中文摘要：

本文采用密度泛函理论（DFT-B3LYP），在6-31G^＋的水平上对C2nH3和C2nH3^＋（n=4，5，6，7）所有可能的几何构型进行了优化和振动频率的计算，进一步得到了C2nH3（n=4，5，6，7）最稳定构型的垂直和绝热电离能，并在这个水平上用含时的密度泛函理论（TD-B3LYP）计算它们的垂直激发能．结果表明中性分子和相对应的阳离子的基态构型几乎是一样的，具有聚炔烃线性链结构的C2H3和C12H3自由基及其他们的阳离子都具有30～60cm。的低频振动模式．通过激发能和电离能的研究发现C2nH3自由基（n=4，5，6，7）基态平衡几何结构的电离能较大，中性分子不易发生电子跃迁，而且不易失去一个电子变为阳离子，所以C2nH3自由基（n=4，5，6，7）基态平衡几何结构比相对应的阳离子要稳定．

英文摘要：

Density functional theory （DFT） has been used to investigate the geometries, vebrational frequencies of C2nH3 and C2nH3 （n =4, 5, 6, 7）. Vertical electronic transition energies of C2nH3 and C2nH3 （n = 4, 5, 6, 7） are calculated by the time-dependent density functional theory （TD-DFT）. In addition, the adiabatic ionization potential and vertical ionization potential of C2nH3（n =4, 5, 6, 7） are obtained. The results show that the ground state structures of C2nH3 and C2nH3^＋ （n = 4, 5, 6, 7） are same. The structure is open chain with C2v symmetry showing polyacetylenic structure, the number of carbon atom of which is 8 and 10. The structure is single ring with Cs symmetry, the number of carbon atom of which is 12 and 14. The stability of C2nH3 and C2nH3 （n = 4, 5, 6, 7） increase as the number of carbon atoms increase, respectively. The lowest frequency origins in the 30cm^- 1～60cm^-1 of C2a H3 and C2nH3 （ n = 4,5,6,7 ） which show polyacetylenic structure. Theoretical investigations of vertical transition energies and ionization potentials found that C2nH3 （n =4,5,6,7 ） has the large ionization potential （IP）. It is opposite of the report for C2n＋1H3. And the electron of C2nH3 （n= 4, 5, 6, 7 ） is difficult to transfer and lose an electron. So the ground state isomer of C2nH3 （n = 4,5,6,7 ） radical is stable than that of C2nH3 （n =4,5,6,7） cation.