Molecular anisotropy

There are two factors that determine chemical shifts - electron distribution and molecular anisotropy. We have already seen how electronics define chemical shifts in previous sections. When we use Table 

4 to estimate shifts around an aromatic ring, for example, the predictions we arrive at are based on the known electron withdrawal or supply of the various substituents on the ring. No allowance is made for unusual anisotropy. Similarly, predictions of chemical shifts of alkyl protons using Table 5.8 will be calculated on the basis of electronic factors only as it would be impossible to vector anisotropy into the prediction since it varies in each individual molecule. They will be reasonably accurate in molecules where electronic factors predominate and molecular anisotropy has little or no influence. A typical example of such a molecule is shown in Structure 6.6. Note the lack of steric crowding in the structure. 

However, in molecules where groups are constrained for whatever steric reasons, molecular anisotropy can play a large part in determining chemical shifts. Take for example, the molecule in Structure 6.7. 

Structure 6.6 Typical molecule where electronic factors predominate. 

Structure 6.8 demonstrates a most extreme example of anisotropy. In this unusual metacyclophane, the predicted chemical shift (Table 5.8) of the methine proton that is suspended above the aromatic ring would be 1.9 ppm. In fact, the observed shift is -4 ppm, i.e., 4 ppm above TMS The discrepancy between these values is all down to the anisotropic effect of the benzene ring and the fact that the proton in question is held very close to the delocalised p electrons of the pi cloud. 

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Molecular anisotropy affects proton chemical shifts to a far greater extent than 13C chemical shifts. This is because the protons occupy the outer extremities of a molecule whilst the carbon framework is far more internal and to a large extent, removed from the influences of anisotropy. 

So if this all sounds a bit bleak, what s the good news Well, strangely, there is quite a lot. For a start, let s not forget that had the 13C nucleus been the predominant carbon isotope, the development of the whole NMR technique itself would have been held back massively and possibly even totally overlooked as proton spectra would have been too complex to interpret. Whimsical speculation aside, chemical shift prediction is far more reliable for 13C than it is for proton NMR and there are chemical shift databases available to help you that are actually very useful (see Chapter 14). This is because 13 C shifts are less prone to the effects of molecular anisotropy than proton shifts as carbon atoms are more internal to a molecule than the protons and also because as the carbon chemical shifts are spread across approximately 200 ppm of the field (as opposed to the approx. 13 ppm of the proton spectrum), the effects are proportionately less dramatic. This large range of chemical shifts also means that it is relatively unlikely that two 13C nuclei are exactly coincident, though it does happen. 

The Tfj-symmetrical hexaaddition pattern also represents an attractive core tecton for dendrimer chemistry [26, 31, 63-67]. Examples for such dendrimers, involving a core branching multiplicity of 12, are 38 and 39 [63, 64], Addition of six mesotropic cyanobiphenyl malonate addends produced the spherical thermotropic liquid crystal 40 [65], DSC and POM investigations revealed a smectic A phase between 80 and 133 °C. Interestingly, this spherical and highly symmetrical compound gives rise to liquid crystallinity despite the absence of molecular anisotropy. 

Some diamagnetic crystals (graphite, bismuth, naphthalene and other aromatic substances) show prohounced diamagnetic anisotropy. The observed anisotropy of crystals of benzene derivatives correspond to the molar diamagnetic susceptibility —54 X 10 with the field direction perpendicular to the plane of the benzene ring and —37 X lO"6 with it in the plane. This molecular anisotropy has been found to be of some use in determining the orientation of the planes of aromatic molecules in crystals.1... 

At once, the previous discussion explains why the polar molecules do not exhibit a polarization effect at all the strong anisotropy of the CO-Na potential leads to a complete mixing of 2 and II states. Somewhat less easily explained is the dependence of the polarization effect on CM and its disappearance at larger scattering angles. One possibility is to ascribe small quenching cross sections to small collision parameters and thus to deeper penetration, where the molecular anisotropy is dominant and thus mixes the initial state preparation as discussed previously. 

Both the amplitude and the direction of the total poling field are functions of position x. The dipole chromophore will align in response to the field Et(x), and a periodic refractive index modulation will be formed due to both molecular anisotropy and the Pockels effect [37], The contribution from molecular anisotropy is an important, sometimes even dominant, mechanism. 

Bain, A.J., McCaffery, A.J., Proctor, M.J. and Whitaker, B.J. (1984). Laser-interrogated dichroic spectroscopy A sensitive probe of molecular anisotropies, Chem. Phys. Lett., 110, 663-665. 

Bain, A.J. and McCaffery, A.J. (1985). On the measurement of molecular anisotropies using laser techniques. I. Polarized laser fluorescence, J. Chem. Phys., 83, 2627-2631. 

There are two angles of incidence (36° and 75°) that maximize the surface-absorption detection. Although these two maxima have theoretically almost the same absolute values, experimentally it was found that 75° is the more appropriate angle of incidence for both the S/N ratio and the sensitivity to the molecular anisotropy of the monolayer [20,83]. 

Compared with C6o, the structures and phase diagram of C7o are less well understood [53-55], This is because of the molecular anisotropy of C70, which allows various degrees of rotational freedom, and of the presence of the impurities, which makes... 

Fig. 27 defines the principal directions of molecular anisotropy (I, II and HI) with respect to the principal directions of shear (x, y and z), x being the direction of velocity and y the normal to the plates. Symmetry of the flow leads to in=z and the relative orientation of (I,H) with respect to (x,y) is defined by the angle %. 

They have done calculations for C2H6, Si2H6 and Gc2H6 geometries and internal rotation barriers. They also used the model potential in eqn (2-L-22) for calculation of equilibrium geometries of some homonuclear diatomics," such as O2, Bc2, B2, C2, N2, O2, F2, P2 and CI2. Molecular anisotropy in some diatoms such as H2, O2, C2, N2, O2 (singlet), F2, HF and CO have been evaluated with same model potential. A nonlocal pseudopotential in the FSGO model ... 

Samples oriented by photoselection have been used for studies of molecular anisotropy by polarized absorption spectroscopy. For instance, the... 

Table 6. Dipolar and octupolar spherical components and molecular anisotropy ratio, p, for compounds la-d...

In order to quantify the dipolar to octupolar conbibution to the NLO response, two spherical components of j3 related to the dipolar and octupolar molecular anisotropy are employed. They represent the dipolar and octupolar contributions to the NLO response of the molecules. The expressions which define them are, respectively ... 

The nonlinear molecular anisotropy ratio p is then defined from the previous equations in the following manner ... 

The NMR lineshape readily reveals molecular anisotropy and motional averaging at a segmental level. It provides information about fast and intermediate dynamics, and about molecular order (see Section 6.2.5). 

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