The regioselectivity of photoinduced electron-transfer (PET) reactions of unsymmetrical phthalimides is controlled by the spin density distribution of the intermediate radical anions. ROHF ab initio calculations were found to be most suitable for atomic spin density analysis. Intramolecular PET reactions of quinolinic acid imides were studied with the potassium butyrate and hexanoate 1a,b and a cysteine derivative 3. The photocyclizations products 2a,b and 4 were formed with moderate regioselectivities (68:32, 57:43, and 81:19) showing preferential ortho cyclization. The intermolecular reaction of potassium propionate and potassium isobutyrate with N-methylquinolinic acid imide (5) yielded as addition products the dihydropyrrolo[3,4-b]pyridines 6a,b with slight ortho regioselectivity (55:45). In contrast to these low regioselectivities, the PET reaction of potassium propionate with the methyl ester of N-methyltrimellitic acid imide (9) yielded solely the para addition product 10. Likewise, the intramolecular photoreaction of the cysteine derivative 7 gave a 75:25 (para/meta) mixture of regioisomeric cyclization products 8. The regioselectivity originates from donor-acceptor interactions prior to electron transfer and differences in spin densities in the corresponding imide radical anions. The results of DFT and ab initio calculations for the radical anions of the quinolinic acid imide (11(*)(-)) and the methyl ester of trimellitic acid imide (12(*)(-))( )()were in agreement with the latter assumption: spin densities in 11(*)(-) were higher for the imido ortho carbon atoms (indicating preferential ortho coupling); for 12(*)(-) the spin densities were higher for the imido para carbon atoms (indicating preferential para coupling). These correlations became more significant when the additional spin densities at the carbonyl oxygen and the adjacent carbon atoms were taken into account. The cyclization selectivities for 2, 4, and 8 deviate from the intermolecular examples probably because of ground-state and solvent effects.

No abstract

A detailed spectroscopic study of the photochemically induced electronic-to-electronic energy transfer reported by us earlier (Gudipati, M. S. J. Phys. Chem. A 1997, 101, 2003) in O···CO geminate van der Waals complexes in Ar matrices revealed that these complexes are stabilized in two distinct sites, a singly substitutional (SS) site and a doubly substitutional (DS) site. In the SS site the spectra of O and CO are strongly perturbed, whereas in the DS site spectroscopic features nearly identical to the isolated O and CO have been observed. Theoretical calculations on the ground-state O···CO complex in the SS and DS sites of the Ar lattice are in agreement with the experimental observations. Photolysis of CO2 in Xe/CO2/Ar matrices leads to the formation of Xe···O···CO complexes, and energy transfer from the excited CO to XeO that results in emission from XeO has been observed. Electronic spectra corresponding to the O···CO complexes in Kr matrices have also been observed, but only as weak features superposed on strong charge-transfer transitions in KrO in Kr matrices. We proposed to unify the nonresonant excitation energy transfer processes that involve the formation of an excited species under photochemically induced energy transfer.