Comparative Analysis of the Multicenter, Long Bond in [TCNE]^·−and Phenalenyl Radical Dimers: A Unified Description of Multicenter, Long Bonds

Abstract

The nature of the multicenter, long bond in neutral phenalenyl dimers is analyzed in detail and compared to the multicenter, long bond in [TCNE](2)(2-). These dimers are prototypes of multicenter, long bond in dimers of neutral and anion radicals. This was done by examining the number of electrons (m) and atomic centers (c) involved in the long bond for these dimers, as well as identifying the dominant attractive components of their interaction energy (SOMO-SOMO bonding, dispersion, and the sum of the exchange-repulsion and electrostatic components) in accord with Pauling's focus on total bond energies. The long bond in [TCNE](2)(2-) is a 2e(-)/4c bond, the electrostatic component is repulsive, and the dominant attractive component is the dispersion component (-27.7 kcal/mol), about two times larger than the bonding component. In phenalenyl dimers the dispersion component (-31.7 kcal/mol) is about 2.5 times stronger (than the SOMO-SOMO bonding component; hence, the multicenter, long bond in these dimers is closer to a van der Waals bond than to a covalent bond. Consequently, it possesses a two-electrons/fourteen center 2e(-)/14c bond, rather than the 2e(-)/12c bond suggested by the SOMO-SOMO bonding component. The covalent-like properties in phenalenyl dimers result from the dominant dispersion component that enable the fragments to approach each other so that their SOMOs overlap and produce a qualitative MO diagram identical to that found in conventional covalent bonds.

Unusually Long, Multicenter, Cationδ+⋅⋅⋅Anionδ−Bonding Observed for Several Polymorphs of [TTF][TCNE]

“…[39] Attributing a bonding component to atoms that lack orbital overlap is counterintuitive, but has been reported for the central carbons on each 2,5,8-tri-tert-butylphenalenyl in di(2,5,8-tri-tert-butylphenalenyl) due to a node. [12] In this case this bonding component is ascribed to dispersion forces, and this C À C separation is 3.20 , which is shorter than the other sub van der Waals CÀC separations that are ! 3.30 .…”

“…Additionally, dispersion also adds energetic stability to the [TCNE]C À ···A C H T U N G T R E N N U N G [TCNE]C À interaction. [10][11][12] The interaction between two [TCNE] ·À moieties is thus a long, multicenter C À C bond, because it differs from a conventional covalent bond in the genesis of the energetic stabilization, and longer equilibrium distances. Note that the [TCNE] 2 2À dimers present in [cation + ] 2 A C H T U N G T R E N N U N G [TCNE] 2 2À are diamagnetic, as they have a closed-shell singlet ground state, in good agreement with the experimental observations.…”

“…[12] The (Table 3). A detailed analysis of the nature of the bonding can be done through an AIM analysis [30] of all (3,À1) bcps in the electron density between bonded atoms between the [TTF]…”

“…The nature of the interaction energy in p-[TTF···TCNE] was also computed by using the same analysis as for [TCNE] 2 2À and phenalenyl dimers, [12] that is, the interaction energy (E int ) can be expressed as the sum of the exchangerepulsion (E er ), electrostatic (E ele ), dispersion (E disp ), and bonding (E bond ) components:…”

London’sche Dispersionswechselwirkungen in der Molekülchemie – eine Neubetrachtung sterischer Effekte

“…Aufsätze P. R. Schreiner und J. P. Wagner accounted in B3LYP calculations,isessential for the stability of the dimer …" [140] {der Dispersionsanteil, der von HF-Rechnungen gar nicht und von B3LYP-Rechnungen nur unvollständig berücksichtigt wird, ist entscheidend für die Stabilität des Dimers}. Während die Autoren eine Dispersionskorrektur von 32 kcal mol À1 berechneten, wurde diese nicht der Gegenwart der tert-Butylgruppen zugeschrieben, sondern vielmehr als Merkmal der p-p-Bindung angesehen, das in einer anderen kritischen theoretischen Analyse als nichtspezifische vdW-Wechselwirkung beschrieben wurde.…”