Prior Likelihoods and Space-Group Preferences of Solvates

For
a range of organic solvents, the likelihood of the solvent
forming solvates has been estimated using the recrystallization solvent
(RS) data in the Cambridge Structural Database (CSD). Although RS
data are viewed with caution by some crystallographers, most of the
likelihood estimates are shown to have good precision. Strong trends
are apparent in the results. For example, high likelihoods are found
for aromatic solvents with electron-withdrawing substituents and low
likelihoods for acyclic aliphatic hydrocarbons. Results for different
CSD subsets, such as organic and metalloorganic, are highly correlated.
The likelihood that a solvent will form solvates is almost always
higher when the solvent is part of a mixture than when it is pure.
The likelihood of two solvents forming a heterosolvate (i.e., both
solvents in the structure) can be well estimated by the product of
the likelihoods of the solvents forming normal solvates (i.e., only
one solvent in the structure). The space-group preferences of solvates
vary significantly with the nature of the cocrystallized solvent.
Those of nonsolvates vary significantly with the solvent(s) from which
they were crystallized. Solvents with inversion centers favor solvate
crystallization in centrosymmetric space groups, and solvents with
2-fold rotational symmetry promote crystallization in space groups
with 2-fold proper rotational axes. The inclusion of cyclohexane and
carbon tetrachloride in a lattice can facilitate crystallization in
trigonal and tetragonal space groups, respectively. Our results can:
(a) guide solvent selection when solvates are undesired; (b) assist
in predicting solvate formation, e.g., using Bayesian algorithms;
(c) assist in the choice of space groups for solvate crystal structure
prediction; and (d) suggest ways in which solvent incorporation can
be used to influence space groups.

针对一系列有机溶剂，研究人员借助剑桥结构数据库（Cambridge Structural Database, CSD）中的重结晶溶剂（recrystallization solvent, RS）数据，评估了各类溶剂形成溶剂化物（solvate）的可能性。尽管部分晶体学家对RS数据持审慎态度，但多数可能性评估结果均展现出良好的精度。研究结果呈现出显著的趋势：例如，带有吸电子取代基的芳香族溶剂形成溶剂化物的概率较高，而无环脂肪烃类溶剂的对应概率则较低。针对不同CSD子集（如有机体系与金属有机体系）得到的结果具有高度相关性。当溶剂以混合形式存在时，其形成溶剂化物的可能性几乎始终高于纯溶剂状态。两种溶剂形成异溶剂化物（heterosolvate，即晶体结构中同时包含两种溶剂分子）的可能性，可通过二者各自形成普通溶剂化物（normal solvate，即晶体结构中仅包含单一组分溶剂分子）的可能性的乘积进行可靠估算。溶剂化物的空间群（space group）偏好会随共结晶溶剂的性质发生显著变化；而非溶剂化物的空间群偏好，则与其结晶所用的溶剂密切相关。带有对称中心的溶剂倾向于使溶剂化物在中心对称空间群中结晶，而具有二重旋转对称性的溶剂则会促进晶体在带有二重固有旋转轴的空间群中结晶。环己烷与四氯化碳的晶格掺入，可分别促进晶体在三方晶系与四方晶系的空间群中结晶。本研究结果可实现以下应用：(a) 在无需生成溶剂化物的场景下指导溶剂选择；(b) 辅助预测溶剂化物的形成，例如结合贝叶斯算法开展相关工作；(c) 为溶剂化物晶体结构预测中的空间群选择提供参考；(d) 提出利用溶剂掺入来调控晶体空间群的可行思路。