Lo Presti Leonardo, Ellern Arkady, Destro Riccardo, Lunelli Bruno
Dipartimento di Chimica Fisica ed Elettrochimica, Universita di Milano, Via Golgi 19, 20133 Milano, Italy.
J Phys Chem A. 2009 Apr 2;113(13):3186-96. doi: 10.1021/jp8084809.
The effect of the methoxy substituent on the structure, crystal packing, and electrostatic properties of hexafluorocyclobutene (C(4)F(6)) was investigated in the solid-state with DFT-B3LYP calculations. Full geometry optimizations were done for the parent compound and its two vinyl methoxy derivatives C(4)F(5)OCH(3) and C(4)F(4)(OCH(3))(2), starting from the structures obtained by single-crystal X-ray diffraction at low temperature. A full topological analysis, followed by the calculation of several electrostatic properties, was performed on the periodic electron density using the quantum theory of atoms in molecules. Eventually, the cohesive energies of the three crystals were estimated. In the cyclobutene plane, the methoxy substitution yields a significant electronic rearrangement involving the pi-electrons. The solid-state (periodic) results agree with those obtained by gas-phase calculations on C(4)F(6) and its derivatives at a comparable level of theory. It was found that the substitution of one or two vinylic fluorine atoms with the OCH(3) group considerably influences the molecular dipole moment, which undergoes an enhancement in both the solid and the gas phase as large as 200% and 235% for C(4)F(5)OCH(3) and C(4)F(4)(OCH(3))(2), respectively, with respect to that calculated for C(4)F(6). The charge rearrangement due to the substituents provides a significant electrostatic contribution to the lattice energy, and in turn it can be related to the change in the observed crystal packing on going from C(4)F(6) (space group P2(1)/c) to both of its derivatives (space group P1). It is also shown that the dispersion energy significantly contributes to the lattice stability in all three compounds. Since the DFT calculations, in the limit of large separations, entirely miss the dispersion term, this was estimated by applying a recently proposed dampening function to the semiempirical atom-atom C(6) R(-6) potentials in the mainframe of Spackman's energy decomposition scheme for Mulliken multipoles.
采用密度泛函理论B3LYP方法，在固态条件下研究了甲氧基取代基对六氟环丁烯（C(4)F(6)）结构、晶体堆积及静电性质的影响。从低温下单晶X射线衍射得到的结构出发，对母体化合物及其两种乙烯基甲氧基衍生物C(4)F(5)OCH(3)和C(4)F(4)(OCH(3))(2)进行了完全几何优化。利用分子中的原子量子理论，对周期电子密度进行了完整的拓扑分析，并计算了几种静电性质。最后，估算了三种晶体的内聚能。在环丁烯平面上，甲氧基取代导致了涉及π电子的显著电子重排。固态（周期性）结果与在类似理论水平下对C(4)F(6)及其衍生物进行气相计算得到的结果一致。研究发现，用OCH(3)基团取代一个或两个乙烯基氟原子会显著影响分子偶极矩，相对于C(4)F(6)计算值，C(4)F(5)OCH(3)和C(4)F(4)(OCH(3))(2)在固态和气态下分子偶极矩分别增大了200%和235%。取代基引起的电荷重排对晶格能有显著的静电贡献，进而与从C(4)F(6)（空间群P2(1)/c）到其两种衍生物（空间群P1）观察到的晶体堆积变化相关。研究还表明，色散能对所有三种化合物的晶格稳定性有显著贡献。由于在大间距极限下密度泛函理论计算完全忽略了色散项，因此在斯帕克曼多极子能量分解方案的主机框架内，通过对半经验原子 - 原子C(6)R(-6)势应用最近提出的阻尼函数来估算色散能。
