ИСТИНА

The structure and reactivity of alkyne radical cations present considerable interest from both general and practical points of view. Meanwhile, to our knowledge, direct information on such species is quite limited, probably, because of their relatively high ionization potential and tendency to dimerization. Here we report the first observation of the radical cations of two terminal hexynes (3,3-dimethyl-1-butyne and 1-hexyne) and their photochemical transformations. The radical cations were generated by X-ray irradiation of dilute frozen solutions of the corresponding alkynes in Freon matrices at 77 K and characterized by EPR and UV/vis-spectroscopy. The structure of primary radical cations and products of their transformations was analyzed using quantum-chemical calculations at the DFT-PBE0/L22 level. The radical cation of 3,3-dimethyl-1-butyne (tert-butylacetylene, TBAC) was found to give an anisotropic EPR signal with the following hyperfine coupling parameters: axx = 2.5 mT, ayy = 1.9 mT, azz = 1.0 mT (for ethynyl proton), a(1H)iso = 3.0 mT (for a strongly coupling “selected” methyl proton), a1(2H)iso = 0.8 mT and a2(2H)iso = 0.75 mT (for two pairs of other methyl protons). The best resolution was achieved in a Freon-11 (CFCl3) matrix. The quantum-chemical calculations reveal significant delocalization of spin density and remarkable weakening of one C—CH3 bond in the TBAC radical cation. Photolysis of the primary TBAC cation with visible light (wave-lenght > 600 nm) results in dramatic spectral change, which was attributed to formation of the trimethylallene radical cation. The hyperfine coupling constants for the latter species determined from fitting of the experimental spectrum are a(1H) = 2.5 mT, a(6H) = 1.14 mT and a(3H) = 0.84 mT , in very good agreement with the results of quantum-chemical calculations. 1-hexyne radical cation was also found to be quite sensitive to the red-light photolysis. Possible mechanisms of low-temperature transformations of alkyne radical cations and evolution of the optical absorption spectra are discussed.