O2 molecule as compared to the others. The weak activation of O2 molecule in these clusters can be attributed to the nature of interaction of O2 with the host clusters, and this is roughly reflected in O O bond order (Mayer). The O O bond order in C2 ScO2 and C2 CuO2 are found to be slightly greater than 1.0 whereas in other cases it is less than 1.0. That means electron density between oxygen atoms in O2 fragment is more, giving rise to slightly larger bond energy, hence less activation. However, the amount of population to be transferred

In the context of the population transfer from the host C2 X to O2 molecule, we use the computational parameter C2 X−O2 as defined by Joshi et al. which is considered to be an important indicator of reactivity and represents the relative energy difference between the HOMO of the bare-clusters and the LUMO of O2 [40]. In the cases of C2 Xlate O2 , the decreasing order of C2 X−O2 is almost marked by the increasing order of population transfer to O2 (Q). However, no specific correlation is found between C2 X−O2 and Q for the whole C2 XO2 clusters. One of the reasons is the frontier orbital picture (FOP) or the bonding mechanism in C2 X clusters which is likely to be the important deciding factor for charge transfer to O2 molecule. The detailed bonding mechanism discussed by Largo et al. clearly show C2 X clusters to have no specific bonding pattern [20]. This is also qualitatively reflected on the FOP of C2 X depicted in Figure 3 (HOMOs are also given in Supplementary material). Composition of HOMOs of C2 X has also been depicted in Figure 3. We can see, for example, HOMO of C2 Sc is found to be largely contributed from s (61.5 %) and d (29.2%) orbital of Sc with negligible contribution from the carbon atoms, whereas for the dicarbides doped with late transition-metal elements, the HOMOs are roughly contributed from s (11–16 %) and p (21–27%) orbitals of C and d (9–24%) orbital of TM elements. On the other hand in the case of C2 Cu, HOMO is found to contributed solely from p (31.2%) of C and d (35 %) orbital of Cu [41]. This also provides a qualitative idea about the different interaction mode of O2 with early and late bare transition metal carbides. The difference in bonding pattern of early and late transition dicarbides with O2 molecule arises from the nature of the frontier orbitals of the bare clusters. The most active regions for O2 adsorption can be understood by detecting the sites of the bare C2 X clusters where HOMO or SOMO (single occupied molecular orbital) are mainly localized. We can see in the Figure 3 that in the cases of early transition-metal dicarbides, HOMOs or SOMOs are mostly localized at X atoms, whereas for late-transition-metal dicarbides, these frontier orbitals are largely concentrated at C with some contribution from X atoms. This fact of binding orientation of O2 in terms of FOP has also been discussed in detail by Joshi et al. [40]. In addition, the above bonding orientations can also be expressed in terms of Mulliken population analysis. For example, in C2 Sc and C2 ScO2 change in Mulliken charge is found to be more prominent in Sc rather than in C, whereas in C2 Mn and C2 MnO2 ,

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Table 2 E(HOMOcluster ) − E(LUMOO2 ) gap (C2 X−O2 in eV), adsorption energies (Eads in eV) corrected with the basis set superposition error (BSSE) and change in Gibb’s free energies (dG, in Kcal/mol) of most stable C2 XO2 isomers at B3LYP/6-311+3df levels. X↓

C2 X−O2

Eads

dG

Sc Ti V Cr Mn Fe Co Ni Cu Zn

−0.788 −1.627 −2.698 −2.996 −3.021 −3.183 −3.718 −4.030 −3.927 −3.401

−3.323 −3.652 −2.664 −2.347 −1.938 −2.127 −1.800 −1.551 −1.905 −1.534

−69.423 −75.690 −52.745 −44.183 −36.093 −40.793 −32.813 −27.071 −35.819 −27.142

the difference is found to be more at C (detailed Mulliken analysis is provided in the Supplementary material). In Table 2 we present adsorption energies (Eads ) corrected with basis set superposition error (BSSE) and Gibbs’ free energies (dG). It is  found  that  for  the clusters having odd number of electrons, both Eads  and dG follow the order C2 ScO2 > C2 VO2 > C2 MnO2 > C2 CuO2 > C2 CoO2 at B3LYP/6-311+G(3df) and B3LYP/LANL2DZ levels, whereas the order is changed at other levels. However, at all  the computational levels, C2 TiO2 is found to have maximum Eads  and

    dG value. It can also be noted that Eads  in the cases of C2 Xearly O2 is more as compared to those in C2 Xlate O2 and is marked by weak activation  of O2 molecule. However, no specific correlation between Eads  and O O bond length is noted. 3.3. Topological analysis

Topological analysis of electron localization function (ELF) can also be used to investigate the nature of bonding in the C2 XO2 complexes. Because of the existence of disynaptic and polysynaptic basins V (X, O), V(X,O,O) in C2 Xearly O2 clusters, the bonding between O2 molecule and transition metal elements are supposed to be shared-electron interactions. However, the relative fluctuations () of V (X, O), V(X,O,O) is found to be lying between 0.20 and 0.45 indicating that the interactions are intermediate type (TopMod data are provided in the Supplementary material). In C2 Xlate O2 clusters, on the other hand, the disynaptic basins V (X, O) and V (C, O) point out the existence of two-center bondings between both C,O and X,O. Likewise, the computed relative fluctuations for V (X, O) and V (C, O) are found to 0.35 and 0.65 respectively. This infer that the interactions between C,O and X,O are covalent and intermediate-type respectively [42,43]. This fact is also somewhat reflected in the ELF pictures (Figure 4), which show the existence of an isosurface in the bonding region between C and O atoms of the C2 Xlate O2 clusters inferring that the electrons are delocalized in between C and O resulting in the shared-type bondings [44,45]. However, no such

