Rationalizing the Effect of Halogenation on the Molecular Structure of Simple Cyclobutene Derivatives by Topological Real-Space Analysis of Their Electron Density

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• 作者：Leonardo Lo Presti ; Arkady Ellern ; Riccardo Destro ; Raffaella Soave ; Bruno Lunelli
• 刊名：Journal of Physical Chemistry A
• 出版年：2011
• 出版时间：November 17, 2011
• 年：2011
• 卷：115
• 期：45
• 页码：12695-12707
• 全文大小：1391K
• 年卷期：v.115,no.45(November 17, 2011)
• ISSN：1520-5215

文摘

The accurate gas-phase equilibrium structures on the ground-state potential energy surface of the complete series of fluorinated and chlorinated cyclobutene derivatives with C_2v symmetry have been evaluated at DFT PBE0/6-311++G(d,p) theory level. The optimized geometries have been compared with all the available experimental data reported in the literature, as obtained by microwave spectroscopy (MW) and gas-phase electron diffraction (GED) techniques. For hexafluorocyclobutene and 1,2-dichloro-3,3鈥?4,4鈥?tetrafluorocyclobut-1-ene, the results of accurate low-temperature single-crystal X-ray diffraction experiments have also been considered. Structural changes within the cyclobutene ring, as induced by fluorination and chlorination at allylic and vinylic positions, have been correlated with changes in the corresponding theoretical charge densities. To this aim, several local and nonlocal topological descriptors provided by the quantum theory of atoms in molecules, QTAIM, have been employed, with particular emphasis on the delocalization indices and integrated source function decomposition schemes. Key factors for the resulting molecular structures are the chemical nature and the steric hindrance of the substituents, as well as quantum-mechanical effects, such as delocalization and partial conjugation. When fluorine atoms replace hydrogens at allylic or vinylic positions, the corresponding Csp³鈥擟sp³ or Csp²鈺怌sp² bonds between the substituted carbons undergo a significant strengthening, while chlorination has just the opposite effect. In the latter case the steric hindrance between bulky chlorine atoms occupying vicinal positions is crucial in determining the single Csp³鈥擟sp³ bond length. These findings are discussed in the context of the reactivity of chemically related chlorofluorocarbon compounds.