C60富勒烯-巴比妥酸及其二聚体几何结构和电子结构的密度泛函计算研究

Density functional study on the structural and electronic properties of fullerene-barbituric acid and its dimmer

采用基于密度泛函理论的广义梯度近似,对C60富勒烯-巴比妥酸及其二聚体的几何结构和电子结构进行了计算研究.发现:C60富勒烯-巴比妥酸只有一种稳定结构,且掺杂巴比妥酸基团对C60分子构型的影响是局域的.C60富勒烯-巴比妥酸的二聚体有三种同素异构体,分别以[6,5]—[6,5],[6,6]—[6,5]和[6,6]—[6,6]三种方式键合,从能隙大小顺序和总能相对大小来看,[6,6]—[6,6]结构最为稳定.电子结构方面,在C60富勒烯-巴比妥酸单体中,Donor-Acceptor电荷转移体系为C60富勒烯-巴比妥酸,即电荷是从C60向巴比妥酸转移.由前线轨道和自旋布居数得知,C60富勒烯-巴比妥酸单体很好地保留了C60的电磁性质,但稳定性下降,易发生二次加成反应形成二聚体.对于C60富勒烯-巴比妥酸二聚体,Mulliken电荷分析显示,在加成四元环处的碳原子分别得到0·104e和0·106e电荷,而与它们邻近的碳原子则失去电子,带有正电荷,且距加成位置越近的碳原子失去的电荷越多.在远离加成位置处,碳原子的净电荷变化相对较小.与单体1·52eV能隙相比,二聚体中的能隙为1·45eV.其前线轨道分布与单体相比,最高占据轨道几乎未变,但最低未占据轨道发生了很大变化.

Geometric and electronic properties of fullerene-barbituric acid and its dimmer have been studied using density functional theory （DFT） at BLYP level with DNP basis sets. The calculation results indicate that the only one stable geometry of the fullerene-barbituric acid is that with [6,6] close-bond structure, and the effects of the barbituric acid on the cage structure is mainly in the local region neighboring the barbituric acid. Among the three isomers of the fullerene-barbituric acid dimmers, one can come to the conclusion both from the energy gap and from relative energy that the [6,6]—[6,6] structure is the most stable one. With respect to the electronic structures, the donor-acceptor system corresponds to C60 fullerene-barbituric acid, ie. electron transfers from the fullerene cage to the barbituric acid. The frontier orbitals and spin population show that C60 fullerene-barbituric acid has similar electromagnetic properties of C60, but its stability is reduced, so that the addition reaction would easily take place to form its dimmer. For the C60 fullerene-barbituric acid dimmer, the Mulliken analysis shows there are 0.104e and 0.106e charge transfers to the four carbons from their neighboring carbons, and the farther the atoms is, the less charge it loses. Compared with those of the C60 fullerene-barbituric acid, the energy gap is changed from 1.52 eV to 1.45 eV, while the distribution of highest occupied molecular orbital （HOMO） is little changed but that of lowest unoccupied molecular orbital （LUMO） is changed a lot.