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.