Resonance contributions

Schaertel S A and Albrecht A C 1994 Interferometric coherent Raman spectroscopy resonant and non-resonant contributions J. Raman Spectrosc. 25 545-55... 

The unpaired electron in benzyl radical is shared by the benzylic carbon and by the nng carbons that are ortho and para to it as shown by the spin density surface in Figure 119 Delocalization of the unpaired electron from the benzylic carbon to the ortho and para positions can be explained on the basis of resonance contributions from the fol lowing structures... 

The additional parameter r is adjusted from reaction to reaction it reflects the extent of the additional resonance contribution. A large r corresponds to a reaction with a large resonance component, whereas when r goes to zero, the equation is identical to the original Hammett equation. When there is direct conjugation with an electron-rich reaction center, an equation analogous to Eq. (4.17) can be employed, but a is used instead of [Pg.210]

In this equation, the substituent parameters and reflect the incremental resonance interaction with electron-demanding and electron-releasing reaction centers, respectively. The variables and r are established for a reaction series by regression analysis and are measures of the extent of the extra resonance contribution. The larger the value of r, the greater is the extra resonance contribution. Because both donor and acceptor capacity will not contribute in a single reaction process, either or r would be expected to be zero. 

Although cTi estimates by different methods or from different data sets may disagree, it is generally held that the inductive effect of a substituent is essentially independent of the nature of the reaction. It is otherwise with the resonance effect, and Ehrenson et al. have defined four different ctr values for a substituent, depending upon the electronic nature of the reaction site. An alternative approach is to add a third term, sometimes interpreted as a polarizability factor, and to estimate the inductive and resonance contribution statistically with the added parameter the resonance effect appears to be substantially independent of reaction site. " " ... 

The net effect is, therefore, a balance of these two components. Some of the results of the separation may be surprising, such as the relatively small contribution that resonance makes to the overall substituent effect of p-NOj. Another interesting result is the predominant resonance contribution in the alkyl series this (if real) must be attributed to hyperconjugation. 

The only groups in Table 9.5 with negative values of CT/ are the alkyl groups methyl and tert-butyl. There has been some controversy on this point. One opinion is that CT/ values decrease in the series methyl, ethyl, isopropyl, /ert-butyl (respectively, — 0.046, —0.057, —0.065, —0.074). Other evidence, however, has led to the belief that all alkyl groups have approximately the same field effect and that the a/ values are invalid as a measure of the intrinsic field effects of alkyl groups. Another attempt to divide ct values into resonance and field contributions is that of Swain and Lupton, who have shown that the large number of sets of ct values (cTm, <3p, a, a+, CT/, csp etc., as well as others we have not mentioned) are not entirely independent and that linear combinations of two sets of new values F (which expresses the field-effect contribution) and R (the resonance contribution) satisfactorily express 43 sets of values. Each set is expressed as... 

The effect of a methoxy group in the -phenyl ring, 146, X = OCH3, Y = H, upon the reaction rate was minimal (about 40%), which indicates that resonance contributions from the carbene species 148b are insignificant. Such... 

TTie solvolysis of propargylic substrates (199) and formation of alkynylcarbonium ions (200) has been extensively investigated. Particularly good evidence for the formation of alkynylcarbonium ions comes from the nuclear magnetic resonance spectra of alkynyl alcohols in strong acid media (200, 201). The downfield shifts of 4ppm for the proton of HC=C— and 1 ppm for CH3C=C- relative to their neutral precursors is indicative of carbonium-ion formation and shows the importance of the allenyl resonance contribution. 

The emitted SFG light is a sum of resonant contributions with nth vibrational states,... 

Electronically related to such borirenes are the salts of the dianion of 1,2-di-tert-butyl-3-[bis(trimethylsilyl)]methyl]-l, 2-diborirane.85 An XRD analysis of the dipotassium salt (86a,b) uncovered shortened B—B (1.58 A) and B—C (1.50 A) bonds, as suggested by resonance contributions 86a and 86b. Salt 86 can be considered to have largely a 2- r-electron Hiickel aromatic ring. 

Fig. 19. The computed differential cross-section for F + HD —> HF + D. (a) is the result of QM while (b) is the pure resonance contribution.

Fig. 3. The normalized excitation functions in A2 versus collision energy for the two isotopic channels for the F+HD reaction. The solid line is the result of quantum scattering theory using the SW-PES. The QCT simulations from Ref. 71 are plotted for comparison. The experiment, shown with points, is normalized to theory by a single scaling factor for both channels. Also shown in (a) is the theoretical decomposition of the excitation function into direct and resonant contributions using the J-shifting procedure.

Scheme 4) <02CC1816>. The reaction occurred selectively in the peripheral carbon-nitrogen bond, showing that this bond is more reactive than the other carbon-carbon double bonds. This can be understood by the resonance contributions to the overall structures. Figure 3 shows two such resonance structures. In the canonical form II the C=N is both cross conjugated and in an iminium form which is known to be electron-deficient and an active dienophile <02CC1816>.

Of course, at a higher level of detail there may also be significant ionic resonance contributions involving equatorial ligands. In SiFs, for example, such contributions correspond to resonance structures of the form... 

The net consequence of n0->-CTMH delocalizations is that OIrH3 and OPH3 acquire partial double-bond character due to resonance contributions as shown in... 

Pauling later expressed the view that partial covalency or ionic-covalent-resonance contributions of the form... 

A typical, experimentally obtained, output spectrum of a waveguide with five resonators of differing sizes is shown in Fig. 16.5a. In this first case, all the five resonators had water as the surrounding medium. As can be seen, each resonator contributes a sharp dip to the output spectrum of the device. We observe that each ID resonator possesses a large -factor varying from 1,500 to 3,000 and a full... 

These results also indicate that the protonation of the imine group is important for the reaction. In the o-OH and p-OH isomers, resonance between the protonated imine and quinoido species would contribute in facilitating the protonation of the imine portion (Scheme 5.17). In the m-OH isomer, however, no such resonance contribution is possible. Thus, the poor additive effect of phenol in the reaction of N-benzylideneaniline with the acylzirconocene chloride (entry 1, Table 5.3) might imply less efficient protonation of the imine, as in the case of the m-OH isomer (Scheme 5.16). 

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