Studies on the interaction of selenite and selenium with sulphur donors. Part 5. Thiocyanate

1996 ◽  
Vol 74 (10) ◽  
pp. 1889-1895 ◽  
Author(s):  
Christopher Milne ◽  
John Milne
Keyword(s):  
Raman Spectroscopy ◽  
Nmr Spectroscopy ◽  
Nmr Spectra ◽  
Chemical Exchange ◽  
Acid Oxidation ◽  
Relaxation Processes ◽  
Line Broadening ◽  
Linkage Isomerism ◽  
Selenous Acid ◽  
Scalar Relaxation

The oxidation of thiocyanic acid by selenous acid has been shown to take place in an analogous way to that found for selenous acid oxidation of most thiols and thiolates to give the corresponding selenotrisulfide and disulfide products[Formula: see text]The products of this reaction have been studied by IR, Raman, and Se-77, C-13, and N-15 NMR spectroscopy. The vibrational spectra of the solid Se(SCN)2 have been assigned based on S-bonded thiocyanate ligands and, through NMR and vibrational spectroscopy, both this linkage isomer and the isomer with N- and S-bonded thiocyanate have been identified and characterized in solutions of Se(SCN)2. A complex combination of scalar relaxation processes, involving chemical exchange and quadrupolar effects, accounts for the line broadening observed in the carbon-13 NMR spectra. Key words: selenite, thiocyanate, selenium dithiocyanate, Se-77, N-15, and C-13 NMR; Raman spectroscopy; linkage isomerism

Studies on the interaction of selenite and selenium with sulphur donors. Part 4. Thiosulfate

1996 ◽  
Vol 74 (5) ◽  
pp. 753-759 ◽  
Author(s):  
Shermin Rahim ◽  
John Milne
Keyword(s):  
Raman Spectroscopy ◽  
Nmr Spectroscopy ◽  
Key Words ◽  
Bond Formation ◽  
Acid Solutions ◽  
Linkage Isomers ◽  
Linkage Isomerism ◽  
Selenous Acid ◽  
Direct Attack ◽  
Incoming Ligand

Raman and Se-77 NMR spectroscopy confirm that when selenous acid is reduced by thiosulfate in water selenopentathionate and tetrathionate are formed.[Formula: see text]Depending upon the stoichiometry and pH, two isomers of the selenopentathionate ion, O- and S-bonded, are formed. Insufficiently acid solutions cause decomposition to selenium and tetrathionate ion.[Formula: see text]Fresh solutions prepared from crystalline sodium selenopentathionate and water undergo slow decompositon. NMR and Raman spectra show the presence of both the O-bonded and S-bonded linkage isomers. The O-bonded isomer facilitates the formation of tetrathionate. Addition of thiosulfate to selenotrithionate solution or sulfite to selenopentathionate solution yields trithionate with no indication of dithionate or tetrathionate formation. This suggests that simple S—S bond formation at selenium does not occur but that there may be direct attack of the incoming ligand on the attached ligand. Key words: selenite, thiosulfate, selenopentathionate, Se-77 NMR, Raman spectroscopy, linkage isomerism.

Studies on the interaction of selenite and selenium with sulphur donors. Part 2. A kinetic study of the reaction with 2-mercaptoethanol

1994 ◽  
Vol 72 (12) ◽  
pp. 2506-2515 ◽  
Author(s):  
Wimal Amaratunga ◽  
John Milne
Keyword(s):  
Reaction Mechanism ◽  
Nmr Spectroscopy ◽  
Nmr Spectra ◽  
Nucleophilic Attack ◽  
Elemental Selenium ◽  
Sulfinic Acid ◽  
Reaction Stage ◽  
Second Stage ◽  
Selenous Acid ◽  
Two Stages

Selenous acid is reduced by 2-mercaptoethanol in water to elemental selenium in two stages.[Formula: see text]The reactions have been followed by spectrophotometry and by proton and 77Se NMR spectroscopy. The mechanism of the first stage parallels that proposed for butyl thiols in 60% dioxane but nucleophilic attack by RS− rather than RSH on the Se(IV) intermediate, RSSeO2−, is of greater significance for 2-mercaptoethanol than for n-butylthiol. The signals of three intermediates are observed in the Se-77 NMR spectra of a mixture undergoing the first reaction stage. These arise from, HOC2H4SSeO2−, (HOC2H4S)2SeO, and an asymmetric intermediate that chemical shift evidence suggests is the sulfinic acid ester, HOC2H4S(O)SeSC2H4OH. The second stage involves coordination of the bis(thio)selenide by thiolate followed by decomposition to disulfide and selenium. Evidence is presented to support a reaction mechanism involving nucleophilic attack exclusively at Se(II) and not at S(II) as suggested in previous work. This stage is strongly accelerated at basic pH and, at pH 8.3 and above, both the first and second stages can take place simultaneously.

