Impact of Chlorine Substitution on Electron Spin Relaxation of a Trityl Radical.

A perchlorotriarylmethyl tricarboxylic acid radical 99% enriched in 13 C at the central carbon ( 13 C 1 -PTMTC) was characterized in phosphate buffered saline solution (pH = 7.2) (PBS) at ambient temperature. Samples immobilized in 1:1 PBS:glycerol or in 9:1 trehalose:sucrose were studied as a function of temperature. Isotope enrichment at C 1 creates a trityl that can be used to accurately measure microscopic viscosity. Understanding of the impact of the 13 C hyperfine interaction on electron spin relaxation is important for application of this trityl in oximetry and distance measurements. The anisotropic 13 C 1 hyperfine couplings (A x = A y = 24 ± 2 MHz, A z = 200 ± 1 MHz) are larger than for the related 13 C 1 -perdeuterated Finland trityl ( 13 C 1 -dFT) and the g anisotropy (g x = 2.0013, g y = 2.0016, g z = 2.0042) is slightly larger than for 13 C 1 -dFT. The tumbling correlation times (τ R ) for 13 C 1 -PTMTC are 0.20 ± 0.02 ns in PBS and 0.40 ± 0.05 ns in 3:1 PBS:glycerol, which are shorter than for 13 C 1 -dFT in the same solutions. T 1 for 13 C 1 -PTMTC is 3.5 ± 0.5 μs in PBS and 5.3 ± 0.4 μs in 3:1 PBS:glycerol, which are shorter than for 13 C 1 -dFT due to faster tumbling, larger anisotropy of the 13 C 1 hyperfine, and about 30% larger contribution from the local mode. In immobilized samples T 1 for 13 C 1 -PTMTC is similar to that for 13 C 1 -dFT and other trityls without chlorine or 13 C 1 substituents, indicating that the 13 C 1 and Cl substituents on the phenyl rings have little impact on T 1 . The temperature dependence of T 1 was modeled with contributions from the direct, Raman, and local mode processes. Broadening of CW linewidths of about 0.6 G in fluid solution and about 2 G in rigid lattice is attributed to unresolved 35,37 Cl hyperfine couplings.