Spectra calculations optical activity

Fig. 33 Optical activity of a chiral ligand-protected gold nanocluster [Au25(SRcys)18]. (a) Optimized geometry, (b) Calculated CD spectrum of the cysteine-protected cluster black), and experimental spectrum for a glutathione-protected cluster (red). Reprinted with permission from [299]. Copyright 2010 American Chemical Society...

The CD spectral investigation of optically active bi[10]paracyclophanes (47), in which two planar and one axial chirality elements are incorporated, was reported in the literature [57]. The experimental CD spectrum of enantiomerically pure (RV,SP)-47, which was derived from the corresponding diastereomeric tetra-(S )-camphanoate, was found to be in good agreement with that calculated for the (aA)-isomer, but approximately the mirror image of the spectrum computed for the (aSHsomer (Fig. 11). Thus, the CD spectrum can be interpreted mostly in terms of the axial chirality, indicating that the effects from the planar chirality of cyclophane units were more or less cancelled out. 

Thermal reaction mixtures in this work were analyzed by FTIR spectroscopy, tunable diode laser spectroscopy and vibrational circular dichroismA pair of representative VCD spectra are shown in Figure 1. The more intense spectrum is an optically pure reference sample of the (25, 35) isomer at 53.59 torr the other is a thermal reaction product mixture from this isomer after 360 min at 407 °C and gas chromatographic isolation, recorded at 81.30 torr . From the measured Ay4/A 4ref absorption intensity ratio and the pressures, one may calculate that the sample retains 51.65% of its original optical activity, which compares well with the value calculated from the value obtained from the least-squares fit of all five VCD experimental points (51.2%). These spectra, obtained in the gas phase with a few mg of chromatographically purified labeled cyclopropanes, demonstrate the promise of VCD for assessing enantiomeric excess in situations where classical polari-metric methods would be of limited utility. 

In vibrational optical activity, this comparison is usually carried out in a reversed manner. A common plausible value, for example, 10 cm", is tentatively assigned to bandwidths, and the theoretical spectrum is simulated using calculated band positions and rotatory strengths or ROA intensities. The resulting theoretical VCD or ROA curve is then compared with the experiment and the difficult curve fitting is therefore avoided (Fig. 8.5). 

Figure 20. The 5j 12j dispersed fluorescence of ultracold anthracene in a supersonic beam. The available vibrational energy is 1792 cm1. The parameters of the optically active modes are given in Table II. The top figure is the experimental spectrum.60 The bottom figure is the emission in the harmonic approximation (y6.6 = 0). The calculation clearly fails to reproduce the broad redistributed emission. The middle figure was calculated with IVR [Eqs. (131)]. Only one b ) state (the ground vibrational state f/> = 0 was used. yt.t/y, = 40. The relaxed emission was calculated in the fast modulation limit [Eq. (116a)], with f0 = 2f = 75 cm-1.61...

This work demonstrated (a) that there are distortions in the CD of particulate systems (this had not previously been appreciated and the distorted spectra had been interpreted in terms of unique conformational features), (b) that the distorted spectra can be calculated and hence corrected, and (c) that there is a measurable differential scatter of left and right circularly polarized light by optically active particles. Thus, in addition to correcting spectra for suspensions of particulate systems that may be interpreted in terms of biomolecular conformation, the third point makes it possible to obtain an optical rotatory dispersion spectrum for the particle surface. In the case of membranes this will allow determination of relative amounts of surface area which are covered by ordered protein. This information coupled with the CD spectrum for the whole membrane will provide considerable information on the structure of membranes. 

In the spectrum, the SiCH signal is split into three singlets at 0.120, 0.125, and 0.131 ppm according to the triad tacticity, which were assigned to the isotactic, heterotactic, and syndiotactic triad, respectively. The calculated concentration of each triad starting from the optically active monomer with 60.8% ee assuming complete retention of Si stereochemistry in the reduction and in the polymerization via hydrosilylation is S H I = 1.0 2.0 3.3 (0.16 0.32 0.52). The actual concentration was 1.0 2.0 2.3 (0.19 0.37 0.44). 

