Structure, Stability, and Properties of Boron Encapsulated Complexes

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Structure, Stability and Properties of Boron Encapsulated Complexes of C , C B and C N 60

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Vinit Pundir, and Chelat Narayana Panicker Ramachandran J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b10649 • Publication Date (Web): 02 Feb 2017 Downloaded from http://pubs.acs.org on February 3, 2017

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The Journal of Physical Chemistry

Structure, Stability and Properties of Boron Encapsulated Complexes of C60, C59B and C59N Vinit and C. N. Ramachandran* Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand 247667, India *Email: [email protected], Phone No.: +91-1332284773

Abstract Density functional studies are performed for the boron atom encapsulated complexes of C60, C59B and C59N using B3LYP and B3LYP-GD2 functionals with 6-311G* basis set. The study shows that the complexes B@C59B and B@C59N can exist in different forms which differ in their structure, electronic and magnetic properties. The nature of interaction between the guest and the host is analyzed based on the stabilization energies of complexes for their different spin states. The electronic properties such as vertical electron affinity (VEA), vertical ionization energy (VIE) and energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the complexes are calculated. The transfer of electron spin density and spin-spin coupling of electrons between the guest and the host are examined. The study reveals that ferromagnetic core@shell spin coupling occurs between the host and the guest species of the complexes B@C59B and B@C59N in their triplet state without any transfer of spin density. The values of hyperfine coupling constant (hfcc) for the heteroatom of the cage and for the encapsulated boron atom of the complexes are analyzed and it indicates that the value of hfcc for the encapsulated boron decreases significantly due to confinement.

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Introduction Endohedral fullerenes have taken wide attention during the last three decades due to their variety of applications in various fields1,2. There are several experimental and theoretical studies available in literature about the encapsulation of various species such as alkali metals, alkaline earth metals, transition metals, p-block elements, noble gases, lanthanides and small molecules (H2, H2O, CH4, NH3) inside fullerenes1-12. The substitution of one or more carbon atoms of fullerene by heteroatom such as boron, nitrogen, phosphorous etc. leads to the formation of heterofullerenes with unique properties13-16. The nitrogen doped azafullerene (C59N) was produced for the first time by arc vaporization of graphite in an atmosphere of pyrrole17. Similarly, borofullerene (C59B) was synthesized by the laser vaporization of graphite in the presence of boron nitride18. Several theoretical studies have been reported on different types of heterofullerenes to investigate their structure, electronic and spectroscopic properties19-25. Liu et. al. investigated the electronic structure of nitrogen and boron doped fullerenes using the tight-binding approximation methods19 and the results suggested that C59B and C59N show electron acceptor and electron donor properties, respectively. Similar results were observed by Andreoni et. al. using Car-Parrinello method20. Using Hartree Fock (HF) and density functional theoretical (DFT/B3LYP) methods, Mekky et. al. studied about the structure and electronic properties of substituted fullerenes C59X (X=B, Al, Ga and In) 21. The study showed that the substitution of carbon atom by a heteroatom leads to a decrease in the HOMO-LUMO energy gap. Density functional theoretical studies have also been performed on various heterofullerenes C59X (X=B, N, Al, Si, P, Ga, Ge, and As) to investigate their relative stability, electronic properties, vibrational frequencies and dielectric constant22. These studies revealed that the chemical and physical properties of heterofullerenes are different from those of bare C60. Time dependent density functional theoretical studies at B3LYP/6-31G* level for various BN-substituted C60 fullerenes indicated that they absorb in the visible region suggesting that the optical and the electronic properties of C60 can be tuned by suitable BN substitution23. To know about the hyperfine coupling constant of the heteroatom present, the electron spin resonance spectroscopic studies of heterofullerenes have been examined24,25. These studies suggested the value of isotropic hyperfine coupling constant of 14N in C59N as 3.60 G (Gauss) with a g-factor value of 2.000424. Later, Schrier et. al. calculated the hyperfine coupling constant for the azafullerenes C19N, C59N, C69N and C75N using B3LYP functional in conjunction with different basis set such as 6-31G, 6-31G* and TZVP25. 2


