Search

Optimal conditions are found for the simultaneous determination of tranexamic acid, ampasse, and ethylmethylhydroxypyridine succinate by reversed-phase HPLC. As is shown, under these conditions the reversed-phase mechanism of retention prevails and the analytes occur mainly in a specific ionic state with an optimal retention time. The possibility of using chromatography columns from various manufacturers is estimated. The method was used to analyze title materials and impurities in drug formulations. Keywords: HPLC, API analysis, tranexamic acid, ethylmethylhydroxypyridine succinate, ampasse DOI: 10.1134/S1061934814020051, Tranexamic acid, trans-4-(aminomethyl)-cyclohexane carboxylic acid, T, 2-ethyl-6-methyl-3-hydroxypyridine succinate, M, and ampasse, 5-hydroxy-3-carboxypyridin-L-glutamic acid, A, possess a wide variety of pharmacological effects. They demonstrate antioxidant, anti-hypoxic, nootropic, and anti-inflammatory activity; tranexamic [...]

Condensation of 2-pyridinecarboxaldehyde with six primary amines containing aliphatic groups gave the corresponding pyridinecarboxaldimines (N-N'R). Addition of these ligands to [Pt[Cl.sub.2](coe)]2 (coe = ris-cyclooctene) gave complexes of the type ris-Pt[Cl.sub.2](N-N'R) (1: R = propyl, 2: R = hexyl, 3: R = octyl, 4: R = nonyl, 5: R = hexadecyl, and 6: R = octadecyl) in moderate yields. The molecular structure of the hexyl derivative (2) has been confirmed by an X-ray diffraction study. Crystals of 2 were triclinic with a = 8.6858(19) Å, b = 8.7567(19) Å, α = 9.5080(19) Å, a = 76.546(3)°, β = 87.151(3)°, and γ = 78.586(3)° in the space group P1. All platinum complexes show considerable anti-bacterial and anti-mycobacterial properties. Key words: aliphatic amines, anti-microbial, anti-mycobacterial, pyridinecarboxaldimines, platinum. La condensation du pyridine-2-carboxaldehyde avec six amines primaires contenant des groupes aliphatiques conduit a la formation des pyridinecarboxaldimines correspondantes (N-N'-R). L'addition de ces ligands au [Pt([Cl.sub.2])(coe)]2 (coe = riscyclooctene) conduit a la formation de complexes de type cis-Pt[Cl.sub.2](N-N'R) (1: R = propyle, 2: R = hexyle, 3: R = octyle, 4: R = nonyle, 5: R = hexadecyle et 6: R = octadecyle), avec des rendements moyens. On a pu confirmer la structure moleculaire du derive hexyle (2) par des etudes de diffraction des rayons X. Les cristaux du compose 2 sont tricliniques, dans le groupe d'espace P1, avec a = 8,6858(19) Å, b = 8,7567(19) Å, c = 9,5080(19) Å, [??] = 76,546(3)°, [??] = 87,151(3)° et [??] = 78,586(3)°. Tous les complexes de platine presentent des proprietes antibacteriennes et antimycobacteriennes importantes. Mots-cles: amines aliphatiques, antimicrobien, antimicybacterien, pyridinecarboxaldimines, platine., Introduction Cisplatin, cis-[Pt[Cl.sub.2][(N[H.sub.3]).sub.2]], and related platinum-based complexes are currently used as anti-cancer agents against testicular and ovarian malignancies. (1) The primary mechanism of action in these platinum drugs is believed [...]

The tereochemical determination and the preparation of 1-(4-(4-((1R,5R,6R)-6-hydroxy-3-oxa-8-azabicyclo[3.2.1]octan-8-yl)-6-morpholino-1,3,5-triazin-2-yl)phenyl)-3-(pyridine-4-yl)urea, an active metabolite of the potent PI3 kinase inhibitor PKI-179, is described. Mosher's method is used for determining the absolute stereochemistry of the enantiomer, thus establishing the stereochemistry of the active metabolite.

