Intermolecular coordination

In the case of a strongly exothermic reaction the final term turns into an absorbing wall, and the transition is completed whenever the distance AB reaches a certain value and the A-B bond is broken. The intra- and intermolecular coordinates Q and q are harmonic and have frequencies (Oo and oji, and reduced masses mo and mi. At fixed intermolecular displacement the tunneling probability equals... 

The preference for formation of dihalocarbenes (but not the trihalomethyl radicals) upon thermolysis of trihalomethyl mercury, silicon and germanium derivatives seems to be a result of intermolecular coordination, of type [1], and of a thermodynamic preference for the carbene-forming pathway. The... 

A polymeric structure can be generated by intermolecular coordination of a metalloporphyrin equipped with a suitable ligand. Fleischer (18,90) solved the crystal structure of a zinc porphyrin with one 4-pyridyl group attached at the meso position. In the solid state, a coordination polymer is formed (75, Fig. 30). The authors reported that the open polymer persists in solution, but the association constant of 3 x 104 M 1 is rather high, and it seems more likely, in the light of later work on closed macrocycles (see above), that this system forms a cyclic tetramer at 10-3 M concentrations in solution (71,73). 

A shift of stretching vibrations vc=0 from 1590 to 1640 cm-1 is observed in the mixture of model compounds which points to intermolecular coordination interactions. 

According to this scheme the fundamental difference in the mechanism of free-radical copolymerization of MA with TASM and of MA with alkyl acrylates is due to the fact that in the former copolymerization intermolecular coordination is involved. This coordination is similar to the effect of various complexing agents (ZnCl2, SnCft and A1C13) on free-radical homo- and copolymerization of vinyl monomers. This effect seems to favor the appearance of isotactic configurations along the main chain. 

As indicated above, the observed effect of intermolecular coordination results in the formation of alternating copolymers having coordination bonds between fragments of the corresponding comonomers. ... 

The obtained value of a indicates the proximity of the rate constant values of the addition of TBSM to the macroradicals MA and of MA to TBSM This can be explained by a similar influence of intermolecular coordination on chain propagation. The values of pt and p2 indicate that in free-radical copolymerization of TBSM with MA both free and complex-bound monomers are involved in chain propagation with a higher contribution of the latter. 

A noticeable change in the TBSM activity as compared with that of its organic analog BMA (rBMA = 0.64 and rSt = 0.54)89) in the free-radical copolymerization with styrene may be ascribed to steric factors and the effect of intermolecular coordination. 

A considerable viscosity increase in copolymers of tributylstannyl methacrylate with methyl methacrylate, butyl acrylate and styrene upon prolongated storage has been observed and special agents to eliminate this effect have been proposed 108). It is likely that the destruction of intermolecular coordination complexes formed by involvement of tin and carbonyl groups in comonomer units takes place in this case. 

Silanethione 38 was characterized by H, 13C, and 29Si NMR, Raman, and UV-vis spectroscopic methods. The 29Si NMR chemical shift of 38 (8Si 166.56/C6D6) for the silathiocarbonyl unit is much downfield shifted from those of the thermodynamically stabilized silanethiones, 31, 34, 35,28 and 36,29 mentioned in the previous sections, clearly indicating a genuine Si=S double bond in 38 without any intra- or intermolecular coordination. The molecular structure of 38 was successfully established by X-ray crystallographic analysis, and the detailed structural parameters are discussed in the following section. 

Commonly, any experimental study of dihydrogen bonds is undertaken to clarify the following aspects (1) establishment of intra- or intermolecular coordination, (2) determination of its stoichiometry, (3) reliable establishment of a proton-donor site and a proton-acceptor center, (4) description of the geometry of dihydrogen-bonded complexes, and (5) correct measurement of bonding energies. In this chapter we demonstrate how to approach these factors using various experimental methods that work in the solid state, the gas phase, and in solution. 

In a number of cases, e.g. for CO, PH3, PF3 and thioethers, the existence of complexes or their surprising stability was explained in terms of a special n bond.16 However, experimental data and theoretical considerations seem to contradict the assumption that such kinds of n bonds exist.25 Borane complexes in which donor atoms of appropriate nucleophilicity are part of the group attached to the boron, e.g. BCl2(NMe2), BH2(PMe2), BBr2SMe, constitute a special case and exist due to intermolecular coordination, in the form of dimers, trimers or polymers.17-19... 

Figure 1.1 illustrates the two basic types of photodissociation of a chemically bound molecule. In Figure 1.1(a) the photon excites the molecule from the ground to a higher electronic state. If the potential of the upper electronic state is repulsive along the intermolecular coordinate RaBi the... 

Complex 26 is among the first examples of intermolecular coordination of a single B-H bond in a neutral borane to a transition metal, and these species can be regarded as models for alkane coordination. The crystal structures of borane complexes such as 26 are of interest to compare with those for other octahedral a complexes, particularly... 

The X-H (X = B, C, Si) bonds in some ligand molecules, like the H-H bond in H2, provide their a bonding electron pairs to bind the ligands to metal centers. They form intermolecular coordination compounds (i.e., sigma-bond complexes) or intramolecular coordination compounds (i.e., agostic-bond complexes). All such compounds involve non-classical 3c-2e bonding and are collectively termed a complexes. 

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