Electronic spectra ketones

Another unique compound was found7(g) among the products of the reaction between 3,5,6-tri-O-methyl-D-glucose and acetylmethylene-phosphorane. An inflection at 222 nm in the electronic spectrum, and strong infrared absorption at 5.8 /am, indicated an a,/3-unsaturated, cyclic ketone structure.89 When this compound was allowed to react with methylmagnesium bromide, the product showed7 8 an absence... 

Electronic Spectrum. Acetone is the simplest ketone and thus has been one of the most thoroughly studied molecules. The it n absorption spectrum extends from 350 nm and reaches a maximum near 270 nm (125,175). There is some structure observable below 295 nm, but no vibrational and rotational analysis has been possible. The fluorescence emission spectrum starts at about 380 nm and continues to longer wavelengths (149). The overlap between the absorption and the fluorescence spectra is very poor, and the 0-0 band has been estimated to be at - 330 nm (87 kcal/mol). The absorption spectra, emission spectra, and quantum yields of fluorescence of acetone and its symmetrically methylated derivatives in the gas phase havbe been summarized recently (101). The total fluorescence quantum yield from vibrationally relaxed acetone has been measured to be 2.1 x 10 j (105,106), and the measurements for other ketones and aldehydes are based on this fluorescence standard. The phosphorescence quantum yield is -0.019 at 313 nm (105). 

Electronic Spectrum. The absorption spectrum of 2-pentanone is typical of the it - n band of the alkyl ketones. A structureless band extends from 330 to 220 nm and there is a maximum at 280 nm. 

Aldehydes and ketones are important chromophoric groups, which play a central role in many different areas of chemistry. Formaldehyde is the prototype molecule for these kinds of compounds. Its electronically excited states have therefore been investigated extensively both experimentally and theoretically (see Refs. 63-65 and references cited therein). Acetone is the simplest aliphatic ketone. It is probably the best experimentally studied system of this group of important organic systems. The interpretation of its electronic spectrum has been and remains a subject of experimental interest [66-73]. In contrast to formaldehyde, acetone has been much less studied theoretically, undoubtedly due to the larger size of the molecule. To our knowledge there exist only two previous ab initio studies [74, 75]. Formaldehyde, on the other hand, is frequently used for testing new theoretical methods developed to treat excited states, because of its apparent simplicity and the numerous studies available. 

Reductions of certain aromatic ketones with metal hydrides have been shown to involve radical intermediates formed by an electron-transfer mechanism (25). For example, the reaction of aluminum hydride with dimesityl ketone in THF produced a violet solution that gave an EPR spectrum indicative of the presence of a paramagnetic species. The paramagnetic species is an intermediate in the reduction of the ketone, and is believed to be a radical cation-radical anion pair (25). 

In some cases where bridged dianhydrides were used, four signals were observed in NMP, evidently because of the ability to transfer K-electron density through such bonds as ether, sulfone, or ketone as discussed above. As an example of such behavior, the diamic acid isomeric solution based on l,l -bis(3,4-carboxyphenyl)sulfone dianhydride (SDA) displayed the spectrum illustrated in... 

Conversely, in the case of the n >7T transition of a ketone, the interaction of the lone pair of the ground state carbonyl with a polar solvent lowers the energy of the lone pair n orbital, and thereby increases the energy required to promote an electron to the ti energy level. Consequently, there is often a blue (hypsochromic) shift - to lower wavelength - observed when the UV spectrum of a ketone is recorded in ethanol rather than cyclohexane. 

McLafferty rearrangements are common for aliphatic aldehydes and ketones, providing that an alkyl group of at least three carbons long is attached to the carbonyl group. Odd-electron ions are formed which are useful in the analysis of the spectrum. 

The reaction of 1 -benzyl- 1,4-dihydronicotinamide (BNAH) with a series of 1,1-di-para-substituted-phenyl-2,2-dinitroethylenes in oxygen-saturated acetonitrile produced various amounts of the corresponding ethanes and diaryl ketones depending on the electronic structure of the substituent groups, thereby indicating a spectrum of behaviour intermediate between polar and SET mechanisms (Scheme ll).261... 

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