A series of new π-conjugated donor (D) and acceptor (A) dyad comonomers were prepared using Suzuki coupling protocols. The D-A comonomers consisling of thiophene/bilhiophene as donors and pyridine/pyrimidine as acceptors were prepared 10 investigate their photophysical and electrochemical properties. The dyads were spectroscopically confirmed to be highly conjugated. This was further supported by the X-ray crystal structure of the bithophene-pyridine dyad that showed all the heterocycles to be coplanar. It was further found that the fluorescence yields (On) of the dyads were highly dependent on the number of thiophenes. The bithiophene derivatives exhibited [Φ.sub.n] values ≥ 0.3 while the thiophene derivatives did not fluoresce. The suppressed fluorescence observed for the thiophene derivatives was due to their higher triplet energy resulting in efficient intersystem crossing (ISC) to the triplet state with [Φ.sub.ISC] ≥ 0.8. This was confirmed both by time-resolved and steady-state measurements. The singlet excited state of both thiophene and bithiophene dyads was deactivated solely by either fluorescence and (or) ISC. Owing to their donor and acceptor character, the dyads could be oxidized and reduced both electrochemically and photochemitally to afford the radical cation and anion, respectively. Key words: comonomer, thiophene, pyridine, pyrimidine. photophysics, fluorescence. A l'aide d'un couplage de Suzuki, une serie de comonomeres conjugues contenant une fonction donneur d'electrons (D) et une fonction accepteur d'electrons (A) out ete prepares. Ces comonomeres, dits D-A, qui utilisent le thiophene/bithiophene comme donneurs d'electrons et la pyridine/pyrimidine comme accepteurs d'electrons, ont ete synthetises dans le but d'analyser leurs proprietes photophysiques et electrochimiques. 11 a ete lvalue spectroscopiquement que les dimeres sont grandement conjugues. Ceci est d'ailleurs soutenu par la structure rayons-X du compose bithiophenepyridine qui montre que tons les heterocycles sont coplanaires. II a ete determine que les rendements quantiques de fluorescence ([Φ.sub.n]) des composes dependent du nombre de thiophenes presents dans la molecule. En effet, les derives du bithiophene ont des [Φ.sub.n] ≥ 0,3 tandis que les derives du thiophene ne fluorescent pas. Cette absence de fluorescence est causee par la formation d'un etat triplet de haute energie par croisement iutersysteme (ISC pour << intersystem crossing >>) avec un [Φ.sub.ISC] ≥ 0,8. L'etat singulet excite des dimeres de thiophene et de bithiophene est uniquement desactive par la fluorescence ou le ISC. Grace a leur caracterislique donneur/accepteur, les comonomeres ont pu etre oxydes el reduits electrochimiquement et chimiquement aim de former le radical cation et le radical anion. Mots-cles: comonomere, thiophene, pyridine, pyrimidine, photophysique, fluorescence. [Traduit par la Redaction], Introduction Conjugated materials have attracted much attention because of their electrochemical and photophysical properties that are ideally suited for plastic electronics, including organic field effect transistor, (1-4) photovoltaics, (5,6) emitting [...]

The paper reports on the development of an accurate hydrophilic liquid interaction chromatography tandem mass spectrometry (HILIC-MS/MS) based stable isotope dilution analysis for the simultaneous quantitation of the food-derived bioactive pyridines trigonelline, nicotinic acid, nicotinamide, and N-methylpyridirdum, as well as their key metabolites nicotinamide-N-oxide, N-methylnicofinamide, N-methyl-2-pyridone-5-carboxamide, N-methyl-4-pyridone-5-carboxamide, and N-methyl-2-pyridone-5carboxylic acid in human plasma and urine. Precision of the stable isotope dilution analysis (SIDA) was 1.9% and 11.9% relative standard deviation (n = 6), and accuracy was between 92.4% and 113.0%. The lower limit of quantitation (LLOQ) was 50 fmol (10 pmol/mL) injected onto the column for all analytes with the exception of N-methyl-2-pyridone-5-carboxylic acid and N-methyl-2-pyridone-5-carboxamide, for which an LLOQ of 100 fmol (20 pmol/mL) was found. The method was applied to monitor the plasma appearance and urinary excretion and to determine pharmacokinetic parameters of the bioactive pyridines as well as their metabolites in a clinical human intervention study with healthy volunteers (six women, seven men) after oral administration of 350 mL of a standard coffee beverage. Trigonelline plasma levels increased from 160 nmol/L to maximum concentrations of 5479 (males) or 6547 nmol/L (females), and N-methylpyridinium plasma levels raised from virtually complete absence to maximum values of 777 (females) or 804 nmol/L (males) within 2-3 and 1 2 h after coffee consumption, respectively. The high plasma levels of N-methyipyridinium found after coffee consumption dearly demonstrate for the first time that this cation is entering the vascular system, which is the prerequisite for biological in vivo effects claimed for that compound. In contrast, the coffee intervention did not significantly influence the plasma concentrations of N-methyl-2-pyridone-5-carboxamide and N-methyl-4-pyridone-5-carboxamide, the major niacin metabolites. Within 8 h after coffee intervention, an urinary excretion of 57.4 [+ or -] 6.9% of trigonelline and 69.1 [+ or -] 6.2% of N-methylpyridinium was found for the male volunteers, whereas females excreted slightly less with 46.2 [+ or -] 7.4% and 61.9 [+ or -] 12.2% of these pyridines. 10.1021/ac902616k

