Cite this Article
Zhang Zhu-Xia, Zhang Yong, Xue Wen-Hua, Jia Wei, Zhang Cai-Li, Li Chun-Xia, Cui Peng. CN bond orientation in metal carbonitride endofullerenes: A density functional theory study . Chinese Physics B, 2017, 26(12): 123102  
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CN bond orientation in metal carbonitride endofullerenes: A density functional theory study
Zhang Zhu-Xia1, 2, 3, †, Zhang Yong1, 2, Xue Wen-Hua1, 2, Jia Wei1, 2, Zhang Cai-Li1, Li Chun-Xia4, Cui Peng5, ‡
Key Laboratory of Interface Science and Engineering in Advanced Materials of Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China
Research Center of Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan 030024, China
Collaborative Innovation Center for Shanxi Advanced Permanent Magnetic Materials and Technology, Linfen 041004, China
School of Electronic Information Engineering, Yangtze Normal University, Chongqing 408100, China
School of Information, Guizhou University of Finance and Economics, Guiyang 550025, China

 

† Corresponding author. E-mail: zhangzhuxia@tyut.edu.cn pcui@ustc.edu.cn

Project supported by the National Natural Science Foundation of China (Grant Nos. 21503208, 61604104, and 51002102), the Natural Science Foundation of Shanxi Province, China (Grant Nos. 2015011034, 201601D202034, and 201601D202029), and the Natural Science Foundation Project of Chongqing Science and Technology Commission, China (Grant No. cstc2014jcyjA00032).

Abstract

The geometric and electronic structures of scandium carbonitride endofullerene Sc3CN@C2n (2n = 68, 78, 80, 82, and 84) and Sc(Y)NC@C76 have been systematically investigated to identify the preferred position of internal C and N atoms by density functional theory (DFT) calculations combined with statistical mechanics treatments. The CN bond orientation can generally be inferred from the molecule stability and electronic configuration. It is found that Sc3CN@C2n molecules have the most stable structure with C atom locating at the center of Sc3CN cluster. The CN bond has trivalent form of [CN]3− and connects with adjacent three Sc atoms tightly. However, in Sc(Y)NC@C76 with [NC]−, the N atom always resides in the center of the whole molecule. In addition, the stability of Sc3CN@C2n has been further compared in terms of the organization of the corresponding molecular energy level. The structural differences between Sc3CN@C2n and Sc3NC@C2n are highlighted by their respected infrared spectra.

PACS: 31.15.E-;36.20.Kd;36.40.Mr;68.55.ap
Keyword:geometric and electronic structures;metal carbonitride endofullerenes;theoretical calculations;CN bond orientation
1. Introduction

To date, numerous endohedral metallofullerenes (EMFs) with diversified encaged species and various carbon cages have been isolated owing to their complicated structures, diversified electronic properties, and potential applications in biomedicine, electronics, photovoltaics, and many other fields.[1–7] The structure and electronic properties of EMFs encaging metal atom(s), trimetallic nitride, metal carbide, metal oxide, metal sulfide, metal carbonitride, and metal cyanide have been extensively investigated.[6,8–10] For instance, the charge distribution and aromaticity of these systems are key in determining the most favorable isomeric cages for EMF encapsulation.[11,12] The chemical reactivity of some of the studied systems has also been explored.[13,14]

