Line-broadening Techniques Research Articles

Electron transfer in non-oxovanadium(IV) and (V) complexes: Kinetic studies of an amavadin model

Electron-transfer reactions of the eight-coordinate vanadium complex, bis-(N-hydroxyiminodiacetate)vanadium(IV) [V(HIDA)2]2–, a synthetic analog of amavadin with ascorbic acid and hexachloroiridate(IV), have been studied. The self-exchange rate constant for this analog has been calculated from oxidation and reduction cross-reactions using Marcus theory and directly measured using 51V NMR paramagnetic line-broadening techniques. The average self-exchange rate constant for the bis-HIDA vanadium(IV/V) couple equals 1.5 × 105 M–1 s–1. The observed rate enhancements are proposed to be due to the small structural differences between the oxidized and reduced forms of the HIDA complex and inner-sphere reorganizational energies. The electron-transfer reaction of this synthetic analog is experimentally indistinguishable from amavadin itself, although significant differences exist in the reduction potential of these compounds. This suggests that ligand modification effects the thermodynamic driving force and not the self-exchange process.

Comparison of the Self-Exchange and Interfacial Charge-Transfer Rate Constants for Methyl- versus tert-Butyl-Substituted Os(III) Polypyridyl Complexes

Differences in the self-exchange and interfacial electron-transfer rate constants have been evaluated for a relatively unhindered Os(III/II) redox system, osmium(III/II) tris(4,4'-di-methyl-2,2'-bipyridyl), [Os(Me2bpy)3]3+/2+, relative to those of a relatively hindered system, osmium(III/II) tris(4,4'-di-tert-butyl-2,2'-bipyridyl), [Os(t-Bu2bpy)3]3+/2+. In contrast to the predicted increase in rate constant by a factor of 2-3 due to the difference in reorganization energy of the two complexes, introduction of the tert-butyl functionality decreased the self-exchange rate constant, as measured by NMR line-broadening techniques, by a factor of approximately 50 as compared to that of the analogous methyl-substituted osmium complex. Steady-state current density versus potential measurements, in conjunction with differential capacitance versus potential measurements, were used to compare the interfacial electron-transfer rate constants at n-type ZnO electrodes of [Os(t-Bu2bpy)3]3+/2+ and [Os(Me2bpy)3]3+/2+. The interfacial electron-transfer rate constant for the reduction of [Os(t-Bu2bpy)3]3+ was 100 times smaller than that for [Os(Me2bpy)3]3+. The results indicate that the tert-butyl group can act as a spacer on an outer-sphere redox couple and significantly decrease the electronic coupling of the electron-transfer reaction in both self-exchange and interfacial electron-transfer processes.

X-ray line-broadening investigation of deformation during hot rolling of Ti–6Al–4V with a colony-alpha microstructure

X-ray line-broadening techniques that were previously developed and applied to quantify deformation behavior during the hot rolling of commercial-purity titanium were applied to Ti–6Al–4V plate with a colony-alpha preform microstructure. The work quantified the challenges in using line-broadening techniques for two-phase titanium alloys which undergo a phase transformation during cooling following hot working.

Use of Phosphorus Ligand NMR Probes To Investigate Electronic and Second-Sphere Solvent Effects in Ligand Substitution Reactions at Manganese(II) and Manganese(III)

Manganese/ligand association dynamics were studied using a series of structurally related anionic phosphorus ester ligand probes [CH(3)OP(O)(X)(Y)(-), where X = CH(3)O, CH(3)CH(2), or H and Y = O, S, or BH(3)]. Reactions of the probe ions with Mn(H(2)O)(6)(2+) and a manganese(III) porphyrin (Mn(III)TMPyP(5+)) were studied in aqueous solution by paramagnetic (31)P NMR line-broadening techniques. A satisfactory linear free energy relationship for reactions of the probe ions with Mn(H(2)O)(6)(2+) and Mn(III)TMPyP(5+) required consideration of both the basicity and solvent affinity of the probe ligands: log(k(app)) = log(k(0)) + alpha pK(a) + beta log(K(ext)), where k(0), alpha, and beta are metal complex dependent parameters and pK(a) and K(ext) represent the measured Bronsted acidity and water/n-butanol extraction constant for the probe anions, respectively. Reactions of Mn(H(2)O)(6)(2+) were relatively insensitive to changes in ligand basicity (alpha = -0.04) and favored the more hydrophilic anions (beta = -0.54). These observations are consistent with a dissociative ligand exchange mechanism wherein the outer-sphere complex is stabilized by hydrogen bonding between Mn(H(2)O)(6)(2+) and the incoming ligand. In contrast, reactions with Mn(III)TMPyP(5+) are accelerated by decreases in both the basicity (alpha = -0.43) and the hydrophilicity (beta = +0.97) of the probe. We conclude that reactions of Mn(III)TMPyP(5+) are also dissociative but that the aromatic groups of the porphyrin provide a hydrophobic environment surrounding the ligand binding site in Mn(III)TMPyP(5+). Thus, the probe/water solvent interactions must be significantly weakened in order to form the outer-sphere complex that leads to ligand substitution. This work demonstrates the utility of phosphorus relaxation enhancement (PhoRE) techniques for characterizing the second coordination sphere environment of metal complexes leading to ligation and will allow comparison of the second coordination spheres of Mn(H(2)O)(6)(2+) and Mn(III)TMPyP(5+) to those of other metal complexes.

