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It has been established that the photoinduced process observed in EPR and in optical absorption spectra for 1,4-dithiane radical cations in the CF3CCl3 matrix at 77 K can be ascribed to a “boat”—“twist boat” conformational transition.

It has been established that transformations of azetidine radical cations observed in freonic matrices under the action of light with λ = 436 nm (T = 77 K) are associated with C-N bond cleavage which corresponds to the cyclic form yielding a mixture of open distonic C-centered radical cations of the following structure: ·CH2CH2CH=NH2+

It has been established that reversible photoinduced transformations of 2,3-dimethyloxirane and methyloxirane radical cations (RCs), observed in freonic matrices at 77 K, are related to the conversion between the open and cyclic forms of the RCs. For the trimethyloxirane RC the action of light on the trans-isomer of the open form results in its photoinduced transformation into a C-centered radical with low quantum efficiency (≈4 × 10−3). Upon the X-ray irradiation of 2,2-dimethyloxirane in freonic matrices at 77 K, a cyclic form of the RC is stabilized (presumably, as part of a complex with matrix molecules) which transforms into a distonic C-centered RC under the action of light with the quantum yield of ≈10−3. Tetramethyloxirane RC, stabilized in its open form, is resistant to the action of light. Probable causes of the observed effects are discussed.

It was shown that trimethylene oxide (oxetane) radical cations were converted at 77 K into either distonic radical cations ·CH2CH2CH=OH+ or 2-oxetanyl radicals, depending on the freonic matrix used, by the action of light at λ = 546 nm and trimethylene sulfide radical cations transformed into distonic radical cations CH2CHSH+CH 2 · under 436-nm irradiation. The quantum yields of the photochemical reactions were determined. Quantum-chemical calculations on the structure and HFC constants of the radical cations and possible paramagnetic products of their transformation were performed. The reasons behind the observed difference in reactivity between the radical cations under the action of light are discussed.

Assignment of the bands in the optical absorption spectrum of ≡SiNNO• radicals grafted to activated Aerosil surface was performed. A photochemical transformation mechanism was proposed, and the quantum yields of the reaction of ≡SiNNO• radicals relevant to the long-wavelength absorption band at 77 K were determined in the absence and presence of adsorbed CO2 and SF6 on the surface. For the model radical F3SiNNO•, the parameters characterizing the equilibrium geometry of excited states and the geometry of transition states involved in the reaction were determined by quantum-chemical calculations.

It was found that the principal photochemical reaction of 1,3,5-trioxane radical cations in freonic matrices at 77 K is their cycle-opening dissociation yielding the distonic radical cation in which the unpaired electron is preferentially localized on the oxygen atom. The dissociation of the trioxane radical cations at 77 K is characterized by high quantum yields, which vary from 0.24 to 0.36 in different matrices. The distonic radical cations produced during photolysis are unstable at 77 K and undergo further transformations, which occur at different rates in freonic matrices. The structure of the intermediates produced and a possible mechanism of the processes are discussed with the use of quantum-chemical calculation data.

Charge transfer to matrix molecules was found to be the principal photochemical reaction of the radical cations Me4Sn+· and Me6Sn 2 +· . It was shown that the difference in the localization of positive charge in the ground state of these radical cations (at the Sn-C bond in Me4Sn+· and at the Sn-Sn bond in Me6Sn 2 +· ) did not affect the pathways of their photochemical reactions.

It has been shown that the quantum yield of the photochemical reaction of bromine replacement by the sulfo group in 1-bromo-2-hydroxynaphthalene decreases with an increase in the magnetic flux density. The participation of the triplet radical ion pair in the initiation of the chain photosubstitution reaction was directly confirmed. The lifetime of the radical ion pair was estimated.

The mechanism of photoinduced transformations of nitroxide radicals ≡SiON(O·)CH3 (I) and ≡SiOCH2N(O·)CH3 (II) obtained on the activated aerosil surface was studied. The nitroxides were photolyzed with 436-nm light at 77 K. It was shown that the action of light in the long-wavelength absorption band corresponding to the n–π* transition resulted in the dissociation of the O–N or C–N bond in radicals I or II, respectively. The quantum yields of these reactions were found to be ≈0.6 and ≈0.002, respectively.

Photolysis of the alkylperoxy radicals ≡SiOCH2OO and ≡SiOCH2CH2OO grafted onto an activated surface of silica was found to be accompanied by photochemical radical chain reactions.