Chromophores carbonyl

The epoxyketone 3 is a versatile precursor for a variety of tricycloundecane systems having a p,y-unsaturated carbonyl chromophore, which are not so readily accessible. The contiguous epoxy ketone functionality and the double bond present in the five-membered ring provide opportunities for further manipulation. Adduct 3 may be transformed into a variety of molecular frameworks such as linearly fused cis anti cis tricyclopentanoids, protoilludanes, and marasmanes in a stereoselective fashion after suitable chemical and photochemical manipulation.13... 

Bis(tributyltin) oxide is known to break down to inorganic tin under UV irradiation in laboratory conditions (509, 510), and the decomposition may be accelerated by absorbing the organotin compound on a cel-lulosic matrix (511). As bis(tributyltin) oxide is known to react rapidly with carbon dioxide (atmospheric, or trapped in various cellulosic materials, such as cotton or wood) (512), to form bis(tributyltin) carbonate, (BusSnO)2CO, the observed UV degradation pattern may be rationalized in terms of more-ready breakdown of the carbonate than of the oxide, due to the presence of the carbonyl chromophore. The half-life of bis(tributyltin) oxide in pond water has recently been given as 16 days (513). Diorganotin compounds have also been shown to decompose to inorganic tin under UV irradiation (514, 515). 

A carbonyl chromophore in a macromolecule can participate in a variety of photochemical processes that can have as end result the degradation of the polymer via processes like the Norrish Type I or Type II reaction, the triggering of a chain reaction leading to peroxidation, the transfer of energy to another chromophore or, it can also behave as an energy sink if a suitable, non-degradative path, is available to the triplet state. 

The triplet state of carbonyl chromophores frequently shows a high reactivity in hydrogen abstraction reactions (l ). These processes can take place intermolecularly (photoreduction) ( l) or intramolecularly, for example in the Norrish Type II process, reaction 1 (.2,3.). 

We have also examined the behavior of copolymers of o-tolyl vinyl ketone and methyl vinyl ketone (CoMT). In this case the light is absorbed exclusively at the aromatic carbonyl chromophore and the reaction proceeds from this site, while the methyl vinyl ketone moieties provide a relatively constant environment but prevent energy migration along the chain. The values of Tg and Tip in benzene have been included in Table II. These copolymers axe also soluble in some polar solvents for example, we have used a mixture of acetonitrile acetone methanol (30 30 Uo, referred to as AAM). This mixture is also a good solvent for the electron acceptor paraquat (PQ++) which has been shown to be good biradical trap in a number of other systems (9.). 

Let us now examine a simple numerical model for the mixing of substituent orbitals into the spectroscopic orbitals of the carbonyl chromophore. For this purpose we consider the triplet 3n— Tt excited state of the model amine derivative of methylethylketone shown in Fig. 3.75. 

With many other ketones and aldehydes, reaction between the photoexcited carbonyl chromophore and alkene can result in formation of four-membered cyclic ethers (oxetanes). This reaction is often referred to as the Paterno-Biichi reaction.127 128 129... 

Concerning the interaction of solvents with bichromophoric compounds such as 98, solvent polarity affects mainly the carbonyl chromophore, rather than the anthracene Jt-system, and alters its electronic character. This conclusion can be drawn from the solvent dependence of the n-n absorption of the model compound 10,10-diethylanthrone. Increased solvent polarity, (though not necessarily increased dielectric constant alone) lowers the energy of n-n transition and concomitantly raises the energy of n-n transition (see Figure 24). As a result, the lowest excited state of the carbonyl chromophore in polar solvents will no longer be dominantly n-n in nature,... 

Most of the research on optical rotatory dispersion to date has been with optically active ketones because the carbonyl chromophore conveniently has a weak absorption band in the 300 nm region. Compounds with chromophores that absorb light strongly in the ultraviolet usually are unsatisfactory for rotatory dispersion measurements because insufficient incident light is transmitted to permit measurement of optical rotation. Weak absorption bands below about 210 nm have not been exploited because of experimental difficulties in making the necessary measurements. 

The validity of the octant rule is supported largely and convincingly by ORD and CD spectra from numerous examples where the dissymmetric perturbers (the groups perturbing the carbonyl chromophore in a nonsymmetric way), such as the methyl group of... 

Sotiropoulos and colleagues371 reported recently the crystal structure and CD spectrum of (l/ ,3/ ,4/ )-3-((l/ ,3/ ,4/ )-l,7,7-trimethylbicyclo[2.2.1]-2-oxohept-3-yl)-l,7,7-trimethylbicyclo[2.2.1]heptan-2-one (174, 3,3 -dicamphor). This compound appears to be the first example of exciton coupling between two n -+ Tt transitions from isolated carbonyl chromophores, as incorporated in a,<5-diketone 174. 

Electronically excited carbonyl chromophores in ketones, aldehydes, amides, imides, or electron-deficient aromatic compounds may act as electron acceptors (A) versus alkenes, amines, carboxylates, carboxamides, and thioethers (D, donors). In addition, PET processes can also occur from aromatic rings with electron-donating groups to chloroacetamides. These reactions can be versatile procedures for the synthesis of nitrogen-containing heterocyclic compounds with six-membered (or larger) rings [2],... 

Figure 2. CD spectra obtained within the n->w band system of the carbonyl chromophore upon dissolution of (a) acetone, (b) acetic acid, (c) acetamide, and (d) urea in (2R,3R)-2,3-butanediol (data adapted from reference [7]).

In Schemes 30 to 35, products of the irradiation of several systems containing carbonyl chromophores (oxoamides, adamantyl phenyl ketones, a-mesitylaceto-phenones, and a-benzonorbomyl aceotophenones) are presented [282,301-303], Primary photoreaction in every one of these cases is intramolecular 7- or 8-hydrogen abstraction. In the majority of these cases the final products of interest are cyclobutanols (Scheme 36). Of the forty compounds examined, the de of the cyclobutanols or cyclopentanols in solution is less than 15%. On the other hand,... 

As in achiral ketone 54, the systems with a chiral auxiliary attached to the C2 carbon should have two conformations in which the carbonyl chromophore is tilted toward one or the other diastereotopic hydrogens at C4 and C. The two lowest energy conformations computed [B3LYP/6-31G(d)] for the menthyl ester of 2-benzoyladamantane-2-carboxylic acid are shown in Fig. 11. Clearly the carbonyl is tilted toward either side, and the two conformers do not have the same energy. As indicated in Fig. 11, the energy difference between the two structures is 2.3 kcal/mol. This small difference allows equilibrium between the two con-formers favoring low de in solution. 

A conclusion which can be drawn from these observations is that a nonhalogen substituent will affect the nonradiative decay rate only when the substitution is directly on the carbonyl chromophore. Deuterium substitution has been shown to have a drastic effect on the nonradiative rates of formaldehyde and glyoxal, with the decrease in kjjR due mainly to a large decrease in Franck-Condon factors. Perdeuteration of acetone, on the other hand, has only a slight effect on kup, the value of kjgc... 

Tr. To date, all the photoexcited triplet CIDEP observed satisfy these conditions with k-p having a value of 10 -10 s-- -and D < coz. While almost all of the photoexcited triplet CIDEP in solution involved the carbonyl chromophore, the singular case of solid-state photochemical decomposition of diphenyldiazo-methane at 1.2°K giving a ground-state triplet diphenylmethylene with electron spin polarization was reported by Doetschman et al. in 1976 (44). There is a genuine potential that the photoexcited triplet mechanism can operate in systems containing chromophores other than the carbonyl and in solid-state photochemical systems. 

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