title: Quantitative assessment of the multiplicity of carbon-halogen bonds: carbenium and halonium ions with F, Cl, Br, and I.
abstract: CX (X = F, Cl, Br, I) and CE bonding (E = O, S, Se, Te) was investigated for a test set of 168 molecules using the local CX and CE stretching force constants k(a) calculated at the M06-2X/cc-pVTZ level of theory. The stretching force constants were used to derive a relative bond strength order (RBSO) parameter n. As alternative bond strength descriptors, bond dissociation energies (BDE) were calculated at the G3 level or at the two-component NESC (normalized elimination of the small component)/CCSD(T) level of theory for molecules with X = Br, I or E = Se, Te. RBSO values reveal that both bond lengths and BDE values are less useful when a quantification of the bond strength is needed. CX double bonds can be realized for Br- or I-substituted carbenium ions where as suitable reference the double bond of the corresponding formaldehyde homologue is used. A triple bond cannot be realized in this way as the diatomic CX(+) ions with a limited π-donor capacity for X are just double-bonded. The stability of halonium ions increases with the atomic number of X, which is reflected by a strengthening of the fractional (electron-deficient) CX bonds. An additional stability increase of up to 25 kcal/mol (X = I) is obtained when the X(+) ion can form a bridged halonium ion with ethene such that a more efficient 2-electron-3-center bonding situation is created.
authors.affiliation: Computational and Theoretical Chemistry Group (CATCO), Department of Chemistry, Southern Methodist University , 3215 Daniel Avenue, Dallas, Texas 75275-0314, United States.
authors.forename: Robert
authors.initials: R
authors.lastname: Kalescky
authors.forename: Wenli
authors.initials: W
authors.lastname: Zou
authors.forename: Elfi
authors.initials: E
authors.lastname: Kraka
authors.forename: Dieter
authors.initials: D
authors.lastname: Cremer