Center for Computational Materials Science, Institute of Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, 980-8577 Sendai, Japan.
J Chem Phys. 2012 Aug 7;137(5):054715. doi: 10.1063/1.4742193.
The properties (geometry, spin, and charge distribution) of a series of flat hexagonal zigzag edged graphene nanodots (GNDs), with interiors modified by centrally located substituent atoms boron and nitrogen and by positive and negative charge, have been calculated using ab initio density functional theory. The doped series X-GND has the stoichiometry C(6m(2)-1)XH(6m), zigzag size index m = 2, 4, 6, 8, 10 and substituent X = B or N. The undoped parents C(6m(2))H(6m) with m ≤ 8 have spin paired ground states and the parent m = 10 has a spin polarized singlet ground state with edges that alternate α- and β-spin. The spin on the substituent atom decreases to zero with size index m and magnetization builds on the edges of all the X-GND. This demonstrates translocation of substituent spin and a proximity or directional effect for small m as the edges show different degrees of magnetization. For the largest X-GND (m = 10) the magnetization on edges resembles the calculated triplet S = 1(a) configuration of the parent (four edge spins up and two down) and has a higher apparent symmetry than the C(2v) point group of X-GND. For charged (m = 10) GNDs the edge magnetization has strength comparable to the parent on two parallel edges and weak on the other four in a perimeter pattern that resembles the triplet S = 1(b) configuration of the undoped parent and not the ground configuration of the isoelectronic X-GND molecule. Many of the results can be interpreted by simple Kekulé valence bond structures for an unpaired spin on a network where the substituent site group symmetry is not compatible with the perimeter. A deeper understanding is provided by the properties of the Kohn-Sham orbitals. The calculations of the X-doped GNDs reveal limitations in the use of the hex-radical hypothesis of the parent ground state to systems where foreign atoms lower symmetry and perturb the π- and σ-bond manifolds.
使用从头算密度泛函理论计算了一系列具有中心取代原子硼和氮以及正电荷和负电荷的平面六方锯齿形边缘石墨烯纳米点（GND）的性质（几何形状、自旋和电荷分布）。掺杂系列 X-GND 的化学计量为 C(6m(2)-1)XH(6m)，锯齿形尺寸指数 m = 2、4、6、8、10，取代基 X = B 或 N。未掺杂的母体 C(6m(2))H(6m) 具有 m ≤ 8 的自旋配对基态，母体 m = 10 具有自旋极化单重态基态，边缘交替具有α-和β-自旋。随着尺寸指数 m 的增加，取代原子上的自旋减小到零，并且所有 X-GND 的边缘都产生了磁化。这表明取代基自旋的迁移和小 m 的邻近或定向效应，因为边缘显示出不同程度的磁化。对于最大的 X-GND（m = 10），边缘的磁化类似于母体的计算三重态 S = 1(a)构型（四个边缘自旋向上和两个向下），并且具有比 X-GND 的 C(2v)点群更高的表观对称性。对于带电的（m = 10）GND，边缘的磁化强度与两个平行边缘上的母体相当，而在其他四个边缘上较弱，形成类似于未掺杂母体的三重态 S = 1(b)构型的周长图案，而不是等电子 X-GND 分子的基态构型。许多结果可以通过网络上未配对自旋的简单凯库勒价键结构来解释，其中取代基位组对称性与周长不兼容。Kohn-Sham 轨道的性质提供了更深入的理解。X 掺杂 GND 的计算揭示了在使用母体基态的六价自由基假设来处理降低对称性和扰乱π和σ键系的外来原子的系统时存在局限性。
