Polarization patterns

As for CIDNP, the polarization pattern is multiplet (E/A or A/E) for each radical if Ag is smaller than the hyperfme coupling constants. In the case where Ag is large compared with the hyperfmes, net polarization (one radical A and the other E or vice versa) is observed. A set of mles similar to those for CIDNP have been developed for both multiplet and net RPM in CIDEP (equation (B1.16.8) and equation (B1.16.9)) [36]. In both expressions, p is postitive for triplet precursors and negative for singlet precursors. J is always negative for neutral RPs, but there is evidence for positive J values in radical ion reactions [37]. In equation (B 1.16.8),... 

Figure Bl.16.22 shows a stick plot siumnary of the various CIDEP mechanisms and the expected polarization patterns for the specific cases detailed in the caption. Each mechanism clearly manifests itself in the spectrum in a different and easily observable fashion, and so qualitative deductions regarding the spin multiplicity of the precursor, the sign of Jin the RP and the presence or absence of SCRPs can innnediately be made by examining the spectral shape. Several types of quantitative infonnation are also available from the spectra.

Figure 3.1 The reactions of ethylene and menthene with bromine. In both molecules, the carbon-carbon doublebond functional group has a similar polarity pattern, so both molecules react with Br2 in the same way. The size and complexity of the remainders of the molecules are not important.

The polarity patterns of some common functional groups are shown in Table 5.1. Carbon is always positively polarized except when bonded to a metal. 

Table 5.1 Polarity Patterns in Some Common Functional Groups...

As a result of these substituent-induced polarizations, the complementary conjugative interactions at each ring site become somewhat imbalanced (so that, e.g., the donor-acceptor interaction from C3—C4 to C5—C(, is 23.1 kcal mol-1, but that in the opposite direction is only 16.4 kcal mol-1). From the polarization pattern in (3.133) one can recognize that excess pi density is accumulated at the ortho (C2, C6) and para (C4) positions, and thus that the reactivity of these sites should increase with respect to electrophilic attack. This is in accord with the well-known o, /(-directing effect of amino substitution in electrophilic aromatic substitution reactions. Although the localized NBO analysis has been carried out for the specific Kckule structure of aniline shown in Fig. 3.40, it is easy to verify that exactly the same physical conclusions are drawn if one starts from the alternative Kekule structure. 

However, the larger effect of polarity on the one-center term (3.177a) comes from the dependence on the cAcy prefactor of FAA-. The cacy product is strongest for electropositive X and electronegative Y, corresponding to a concerted (cooperative) polarization pattern of the form... 

From polarity considerations, one can recognize that the prefactor CxeA> of the nonbonded term is minimized by the same concerted polarization pattern as that shown in (3.179), which is consistent with maximization of the one-center term. This is also consistent with higher ionic character and the X+ A Y resonance pattern shown in Fig. 3.78. 

Figure 5.24 Monomer charges ( (2hf) in model (HF) chain clusters (Figs. 5.23(a)-(c)). For each n, distinct symbols identify monomers at the end (circle), second from end (triangle), and third from end (square) positions of the chain, with connecting dotted lines to aid visualization. Note the strong CT-polarization pattern from the cationic to the anionic end of chain.

Figure 9.13 (a) and (b) Examples of polarization patterns obtained from quarter waveplate... 

The nonlinearity of the sample was analyzed using the experimental procedure described in Section 3.3 The polarization of the fundamental beam of a YAG laser was continuously varied by means of a quarter waveplate, and the intensity of the second-harmonic signal was measured as a function of the rotation angle of the quarter waveplate. The obtained polarization pattern were then fitted to Eq. (42), which yields the relative values of the expansion coefficients /, g, and h. The experimental results for the transmitted, glass-side-incidence, s-polarizcd signal are shown in Figure 9.20. 

A detailed analysis of the experimentally obtained polarization patterns allows to determine all susceptibility components of the LB films.16,49 The values of the susceptibility were calibrated using a quartz wedge ([Pg.558]

The polarization patterns are dependent upon the strength of the magnetic field, in which the reactions are carried out. If the reactions are carried out at high fields (i.e., notably within the NMR spectrometer), the resonances appear in antiphase - that is, there is an equal number of absorption and emission lines and no net polarization. At low field however (i.e., when the reaction is carried out at zero or a very low field and then transferred into the high field of the NMR spectrometer for subsequent investigation), the resonances display net polarization, as has been outlined by Pravica and Weitekamp [9]. 

For more complex spin systems, a computer program PHIP+ has been developed [13, 45] which allows the expected PHIP spectra to be calculated from the chemical shifts and coupling constants of the products. Depending upon which proton pair in the product molecule stems from p-H2, different - but characteristic - polarization patterns result [14]. The patterns also depend on the sign of the coupling constants. Simple sign rules governing the relative sequence of the emission and absorption lines in the PHIP spectra (i.e., their phase ) can be formulated in similar manner to the Kaptein Rules of chemically induced dynamic nuclear polarization (CIDNP) [15]. 

Computer-Assisted Prediction and Analysis of the Polarization Patterns DYPAS2... 

The effects observed for 23 (Figure 8) are particularly clear-cut, since the spectrum is fully resolved." The key to the structure of 23 lies in the prominent enhanced absorption signals of H3 (5.2 ppm) and (-0.2 ppm) and the strong emission of (2,4 ppm), H4 (2.2 ppm), and H5 (1.5 ppm). This polarization pattern supports a spepies with spin density on C3 and C6, indicating the delocalization of spin and charge into the lateral cyclopropane bond. Weakly enhanced absorption observed for H2 (5.8 ppm), (0.8 ppm), and Hi (1.75 ppm), and weak emission for H5 further support this structure type. ... 

Fig. 4. Comparison of the raw data from 49-point diameter, 21-point contour, and 9-point contour measurement patterns to 49-point polar pattern data, showing (a) mean and (b) the nonuniformity (standard deviation divided by mean in percentage) using reflectometry.

It is pertinent to mention the formation of y-ketols by photochemical addition of alcohols to enones follows the same polarity pattern [193],... 

The first detection of the RP state in PS I single crystals was reported by Kamlowski et al.2il using transient EPR at X-band. Berthold et al.2 3 oriented PS I RCs in the B0 field of the W-band spectrometer. From the analysis of the polarization pattern they reproduced the RP geometry obtained in the single crystal data. High field EPR studies proved to be very useful as shown by Stehlik and coworkers who first used K-band EPR144,284, later the work was extended to a multifrequency approach including W-band by van der Est et al.285 and Zech et al.192. In the latter paper a complete analysis of the structure of the RP P 7o0 A -... 

Fig. 3. Mirror-image polar patterns arising through a primary bifurcation.

Two electrostatic potential maps, one of methyllithium (CH3Li) and one of chloromethane (CH3C1) are shown. Based on their polarity patterns, which do you think is which ... 

First, the polarization pattern of the 3-line phenacyl radicals appeared very different. Actually the difference is only due to the increased contribution of the E/A Radical-Pair polarization which now increases the relative emissive intensity of the low-field hyperfine line and essentially cancels the high-field line. As mentioned earlier E/A Radical-Pair polarization has no effect on the centre line. The relative Radical-Pair... 

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