Considering pronounced Coulomb repulsion between two or more metal atoms, the presence of the negatively charged nonmetal atoms or clusters, e.g., carbon, nitrogen, oxygen, sulfur, carbonitride, and cyanide, is necessary to stabilize multi-metal clusters. In 1999, the first metal nitride clusterfullerene Sc3N@C80-Ih was reported.[15] In 2001, the metal carbide clusterfullerene Sc2C2@C84 was isolated.[16] Since then, more EMFs such as Sc3C2@C80−Ih,[17] Sc2C2@C82−C3v(8),[18] Sc2C2@C84−D2d(23),[19] and Sc4C2@C80−Ih[20] were revealed. Among them, the encapsulated C2 moiety displays complicated charge states. was found in Sc2C2@C84,[19] Sc2C2@C68,[21] Ti2C2@C78,[22,23] and La2C2@C90 − 98.[24] A moiety was disclosed in Sc3C2@C80[17] and a anion could exist in Sc4C2@C80.[20] In 2010, metal carbonitride endofullerene Sc3CN@C80-Ih with a CN bond was predicted by density functional theory (DFT) calculation.[25,26] Soon after that, Sc3CN@C80[27] and Sc3CN@C78[28] containing planar quinary cluster Sc3CN were experimentally verified. The single crystal x-ray crystallographic study on Sc3CN@C80 failed to distinguish the position of endohedral carbon atom and nitrogen atom because of their similar electron density and size. In this context, theoretical calculations with DFT have been quite helpful. For example, Lu et al. explored the Sc3CN@C2n (2n = 68, 78, and 80) and reported that all initial isomers with C atom residing at the center of the Sc3 plane are all energetically favorable.[25] Recently, theoretical calculations have also been performed to explore the structures and electronic properties of Sc3CN@C2n (2n = 82 and 84) with N atom residing at the center of the Sc3 plane.[29,30] Moreover, the monometallic carbonitride endofullerene (or metal cyanide endofullerene) of YCN@Cs(6)-C82 and TbCN@Cs(6)-C82 has been successfully synthesized and isolated, whose electronic structures were proposed as [Y(Tb)3+(CN)−]2+@[C82]2− by DFT calculation with a special triangular Y(Tb)CN cluster.[31–34] Wang et al. explored MCN@C76 (M = Sc, Y) using DFT calculations and demonstrated that the entrapped MCN cluster is linear with C atom locating in the center of molecule.[35] Recently, Yang et al. report the syntheses and isolations of novel non-IPR (IPR: isolated pentagon rule) mononuclear clusterfullerenes MNC@C76 (M = Tb, Y), in which one pair of strained fused-pentagon is stabilized by a nearly linear MNC mononuclear cluster.[36] YCN@C78 has also been studied theoretically with a triangular YCN cluster.[37] Apparently, these metal carbonitride endofullerene could have variable inner CN unit with different charging states and coordinating modes. It is thus highly desirable to carry out a systematical investigation on the family of metal carbonitride endofullerenes Sc3CN@C2n (2n = 68, 78, 80, 82, and 84) and Sc(Y)NC@C76 to define the general relationship between the molecule structure and CN bond orientation.

2. Computational method

All isomers were derived from the reported results without considering the different cages. The endohedral C and N positions were alternated, named as Sc3CN@C2n (with C atom at the center of Sc3 plane) and Sc3NC@C2n (with N atom at the center of Sc3 plane), respectively, to understand their relative stability. For monometal cyanide endofullerene, MCN@C76 and MNC@C76 also represent two isomers with different CN orientations defined in a similar way as for Sc3CN@C2n.

The geometry optimizations were carried out with no symmetry constrain using the Gaussian 09 program. DFT methods, including PW91PW91, BP86, TPSSTPSS, M06L, B3LYP, B3PW91, PBE1PBE, BHandHLYP, CAM-B3LYP, and wb97xd, were employed for full geometry optimization. The calculated relative energies and the key structure and electronic parameters are shown in Table 1. Depending on the HF exchange contributions in these functionals, the HOMO–LUMO (highest occupied molecular orbital–lowest unoccupied molecular orbital) energy gap changes significantly. However, all DFT results indicated that Sc3CN@C2n isomers are energetically stable compared to Sc3NC@C2n, and the calculated C–N distances in Sc3CN cluster by each DFT method are almost same, which suggests that all DFT methods employed here show similar performance in the description of the geometry stabilities of metal carbonitride endofullerenes system. Moreover, all geometries were characterized by harmonic vibrational frequency analysis to enable to investigate enthalpy-entropy effect and infrared spectroscopy (IR) at the B3LYP level. The standard 6-31G(d) basis set for C, N, and Sc with the effective core potentials of Dolg et al.,[38,39] together with the optimized basis set for the valence shells with a contraction scheme of (8s7p6d2f1g)/[6s5p3d2f1g] for Y, was employed.

Table 1.

