2603
ESR Study of 1,8-Di-tert-butylnaphthalene Anions
Electron Spin Resonance Study of the Association of 1,8-Di-terl-butyInaphthalene Anions with Sodium and Potassium Cations Ira 6. Goldberg' and Harry R. Crowe Science Center, Rockwell International, Thousand Oaks, California 9 1360 (Received May 24, 1976) Publication costs assisted by Rockwell hternational
Electron spin resonance studies of the association of the nonplanar 1,3,8-tri-tert-butylnaphthalene or 1,3,6,8-tetra-tert-butylnaphthalene anions with alkali metal cations exhibit an alternating line width arising from cation motion between opposite sides of the median plane. The rates of motion were determined from simulation of the spectra, and were found to increase or decrease with temperature depending on the solvent and cation. The range of activation energies derived from the Arrhenius treatment were attributed to differences in the structure of the ion pair. Tight ion pairs exhibit no significant line width variation, but do exhibit anomalous changes in proton hyperfine splittings which suggest thatthe aromatic system can bend o accommodate the alkali metal.
Introduction Electron spin resonance (ESR) has been used extensively during the past decade to study ion pairs formed between aromatic hydrocarbon radical anions and alkali metal cations. In all of these studies, the aromatic systems were planar or averaged to a planar configuration except for alkyl substituents. The effect of large groups which restrict the motion of the alkali metal relative to the aromatic system was not studied. For this reason, we have studied ion association of sodium and potassium ions with 1,3,6,8-tetra-tert-butylnaphthalene (TBIN) and 1,3,8-tri-tert-butylnaphthalene (TB3N) anions. The low symmetry and nonplanarity of the hydrocarbon permit the elucidation of the effect of localization of the cation. This also permits correlation of experiment with theoretical models which invoke a static electric field perturbation on the radical ion to calculate spin distribution. Experimental Section TB3N and TB4N were prepared as described in the literature.' Radical anions were generated in either dry dimethoxyethane (DME), tetrahydrofuran (THF), or mixtures thereof, by reduction with metallic sodium or potassium at -10 "C. The ESR spectrometer and computer interface utilized in these experiments has been described elsewhere.2 The lifetimes of these radical anions were found to decrease as the temperature is increased.3 In order to avoid decreasing line amplitudes during the scan of the spectrum, it was necessary to average many sweeps, each of much shorter duration than the lifetime of the anion. At temperatures between 30 and 50 "C, the signal of the anion decreased significantly during the time (-3 min) required for reasonable temperature equilibration. Scan rates of up to 2000 G/min were used to acquire the spectra. In some cases, the spectra of several different samples were recorded, and the spectra were subsequently added together and/or smoothed. Spectrum simulations and data processing were done either on the dedicated PDP 8/m computer or a CDC 6600 computer. The latter system was utilized for solving the modified block equations for the alternating line width.
Results Effects of Ion Association on the ESR Spectra. In a previous report3 the proton hyperfine splittings (hfs) of the ESR spectra of anions of TB4N and TB3N were used to derive a simple model for the structure in which the 1 and 8 carbon atoms were forced onto opposite sides of the median plane of the naphthalene because of steric interactions between the 1and 8 tert-butyl groups. The planes containing the 1,2, and 9 carbon atoms and the plane containing the 7,8,and 9 carbon atoms each intersected the median plane at an angle of 20-25". In addition, ESR spectra of both anions exhibit line width variations and nonintegral intensity ratios. These effects were tentatively attributed to the result of dynamic processes occurring within ion pairs. Under conditions of loosely bound, solvated ion pairs, the ESR spectra of TB3N.- was interpreted on the basis of five inequivalent protons. The spectrum of TB4N.- under similar conditions was interpreted on the basis of two pairs of equivalent protons. For TBCNe-, a pair which exhibited an hfs of -3.65 G was assigned to the protons at the 4 and 5 positions, while the second pair which exhibited an hfs of less than 10.151 G was assigned to the 2 and 7 positions. Under conditions in which tight ion association between the cation and the TB4N.- might occur, such that the cation is localized either above or below the median plane each of the four a protons may exhibit different hfs. If the cation moved to the opposite side of the plane, the hfs of the protons at the 2 and I positions as well as the 4 and 5 positions would be interchanged. This is schematically illustrated in Figure 1where A, A', B, and B' represent the different hfs. Under conditions in which the cation moved slowly between sites above and below the plane, such that T >> [ylr17~- azHI]-' and 7 >> [ylabH - ~ 4 ~ 1 1 -where l T is the lifetime of the particular conformation, y is the magnetogyric ratio, and a L His the hfs of the proton a t the ith position, a spectrum resulting from four inequivalent protons would be observed. Under the condition in which the cation motion is rapid, where T