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The magnetic properties of fullerenes and their derivatives have taken much attention in the scientific community, recently. Several theoretical studies about the paramagnetic endohedral fullerene complexes have been reported26-34. For example, Park et. al. applied quantum mechanical and density functional methods to investigate endohedral complexes of C60 having paramagnetic atoms as the guest species26. The calculated values of stabilization energy for the above complexes suggested that the stability arises mainly due to the dispersive interactions between the guest and the host species. The study also showed that there is no transfer of spin density from the guest to the host or vice-versa. Studies on the trapping of non-metallic paramagnetic atoms inside C60 indicated that the guest atoms retain their electronic configuration without making any covalent bonds with the atoms of the host cage27. To know the effect of confinement on the relative stability of the singlet state with the triplet state of the paramagnetic molecules B2, O2 and Ge2, Asif et. al. studied about the endohedral fullerene complexes B2@C60, O2@C60, and Ge2@C60 28. Their results showed that the guest molecules retained the triplet spin state inside the cage as in their free state. In Po@C60, it is reported that the Po atom locates at the centre of the cage retaining its free state electronic configuration ([Xe] 6s2 4f14 5d10 6p4) without any electron spin density transfer between Po and C60 cage29. Studies on the electronic structure of M@C60 (where, M = Ca, Sc, and Y) suggested the transfer of two s-electrons from Ca, Sc and Y to the cage30. Lu et. al. applied density functional theoretical methods to infer about the electronic structure of Gd@C60 and found that the complex is stable in its septet state31. The interaction between the components of Gd@C60 was found to be ionic in nature, in which three electrons of Gd is transferred to C60 fullerene. They also reported an antiferromagnetic coupling between the guest and the host cage. Dai et. al. theoretically investigated the defect on the surface of U2@C61 and showed that its electronic properties differ from those of the uranofullerene U2@C6032,33. They also reported that the ferromagnetic or antiferromagnetic coupling between the unpaired electrons present in guest and host species occurs depending on the additional carbon atom is attached to the hexagon-hexagon bond or the hexagonpentagon bond of C60. Buchachenko et. al. studied the encapsulation of a hydrogen atom inside the heterofullerenes C59B and C59P using the hybrid density functional B3LYP and 6-31G* basis set34. The study showed that the hydrogen atom can make a bond with one of the carbon atoms of the cage either endohedrally or exohedrally, although the former leads to a more stable complex. Recently, we have investigated the nitrogen atom encapsulated complexes of C60, C59N, and their respective dimers35. Our results showed that unlike in N@C60, either an antiferromagnetic or a ferromagnetic coupling between the guest and the host occurs in N@C59N, depending upon the spin 3



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state of the complex. Similarly, for the complexes (N@C60)2 and (N@C59N)2, the spins of the electrons of the guest species are either ferromagnetically or antiferromagnetically coupled. To the best of our knowledge, studies on the endohedral complexes of paramagnetic heterofullerenes with boron as guest are not reported. Keeping this in mind, here we investigate the structure, electronic, magnetic and spectral properties of boron encapsulated complexes of C60, C59B and C59N. For this purpose, different spin states of the complexes B@C60, B@C59B and B@C59N are considered. The interaction between the guest and the host in the above complexes are analyzed. The electron spin-spin coupling between the guest and the host cage as well as the spin density transfer between them is considered. The effect of confinement on the isotropic hyperfine coupling constant (hfcc) of boron atom due to different cages is studied. The optical properties of various endohedral fullerenes are investigated using time dependent density functional (TDDFT) method. Computational Methods All calculations were performed using the Gaussian 09 program36. Geometry optimization was performed for C60, C59N, C59B and their boron encapsulated derivatives using the hybrid density functional, B3LYP with and without Grimme’s empirical dispersion correction (B3LYP-GD2) in conjunction with 6-311G* basis set37,38. The HOMO-LUMO energy gap of C60 obtained by these methods is 2.7 eV and is in agreement with experimental values reported earlier39,40. The calculated vertical electron affinity (2.44 eV) and vertical ionization energy (7.63 eV) are also found to be in agreement with the experimental values of 2.46 and 7.6 eV, respectively41,42. The value of hyperfine coupling constant for