The photochemical dynamics of the pyrrole-pyridine hydrogen-bonded complex has been investigated with computational methods. In this system, a highly polar charge-transfer state of [sup.1][pi][[pi].sup.*] character drives the proton transfer from pyrrole to pyridine, leading to a conical intersection of [S.sub.1] and [S.sub.0] energy surfaces. A two-sheeted potential-energy surface including 39 in-plane nuclear degrees of freedom has been constructed on the basis of ab initio multiconfiguration electronic-structure data. The non-Born-Oppenheimer nuclear dynamics has been treated with time-dependent quantum wave-packet methods, including the two or three most relevant nuclear degrees of freedom. The effect of the numerous weakly coupled vibrational modes has been taken into account with reduced-density-matrix methods (multilevel Redfield theory). The results provide insight into the mechanisms of excited-state deactivation of hydrogen-bonded aromatic systems via the electron-driven proton-transfer process. This process is believed to be of relevance for the ultrafast excited-state deactivation of DNA base pairs and may contribute to the photostability of the molecular encoding of the genetic information. conical intersection | excited-state hydrogen transfer | nonadiabatic transition

A study was conducted to analyze the photoreactions of 4-nitroanisole and the 2-halo-4-nitroanisoles with the nucleophiles hydroxide ion and pyridine. The findings suggest that the efficiency-determining step to the halogen photosubstitution is attributed to simple formation of a [sigma]-complex through electron-paired bonding within the triplet manifold.

The full details, scope, and limitations of copper-catalyzed one-step functionalization of pyridines and related nitrogen-containing heterocycles with various simple alkynes is reported. The method provides an alternative catalytic route to nucleophilic or cross-coupling to enantioenriched alkynylated heterocycles.

2-Bromopyridine is used as a starting point for a four-step synthesis of (-)-lentiginosine and its epimers, which is based on the Mitsunobu methodology. The stereochemically flexible strategy is shown by the concise total synthesis of non-natural products derivatives like (-)-lentiginosine and its stereoisomers.

The 2-bromo-5(or 6)-tri-n-butylstannylpyridines are prepared from dibromopyridines and i-PrMgAl at room temperature and they have undergone Negishi coupling with either alkyl or arylzinc chlorides. A group of new liquid crystalline materials with aromatic cores containing pyridine and thiophene rings are prepared by using the new pyridinylstannanes as key intermediates.

The pH-controlled intramolecular reversible cyclization reaction between the pyridine and aldehyde moieties to form a pyridinium-fused heterocycle is reported.

A microwave-assisted one-pot, two-step or one-pot, three-step cylization/Suzuki/heteroarylation method was developed to synthesize the 2,3,6-trisubstituted imidazo[1,2-a]pyridine derivatives. The newly developed method could be applied to obtain other polysubstituted compounds from 2-amino-5-halogenopyridines, 2-halogenocarbonyl derivatives, boronic acids and heteroaryl bromides.

A novel modular and practical methodology is developed for preparing 6-substituted pyridin-3-yl C-nucleosides. The deprotection of all the silylated nucleosides by [Et.sub.3]N.3HF has given a series of free C-nucleosides.