Membrane Dynamics of the Amphiphilic Siderophore, Acinetoferrin

Acinetobacter haemolyticus is an antibiotic resistant, pathogenic bacterium responsible for an increasing number of hospital infections. Acinetoferrin (Af), the amphiphilic siderophore isolated from this organism, contains two unusual trans-2-octenoyl hydrocarbon chains reminiscent of a phospholipid structural motif. Here, we have investigated the membrane affinity of Af and its iron complex, Fe-Af, using small and large unilamellar phospholipid vesicles (SUV and LUV) as model membranes. Af shows a high membrane affinity with a partition coefficient, K(x)= 6.8 x 10(5). Membrane partitioning and trans-membrane flip-flop of Fe-Af have also been studied via fluorescence quenching of specifically labeled vesicle leaflets and (1)H NMR line-broadening techniques. Fe-Af is found to rapidly redistribute between lipid and aqueous phases with dissociation/partitioning rates of k(off) = 29 s(-1) and k(on) = 2.4 x 10(4) M(-1) s(-1), respectively. Upon binding iron, the membrane affinity of Af is reduced 30-fold to K'(x) = 2.2 x 10(4) for Fe-Af. In addition, trans-membrane flip-flop of Fe-Af occurs with a rate constant, k(p) = 1.2 x 10(-3) s(-1), with egg-PC LUV and a half-life time around 10 min with DMPC SUV. These properties are due to the phospholipid-like conformation of Af and the more extended conformation of Fe-Af that is enforced by iron binding. Remarkable similarities and differences between Af and another amphiphilic siderophore, marinobactin E, are discussed. The potential biological implications of Af and Fe-Af are also addressed. Our approaches using inner- and outer-leaflet-labeled fluorescent vesicles and (1)H NMR line-broadening techniques to discern Af-mediated membrane partitioning and trans-membrane diffusion are amenable to similar studies for other paramagnetic amphiphiles.

X-ray line-broadening analysis of deformation mechanisms during rolling of commercial-purity titanium

X-ray line-broadening techniques were used to establish deformation mechanisms in textured commercial purity (CP) titanium specimens that were rolled at various temperatures. The results indicated that conventional line-broadening techniques developed specifically for polycrystalline powders could be used on textured materials to deduce the slip-system activity and dislocation density that develop during thermomechanical processing if appropriate averaging procedures are followed. The deduced slip-system activity and densities revealed a transition in deformation behavior from a heavily-twinned to an untwinned mode with an increase in deformation temperature for CP titanium.

Kinetics and Mechanism of Formation of Yttrium Alkyl Complexes from (Cp*2YH)2 and Alkenes

