Intramolecular Magnetic Interactions Research Articles

Syntheses, structures, magnetic and spectroscopic properties of bis(pentane-2,4-dionato)chromium(iii) complexes incorporating 3- or 4-pyridyl-substituted nitronyl and imino nitroxides

Preparation and characterization by X-ray analysis and 2H NMR spectra have been described for twelve chromium(III)–nitroxide complexes of cis-[Cr(acac)2Cl(L)] and cis- and trans-[Cr(acac)2(L)2]PF6 with monodentate coordination through the pyridyl-N atom in the nitroxides, where acac = pentane-2,4-dionate and L = 4,4,5,5-tetramethyl-2-(3- or 4-pyridyl)imidazolin-1-oxyl 3-oxide (NIT3py or NIT4py) and 4,4,5,5-tetramethyl-2-(3- or 4-pyridyl)imidazolin-1-oxyl (IM3py or IM4py). An intramolecular magnetic interaction between CrIII and the nitroxide was found to be weakly antiferrromagnetic and ferromagnetic, respectively, in the NIT3py or IM3py complexes and the NIT4py or IM4py complexes. The difference in sign of these magnetic interaction constants between the 3- and 4-pyridyl-substituted nitroxides can be interpreted by a spin polarization mechanism for the π-electron system including the nitroxide SOMO π* and CrIII dπ orbitals. The inherently weak magnetic interactions and absence of an intensity enhancement of the spin-forbidden d–d transition band in these nitroxides complexes are in contrast to our previous results for the chelated NIT2py [4,4,5,5-tetramethyl-2-(2-pyridyl)imidazolin-1-oxyl 3-oxide] and IM2py [4,4,5,5-tetramethyl-2-(2-pyridyl)imidazolin-1-oxyl] complexes. This may be accounted for by the difference in the mechanism of magnetic interactions; spin-polarization mechanism and exchange mechanism, respectively.

Reversed Photoswitching of Intramolecular Magnetic Interaction Using A Photochromic Bis(2-thienyl)ethene Spin Coupler

Bis(3-thienyl)- and bis(2-thienyl)ethenes having nitronyl nitroxide radicals at both ends of the molecules were synthesized. Bis(3-thienyl)ethene diradicals underwent a reversible photochromic reaction by irradiation with UV and visible light. Photoswitching of intramolecular magnetic interaction was proved by EPR spectroscopy. By choosing an appropriate spacer, the intramolecular exchange interaction in the closed-ring isomer was detected to be more than 150 times larger than that in the open-ring isomer. In the case of bis(2-thienyl)ethene diradicals, the photocyclization reaction was prohibited, and only the cycloreversion reaction took place by irradiation with visible light. The closed-ring isomers of the diradicals were synthesized from the closed-ring isomers of the precursors. From the analysis of the EPR spectra of the system with an appropriate spacer, it was found that on/off switching behavior was reversed by changing the substitution position of the thiophene rings to the ethene moiety from t...

Photoswitching of intramolecular magnetic interaction using diarylethene with oligothiophene pi-conjugated chain.

Diarylethenes having two nitronyl nitroxide radicals at both ends of the molecule with oligothiophene spacers were synthesized. The diradicals underwent photochromic reactions upon alternate irradiation with UV and visible light. ESR spectra of the diradicals reversibly changed with the photochromism. The magnetic interaction between spins through oligothiophene spacers was stronger than that through oligophenylene spacers, and the photochromic reactivity of the diradical diarylethenes with oligothiophene spacers was much reduced. The difference of the exchange interaction between open- and closed-ring form isomers was estimated to be more than 150-fold.

Cover Picture

The cover picture shows the photoswitching of an intramolecular magnetic interaction by means of a photochromic diarylethene spin coupler. The magnetic interaction between two nitronyl nitroxide radicals placed at both ends of the diarylethene was switched by alternate irradiation with UV and visible light (top). The open-ring isomer (middle left) has disconnected π-conjugated systems, while the closed-ring isomer (middle right) has a delocalized π-system throughout the molecule. The change in exchange interaction between the open- and closed-ring isomers was determined to be more than 30-fold by ESR spectroscopy (bottom). The study of magnetic interaction and photochromic reactivity is described in more detail by K. Matsuda and M. Irie on p. 3466 ff.

