Electronic spectra aldehydes

The effect of the electronic properties of the substituted benzaldehydes (la-c) on the allylation reaction is another interesting issue. Whilst the majority of catalysts shown in Figures 7.1 and 7.2 generally exhibit rather minor variation of the ee-value (typically with less than 20% difference between the electron-rich and electron-poor aldehydes), METHOX (22) appears to be a particularly tolerant catalyst, exhibiting practically the same enantioselectivity (93-96% ee Table 7.2, entries 1-3) and reaction rate across the range of substrates [28b, 29]. By contrast, QUINOX (24) stands on the opposite side of the spectrum, showing the most dramatic differences between the electron-poor and electron-rich substrate aldehyde (12-96% ee entries 4-6) [30]. 

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). 

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. 

Aldehydes - The destruction of cytochrome P-A50 by aromatic aldehydes is accompanied by equimolar loss of microsomal heme. > Aliphatic aldehydes also destroy cytochrome P-450 but, unlike the aromatic analogues, only appear to be active in vitro.Enzyme destruction by these monoaldehydes is distinguished from that mediated by phthalaldehyde by a requirement for NADPH. The incubation of octanal with hepatic microsomes from rats pretreated with radiolabeled levulinic acid to tag the heme groups causes the formation of a radiolabeled "green" pigment, but the electronic spectrum of the pigment lacks the features that characterize N-alkylprotoporphyrin IX derivatives. ... 

Compound 37a showed the absence of an aldehydic proton and the singlet around 8.15 ppm was assigned to the ethylenic proton located p with respect to the electron-withdrawing cyano and ester groups. The benzofuranyl coumarins 38 exhibited the carbonyl-stretching band around 1690 cm in the IR spectra (Table 6). PMR data for 13 compounds are given in Table 2. The El mass spectrum of 36a showed a molecular ion peak at m/z 324 (41%). 

Fig. 11.4. Electron ionization mass spectrum of nonanal. Unlike the previous example (toluene, Fig. 11.3), this 9-carbon alkyl aldehyde displays extensive fragmentation and a very low abundance molecular ion at mlz 142. The extensive degree of fragmentation exhibited by many compounds under El conditions makes manual interpretation complex and tedious. Consequently, computerized searches of spectral libraries find extensive use in compound identification.

Upon purification of the XDH from C. purinolyticum, a separate Se-labeled peak appeared eluting from a DEAE sepharose column. This second peak also appeared to contain a flavin based on UV-visible spectrum. This peak did not use xanthine as a substrate for the reduction of artificial electron acceptors (2,6 dichlor-oindophenol, DCIP), and based on this altered specificity this fraction was further studied. Subsequent purification and analysis showed the enzyme complex consisted of four subunits, and contained molybdenum, iron selenium, and FAD. The most unique property of this enzyme lies in its substrate specificity. Purine, hypoxanthine (6-OH purine), and 2-OH purine were all found to serve as reductants in the presence of DCIP, yet xanthine was not a substrate at any concentration tested. The enzyme was named purine hydroxylase to differentiate it from similar enzymes that use xanthine as a substrate. To date, this is the only enzyme in the molybdenum hydroxylase family (including aldehyde oxidoreductases) that does not hydroxylate the 8-position of the purine ring. This unique substrate specificity, coupled with the studies of Andreesen on purine fermentation pathways, suggests that xanthine is the key intermediate that is broken down in a selenium-dependent purine fermentation pathway. ... 

Identification. Identification of the carbonyl PFBOA derivatives was performed by mass spectrometry using electron impact ionization running in the scan mode. It was confirmed that fragment m/z 181 was the main fragment of all analyzed aldehydes (6). Figure 1 shows as an example the mass spectrum of the PFBOA derivative of methi-onal. To increase the selectivity of the method, all aldehyde analyses were run in the... 

There is good evidence from 13C NMR and electronic spectra for an enzyme-bound reduced flavin hydroperoxide as in Eq. 15-31. While this hydroperoxide can decompose slowly to flavin and H202 in the dark, it can also carry out the oxidation of the aldehyde with emission of light.685/685a The luminescent emission spectrum resembles the fluorescence spectrum of the 4a -OH adduct (Eq. 23-49), which is probably the light-emitting species.686-688... 

The resonance Raman spectrum of a similar complex has been reported, that of a ternary complex of LADH, NADH and 4-(7V 7V-dimethylamino)benzaldehyde (DABA) with the disappearance of the carbonyl stretching frequency of the DABA at 1664 cm-1 also indicating strongly that inner sphere complexation of the substrate occurs, the zinc withdrawing electron density from the aldehyde oxygen forming a zinc-oxygen coordinate bond.1397... 

In contrast to N,N-dimethylaminoacrolein, restricted rotation of the thioaldehyde group is observed in the 13C NMR spectrum of N,N-dimethylaminothioacrolein [336]. The C-2 carbon shift of the thioaldehyde (118.5 ppm) also indicates that electron release by the dimethylamino group is attenuated when compared with the aldehyde (101.3 ppm). 

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. 

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