Asymmetric shifts

FERROELECTRIC EFFECT. The phenomenon whereby certain crystals may exhibit a spontaneous dipole moment twhich is called ferroelectric by analogy with ferromagnetic—exhibiting a permanent magnetic moment). The effect in the most typical case, barium manate. seems to he due to a polarization catastrophe, in which the local electric fields due lo the polarizuiion itself increase faster than die elastic restoring forces on the ions in Ihe crystal, thereby leading to an asymmetrical shift in ionic positions, and hence lo a permanent dipole moment. Ferroelectric crystals... 

The photorefractive effect is usually probed by two beam-coupling experiments, in which one beam gains intensity at the expense of the other. This coupling is a characteristic property of the photorefractive effect. Such an asymmetric coupling requires an asymmetric shifting of the refractive index grating... 

Figure 3.2.10 Schematic representation of theoretical powder line shapes for the chemical-shift tensor, (a) - asymmetric shift anisotropy, (b) axially symmetric shift anisotropy.

We are currently working to extend these results to account for axially asymmetric shift tensors and higher order sidebands such as those we observe. In summary, our work indicates MASS together with dilute spin double resonance is an effective means for obtaining high resolution NMR spectra of a number of different nuclear spins in solids. Contrary to popular beliefs, it is effective at high as well as low magnetic fields. 

In every case a bathochromic shift that is greater for asymmetrical than for symmetrical dyes has only been observed. 

It has been found (7,9) that Z) > 0, ie, the maximum of an asymmetrical dye, X, is shifted to the short-wavelength region with respect to the arithmetical mean of the parent dye maxima. The phenomenon has been named a deviation (7). The positive deviations in PMDs are explained by the bond order alternation within the polymethine chain caused by different contributions of both end groups to the dye energetic stabiUty (7,9,10). The deviation reaches its maximum at Tqi > 45° and Tqi < 45° if the end groups have > 45° and 4>q2 > 45°, or < 45° and 4>q2 < 45°, then the... 

A power circuit is basically an R-L circuit. In the event of a fault, the system voltage (V , sin ft))) may occur somewhere between V = 0 and V = on its voltage wave. This will cause a shift in the zero axis of the fault current, 7sc> and give rise to a d.c. component. The fault current will generally assume an asymmetrical waveform as illustrated in Figure 13.27. 

The generation of an asymmetrical current on an a.c. system, leads to the inference that a short-circuit condition will give rise to a d.c. component due to a shift in its zero axis. During the sub-transient state the value of the asymmetrical current will be the phasor sum of the symmetrical /sc and the asymmetrical current components. For details refer to Section 14.3.6. 

If a molecule contains several asymmetric C atoms, then the diastereomers show diastereotopic shifts. Clionasterol (28a) and sitosterol (28b) for example, are two steroids that differ only in the absolute configuration at one carbon atom, C-24 Differing shifts of C nuclei close to this asymmetric C atom in 28a and b identify the two diastereomers including the absolute configuration of C-24 in both. The absolute configurations of carboxylic acids in pyrrolizidine ester alkaloids are also reflected in diastereotopic H and C shifts which is used in solving problem 54. 

The presence of asymmetric C atoms in a molecule may, of course, be indicated by diastereotopic shifts and absolute configurations may, as already shown, be determined empirically by comparison of diastereotopic shifts However, enantiomers are not differentiated in the NMR spectrum. The spectrum gives no indication as to whether a chiral compound exists in a racemic form or as a pure enantiomer. 

Figure 3 Characteristic solid state NMR line shapes, dominated by the chemical shift anisotropy. The spatial distribution of the chemical shift is assumed to be spherically symmetric (a), axially symmetric (b), and completely asymmetric (c). The top trace shows theoretical line shapes, while the bottom trace shows rear spectra influenced by broadening effects due to dipole-dipole couplings.

The formal derivation of the analogs, described in the foregoing, represents, from the point of view of systematic organic chemistry, a shift to the derivatives of other heterocyclic systems. In the case of pyrimidine aza analogs we arc dealing with derivatives of symmetrical or asymmetrical triazine in the case of purine aza analogs, the derivatives produced are those of imidazo[4,5]-i -triazine and z -triazolo [4,5-d] pyrimidine. 

Titanium(IV) chloride induced re.ictionsof(1 -methyl-3-trimethylstannyI-l-butenyl)diisopropyl-carbamate and aldehydes proceed with a double 1,3-shift to give ami-products, also with effective asymmetric induction118. 1-Alkoxyallyltitanium trichlorides are involved. 

Sesquinorbornatrienes 949 Sharpless asymmetric epoxidation 826 Sharpless reagent 73, 289, 291 Shift reagents, coordination of sulphoxides with 573... 

The quasi-classical description of the Q-branch becomes valid as soon as its rotational structure is washed out. There is no doubt that at this point its contour is close to a static one, and, consequently, asymmetric to a large extent. It is also established [136] that after narrowing of the contour its shape in the liquid is Lorentzian even in the far wings where the intensity is four orders less than in the centre (see Fig. 3.3). In this case it is more convenient to compare observed contours with calculated ones by their characteristic parameters. These are the half width at half height Aa)i/2 and the shift of the spectrum maximum ftW—< > = 5a>+A, which is usually assumed to be a sum of the rotational shift 5larger scale A determined by vibrational dephasing. 

NMR is the only technique capable of assigning the cysteines ligating specific Fe atoms, since the asymmetric coupling of the three Fe + ions results in different temperature dependence for the contact shifts... 

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