Lengvinaitė Dovilė, Aidas Kęstutis, Kimtys Liudvikas
Institute of Chemical Physics, Faculty of Physics, Vilnius University, Saulėtekio al. 3, LT-10257 Vilnius, Lithuania.
Phys Chem Chem Phys. 2019 Jul 10;21(27):14811-14820. doi: 10.1039/c9cp01892a.
The 1H NMR signal of the acidic proton in acetic acid molecules shows a marked upfield shift in the neat liquid as compared to that in low-concentration acetic acid solution in inert solvents where acetic acid cyclic dimers predominate. The underlying reasons for this phenomenon are analyzed in this work by considering classical molecular dynamics simulations and combined quantum mechanics/molecular mechanics calculations of the 1H NMR chemical shift of the acidic proton in the neat liquid and in the cyclic dimer of acetic acid in cyclohexane solution. Recorded trajectories were quantitatively analyzed in terms of different types of molecular aggregates formed in the neat liquid by using a geometrical definition of the hydrogen bond. Both the geometrical analysis and the computational NMR results indicate that the cyclic dimer cannot be the dominating aggregation pattern for acetic acid molecules in the neat liquid. The applied computational approach reproduces the lowering of the 1H NMR chemical shift of the acidic proton in acetic acid when going from cyclohexane solution to the neat liquid very well. The presence of acetic acid aggregates with hydrogen bonding between hydroxyl moieties and of monomeric acetic acid molecules in the neat liquid is found to lead to the observed lowering of the chemical shift, with lesser contribution from the formation of open acetic acid aggregates.
与在惰性溶剂中以乙酸环状二聚体为主的低浓度乙酸溶液相比，乙酸分子中酸性质子的1H NMR信号在纯液体中显示出明显的高场位移。本文通过考虑经典分子动力学模拟以及对纯液体和环己烷溶液中乙酸环状二聚体中酸性质子的1H NMR化学位移进行量子力学/分子力学联合计算，分析了这一现象的潜在原因。利用氢键的几何定义，对记录的轨迹进行定量分析，以研究纯液体中形成的不同类型分子聚集体。几何分析和计算NMR结果均表明，环状二聚体并非纯液体中乙酸分子的主要聚集模式。所应用的计算方法能够很好地再现从环己烷溶液到纯液体时乙酸中酸性质子的1H NMR化学位移降低的情况。研究发现，纯液体中存在羟基部分之间通过氢键结合的乙酸聚集体以及单体乙酸分子，这导致了观察到的化学位移降低，而开放乙酸聚集体的形成贡献较小。