Photochromism of diarylethenes with two nitronyl nitroxides: photoswitching of an intramolecular magnetic interaction.

Photochromic diarylethenes that have p-phenylene-substituted benzothiophene aryl groups with and without nitronyl nitroxide radicals at both ends of the molecules were synthesized. The absorption maxima of the closed-ring isomers showed a hypsochromic shift with the increase in the pi-conjugated chain length. The unique behavior was attributed to the stabilization by the resonant quinoid structures. Both photocyclization and photocycloreversion quantum yields of the diarylethene with nitronyl nitroxide radicals were found to increase with the increase in the pi-conjugated chain length. Photoswitching of the magnetic interaction between two nitronyl nitroxide radicals was studied by means of ESR spectroscopy. The change in exchange interaction between open- and closed-ring isomers of 1,2-bis[6-[4-[4-(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin-2-yl)phenyl]phenyl]-2-methyl-1-benzothiophen-3-yl]hexafluorocyclopentene was determined to be more than 30-fold.

Photoswitching of Intramolecular Magnetic Interaction Using Photochromic Diarylethene Spin Coupler: Introduction of Thiophene Spacer

Abstract Photochromic 1,2-bis(2-methyl-1-benzothiophen-3-yl)perfluorocyclopentene having two nitronyl nitroxide radicals with a thiophene-2,5-diyl spacer was prepared and the intramolecular magnetic interaction was compared with the derivative having the radicals with a p-phenylene spacer. The stronger magnetic interaction was observed for the compound with the thiophene spacer.

Photoswitching of magnetic interaction: diarylethene photochromic spin couplers

We have synthesized 1,2-bis[6-(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin-2-yl)-2-methylbenzo[ b]thiophen-3-yl]hexafluorocyclopentene ( 2a) which shows photoswitching of intramolecular magnetic interaction between two nitronyl nitroxide radicals. Compound 2 showed ideal photochromic performance. The conversions from open-ring form isomer 2a to closed-ring form isomer 2b, and from 2b to 2a were both almost 100%. From the magnetic measurement it was found that while the two spins of the open-ring form isomer 2a weakly interact with each other (2 J/ k B=−2.2 K) due to the disconnection of the π-systems, spins of 2b have remarkable antiferromagnetic interaction (2 J/ k B=−11.6 K). The EPR spectra were measured for both 2a and 2b in benzene solution at room temperature. In both cases, the ESR spectra showed nine lines indicating that the exchange interaction between the two radicals was much stronger than the hyperfine coupling constant. We have also synthesized 3a, in which two nitronyl nitroxides were located at both ends of 1,2-bis(6-phenyl-2-methyl-1-benzothiophen-3-yl)hexafluorocyclopentene. Two p-phenylene groups are introduced to regulate the strength of the exchange interaction between spins at nitronyl nitroxide and to detect the change of the interaction by ESR spectroscopy. Compound 3 underwent reversible photochromic reactions in ethyl acetate by irradiation with 313 and 578 nm light. The photochromic reaction proceeded both ways almost completely. While the ESR spectrum of 3a showed complex 15 lines, the spectrum of 3b showed nine lines. The double integrals of the spectra at the initial and photostationary states were almost identical, indicating that there is no gain or loss of the amount of the spins during the photochromic reaction.

Syntheses, structures, magnetic, and spectroscopic properties of cobalt(ii), nickel(ii) and zinc(ii) complexes containing 2-(6-methyl)pyridyl-substituted nitronyl and imino nitroxide†

