On the irrelevance of electrostatics for the crystal structures and polymorphism of long even n‐alkanes

Abstract

It is known that the experimental triclinic crystal structures of even n-alkanes are not well reproduced upon energy minimization with current force fields. The inclusion of electrostatics does not solve this, and, moreover, some charge schemes show unphysical features such as positively charged carbon atoms or charge alternation. The effect of the electrostatics on the energies of the crystal structures of the even n-alkanes, and thereby on their polymorphism, has never been established. A new charge scheme is introduced that yields physically sensible charges without constraints. It will also be shown, however, that electrostatics are relevant neither for the structures of the crystals, nor for their energies.

Derivation of a New Force Field for Crystal-Structure Prediction Using Global Optimization: Nonbonded Potential Parameters for Hydrocarbons and Alcohols

“…The results of global-energy minimizations presented here support the conclusion 32 that the interatomic interactions in crystals of aliphatic hydrocarbons are rather weak, which leads to a “flat” potential energy surface with very broad and shallow minima. The results obtained in this work are also in agreement with those presented by other authors. , Taking into account the role of thermal effects in crystal structures of aliphatic hydrocarbons, , it appears that probably no further improvement of the results can be achieved by using a simple static model. The “ab initio” potential parameters were accepted as final for aliphatic carbon and hydrogen and used later for other molecules containing the same atom types.…”

“…The observed crystal structures were also predicted as the global or one of the lowest-energy minima. However, for ethane and butane, the observed relative stabilities of different crystal polymorphs were not reproduced, which is consistent with results obtained by van de Streek et al , and indicates that static calculations with a simple “6-exp-1” potential are not completely adequate for this type of molecule.…”

“…The largest deviations were obtained for the unit cell parameter a of the ethane and propane crystal structures (8.0% and 5.8%, respectively). It was shown , that thermal motion plays an important role in hydrocarbon crystals. Because ethane and propane are the smallest in the homologous series of aliphatic hydrocarbons tested, it can be expected that thermal vibrations can have a strong effect on their crystal structures.…”

Development of a new physics‐based internal coordinate mechanics force field and its application to protein loop modeling

“…Excellent agreement between the energy‐minimized and experimental structures (with the average deviation of 1.2%) was also obtained for alcohols (Supporting Information Table S3). The larger than average deviations observed for some crystals [e.g., for propane and pentane (Supporting Information Table S2) with the deviations of the unit cell parameters of −5.7% and −6.1%, respectively, and for molecular rotation in methanol crystal (CSD ID methol02, Supporting Information Table S3)] can be explained by physical effects such as thermal motion which has stronger effect on crystal of very small molecules and on those with relatively weak intermolecular interactions 76, 77…”

Global Optimization-Based Method for Deriving Intermolecular Potential Parameters for Crystals

“…However, only a small improvement of the crystal structure parameters was achieved. Van de Streek at el. , came to the conclusion that the crystal structures for long even- n -alkanes are not sensitive to electrostatic effects. Moreover, it was shown 27 that electrostatic interactions are not important for polymorphism of saturated hydrocarbons.…”