Frequency dependent study of the correlation functions in EPR spectroscopy - The Cole-Davidson approach. II. 2,N-(4-n-Butyl benzilidene) 4-amino 2,2,6,6-tetramethyl piperidine 1-oxide in toluene

EPR linewidth measurements of 2,N-(4-n-butyl benzilidene) 4-amino 2,2,6,6-tetramethyl piperidine 1-oxide (BBTMPO) in toluene at 1 GHz (L-Band), 4 GHz (S-Band), 9 GHz (X-Band) and 34 GHz (Q-Band) microwave frequencies indicate the presence of a distribution of relaxation times. The empirical response parameter introduced by Cole-Davidson for the analysis of dielectric relaxation in liquids has been used for the analysis of EPR relaxation data in the L-Band and S-Band frequency regions. The Cole-Davidson parameter can assume values in the range 0 < β ≤ 1. When β = 1, one obtains the Debye-type spectral density. The calculated linewidth data at 1 GHz and 4 GHz agree with a Cole-Davidson parameter of 0.7 for the spherocone shaped BBMTPO solute. β < 1 at the L- and S-bands suggests the presence of an asymmetrical distribution of relaxation times associated with different modes of relaxation mechanisms or internal molecular motions. This study shows EPR experiments at low microwave frequencies are more sensitive to the shape of the correlation function. Differences between this study and an earlier study [6] on perdeuterated 2,2,6,6-tetramethly-4-piperidone N oxide (PD-Tempone) in toluene are attributed to the size of and absence of deuteration in the BBTMPO probe.