14

N (3.6 G) of C59N obtained by these methods agrees well with the

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experimental value and that reported at B3LYP/6-31G* level25. For the complexes B@C59B and B@C59N, various possible spin states were considered. The stabilization energy was calculated for each of these complexes for their different spin states and from which the most stable spin state was determined. The broken symmetry calculations were performed to analyze the antiferromagnetic singlet state of the complexes using their high spin state wave function and it was found to be higher in energy than the closed shell singlet. The wave functions corresponding to various spin states of different complexes were tested for their stability and confirmed that they are stable. The frequency analysis confirmed that the optimized geometry of the most stable state of each type of complexes corresponds to minima on the respective potential 4


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energy surface. The stabilization energy of the complex was calculated as follows: ‫ܧ‬௦௧௔௕ = ‫ܧ‬௖௢௠௣௟௘௫ − ‫ܧ‬௖௢௠௣௢௡௘௡௧௦ where, Ecomplex and Ecomponents are the energy of the complex and their components, respectively. The stabilization energy is further corrected for basis set superposition error (BSSE). The vertical ionization energy (VIE) and the vertical electron affinity (VEA) of the species are further calculated from the energy of the neutral, cationic, and anionic species using the optimized geometries of the respective neutral species. The electron spin density associated with the heteroatom of the cage and the encapsulated boron atom of the complexes was also calculated. The spin density difference distribution and the isodensity surface of HOMO and LUMO were plotted using the program GaussView 543. The values of hyperfine coupling constant corresponding to the heteroatom of the cages and the guest species were calculated. The excited state calculations were performed using the TDDFT formalism at the B3LYP/6-311G* level. Results and Discussion 1. Electronic Structure and Host-guest interaction The optimized geometries of different complexes for their various spin states are depicted in Figure 1 and the stabilization energies for the corresponding complexes are listed in Table 1. The complex B@C60 in its doublet spin state, where the boron atom is located at the centre of the cage is found to be stable. The stabilization energy of -11.52 kcal/mol for B@C60 obtained at B3LYPGD2/6-311G* level compared to that of 6.0 kcal/mol at B3LYP/6-311G* level suggests that the stabilization of the complex is mainly due to the dispersion interaction between the guest and the host. The stabilization energy of the complex that formed by the encapsulation of boron atom inside the paramagnetic cage C59B shows that among the possible two spin states of the complex B@C59B, the singlet state is ~40.0 kcal/mol more stable than the triplet one. In the complex 1[B@C59B], the encapsulated boron is located near to the C5B ring of the cage forming a nearly hexagonal pyramidal geometry (edge length 1.95-2.13 Å) as shown in Figure S1 of the supplementary information. In the triplet state of the complex, the encapsulated boron is located near to the centre of the cage. The studies also showed that the complex B@C59N is stable in its singlet and triplet states, although the former is more stabilized (by ~10 kcal/mol) than the latter. In the complex 5



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1

[B@C59N], the encapsulated boron is located adjacent to a C5 ring that is bonded to nitrogen of the

cage thereby forming an approximate pentagonal pyramidal geometry (edge length 1.84-1.96 Å) as shown in Figure S1 of the supplementary information.