All the optimized Sc3CN@C2n/Sc3NC@C2n and Sc(Y)NC@C76/Sc(Y)CN@C76 isomers, their relative bonding energies (ΔEb, in units kcal·mol−1), HOMO–LUMO gaps (gap, in unit eV), encaged C–N bond length (d(C–N), in unit Å), and the dipole moment (D, in unit Debye).

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3. Results and discussion

The C68 (D3: 6140),[25] C78 (C2: 22010),[25,28] C80 (Ih: 7),[27] C82 (C2v: 39705),[29] and C84 (Cs: 51365)[30] were assigned in Sc3CN@C2n molecules. Among them, C80 (Ih: 7) has IPR-type fullerene cages, and C68 (D3: 6140), C78 (C2: 22010), C82 (C2v: 39705), and C84 (Cs: 51365) have non-IPR-type cages. In Table 1, the calculated relative bonding energies, HOMO–LUMO gaps, and other parameters at B3LYP level are listed. The results show that Sc3CN@C2n species are about 10.8 kcal · mol−1 ~ 17.1 kcal · mol−1, more stable than Sc3NC@C2n molecules. Figure 1 presents all of the most stable isomers under investigations. The Sc3CN clusters in Sc3CN@C2n are quasi-planar with the carbon atom in the center and nitrogen atom locating on one side of the triangle formed by the Sc atoms. The HOMO–LUMO gaps and C–N bond lengths found in this study are in good agreement with previous reported data.[25,28,29,35] The change of the dipole moment is normally small when altering the CN orientation, except for that of Sc3CN@C80. The large variation in Sc3CN@C80 is due to the slight fold of Sc3CN compared to the planar Sc3NC.

Fig. 1. (color online) The calculated Sc3CN@C2n (2n = 68, 78, 80, 82, and 84) and Y(Sc)NC@C76. N, Sc, and C atoms in cage are blue, white, and grey, respectively. The C atom inside cage is highlighted in purple.

Wang et al. investigated the relative energies and stabilities of Sc(Y)CN@C76.[35] Here, we also made a comparative study on this type of molecules. Our calculations indicate that ScNC and YNC are still put in the most stable (C2v: 19138) structure. N atom is in the center of cage in both structures. Sc(Y)NC@C76 are more stable than Sc(Y)CN@C76 by 2.2 and 3.3 kcal · mol−, respectively. The calculated N–C and Y–N bond lengths for YNC@C76−C2v(19138) are 1.18 Å and 2.21 Å and the M–N–C bond angle is 144.7°, which agree well with the experimental results of Yang.[36]

3.1. Temperature-dependent yield

Although Sc3CN@C2n isomers are energetically stable compared to Sc3NC@C2n, their relative stabilities at high temperature is still not known. We have thus performed statistical thermodynamic analysis for these metal carbonitride endofullerenes. To evaluate the temperature effect, the relative concentrations of Sc3CN@C2n isomers were calculated according to the formula given in Ref. [40] with the consideration of the enthalpy–entropy effect. As depicted in Fig. 2, the concentration of Sc3NC@C2n is noticeably increased with the increase of temperature. Even so, it is obvious that all Sc3CN@C2n structures exhibit superior concentration at all temperatures. It is thus reasonable to conclude that Sc3CN@C2n is the major isomer in the products.

Fig. 2. (color online) Temperature-dependent molar fractions (xi) of the carbonitride endofullerenes.

YNC@C76 isomer is about 3.3 kcal ·mol− more stable than YCN@C76 at 0 K and is also highly stable in the whole temperature range (Fig. 2). The curve of the YCN@C76 structure slides up slowly after 500 K and reaches only 20.3% in 4000 K. The same trend can be observed for ScCN@C76.

3.2. The charge state of the cyanide

It is natural that Sc3CN is more stable (8.29 kcal · mol−1 than Sc3NC, because C is four-coordinated and N likes three-coordination. The charge state of the trapped CN unit in fullerene is an important factor for determining the Sc3CN geometric configuration. For Sc(Y)NC@C76, the valence state can be approximately described as [Sc(Y)3+(NC)−]2+@[C76]2−. As shown in Fig. 3, the natural population of the N atom is more negative than C atom in CN− anion. It is thus more reasonable that the N atom locates in the center of the cage and coordinates with Sc(Y) atoms directly.

