Isomer selective excitation

Disrotatory closure of the substituted benzene to produce a Dewar benzene is photo-chemically allowed, as is, of course, the reverse process. However, because benzene is conjugated, it absorbs UV light at longer wavelengths than the Dewar benzene isomer. Therefore, it is possible to selectively excite the benzene chromophore and produce the less stable Dewar isomer. In this particular case the rm-butyl groups favor the reaction because they destabilize the benzene isomer somewhat, owing to steric hindrance. Because the two adjacent im-butyl groups in the Dewar isomer do not lie in the same plane, this steric strain is decreased in the product. Because of this steric effect and the forbidden nature of the conversion back to benzene, the Dewar isomer is relatively stable. However, when it is heated to 200°C, it is rapidly converted to the benzene isomer, probably by a nonconcerted pathway. 

Hoechst 33258 binds to the minor groove of double-stranded DNA with a preference for the A-T sequence (Pjura et al. 1987). Interaction between DNA and proteins very often induces structural modifications in both interacting molecules. Such modifications in DNA can be characterized with 2-aminopurine (2AP), which is a highly fluorescent isomer of adenine. 2AP does not alter the DNA structure. It forms a base pair with thymine and can be selectively excited, since its absorption is red-shifted compared to that of nucleic acids and aromatic amino acids. In addition, its fluorescence is sensitive to the conformational change that occurs within the DNA (Rachofsky et al. 2001). 

Lewis acid (BF3 or EtAlC ) complexes of a,(3-unsaturated esters can shift the photoequilibrium (PSS) toward the thermodynamically less stable Z-isomer even more and may inhibit other competing unimolecular photochemical processes.563 Such enhanced isomerization results are explained by selective excitation of the ground-state Lewis acid ester carbonyl complex, which exhibits a red shift in the long-wavelength k,k absorption band (/lmax) and higher molar absorption coefficients ( 313) (Scheme 6.6). 

Davis DG, Bax A (1985a) Assignment of complex H NMR spectra via two-dimensional homonuclear Hartmann-Hahn spectroscopy. J Am Chem Soc 107 2820-2821 Davis DG, Bax A (1985b) Identification of H NMR spectra by selective excitation of experimental subspectra. J Am Chem Soc 107 7197-7198 De Bruyn A, Zhang W, Budesinsky M (1989) NMR Study of three heteroyohimbine derivatives from Rauwolfia serpentina stereochemical aspects of the two isomers of reserpiline hydrochloride. Magn Reson Chem 27 935-940... 

Isomer selectivity At a finer level of discrimination, vdW complexes or clusters of a given size may occur as a distribution of structural isomers. Isomers of van der Waals and H-bonded complexes have been detected by rotationally resolved UV or IR spectroscopy [29-34], as have isomers of vdW solvent clusters with aromatic molecules [34-38]. The ionization potentials of vdW isomers can differ substantially [17,18,39], allowing mass- and isomer-selective electronic spectroscopy to be performed in a mixture of clusters by selective-ionization (SI) two-color R2PI combined with mass spectrometry in a mixture of clusters [17]. This increase in selectivity is quite general and may also be applied in IR-UV and microwave-UV excitation-ionization schemes. 

In order to derive structural information from infrared frequencies, input is required from quantum chemical calculations at computational levels which match the experimental resolution. Experimentally, gas-phase conditions imply extremely low sample densities, requiring special techniques in order to acquire infrared data. Some of those techniques involve double resonance approaches which provide unique opportunities for isomer selective IR spectroscopy. This facet is among the advantages of gas-phase experiments, making it possible to follow certain properties, such as excited state dynamics, as a function of molecular structure. At the same time, the availability of gas-phase data provides opportunities to calibrate computational methods, force fields, and functionals. 

The triplet excitation energy of benzophenone is approximately 69 k cal/mol and makes the cis- isomer in dominance, from this fact it is concluded the energy required for excitation of trans- isomer is less than that for cis- isomer. Also the sensitiser having triplet energy in the range of 52-58 k cal/mol, selectively excites the trans- isomer. Since the rate of trans to cis conversion increased, the photostationary state is enriched with cis- isomer. 

The isomerization takes place because the excited states, both 5i and T, of many alkenes have a perpendicular instead of a planar geometry (p. 311), so cis-trans isomerism disappears upon excitation. When the excited molecule drops back to the So state, either isomer can be formed. A useful example is the photochemical conversion of c/s-cyclooctene to the much less stable trans isomer." Another interesting example of this isomerization involves azo crown ethers. The crown ether (5), in which the N=N bond is anti, preferentially binds NH4, Li, and Na, but the syn isomer preferentially binds and Rb (see p. 105). Thus, ions can be selectively put in or taken out of solution merely by turning a light source on or off." ... 

In summary, all the experiments expressly selected to check the theoretical description provided fairly clear evidence in favour of both the basic electronic model proposed for the BMPC photoisomerization (involving a TICT-like state) and the essential characteristics of the intramolecular S and S, potential surfaces as derived from CS INDO Cl calculations. Now, combining the results of the present investigation with those of previous studies [24,25] we are in a position to fix the following points about the mechanism and dynamics of BMPC excited-state relaxation l)photoexcitation (So-Si)of the stable (trans) form results in the formation of the 3-4 cis planar isomer, as well as recovery of the trans one, through a perpendicular CT-like S] minimum of intramolecular origin, 2) a small intramolecular barrier (1.-1.2 kcal mol ) is interposed between the secondary trans and the absolute perp minima, 3) the thermal back 3-4 cis trans isomerization requires travelling over a substantial intramolecular barrier (=18 kcal moM) at the perp conformation, 4) solvent polarity effects come into play primarily around the perp conformation, due to localization of the... 

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