Donor-acceptor bond, effect

In the ethane case, however, the AIM analysis helps in understanding the overlap of the bonds and the location of the electrons as derived from the density picture, but it does not tell us anything about the origin of the rotational barrier. For that, we need methods that quantitatively give us energies that can be associated with the effects of donor-acceptor bonding (hyperconjugation) and electron-electron repulsion (Pauli repulsion) as noted above. 

The donor/acceptor properties and the electronic coupling interactions determine the redistribution of electron density between the aromatic donor and the electron acceptor upon complexation. Significant changes in structure and reactivity of the coordinated arene can be rationalized in terms of spectral and thermodynamic properties within the framework of the CT formalism. This section is devoted to a consideration of the structural effects of arene coordination (in terms of donor/acceptor bond distance and type of bonding, distortion of arene planarity, expansion of the aromatic ring, and re-bond localization). 

The frequency shift Av = v(OH) — v(OH O), where v(OH) is the frequency of the stretching mode of the O—H bond of the isolated phenol and v(OH O) in the presence of the electron donor 0(X,)2, is very informative for this series. The quantity Av is linearly correlated with the change of enthalpy (energy of donor-acceptor bond in the H-complex) and free energy (stability of the H-complex) , as well as with the value of effective charge q on the donor centre B, which was calculated by quantum-chemical... 

Note that in contrast to the complexes of SiCU with neutral phosphines, as shown above, the complexes with the carbanionic ligands C have at least one silicon-carbon bond. Apparently, this is enough to effectively make an additional P->Si donor-acceptor bond impossible. It is somewhat surprising that this is the case also in solution where an intramolecular P-Si bond should be particularly favored by the chelate effect. 

Poteau R, Alary F, El Makarim HA, Heully JL, Barthelat JC, Daudey JP. Effective group potentials. 2. Extraction and transferability for chemical groups involved in covalent or donor-acceptor bonds. J Phys Chem A 2001 105 206-214. 

Ten adducts L->BH Cl3 in = 0—3), L->BH Br3 in = 1—3), L- -BCl Br3 ( = 1 or 2), and L- -BHBrCl (L = PMe3) have been subjected to intensive n.m.r. study CH, B, and 3ip) 181 only parameter which correlated with the thermodynamic stability of these adducts was the screening constant of the protons P—CH3. This is consistent with the idea that in studying the formation and behaviour of such complexes one must consider the whole molecule and not just effects on the donor-acceptor bond itself. 

As an example, the results for formation of the donor-acceptor bond in MejN-BMCj are shown in Figure 16.4. Such bonds (and their strengthening by dispersion and weakening by EXR effects) play an important role in the so-called frustrated Lewis pairs (FLP), which have attracted enormous attention recently because of their ability to activate small molecules such as H2 [35-37]. Because the bond dissociates heterolyticaUy into closed-shell fragments, static electron correlation effects play no important role and a standard restricted DFT treatment... 

The hydrophilicity of nanooxides, which plays a very important role in their applications and affects many of their properties, was analyzed using calorimetry (oxides were degassed at 473 K at low pressures for several hours) and H NMR spectroscopy (oxides were equilibrated in air) methods applied to samples after different pretreatments. This characteristic is linked to the possibility of the formation of strong hydrogen and donor-acceptor bonds or/and dissociative adsorption of water. The treatments before the calorimetric measurements resulted in desorption of intact water and a portion of dissociatively adsorbed water (=MOH, M 0(H)M"=, where M=Si, Al, or Ti) from both surface and volume of oxide nanoparticles. However, in the case of the NMR measuranents, surface and volume water was readsorbed from air. Therefore, one could expect that the heat effects on the adsorption of water on the calorimetric measuranents should be stronger than that on the NMR measurements. This is typically observed for the samples studied with the exception of SA8 and ST20 (Table 2.12). 

It is well-known that the halides of the Group III elements are good electron acceptors and previous work (1,2) has explored the thermochemistry of adduct formation in order to obtain information about the strength of the donor-acceptor bond. The results of this work have been interpreted in terms of the steric, inductive, and tt-bonding effects of the groups attached to the donor and acceptor atoms. The dominant effect, at least in the halide... 

The influence of electronic effects on the binding mode of tetraalkynylplatinate anions is apparent in the reactions of compounds of general formula Li2[Pt (C=CR)4] with thallium(l) nitrate [72]. For electrrai-rich alkynyl moieties (R = p-tolyl, 1-naphtyl), discrete hexanuclear structures are obtained (Scheme 16, top), in which each Tl center interacts with four C=C triple bonds and not with the platinum atom. The introduction of a remote electron-withdrawing substituent oti the alkynyl unit (R = p-CFsC lU) induces a dramatic change in the structure of the Lewis acid-base complex each platinum atom now involves in donor-acceptor bonds with two thaUium(I) ions (Pt-Tl 2.9355(5) and 3.0272(5) A), one oti each face of its coordination plane. These Tl2Pt units are interconnected by weak interactions of thallium with the Jt-elecfron density of the triple bonds. 

The donor-acceptor interaction is one of the main mechanisms for metal compound extraction. The extraction ability correlates with the distribution of electron density in extracting agents, including phosphorus-organic compounds [87]. Correlations between effective extraction parameters defined by thermodynamics of the donor-acceptor bond and the so-called effective charge at phosphorus by which electron density distribution in... 

The contribution of this polar structure to the bonding lowers the energy of the transition state. This may be viewed as a lower activation energy for the addition step and thus a factor which promotes this particular reaction. The effect is clearly larger the greater the difference in the donor-acceptor properties of X and Y. The transition state for the successive addition of the same monomer (whether X or Y substituted) is structure [V] ... 

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