Chelate complexation of 4,4,5,5-tetramethyl-2-(6-methyl-2-pyridyl)imidazolin-1-oxyl 3-oxide (NITmepy) and 4,4,5,5-tetramethyl-2-(6-methyl-2-pyridyl)imidazolin-1-oxyl (IMmepy) to cobalt(II) chloride (bromide) or zinc(II) chloride afforded tetrahedral four-co-ordinate (T-4) complexes, [MX2(NITmepy)] and [MX2(IMmepy)] (MX = CoCl (1 and 2), CoBr (1Br and 2Br) or ZnCl (3 and 4), respectively) containing either a six- or a five-membered chelate ring as determined by single-crystal X-ray analysis. The formation of such T-4 complexes arises from the steric requirements of the methyl group at the o-position of the pyridyl-N donor. The UV-vis-NIR spectra suggest that both complexes 1 and 2 in dichloromethane retain their solid state structure. In acetonitrile, however, 1 exists as an equilibrium mixture due to dissociation of the co-ordinated NITmepy, while 2 is stable. The ligand-field spin-allowed and spin-forbidden d–d transitions of 1 and 2 are not influenced by the coordinated radical ligands. The reaction of nickel(II) chloride and thiocyanate with IMmepy gave a dichloro-bridged dinuclear complex with a square pyramidal five-co-ordinate (SPR-5) geometry and an OC-6 complex, respectively, [{NiCl(IMmepy)}2(μ-Cl)2] (5) and [Ni(NCS)2(IMmepy)2] (6). The intramolecular magnetic interactions in 1 and 2 are estimated to be antiferromagnetic, whereas 5 and 6 give ferromagnetic interactions. Complex 3, with one-dimensional columns, forms a dimer so that a moderate intermolecular antiferromagnetic coupling is demonstrated, while complex 4 exists as a discrete molecule in the crystal and shows paramagnetic behaviour.

A Diarylethene with Two Nitronyl Nitroxides: Photoswitching of Intramolecular Magnetic Interaction

A diarylethene having two nitronyl nitroxides, 1,2-bis[6-(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin-2-yl)-2-methyl-1-benzothiophen-3-yl]hexafluorocyclopentene (2a), was synthesized in an attempt to control the intramolecular magnetic interaction by photoirradiation. The photochemical conversions from open-ring isomer 2a to closed-ring isomer 2b and from 2b to 2a were both almost 100%. Magnetic measurement revealed the antiferromagnetic interaction between two nitronyl nitroxides remarkably increased from 2J/kB = −2.2 K to 2J/kB = −11.6 K when the diarylethene spin coupler was switched from the open-ring isomer 2a to the closed-ring isomer 2b. ESR measurements were also carried out for both 2a and 2b in benzene solutions at room temperature and in MTHF solid solutions at cryogenic temperature. Both ESR and magnetic measurement indicated that the intramolecular interaction was switched by the photochromic spin coupler.

Photoswitching of Magnetic Properties by using Diarylethene Photochromic Spin Coupler

We have designed and synthesized 1,2-bis[6-(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin-2-yl)-2-methylbenzo[b]-thiophen-3-yl]hexafluorocyclopentene which shows photo-switching of intramolecular magnetic interaction by incorporating two nitronyl nitroxide into diarylethene photochromic spin coupler.

Photoswitching of Intramolecular Magnetic Interaction: A Diarylethene Photochromic Spin Coupler

Abstract We have designed and synthesized 1,2-bis[6-(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin-2-yl)-2-methylbenzo[b]thiophen-3-yl]hexafluorocyclopentene which shows photoswitching of intramolecular magnetic interaction by incorporating two nitronyl nitroxide into diarylethene photochromic spin coupler.

Theoretical Studies of Intra- and Inter- Magnetic Interactions in TMAO(1,3,5,7 - Tetramethyl - 2, 6- Diazaadamantane N, N′ - Dioxyl)

Ferromagnetic properties of TMAO organic crystal, especially α-form crystal, are studied theoretically. There are two nitroxide groups, that is, both intramolecular magnetic interaction in one molecule and inter-molecular interaction between two molecules are expected in contrast to many organic ferromagnetic crystalline species that have been reported by other groups. Obtained Jab values for intra- and inter- molecular interactions are positive and small positive for only a few pairs, respectively. Cooperation of two types of magnetic coupling is essential to realize bulk ferromagnetic behavior.