Fig. 3. (color online) Natural population of C and N atoms in CNn−.

With the increasing of negative charge of CN, C atom possesses more electrons, which means that the C can tend to coordinate with metal cations. This explains why the C atoms always locate at the center of Sc3 plane to form more bonds with Sc in Sc3CN@C2n molecules whose electronic structures were described as [Sc . (CN)3−]6+@[C2n]6−.

3.3. The electronic configurations of Sc3CN and Sc3NC

We have also paid attention to the electronic structure of the isolated Sc3CN and Sc3NC clusters. It is helpful to understand the structural stability by analyzing their molecular orbitals (MOs). Figure 4 shows their frontier MOs in the ground state. In general, MOs in both clusters have very similar organization, except the relative positions of HOMO-2 and HOMO-3. Sc3CN has the large gap between HOMO-2 and HOMO-3 and is easier to transfer six electrons to cage when it is encaged in cage, which can be an important factor to determine Sc3CN@C80 superior stability.

Fig. 4. (color online) The molecular energy levels of Sc3CN and Sc3NC.
3.4. IR spectrum

IR spectroscopy is a powerful tool for distinguishing the geometric structure of EMFs, from which the C–N bond orientation in scandium cyanide clusters can also be inferred. Here we focus on analyzing the vibrations of CN unit in above mentioned two isomers. In Fig. 5, the simulated IR spectra of Sc3CN@C2n and Sc3NC@C2n are presented for comparison.

Fig. 5. (color online) The calculated IR vibration spectra. The C–N stretching frequencies were marked with star.

For most EMFs, their IR spectra can be roughly classified into three bands: from 1000 cm−1 to 1700 cm−1, from 300 to 900 cm−1, and blow 200 cm−1. The first two bands are contributed by the vibrations of the carbon cages. The third band mainly comes from the inner clusters. Notably, the IR spectral features of Sc3CN@C68 in the high-energy range (related to the internal cluster) are similar to those of Sc3NC@C68, because large strain between embedded cluster Sc3CN (Sc3NC) and cage masks the differences from different positions of C and N atoms. However, for those with larger fullerene cages, the IR spectral features in the high-energy range show slight differences between Sc3CN@C2n and Sc3NC@C2n. For example, the IR vibrations in high-energy range for Sc3CN@C78 are different from those in Sc3NC@C78. In detail, the signals (405 and 265 cm−1 for Sc3CN@C78 and Sc3NC@C78, respectively) are assigned to the C (or N)-dominated (Sc–C–Sc or Sc–N–Sc) stretch modes.

The C–N stretching frequencies (marked with black star) of the central CN units (1767, 1809, 1689, 1618, and 1588 cm−1 for Sc3CN@C68, Sc3CN@C78, Sc3CN@C80, Sc3CN@C82, and Sc3CN@C84, respectively) have very weak absorption intensities. The corresponding stretching modes (marked with red star) from Sc3NC@C68 to Sc3NC@C84 for Sc3NC@C2n species change to 1653, 1699, 1534, 1515, and 1504 cm−1, respectively. Their absorption intensities are slighter stronger than those in Sc3CN@C2n. That is more obvious in Sc(Y)NC@C76. There are characteristic peaks for C–N stretching frequencies at 2142 and 2141 cm−1 in ScNC@C76 and YNC@C76, respectively.

4. Conclusion

The positions of C and N atom in metal carbonitride endofullerenes were investigated. In Sc3CN@C2n (2n=68, 78, 80, 82, and 84), CN was found to have the 3-valence state, resulting in more electrons located at C atom and placing C atom at the center of Sc3 plane. In Sc(Y)NC@C76, the N atom becomes more negative to favor its direct bonding with metal atoms. The enthalpy–entropy effect was also considered to examine the relatively high stabilities of Sc3CN@C2n and Sc(Y)NC@C76 at elevated temperatures. It is expected that the present results will enrich the research area of the stability of metallofullerenes with complex composition and geometry.

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