Coherent manipulation and decoherence of s=10 single-molecule magnets.

We report coherent manipulation of S=10 Fe8 single-molecule magnets. The temperature dependence of the spin decoherence time T2 measured by high-frequency pulsed electron paramagnetic resonance indicates that strong spin decoherence is dominated by Fe8 spin bath fluctuations. By polarizing the spin bath in Fe8 single-molecule magnets at magnetic field B=4.6 T and temperature T=1.3 K, spin decoherence is significantly suppressed and extends the spin decoherence time T2 to as long as 712 ns. A second decoherence source is likely due to fluctuations of the nuclear spin bath. This hints that the spin decoherence time can be further extended via isotopic substitution to smaller nuclear magnetic moments.

A multifrequency high-field pulsed electron paramagnetic resonance/electron-nuclear double resonance spectrometer.

We describe a pulsed electron paramagnetic resonance spectrometer operating at several frequencies in the range of 110-336 GHz. The microwave source at all frequencies consists of a multiplier chain starting from a solid state synthesizer in the 12-15 GHz range. A fast p-i-n-switch at the base frequency creates the pulses. At all frequencies a Fabry-Perot resonator is employed and the pi/2 pulse length ranges from approximately 100 ns at 110 GHz to approximately 600 ns at 334 GHz. Measurements of a single crystal containing dilute Mn(2+) impurities at 12 T illustrate the effects of large electron spin polarizations. The capabilities also allow for pulsed electron-nuclear double resonance (ENDOR) experiments as demonstrated by Mims ENDOR of (39)K nuclei in Cr:K(3)NbO(8).

A high-frequency EPR study of frozen solutions of Gd(III) complexes: straightforward determination of the zero-field splitting parameters and simulation of the NMRD profiles.

Gd(III) (S = 7/2) polyaminocarboxylates, used as contrast agents for Magnetic Resonance Imaging (MRI), were studied in frozen solutions by High-Frequency-High-Field Electron Paramagnetic Resonance (HF-EPR). EPR spectra recorded at 240 GHz and temperatures below 150 K allowed the direct and straightforward determination of parameters governing the strength of zero-field splitting (ZFS). For the first time, a correlation has been established between the sign of the axial ZFS parameter, D, and the nature of the chelating ligand in Gd(III) complexes: positive and negative signs have been observed for acyclic and macrocyclic complexes, respectively. Furthermore, it has been shown that complexes of the less symmetric acyclic DTPA derivatives possess a substantial rhombicity, E, in contrast to the more symmetric macrocyclic DOTA derivatives, where E is negligible. The results obtained are compatible with recent results of liquid-state EPR and allowed to simulate 1H Nuclear Magnetic Relaxation Dispersion (NMRD) profiles with more directly physically meaningful EPR and NMR parameters over the full frequency range from 0.01 to 50 MHz.

High-field/high-frequency EPR of paramagnetic functional centers in Cu2+- and Fe3+-modified polycrystalline Pb[Zr(x)Ti(1-x)]O3 ferroelectrics.

Copper(II)- and iron(III)-modified Pb[Zr(x)Ti(1-x)]O3 ferroelectrics were investigated by means of high-field/high-frequency EPR. The results obtained suggest that Cu2+ and Fe3+ both substitute as acceptor centers for [Zr,Ti]4+. Whereas for the iron-doped system the charge compensating oxygen vacancies (V(O)**) lead to the formation of charged (Fe'(Ti-)V(O)**)* defect associates, no such associates have been observed for the copper-modified system. As regards the morphotropic phase boundary, the model of a mesoscopic mixing of the pure-member phases has been refined to a picture in which a nanoscale composition distribution prevails.

Synthesis, crystal structure, and high-precision high-frequency and -field electron paramagnetic resonance investigation of a manganese(III) complex: [Mn(dbm)2(py)2](ClO4).

