Reaction cavity effective

As noted previously, rate accelerations imposed by the cycloamyloses may be competitively inhibited by the addition of inert reagents to the reaction medium. The inhibitor, by competing with the substrate for the cycloamylose cavity, effectively removes a fraction of the catalyst from the reaction coordinate. This observation lends additional force to the mechanism illustrated in scheme I. 

In this respect, the solvatochromic approach developed by Kamlet, Taft and coworkers38 which defines four parameters n. a, ji and <5 (with the addition of others when the need arose), to evaluate the different solvent effects, was highly successful in describing the solvent effects on the rates of reactions, as well as in NMR chemical shifts, IR, UV and fluorescence spectra, sol vent-water partition coefficients etc.38. In addition to the polarity/polarizability of the solvent, measured by the solvatochromic parameter ir, the aptitude to donate a hydrogen atom to form a hydrogen bond, measured by a, or its tendency to provide a pair of electrons to such a bond, /, and the cavity effect (or Hildebrand solubility parameter), S, are integrated in a multi-parametric equation to rationalize the solvent effects. 

All of the interaction mechanisms described above are expected to produce electric fields in the solute cavity. In the case of specific interactions and reaction field effects these electric fields are expected to have some specific orientation with respect to the solute coordinate system. Dispersion forces and Stark effects are not expected to have any specific orientation with respect to the solute. Magnetic field effects seem unlikely to be important in light of the well-known invariance of coupling constants to changes of the external magnetic field. However, it is conceivable that a solvent magnetic reaction field might... 

According to the model, a perturbation at one site is transmitted to all the other sites, but the key point is that the propagation occurs via all the other molecules as a collective process as if all the molecules were connected by a network of springs. It can be seen that the model stresses the concept, already discussed above, that chemical processes at high pressure cannot be simply considered mono- or bimolecular processes. The response function X representing the collective excitations of molecules in the lattice may be viewed as an effective mechanical susceptibility of a reaction cavity subjected to the mechanical perturbation produced by a chemical reaction. It can be related to measurable properties such as elastic constants, phonon frequencies, and Debye-Waller factors and therefore can in principle be obtained from the knowledge of the crystal structure of the system of interest. A perturbation of chemical nature introduced at one site in the crystal (product molecules of a reactive process, ionized or excited host molecules, etc.) acts on all the surrounding molecules with a distribution of forces in the reaction cavity that can be described as a chemical pressure. 

When guest molecules are able to explore more space during their transformation to products than is available in the cavity in which they are accommodated at the time of excitation (initial reaction cavity), their behavior may depend upon the effective space explored. The effective reaction cavity, the space explored, will depend on the lifetime of the excited state, the nature of the mobility, and the structure of the guest molecule and the intermediate(s) derived therefrom. The initial and effective reaction cavity... 

Initial Reaction Cavity = a Effective Reaction Cavity = a+b+c+d Final Reaction Cavity = d... 

Figure 19. An illustration of three possible reaction cavities as the reaction occurs— initial, effective, and final reaction cavities.

Our concept of a reaction cavity in organized media has been considerably modified from that of Cohen [13]. It requires the inclusion of more factors to be used effectively, but it provides a base for discussion of a myriad of reaction environments. 

It is very important to note that the exact size and shape of a reaction cavity (initial, effective, and final) that control the excited state behavior of guest reactants will depend on the particular reaction as well as on the guest and intermediate s) themselves. Whether the information regarding the space explored (effective reaction cavity) by the excited molecule will be registered in the distribution or stereochemistry of the products will depend on the nature of the mechanism involved in the product formation. In some cases, explorations over a larger space by excited state species and their intermediates may not be germane to the distribution and types of products formed. In certain cases, especially those that involve the probability of encounters, all of the space excited molecules and their intermediates explore before they yield final products may be important. In cases for which the distribution of specific product types is being probed, only the site in which... 

The cage effects measured in various media are compiled in Table 2. Results clearly show that all of the organized media listed in the table have a cage effect larger than is observed in benzene ( 0%). Also note that the magnitudes of the cage effect and the yields of the rearrangement product depend on the medium (probably a reflection of the differences in the characteristics of the reaction cavity in various media). 

The importance of the size of the enclosure (reaction cavity) on a reaction course has been a subject of investigation in several laboratories. On the basis of the proposed mechanistic scheme for DBK fragmentation and on the basis of the effective reaction cavity model presented in Section III, the following predictions can be made (a) a relationship will exist between the cage effect and the reaction cavity size (b) the cage effects observed for singlet and triplet benzyl radical pairs will be different (assuming very similar diffusion rates)... 

We illustrate in this section with a number of examples how the presence or absence of free volume within a reaction cavity determines the feasibility of a reaction in organized media. Presence of free volume alone may not be sufficient to effect a reaction within a reaction cavity. Its location and its directionality (presence of free volume in the critical dimension) are extremely important, as revealed by a number of examples discussed in Section D. 

The influence of these various effects may be manifested in measurable parameters of the reaction like the overall quantum yields (On) and the photoproduct ratios for fragmentation to cyclization (E/C) and for trans to cis cyclobutanol formation (t/c) as shown in Scheme 41. The values of these quantities and their variations as the media are changed can provide comparative information concerning the relative importance of solvent anisotropy on Norrish II reactions, also. Specifically, they reveal characteristics of the activity of the walls and the size, shape, and rigidity of the reaction cavities occupied by electronically excited ketones and their BR intermediates. 

A similar study of Norrish II reactions has been conducted on complexes of aryl ketones in Dianin s compound 1 [295], a nonpolar host whose channels are effectively truncated at each 11A of length by a 2.8-A constriction from 6 hydrogen-bonding hydroxyl groups (see Figure 3) [296]. Table 13 summarizes the results from complexes with ketones expected to undergo primarily the Norrish II reactions [297]. As befits the rather large (and mostly) nonpolar reaction cavities, the E/C and t/c ratios in Table 13 provide evidence for relatively little control by the channels of Dianin s compound over the fate BRs. Even in the most selective case from 5-methyl-... 

The formation of a 1 1 complex between open form of 16b,c and Ba2+, Pb2+ leads to a large bathochromic effect and the decrease in the rate constant for dark ring-closure reaction. The effect is explained by the formation of the anion- capped complex of metal cation located in crown ether cavity with carbonyl oxygen atom (Scheme 20). Substantial difference in the changes of magnitude of /c,/s 1 was obtained for the complexes of mono- and bischromene 16b,c with Ba2+ (Table 3). The difference is due to in the complex of 16c with Ba2+ the carbonyl atoms of both chromene units participate in the formation of anion- capped complex, whereas, the anion- capped ... 

The corresponding PCM expressions (2.193) and (2.194) show that the same physical effects are considered the static cavity field effects are explicitly represented by the matrices m°, while the static reaction field effects are implicit in the coupled perturbed HF (or KS) equations which determine the derivative of the density matrix. 

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