N8H8环状异构体的结构与稳定性的理论研究

Theoretical Study on the Structures and Stabilities of the Ring Isomers of N_8H_8

采用密度泛函理论的B3LYP方法在6-311++G**基组水平上对N8H8氮氢环状化合物可能存在的构型进行了几何优化,得到74种稳定异构体,应用自然键轨道理论NBO和分子中的原子理论AIM分析了这些化合物成键特征和相对稳定性,G3MP2方法计算了各异构体的能量及生成热.研究结果表明:N原子孤对电子到相邻的氮氮键的超共轭作用是影响氮氮键长变化的主要因素;N8H8环状异构体的稳定性顺序为:六元环>七元环>八元环,五元环>三元环>四元环,六元环是这些N8H8环状异构体中最稳定的,最不稳定的是四元环,G19是所有环状异构体中能量最低的;M3能量最高,稳定性最差,A7密度最大.

Seventy-four possible steady isomers of ring-N8H8 have been optimized by a density function B3LYP method at 6-311 ＋ ＋G＊＊ level NBO （nature bond orbital） and AIM （atoms in molecules） were used to discuss bond nature and relative stabilities of these conformers. Energies and heats of formation were calculated by a G3MP2 method. The result indicates that the hyperconjugation effect from lone pairs of the nitrogen atoms on its neighboring N--N bonds is the main factor that affects the length of N-4-N bond. The relative stability order of the isomers of ring-N8H8 is as follows： six-membered-ring 〉 seven-membered-ring 〉eight-membered-ring; five-membered-ring〉three-membered-ring〉four-membered-ring. Among all the conformers, six-membered-ring isomers are the most stable ones while the four-member-ring isomers are the most unstable ones. In addition, G19 has the lowest energy. M3 has the highest energy, so it is the most unstable isomer. The density of A7 is the largest.