The complex [Mn(dbm)(2)(py)(2)](ClO(4)) (dbm = anion of 1,3-diphenyl-1,3-propanedione (dibenzoylmethane), py = pyridine) was synthesized and characterized by X-ray crystallography. It has tetragonally distorted geometry with the axial positions occupied by the py ligands and the equatorial positions by the dbm ligands. This mononuclear complex of high-spin Mn(III) (3d(4), S = 2) was studied by high-frequency and -field electron paramagnetic resonance (HFEPR) both as a solid powder and in frozen dichloromethane solution. Very high quality HFEPR spectra were recorded over a wide range of frequencies. The complete dataset of resonant magnetic fields versus transition energies was analyzed using automated fitting software. This analysis yielded the following spin Hamiltonian parameters (energies in cm(-1)): D = -4.504(2), E = -0.425(1), B(4)(0) = -1.8(4) x 10(-4), B(4)(2) = 7(3) x 10(-4), B(4)(4) = 48(4) x 10(-4), g(x) = 1.993(1), g(y) = 1.994(1), and g(z) = 1.983(1), where the B(4)(n) values represent fourth-order zero-field splitting terms that are generally very difficult to extract, even from single-crystal measurements. The results here demonstrate the applicability of HFEPR at high-precision measurements, even for powder samples. The zero-field splitting parameters determined here for [Mn(dbm)(2)(py)(2)](+) are placed into the context of those determined for other mononuclear complexes of Mn(III).

Electron spin-echo envelope modulation and pulse electron nuclear double resonance studies of Cu2+...beta-carotene interactions in Cu-MCM-41 molecular sieves.

Perdeuterated all-trans beta-carotene imbedded in activated Cu-MCM-41 was examined by electron paramagnetic resonance (EPR) and electron spin-echo envelope modulation (ESEEM) spectroscopies. The EPR study showed that complexation and electron transfer between Cu2+ and deuterated beta-carotene occurs. The interaction was confirmed by detecting the spin-echo modulation of deuterium in the ESEEM spectra of Cu2+. Ratio analysis of ESEEM was used to determine the number of deuterons which interact with Cu2+ and the distance between deuteron(s) and Cu2+. The bonding site of beta- carotene determined by ESEEM and pulse electron nuclear double resonance is the C15=C15' double bond.

High-affinity metal-binding site in beef heart mitochondrial F1ATPase: an EPR spectroscopy study.

The high-affinity metal-binding site of isolated F(1)-ATPase from beef heart mitochondria was studied by high-field (HF) continuous wave electron paramagnetic resonance (CW-EPR) and pulsed EPR spectroscopy, using Mn(II) as a paramagnetic probe. The protein F(1) was fully depleted of endogenous Mg(II) and nucleotides [stripped F(1) or MF1(0,0)] and loaded with stoichiometric Mn(II) and stoichiometric or excess amounts of ADP or adenosine 5'-(beta,gamma-imido)-triphosphate (AMPPNP). Mn(II) and nucleotides were added to MF1(0,0) either subsequently or together as preformed complexes. Metal-ADP inhibition kinetics analysis was performed showing that in all samples Mn(II) enters one catalytic site on a beta subunit. From the HF-EPR spectra, the zero-field splitting (ZFS) parameters of the various samples were obtained, showing that different metal-protein coordination symmetry is induced depending on the metal nucleotide addition order and the protein/metal/nucleotide molar ratios. The electron spin-echo envelope modulation (ESEEM) technique was used to obtain information on the interaction between Mn(II) and the (31)P nuclei of the metal-coordinated nucleotide. In the case of samples containing ADP, the measured (31)P hyperfine couplings clearly indicated coordination changes related to the metal nucleotide addition order and the protein/metal/nucleotide ratios. On the contrary, the samples with AMPPNP showed very similar ESEEM patterns, despite the remarkable differences present among their HF-EPR spectra. This fact has been attributed to changes in the metal-site coordination symmetry because of ligands not involving phosphate groups. The kinetic data showed that the divalent metal always induces in the catalytic site the high-affinity conformation, while EPR experiments in frozen solutions supported the occurrence of different precatalytic states when the metal and ADP are added to the protein sequentially or together as a preformed complex. The different states evolve to the same conformation, the metal(II)-ADP inhibited form, upon induction of the trisite catalytic activity. All our spectroscopic and kinetic data point to the active role of the divalent cation in creating a competent catalytic site upon binding to MF1, in accordance with previous evidence obtained for Escherichia coli and chloroplast F(1).

Mutation of the putative hydrogen-bond donor to P700 of photosystem I.

