Thiazolylblue tetrazolium bromide — a new spectrophotometric reagent for determination of manganese

A method has been developed for the micro-determination of Mn(VII) by the selective extraction of the MnO₄ ^– anion with thiazolylblue tetrazolium bromide from acidic medium. The molar absorptivity of the complex is 1 × 10⁴lmol^–1cm^–1 at 250 nm and the system obeys Beer's law in the range 0.1–1.75 gml^–1 Mn(VII). The composition and stability of the complex are discussed. The characteristic values for the extraction equilibrium and the equilibrium in the aqueous phase are: extraction constantK _ex = 3.16 × 10⁴, distribution constantK _D = 21.85 and association constant =1.45 × 10³. Manganese(VII) was determined in soils and plants. The determination was carried out without preliminary separation of manganese.     

Investigation of the spectral properties of a squarylium near-infrared dye and its complexation with Fe(III) and Co(II) ions

The spectral features of the squarylium near-infrared (NIR) dye NN525 in different solutions and its complexation with several metal ions were investigated. The absorbance maximum of the dye is λ=663 nm in methanol. This value matches the output of a commercially available laser diode (650 nm), thus making use of such a source practical for excitation. The emission wavelength of the dye in methanol is λ_em=670 nm. The addition of either Fe(III) ion or Co(II) ion resulted in fluorescence quenching of the dye. The Stern–Volmer quenching constant, K_SV, was calculated from the Stern–Volmer plot to be K_SV=2.70×10⁷ M⁻¹ for Co(II) ion. The K_SV value for Fe(III) ion could not be established due to the non-linearity of the Stern–Volmer plot and the modified Stern–Volmer plot for this ion. The detection limit is 6.24×10⁻⁸ M for Fe(III) ion and 1.55×10⁻⁵ M for Co(III) ion. The molar ratio of the metal to the dye was established to be 1:1 for both metal ions. The stability constant, K_S, of the metal–dye complex was calculated to be 3.14×10⁶ M⁻¹ for the Fe–dye complex and 2.64×10⁵ M⁻¹ for the Co–dye complex.     

Ion and solvent transfer discrimination at a nickel hydroxide film exposed to LiOH by combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) techniques

A combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) instrument was used to monitor the mobile species transfers associated with the redox processes of thin (Γ100–150 nmol cm⁻²) α- and β-nickel hydroxide films exposed to aqueous LiOH solution. A comparison of the measured PBD signal with the predicted PBD profiles, calculated by temporal convolution analysis of the current and mass responses, enabled the contributions to redox switching of anion (OH⁻) and solvent (H₂O) transfers to be discriminated quantitatively. The responses from the combined instrument are reconciled in terms of H⁺ deintercalation/intercalation within the nickel hydroxide structure as OH⁻ ions enter/exit the film. Hydroxide ion movement is associated with a counterflux of water. Thin nickel hydroxide films show a gradual α→β phase transformation with continuous voltammetric cycling, especially when the films are exposed to high concentrations of electrolyte. α-Films are characterised by OH⁻ transfers that dominate the H⁺ and H₂O movements; β-films are characterised by an increased participation of water and protons to the exchange dynamics.     

Thermodynamics and Kinetics of Reaction of (Oxo)(hydroxo)molybdenumtetraphenylporphyrin with Pyridine

The reaction of meso-tetraphenylporphyrin with Mo(VI) oxide in boiling phenol resulted in a stable complex O=Mo(OH)TPP. Thermodynamics and kinetics of the reaction between (oxo)(hydroxo)molybdenumtetraphenyporphyrin with pyridine in toluene were studied by spectrophotometric method. This reaction was found to occur in three equilibrium elementary stages: replacement of OH^– by Py (K ₁=9.1 × 10³ l/mol, k ₁=5.25 s^–1 mol^–1 l), the formation of dication (dipyridine)(hydroxo)molybdenumtetraphenylporphyrin as a result of cleavage of a double bond Mo=O (K ²=39.3 l/mol, k ₂=1.83 × 10^–2 s^-1 mol^–1 l), and the formation of cationic complex[Mo(Py)₃TPP]³⁺ · 3OH^– (K ₃=1.0 l/mol, k ₃=1.19 × 10^–3 s^–1 mol^–1 l).__________Translated from Koordinatsionnaya Khimiya, Vol. 31, No. 5, 2005, pp. 380–386.Original Russian Text Copyright © 2005 by Tipugina, Lomova, Motorina.     