In the triplet state of Be@C59N, the

encapsulated boron atom is located near the centre of the cage. To get further insight about the nature of interaction, we have carried out natural bond orbital (NBO) analysis of the above complexes. It revealed that a strong interaction exists between the encapsulated boron and the nearby hexagonal or pentagonal ring of the cages mentioned above in the singlet state of the respective complexes. The analysis also indicated that the interaction of the guest atom with the heteroatom of the cage is more prominent in 1[B@C59B] than in 1[B@C59N], thereby showing a higher stabilization energy for the former. It should mention that, although the substitution of one of the carbon atoms of C60 by a heteroatom (B/N) causes slight distortion of the cage, encapsulation of boron atom inside the cage did not show significant distortion further. The changes in C-C and C-B (or C-N) bond lengths for the nearby hexagonal ring of B@C59B (or pentagonal ring of B@C59N) are less than ~0.03 Å as evident from Figure S1 of the supplementary information. The change in size of the cage due to encapsulation of boron is also negligible. 2. Electronic Properties 2a. Electron Affinity and Ionization Energy To study the electronic properties of the complexes, the vertical electron affinity (VEA) and the vertical ionization energy (VIE) of different spin states of the complexes are calculated and are listed in table 2. From the table, it can be seen that, the VEA of B@C60 is 0.68 eV less than that of C60. On the other hand, the VIE of C60 is not much affected due to the encapsulation of boron atom. Similarly, the electron affinity of the most stable state of the complex B@C59B is 1.07 eV less than that of its host cage C59B. With the encapsulation of boron, the vertical ionization energy of C59B is increased by 0.21 eV for the most stable state of the resultant complex. This suggests that the electron donating propensity is reduced significantly with the encapsulation of boron, probably due to the electron deficiency of boron atom. However, the encapsulation of boron in C59N does not change the vertical electron affinity of C59N. In contrast, the vertical ionization energy of C59N is increased with the encapsulation of boron.

The ionization energy of the most stable state of the

resultant complex suggests that C59N has a more tendency to donate an electron than that for 6


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B@C59N. In conclusion, the complex C59B has a more propensity to accept electrons among all the species considered. 2b. Energy gap between HOMO and LUMO To know more about the electronic properties of the complexes, the isodensity surfaces of HOMO and LUMO as well as the energy gap between them are analyzed. The isodensity surfaces of HOMO and LUMO of the complexes in their different spin states are depicted in Figure 2. For the complexes with unpaired electrons, the α-spin orbitals are given. The corresponding β-spin orbitals are shown in Figure S2 of Supporting Information. The energy gap between the respective HOMO and LUMO of C60, C59B, C59N and their boron encapsulated complexes is listed in Table 2. It can be inferred from the figure that the HOMO corresponding to α-spin of B@C60 is mainly formed by the orbitals of boron atom. The LUMO of the above is formed by the orbitals of carbon atoms of C60, and resembles to the LUMO of C60. Due to the encapsulation of boron, the HOMO-LUMO energy gap of C60 is decreased by ~1.0 eV. It was also found that the HOMO-LUMO energy gap of the complexes varies with their spin states. For example, the HOMO-LUMO energy gap of B@C59B is 2.52 eV for its singlet state, whereas, that of triplet state is 1.79 eV. The frontier molecular orbitals of the singlet state of B@C59B are formed by the orbitals of both the guest and the host species. The HOMO-LUMO energy gap of B@C59N in its triplet state remains nearly the same as that of C59N. However, the HOMO-LUMO energy gap of B@C59N in its singlet state is increased slightly by 0.21 eV with respect to the host cage. It can be inferred from the Figure 2 that the frontier molecular orbitals of B@C59N in its singlet state are formed from the orbitals of both components of the complex. In short, the above results showed that the HOMO-LUMO energy gap for the singlet state of B@C59B and B@C59N, is slightly more compared to the corresponding host cages. 3. Spin-Spin Coupling between Guest and Host To get insight on the spin-spin coupling and the electron spin density transfer between the guest and the host cage, the electron spin density of the guest and the heteroatoms of the cage for various complexes are calculated and are listed in table 3. The spin density difference plots for complexes are depicted in Figure 3. It can be seen from the table that boron atom of B@C60 has an electron spin density of 1.0 e/au3 keeping its original electronic configuration. This also indicates that there is no transfer of spin density from the trapped boron to the host cage or vice versa. It is also evident 7