The primary electron donor of photosystem I (PS1), called P(700), is a heterodimer of chlorophyll (Chl) a and a'. The crystal structure of photosystem I reveals that the chlorophyll a' (P(A)) could be hydrogen-bonded to the protein via a threonine residue, while the chlorophyll a (P(B)) does not have such a hydrogen bond. To investigate the influence of this hydrogen bond on P(700), PsaA-Thr739 was converted to alanine to remove the H-bond to the 13(1)-keto group of the chlorophyll a' in Chlamydomonas reinhardtii. The PsaA-T739A mutant was capable of assembling active PS1. Furthermore the mutant PS1 contained approximately one chlorophyll a' molecule per reaction center, indicating that P(700) was still a Chl a/a' heterodimer in the mutant. However, the mutation induced several band shifts in the visible P(700)(+) - P(700) absorbance difference spectrum. Redox titration of P(700) revealed a 60 mV decrease in the P(700)/P(700)(+) midpoint potential of the mutant, consistent with loss of a H-bond. Fourier transform infrared (FTIR) spectroscopy indicates that the ground state of P(700) is somewhat modified by mutation of ThrA739 to alanine. Comparison of FTIR difference band shifts upon P(700)(+) formation in WT and mutant PS1 suggests that the mutation modifies the charge distribution over the pigments in the P(700)(+) state, with approximately 14-18% of the positive charge on P(B) in WT being relocated onto P(A) in the mutant. (1)H-electron-nuclear double resonance (ENDOR) analysis of the P(700)(+) cation radical was also consistent with a slight redistribution of spin from the P(B) chlorophyll to P(A), as well as some redistribution of spin within the P(B) chlorophyll. High-field electron paramagnetic resonance (EPR) spectroscopy at 330-GHz was used to resolve the g-tensor of P(700)(+), but no significant differences from wild-type were observed, except for a slight decrease of anisotropy. The mutation did, however, provoke changes in the zero-field splitting parameters of the triplet state of P(700) ((3)P(700)), as determined by EPR. Interestingly, the mutation-induced change in asymmetry of P(700) did not cause an observable change in the directionality of electron transfer within PS1.

Pseudooctahedral complexes of vanadium(III): electronic structure investigation by magnetic and electronic spectroscopy.

A variety of physical methods has been used to probe the non-Kramers, S = 1, V(III) ion in two types of pseudooctahedral complexes: V(acac)(3), where acac = anion of 2,4-pentanedione, and VX(3)(thf)(3), where thf = tetrahydrofuran and X = Cl and Br. These methods include tunable frequency and high-field electron paramagnetic resonance (HFEPR) spectroscopy (using frequencies of approximately 95-700 GHz and fields up to 25 T) in conjunction with electronic absorption, magnetic circular dichroism (MCD), and variable-temperature variable-field MCD (VTVH-MCD) spectroscopies. Variable-temperature magnetic susceptibility and field-dependent magnetization measurements were also performed. All measurements were conducted on complexes in the solid state (powder or mull samples). The field versus sub-THz wave quantum energy dependence of observed HFEPR resonances yielded the following spin Hamiltonian parameters for V(acac)(3): D = +7.470(1) cm(-1); E = +1.916(1) cm(-1); g(x) = 1.833(4); g(y) = 1.72(2); g(z) = 2.03(2). For VCl(3)(thf)(3), HFEPR detected a single zero-field transition at 15.8 cm(-1) (474 GHz), which was insufficient to determine the complete set of spin Hamiltonian parameters. For VBr(3)(thf)(3), however, a particularly rich data set was obtained using tunable-frequency HFEPR, and analysis of this data set gave the folowing: D = -16.162(6) cm(-1); E = -3.694(4) cm(-1); g(x) = 1.86(1); g(y) = 1.90(1); g(z) = 1.710(4). Analysis of the VTVH-MCD data gave spin Hamiltonian parameters in good agreement with those determined by HFEPR for both V(acac)(3) and VBr(3)(thf)(3) and in rough agreement with the estimate for VCl(3)(thf)(3) (D approximately 10 cm(-1), |E/D| approximately 0.18), together with the finding that the value of D is negative for both thf complexes. The electronic structures of these V(III) complexes are discussed in terms of their molecular structures and the electronic transitions observed by electronic absorption and MCD spectroscopies.

High-frequency and -field EPR of a pseudo-octahedral complex of high-spin Fe(II): bis(2,2'-bi-2-thiazoline)bis(isothiocyanato)iron(II).

