Relative electronic and free energies of octane's unique conformations

This study reports the geometries and electronic energies of n-octane's unique conformations using perturbation methods that best mimic CCSD(T) results. In total, the fully optimised minima of n-butane (2 conformations), n-pentane (4 conformations), n-hexane (12 conformations) and n-octane (96 conformations) were investigated at several different theory levels and basis sets. We find that DF-MP2.5/aug-cc-pVTZ is in very good agreement with the more expensive CCSD(T) results. At this level, we can clearly confirm the 96 stable minima which were previously found  using a reparameterised density functional theory (DFT). Excellent agreement was found between their DFT results and our DF-MP2.5 perturbation results. Subsequent Gibbs free energy calculations, using scaled MP2/aug-cc-pVTZ zero-point vibrational energy and frequencies, indicate a significant temperature dependency of the relative energies, with a change in the predicted global minimum. The results of this work will be important for future computational investigations of fuel-related octane reactions and for optimisation of molecular force fields (e.g. lipids).

本研究采用最贴合耦合簇单双三重激发（CCSD(T)）结果的微扰方法，报道了正辛烷特有构象的几何结构与电子能量。本研究在多种不同的理论级别与基组下，对正丁烷（2种构象）、正戊烷（4种构象）、正己烷（12种构象）及正辛烷（96种构象）的全优化极小值结构进行了系统考察。研究发现，DF-MP2.5/aug-cc-pVTZ水平下的计算结果与计算成本更高的CCSD(T)结果吻合极佳。在该理论级别下，我们可明确确认此前通过重参数化密度泛函理论（DFT）得到的96个稳定极小值结构，且其DFT计算结果与本研究的DF-MP2.5微扰计算结果同样表现出优异的一致性。后续采用标度后的MP2/aug-cc-pVTZ零点振动能与振动频率开展的吉布斯自由能计算表明，相对能量具有显著的温度依赖性，且预测的全局极小值结构会随温度发生变化。本研究的结果对于未来与燃料相关的辛烷反应计算研究，以及分子力场（如脂质相关力场）的优化具有重要意义。