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from the spin density difference plot of the above complex that the spin density is distributed only on the guest. For the triplet state of B@C59B, the spin of guest and that of host is ferromagnetically coupled without the transfer of spin density between the guest and the host. Similarly, ferromagnetic coupling also occurs in the triplet state of B@C59N without any spin density transfer between its components. Thus, the above observation clearly indicates the possibility of a core@shell type ferromagnetic coupling between the guest and the host cage in the triplet state of B@C59B and B@C59N. 4. Hyperfine coupling constant (hfcc) The effect of confinement on the hyperfine coupling constant (hfcc) of boron due to the diamagnetic cage (C60) and the paramagnetic cages (C59B and C59N) was examined. In addition, we calculated the hfcc of the heteroatom of the cages both in complexes and in the free state. The calculated values of hfcc for the guest and the heteroatom of the cages of various complexes are listed in table 3. From the table, it can be seen that the value of hfcc obtained for free boron is 4.52 G at B3LYP/6-311G* and B3LYP-GD2/6-311G* levels of theory. The value of hfcc of boron is decreased to 1.07 G when it is encapsulated inside C60, although there was no change in its electron spin density. For the complexes 3[B@C59B] and 3[B@C59N], the value of hfcc of the trapped boron is further reduced to 0.47 G and 0.52 G, respectively, showing the effect of encapsulation by a paramagnetic cage. Despite a decrease in the hfcc, the electron spin density of the encapsulated boron for the complexes B@C59B and B@C59N remained the same as in its free state. Further, the hyperfine coupling constant of the heteroatoms of the cages C59B and C59N with and without the guest species are also analyzed. As evident from the table, the value of hfcc of boron in C59B is 1.09 G, which is increased to -0.54 G by encapsulating a boron atom inside. On the other hand, hfcc of nitrogen in C59N (3.67 G) is decreased to 1.90 G on encapsulating the guest species inside. Thus, hfcc of encapsulated boron atom of the complexes decreased in the order B@C60 > B@C59N > B@C59B. 5. UV-Visible Absorption The absorption spectra of C60, C59B, C59N and their boron encapsulated complexes were obtained using the time dependent density functional theoretical method and are depicted in Figure S3 of the Supporting Information. The value of maximum absorption wavelength at 317 nm obtained for C60 is in reasonable agreement with the experimental value of 337 nm in hexane reported earlier44. The 8


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study showed that the boron encapsulated complexes of C60, C59B and C59N do not absorb in visible region. To investigate the stability of antiferromagnetic singlet state of the complex, we carried out broken symmetry calculations for the complexes. The stabilization energy obtained for these complexes are listed in table S1 of Supporting Information. From the values listed in table S1, it is clear that the antiferromagnetic singlet states have higher energy than the corresponding closed shell singlet states. The antiferromagnetic singlet and triplet states were found to be of same energy. Conclusion The structure, electronic, magnetic and spectral properties of boron encapsulated C60, C59B and C59N are investigated using density functional theoretical methods. The study showed that in B@C60, boron atom locates at the centre of the cage. The complex was found to be stable due to dispersion interaction between the guest and the host. The complexes B@C59B and B@C59N exist in two different stable spin states, viz singlet and triplet. For both types of complexes, the singlet state was found to be more stable than the triplet state analogue. The calculated values of vertical electron affinity (VEA) and vertical ionization energy (VIE) obtained for the complexes indicate that C59B has more propensity to accept the electrons than C60, B@C60, B@C59B, C59N and B@C59N. On the other hand, C59N and B@C59N have more tendency to donate electrons. With the encapsulation of boron inside C60, C59B and C59N, the HOMO-LUMO energy gap of the respective cages is either increased or decreased depending upon the spin states of complexes. The calculated electron spin density values of the guest and the host indicated the absence of any spin density transfer between the guest and the host. In B@C60, the boron atom retained its atomic spin as in its free state. The studies also showed ferromagnetic spin coupling between the guest and the host in the triplet state of the complexes B@C59B and B@C59N. The value of the hyperfine coupling constant of boron is decreased due to its encapsulation in a cage and the change was found to be more prominent inside the paramagnetic cages. Supporting Information Various internuclear distances, the HOMO and LUMO plots for the β-spin orbitals of the relevant complexes, calculated absorption spectra of the complexes, stabilization energy of the antiferromagnetic singlet states of the complexes B@C59B and B@C59N. 9



