Response to “Comment on ‘A 250 GHz ESR study of o-terphenyl dynamic cage effects above TC’ ” [J. Chem. Phys. 109, 10523 (1998)]

Abstract

We address the points raised by Giordano and Leporini (GL) and show that accounting properly for the nonexponential decay of the rotational correlation function leads to improved agreement with the Stokes–Einstein–Debye (SED) relation above the crossover temperature TC for those probes 3,3′-dimethyloxazolidinyl-N-oxy-2′,3-5α-cholestane (CSL), and perdeuterated 2,2′,6,6′-tetramethyl-4-methyl aminopiperidinyl-N-oxide) (MOTA) that are well-coupled to the viscous modes of o-terphenyl (OTP) when the average relaxation rate 16〈τ〉 is plotted versus 1/T. On the other hand, 2,2′,6,6′-tetramethyl-4-piperidine-N-oxide (PDT) shows simple Arrhenius behavior in this regime, because of weak coupling to the solvent cage, inconsistent with SED, which was clearly shown in our paper. We also suggest that the difference in chemical structure of the PDT probe, studied by us, compared to 2,2′,6,6′-tetramethylpiperidine-N-oxyl (TEMPO), studied by GL, accounts for the difference in the low-temperature relaxation behavior of the two probes.

New Technologies in Electron Spin Resonance

“…Improved Orientational Resolution We now address the application of HFHF-ESR to studies of dynamics in fluids. One of the main virtues of FIR-ESR over ESR at conventional microwave frequencies is the excellent orientational resolution it provides for studies utilizing nitroxide spin labels (87,(96)(97)(98). This is clearly shown in Figure 5 (upper), which shows the positions of the resonant ESR absorptions for the canonical orientations of a typical spin probe in a powder simulation at 9.1 and at 250 GHz.…”

Molecular mobility of nitroxide spin probes in glassy polymers. Quasi‐libration model

“…Numerical spectra simulation is used to verify developed models and to obtain quantitative characteristics of dynamic and structural properties of the medium 1, 2, 13, 25–32. Numerical spectra simulation at present time provides the most detailed information on molecular mobility of nitroxides in different media 33–40. In spite of considerable progress in this field, there is some uncertainty regarding which models of molecular mobility should be used for simulation of ESR spectra of nitroxide spin probes in glassy polymer media, and what are quantitative characteristics of corresponding probe movements.…”