间位聚苯及其衍生物的构象与电子结构的理论研究

用分子力学方法(MM)对间位聚苯(PMP)及其衍生物的势能随扭曲角(f)变化规律进行了研究.结果表明，势能曲线上出现四个能量最小值，分别对应于扭曲角f ≈－135°,－45°,45°,135°.进一步对以上构建的全部PMP及其衍生物用分子力场Drieding 2.11进行了分子力学能量优化.最终得到主要的四种构象，其中螺旋构象的能量最低，而且以螺旋构象为优势分布构象.在真实条件下，PMP及其衍生物长链可能采取以上四种构象的混合片段组成.用量子化学方法(GGA-DFT)研究了PMP及其衍生物的电子结构能隙随相邻苯环之间的扭曲角的变化趋势.用量子化学半经验方法(AM1)对四种构象分别进行几何优化，优化结果与分子力学优化结果基本一致，并运用混合密度泛函方法(DFT/B3LYP/6-31G)进一步对AM1优化的构象结构进行更精确的电子结构能隙计算.最终得出影响间位聚苯及其衍生物电子结构能隙的主要因素为连接苯环间扭曲角的大小.

A Theoretical Invstigation of Conformation and Electronic Structure of Poly(m-phenylene) and Its Derivatives

Potential energy of poly(m-phenylene)(PMP, P1) and the related derivatives (P2,P3) as a function of torsion angles(f) were constructed by using molecular mechanics method based on force field(Drieding 2.11). It is found that there are four minima in potential energy curve, which correspond to four torsion angles (f ≈－135°, －45°, 135°, 45°) respectively. Furthermore, all constructed PMP and derivatives are minimized by molecular force field, and four main conformations are found. For PMP, two helical conformations have minimal energy, and they have preponderant population. Under the real conditions, PMP and derivatives polymer chain may consist of above stable conformation segments with an equilibrium ratio, which depends on polymer surroundings. The electronic structure (HOMO-LUMO gap) prediction depending on torsion angles was calculated by GGA-DFT method using Dmol3 program. The AM1 method was used to investigate geometry optimization, and the optimized conformations are basically consistent with that simulated by molecular mechanics. In order to get more accurate HOMO-LUMO gap of above optimized geometries, the hybrid functional B3LYP was adopted at the level of 6-31G basis set using Gaussian98 software. At last, it is found that the twist extent of torsion angle between phenyl rings is the main factor that affects HOMO-LUMO gap of PMP and its derivatives.