The dissociation of the dimer (Cp* 2 YH) 2 (2) to the Cp* 2 YH monomer is an important process in reactions of 2 with alkenes. The rate of dissociation of 2 was measured by NMR line-broadening techniques (14 s - 1 at 0 °C, ΔG = 14.5 kcal mol- 1 , ΔH = 14.9(8) kcal mol - 1 , and ΔS = 3(2) eu). A full range of dissociative and associative mechanisms for reaction of alkenes with 2 was found. For the most crowded and least reactive alkenes studied, 2-butene and 2-methylpropene, reaction with 2 occurred slower than dissociation of dimer 2; kinetic studies established reversible dissociation of 2 to monomeric Cp* 2 YH followed by competitive trapping by alkene and recombination to regenerate 2. Kinetic studies of the less crowded alkene 3-methyl-1-butene are consistent with rate-limiting dissociation of dimer 2 followed by efficient trapping of the intermediate Cp* 2 YH by alkene. The least crowded terminal alkenes such as 1-hexene reacted with 2 at a rate faster than dimer dissociation; kinetic studies established a two-component rate law involving a second-order term for direct attack of alkene on the dimer and a first-order term involving rate-determining dimer dissociation followed by rapid alkene reaction with monomeric Cp* 2 YH. The reactions of terminal alkenes with 2 initially gave mixtures of single- and double-alkene-insertion products but no triple-insertion products. The initially formed n-alkyl yttrium complex reacts with terminal alkenes at a rate similar to the reaction of yttrium hydride dimer 2 with terminal alkenes. The more crowded β-alkyl yttrium double-insertion product is much less reactive toward terminal alkenes.

Examination of Electron Transfer Self-Exchange Rates Using NMR Line-Broadening Techniques: An Advanced Physical Inorganic Laboratory Experiment

This article describes the determination of the decamethylferrocene(Fc*)/decamethylferrocenium (Fc*+) electron transfer self-exchange rate by NMR line-broadening techniques. NMR spectra of Fc*, Fc*+, and several mixtures yield frequency and peak-width data that are used to solve for the self-exchange rate constant. Students obtain rate constants in the range 1.6 x 107 to 2.4 x 107 M-1 s-1 (38 °C in acetone-d6), which is in good agreement with the literature value of 2.4 x 107 M-1 s-1 (25 °C). The experiment represents an alternative to the classic rate of amide bond rotation as a demonstration of the effectiveness of NMR in analyzing dynamic molecular processes.

Rhodium(III) and Iridium(III) Solvates of the Form [(η5-C5Me5)M(S)3]2+ as Models for the Reactivity of Half-Sandwich Compounds1

Solvent exchange on the half-sandwich organic solvates [(eta(5)-C(5)Me(5))M(S)(3)](2+) (M = Rh, S = MeCN (1) or Me(2)SO (3); and M = Ir, S = MeCN (2) or Me(2)SO (4)) has been investigated as a function of temperature, pressure, and concentration of free solvent by (1)H NMR line-broadening techniques in CD(3)CN and/or CD(3)NO(2). The exchange rates span several orders of magnitude, from k(ex)(298) = 8.8 x 10(-)(2) s(-)(1) for 2 to 3.6 x 10(3) s(-)(1) for 3, as a result of changes in the electronic and steric properties of the ligands. Nevertheless, the volume of activation remains consistently positive for compounds 1-4 with values ranging from +0.8 to +3.3 cm(3) mol(-)(1). In combination with the positive activation entropies obtained and the first-order rate law established for these systems, it was concluded that regardless of the nature of the ligand the solvent exchange process on 1-4 proceeds via a dissociative D mechanism. Of note, the intermolecular exchange with free Me(2)SO on 4 takes place exclusively from a conformational isomer of 4 (structure 4.2), which is itself in equilibrium with a second, more compact conformer (structure 4.1).

Formation, Properties, and Characterization of a Fully Reduced Fe(II)Fe(II) Form of Spinach (and Parsley) [2Fe-2S] Ferredoxin with the Macrocyclic Complex [Cr(15-aneN(4))(H(2)O)(2)](2+) as Reductant.

Reduction of spinach and parsley ferredoxin FdI in the Fe(III)Fe(III) state with the 1,4,8,12-tetraazacyclopentadecane complex [Cr(15-aneN(4))(H(2)O)(2)](2+), here written as Cr(II)L, provides the first evidence for two 1-equiv steps yielding an Fe(II)Fe(II) product. Rate constants (25 degrees C) for spinach FdI are 2760 and 660 M(-)(1) s(-)(1), respectively, at pH 7.5, I = 0.100 M (NaCl). An important observation is that the Cr(III)L generated in the first step remains attached to the Fe(II)Fe(III) product and perturbs the protein active site sufficiently to make the second stage possible. The second Cr(II)L reduction is of the "outer-sphere" type, and the Cr(III)L generated is not attached to the protein. Anaerobic reoxidation of the fully reduced protein with [Co(NH(3))(6)](3+) is rapid and can be achieved with approximately 80% recovery of the Fe(III)Fe(III) oxidation state over 40 min. Air oxidation yields the Cr(III)L product Fe(III)Fe(III).Cr(III)L (Fe:Cr = 2:1). With Anabaena variabilis only a one-step reduction is observed and there is no Cr(III)L attachment. From a comparison of amino acid sequences with spinach (and parsley) FdI, a likely point of Cr(III)L attachment is indicated. Comparisons are made with dithionite as reductant. In addition, square-wave voltammetry on spinach Fe(III)Fe(III).Cr(III)L gives two reduction potentials -273 and -410 mV vs NHE. The different redox products have been characterized by EPR. Using (1)H NMR line-broadening techniques, evidence for Cr(III)L binding at a surface site close to Tyr-25/Tyr-82 is obtained. Also from investigations with redox-inactive [Cr(en)(3)](3+) as a competitive inhibitor for Cr(II)L reduction of spinach Fe(III)Fe(III), Tyr-25/Tyr-82 is proposed as the site for Cr(II)L reduction. From an extension of studies to include reduction of Fe(III)Fe(III).Cr(III)L with Cr(II)L, evidence is obtained for a second reaction site when that at Tyr-25/Tyr-82 is no longer available.

