Dipolar process

Although most of the examples of [3S+2C] cycloaddition reactions with carbene complexes are referred to as 1,3-dipolar processes, we should include in this section another kind of non-dipolar transformation dealing with the reaction of pentacarbonyl(methoxymethylcarbene)chromium with a base followed by treatment with an epoxide in the presence of boron trifluoride. This reaction gives cyclic carbene complexes in a process that can be considered a [3S+2C] cycloaddition [44] (Scheme 14). 

The magnetization decay rate constant in the x-y plane, 1/T, consists of a term related to the recovery rate constant for the magnetization along z, 1/Tj, as well as other mechanisms which give rise to a dispersal of the magnetization only in the x-y plane. Two examples of the latter are 1/T which results from dipolar processes (to be discussed later) and YAHq which is due to the applied magnetic field inhomogeneity AHq. In this case the relaxation rate is approximately... 

Figure 11, Relaxation map for the dipolar processes detected in TeEGDMA evidencing the curvature of the cooperative a process in contrast to linear temperature dependence of the relaxation times of P and y local processes described previously solid lines - fitting by VFTH (a) and Arrhenius (P and y) laws (adapted from [47]).

A dipolar process is described in terms of three quantities the orientation polarization, the vibrational polarization, and the relaxation time. The first of the three is expressed as A =85-8oc, where is as defined earlier, and is the limiting high-frequency permittivity. The vibrational polarization is expressed as A8vib=eoo- op. where op is the refractive index of the material, usually at the... 

In most supramolecular structures, the temperature dependence of the characteristic dielectric relaxation time follows the Arrhenius equation, r = Toexp(A dip/ T). where tq is the preexponential factor that is often of the magnitude of the vibrational time scale and A dip is the activation energy of the dipolar process.The dipolar process of the host lattice and the trapped molecules follows this behavior, but A trapped molecules is less than that for the host lattice molecules. In ice ciathrates, the dipolar processes of the water molecules that form the host lattice and the guest molecules inside the cages of this lattice occur at widely different time scales. This allows for a reliable attribution of the dielectric spectra features to water molecules and to the guest molecules. As an example of the magnitude of the dielectric properties of supiainolecular structures, the data on selected ice clathrates and other inclusion compounds are summarized in Tables 1 and 2. 

Table 1 The various properties of the dipolar processes in the two types of host lattices of some clathrate structures...

Table 2 The various properties of the dipolar processes of the guest (or included) inolecules in the lattice of selected...

A facile dearomatization of nitrobenzene derivatives and other nitroarenes with A-benzyl azomethine ylide has been reported (Scheme 6)." The fact that all fused rings arising from the cyclization have a cis relative stereochemistry is in line with a concerted 1,3-dipolar process. 

Depending on the electromagnetic nature of Ti , a double-electron exchange (Dexter) mechanism or an electrostatic multipolar (Forster) mechanism have been proposed and theoretically modeled. They are sketched on Fig. 8 for the simple S - T -Ln path. Their specific dependences on the distance d separating the donor D from the acceptor A, i.e., for double-electron exchange and for dipole-dipolar processes, respectively, often limit Dexter mechanism to operate at short distance (typically 30-50 pm) at which orbital overlap is significant, while Forster mechanism may extend over much longer distances (up to 1,000 pm). 

Otvos and Armitage (46a) have observed Tj values similar to those presented here in their study of alkaline phosphatase. They attempted to apply simple models to their Tj data using rotational correlaton times for alkaline phosphatase. These models failed to explain the observed Ty and NOE, and their conclusion was that the relaxation was probably dominated by dipolar processes mediated by internal motions within the protein, This conclusion may also be consistent with the data presented here. For metalloproteins where one can "readily" exchange the metal(s) out of the protein, the "internal motions" may be the formation of "abortive" complexes similar to those described above. 

Only a very few measurements have been reported on phosphorus coordination compounds. The relaxation is probably dominated by dipolar processes since NOE measurements gave values of 0.68-1.00 for the palladium complexes listed in Table 1. Decrease in the relaxation time with decreasing temperature, decease with molecular weight in alkyl phosphine complexes, and longer Jj for aryl phosphines relative to alkyl, are all consistent with the dipolar mechanism. The results also showed a marked solvent dependence—which may be important from the practical point of view—with up to 50% longer in CD2CI2 than in CDCI3. 

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