取代基对1-甲基尿嘧啶与N-甲基乙酰胺氢键复合物中氢键强度的影响

使用密度泛函理论B3LYP方法和二阶微扰理论MP2方法对由1-甲基尿嘧啶与N-甲基乙酰胺所形成的氢键复合物中的氢键强度进行了理论研究, 探讨了不同取代基取代氢键受体分子1-甲基尿嘧啶中的氢原子对氢键强度的影响和氢键的协同性. 研究表明: 供电子取代基使N－H…O＝C氢键键长r(H…O)缩短, 氢键强度增强; 吸电子取代基使N－H…O＝C氢键键长r(H…O)伸长, 氢键强度减弱. 自然键轨道(NBO)分析表明: 供电子基团使参与形成氢键的氢原子的正电荷增加, 使氧原子的负电荷增加, 使质子供体和受体分子间的电荷转移量增多; 吸电子基团则相反. 供电子基团使N－H…O＝C氢键中氧原子的孤对电子轨道n(O)对N－H的反键轨道σ^*(N－H)的二阶相互作用稳定化能增强, 吸电子基团使这种二阶相互作用稳定化能减弱. 取代基对与其相近的N－H…O＝C氢键影响更大.

Effect of Substituents on Hydrogen Bond Strength in Hydrogen-Bonded N-methylacetamide and Uracil Complexes

Theoretical calculations on a series of N－H…O＝C hydrogen-bonded complexes containing 1-methyluracil and N-methylacetamide were carried out using B3LYP and MP2 methods. Substituent effects in the hydrogen bond acceptor molecule (1-methyluracil) on the hydrogen bond strength and hydrogen bond cooperativity were explored. The calculation results show that electron donating groups shorten the H…O distance and strengthen the N－H…O＝C hydrogen bond, whereas electron withdrawing groups lengthen the H…O distance and weaken the N－H…O＝C hydrogen bond. Natural bond orbital (NBO) analysis further indicates that electron donating groups result in a larger positive charge on the H atom and a larger negative charge on the O atom in the N－H…O＝C bond, and result in increased charge transfer between the proton donor and acceptor molecules. Electron withdrawing groups show the opposite results. NBO analysis also indicates that electron donating groups result in larger second-order interaction energies between the oxygen lone pair and the N － H antibonding orbital when compared to the 1-methyluracil-containing complex (R=H), while electron withdrawing groups result in smaller second-order interaction energies.