Factors influencing the stability of ATP in ternary complexes: Spectroscopic investigation of the interaction of certain biomimetic copper(II) complexe with ATP and AMP

The interaction of Cu(salgly)(H 2O) (1), Cu(sal-α-ala)(H 2O) (2), Na(salglygly)Cu(II) (3), Cu(glygly(H 2O) (4), [Cu(salen)H 2O)](ClO 4) (5), [Cu(sal-N-Meen)](ClO 4) (6), and [Cu(sal-N,N-Me 2en)(H 2O)](ClO 4) (7) with ATP and AMP has been studied using FTIR, ligand field and EPR spectroscopy, and 1H NMR line-broadening techniques. These complexes interact equatorially with ATP through the phosphate group and with AMP through N(7), forming 1:1 species and stabilizes it from dephosphorylation. The extent of interaction of the complexes with ATP decreases in the order 1 >- 2 > 7 > 3 >- 4. EPR studies also indicate the formation of ternary species involving the γ-phosphate group of ATP and N(7) of AMP. However, the preferential broadening of H(8) resonance for all the complexes indicates the binding of ATP through N(7) of the purine base moiety and possibly through phosphate also. ATP was found to be dephosphorylated by [Cu(salen)H 2O)] + and [Cu(sal-N-Meen)] + complexes. Our observations indicate that the primary amino group having the potential to form hydrogen bonds with ATP coupled with the positive charge on the chelate in addition to N(7) coordination may facilitate dephosphorylation.

Pressure dependence of the rate constants for the electron/atom self-exchange between MII (cp2 and MIV) (cp)2 = X+ (M = ruthenium, osmium; X = bromide, iodide; cp = cyclopentadienide) as a function of solvent

The rate constants for the atom/electron self-exchange between Ru(cp) 2 and Ru(cp) 2 Br + (cp represents the cyclopentadienide anion), Ru(cp) 2 and Ru(cp) 2 I + , and Os(cp) 2 and Os(cp) 2 I + were measured by 1 H NMR line-broadening techniques as a function of pressure and solvent

Alkene exchange kinetics in bis(.eta.2-alkene)(2,4-pentanedionato)rhodium(I) investigated by NMR spectroscopy

The alkene exchange kinetics in bis(η 2 -alkene) (2,4-pentanedionatro) rhodium(I) has been studied by 1 H and 13 C NMR line-broadening techniques for ethylene and cis-2-butene. The ethylene exchange goes by an overall second-order reaction, first order in both complex and ethylene

Electron self-exchange study of the coordination-number-invariant pentacoordinate copper(I/II) couple [CuI,II((5-meimidh)2DAP)]+/2+

The electron self-exchange rate of the [Cu I,II ((5-MeimidH) 2 DAP)] +/2+ cation coupled has been determined in CD 3 CN, as a function of temperature, by dynamic NMR line-broadening techniques

Ligand-substitution and electron-transfer reactions of pentacoordinate copper(I) complexes

