HINDERED INTERNAL ROTATION OF THE CF<sub>3</sub> GROUP IN THE 2-TRIFLUOROMETHYLNITROBENZENE RADICAL ANION IN DMF:H<sub>2</sub>O MIXTURES
The temperature dependences of the EPR spectrum of the 2-trifluoromethylnitrobenzene radical anion in DMF:H<sub>2</sub>O mixtures, caused by the dynamic modulation of the fluorine isotropic hyperfine interaction by the hindered internal rotation of the CF<sub>3</sub> group, have been measured and reconstructed numerically. The activation energy of rotation (<i>E</i><sub>F</sub>) and the dynamic mode depended on the water content in the mixture. For mixtures with a molar fraction of water χ = 0, 0.186, 0.315, 0.409, 0.534, 0.650, 0.810, and 0.910, <i>E</i><sub>F</sub> = 34.70 kJ/mol, 41.31 kJ/mol, 42.30 kJ/mol, 38.41 kJ/mol, 37.01 kJ/mol, 34.51 kJ/mol, 24.10 kJ/mol, and 21.78 kJ/mol, respectively. For χ = 0.186 in the temperature ranges accessible for measurements, the dynamic exchange is slow; for χ = 0.315, 0.409, 0.534, and 0.650, transitions from slow to intermediate and fast exchange take place; for χ = 0.810 and 0.910 in the temperature ranges under study <i>T</i>∈[252, 309]; [254, 297] (K), the exchange is fast. In the range 0.6 < χ < 0.9, <i>E</i><sub>F</sub> decreased drastically, and the activation energy of rotational diffusion (<i>E</i><sub>r</sub>) of the radical anion became maximum, which corresponds to the range of the compositions of DMF:H<sub>2</sub>O with maximum deviations from the ideal state.