石油中含氮化合物碱性的分子模拟研究

Molecular simulation study of basicity of nitrogen-containing compounds

利用密度泛函理论的量子力学从头算法,计算石油中含氮化合物与H+质子的结合能,考察其碱性的强弱。计算中采用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+质子结合的强弱与其碱性强弱具有线性关系;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～-640)kJ/mol之间,吡啶、芳香胺和吡咯类氮化物分别约为(-620,-590,-510)kJ/mol,它们的碱性依次减弱。氮原子上连接烷基、苯基基团会对含氮化物的碱性有一定的影响。随着芳香胺结构中苯基数的增加,与H+结合能力逐渐减弱,碱性也逐渐减弱。芳香环的并入对五员和六员杂环氮化物碱性的影响有所不同。溶剂性质会影响含氮化合物的碱性;随着溶剂极性的减弱,含氮化合物与H+结合得越强,其碱性也越强;在不同溶剂中,含氮化合物碱性相对强弱的顺序会发生变化。

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, COSMO （conductor-like screen model） is selected to simulate solution environments in the quantum mechanical calculation. The results show that there is a distinct relationship between the protonation energies of nitrogen-containing compounds in aqueous solution and their basicity . The stronger the combination of nitrogen-containing compounds with H^＋, the higher their basicity. Therefore, the relative basicity 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, -590 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. Alkyl substituents or the addition of phenyl group may change the basicity of the nitrogen-containing compounds. The addition of the aromatic rings influences the basicity of 5- and 6-membered heterocyclic compounds differently. The solvent properties may affect the basicity of these nitrogen-containing compounds. The lower the solvent polarity, the more negative the protonation energy, the stronger the basicity. The relative basicity of these compounds are altered somewhat in the different solvents.