A pseudo-octahedral complex of high-spin Fe(II), bis(2,2'-bi-2-thiazoline)bis(isothiocyanato)iron(II), which has a cis-FeN'2N4 chromophore, has been investigated by high-frequency, high-field electron paramagnetic resonance (HFEPR). Complementary Mössbauer and DC magnetic susceptibility studies were also performed. HFEPR spectra of powder samples were recorded at frequencies up to 700 GHz and over a magnetic field range of 0-25 T. Analysis of the field-frequency data set yields the following set of spin Hamiltonian parameters for S = 2: D = +12.427(12) cm-1, E = +0.243(3) cm-1; gx = 2.147(3), gy = 2.166(3), gz = 2.01(1). The parameters are analyzed by use of a simple crystal-field model. This study represents the first precise determination by HFEPR of spin Hamiltonian parameters in six-coordinate high-spin Fe(II) and indicates the applicability of HFEPR to the study of high-spin Fe(II) in coordination complexes and biological model compounds.

A high-field EPR study of P700+ in wild-type and mutant photosystem I from Chlamydomonas reinhardtii.

High-frequency, high-field EPR at 330 GHz was used to study the photo-oxidized primary donor of photosystem I (P(700)(+)(*)) in wild-type and mutant forms of photosystem I in the green alga Chlamydomonas reinhardtii. The main focus was the substitution of the axial ligand of the chlorophyll a and chlorophyll a' molecules that form the P(700) heterodimer. Specifically, we examined PsaA-H676Q, in which the histidine axial ligand of the A-side chlorophyll a' (P(A)) is replaced with glutamine, and PsaB-H656Q, with a similar replacement of the axial ligand of the B-side chlorophyll a (P(B)), as well as the double mutant (PsaA-H676Q/PsaB-H656Q), in which both axial ligands were replaced. We also examined the PsaA-T739A mutant, which replaces a threonine residue hydrogen-bonded to the 13(1)-keto group of P(A) with an alanine residue. The principal g-tensor components of the P(700)(+)(*) radical determined in these mutants and in wild-type photosystem I were compared with each other, with the monomeric chlorophyll cation radical (Chl(z)(+)(*)) in photosystem II, and with recent theoretical calculations for different model structures of the chlorophyll a(+) cation radical. In mutants with a modified P(B) axial ligand, the g(zz) component of P(700)(+)(*) was shifted down by up to 2 x 10(-4), while mutations near P(A) had no significant effect. We discuss the shift of the g(zz) component in terms of a model with a highly asymmetric distribution of unpaired electron spin in the P(700)(+)(*) radical cation, mostly localized on P(B), and a deviation of the P(B) chlorophyll structure from planarity due to the axial ligand.

High-frequency and -field EPR spectroscopy of tris(2,4-pentanedionato)manganese(III): investigation of solid-state versus solution Jahn-Teller effects.

High-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy of a classical coordination complex, Mn(acac)(3) (Hacac = 2,4-pentanedione), has been performed on both solid powder and frozen solution (in CH(2)Cl(2)/toluene, 3:2 v/v) samples. Parallel mode detection X-band EPR spectra exhibiting resolved (55)Mn hyperfine coupling were additionally obtained for frozen solutions. Magnetic susceptibility and field-dependent magnetization measurements were also made on powder samples. Analysis of the entire EPR data set for the frozen solution allowed extraction of the relevant spin Hamiltonian parameters: D = -4.52(2); |E| = 0.25(2) cm(-1); g(iso) = 1.99(1). The somewhat lower quality solid-state HFEPR data and the magnetic measurements confirmed these parameters. These parameters are compared to those for other complexes of Mn(III) and to previous studies on Mn(acac)(3) using X-ray crystallography, solution electronic absorption spectroscopy, and powder magnetic susceptibility. Crystal structures have been reported for Mn(acac)(3) and show tetragonal distortion, as expected for this Jahn-Teller ion (Mn(3+), 3d(4)). However, in one case, the molecule exhibits axial compression and, in another, axial elongation. The current HFEPR studies clearly show the negative sign of D, which corresponds to an axial (tetragonal) elongation in frozen solution. The correspondence among solution and solid-state HFEPR data, solid-state magnetic measurements, and an HFEPR study by others on a related complex indicates that the form of Mn(acac)(3) studied here exhibits axial elongation in all cases. Such tetragonal elongation has been found for Mn(3+) and Cr(2+) complexes with homoleptic pseudooctahedral geometry as well as for Mn(3+) in square pyramidal geometry. This taken together with the results obtained here for Mn(acac)(3) in frozen solution indicates that axial elongation could be considered the "natural" form of Jahn-Teller distortion for octahedral high-spin 3d(4) ions. The previous electronic absorption data together with current HFEPR and magnetic data allow estimation of ligand-field parameters for Mn(acac)(3).