Solvent Extraction–Photometric Determination of Palladium Using Complexation with 1-(2-Pyridylazo)-2-Naphthol

Palladium(II) complexation with 1-(2-pyridylazo)-2-naphthol (PAN) in aqueous solutions followed by extraction with chloroform and photometric detection was studied. The best conditions were found for the formation of the complex in an aqueous solution and for its extraction with chloroform that provided a sufficient degree of binding palladium ions. The stability constant of the complex cation PdX⁺, which is extracted by chloroform as an ion pair [PdX]⁺[An^–] (An^–– CH₃COO^–), was calculated using the methods proposed by Rossotti (log K _stab= 18.73) and Komar' (log K _stab= 18.82). The equilibrium constant of the complexation reaction was also calculated (5.45 × 10⁴). It was shown that components of nonferrous alloys affect the determination of palladium by photometry as its complex or ion pair with PAN in chloroform.     

Sorption of hafnium on hydrous titanium oxide using radiotracer technique

The sorption of hafnium on hydrous titanium oxide (TiO₂·1.94 H₂O) has been studied in detail. Maximum sorption of hafnium can be achieved from a pH 7 buffer solution containing boric acid and sodium hydroxide using 50 mg of the oxide after 30 minutes shaking. The value ofk _d, the rate constant of intraparticle transport for hafnium sorption, from 0.01M hydrochloric and perchloric acid and pH 7 buffer solutions has been found to be 17 mmole·g^–1·min^–2. The kinetics of hafnium sorption follows Lagergren equation in 0.01M HCl solution only. The values of the overall rate constantK=6.33·10^–2 min^–1 and of the rate constant for sorptionk ₁=6.32·10^–2 min^–1 and desorptionk ₂=2.28·10^–5 min^–1 have been evaluated using linear regression analysis. The value of correlation factor() is 0.9824. The influence of hafnium concentration on its sorption has been examined from 4.55·10^–5 to 9.01·10^–4 M from pH 7 buffer solution. The sorption data followed only the Langmuir sorption isotherm. The saturation capacity of 9.52 mmole·g^–1 and of a constant related to sorption energy have been estimated to be 2917 dm³·mole^–1. Among all the additional anions and cations tested only citrate ions reduce the sorption significantly. Under optimal experimental conditions selected for hafnium sorption, As(III), Sn(V), Co(II), Se(IV) and Eu(III) have shown higher sorption whereas Mn(II), Ag(I) and Sc(III) are sorbed to a lesser extent. It can be concluded that a titanium oxide bed can be used for the preconcentration and removal of hafnium and other metal ions showing higher sorption from their very dilute solutions. The oxide can also be employed for the decontamination of radioactive liquid waste and for pollution abatement studies.     

Das Gleichgewicht zwischen oktaedrisch und tetraedrisch koordiniertem Kobalt(II) in N,N-Dimethylacetamid

Zusammenfassung In Lösung von [Co(DMA)₆](ClO₄)₂ in wasserfr. N,N-Dimethylacetamid (DMA) wird spektrophotometrisch ein Gleichgewicht zwischen tetraedrisch und oktaedrisch koordiniertem Co²⁺ festgestellt. Die GleichgewichtskonstanteK=[Co _tetr ²⁺ ]/[Co _oct ²⁺ ] wurde bei 25°C zu 0,028 und bei 70°C zu 0,19 bestimmt. Bei –8°C zeigen die Elektronenspektren keinen meßbaren Anteil der tetraedrischen Species. Für den oktaedrischen Komplex ergibt sichD _q =805 cm^–1,B=834 cm^–1.

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The equilibrium between octahedral and tetrahedral coordinated cobalt(II) in N,N-dimethylacetamide

Spectrophotometric results establish an equilibrium between tetrahedral and octahedral coordinated cobalt(II) in anhydrous N,N-dimethylacetamide (DMA). The equilibrium constantK=[Co _tetr ²⁺ ]/[Co _okt ²⁺ ] has been found to be 0,028 at 25°C and 0,19 at 70°C. Electronic spectra indicate no measurable amount of the tetrahedral species at –8°C. For the octahedral complexD _q =805 cm^–1 andB=834 cm^–1.