The kinetics of the electron self-exchange for the Cu(I)/Cu(II) couple of the pentacoordinate complex of 2,6-bis(1-((2-imidazol-4-ylethyl)imino)ethyl)pyridine ((imidH)/sub 2/DAP) has been studied in CD/sub 3/CN by dynamic NMR line-broadening techniques. The rate constant, k/prime//sub 22/ is 1.31 /times/ 10/sup 4/ M/sup /minus/1/ s/sup /minus/1/ at 25/degree/C and an ionic strength of 22.3 mM (Me/sub 4/NBF/sub 4/). Correction for ion pairing gives k''/sub 22/ = 2.8 /times/ 10/sup 4/M/sup /minus/1/ s/sup /minus/1/ at /mu/ = 38 mM and 25/degree/C, which is a factor of 10 greater than that for the (Cu((py)/sub 2/DAP))/sup n+/ analogue. With the use of these two self-exchange rate constants, the Marcus cross relationship accurately predicts the measured cross-exchange rate constant. Transfer of the ligand from (Cu/sup I/((py)/sub 2/DAP))/sup +/ to (Zn/sup II/(CH/sub 3/CN)/sub 6/)/sup 2+/ at /mu/ = 20 mM was investigated at 25/degree/C in CH/sub 3/CN by using the stopped-flow technique. With excess Zn(II), saturation kinetics were observed; this behavior is attributed to rate-limiting dissociation of the copper complex with a rate constant of 310 s/sup /minus/1/. These results are considered in evaluating the possibility of an inner-sphere mechanism of electron transfer. 33 refs., 6 figs., 3 tabs..

Alignment effects in Ca–He(5 1P1–5 3P J) energy transfer collisions by far wing laser scattering

The far wing absorption profiles for excitation on the Ca(4s2 1S0–4s5p 1P01) atomic transition, broadened in collisions with He are measured. We observe strong absorption in both wings and a blue wing satellite near Δ∼125 cm−1. We tentatively identify this satellite as due to a maximum in the CaHe(4s2 1∑+–4s5p 1∑+) difference potential. These line-broadening techniques are used to study electronic energy transfer in the spin-changing collisions of Ca with He: Ca(5p 1P01) +He→Ca(5p 3P0J)+He+ΔE. Measurements of the ‘‘single collision’’ triplet–singlet branching ratio as a function of laser detuning from the atomic resonance frequency indicate a clear red wing/blue wing asymmetry. We interpret this asymmetry in terms of a preferential orbital alignment effect in the energy transfer process [Phys. Rev. Lett. 53, 2296 (1984)]. No clear structure is observed in the range of detunings probed that might indicate the curve crossing responsible for the energy transfer.

Time-resolved electron density measurements in rare-gas-halide laser discharges

Time-resolved spectroscopic measurements are described of the evolution of electron density and other parameters in a UV-preionised XeCl laser discharge. Measurements of electron density by line-broadening techniques indicate the existence of a transition in the nature of the discharge from glow-like to arc-like. Independent measurements of intensities of ionic spectral lines support the existence of a phase of constriction during a period of falling global electron density.

Dislocation-depth distribution in high-temperature creep

The dislocation density as a function of depth from the surface was determined by X-ray diffraction line-broadening techniques on a specimen of 1100 Al allowed to creep to various strains at a temperature above T /sub m/ /2. The dislocation density was found to be higher in the surface layer, /rho/ /sub s/ , than in the interior, /rho/ /sub i/ . With increasing strain, /rho/ /sub s/ increased by /rho/ /sub i/ remained constant. During the annealing of the strained specimens, the recovery occurred in two stages: a fast stage (U /sub I/ ) and a slow stage (U /sub II/ ). The activation energy U /sub I/ equaled 14.3 kJ/mole, whereas U /sub II/ equaled 127.9 kJ/mole. The rate of decrease of dislocations in the interior was impeded by the presence of the surface layer.

Petrology, mineralogy, geochemistry, and origin of the Precambrian Aravallian phosphorite deposits of Udaipur and Jhabua, India