Quaternized poly(4-vinylpyridine) coated activated carbon has been proven to be successful in removal and recovery of Cr(VI) from aqueous solution. However, the key component of the mass transfer operations and the diffusion coefficient has not been determined. In this study, adsorption of Cr(VI) was investigated at different initial concentrations. A finite bath diffusion control model with changing bulk concentrations was derived analytically, and a constant related to radius of the absorbent particle and the fractional attainment of adsorption was first introduced into this model. Results indicated that the adsorbent provided fast adsorption kinetics and the modified diffusion-control model fitted the experimental data well. The observed sorption kinetics was consistent with the finite bath diffusion, with an average value of 4.10 x [10.sup.-6] [cm.sup.2]/s for the product of the distribution coefficient and the effective diffusivity in the adsorption of Cr(VI) on the quaternized poly(4-vinylpyridine) coated activated carbon. DOI: 10.1061/(ASCE)0733-9372(2007)133:8(834) CE Database subject headings: Chromium; Sorption; Activated carbon; Kinetics.

The effects of anchoring groups on electron injection from adsorbate to nanocrystalline thin films were investigated by comparisons injection kinetics through carboxylate versus phosphonate groups to Ti[O.sub.2] and Sn[O.sub.2]. The anchoring groups were shown to have negligible effects on the energetics of relevant orbitals in the Re complexes.

Experimental and theoretical methods are used for characterizing a series of Ir(III)-based heteroleptic complexes with phenylpyridine (ppy) and 2-(5-phenyl-4H-[1,2,4]-triazol-3-yl)pyridine (ptpy) derivatives as coordinating ligands. The results have shown that the extent of charge-transfer contributions have depended on the energy gaps between the relevant molecular orbitals, which can be modified by the substitution pattern.

Femtosecond fluorescence up-conversion and transient absorption (TA) studies are used for characterizing the ultrafast relaxation of aqueous iron(II)-tris(bipyridine) upon excitation into the singlet metal-to-ligand charge-transfer band (MLCT). A simple model of avoided crossings between singlet and triplet potential curves is proposed that is induced by the strong spin-orbit interaction.

The synthesis and characterization of the pivalic acid ester N-pivaloxy-2-amino-1-methyl-6-1H-imidazo[4,5-b]pyridine and the characterization of its reactions in aqueous media are presented. The correlation of nitrenium ion selectivity with the mutagenicity and carcinogenicity of PhIP is examined.

The stability of silver(I) complexes with pyridine ligands is affected by solvation. Pyridine exhibits differential affinity for silver(I) ion in water and dmso.

A study was conducted to investigate the effect of alkylation on the strong collisional energy transfer between C[O.sub.2] and a series of highly vibrationally excited alkyl pyridines where alkyl = methyl, ethyl, and propyl. The results have shown that the alkylated donors have undergone strong collision with C[O.sub.2] via a less repulsive part of the intermolecular potential than does pyridine.

The stereochemistry of the aminopalladation step of various different catalyst systems of alkenes with two-faced reactivity is studied to analyze and report the cis- versus trans-aminopalladation in several aerobic Pd-catalyzed intramolecular Aza-Wacker reactions. The analysis reveals that the nitrogen nucleophile in such systems highly affect the aminopalladation pathway, hence leading to successful oxidative amination of alkenes.

Three pathways for the reaction of bispidine-iron(II) complexes are studied where bispidine is a rigid tetradentate amine/pyridine ligand with [H.sub.2][O.sub.2]. The findings reveal that there is a direct conversion of the dispidine-iron(II)-hydrogen peroxide complex to the corresponding iron(IV)-dihydroxo compound which is a novel possible oxidant for the dihyroxylation of olefins.

The reactivity of the pyridyl-substituted aziridine with different N-, S-, and O-nucleophiles is examined in order to understand the optimal conditions allowing for a selective ring-opening. It is shown that complete or prevalent regioelectivity can be obtained by using cerium trichloride heptahydrate as a catalyst and the N-substituent can be removed by an oxidative protocol.

Nanosecond time-resolved absorption in a magnetic field ranging from 0 to 2.0 were performed on a series of covalently linked donor(PXZ)-Ru[(bipyridine).sub.3]-acceptor(diquat) complexes to study spin-orbit coupling effects. The spin-rotational interaction is found to be responsible for the saturating limit of the magnetic-field effect in high fields for PSZ.

The guided ion beam tandem mass spectrometry was used to study the collision-induced dissociation of complexes of [Cu.sup.+] bound to a variety of N-donor ligands (N-L) such as pyridine, 4,4-dipyridyl, 2,2-dipyridyl, and 1,10-phenanthroline. The primary and lowest-energy dissociation channel observed in all cases corresponds to the endothermic loss of a single intact N-L ligand.