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Mit 4 Abbildungen     

Effect of chain length on the glyme-separated ion-pair formation in bolaform salts of α,ω-(9-fluorenyl)polymethylenes

Formation constants of tetraglyme (glyme 5) separated ion pairs of bolaform electrolytes of the type Na⁺,^–Fl(CH₂) _n Fl^–, Na⁺ (Fl^– denoting a fluorenyl carbanion) were measured spectrophotometrically in tetrahydrofuran (THF) and tetrahydropyran (THP) at 25°C as a function of the chain lengthn, withn=2, 3, 4, or 6. The ratiosK ₁/K ₂ of the first and second glyme ion-pair separation steps were found to be equal to the statistical factor 4 in all cases except in THP for the compoundn=2, where the ratio is 15. Values ofK ₁ as well as K ₂ increase considerably with chain length, the former by a factor 19 asn increases from 2 to 6. The ion-pair separation with glyme 5 is easier in THP than in THF. The results are discussed in terms of differences in external cation solvation and the possible involvement of cyclized structures or curled conformations as a result of interactions between the terminal ion pairs.     

Katalytische Reaktionen in der Spurenanalyse und Untersuchung ihrer Mechanismen. VII

Zusammenfassung Für die Bestimmung von Vanadinspuren eignet sich die Bromat-Ascorbinsäure-Reaktion vom Landolt-Typ, für die das Gesehwindigkeitsgesetz in folgender Form Gültigkeit hat: –d[BrO₃ ^–]/d=[BrO₃ ^– K ₁([(₆H₈O₆+K _v[v]+K ₂ [Br^–][H⁺]²). Die Verfasser bestimmten auf Grund der Daten für die Reaktionszeit der Landolt-Reaktion den Wert vonK ₁ und die Aktivierungsenergie der Bromat-Ascorbinsäure-Reaktion.K ₁=2,5·10^–/Mol^–1 · min^–1 (25° C) bzw. 15,408 Kcal/Mol.

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Catalytic reactions in trace analysis and investigation of their mechanisms. VII

Summary The bromate-ascorbic acid reaction of the Landolt type is suited to the determination of traces of vanadium. The velocity law is valid in the following form: –d[BrO₃ ^–]/d=[BrO₃ ^– K ₁([(₆H₈O₆+K _v [v]+K ₂[Br^–][H ⁺]²). Using the data for the reaction time of the Landolt-reaction, the authors determined the value ofK ₁ and the activation energy of the bromate-ascorbic acid reaction.K ₁=2.5 · 10^–2/mol^–1 · min^–1 (25° C) or 15.408 kcal/mol.

     

NMR and potentiometric determination of the high pK values and protonation sequence of dipyridino-hexaaza-28-crown-8 and its interactions with selenate,sulfate and nitrate ions in aqueous solution

The aqueous protonation and anion-binding SeO ₂ ^–/4 SO _2– ⁴ , and NO _– ³ ) constants of the macrocyclic polyamine ligand, dipyridino-hexaaza-28-crown-8(L), were measured in 0.1M KCl using a potentiometric titration technique. The protonation sequence of the aza groups of L was studied in D₂O from the chemical shifts of the nonlabile protons so as to find the charge distribution geometry as a function of pD. The study indicates that in 0.1M KC1 fully protonated L forms stable l: 1 complexes with SeO _2– ⁴ (logK=3.68) and SO _2– ⁴ (logK=3.55), but not with NO _– ³ (logK < l.5). All of the amine pK values were above 6.3, thus allowing the use of the protonated form of this ligand over a wide pH range.     

Solubility and Phase Behavior of Cr(III) Oxides in Alkaline Media at Elevated Temperatures

