AM1 Investigation on Stabilities of C26BN Isomers

Li Jianhong, Wang Zhichang, Zhang Xie, Li Jianqiang
(Department of Applied Chemistry, East China Institute of Technology, Fuzhou, 344000)

Abstract The possible stable structures of substituted fullerene C26BN formed on the initial C28 cage with Td symmetry have been systematically studied by AM1 method included in Gaussian 98W package. The symmetries(Cs or C1) were constrained and the charge and multiplicity of all the isomers are zero and five, respectively. The geometrical optimization and the vibrational frequencies analysis were performed on the same level with constrained symmetry(Cs or C1). The results show that the most stable isomer of C26BN is formed by boron and nitrogen atoms doping at 5,2-sites. The stabilities of C26BN isomers were determined mainly by B-N and conjugative effect.

Heterofullerenes which have one or more carbon atoms substituted by other elements such as boron and nitrogen are expected to be used as semiconductors and building materials for nanometer electronics due to their potential novel properties [1-2], such as superconductivity, hardness, photoinduced electron transfer and nonlinear. The first heterofullerene of C60-xBx(x=1-6) were prepared by Smalley[3] and co-worker in 1991.
Considering BN and CC are isoelectronic species, boron and nitrogen atoms substituted fullerenes may be stable, which were also surmised from a large number of research results. In fact, BN substituted C60-n(BN)n have already been reported by Piechota early in 1996. Theoretical studies on the structures and stabilities of C34BN isomers[4], C38BN[5] have been performed recently. AM1 method is proved to be an effective means to forecast the structure and stabilities of the heterofullerene isomers[4]. However, it turns out that little have been known so far about the boron and nitrogen substituted C26BN based on fullerene C28 with Td symmetry[5] and four unpaired electronics on ground state, the smaller fullerene relatively to C36 and C60.

The C26BN isomers were constructed by boron and nitrogen substituted the carbon atoms of the C28 cage with Td symmetry. The geometrical optimization and the frequencies of all the possible isomers of C26BN have been performed at the AM1 level supplied by Gaussian 98W package on P4 computer, the keyword scf(conver=9) was used in the calculations, the charge and the multiplicity of the isomers of C26BN are zero and five, respectively. Symmetry (Cs or C1) of all the isomers were constrained during the optimizing process.

The geometry configuration of C28 with Td symmetry was shown in Fig.1. For the convenience of discussion, all the carbon atoms of C28 cage were numbered, and heterofullerenes C26BN were labeled as C26BN: i-j which indicated that boron the special symmetry, C28 molecule with Td symmetry has three kinds of atoms, the first kind of carbon atoms labeled as 1,8,15 and 22 are the juncture of three five-membered rings, the second kind of carbon atoms connect the first kind of carbon atoms labeled as 2,3,4, and so on, the third kind connect the second kind. Except the isomers with the same energy brought by symmetry, forty C26BN isomers were obtained. The symmetry and heat of formation(HOF) at AM1 level were listed in Table 1. The vibrational analysis indicated all the isomers have no imaginative frequencies.

Figure 1 The structure and numbering of C28 with Td symmetry

Table 1 The symmetry and heat of formation for C26BN isomers at AM1 level

No. Isomers Sym. HOF
No. Isomers Sym. HOF
1 C26BN: 1-2 Cs 3399.546 21 C26BN: 11-2 C1 3506.737
2 C26BN: 1-5 C1 3452.388 22 C26BN: 13-2 Cs 3418.987
3 C26BN: 2-5 C1 3410.013 23 C26BN: 14-2 C1 3415.022
4 C26BN: 2-6 C1 3476.896 24 C26BN: 14-3 Cs 3412.698
5 C26BN: 2-7 C1 3489.323 25 C26BN: 8-4 Cs 3451.153
6 C26BN: 2-1 Cs 3425.170 26 C26BN: 9-4 C1 3439.566
7 C26BN: 5-1 C1 3442.078 27 C26BN: 11-4 Cs 3443.259
8 C26BN: 2-3 C1 3534.340 28 C26BN: 13-4 C1 3414.304
9 C26BN: 5-2 C1 3363.465 29 C26BN: 12-5 Cs 3487.880
10 C26BN: 6-2 C1 3414.490 30 C26BN: 13-5 C1 3451.747
11 C26BN: 7-2 C1 3426.489 31 C26BN: 14-5 C1 3440.147
12 C26BN: 5-6 C1 3439.850 32 *C26BN: 8-6 C1 3441.286
13 C26BN: 8-1 Cs 3541.287 33 C26BN: 9-6 Cs 3480.335
14 C26BN: 10-1 C1 3467.902 34 C26BN:10-6 C1 3481.347
15 C26BN: 11-1 Cs 3471.044 35 C26BN: 13-6 C1 3484.843
16 C26BN: 13-1 Cs 3438.183 36 C26BN: 14-6 C1 3480.197
17 C26BN: 14-1 Cs 3462.749 37 *C26BN: 8-7 C1 3520.526
18 C26BN: 8-2 C1 3449.878 38 C26BN: 9-7 C1 3488.314
19 C26BN: 9-2 C1 3497.104 39 C26BN: 10-7 Cs 3488.535
20 C26BN: 10-2 C1 3453.640 40 C26BN: 13-7 C1 3439.850

* Symmetry is debased from Cs to C1 for convergence.

As far as C26BN isomers are concerned, heteroatoms bring about deformation more or less, the deformation of C26BN isomers with bonded heteroatoms is small, whereas the deformation of C26BN isomers with isolated heteroatoms is large. The C26BN: 5-2 isomer has very similar structure to C28(Td) and is the most stable isomer. The difference of boron and nitrogen atomic charge is affected mainly by the distance of boron and nitrogen atoms, when boron and nitrogen are bonded or near, s-bond inducing effect makes electron transfers from boron atom to nitrogen atom, this is primary; on the other hand, conjugative effect makes electron transfers from carbon and nitrogen atoms to boron atom, because two electrons of nitrogen atom, one electron of carbon atom and empty orbit supported by boron atom participate in the conjugative effect. It can be concluded that the stabilities of the C26BN isomers are affected by both inducing effect and conjugative effect, since the bonded boron and nitrogen atoms come up to one carbon-carbon bond to a great extent. This shows that bonded B-N unit make a great contribution to the stabilities of the C26BN isomers, which is in good agreement with the former conclusion[7]. When nitrogen atom and boron atom replace C2 and C5 atom, respectively, nitrogen atom can provide electron both to boron and also to C1 atom, this forms remarkable conjugative effect, keeps the analogous structure of C28(Td), whereas, boron atom of another isomer C26BN: 2-5 cannot provide electron to C1 atom to conjugate although nitrogen can provide electron to boron atom. The heat of formation of the other isomers with unbonded heteroatoms are high, greater than the most stable one due to lack of B-N σ-bond inducing effect and lack of conjugative effect. What is more, It is worthwhile to mention that C26BN: 12-5 isomer has a considerably high heat of formation although boron and nitrogen atoms are adjacent, yet this substitution mode will destruct the conjugative effect. If you have any chemistry news articles and you want to share them, please contact us.