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Optically inactive states

However, when (+)-methylneophylphenyltin deuteride, (+)-(56) ([ot] s + 10.7) is kept in the dark mixed with five equivalents of diethylaluminum hydride for ten hours at room temperature in benzene, optically inactive (72) is formed 44). (In the absence of (Et2AlH)2 less than 3 % of (12) is racemized under these conditions). The four-center transition state is therefore very unlikely. [Pg.106]

Aq. soln. of sodium chlorate are optically inactive no matter whether the soln. be made from the right- or left-handed crystals—presumably because for a state of stable equilibrium—dynamic isomerism—equal quantities of the two isomers are required, and when the aq. soln. of either form is allowed to recrystallize, H. Landolt7 found equal proportions of the two forms are obtained but D. Gernez showed that if a right- or left-handed crystal be added to the undercooled soln., only one... [Pg.325]

Although pure compounds are always optically active if they are composed of chiral molecules, mixtures of equal amounts of enantiomers are optically inactive since the equal and opposite rotations cancel. Such mixtures are called racemic mixtures6 or racematesP Their properties are not always the same as those of the individual enantiomers. The properties in the gaseous or liquid state or in solution usually are the same, since such a mixture is nearly ideal, but properties involving the solid state,8 such as melting points, solubilities, and heats of fusion, are often different. Thus racemic tartaric acid has a melting point of 204-206°C and a solubility in water at 20°C of 206 g/liter, while for the ( + ) or the ( —)... [Pg.95]

We observe the coherent excitation of an optically inactive mode proving that the reactive process itself and not only the optical excitation drives the observed vibrational motions. Further we demonstrate that during the ESIPT the proton is adiabatically shifted from one site to the other and tunneling of the proton is not rate determining. The dynamics is entirely controlled by the skeletal modes. Interestingly, this is quite similar to ground state proton transfer of HC1, where the fluctuations of the water environment enable the adiabatic process [8]. [Pg.196]

X lT + E r Xij ki, which refer in their order of appearance to the Cartesian polarization components of the CRS, pump, probe, and Stokes fields in the four-wave mixing process [31]. In transparent and optically inactive media, where the input frequencies are away from any electronic transition frequencies, and only the molecular ground state is populated, the selection rules of both resonant coherent and spontaneous Raman scattering are identical... [Pg.115]

It was Pasteur, in the middle of the 19th century, who first recognized the breaking of chiral symmetry in life. By crystallizing optically inactive sodium anmonium racemates, he separated two enantiomers of sodium ammonium tartrates, with opposite optical activities, by means of their asymmetric crystalline shapes [2], Since the activity was observed even in solution, it was concluded that optical activity is due to the molecular asymmetry or chirality, not due to the crystalline symmetry. Because two enantiomers with different chiralities are identical in every chemical and physical property except for optical activity, in 1860 Pasteur stated that artificial products have no molecular asymmetry and continued that the molecular asymmetry of natural organic products establishes the only well-marked line of demarcation that can at present be drawn between the chemistry of dead matter and the chemistry... [Pg.98]

A noncovalent chiral amplification based on the sergeants and soldiers effect between chiral and achiral polymers in the film state has been developed by Fujiki et al., which results in the formation of helix bundles (Fig. 35) [ 159]. An optically active, helical polysilane 84 with an almost single-handed screw-sense was chemically bonded or spin-coated on the surface of a quartz plate, followed by further spin-coating of an optically inactive polysilane 85 to give... [Pg.82]

A distinction arises in the description of the lineshape when the dense, optically inactive manifold is replaced by a sparse one, corresponding to the intermediate case 15>. This may apply to small molecules such as SO2 and NO2, and also to large molecules characterized by a small energy gap between the two interacting electronic states, such as naphthalene 16> V) and 3,4-benzpyrene 18>. According to Jortner, Rice and Hochstrasser 8>, separate, inhomogenously broadened lines are predicted for the intermediate case. [Pg.122]

The reaction kinetics of the even more reduced scheme R S were first developed by Stevenson and Verdieck [34]. It is evident that the total concentration c0 in reactions following Scheme 2 will remain constant, and so does the absorbance. With 1- or r-cpl, the ratio of the concentrations of R and S forms becomes larger or smaller than 1, depending on which enantiomer absorbs more strongly, and for long irradiation times, a photostationary state with a certain CD value is reached. Of course, photoenantiomerization occurs also with nonpolarized light (npl), but for an optically inactive educt mixture it does not become apparent, because of the symmetry of the reaction system. If, however, pure enantiomers or an unequal mixture of the enantiomers is irradiated with npl, this photoenantiomerization leads to a racemic mixture of R and S. [Pg.10]

Meso configurations can have enantiomeric conformers in crystals. Ribitol and xylitol have meso configurations and are optically inactive in solution. In the crystalline state, they have the bent-chain enantiomorphic conformations. Xylitol crystallizes in space group P212121, so each crystal contains either only the optically active left-handed or the optically active right-handed conformers. There are, of course, an equal number of left- and right-handed crystals in any one batch, which on dissolution give no optical activity.1... [Pg.175]

We know the location of the -OH group of tropine because it is stated that tropine is an optically inactive alcohol. This hydroxyl group results from basic hydrolysis of the ester that is composed of tropine and tropic acid. [Pg.674]

Rosenfeld[187] A system (state) with sharp energy is optically inactive. [Pg.198]

These authors observed that binaphthyl crystallize in two polymorphs. The one is stable at lower temperature, is centrosymmetric and is not optically active. This polymorph melts at 145 °C. The second polymorph is stable at higher temperature but is metastable at room temperature. It is optically active and melts at 158 °C. Wilson and Pincock show that as one cycles in temperature between room temperature and 150 °C a sample which is initially the optically inactive low temperature polymoiph transforms to an optically active solid. After three or four cycles one achieves the maximum optical resolution which corresponds to 56% ee. The crux of the Wilson and Pincock experiment is that at 150 °C, the reaction physically resembles a solid state reaction in which the low temperature form is melting in the near presence of high temperature polymorph crystals. These chiral crystals are therefore nucleating sites for further chiral crystal growth. As at room temperature binaphthyl retains its chirality, the resultant samples can then be dissolved with retention of stereochemistry. [Pg.85]


See other pages where Optically inactive states is mentioned: [Pg.270]    [Pg.71]    [Pg.270]    [Pg.71]    [Pg.33]    [Pg.47]    [Pg.85]    [Pg.41]    [Pg.43]    [Pg.131]    [Pg.126]    [Pg.26]    [Pg.871]    [Pg.215]    [Pg.244]    [Pg.20]    [Pg.704]    [Pg.27]    [Pg.12]    [Pg.38]    [Pg.38]    [Pg.228]    [Pg.65]    [Pg.128]    [Pg.157]    [Pg.160]    [Pg.160]    [Pg.532]    [Pg.356]    [Pg.196]    [Pg.16]    [Pg.490]    [Pg.564]    [Pg.632]    [Pg.194]    [Pg.138]    [Pg.196]   
See also in sourсe #XX -- [ Pg.270 ]




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