A platinum-lined, flowing autoclave facility is used to investigate the solubility behavior of Cr₂O₃ and FeCr₂O₄ in alkaline sodium phosphate, sodium hydroxide, and ammonium hydroxide solutions between 21 and 288°C. Baseline Cr(III) ion solubilities were found to be on the order of 0.1 nmolal, which were enhanced by the formation of anionic hydroxo and phosphato complexes. At temperatures below 51°C, the activity of Cr(III) ions in aqueous solution is controlled by a Cr(OH)₃·3H₂O solid phase rather than Cr₂O₃; above 51°C the saturating solid phase is -CrOOH. Measured chromium solubilities were interpreted via a Cr(III) ion hydrolysis/complexing model and thermodynamic functions for the hydrolysis/complexing reaction equilibria were obtained from least-squares analyses of the data. The existence of four new Cr(III) ion complexes is reported: Cr(OH)₃(H₂PO₄)^–, Cr(OH)₃(HPO₄)^2–, Cr(OH)₃(PO₄)^3–, and Cr(OH)₄(HPO₄)-(H₂PO₄)^4–. The last species is the dominant Cr(III) ion complex in concentrated, alkaline phosphate solutions at elevated temperatures.     

Electromigration of carrier-free radionuclides

Dependences of La(III) overall ion mobilities on concentrations of ox^2– and tart^2– anions of oxalic and tartaric acid in aqueous solutions of 0.01 overall ionic strength and temperature 298.1 K were obtained by direct measurements of electromigration of carrierfree¹⁴⁰La-lanthanum(III). Concentration stability constants K_n and individual ion mobilities u _i ^o of oxalate and tartrate complexes of La(III) have been calculated for nitrate and perchlorate electrolytes, respectively: [La(ox)]⁺, lg K₁=5.63(9), u⁰[La(ox)]⁺=1.95(15)·10^–4 cm²·s^–1·V^–1; [la(ox)₂]^–, lg K₂=4.05(19) u⁰[La(ox)₂]^–=–1.76(20)·10^–4 cm²·s^–1·V^–1; [La(tart)]⁺, lg K₁=4.40(5), u⁰[La(tart)]⁺=+1.99(18)·10^–4 cm²·s^–1·V^–1.Results are compared with literature data. Additional, limiting individual La³⁺ ion mobility was calculated: =+6.9(1)·10^–4 cm²·s^–1·V^–1 for pure water at 298.1 K.     

Photometrisch-potentiometrische Bestimmung der Hydrolysenkonstanten vonN-Bromverbindungen

The equilibrium concentrations of all reaction products emerging from the hydrolysis ofN-bromo compounds in the presence of bromide and thereby also the hydrolysis constants (K ₁) have been calculated from the absorbance at 392.8 nm, thepH-value and the initial concentrations of theN-bromo compound and the bromide. The following compounds have been investigated:N-bromo-succinimide:K ₁=2.2·10^–6, 1,3-dibromo-5,5-dimethylhydantoin:K ₁=1.7·10^–5,N-bromoacetamide:K ₁=1.8·10^–6,N-bromo-monochloroacetamide: 5.2·10^–6,N-bromo-dichloroacetamide:K ₁=8.9·10^–6 andN-bromo-trichloroacetamide:K ₁=1.8·10^–5. The precision of the method, which is mainly suited for weak hydrolizingN-bromocompounds (K ₁<10^–4) are discussed and the overall error of the calculated values was found to be in the range of ±5–12%. The reactivities in aqueous solution of the most frequently usedN-bromo compounds are compared by means of the calculated HOBr equilibrium concentrations. The differences to be expected on the basis of the latters are at concentrations >10^–5 mol/l rather great, while they can be neglected in very dilute solutions (-10^–6 mol/l).

     

New Layered Materials in the K–In–Ge–As System: K8In8Ge5As17and K5In5Ge5As14

Exploratory synthesis in the K–In–Ge–As system has yielded the unusual layered compounds K₈In₈Ge₅As₁₇(1) and K₅In₅Ge₅As₁₄(2), both of which contain In–Ge–As layers with interleaved potassium ions, Ge–Ge bonds, InAs₄tetrahedra, As–As bonds, and rows of Ge₂As₆dimers. Compound 1 has As₃groups, while compound 2 has infinite As ribbons on both faces of each layer. Unlike compound 1, compound 2 has substitutional defects where indium partially occupies each of the three independent germanium sites in the ratio of 1:5 for In:Ge. This partial occupancy makes 2 an electron-precise compound. The Ge(In)–Ge(In) bond of 2 is longer than the Ge–Ge bond of 1, and this bond lengthening effect was confirmed by performing DFT-MO calculations on the model compounds H₃Ge–GeH₃and H₃Ge–InH⁻₃. Possible implications of electron imprecise formulas determined by X-ray crystal structure determinations are discussed. Compound 1: space groupP2₁/cwitha=18.394 (8) Å,b=19.087 (7) Å,c=25.360 (3) Å,β=105.71 (2)°,V=8571 (4) ų, andD_calcd=4.45g/cm³forZ=4. Refinement on 4455 reflections yieldedR(R_w)=6.8%(7.8%). Compound 2: space groupC2/mwitha=40.00 (1) Å,b=3.925 (2) Å,c=10.299 (3),β=99.97 (2)°,V=1592 (1) ų, andD_calcd= 4.55g/cm³forZ=8. Refinement on 1206 reflections yieldedR(R_w)=5.6% (5.7%).     