Oligopyridine bis(nitronyl nitroxides): synthesis, structures, electrochemical, magnetic and electronic properties

A family of pyridine, 2,2′-bipyridine, 1,10-phenanthroline, 3,6-bis(2-pyridyl)pyridazine ligands bearing two nitronyl nitroxide radicals was synthesized and characterized. The pincer ligands having two radicals in ortho positions relative to the nitrogen atoms form stable nickel(II) and copper(II) complexes. The pyridine based 2,6- and 2,5-biradicals crystallized in the monoclinic space group P2 1 /c while the bipyridine 5,5′-biradical crystallized in the monoclinic space group P2 1 /n (radical is 4,4,5,5-tetramethylimidazoline 3-oxide 1-oxyl). The highest dihedral angle between the radicals and the pyridine ring is found in the pyridine 2,6-biradical (ca. 47°), followed by the 2,5-biradical (ca. 38°) and then by the bipyridine 6,6′- (ca. 27°) and 5,5′-biradicals (ca. 24°). All the biradicals exhibit similar EPR spectra, in solution and at room temperature, with nine lines due to coupling of the unpaired electrons with four equivalent nitrogen nuclei (hyperfine coupling constant A N 3.3-3.8 G). In some cases these nine lines are reduced to five lines (A N 7.5-7.6 G) at low temperature due to a weakening of the spin-spin exchange interaction J which becomes smaller than the isotropic coupling constant. The χT product for all radicals measured at 300 K lies in the range of 0.70-0.77 emu K mol –1 . For the pyridine 2,6- and 2,5-biradicals, magneto-structural correlation reveals through bond ferro- and antiferro-magnetic interaction (J/k B =+9.4 and –39 K, respectively) due to a favorable spin polarization effect. These intramolecular interactions persist when the radicals are dispersed in a polystyrene matrix. For the bipyridine 6,6′- and 5,5′-biradicals, the experimental data were fitted using a Bleaney-Bowers expression for two interacting spins and the values of J/k B =–24 to –1.9 K, respectively, are representative of an antiferromagnetic through-space interaction between two neighboring radicals. No intramolecular magnetic interactions were found in the oligopyridine based compounds. In all of these biradicals, both nitroxides are oxidized at the same potential (ca. +0.45 V vs. Ag/Ag + ) by a two-electron quasi-reversible process to the nitrosonium dication and irreversibly reduced to the dianions between –1.80 and –1.40 V. In the Cu(II) and Ni(II) complexes of the bipyridine-6,6′- and phenanthroline-2,9-biradical reduction of the nitroxides to the hydroxylamine anions is quasi-reversible and is facilitated by ca. 1.20 V versus the

Magnetic properties of new copper(II) and [copper(II)]3-manganese(II) complexes of [14]N4 macrocyclic oxamides

Four new complexes of [14]N4 macrocyclic oxamides, namely [CuL1], [CuL2], [(CuL1)3Mn](ClO4)3 and [(CuL2)3 Mn](ClO4)3 [L1 and L2 are the dianions of 2,3-dioxo-5,6: 3,14-dibenzo-7,12-bis(ethoxycarbonyl)-1,4,8,11-tetraazacyclotetradeca-7,11-diene (H2L1) and 2,3-dioxo-5,6:13,14-dibenzo-9-methyl-7,12-bis(ethoxycarbonyl)-1,4,8,11-tetraazacyclo-tetradeca-7,11-diene (H2L2), respectively], have been prepared and characterized. The observed magnetic susceptibilities of the [(CuL1)3Mn] (ClO4)3 complex over the ca. 2–300 K range have been fitted to theoretical equations based on the Heisenberg spin Hamiltonian Ĥ=−2JŜMn(ŜCu1+ ŜCu2+ŜCu3) and molecular field approximation. The results indicated that both the intramolecular and intermolecular magnetic interactions are antiferromagnetic with JCuMn= −19.8cm−1 and zJ′=−1.41cm−1.

