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

Abstract

A perchlorotriarylmethyl tricarboxylic acid radical 99% enriched in 13C at the central carbon (13C1-PTMTC) was characterized in phosphate-buffered saline solution (pH\u2009=\u20097.2) (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 C1 creates a trityl that can be used to accurately measure microscopic viscosity. Understanding of the impact of the 13C hyperfine interaction on electron spin relaxation is important for application of this trityl in oximetry and distance measurements. The anisotropic 13C1 hyperfine couplings (Ax\u2009=\u2009Ay\u2009=\u200924\u2009±\u20092\xa0MHz, Az\u2009=\u2009200\u2009±\u20091\xa0MHz) are larger than for the related 13C1-perdeuterated Finland trityl (13C1-dFT) and the g anisotropy (gx\u2009=\u20092.0013, gy\u2009=\u20092.0016, gz\u2009=\u20092.0042) is slightly larger than for 13C1-dFT. The tumbling correlation times (τR) for 13C1-PTMTC are 0.20\u2009±\u20090.02\xa0ns in PBS and 0.40\u2009±\u20090.05\xa0ns in 3:1 PBS:glycerol, which are shorter than for 13C1-dFT in the same solutions. T1 for 13C1-PTMTC is 3.5\u2009±\u20090.5\xa0μs in PBS and 5.3\u2009±\u20090.4\xa0μs in 3:1 PBS:glycerol, which are shorter than for 13C1-dFT due to faster tumbling, larger anisotropy of the 13C1 hyperfine, and about 30% larger contribution from the local mode. In immobilized samples, T1 for 13C1-PTMTC is similar to that for 13C1-dFT and other trityls without chlorine or 13C1 substituents, indicating that the 13C1 and Cl substituents on the phenyl rings have little impact on T1. The temperature dependence of T1 was modeled with contributions from the direct, Raman, and local mode processes. Broadening of CW linewidths of about 0.6\xa0G in fluid solution and about 2\xa0G in rigid lattice is attributed to unresolved 35,37Cl hyperfine couplings.