Determination of ion-pair formation constants of univalent metal–crown ether complex ions with anions in water using ion-selective electrodes: application of modified determination methods to several salts

Ion-pair formation constants (mol^–1 dm³ unit), K_MX for a univalent metal salt (MX) and K_MLX for its ion-pair complex (ML⁺X^–) with a crown ether (L) in water, were determined at various ionic strengths (I) and 25°C by potentiometry with ion-selective electrodes for MX=NaPic, NaMnO₄, NaBPh₄, KPic, and KMnO₄; and MLX=Na(18C6)Pic, K(18C6)Pic, and Na(18C6)BPh₄, where Pic^– and 18C6 denote a picrate ion and 18-crown-6 ether, respectively. Equations for analyzing I-dependence of logK_MLX and logK_MX were derived and fitted well to the I-dependence using a non-linear regression analysis. The equilibrium constants at I=0 mol dm^–3, K_MLX° and K_MX°, were simultaneously obtained from the analysis. The experimental values of K_MLX and K_MX were only in agreement with the values calculated from K_MLX° and K_MX°, respectively, in the ranges of higher I.     

Dibenzophenoxazon-(5)-derivate als Neutralisationsindicatoren in wasserfreier Essigs?ure

Zusammenfassung Die relativen Basizitätskonstanten von 5H-Dibenzo(a,h)phenoxazon-(5) (I) (K=3,2 · 10^–2), 5H-Dibenzo(a,j)phenoxazon-(5) (II) (K=6,5 · 10^–2), 9-(N-1-Naphthylamino)-5H-dibenzo(a,j)phenoxazon-(5) (III) (K=1,12), 9-(N-2-Naphthylamino)-5H-dibenzo(a, j)phenoxazon-(S) (IV) (K=1,22), 9-Anilino-5H-dibenzo(a,j) phenoxazon-(5) (V) (K=1,28) und 9-(p-Tolylamino)-5H-dibenzo(a,j)phenoxazon-(5) (VI) (K=1,45) wurden für das Puffersystem Acetat-Antipyrinperchlorat in wasserfreier Essigsäure bestimmt. Die Verbindungen II, V und VI wurden zur visuellen Indikation von Titrationen schwacher Basen mit Perchlorsäure in wasserfreier Essigsäure benutzt. Mit Indicator II können Basen mit pK_a(H₂O)-Werten von 2–4 und mit den Indicatoren V und VI stärkere Basen mit pK_a(H₂O)-Werten von 4–7 bestimmt werden.

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Dibenzophenoxazone-(5)-derivatives as neutralisation indicators in non-aqueous acetic acid

The relative basicity constants of 5H-dibenzo(a,h)phenoxazone-(5) (I) (K=3.2×10^–2), 5H-dibenzo(a,j)phenoxazone-(5) (II) (K=6.5×10^–2), 9-(N-1-naphthylamino)-5H-dibenzo(a,j)phenoxazone-(5) (III) (K=1.12), 9-(N-2-naphthylamino)-5H-dibenzo(a,j)phenoxazone-(5) (IV) (K=1.22), 9-anilino-5H-dibenzo(a,j)phenoxazone-(5) (V) (K=1.28) and 9-(p-tolylamino)-5H-dibenzo(a,j)phenoxazone-(5) (VI) (K=1.45) have been determined with respect to the buffer system antipyrine acetate-antipyrine perchlorate in non-aqueous acetic acid. The compounds II, V and VI were employed for visual indication of titrations of weak bases with perchloric acid in non-aqueous acetic acid. Indicator II is convenient for the titration of bases with pK _a (H₂O) values 2–4 and indicators V and VI for bases with pK_a(H₂O) values 4–7.

