Electronic chromophores

The condensation gives the orange-colored macrocycle 39 in reasonable yield with a fully cyclically conjugated, but nonaromatic, 2071 electron chromophore. The cyclic conjugation is formed directly upon condensation so that no further oxidation is required. 

Among the spectroscopic methods applicable to polysaccharides, u.v. spectrophotometry is of little value for characterizing heparin, whose main, electronic chromophore (the C02 group) displays a band at 220 nm, that is, in a region where all glycosaminoglycans absorb (also through their N-acetyl chromophores), and where minor proportions of unsaturated or aromatic contaminants cause serious interference.77 With pure heparin preparations, the carboxylate chromophore is most useful for chiroptical measurements, and a semi-quantitative evaluation of the extent of N-acetylation of 2-amino-2-deoxy-D-glucose residues is also possible.78... 

Since the ethyl and propyl groups are not the electronic chromophore at 248 nm and 193 nm, the photon absorption corresponds to the excitation... 

The corrole ring system [188], like porphyrin, contains an aromatic 18 ir-electron chromophore as shown by its electronic and... 

In ( )-49 it-electron conjugation between the two free poles is maintained through two tran.v-stilbcnc-likc bridges. As a result of this extended conjugation, the compound is red in solution, with an end absorption extending to 600 nm. This contrasts with the light-yellow color (end adsorption around 450 nm) of the hexakis adducts with a pseudo-octahedral addition pattern, in which the residual tt-electron chromophore is reduced to a benzenoid cubic cyclophane -type substructure [15,54],... 

In order to use its CD to determine the AC of a chiral molecule, a theory is required which predicts the sign of the CD of a given enantiomer. The utilization of CD by organic chemists was greatly stimulated by the development of the Octant Rule, which predicts the CD of the n-tt electronic excitation of carbonyl functional groups [3], Subsequently, so-called Sector Rules were developed for many other electronic chromophores, extending the applicability of CD [4],... 

In the case of (8aS)-(+)-l,8a-dihydroazulene 10,15 the composition of the apparent CD and UV bands was rather simple, because each of the apparent bands was composed of a single electronic transition. The case of chiral troponoid spiro compounds (15aS)-(-)-14 and (18a5)-(-)-15 was also simple because of their Ci symmetrical structures. 16 On the other hand, the Ji-electron chromophores of the twisted naphthalene-diene systems 22-26 are complex and have no symmetric character.18 Therefore, to clarify the applicability of the 7i-electron SCF-CI-DV MO method to such complicated systems, it is important to analyze the composition of the apparent CD and UV bands theoretically obtained. As illustrated in Figure... 

Tetraethynylethenes, Fully Cross-Conjugated n-Electron Chromophores, and Other Perethynylated Molecules... 

Apart from the number of theoretical studies on n-glucopyranose [257, 258], only one vibrational spectroscopic study of a-D-glucopyranose isolated in Ar matrix has been reported [259]. Laser spectroscopy through UV—UV and IR—UV doubleresonance techniques has contributed to the description of the conformations of some p-phenylglucopyranosides and their hydrates [219, 220, 260, 261] but these studies are limited to vibrational resolution and the structural conclusions are not totally transferable to D-glucose because of the electronic chromophore at the anomeric position. 

In principle, no special Raman instrumentation is needed to perform RRS because RR spectra can be obtained with conventional Raman spectrometers, if only the suitable excitation wavelength is applied. However, resonance Raman scattering is experimentally more difficult to implement than normal spontaneous Raman scattering. The excitation wavelength must be made to match the absorption band of the electronic chromophore of interest. The absorption band makes both the excitation intensity and Raman scattered intensity dependent on sample thickness, complicating quantitative analysis. Absorption of the excitation intensity can damage the sample due to heating and/or photochemistry. 

A strong intermolecular interaction among Jt-electron chromophores is observed as Davydov splitting in the absorption spectrum. Kasha proposed a molecular exciton theory for the strong n-n interaction in molecular aggregates, e.g., molecular crystals, and the spectral shift due to the Davydov splitting can be described as a function of chromophore orientation [35,36]. Due to the two-... 

The most obvious advantage of resonance Raman spectroscopy is the enhancement of the observed UV excitation by 6 to 7 orders of magnitude. These high intensity enhancements allow trace analysis at the 20-ppb level. The enhanced vibrational modes are generally totally symmetric and are associated only with the electronic chromophore being excited. This condition results in a considerable simplification of the Raman spectrum and allows a selected chromophore to be used as a probe in the molecule. 

At this stage it may be desirable to compare resonance Raman spectroscopy with FT-Raman spectroscopy. FT-Raman spectroscopy allows vibrations that are not associated with the electronic chromophore to be observed. Additionally, FT-Raman spectroscopy exhibits linear scattering intensities that allow quantitative analysis. On the other hand, resonance Raman spectroscopy is much more sensitive than FT-Raman spectroscopy [28]. 

---