Transferable Potentials for Phase Equilibria–United Atom Description of Five- and Six-Membered Cyclic Alkanes and Ethers

While the transferable potentials for phase equilibria–united
atom (TraPPE–UA) force field has generally been successful
at providing parameters that are highly transferable between different
molecules, the polarity and polarizability of a given functional group
can be significantly perturbed in small cyclic structures, which limits
the transferability of parameters obtained for linear molecules. This
has motivated us to develop a version of the TraPPE–UA force
field specifically for five- and six-membered cyclic alkanes and ethers.
The Lennard-Jones parameters for the methylene group obtained from
cyclic alkanes are transferred to the ethers for each ring size, and
those for the oxygen atom are common to all compounds for a given
ring size. However, the partial charges are molecule specific and
parametrized using liquid-phase dielectric constants. This model yields
accurate saturated liquid densities and vapor pressures, critical
temperatures and densities, normal boiling points, heat capacities,
and isothermal compressibilities for the following molecules: cyclopentane,
tetrahydrofuran, 1,3-dioxolane, cyclohexane, oxane, 1,4-dioxane, 1,3-dioxane,
and 1,3,5-trioxane. The azeotropic behavior and separation factor
for the binary mixtures of 1,3-dioxolane/cyclohexane and ethanol/1,4-dioxane
are qualitively reproduced.

尽管相平衡可迁移势-联合原子（TraPPE–UA）力场通常可成功提供可在不同分子间高度迁移的力场参数，但给定官能团的极性与极化率在小型环状结构中会发生显著扰动，这限制了从线性分子中获取的参数的可迁移性。为此，我们开发了专门针对五元环与六元环环烷烃及醚类的TraPPE–UA力场版本。我们从环烷烃中拟合得到的亚甲基伦纳德-琼斯（Lennard-Jones）参数，将按环的尺寸转移至对应醚类分子中；而氧原子的伦纳德-琼斯参数则对给定环尺寸的所有化合物通用。不过，部分电荷具有分子特异性，需通过液相介电常数进行参数化拟合。该模型可准确预测以下分子的饱和液相密度、蒸气压、临界温度与临界密度、正常沸点、热容及等温压缩率：环戊烷、四氢呋喃、1,3-二氧戊环、环己烷、四氢吡喃、1,4-二氧六环、1,3-二氧六环及1,3,5-三氧六环。对于1,3-二氧戊环/环己烷与乙醇/1,4-二氧六环这两组二元混合物，其共沸行为与分离因子均可得到定性重现。