Bimodality possible explanations

A conspicuous finding in these studies is that Et2AlCl-based initiator systems lead to bimodal distributions whereas those with EtjAlBr and Et2All lead to mono-modal distributions. Also, the MWD for Et2AlCl was broader ( 1.4 to 5.9) than those for Et2AlBr ( 1.7 to 2.2) and Et2AlI ( 1.9 to 3.0). A possible explanation for the bimodal MWD is given in Section 5. The reason(s) for the relatively narrow MWD s (<2) of LMWF and HMWF remains obscure. 

The movement of chemicals undergoing any number of reactions with the soil and/or in the soil system (e.g., precipitation-dissolution or adsorption-desorption) can be described by considering that the system is in either the equilibrium or nonequilibrium state. Most often, however, nonequilibrium is assumed to control transport behavior of chemical species in soil. This nonequilibrium state is thought to be represented by two different adsorption or sorption sites. The first site probably reacts instantaneously, whereas the second may be time dependent. A possible explanation for these time-dependent reactions is high activation energy or, more likely, diffusion-controlled reaction. In essence, it is assumed that the pore-water velocity distribution is bimodal,... 

Breckenridge and Umemoto measured the nascent rotational state distributions of MgH(t) = 0,1) and MgD(u = 0) produced in the reactions between Mg( Pi) and H2/D2. They observed bimodal distributions, with the major components (ca. 90% of the total) showing very high rotational excitation and the minor components lower rotational excitation. The reactions producing highly rotationally excited MgH(MgD) were suggested to involve side-on insertion of the Mg( Pi) into H2(D2), consistent with at initio studies of MgHa potential surfaces. The minor components were discussed in terms of less efficient end-on insertion, but other possible explanations were also considered. 

Interestingly, the elution pattern of the pigmented tannin was bimodal. Given that the molecular weight distribution of seed and skin tannins is very different, one possible explanation for the bimodal distribution of the tannin is that it represents seed (later eluting material) and skin (earlier eluting material) extraction. 

None of the above four models reproduced the observed Nv- distribution well. Although the N population decreases rapidly for v = 0-2, it becomes rather flat above that. The deviation from FC factors is relatively small for the low V = 0-2 levels, while it is very large for the high V = 3-5 levels. Thus, it is reasonable to assume that the Nv< distribution is bimodal. There exist both singlet and triplet entrance Hef + NO surfaces, which can interact with the exit NO (A v )-l-2He potentials. Therefore, one possible explanation for the bimodal distribution is the different entrance surfaces, which preferentially provide the low and high vibrational levels. The other possible explanation is different reaction dynamics for the low and high vibrational levels. 

A possible explanation of the bimodal character of the y-relaxation in the quenched HTH-C q polymer is given below. 

However, the argument that the cyclic nature of the perturbation ehminates the intrusion of heat effects must be treated with caution. For both p-xylene and 2-butyne in silicalite Shen and Rees [31,32] observed a bimodal response spectrum and they interpreted the two peaks as indicative of two different transport processes corresponding to diffusion through the straight and sinusoidal channels. There is some NMR evidence to support the view that such molecules cannot easily reorient themselves at the channel intersections, and for silicalite-2, which contains only straight channels of similar dimensions, only a single response peak is observed so this hypothesis is certainly plausible. However, Sun and Bourdin [34] have shown that an alternative explanation is also possible. If the heat balance equations are included in the theoretical model, the predicted response assumes a bimodal form and the heat-transfer parameter required to match the experimental data appears to be quite reasonable. 

The existence of a bimodal linewidth distribution may be attributed to several factors. When a polymer is large, interference between segments of the same chain will give rise to an intramolecular scattering contribution to the linewidth. We have ruled out this possibility since K has a maximum value of 1.2 and is often much less than one in our experiments. Thus, our experiments cannot observe the contributions due to internal motions and they amount to, at most, one to two percent of the total scattered intensity.(lO) We have also made other studies whereby a second faster peak can be attributed to a pseudo-gel motion in semidilute solutions (l ). This explanation is unreasonable because the concentrations of our solutions are very small. We should not have reached the semidilute regime. 

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