EPR spectra from "EPR-silent" species: high-frequency and high-field EPR spectroscopy of pseudotetrahedral complexes of nickel(II).

High-frequency and high-field electron paramagnetic resonance (HFEPR) spectroscopy (using frequencies of approximately 90-550 GHz and fields up to approximately 15 T) has been used to probe the non-Kramers, S = 1, Ni(2+) ion in a series of pseudotetrahedral complexes of general formula NiL(2)X(2), where L = PPh(3) (Ph = phenyl) and X = Cl, Br, and I. Analysis based on full-matrix solutions to the spin Hamiltonian for an S = 1 system gave zero-field splitting parameters: D = +13.20(5) cm(-1), /E/ = 1.85(5) cm(-1), g(x) = g(y) = g(z) = 2.20(5) for Ni(PPh(3))(2)Cl(2). These values are in good agreement with those obtained by powder magnetic susceptibility and field-dependent magnetization measurements and with earlier, single-crystal magnetic susceptibility measurements. For Ni(PPh(3))(2)Br(2), HFEPR suggested /D/ = 4.5(5) cm(-1), /E/ = 1.5(5) cm(-1), g(x) = g(y) = 2.2(1), and g(z) = 2.0(1), which are in agreement with concurrent magnetic measurements, but do not agree with previous single-crystal work. The previous studies were performed on a minor crystal form, while the present study was performed on the major form, and apparently the electronic parameters differ greatly between the two. HFEPR of Ni(PPh(3))(2)I(2) was unsuccessful; however, magnetic susceptibility measurements indicated /D/ = 27.9(1) cm(-1), /E/ = 4.7(1), g(x) = 1.95(5), g(y) = 2.00(5), and g(z) = 2.11(5). This magnitude of the zero-field splitting ( approximately 840 GHz) is too large for successful detection of resonances, even for current HFEPR spectrometers. The electronic structure of these complexes is discussed in terms of their molecular structure and previous electronic absorption spectroscopic studies. This analysis, which involved fitting of experimental data to ligand-field parameters, shows that the halo ligands act as strong pi-donors, while the triphenylphosphane ligands are pi-acceptors.

High-frequency and -field electron paramagnetic resonance of high-spin manganese(III) in tetrapyrrole complexes.

High-field and -frequency electron paramagnetic resonance (HFEPR) spectroscopy has been used to study three complexes of high spin Manganese(III), 3d4, S = 2. The complexes studied were tetraphenylporphyrinatomanganese(III) chloride (MnTPPCI), phthalocyanatomanganese(III) chloride (MnPcCl), and (8,12-diethyl-2,3,7,13,17,18-hexamethylcorrolato)manganese(III) (MnCor). We demonstrate the ability to obtain both field-oriented (single-crystal like) spectra and true powder pattern HFEPR spectra of solid samples. The latter are obtained by immobilizing the powder, either in an n-eicosane mull or KBr pellet. We can also obtain frozen solution HFEPR spectra with good signal-to-noise, and yielding the expected true powder pattern. Frozen solution spectra are described for MnTPPCl in 2:3 (v/v) toluene/CH2Cl2 solution and for MnCor in neat pyridine (py) solution. All of the HFEPR spectra have been fully analyzed using spectral simulation software and a complete set of spin Hamiltonian parameters has been determined for each complex in each medium. Both porphyrinic complexes (MnTPPCl and MnPcCl) are rigorously axial systems, with similar axial zero-field splitting (zfs): D approximately -2.3 cm(-1), and g values quite close to 2.00. In contrast, the corrole complex, MnCor, exhibits slightly larger magnitude, rhombic zfs: D approximtely -2.6 cm(-1), absolute value(E) approximately 0.015 cm(-1), also with g values quite close to 2.00. These results are discussed in terms of the molecular structures of these complexes and their electronic structure. We propose that there is a significant mixing of the triplet (S = 1) excited state with the quintet (S= 2) ground state in Mn(III) complexes with porphyrinic ligands, which is even more pronounced for corroles.

Structural investigation of oxidized chlorosomes from green bacteria using multifrequency electron paramagnetic resonance up to 330 GHz.