A series of trinuclear [Ni.sub.2.sup.II][Ln.sup.III] complexes of 2,6-di(acetoacetyl)pyridine ([H.sub.2]L) (Ln = La-Lu except for Pm) are synthesized by a 'one-pot reaction' of [H.sub.2]L, Ni[(N[O.sub.3]).sub.2].6[H.sub.2]O and Ln[(N[O.sub.3]).sub.3].n[H.sub.2]O in methanol and their structures and magnetic properties are characterized. The results have shown that the [Ni.sup.II]-[Ln.sup.III] interaction is weakly antiferromagnetic for Ln = Ce, Pr, and Nd and ferromagnetic for Ln = Gd, Tb, Dy, Ho and Er.

The synthesis of quinquedentate ligand incorporating three thiaether sulfurs and a bipyridine moiety within a macrocyclic structure, namely, cyclo-6,6'-(2,5,8-trithianonane)]-2,2'-bipyridine ([15]ane[S.sub.3]bpy = L], is described. The combination of a nearly constant geometry and a large self-exchange rate constant has indicated that the C[u.sup.II/I] redox system has represented a true geometric entatic state.

A unique thiol, especially 4-pyridinethiol (4-PS), assisted dissociation of Au in alcohol solutions is described. A mechanism is proposed for the dissolution reaction, which can be applied in selective preparation of self-supporting nanometer-thick Au foils.

A series of novel organic-inorganic assemblies are constructed from organically modified Zn-substituted polyoxovanadates and zinc organoamine subunits. The rigid/flexible ligands have crucial influence on the formation of products and different coordination environments of the secondary metal and the pH value play vital roles in the structures of the products.

The synthesis, characterization and analysis of a family of N-crowned 4-p-(aminophenyl)-2,6-diphenylpyriidines DA (1-6) as potential hosts for the signaling of cationic and anionic guests is described. The results have yielded recognition patterns for acetate, [F.sup.-] and C[N.sup.-], indicating the potential of simple chromogenic host-guest pairs for differential anion signaling.

The interaction between tetramethylcucurbit[6]uril (TMeQ[6]) with hydrochloride salts of 2-phenylpridine (G1), 2-benzylpyridine (G2), and 4-benzylpyridine (G3) is examined by using H NMR spectroscopy and electron absorption spectroscopy. The calculations show that when the phenyl moiety of the three pyridine derivative guests are included, the host-guest complexation reached the minimum, and the corresponding energy differences for the formation of the title host-guest inclusion complexes are consistent with experimental results.

The synthesis and photophysical properties of new chormophore based pyridine-dicarboxamide ligands and related [D.sub.3] symmetric europium(III) complexes are described. The luminescence of Eu(III) was sensitized by two-photon absorption of the ligand showing the two-photon antenna effect dphenomenon.

Novel iron (II) complexes with pyridine-containing macrocyles having an aminopropyl pendant arm were synthesized and characterized. It was seen that reversible protonation of the pendant arm regulates the catalytic activity of complexes 1-3 in olefin epoxidation with [H.sub.2][O.sub.2] and five coordinate macrocycles were catalytically inactive because of large out-of-plane displacement of iron.

The spectroscopic study of three types of protonated corroles [[(Cor)[H.sub.4]].sup.}, [[(Cor)[H.sub.5]].sup.2}, [[(Cor)[H.sub.6]].sup.3} in acidic nonaqueous media is described. The three corroles with one or two meso pyridyl substituents underwent multiple protonation both at the central nitrogens and at the nitrogens of the pyridyl substituents and a relationship is observed between the pKa values and the oxidation potential for the fully protonated corroles.

A new synthetic strategy is described for the construction of boron-based macrocycles and dendrimers. The results have highlighted the importance of boron-based compounds in supramolecular chemistry.

A study determines the first-order rate constants for deprotonation of the [alpha]-amino carbon of the adduct between 5'-deoxypyridoxal and glycine. No significant deprotonation is found at pH 7.1 of the form of the 1-glycine iminium ion with the pyridine nitrogen in the basic form.

