Other Mixed Trimers

The HjO HP-HF complex was reexamined more recently in conjunction with a comparison with H O H O -HF. The focus of this work was an exploration of the potential energy surface to identify all the local minima. In addition to the cyclic structure of Fig. 5.27, a bifurcated geometry, in which the water serves as double proton acceptor, was also located as a minimum of the PES of H O -HF—HF. In the complex containing a pair of water molecules and one HF, the only minimum located was of cyclic type. Both of these structures are illustrated in Fig. 5.28. 

The binding energy for the cyclic HjO—HF---HF complex in Fig. 5.27 and the analogous H20- -H20 - HF in Fig. 5.28 are calculated to be quite similar to one another. As illustrated in Table 5.30, the SCF-level calculations suggest the former is marginally more strongly bound, whereas including correlation shifts the balance toward the latter. In any case, it is clear that there is little difference between the two. Computation of the cooperativity within 

The energetics of formation of the H20 --H20--HC1 complex are reported in Table 5.31, in a format comparable to that for H20 -HF HF in Table 5.29. All data were obtained at the MP2 level and are corrected for BSSE by the counterpoise procedure. The three basis sets all contain polarization functions and yield comparable results. The binding of HCl to the pre-formed water dimer (step lb) is considerably more favorable than its binding to a water monomer, as in step 2a. This enhancement is referred to as E p in Table 5.31, and amounts to between 2 and 3 kcal/mol. (The same quantity is identically equal to the enhancement of binding of water to HjO—HCl as compared to HCl.) It is worth noting that whereas the total binding energy in the appropriate row of Table 5.31 is rather insensitive 

Data computed at MP2 level and corrected for BSSE, in kcal/raol °. 

It is intriguing that this 2-3 kcal range of is quite similar to the 2.4 kcal/mol computed for HjO -HjO- HF, also at the MP2 level and with BSSE correction, despite the substitution of HE by HCl in the present system. Perhaps E p would have been larger in the H20- H20 HF system had a basis set more flexible than 6-31 -I- -l-G been used. At any rate, this cooperativity is significantly smaller than the 4 kcal/mol reported in the last column of Table 5.29 for H20 - HF - HF where there are two HF molecules present, again obtained at the MP2 level with BSSE corrections. 

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There are fewer reports on the trimerization of aryl thiocyanates, although they are used in mixed trimerization reactions in the presence of other nitriles (vide infra).1 7... 

The mixed trimerization depends on the ability of the nitriles to form dienes and on the relative dienophilic character of R1CN versus R2CN. Thus, if formation of the diene from R CN exceeds that of R2CN, then products 22 and 24 are most likely to be formed. Alternatively, if both nitriles easily form dienes , and the addition of the diene -derived nitrile exceeds the addition of the other nitrile, then only the homotrimerization products 24 and 25 should be detected however, if both dienes prefer to react with the other nitrile, then the mixed trimerization products 22 and 23 are formed. In a pure sense, the latter reaction types have not been reported. In the third case, if the tendency to form the diene of both nitriles is comparable, and the dienophilicity is also almost equal, then all four trimerization compounds 22-25 would be formed. 

We have already shown how simultaneous codeposition of two metals in inert-gas matrices can lead to the formation of mixed-metal dimers. As in the case of silver, it was found that irradiation into the atomic absorptions of Cr or Mo results in formation of their respective dimers and trimers (114). In addition to this, however, irradiation into the atomic resonances of the two metals in the presence of each other results (114) in formation of the mixed-metal species CrMo, CrjMo, and CrMo2. It would seem that selective irradiation into the 300-400-nm bands of atomic Cr or Mo excites the 3d 4p, 3dMs 4p , or 4d 5p, ... 

The reaction of [AuCl(THT)] with 2-(4,5,-dihydro-4,4,-dimethyl-2,-oxazolinyl)-thien- -yllthium (n = 3,5) affords dimeric (586) or trimeric (587) gold(I) derivatives with mixed /V,C-donor ligands.2263 Other complexes with mixed N,C donor ligands which have been prepared in a similar manner are [Au2 Fc(CFl2NMe2 2], 7 [Au2 C6Fl4(CFl2NMe2) 2],3132... 

Other possible compounds may be formed including mixed-valent1566 and hydroxy-bridged trimeric complexes of type [Pt3(ju-0H)3(NH3)6](N03)31567 and [Pt3( -0H)3(NH3)6](S04)3-6H20.1568... 

One property of hydrophobic interactions is that they tend not to be specific. In other words (and as evident in natural coiled-coil structures), the basic coiled coil pattern HPPHPPP is compatible with a number of helix-bundle quaternary structures. Dimeric, trimeric, tetrameric, penta-meric, and dodecameric coiled coils are all known as are homomeric and heteromeric complexes and topologies with parallel, antiparallel, or mixed arrangements of helices (Burkhard et at, 2001 Gruber and Lupas,... 

Figure 8.12. The two pertinent empty orbitals are of b2g symmetry (cluster and ligand antibonding) and a2g symmetry (cluster antibonding) and cannot mix. Bending the Ge-H bonds to model the doubly connected Ge9 cluster units in the dimer and trimer lowers the symmetry to D2h and these two orbitals now are of big symmetry and can mix. One is stabilized and the other destabilized as shown. The former is now available at low energy to accommodate the extra lone pair. An important consequence of this exocluster orbital mixing is that the oligomers are not viewed as delocalized clusters connected by localized bonds but single delocalized entities.

Organic nitriles associate to dimers by dipole-dipole interaction of their nitrile groups (Sect. 2.1) water molecules, on the other hand, associate by formation of hydrogen bonds (preceding section). However, if water and nitrile (in particular acetonitrile ) are mixed, heterodimers and -trimers are formed ... 

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