High-level ab initio molecular orbital calculations of imine formation
High-level ab initio molecular orbital calculations of imine formation
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DOI：
10.1021/jp9810825
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发表时间：
1998-06-18
影响因子：
2.9
通讯作者：
Smith, BJ
中科院分区：
文献类型：
--
作者：
Hall, NE;Smith, BJ
本文报道了甲胺与甲醛反应生成席夫碱N-甲基甲亚胺的G_2（MP_2，SVP）水平的从头算分子轨道计算。通过分子内质子转移形成甲醇胺的气相势垒被发现显着降低时，明确的水分子被用来促进质子转移。最低的势垒位于甲胺、甲醛和两个水分子之间的复合物上方14.7 kJ mol（-1），并通过两性离子中间体。该过程的过渡态的结构特征与实验预测的非常相似。虽然分子内质子通过水分子转移以从甲醇胺形成亚胺所需的能量比没有水时少得多，但势垒仍然很大。对于两个沃茨，势垒计算为111.9 kJ mol（-1）。一个相当小的障碍是预测质子化甲醇胺，67.9 kJ mol（-1），虽然没有实验证据的内部质子转移，计算的结构显示实验预测的过渡态的特性。计算的水相离解能与实验观察到的平衡常数相比太小。
Ab initio molecular orbital calculations at the G2(MP2,SVP) level are reported for the reaction between methylamine and formaldehyde to form the Schiff-base, N-methylmethanimine, with loss of water. The gasphase barrier to carbinolamine formation through intramolecular proton transfer is found to be dramatically reduced when explicit water molecules are employed to facilitate the proton transfer. The lowest barrier lies 14.7 kJ mol(-1) above the complex between methylamine, formaldehyde, and two water molecules and passes through a zwitterionic intermediate. The structural features of the transition state for this process closely resemble those predicted experimentally. Although intramolecular proton transfer through water molecules to form the imine from the carbinolamine requires significantly less energy than without the water, the barrier remains quite large. With two waters the barrier is calculated to be 111.9 kJ mol(-1). A considerably smaller barrier is predicted for the protonated carbinolamine, 67.9 kJ mol(-1), and while there is no experimental evidence for internal proton transfer, the calculated structure shows characteristics of the experimentally predicted transition state. Calculated aqueous-phase dissociation energies are too small in comparison with the experimentally observed equilibrium constants.