含氮化合物碱性强弱的量子力学研究

Quantum mechanical studies on basicity of nitrogen-containing compounds

利用密度泛函理论的量子力学从头算法,计算原油及石油产品中不同含氮化合物与H^+的结合能,并采用COSMO(conductor-like screen model)模型模拟溶剂的介质环境。结果表明,在水溶剂中,含氮化合物与H^+结合的强弱与碱性强弱具有密切关系;与H^+结合得越强,其碱性就越强;反之,则越弱。因此,可以算出它们在水溶剂中与H^+结合能的高低,判断其碱性的强弱。脂肪胺、非芳香杂环氮化物与H^+质子的结合能约在-660～-640kJ/mol之间,吡啶、芳香胺和吡咯类氮化物分别约为-620、-600、-510 kJ/mol,它们的碱性依次减弱。随着N-烷基苯胺烷基链中碳原子数从0增长到6,与H^+结合能从-595kJ/mol降低到-610kJ/mol,碱性不断增强;烷基链继续增长,结合能变化很小,对碱性影响不大。芳香环的并入以及芳香环的连接方式,对五员和六员杂环氮化物碱性的影响有所不同。不同氮化物碱性的强弱和其结构有关,尤其电子密度的分布状态,其中电子共轭结构对碱性有很大的影响。

DFT-based （density functional theory） ab initio quantum mechanical methods have been applied to calculate the protonation energies of various nitrogen-containing compounds in petroleum and its products. COSMO （conductor-like screen model） is selected to simulate aqueous solution in the quantum mechanical calculation. Tbe results show that there is a distinct relationship between the protonation energies of nitrogen-containing compounds in aqueous solution and their basicities. The stronger the combination of nitrogencontaining compounds with H^＋ , the higher their basicities. Therefore, the relative basicities of these compounds can be effectively characterized by their protonation energies. The protonation energies of aliphatic amines and nonaromatic heterocycles are about - 660 - -640 kJ/mol. Those of pyridines, aromatic amines and pyrroles are about -620 kJ/mol, -600 kJ/mol, and -510 kJ/mol, respectively. It suggests that aliphatic amines and nonaromatic heterocycles are more basic than pyridines or aromatic amines, and all these compounds are more basic than pyrroles. The protonation energies of N-alkyl anilines are reduced from - 595 kJ/mol to - 510 kJ/mol with carbon atoms in alkyl chain from zero to six to indicate that the basicity of N-alkyl anilines increase with length of alkyl.While the length continues increasing, the protonation energies of N-alkyl anilines change litt｜e, so their basicities are hardly affected by alkyl chain. The fusion of the aromatic rings may differently influence the basicities of 5- and 6-membered aromatic heterocyclic compounds. The base difference among the various nitrogen-containing compounds is perhaps attributed to their structures, especially the distribution of electron density. The electron delocalization may shift the basieity greatly.