Topological Insights into the Nature of the Halogen-Carbon Bonds in Dimethylhalonium Ylides and Their Cations

A. JUBERT, N. OKULIK, M. C. MICHELINI, C. J. A. MOTA

JOURNAL OF PHYSICAL CHEMISTRY A

ACS

Año: 2008 vol. 112 p. 11468 - 11480

1089-5639

In this study the nature of the bonding in a series of dimethylhalonium ylides (fluoronium, chloronium, bromonium and iodonium) was analyzed by means of topological methodologies (AIM and ELF analysis), to document the changes in the nature of the C-X bonds (X ) F, Cl, Br, I) upon the series. For the sake of comparison the same study was performed on the corresponding dimethylhalonium cations (XC2H6+) and the XCH3 series. The wave functions used for the topological analysis were obtained at B3LYP level using extended triple-zeta basis sets. The formation of the cationic C2H6+ structures can be interpreted to arise from the interaction between the XCH3 and CH3+ moieties. The resultant structures can be explained in terms of the superposition of two electrostatically interacting and two dative mesomeric structures. The halogen-carbon bonds have all the characteristics of the charge-shift (CS) bonds. The analysis of the C-X bond in the XC2H5series shows a progressive reinforcing of the CH3X-CH2 bond, from FC2H5 that can be considered as formed from two fragments, FCH3 and CH2, to IC2H5, in which the CH3I-CH2 bond has all the features of a multiple bond involving atoms bearing lone pairs. Particularly interesting is BrC2H5, in which a special type of bond (hybrid covalent-dative double bond) has been characterized. The energetic stability of the XC2H5 structures with respect to the dissociation into the XCH2 + CH3 and XCH3 + CH2 ground-state fragments was studied in detail