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Acknowledgement Vinit is grateful to University Grants Commission (UGC), New Delhi, India for providing a senior research fellowship (SRF). CNR acknowledges Science and Engineering Research Board (SERB), Department of Science and Technology (DST), India and Sponsored Research and Industrial Consultancy (SRIC), Indian Institute of Technology Roorkee, India for the research grants SB/S1/PC-019/2013 and CMD/FIG/100604, respectively. References [1] Lu, X.; Feng, L.; Akasaka, T.; Nagase, S. Current status and future developments of endohedral metallofullerenes. Chem. Soc. Rev. 2012, 41, 7723-7760. [2] Popov, A. A.; Yang, S.; Dunsch, L. Endohedral fullerenes. Chem. Rev. 2013, 113, 5989-6113. [3] Heath, J.R.; O’Brien, S.C.; Zhang, Q.; Liu, Y.; Curl, R.F.; Kroto H.W.; Tittel F. K.; Smalley, R.E. Lanthanum complexes of spheroidal carbon shells. J. Am. Chem. Soc. 1985, 107, 7779-7780. [4] Murphy, T. A.; Pawlik, Th.; Weidinger, A.; Höhne, M.; Alcala, R.; Spaeth, J. -M. Observation of atom like nitrogen-implanted solid C60. Phys. Rev. Lett. 1996, 77, 1075-1078. [5] Komatsu, K.; Murata, M.; Murata, Y. Encapsulation of molecular hydrogen in fullerenes C60 by organic synthesis. Science, 2005, 307, 238-240. [6] Iwamatsu, S.; Uozaki, T.; Kobayashi, K.; Re, S.; Nagase, S.; Murata, S. A Bowl-shaped fullerene encapsulates a water into the cage. J. Am. Chem. Soc. 2004, 126, 2668-2669. [7] Darzynkiewicz, R. B.; Scuseria, G. E. Noble gas endohedral complexes of C60 buckminsterfullerene. J. Phys. Chem A 1997, 101, 7141-7144. [8] Cioslowski, J.; Fleischmann, E. D. Endohedral complexes: atoms and ions inside the C60 cage. J. Chem. Phys. 1991, 94, 3730-3734. [9] Wang, L. S.; Alford, J. M.; Chai, Y.; Diener, M.; Zhang, J.; McClure, S.M.; Guo, T.; Scuseria, G. E.; Smalley R. E. The electronic structure of Ca@C60. Chem. Phys. Lett., 1993, 207, 354-359. [10] Tishchenko, O.; Truhlar, D. G. Atom-cage charge transfer in endohedral metallofullerenes: trapping atoms within a sphere-like ridge of avoided crossings. J. Phys. Chem. Lett. 2013, 4, 422−425. 10