Chemical oxidation of the chlorosomes from Chloroflexus aurantiacus and Chlorobium tepidum green bacteria produces bacteriochlorophyll radicals, which are characterized by an anomalously narrow EPR signal compared to in vitro monomeric BChl c (.+) [Van Noort PI, Zhu Y, LoBrutto R and Blankenship RE (1997) Biophys J 72: 316-325]. We have performed oxidant concentration and temperature-dependent X-band EPR measurements in order to elucidate the line narrowing mechanism. The linewidth decreases as the oxidant concentration is increased only for Chloroflexus indicating that for this system Heisenberg spin exchange is at least partially responsible for the EPR spectra narrowing. For both species the linewidth is decreasing on increasing the temperature. This indicates that temperature-activated electron transfer is the main narrowing mechanism for BChl radicals in chlorosomes. The extent of the electron transfer process among different BChl molecules has been evaluated and a comparison between the two species representative of the two green bacteria families has been made. In parallel, high frequency EPR experiments have been performed on the oxidized chlorosomes of Chloroflexus and Chlorobium at 110 and 330 GHz in the full temperature range investigated at X-band. The g-tensor components obtained from the simulation of the 330 GHz EPR spectrum from Chlorobium show the same anisotropy as those of monomeric Chl a (.+) [Bratt PJ, Poluektov OG, Thurnauer MC, Krzystek J, Brunel LC, Schrier J, Hsiao YW, Zerner M and Angerhofer A (2000) J Phys Chem B 104: 6973-6977]. The spectrum of Chloroflexus has a nearly axial g-tensor with reduced anisotropy compared to Chlorobium and monomeric Chl a in vitro. g-tensor values and temperature dependence of the linewidth have been discussed in terms of the differences in the local structure of the chlorosomes of the two families.

High-Field EPR Study of Resonance-Delocalized [Fe(2)(OH)(3)(tmtacn)(2)](2+).

High-frequency EPR data are reported for the Fe(II/III) valence delocalized dinuclear complex [Fe(2)(OH)(3)(tmtacn)(2)](2+). A full-matrix diagonalization approach is used to derive the spin-Hamiltonian parameters for this S(T) = (9)/(2) complex. At high fields (up to 14.5 T) and high frequencies (189-433 GHz) fine structure peaks due to resonances between the Kramers doublets (M(s) = (9)/(2), (7)/(2),.) are observed. The spacing of the fine structure reveals that the axial zero-field splitting (ZFS) parameter D is +1.08(1) cm(-)(1); a very small rhombic ZFS (|E|

High-Frequency and -Field Electron Paramagnetic Resonance of High-Spin Manganese(III) in Porphyrinic Complexes.

High-field and -frequency electron paramagnetic resonance (HFEPR) spectroscopy has been used to study two complexes of high-spin manganese(III), d(4), S = 2. The complexes studied were (tetraphenylporphyrinato)manganese(III) chloride and (phthalocyanato)manganese(III) chloride. Our previous HFEPR study (Goldberg, D. P.; Telser, J.; Krzystek, J.; Montalban, A. G.; Brunel, L.-C.; Barrett, A. G. M.; Hoffman, B. M. J. Am. Chem. Soc. 1997, 119, 8722-8723) included results on the porphyrin complex; however, we were unable to obtain true powder pattern HFEPR spectra, as the crystallites oriented in the intense external magnetic field. In this work we are now able to immobilize the powder, either in an n-eicosane mull or KBr pellet and obtain true powder pattern spectra. These spectra have been fully analyzed using spectral simulation software, and a complete set of spin Hamiltonian parameters has been determined for each complex. Both complexes are rigorously axial systems, with relatively low magnitude zero-field splitting: D approximately -2.3 cm(-)(1) and g values quite close to 2.00. Prior to this work, no experimental nor theoretical data exist for the metal-based electronic energy levels in Mn(III) complexes of porphyrinic ligands. This lack of information is in contrast to other transition metal complexes and is likely due to the dominance of ligand-based transitions in the absorption spectra of Mn(III) complexes of this type. We have therefore made use of theoretical values for the electronic energy levels of (phthalocyanato)copper(II), which electronically resembles these Mn(III) complexes. This analogy works surprisingly well in terms of the agreement between the calculated and experimentally determined EPR parameters. These results show a significant mixing of the triplet (S = 1) excited state with the quintet (S = 2) ground state in Mn(III) complexes with porphyrinic ligands. This is in agreement with the experimental observation of lower spin ground states in other metalloporphyrinic complexes, such as those of Fe(II) with S = 1.