Radical cations of six-membered ring cyclic ethers. Comparison of calculated hyperfine coupling constants with low-temperature EPR results

Complete text of publication follows. The formation and transformations of radical cations from different cyclic ethers containing six-membered rings (tetrahydropyran, 1,4-dioxane and 1,3,5-trioxane) were studied in the electron-irradiated frozen CF3CCl3 and CF2ClCFCl2 matrices by low-temperature EPR spectroscopy. The irradiation (at 77 K) of dilute solutions of the cyclic ethers containing heteroatoms generates the corresponding radical cations, which are all relatively stable. The presence of heteroatoms in the saturated systems gives a SOMO which involves the lone-pair orbital. In such cases, the radical cations usually have a p-type character. Among the three ethers studied tetrahydropyran is the most interesting case, because the experimental spectra can be explained by two different radical cation structures, namely chair and a planar one, which exist simultaneously in a relatively broad temperature range; the planar structure shows large coupling due to four equivalent protons (hfcc 6.7 mT), typical for cyclic ethers. 1,3,5-trioxane in CF3CCl3 gives after warming from 77 K to 110 K a well resolved spectrum of a cation radical, which can be explained by coupling of the six equivalent protons (vibration averaging) with a large splitting constant of 5.9 mT. Quantum chemical calculations at the DFT level are now routinely applied to assist the identification of reaction products and plausible geometries of the observed species through comparison of experimental and computed hfcc. As the calculated hfcc are sensitive to geometry, the ability of various gradient correction DFT schemes to predict the correct geometry and therefore the correct isotropic hfcc of hydrogen atoms is of great importance. In the present work we compare the various gradient correction DFT schemes available in the Gaussian 98 program package for calculation of geometrical parameters, energies of stable structures and transition states with respect to the experimental results for the six-member ring cyclic ethers