Aldehydes transitions/spectra

The method was first applied by Rothman, Case, and Kearns to the determination of the Ti- -So absorption spectrum of 1-bromonaphthalene. Sixteen photochemically active aromatic ketones cind aldehydes have been investigated by Kearns and Case Transitions from So to two triplet states were located and assigned as n,n ) and... 

RetinalS. The structure and photophysics of rhodopsins are intimately related to the spectroscopic properties of their retiny1-polyene chromophore in its protein-free forms, such as the aldehyde (retinal), the alcohol (retinol or vitamin A), and the corresponding Schiff bases. Since most of the available spectroscopic information refers to retinal isomers (48-55), we shall first center the discussion on the aldehyde derivatives. Three bands, a main one (I) around 380 nm and two weaker transitions at 280 nm and 250 nm (II and III), dominate the spectrum of retinals in the visible and near ultraviolet (Fig. 2). Assignments of these transitions are commonly made in terms of the lowest tt, tt excited states of linear polyenes, the spectroscopic theories of which have been extensively discussed in the past decade (56-60). In terms of the idealized C2h point group of, for example, all-trans butadiene, transitions are expected from the Ta ground state to B , A, and A" excited states... 

Furan-2-carbaldehyde has been much studied. A thorough analysis of the first two major electronic transitions has been carried out. Practical work is hampered by the resinification of the compound and by the presence of a trace impurity which gives rise to a long-lived pressure-independent component in the phosphorescence spectrum.23 The absence of n ->n excited emission and other facts implicate a very efficient double intersystem crossing.14 24 Whether or not sensitized by mercury, photodecomposition of the aldehyde gives much carbon monoxide, propyne, and allene. Small amounts of furan, carbon dioxide, and acetylene are also formed. 

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... 

The high ion yield of benzene and other aromatic molecules found in low pressure gas phase studies suggests the use of these ions in preparative phtochemistry in solution. The absorption spectrum of benzene iy wat r as a solvent shows a weak band around 250 nm (E = 200 1 mole cm ) corresponding to the S S transition (see sec fon II). The photochemistry fo this molecule exhibits a great manifold of processes. Pure benzene can be transformed by irradiation in this spectral region into various ring isomers, open chain valence isomers and other minor products. In aqueous solution benzene may be photochemically oxidized to formyl-cyclopentadiene-1,3. This oxidation is believed to proceed via the formation of benzvalene in the first step which is rapidly oxidized - with and without oxygen, eqn. (1) - to the aldehyde... 

The formation of four-member transition state will be more unfavorable than the six-member transition state. This conclusion is in agreement with the kinetie data, namely, the rate of ester formation is approximately seven times lower than that of alcohol and aldehyde formation. Direct evidence for EOOOH formation was foimd only after prolonged ozonation for 24 h at -78°C [72-75]. The NMR spectrum of file oxidate has a signal at 6 = 13.52 ppm, which is attributed to OOOH. 

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... 

Aldehydes without unsaturation other than the carbon-oxygen double bond have only weak absorption in the portion of the electronic (UV) spectrum readily accessible in the laboratory (i.e., 200 00nm). This band (generally found between about 270 and 300 nm) is the result of an n n transition and has a very low extinction coefficient, e < 30 (Chapter 2). It is of little practical value. Unfortunately,... 

Absorption Spectrum and Cross Sections of HCHO For carbonyl compounds such as aldehydes and ketones, absorption bands due to the electronic transition called n — r transition, in which the isolated pair of lone-pair electrons on the O atom of carbonyl group (—C = O) is excited to the excited Jt orbital of the double bond, appears around 300 nm. Since this transition is a forbidden transition, the absorption cross sections are not very large ( 10 cm molecule ) in general. However, since the absorption bands extend to near 350 nm where solar actinic flux grows, their photolyses are very important in the troposphere. 

The vibronic structure in the spectrum is consistent with but does not prove that the transition is A" <- Aj. Attempts to assign the bands using selection rules appropriate to other electronic transitions either failed or were inconclusive. In the absence of further information, the exact nature of the electronic transition cannot be determined although it can be assigned as A <- X with reasonable certainty. A careful search, out to 800 nm, was made for the corresponding triplet-singlet transition but no further bands were observed. In sharp contrast to the spectra of formaldehyde and thioform-aldehyde, the triplet-singlet system in thioketene is too weak to be observed by... 

The effect of the electronic properties of the substituted benzaldehydes (la-c) on the allylation reaction is another interesting issue. While most catalysts shown in Figures 15.1 and 15.2 generally exhibit a rather minor variation in ee (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 15.2, entries 1-3) and reaction rate across a range of substrates [28f, 28g]. In contrast, QUINOX (23) stands at the opposite side of the spectrum, showing the most dramatic differences between the electron-rich and electron-poor substrate aldehyde (16-96% ee entries 4-6) [30]. Kinetic and computational studies shed some light on the latter behavior it seems that METHOX prefers an ionic transition state with a pentacoordinate silicon (Scheme 15.4), whereas QUINOX favors the neutral, hexacoordinate species. This hypothesis is, inter alia, supported by the choice of solvents, namely. 

The n n transition, typical of aldehydes, is present in 2-methylpropanal see figure IX-B-12. The magnitudes of the cross sections of 2-methylpropanal are similar to those of acetaldehyde and the higher aldehydes, and the unresolved vibrational structure of the excited aldehydes is evident in the spectrum. The recommended cross... 

---