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[24] Simon, F.; Aron, D.; Tagmatarchis, N.; Garaj, S.; Forro, L.; Prassides, K. ESR signal in azafullerene (C59N)2 induced by thermal homolysis. J. Phys. Chem. A 1999, 103, 6969-6971. [25] Schrier, J.; Whaley, K. B. Hyperfine coupling constants of the azafullerenes C19N, C59N, C69N, and C75N J. Phys. Chem. A 2006, 110, 5386-5390. [26] Park, J. M.; Tarakeshwar, P.; Kim, K. S.; Clark, T. Nature of the interaction of paramagnetic atoms (A=4N, 4P, 3O, 3S) with π systems and C60: A theoretical investigation of A...C6H6 and endohedral fullerenes A@C60. J. Chem. Phys. 2002, 116, 10684-10691. [27] Lu, J.; Zhang, X.; Zhao, X. Electronic structure of endohedral N@C60, O@C60, and F@C60. Chem. Phys. Lett. 1999, 312, 85-90. [28] Equbal, A.; Srinivasan, S.; Ramachandran, C. N.; Sathyamurthy, N. Encapsulation of paramagnetic diatomic molecules B2, O2, and Ge2 inside C60. Chem. Phys. Lett. 2014, 610, 251-255. [29] Chi, M.; Han, P.; Fang, X.; Jia, W.; Liu, X.; Xu, B. Density functional theory of poloniumdoped endohedral fullerenes Po@C60. J. Mole. Struc.: THEOCHEM 2007, 807, 121-124. [30] Lu, J.; Zhang, X.; Zhao, X. Electronic structure of endohedral Ca@C60, Sc@C60 and Y@C60. Solid State Communication 1999, 110, 565-568. [31] Lu, J.; Mei, W. N.; Gao, Y.; Zing, X.; Jing, M.; Li, G.; Sabirianov, R.; Gao, Z.; You, L.; Xu, J.; et. al., Structural and electronic properties of Gd@C60: all-electronic relativistic total-energy study. Chem. Phys. Lett. 2006, 425, 82-84. [32] Dai, X.; Cheng, C.; Zhang, W.; Xin, M.; Huai, P.; Zhang, R.; Wang, Z. Defect induced electronic structure of uranofullerene. Scientific Reports, 2013, 3, 1341. [33] Dai, X.; Xin, M.; Meng, Y.; Han, J.; Gao, Y.; Zhang, W.; Jin, M.; Wang, Z.; Zhang, R.; Stable electronic structure of a defective uranofullerene. Carbon 2014, 78, 19-25. [34] Buchachenko, A. L.; Breslavskaya, N. N. Chemical bond inside endohedral complexes H@C59B and H@C59P. Russian Chemical Bulletin, International Edition 2007, 56, 1283-1288. [35] Vinit; Sujith, K.S.; Ramachandran, C.N. Spin-spin coupling in nitrogen atom encapsulated C60, C59N, and their respective dimers. J. Phys. Chem. A 2016, 120, 6990-6997. [36] Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et. al. Gaussian 09, Revision D.01, Gaussian, Inc., Wallingford CT, 2013. [37] Becke, A. D. Density-functional theromochemistry.III. The role of exact exchange. J. Chem. Phys. 1993, 98, 5648-5652. [38] Grimme, S. Semiempirical GGA-type density functional constructed with a long-range 12


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dispersion correction. J. Comp. Chem. 2006, 27, 1788-1799. [39] De Oliveira, O. V.; Gonçalves, A. da S. Quantum chemical studies of endofullerenes (M@C60) where M=H2O, Li+, Na+, K+, Be2+, Mg2+, and Ca2+. Comp. Chem. 2014, 2, 51-58. [40] Schiller, F.; Ruiz-Osés, M.; Orterga, G. E.; Segovia, P.; Martínez-Blanco, J.;Doyle, B. P.; Pérez-Dieste, V.; Lobo, J.; Néel, N.; Berndt, R.; et. al. Electronic structure of C60 on Au(887) J. Chem. Phys. 2006, 125, 144719. [41] Scheier P.; Dünser, B.; Wörgötter, R.; Lezius, M.; Robl, R.; Märk, T. D. Appearance and ionization energies of singly, doubly, and triply charged C60 and its fragment ions produced by electron impact ionization. Int. J. Mass. Spect. and Ion Proc. 1994, 138, 77-93. [42] Palpant B.; Otake, A.; Hayakawa, F.; Negishi, Y.; Lee, G. H.; Nakajima, A.; Kaya, K. Photoelectron spectroscopy of sodium-coated C60 and C70 cluster anions. Phys. Rev., B 1999, 60, 4509-4512. [43] Roy, D.; Todd, K.; John, M. GaussView, Version 5, Semichem Inc., Shawnee Mission, KS, 2009. [44] Pavlovich, V. S.; Shpilevsky, E. M. Absorption and fluorescence spectra of C60 fullerene concentrated solutions in hexane and polustyrene at 77–300 K. J. Appl. Spect. 2010, 77, 335-342.