The late Precambrian phosphorite deposits of the Aravalli Mountain belt occur as discontinuous outcrops within dolomitic limestone and silicified dolomite of the Aravalli Supergroup. They extend from Udaipur in the north to Jhabua in the south. Phosphorites concentrated in algal stromatolitic columns constitute the bulk of the deposit. Petrological, geochemical, and mineralogical studies of the phosphorites around Matoon (including Kharwaria), Kanpur, and Jhamarkotra of the Udaipur district and Khatama, Kelkua, and Amlamal in the Jhabua district have been carried out and the results have been incorporated in this paper.Matoon and Kharwaria phosphorites are stromatolitic, fragmental rocks, composed essentially of microsphorite (authigenic microcrystalline phosphorite mud), embedded in carbonate-chert matrix. Phosphates occur as nodules, intraclasts, and irregular fragments. Kanpur phosphorite is confined to algal stromatolitic columns and is composed of microsphorite grains embedded in a dolomicrite groundmass. Jhamarkotra phosphorite displays a larger spectrum of petrographic types, including algal microsphorite in a micritic groundmass, fragmental phosphatic intraclasts with recrystallized calcareous cement, and microsphorite clasts embedded in silt-sized quartz and pelitic intercalations. Recrystallized apatite crystallites in the fractures of the carbonate host rocks are also common. Phosphorite of Khatama is also microsphorite, rich in silt-sized detritus and pelitic disseminations. In Kelkua, microsphorite layers alternate with chalcedony layers and comprise micrite matrix with some detrital quartz.Bulk chemical analyses reveal the oxide distribution pattern in various deposits. The F/P 2 O 5 ratio is higher in Jhabua compared to the Udaipur deposits. Similarly, Na is more common in Jhabua phosphorite except for a few samples from Udaipur. The 2 content of the bulk rock is more or less uniform except where carbonates constitute a significant fraction of the microsphorite groundmass. 2 determinations in apatite concentrates by the X-ray peak-air method show a low 2 content in most concentrates indicating a very minor substitution in the fluorapatite structure. The most common phosphate mineral identified by X-ray diffraction studies is carbonate-fluorapatite with partial substitution by Mg for Ca in most samples and Na and Mg in samples from Jhabua. Partial substitution for (PO 4 ) by (CO (sub ) 3) is more pronounced in the Kanpur and Jhabua phosphorites. Electron microprobe analysis of minute apatites and discrete crystallites in the microsphorite was utilized for deducing the structural formula of average Precambrian phosphorites of India. The unit cell dimensions indicate a o = 9.362 and c o = 6.868 for Matoon apatites, a o = 9.366 and c o = 6.88 for Kanpur (stromatolitic) apatites, a o = 9.354 to 9.336 and c o -- 6.87 to 6.868 for Jhamarkotra apatites, and a o = 9.35 to 9.36 and c o = 6.88 for average Jhabua phosphorites. Heating of samples to 900 degrees C in an open platinum crucible for one hour resulted in a loss of water of crystallization and a consequent reduction and sharpening of peak heights with some minor shifts of diffraction lines. Crystallite size measurements by line-broadening techniques indicated a size range of a o = 1,500 to 2,000 Aa and c o = 1,700 to 2,000 Aa, whereas SEM photographs suggested a size range between 0.1 to 0.3mu m for the a o and c o axes, respectively. Infrared spectroscopy indicated the presence of one or more discrete carbonate phases mixed with apatite. These data conform with X-ray data presented here, but the CO 2 index does not show any apparent correlation with a o cell dimensions. On the basis of nature of occurrence, and geochemical and petrological characterizations, three important classes of phosphorites have been distinguished. These are (1) stromatolitic-carbonate biostromal phosphorite, (2) massively bedded phosphorite, and (3) fragmental-brecciated phosphorite. Each class is characterized by distinct ranges of P 2 O 5 , CaO, MgO, R 2 O 3 (Fe 2 O 3 and Al 2 O 3 ), and SiO 2 contents. Based on available data it has been postulated that these Aravallian phosphorites formed in protected shallow tidal to intertidal waters. While some stromatolitic assemblages have been recognized as ubiquitous, some deposits display specific and restricted algal forms. Geochemical studies and paleogeographic postulations indicate that the water chemistry in various paleodepressions, where phosphorite accumulated, varied marginally from place to place and supported a primary algal-induced biochemical origin for the microsphorites in these paleobasins. The possibility of phosphorite formation through the transformation of hypophosphites has been suggested as an alternate mechanism to explain the origin of at least some phosphorites of this area.

Rates of hydrogen exchange in thioamides

The rates of acid- and base-catalysed proton exchange in N-methylthioacetamide and of protonation of NN-dimethylthioacetamide have been measured in aqueous solution by using n.m.r. line-broadening techniques. N-Methylthioacetamide shows an increase of ca. 103-fold in the rate constant for hydroxide-catalysed exchange, and a 10-fold decrease in the rate constant for acid-catalysed exchange, relative to N-methylacetamide. Both results are consistent with a considerably greater degree of polarisation in the thioamide. The rate of N-protonation of NN-dimethylthioacetamide is ca. 50 times less than the rate of acid-catalysed proton exchange of N-methylthioacetamide. It is suggested that proton exchange in the latter may proceed via the thionium ion.