In indirect procedures for measurement of the CD spectrum, therefore, the CD-induced elliptically polarized beam emerging from the optically active solution is transformed to plane-polarized light by means of an anisotropic crystal with known optical properties. From the angle of the polarization plane it is then possible to calculate the data relating to the given ellipse and necessary for construction of the CD spectrum. Instruments operating on this principle were constructed, e.g., by Jeffard [Je 48] and Arvedson [Ar 66]. Instruments based on these two principles are currently commercially available. 

Methyl torsion Raman optical activity of trans-2,3-dimethyloxirane has been compared with that of rraws-2,3-dimethylthiirane [5]. Torsion of the two methyl groups in these molecules can occur as in-phase and out-of-phase combinations. The former can be observed as a very weak and broad Raman band at -200 cm" in the dimethyloxirane with positive ROA of medium size, whereas it is observed as a shoulder at -220 cm i in the spectrum of the dimethylthiirane with large positive ROA. Contrary to the dimethyloxirane spectra, the dimethylthiirane spectra also contain the out-of-phase torsion it shows up as a medium size Raman band at -245 cm i with zero or small negative ROA. This is in good agreement with ab initio calculations and even with the old inertial model for methyl torsions. 

From the Raman optical activity of trans-2,3-dimethylthiirane the absolute configuration has been determined. Its experimental spectrum is very similar to that calculated ab initio, basis set 6-3IG ) for the 2R,3R-isomer [5] with best agreement in the skeletal vibration region. 

The vibrational spectrum of (S)-3-methylcyclopen-tanone [6] has been calculated on the 6-3IG and on the 6-3IG " level, whereas Raman optical activity has only been predicted on the 6-31G /6-31G and on the 6-3IG level. The calculations with the larger basis set did not show any improvements. 

In the direct comparison approach, an atmospheric model is assumed, and the radiative transfer equation is used to calculate a spectrum, which is then compared with the measurements along the lines discussed in Chapters 4 and 6. The parameters of the model are varied and calculations are repeated until the theoretical spectrum agrees with the measured spectrum to within prescribed limits. This approach requires an independent knowledge of the temperature as a function of pressure in the atmosphere, which may be obtained either from another portion of the spectmm or from auxiliary measurements. A knowledge of the absorption coefficients for the gases that are optically active in the portion of the spectrum being analyzed is also necessary. The coefficients can be obtained from a combination of theory and laboratory measurements, as discussed in Chapter 3. 

Isotope superlattices of nonpolar semiconductors gave an insight on how the coherent optical phonon wavepackets are created [49]. High-order coherent confined optical phonons were observed in 70Ge/74Ge isotope superlattices. Comparison with the calculated spectrum based on a planar force-constant model and a bond polarizability approach indicated that the coherent phonon amplitudes are determined solely by the degree of the atomic displacement, and that only the Raman active odd-number-order modes are observable. 

We turn to empirical force field calculations in order to choose between these two mechanisms. 44 Such calculations indicate that the two-ring flip mechanism is the lowest-energy pathway, and yield a barrier of 20 kcal/mol for the two-ring flip of 7. 44> The experimental free energy of activation for stereoisomerization derived from the temperature-dependent 1H-nmr spectrum, is AG 67 21.9 kcal/mol 43, in excellent agreement with the calculated value. This high barrier admits of the possibility that 7 is separable into its optical antipodes at moderately low temperatures. 

A detailed analysis of the UV-VIS spectrum of (spinach) plasto-cyanin in the Cu(II) state has been reported (56). A Gaussian resolution of bands at 427, 468, 535, 599, 717, 781, and 926 nm is indicated in Fig. 7. Detailed assignments have been made from low-temperature optical absorption and magnetic circular dichroic (MCD) and CD spectra in conjunction with self-consistent field Xa-scattered wave calculations. The intense blue band at 600 nm is due to the S(Cys) pvr transition, which is intense because of the very good overlap between ground- and excited-state wave functions. Other transitions which are observed implicate, for example, the Met (427 nm) and His (468 nm) residues. These bonds are much less intense. The low energy of the d 2 orbital indicates a reasonable interaction between the Cu and S(Met), even at 2.9 A. It is concluded that the S(Cys)—Cu(II) bond makes a dominant contribution to the electronic structure of the active site, which is strongly influenced by the orientation of this residue by the... 

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