[C_(2)mim][BF_(4)]离子液体^(1)H NMR性质的理论研究

Theoretical Studies on the 1H NMR Properties of[C_(2)mim][BF_(4)]Ionic Liquids

本研究选择1-甲基-3-乙基咪唑四氟硼酸盐(简称[C_(2)mim][BF_(4)])离子液体为研究对象,计算其^(1)H NMR化学位移。首先,我们构建了含有20对阴阳离子的凝聚相体系,采用分子动力学模拟方法获得其凝聚相的平衡态结构。随后,我们等间距的选择5个平衡态结构,用于构建QM/MM模型,详细考察两种QM/MM模型对离子液体^(1)H NMR性质的影响。选取的两种QM/MM模型中的QM分别是一个阳离子、一对阴阳离子。在QM/MM计算中,QM层采用B97-2/pcseg-2模型,MM层采用UFF点电荷模型。我们发现,QM/MM计算结果比仅采用一个阳离子的计算结果与实验值更加的接近。我们考察了MM背景电荷对计算结果的影响,研究表明,当MM层有背景电荷时,其实验值与计算值的偏差为0.30,当MM无背景电荷时,其实验值与计算值的偏差为0.31,表明MM的背景电荷对计算结果的影响较小。

In this work,the 1-methyl-3-ethylimidazolium tetrafluoroborate([C_(2)mim][BF_(4)]in short)ionic liquids were chosen for the study of the^(1)H NMR chemical shifts.Firstly,a condensed phase system,which contains 20 pairs of anions and cations,was constructed to study the^(1)H NMR chemical shifts.Then,molecular dynamics simulations were used to obtain the equilibrium state structure of its condensed phase.Subsequently,five equilibrium structures were randomly selected from MD simulation trajectories to construct the QM/MM models.The QM region is treated with two schemes,one cation and one ion-pair,respectively.Then,the^(1)H NMR chemical shifts were obtained from QM/MM calculations at the B97-2/pcSseg-2:UFF level.We also compared the QM/MM results with those using only one cation with vacuum boundary conditions.We found that for the QM region with an ion-pair,the QM/MM calculations are closer to the experimental values than those with only one cation as the QM region.In addition,we examined the effect of the MM background charge on the calculated results.We found that the average deviation between the experimental and calculated values is only 0.30 when the MM region contains background charges.For the MM region without background charges,the corresponding value is about 0.31.Thus,one can note that the MM background charge has less influence on the calculated^(1)H NMR results.