Synthesis, characterization and some properties of S-bridged polynuclear complexes composed of fac(S)-[Cr(aet) 3] units (aet=2-aminoethanethiolate (aet). Crystal structures of [Ni{Cr(aet) 3} 2] 2+ and [{Cr(aet) 3} 4Zn 4O] 6+

The reactions of newly prepared fac(S)-[Cr(aet) 3] ( 1) with Zn 2+ and Ni 2+ gave the polynuclear complexes [{Cr(aet) 3} 4Zn 4O] 6+ ( 2) and [Ni{Cr(aet) 3} 2] 2+ ( 3), respectively, where aet denotes 2-aminoethanethiolate (NH 2CH 2CH 2S –). The structures of complex cations 2 and 3 are determined by the X-ray diffractions. 2Br 6·6H 2O is spontaneously resolved as single crystals and crystallizes in the cubic space group P2 13, a=19.024(1) Å, Z=4, and R=0.044. 3(NO 3) 2·2H 2O crystallizes in the triclinic space group P1̄ with a=10.228(2), b=12.265(3), c=13.362(3) Å, α=101.08(1), β=108.02(1), γ=102.35(1)°, Z=2, and R=0.031. 2 consists of four fac(S)-[Cr(aet) 3] units, four Zn atoms, and one central μ 4-oxygen atom, forming the T-cage-type octanuclear structure. 3 consists of two fac(S)-[Cr(aet) 3] units and one Ni atom, forming linear-type trinuclear structure. The electronic absorption and CD spectral behavior of the isolated complexes are discussed in comparison with those of the corresponding polynuclear complexes. The effective magnetic moments are 3.99 μ B for 1 at 296 K, and 7.58 μ B for 2, and 5.19 μ B for 3 at 300 K. 3 indicates the presence of intramolecular antiferromagnetic exchange interactions, but 2 does not show the intramolecular magnetic exchange interactions.

Spin density in the tetrahedral copper complex of an enaninoketon nitroxide

The copper complex C 18H 28O 2N 2Cu(NO) 2 carries three spins S = 1 2 . It crystallizes with two molecules in the asymmetric unit. Its susceptibility reveals positive intramolecular magnetic interactions, but negative intermolecular interactions. We have investigated its spin density with polarized neutrons at T = 40 K and T = 4.10 K. At the higher temperature, the spin density is well localized on the Cu atom and the two NO groups of each molecule. At the lower temperature, the spin density exists, for each molecule, on the Cu atom and on one of the two NO groups only. There is a practical dimerization on two NO groups, belonging to the different molecules, facing each other, and distant only by 3.40 Å.

Copper and Bismuth Complexes Containing Dipyridyl gem-Diolato Ligands: Bi(III)(2)[(2-Py)(2)CO(OH)](2)(O(2)CCF(3))(4)(THF)(2), Cu(II)[(2-Py)(2)CO(OH)](2)(HO(2)CCH(3))(2), and Cu(II)(4)[(2-Py)(2)CO(OH)](2)(O(2)CCH(3))(6)(H(2)O)(2), a Ferromagnetically Coupled Tetranuclear Copper(II) Chain.

The reactions of the singly deprotonated di-2-pyridylmethanediol ligand (dpmdH(-)) with copper(II) and bismuth(III) have been investigated. A new dinuclear bismuth(III) complex Bi(2)(dpmdH)(2)(O(2)CCF(3))(4)(THF)(2), 1, has been obtained by the reaction of BiPh(3) with di-2-pyridyl ketone in the presence of HO(2)CCF(3) in tetrahydrofuran (THF). The reaction of Cu(OCH(3))(2) with di-2-pyridyl ketone, H(2)O, and acetic acid in a 1:2:2:2 ratio yielded a mononuclear complex Cu[(2-Py)(2)CO(OH)](2)(HO(2)CCH(3))(2), 2, while the reaction of Cu(OAC)(2)(H(2)O) with di-2-pyridyl ketone and acetic acid in a 2:1:1 ratio yielded a tetranuclear complex Cu(4)[(2-Py)(2)CO(OH)](2)(O(2)CCH(3))(6)(H(2)O)(2), 3. The structures of these complexes were determined by single-crystal X-ray diffraction analyses. Three different bonding modes of the dpmdH(-) ligand were observed in compounds 1-3. In 2, the dpmdH(-) ligand functions as a tridentate chelate to the copper center and forms a hydrogen bond between the OH group and the noncoordinating HO(2)CCH(3) molecule. In 1 and 3, the dpmdH(-) ligand functions as a bridging ligand to two metal centers through the oxygen atom. The two pyridyl groups of the dpmdH(-) ligand are bound to one bismuth(III) center in 1, while in 3 they are bound two copper(II) centers, respectively. Compound 3 has an unusual one dimensional hydrogen bonded extended structure. The intramolecular magnetic interaction in 3 has been found to be dominated by ferromagnetism. Crystal data: 1, C(38)H(34)N(4)O(14)F(12)Bi(2), triclinic P&onemacr;, a = 11.764(3) Å, b = 11.949(3) Å, c = 9.737(1) Å, alpha =101.36(2) degrees, beta = 105.64(2) degrees, gamma = 63.79(2) degrees, Z = 1; 2, C(26)H(26)N(4)O(8)Cu/CH(2)Cl(2), monoclinic C2/c, a = 25.51(3) Å, b = 7.861(7) Å, c = 16.24(2) Å, beta = 113.08(9) degrees, Z = 4; 3, C(34)H(40)N(4)O(18)Cu(4)/CH(2)Cl(2), triclinic P&onemacr;, a = 10.494(2) Å, b = 13.885(2) Å, c = 7.900(4) Å, alpha =106.52(2) degrees, beta = 90.85(3) degrees, gamma = 94.12(1) degrees, Z = 1.