A mechanistic study is used to determine the catalytic cycle and its dependence on the ligand, bipyridine (bipy) or isopropyl-phosphinooxazoline ([super i]PrPHOX). The presence of styrene in this system has increased rate-limiting oxidative addition and has promoted the cycle that proceeds four times more rapidly and with much higher enantioselectivity than its styrene-lacking counterpart.

The compound [Re.sub.2][(hpp).sub.4]F[(TFPB).sub.2] is characterized by both X-ray and neutron diffraction and an EPR spectroscopy of [Re.sub.2][(hpp).sub.4][Cl.sub.2]P[F.sub.6] at multifrequency (9-95 GHz) is studied by using dilute solution, frozen glass and neat powder samples. It is indicated that the unpaired electron in a [Re.sub.2.sup.7+] compound resides in a dominantly metal-based orbital.

The synthesis and characterization of two new isomeric, flexible four-armed thioether pyridine-containing ligands 1,2,4,5-tetrakis(3-pyridylmethylulfanylmethyl)benzene and 1,2,4,5-tetrakis(4-pyridylmethylsulfanylmethyl)benzene are reported. The network has indicated a remarkable lack of dependence on the nature of the counterion and solvent.

The coordination assemblies resulting from the complexation of 1,3,5-tri(4-sulfophenyl)benzene, [L.sup.3-], with copper(II) in the presence of pyridine are analyzed. The results have given insights into the subtleties of designing coordination polymers and open-channel solids and three different single crystalline solids are obtained from an initial crystallization.

The chemical reactivity studies were used to investigate the regioselective deprotonation of pyridine in the gas phase. The results indicate that the nitrogen in the ring acts as a strongly [pi]-electron-withdrawing group but with little inductive effect which is consistent with the observed and computed regiospecific properties of the pyridine ring.

A general method is described for the synthesis of azabiaryls by a one-pot procedure involving a Directed ortho metalation (DoM)-boronation-Suzuki-Miyaura cross coupling sequence. The method is found to avoid the recognized difficult isolation of pyridyl boronic acids and their instability toward deboronation.

Synthesis of nitrosylruthenium complexes containing 2,2':6,'2'-terpyridine was carried out by reactions of alkoxo complexes with acids. Reactions of this complexes P[F.sub.6] with N[O.sub.2.sup.-] and [Br.sup.-] in [H.sub.2]O or C[H.sub.3]OH afforded a trans-(NO, OR), cis-(X, OR) type complex in which the OR ligand came from the solvent.

A study explores the N/[eta.sup.2] equilibrium for TpW(NO)-(P[Me.sub.3])(pyridine) and its dependence on the heterocycle substituents and metal oxidation state. It is found that the ability of tungsten to bind pyridines in an [eta.sup.2] manner is dependent on both the electronic and steric properties of the substituents as well as the oxidation state of the metal.

The effects of hydrogen bonding on the ring stretching modes of pyridine are studied by using noisy light based Raman scattering spectroscopy. It is seen that three systems, pyridine-formamide, pyridine-water and pyridine-acetic acid provide varying degrees of strength for the diluent pyridine hydrogen bond complex.

The preparation, characterization, and photophysical properties of a series of multinuclear complexes of 5,5'-diethynyl-2,2'-bipyridine capped with triangular trinuclear [Cu.sub.3]/[Ag.sub.3] cluster units is described. These heteronuclear or multicomponent complexes were found to exhibit photoluminescence with the emissive lifetimes in the microsecond time scale in both solid states and dichloromethane at room temperature.

A study was conducted to investigate the interaction of polypyridine nitrosyl ruthenium complexes with the DNA upon photoactivation. Two water molecules form strong hydrogen bonds with the nitro and the carboxylic oxygen atoms of two separate units of complex resulting in a dimeric unit.

The preparation of two new heptanuclear Mn clusters, [[Mn.sub.7][O.sub.5][(OMe).sub.2][(O2CPh).sub.9](terpy)] and [[Mn.sub.7][O.sub.5][(OCH2Ph).sub.2][(O2CPh).sub.9](terpy)] from the partial alcoholysis of the trinuclear complex [[Mn.sub.3]O[(O2CPh).sub.6][(py).sub.2]([H.sub.2]O)] in the presence of terpy (terpy = 2,2'; 6',2'-terpyridine) is illustrated. The obtained complexes possess S = 6 ground states and could be rationalized by qualitative considerations of the expected [Mn.sub.2] pairwise coupling.