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Abstract. The orientation dependence of the electron spin phase memory relaxation time (T m ) was measured for vanadyI5-(4-carboxyphenyl)-10,15,20-tritolylporphyrin at 22, 50, and 100 K, copper(II) bis(diethyldithiocarbamate) at 50 and 100 K, and copper(II) 5,10, 15,20-tetratolylporphyrin at 50, 70, and 100 K in frozen solution. Tm was determined by fitting a single exponential to two-pulse electron spin echo data. The values of Tm were strongly dependent on the orientation of the molecule in the magnetic field. Longer values were obtained when the magnetic field was along a principal axis or along a non-canonical turning point in the spectrum. Shorter values of T were observed at intermediate orientations. The orientation dependence of T is attributed to molecular motion. The EPR spectra for the three systems examined~e approximately axial, so the relevant motion is motion of the molecular z axis with respect to the external magnetic field. Longer values of T m (slower relaxation) occur for orientations at which the resonant condition is less sensitive to a change in orientation of the molecular z axis. Shorter values of Tm (faster relaxation) occur at orientations for which the resonant condition is more sensitive to a change in orientation of the molecular z axis. INTRODUCTIONThe effects of motion on EPR lineshapes and electron spin phase memory time (T m ) of organic radicals have been well documented both in fluid solution and in partially immobilized samples. I. 2 In fluid solution it has been shown that the larger anisotropy of transition metal EPR spectra, as compared to organic radicals, provides a longer time-scale for the study of incomplete motional averaging.' However the effect of motion on T m for transition metals in frozen solution does not appear to have been examined. Due to the large anisotropy in transition metal EPR spectra, it is expected that T m for transition metals will be more sensitive to slower motion than that for organic radicals. In this paper we report the orientation dependence of T m for a vanadyl complex and two copper(II) complexes.

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Nitroxyl free radical electron spin relaxation times for spin-labeled low-spin methemoglobins were measured between 6 and 120 K by two-pulse electron spin echo spectroscopy and by saturation recovery electron paramagnetic resonance (EPR). Spin-lattice relaxation times for cyano-methemoglobin and imidazole-methemoglobin were measured between 8 and 25 K by saturation recovery and between 4.2 and 20 K by electron spin echo. At low temperature the iron electron spin relaxation rates are slow relative to the iron-nitroxyl electron-electron spin-spin splitting. As temperature is increased, the relaxation rates for the Fe(III) become comparable to and then greater than the spin-spin splitting, which collapses the splitting in the continuous wave EPR spectra and causes an increase and then a decrease in the nitroxyl electron spin echo decay rate. Throughout the temperature range examined, interaction with the Fe(III) increases the spin lattice relaxation rate (1/T1) for the nitroxyl. The measured relaxation times for the Fe(III) were used to analyze the temperature-dependent changes in the spin echo decays and in the saturation recovery (T1) data for the interacting nitroxyl and to determine the interspin distance, r. The values of r for three spin-labeled methemoglobins were between 15 and 15.5 A, with good agreement between values obtained by electron spin echo and saturation recovery. Analysis of the nitroxyl spin echo and saturation recovery data also provides values of the iron relaxation rates at temperatures where the iron relaxation rates are too fast to measure directly by saturation recovery or electron spin echo spectroscopy. These results demonstrate the power of using time-domain EPR measurements to probe the distance between a slowly relaxing spin and a relatively rapidly relaxing metal in a protein.

T, and T m for nitridochromiwn(V) tetratolylporphyrin and nitridochromiwn(V) octaethylporphyrin were measured by saturation recovery and electron spin echo EPR, respectively, between 10 and 130 K.The temperature dependence of liT, was similar to that observed previously for chromiwn(V) complexes of hydroxycarboxylic acids. The spin lattice relaxation rate was faster in the perpendicular plane (the porphyrin plane) than normal to this plane. IIT m was orientation dependent with the fastest rates observed for orientations intermediate between the principal axes. The orientation dependence of IIT m increased with increasing temperature and decreasing rigidity of the matrix, and is attributed to molecular motion.

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Electrospray ionization mass spectrometry was used to study the photodegradation of an oil sands naphthenic acid (NA) mixture, a commercial Fluka NA mixture and a candidate NA, 4-Methyl-cyclohexaneaceticic acid (4-MCHAA) irradiated with TiO(2) (P25) suspension under both fluorescent and natural sunlight. Under natural sunlight irradiation over the TiO(2) suspension, approximately 75% of compounds in the NA mixtures and 100% of 4-MCHAA were degraded in 8 h. No degradation was observed under dark conditions, regardless of the presence or absence of TiO(2). The structural formula of the NAs is given by C(n)H(2n + z)O(2), where n represents the carbon number and z specifies a homologous family with 0-6 rings (z = 0 to -12). The degree of degradation was noted to vary among the NA mixtures and the candidate NA compound with more efficient degradation achieved for molecules with -z values from 0 to 6. The difference in the efficacy of the photocatalysis was likely due to the structure and size of the compounds. In the case of -z = 6 to 12, steric constraints are a key factor what hinders photocatalysis.