Electronic Spectra, Optical Rotatory Dispersion-Circular Dichroism

4 Electronic Spectra, Optical Rotatory Dispersion-Circular Dichroism 

Absorptions in the UV spectra of thiiranes are observed around 260 nm ( - r e —40) and 205 nm (e—40 00) (75MI50600). A number of other transitions are reported in the vacuum UV spectrum, and the calculated lowest singlet transition energies correspond to n - Ti, n - (T2 and r — r transitions 75BCJ33). Thiirane and oxirane groups behave as electron withdrawing substituents when attached to aromatic rings as indicated by the UV spectra of 2-arylthiiranes. 

The 260 nm band of chiral thiiranes is optically active and a Cotton effect is observed R) (+)-methylthiirane shows a negative Cotton effect at ca. 250 nm followed by a positive effect below 200 nm. An MO analysis indicates that charge transfer contributions are most important in determining the optical activity of the transition (81JCS(F2)503). The 

The UV spectra of thiirane 1-oxide and (15,25)-(+)-2-methylthiirane 1-oxide show a broad maximum at about 205 nm (e —23 000). The latter shows a positive Cotton effect at low energy followed by a negative effect at high energy. The lowest excited states of thiirane 1-oxide involve excitations from the two lone pairs of the oxygen atom (79G19). 2,3-Diphenylthiirene 1-oxide and 1,1-dioxide show absorption due to the 1,2-diphenyl-ethylene chromophore. 

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The circular dichroism (which may be more readily analysed than optical rotatory dispersion) of the prototype resolved coordination compound (+)-[Co(en)3]3+ is shown in Fig. 3. The very similar electronic spectrum of [Co(NHs) 6]3+ (with point group O in the ground state) is known to arise from the transitions 1A g XTig (at lower energy) and... 

The electronic absorption characteristics of chromophores within potential gela-tors can provide an important experimental monitor of the microscopic environment in which they reside. This is especially true when the information includes optical rotatory dispersion (ORD) and circular dichroism (CD) data for potential gelalors that arc chiral. Dichroism relates to the absorptivity difference between the two components of circularly polarized light, w-hich constitutes the incident plane of linearly polarized light as described by the Kronig-Kramers transform. The intensity of UV/vis absorption depends on corresponding quantum transition. The wavelengths at which nonzero circular dichroism may be observable in the CD spectrum can be discerned from the shape of the absorption bands. The... 

The significance of vibrational optical activity becomes apparent when it is compared with conventional electronic optical activity in the form of optical rotatory dispersion (ORD) and circular dichroism (CD) of visible and near-ultraviolet radiation. These conventional techniques have proved most valuable in stereochemistry, but since the electronic transition frequencies of most structural units in a molecule occur in inaccessible regions of the far-ultraviolet, they are restricted to probing chromophores and their immediate intramolecular environments. On the other hand, a vibrational spectrum contains bands from most parts of a molecule, so the measurement of vibrational optical activity should provide much more information. 

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