     

Coupled diffusion produced by hydrolysis of aqueous potassium phosphate

Conductimetric and diaphragm cell techniques have been used to measure diffusion of aqueous potassium phosphate solutions at 25°C from 0.01 to 0.10 mol-dm^–3 (M). A significant portion of the aqueous K₃PO₄ component diffuses as equimolar amounts of potassium hydrogen phosphate and potassium hydroxide produced by hydrolysis: K₃PO₄+H₂O=K₂HPO₄+KOH. Because OH^– diffuses more rapidly than HPO ₄ ^2– , the total flow of KOH exceeds the flow of K₂HPO₄. The extra flow of KOH constitutes coupled transport of a second solute component. Ternary diffusion coefficients that describe interacting flows of K₃PO₄ and KOH components are reported. At low concentrations where phosphate is strongly hydrolyzed, the molar flux of the KOH component produced by diffusion of K₃PO₄ is six times larger than the flux of the K₃PO₄ component. Binary diffusion coefficients for aqueous K₂HPO₄ solutions are also reported. It is shown that ternary transport coefficients for K₃PO₄ solutions can be estimated from the properties of binary solutions of K₂HPO₄ and KOH.     

Spectrophotometric study of the complexation of iodine with 1,7-diaza-15-crown-5 in chloroform solution

The complex formation reaction between iodine and 1,7-diaza-15-crown-5 (DA15C5) has been studied spectrophotometrically in chloroform at 25°C. The resulting 1:2 (DA15C5:I₂) molecular complex was formulated as (DA15C5...;I⁺)I ₃ ^– . The spectrophotometric results, as well as the conductivity measurements, revealed that the gradual release of triiodide ion from its contact ion paired form in the molecular complex into the solution is the rate determining step of the reaction. The rate constant was calculated ask=(8.8±0.2)×10^–3 min^–1. The formation constant of the molecular complex was evaluated from the computer fitting of the absorbance-mole ratio data as logK _f=6.89±0.09.     

Crystal and molecular structure of a 1:1 complex of a chiral α-d-glucosido-benzo-18-crown-6 and potassium thiocyanate

C₂₈H₃₆O₁₀. KSCN is monoclinic, space groupP2₁ withZ=2,a=10.390(3),b=8.959(7),c=16.377(7) Å, =92.49(5)°. FinalR=0.053 for 1437 reflections measured at room temperature. The K^– ion lies on the least-squares plane formed by the six oxygen atoms in the macrocyclic ring. The SCN^– ion was found on the same face of the macrocycle as the chiral glucopyranoside moiety.Methyl-4,6-O-benzylidene-2,3-O-(1,2-bis(ethoxyethoxy)benzenediyl)--d-glucopyranoside.     

Coordination chemistry of alkali and alkaline earth cations: Synthesis and crystal structure of potassium(benzo-15-crown-5)2 [3,5-dinitrobenzoate(3,5-dinitrobenzoic acid)2]

The interaction of K(35-Dnb) (35-Dnb=3,5-dinitrobenzoate) with benzo-15-crown-5 (B15C5) in ethanol yields a charge-separated sandwich structured complex [K(B15C5)₂]⁺[35-Dnb(35-DnbH)₂]^– even when equimolar amounts of reactants were used and no external 35-DnbH was added to the reaction mixture. The complex (KC₄₉H₅₁O₂₈N₆, FW=1211.1), is monoclinic,P2₁/c,a=11.063(2),b=10.680(1),c=46.548(8) Å, =91.629(2)⁰,Z=4,D ₀=1.485 g/cm³,D _c=1.468 g/cm³, CuK =1.5418 Å, =17.01 cm^–1, 2<130⁰,F(000)=2520,T=298 K. FinalR for the 6618 observed reflections was 0.071. In the sandwich moiety, the K⁺ is 10-coordinated through all the oxygens of the crown molecules (K⁺–O, 2.76–3.11 Å). The 35-Dnb anion lies 5.3 Å below the lower crown mean plane and is charge separated with respect to K⁺ (K⁺–O^–>7 Å) but undergoes strong hydrogen bonding (2.59 and 2.49 Å) through each carboxylate oxygen with the carboxylic protons of two separate 35-DnbH molecules. Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82014 (52 pages).