ENDOR Studies of the Hindered Internal Rotation of Alkyl Groups in the 1,4-Dialkyl Naphthalene Radical Anions

The ENDOR spectra were measured for the anion radicals of 1,4-dimethylnaphthalene, 1,4-diethylnaphthalene, and 1,4-dipropylnaphthalene. The β-proton splittings of the alkyl groups were 3.30, 2.85, and 2.44 G at −80 °C for 1,4-dimethyl-, 1,4-diethyl-, and 1,4-dipropylnaphthalene respectively, and the positive temperature dependence of the β-proton splitting was demonstrated. Based on the Heller-McConnell relationship, a1,4β=(B0+B2cos2θ)ρ1.4π the β-proton splittings and their temperature dependence were interpreted in terms of a modified model of the restricted internal rotation of ethyl and propyl groups over two kinds of potential barriers. The rotational potential barriers thus estimated were 3.0 and 0.7 kcal/mol for the 1,4-diethylnaphthalene anion and 6.0 and 1.2 kcal/mol for the 1,4-dipropylnaphthalene anion. In the ENDOR spectra of potassium 1,4-diethylnaphthalene observed in THF solutions, two ENDOR lines attributed to the non-equivalent alkyl β-protons were observed at −110 °C, these lines collapsed to a single line when the temperature was raised to −70 °C. This phenomenon can be understood on the assumption that the steric effect of the solvated cation alters the rotational potential curve from a symmetric to an asymmetric one.