Ketones ultraviolet transitions/spectra

The ultraviolet absorption spectrum of cyclohexanone reflects the n jt transition common to all carbonyls see figure IX-E-1. The data derived from gas-phase measurements of the cross sections for cyclohexanone from two different research groups [National Center for Atmospheric Research (NCAR) and Ford Scientific Laboratories (Ford)] are in reasonable agreement (Iwasaki et al., 2008). The cyclohexanone cross sections as measured in cyclohexane solution by Benson and Kistiakowski (1942) had indicated seemingly low values (cross sections shown here is significantly less than those observed for cyclopropanone, cyclobutanone, and cyclopentanone, and in fact, all other carbonyls considered in this work. It is not obvious why these significant differences exist in the probability for the n -> 7T transition for cyclohexanone and that of the other cyclic ketones and most other carbonyl compounds. Theoretical studies will be important in defining the reasons for these differences. 

TMB (42) was first generated by Roth el al. by photochemical decarbonyla-tion of the ketone 44 in a low-temperature matrix. This preparation was intensely colored, with a main transition at 490 nm and several subsidiary absorptions. Earlier ti-CI quantum chemical computations had predicted ultraviolet-visible (UV-vis) is transitions for the singlet and triplet states of TMB, and the bands observed by the Roth group were in better agreement with the predictions for the triplet. The preparation also showed a narrow ESR spectrum interpreted by the authors as that of a triplet species with D = 0.0042 cm and E = 0.0009 cm, which gave a linear Curie plot. The authors assumed that the carriers of the UV-vis and ESR spectra were the same species, namely, triplet TMB. They concluded that TMB is a ground-state triplet, contrary to the disjoint theory and to the computational results described above. 

Similar sorts of results may be found with the nitrate anion. In this case, the nitrate ion itself has a characteristic absorption in the ultraviolet. When paired with a transition-element cation, in alcoholic solution, this absorption is markedly altered (2). It also shows alterations with other cations. In certain ketone and ether solutions, it has been possible to demonstrate further that the vibrational spectrum of the nitrate ion has been altered in such a pattern as to be consistent with a binding of one of the nitrate oxygens to the cation (2), so that major vibration now occurs between this oxygen and the rest of the bound nitrate group. 

Let us now look at an ultraviolet spectrum, shown in Figure 4.1. The spectrum is that of a saturated ketone, butan-2-one, CH3COCH2CH3. The spectrum shows a single absorption peak at Amax 279 nm, max 16.6. This very low value of max shows us that this is a forbidden transition, an n jr transition, characteristic of an aldehyde or ketone group or a nitro group. These peaks all occur within the general range of 275-290 nm. Note that... 

Simple aldehydes and ketones show only weak absorption in the ultraviolet region of the spectrum owing to an n to tt electronic transition of the carbonyl group. If, however, the carbonyl group is conjugated with one or more carbon-carbon double bonds, intense absorption (g = 8,000 - 20,000 M cm ) occurs as a result of a 77 to tt transition as with polyenes, the position of absorption is shifted... 

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