Trimeric interactions

The three-body contribution may also be modelled using a term of the form i ( AB,tAc,J Bc) = i A,B,c exp(-Q AB)exp(-/i Ac)exp(-7 Bc) where K, a, j3 and 7 are constants describing the interaction between the atoms A, B and C. Such a functional form has been used in simulations of ion-water systems, where polarisation alone does not exactly model configurations when there are two water molecules close to an ion [Lybrand and Kollman 1985]. The three-body exchange repulsion term is thus only calculated for ion-water-water trimers when the species are close together. 

Fig. 10. The receptor—G-protein sequence. An activated receptor interacts with the trimeric GDP-ligated receptor to cause an interchange of GDP by GTP and dissociation into the activated Ga—GTP (left) and G y (right) subunits. These then interact with a variety of effectors. The purpose of the activated...

Figure 13.17 Schematic diagram of the structure of a complex between phosducin and the transducin Gpy dimer. The p subunit of transducin is light red and the seven WD repeats are represented as seven orange blades of a propeller. The y subunit is yellow and the phosducin molecule is blue. The helical domain of phosducin interacts with Gp in the same region that Gq binds, thereby blocking the formation of a trimeric Gapy complex.

However, one should emphasize that the proposed model of the interactions does not exclude a possibility of the formation of higher-order associates. On average, the concentration of trimers and higher-order mers is about 8% in the case of systems presented in Fig. 20. Thus, the presence of associates larger than dimers does not prevent the applicability of the... 

LCo(H20)6] ion, and bidentate /V-donor ligands such as cn, bipy and phen form octahedral cationic complexes [Co(L-L)3] , which are much more stable to oxidation than is the hexaammine [Co(NH3)6l . Acac yields the orange [Co(acac)2(H20)2] which has the tram octahedral structure and can be dehydrated to [Co(acac)2l which attains octahedral coordination by forming the tetrameric species shown in Fig. 26.3. This is comparable with the trimeric [Ni(acac>2]3 (p. 1157), like which it shows evidence of weak ferromagnetic interactions at very low temperatures. fCo(edta)(H20)] is ostensibly analogous to the 7-coordinate Mn and complexes with the same stoichiometry, but in fact the cobalt is only 6-coordinate, 1 of the oxygen atoms of the cdta being too far away from the cobalt (272 compared to 223 pm for the other edta donor atoms) to be considered as coordinated. 

The frustration effects are implicit in many physical systems, as different as spin glass magnets, adsorbed monomolecular films and liquid crystals [32, 54, 55], In the case of polar mesogens the dipolar frustrations may be modelled by a spin system on a triangular lattice (Fig, 5), The corresponding Hamiltonian consists of a two particle dipolar potential that has competing parallel dipole and antiparallel dipole interactions [321, The system is analyzed in terms of dimers and trimers of dipoles. When the dipolar forces between two of them cancel, the third dipole experiences no overall interaction. It is free to permeate out of the layer, thus frustrating smectic order. 

All mechanisms proposed in Scheme 7 start from the common hypotheses that the coordinatively unsaturated Cr(II) site initially adsorbs one, two, or three ethylene molecules via a coordinative d-7r bond (left column in Scheme 7). Supporting considerations about the possibility of coordinating up to three ethylene molecules come from Zecchina et al. [118], who recently showed that Cr(II) is able to adsorb and trimerize acetylene, giving benzene. Concerning the oxidation state of the active chromium sites, it is important to notice that, although the Cr(II) form of the catalyst can be considered as active , in all the proposed reactions the metal formally becomes Cr(IV) as it is converted into the active site. These hypotheses are supported by studies of the interaction of molecular transition metal complexes with ethylene [119,120]. Groppo et al. [66] have recently reported that the XANES feature at 5996 eV typical of Cr(II) species is progressively eroded upon in situ ethylene polymerization. 

Olmstead, M.M., Jiang, F., Attar, S. and Balch, A.L. (2001) Alteration of the aurophilic interactions in trimeric Gold(I) compounds through charge transfer, behavior of solvoluminescent Au3(MeN=COMe)3 in the presence of electron acceptors. Journal of the American Chemical Society, 123, 3260. 

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