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Figure 1 The optimized geometries of the complexes in their different spin states.

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Figure 2. Isodensity surface plots of α-spin highest occupied and lowest unoccupied molecular orbitals of the complexes in their different spin states with isodensity value of 0.02.

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Figure 3. The spin density difference plots of various complexes in their different spin states with isodensity value of 0.0004. The blue colour represents the excess of α spin density and the orange colour represents the excess of β spin density.

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Table 1 The stabilization energy of various endohedral fullerene complexes in their different spin states. The basis set superposition error (BSSE) corrected values are given in parentheses.

Complexes

Multiplicity

B3LYP/6-311G*

B3LYP-GD2/6-311G*

(kcal/mol)

(kcal/mol)

B@C60

2

6.00(7.95)

-11.52(-9.56)

B@C59B

1

-43.20(-41.20)

-52.68(-50.61)

3

5.79 (7.63)

-11.57(-9.67)

1

-13.72 (-11.21)

-24.12 (-21.48)

3

5.13 (7.07)

-12.46 (-10.51)

B@C59N

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Table 2 The values of vertical electron affinity (VEA), vertical ionization energy (VIE) and the energy gap between HOMO and LUMO (∆EHOMO-LUMO) of complexes in their different spin states obtained at B3LYP/6-311G* level. The values obtained at B3LYP-GD2/6-311G* level are given in parentheses.

Complexes

Multiplicity

VEA (eV)

VIE (eV)

∆EHOMO-LUMO (eV)

C60

1

-2.44 (-2.44)

7.63 (7.62)

2.75 (2.74)

B@C60

2

-1.76 (-1.76)

7.72 (7.74)

α=1.77 (1.78) β=2.72 (2.72)

C59B

2

-3.59 (-3.59)

7.23 (7.23)

α=2.41 (2.40) β=1.54 (1.54)

1

-2.52 (-2.52)

7.44 (7.44)

2.52 (2.52)

3

-3.58(-3.61)

7.17(7.17)

α=1.79 (1.79)

B@C59B

β=1.53 (1.53)

C59N

2

-2.72 (-2.72)

6.19 (6.19)

α=1.27 (1.27) β=2.46 (2.45)

1

-2.72 (-2.72)

6.59 (6.60)

1.48 (1.48)

3

-2.68(-2.68)

6.18(6.18)

α=1.26 (1.26)

B@C59N

β=2.46 (2.46)

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Table 3 The spin density values (in e/au3) and hyperfine coupling constant values (hfcc) (in Gauss) on the heteroatom of cage and encapsulated boron atom of various complexes.

Complexes

Multiplicity

B3LYP/6-311G*

B3LYP-GD2/6-311G*

Spin Density

hfcc

Spin Density

hfcc

B

2

1.00

4.52

1.00

4.52

B@C60

2

Bencap=1.00

Bencap=1.07

Bencap=1.00

Bencap=1.07

C59B

2

Bcage=0.14

Bcage=-1.09

Bcage=0.14

Bcage=-1.08

B@C59B

3

Bencap=1.00

Bencap=0.47

Bencap=1.00

Bencap=0.43

Bcage=0.14

Bcage=-0.54

Bcage=0.14

Bcage=-0.54

C59N

2

Ncage=0.08

Ncage=3.67

Ncage=0.08

Ncage=3.63

B@C59N

3

Bencap=1.00

Bencap=0.51

Bencap=1.00

Bencap=0.51

Ncage=0.08

Ncage=1.86

Ncage=0.08

Ncage=1.86

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Structure, Stability, and Properties of Boron Encapsulated Complexes

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