Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602, USA.
J Phys Chem A. 2010 Dec 16;114(49):12952-7. doi: 10.1021/jp108577g. Epub 2010 Nov 18.
Branched alkane hydrocarbons are thermodynamically more stable than straight-chain linear alkanes. This thermodynamic stability is also manifest in alkane bond separation energies. To understand the physical differences between branched and linear alkanes, we have utilized a novel density functional theory (DFT) definition of steric energy based on the Weizäcker kinetic energy. Using the M06-2X functional, the total DFT energy was partitioned into a steric energy term (E(s)[ρ]), an electrostatic energy term (E(e)[ρ]), and a fermionic quantum energy term (E(q)[ρ]). This analysis revealed that branched alkanes have less (destabilizing) DFT steric energy than linear alkanes. The lower steric energy of branched alkanes is mitigated by an equal and opposite quantum energy term that contains the Pauli component of the kinetic energy and exchange-correlation energy. Because the steric and quantum energy terms cancel, this leaves the electrostatic energy term that favors alkane branching. Electrostatic effects, combined with correlation energy, explains why branched alkanes are more stable than linear alkanes.
支链烷烃在热力学上比直链线性烷烃更稳定。这种热力学稳定性也体现在烷烃键的分离能上。为了理解支链烷烃和线性烷烃之间的物理差异，我们利用了一种基于 Weizäcker 动能的新的密度泛函理论（DFT）位能定义。使用 M06-2X 泛函，将总 DFT 能量分解为位能项（E(s)[ρ]）、静电能项（E(e)[ρ]）和费米子量子能项（E(q)[ρ]）。这项分析表明，支链烷烃的 DFT 位能比线性烷烃小（不稳定）。支链烷烃的较低位能被一个相等且相反的量子能项所缓解，该量子能项包含了动能和交换相关能的 Pauli 分量。由于位能和量子能项相互抵消，因此留下了有利于烷烃支化的静电能项。静电效应与相关能量相结合，解释了为什么支链烷烃比线性烷烃更稳定。
