Are carbon—halogen double and triple bonds possible?

Abstract

The carbonAhalogen and carbonAchalcogen bonding of 84 molecules was investigated utilizing local vibrational modes calculated at the M06-2X/cc-pVTZ level of theory including anharmonicity corrections in all cases. The relative bond strength order of each CX or CE bond (X 5 F, Cl; E 5 O, S) was derived from the local CX or CE stretching force constant and compared with trends of calculated bond lengths r and bond dissociation energies (BDE) obtained at the G3 level of theory. It is shown that both bond length r and BDE are not reliable bond strength descriptors. The CX double bond is realized for some Cl-substituted carbenium ions, however, not for the cor-responding F-derivatives. Diatomic CF 1 and CCl 1 possess fully developed double bonds but not, as suggested in the literature, triple bonds. Halonium ions have fractional (electron-deficient) CX bonds, which can be stabilized by r-donor substituents or by an increased polarizability of the halogen atom as with Cl. Bridged halonium ions are more stable than their acyclic counterparts, which results from more effective two-electron-three-center bonding.

C2 in a Box: Determining Its Intrinsic Bond Strength for the X1Σg+ Ground State

“…Alternatively, one could say that the C 2 bond stretching force constant is too small by 1.199 mdyn Å −1 . Clearly, the Badger relationship is just qualitative and there have been many discussions why deviations are found therefrom . A bond shortening by 0.021 Å can be related to a decreased covalent radius in line with the increased electronegativity of a sp‐hybridized C in C 2 that, contrary to the C atoms in acetylene, has not to accommodate charge obtained from a substituent atom.…”

Re‐evaluation of the bond length–bond strength rule: The stronger bond is not always the shorter bond

“…Often the argument is passed forward that the BDE values might describe at least qualitatively the correct trend of the intrinsic bond strength. However, numerous examples have been shown [35,[37][38][39]46,96] that this hope lacks any quantum mechanical basis and therefore it is always desirable to obtain the intrinsic strength of a bond from vibrational spectroscopy as the local stretching force constants can be always derived.…”

“…The preferred tool in this work will be a dynamical model of the bond strength based on vibrational spectroscopy and the theory of local modes, which was introduced by Konkoli and Cremer [28] and which, since then, has been proved to be physically sound, generally applicable, and a direct way of determining the intrinsic strength of a bond. [29][30][31] The usefulness of the local vibrational mode description of the bond strength has been documented by the characterization of CC bonds, [29,[32][33][34] NN bonds, [35] CO bonds, [36] CX bonds with X 5 F, Cl, Br, I, [37][38][39][40] H-bonding, [41][42][43][44] pnicogen bonding, [45,46] and the characterization of isotopomers. [47] The main objectives of this work are (i) to critically reevaluate previously published reverse BLBS relationships, (ii) to introduce reliable bond strength orders (BSOs) needed for characterizing unusual bonds, (iii) to elucidate the interplay of electronic and electrostatic factors determining the stability of electron-rich bonds, and (iv) to derive a rational for any deviation from the commonly known inverse BLBS relationships.…”

Quantitative Assessment of B−B−B, B−Hb−B, and B−Ht Bonds: From BH3 to B12H122−

“…It has been shown that the local stretching force constant k a is a superior bond strength measure compared to the often used bond dissociation energies (BDE) and bond dissociation enthalpies (BDH) as they contain cumulative effects of geometry relaxation, multiple bonding interactions, and also electronic density reorganizations of the dissociated fragments . Local stretching force constants k a have been successfully used to determine the intrinsic bond strength of covalent bonds such as: CC bonds, NN bonds, NF bonds, CO bonds, and CX (X=F, Cl, Br, I) bonds, and weak chemical interactions such as: hydrogen bonding, halogen bonding, pnicogen bonding, chalcogen bonding, tetrel bonding, and recently interactions …”

“…[25,[73][74][75] It has been shown that the local stretching force constant k a is a superior bond strength measure compared to the often used bond dissociation energies (BDE) and bond dissociation enthalpies (BDH) as they contain cumulative effects of geometry relaxation, multiple bonding interactions, and also electronic density reorganizations of the dissociated fragments. [73,76,77] Local stretching force constants k a have been successfully used to determine the intrinsic bond strength of covalent bonds such as: CC bonds, [74,75,78,79] NN bonds, [80] NF bonds, [81] CO bonds, [82] and CX (X = F, Cl, Br, I) bonds, [83][84][85][86] and weak chemical interactions such as: hydrogen bonding, [87][88][89][90] halogen bonding, [91][92][93] pnicogen bonding, [94][95][96] chalcogen bonding, [73,77] tetrel bonding, [97] and recently BH � � � p interactions. [16,17] The main objectives of this paper are: (i) to evaluate the nature of the BÀ B, BÀ H b , and BÀ H t bonds; (ii) to evaluate why intermediate species may or may not facilitate the reversibility reactions in the perspective of thermodynamics and intrinsic bond strength; (iii) to provide a new tool to characterize new (potential) intermediates and their role for the reversibility of the hydrogenation/dehydrogenation reactions; and (iv) to give guidelines whether intermediates may be stable enough to be isolated.…”