Control of Intramolecular Magnetic Interaction by the Spin Polarization of d.pi. Spin to p.pi. Orbital of an Organic Bridging Ligand

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTControl of Intramolecular Magnetic Interaction by the Spin Polarization of d.pi. Spin to p.pi. Orbital of an Organic Bridging LigandHiroki Oshio and Hikaru IchidaCite this: J. Phys. Chem. 1995, 99, 10, 3294–3302Publication Date (Print):March 1, 1995Publication History Published online1 May 2002Published inissue 1 March 1995https://doi.org/10.1021/j100010a046RIGHTS & PERMISSIONSArticle Views412Altmetric-Citations64LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Supporting Info (3)»Supporting Information Supporting Information Get e-Alerts

Intramolecular Magnetic and Electrostatic Interactions in Stepwise Stacked Trinuclear Paramagnetic Metallocenes with the Metal Sequences FeM'Fe and NiM'Ni (M' = V, Cr, Co, Ni) and CoCrCo

Intramolecular Magnetic and Electrostatic Interactions in Stepwise Stacked Trinuclear Paramagnetic Metallocenes with the Metal Sequences FeM'Fe and NiM'Ni (M' = V, Cr, Co, Ni) and CoCrCo

Molecular flexibility demonstrated by paramagnetic enhancements of nuclear relaxation. Application to alamethicin: a voltage-gated peptide channel

A nitroxide spin label attached to the C-terminus of the channel forming peptide alamethicin produces an enhancement of the nuclear spin-lattice relaxation rates of peptide protons as a result of both intermolecular and intramolecular magnetic dipole-dipole interactions. The intermolecular contribution provides evidence that alamethicin monomers collide preferentially in a C-terminal-to-N-terminal configuration in methanol. From the intramolecular paramagnetic enhancement of nuclear spin-lattice relaxation times, effective distances between the unpaired electron on the nitroxide at the C-terminus of alamethicin and protons along the peptide backbone were calculated. These distances are much shorter than distances based on the reported crystal structure of alamethicin, and cannot be accounted for by motion in the bonds that attach the nitroxide to the peptide. In addition, the differences between distances deduced from the nuclear spin relaxation and the distances seen in the crystal structure increase toward the N-terminal end of the peptide. The simplest explanation for these data is that the alamethicin backbone suffers large structural fluctuations that yield shorter effective distances between the C-terminus and positions along the backbone. This finding can be interpreted in terms of a molecular mechanism for the voltage-gating of the alamethicin channel. When the distances between a paramagnetic center and a nucleus fluctuate, paramagnetic enhancements are expected to yield distances that are weighted by r-6, and distances calculated using the Solomon-Bloembergen equations may more nearly represent a distance of closest approach than a time average distance. Therefore, the use of paramagnetic centers such as spin labels or metal ions with long electron T1 values provides a distance measurement that reflects a dynamically averaged structure where the averaging process heavily weights short distances. The results of such measurements, when combined with other structural information, may provide particularly clear evidence for the magnitude of structural fluctuations involving distances greater than 10 A.