Ethylene chromophore

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. 

Absorption bands due to conjugated chromophores are shifted to longer wavelengths bathochromic or red shift) and intensified relative to an isolated chromophore. The shift can be explained in terms of interaction or delocalization of the 7t and % orbitals of each chromophore to produce new orbitals in which the highest ti orbital and the lowest k orbital are closer in energy. Figure 9.8 shows the conjugation of two ethylene chromophores to form 1,3-butadiene. The n—>n transition in ethylene occurs at 165 nm with an s value of 1500 whereas in 1,3-butadiene the values are 217 nm and 2100 respectively. 

Heat catalyzes free radical formation in cellulose. Aldehydes form from C2 and C3 hydroxyls. Aldehydes oxidize to carboxyls, and with dehydration, carbon monoxide (CO) and carbon dioxide (C02) form as well as conjugated carbonyl-ethylenic chromophoric groups that selectively absorb blue light and impart yellowness (35). During the induction stage of cellulose oxidation, yellowness may increase steadily with selective carbonyl and ethylene group formation. By artificially aging... 

Formation and Elimination of Multiple Bond Functionalities. Reactions that involve the formation and elimination of multiple bond functional groups may significantly effect the color of residual lignin in bleached and unbleached pulps. The ethylenic and carbonyl groups conjugated with phenoHc or quinoid stmctures are possible components of chromophore or leucochromophore systems that contribute to the color of lignin. 

Enediynes with ethylene carbonate fragment in the synthesis of dienediyne models of the biradical-forming and DNA-cleaving natural neocarzinostatin chromophore 99SL657. 

Anti-Stokes picosecond TR spectra were also obtained with pump-probe time delays over the 0 to 10 ps range and selected spectra are shown in Figure 3.33. The anti-Stokes Raman spectrum at Ops indicates that hot, unrelaxed, species are produced. The approximately 1521 cm ethylenic stretch Raman band vibrational frequency also suggests that most of the Ops anti-Stokes TR spectrum is mostly due to the J intermediate. The 1521 cm Raman band s intensity and its bandwidth decrease with a decay time of about 2.5 ps, and this can be attributed the vibrational cooling and conformational relaxation of the chromophore as the J intermediate relaxes to produce the K intermediate.This very fast relaxation of the initially hot J intermediate is believed to be due to strong coupling between the chromophore the protein bath that can enable better energy transfer compared to typical solute-solvent interactions. ... 

Another possible source of modification of the HBI optical properties arises from cis-trans (or, more properly, Z-E) isomerization around its exocyclic ethylene bridge (dihedral angle x as depicted in Fig. 3a) [74, 75]. The absorption spectrum of trans HBI in different solvents is red-shifted by 5-10 nm compared to that of the cis conformation [76]. While the trans conformation is thermodynamically unfavorable and contributes only a minor population at room temperature, cis-trans isomerization seems to take place regardless of the chromophore ionization state, and involves a relatively low energy barrier of about 50 kJ/mol [75], a value that appears significantly lower than initially predicted from quantum mechanics [77, 78]. 

It gives an absorption plateau between 282 mp and 295 mp and an absorption peak at 315 mp. It has been amply demonstrated that absorption above 300 mp is due to the presence of a chromophoric group conjugated with an oxygen substituted benzene ring (51), (52), (69), (75). To ascertain the nature of the chromophoric group (carbonyl or ethylenic... 

Hancock and coworicers have studied the photochemistry of a series of compounds in which the -B=N- functional group is the essential chromophore " The -B=N- linkage in aminoboranes is analogous to the isoelectronic (-C=C-) ethylenic linkage in molecules studied extensively by organic photochemists. Many aminoboranes absorb radiation in the near ultraviolet Fluorescence spectra have been reported for a number of compounds with the general formula... 

For less polar compounds, the solvent effect is weak. However, if the dipole moment of the chromophore increases during the transition, the final state will be more solvated. This is the case for n — n transitions in ethylenic hydrocarbons with a slightly polar double bond. A polar solvent has the effect of stabilising the excited state, which favours the transition. A shift towards greater wavelengths is observed unlike the spectrum obtained in a nonpolar solvent. This is the bathochromic effect. 

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