Figure 4. ELF of C2 XO2 , (X = Sc-Zn) clusters at isovalues >7.0.

isosurface is observed in the bonding region between X and O of these clusters similar to what is viewed in the cases of C2 Xearly O2 . Topological analysis of the bonding nature of hetero-atom doped dicarbide clusters with molecular oxygen can also be obtained by using Bader’s quantum theory of atoms in molecules (QTAIM) [46]. In Table 3 we provide different topological parameters such electron density (), ∇ 2 , and delocalization index (ı). It is found that for C2 Xearly O2 clusters,  < 0.20 a.u. (with positive ∇ 2 ) at the bond critical point (BCP) of (X,O) inferring that O2 molecule is more likely to have somewhat closed-shell type interaction in these clusters, whereas for C2 Xlate O2 ,  are found to be more than 0.30

Table 3   Average number of electrons (N) in carbon/oxygen atoms, electron density () in a.u., ∇ 2 , total energy density (HBCP ) in a.u., ( VBCP )/(GBCP ) ratio and delocalization index (ı) at BCP of (C,O) and (X,O).

X Sc Ti V Cr Mn Fe Co Ni Cu Zn

N 6.58/8.32 6.45/8.46 6.41/8.41 6.00/8.62 5.95/8.65 5.90/8.64 5.91/8.64 5.83/8.66 5.86/8.53 5.63/8.52

C

O

– – – 0.33 0.34 0.34 0.34 0.35 0.32 0.34

X

O

0.07 0.15 0.16 0.11 0.09 0.10 0.10 0.09 – 0.08

∇ 2 C – – – −0.39 −0.35 −0.36 −0.37 −0.31 −0.23 −0.34

O

∇ 2 X 0.33 0.58 0.61 0.52 0.44 0.47 0.57 0.47 – 0.39

O

HBCP C – – – −0.51 −0.52 −0.52 −0.53 −0.56 −0.49 −0.53

O

HBCP X -0.01 -0.07 -0.08 −0.03 −0.01 −0.01 −0.01 −0.01 – −0.02

  VBCPC O 

O

GBCP

– – – 2.27 2.20 2.20 2.23 2.19 2.16 2.23

C O

  VBCPX O  GBCP

1.12 1.13 1.36 1.18 1.08 1.07 1.06 1.08 – 1.20

X O

ıC

O

0.03 0.04 0.06 0.06 1.08 1.07 1.07 1.12 1.02 1.08

ıX

O

0.40 0.97 1.03 0.84 0.69 0.75 0.83 0.71 0.02 0.65

S.K. Parida et al. / Chemical Physics Letters 626 (2015) 1–5

a.u. (with negative ∇ 2 ) and less than 0.20 a.u (with positive ∇ 2 ) at the BCP of (C,O) and (X,O) respectively indicating the existence We also provide of both shared-type and closed-shell  interactions.  total energy density (HBCP ) and (VBCP )/(GBCP ) ratio to account for the nature of interaction in transition metal complexes as proposed by Cremer et al. and Espinosa et al. [47,48]. It is found that HBCP is negative in all the cases which suggests that potential energy density (VBCP ) dominates over kinetic  energy density (GBCP ) at the BCP. However, in C2 Xlate O2 , (VBCP )/(GBCP ) for (C O) is found to be greater that 2.0 indicating that the interaction is shared-kind, whereas, the ratio for (X O) in both C2 Xlate O2 and C2 Xearly O2 clusters lies between 1.0 and 2.0 (with ∇ 2  > 0) which suggests that the X O interaction is of intermediate kind. It is noteworthy that the electron density along the bond critical points of C X, C − O and O O bonds correlates well with the computed O O stretching frequencies of the cluster. An analysis of the electron density along the O O bond path shows that the electron density follows the order of C2 ScO2 (0.39 a.u.)> C2 CuO2 (0.35 a.u.) > C2 VO2 (0.29 a.u.) which is also reflected in the the O O stretching frequencies of these three compounds (Table 1). The nature of bonding in C2 XO2 is also characterized by the delocalization index (ı) [49]. Obviously the values of ı(X, O) are more in C2 Xearly O2 as compared to those in the cases of C2 Xlate O2 clusters inferring the fact that O2 is likely to have dominant interaction with X of C2 X in C2 Xearly O2 which is also evident from Figure 1. 4. Conclusion In conclusion, we investigated interaction of first-row transition-metal dicarbides C2 X (X = Sc-Zn) with O2 using density functional theory. It is noted that, in the cases of C2 Xearly O2 clusters, O2 molecule is weakly activated as compared to C2 Xlate O2 (except X = Cu). It is also marked by the net Mulliken charges on O2 and is roughly reflected in the O O bond orders which are analysis comparatively larger in C2 Xearly O2 clusters.  Topological  of C2 XO2 concluded that in C2 Xlate O2 , (VBCP )/(GBCP ) for (C O) is found to be greater that 2.0 (with  > 0.30 a.u. and ∇ 2  < 0) inferring that the interaction is shared-kind, whereas, the ratio for (X O) in both C2 Xlate O2 and C2 Xearly O2 clusters lies between 1.0 and 2.0 (with  < 0.16 and ∇ 2  > 0). This suggests that the X-O interaction is of intermediate kind. Similar conclusion is also obtained by computing delocalization index between (C,O) and (X,O). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cplett. 2015.03.005.

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