Studies on the interaction of selenite and selenium with sulfur donors. Part 3. Sulfite

1995 ◽  
Vol 73 (5) ◽  
pp. 716-724 ◽  
Author(s):  
Sheila Ball ◽  
John Milne
Keyword(s):  
Raman Spectroscopy ◽  
Aqueous Solutions ◽  
Nmr Spectra ◽  
Formation Constants ◽  
The Other ◽  
Elemental Selenium ◽  
Ambient Temperatures ◽  
Selenous Acid ◽  
First Time ◽  
Sulfite Solution

Elemental selenium dissolves in sulfite solution to form selenosulfate ion: Se + SO32− = SeSO32−.The formation constants for this equilibrium at temperatures from 0 to 35 °C are reported for the first time. The isomeric thioselenate anion, SSeO32−, is not, however, produced by the reaction of sulfur with selenite nor is the selenoselenate ion, Se2O32−, formed from selenium and selenite. Selenotrithionate is formed rapidly from the reaction of selenous acid with sulfite and hydrogen sulfite according to: HSeO3− + 3 HSO3− = Se(SO3)22− + SO42− + 2H2O.Two isomers of the selenotrithionate ion are observed by Se-77 NMR and Raman spectroscopy, one with O-bonded Se, Se(OSO2)22−, and the other with S-bonded Se, Se(SO3)22−. Both isomers are formed in reactions with hydrogen sulfite but only the O-bonded isomer is formed in sulfite solutions at ambient temperatures. The Raman and Se-77 NMR spectra of the various sulphur–selenium anions formed are given and the parallel with the reactions of selenous acid and thiols is discussed. Keywords: selenium, sulfite, selenosulfate, selenotrithionate, Se-77 NMR, Raman spectroscopy, equilibria, aqueous solutions.

Hexachlorotellurate(IV) hydrolysis equilibria in hydrochloric acid. Measurement by Raman and 125Te NMR spectroscopy and a reconsideration of earlier spectrophotometric results

1991 ◽  
Vol 69 (6) ◽  
pp. 987-992 ◽  
Author(s):  
J. B. Milne
Keyword(s):  
Raman Spectroscopy ◽  
Nmr Spectroscopy ◽  
Hydrochloric Acid ◽  
Key Words ◽  
Nmr Spectra ◽  
Ion Pairing ◽  
Hydrolysis Constant ◽  
Visible Spectrophotometry ◽  
Hydrolysis Constants ◽  
Uv Visible

Solutions of TeO2 (0.001 M) in HCl over a range of concentrations are shown to contain TeCl2(OH)2 in addition to TeCl62−and TeCl4(OH)−. The hydrolysis constants for TeCl62− and TeCl4(OH)− have been determined from a reconsideration of earlier UV-visible spectrophotometric results (1)[Formula: see text]The hydrolysis constants have also been determined by quantitative Raman spectroscopy (K1 = 2.21 (± 0.16) × 104 M3; K2 = 442 ± 57 M3). The agreement between K2 determined by the two methods is good but K1 from spectrophotometry is much larger than that from Raman studies. This disagreement is attributed to ion-pairing which will be significant at the concentration at which the Raman measurements were made (c(TeO2) = 0.50 M). The hydrolysis constant for TeCl62− was also determined from the 125Te NMR spectra of TeO2/HCl solutions, which consisted of one signal, indicating rapid Te exchange between all species. The hydrolysis constant, determined in this way (K1 = 1.92 (± 0.07) × 104 M3) was also low compared to that determined from spectrophotometry. Key words: solution, equilibria, tellurium, hydrochloric acid, Raman spectroscopy, 125Te NMR spectroscopy, UV–visible spectrophotometry.

scholarly journals Dynamical Models of Chemical Exchange in Nuclear Magnetic Resonance Spectroscopy

2021 ◽  
Author(s):  
Nicolas Daffern ◽  
Christopher Nordyke ◽  
Meiling Zhang ◽  
Arthur G. Palmer ◽  
John E. Straub
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Nmr Spectroscopy ◽  
Magnetic Resonance ◽  
Chemical Exchange ◽  
Random Phase ◽  
Rate Theory ◽  
Resonance Spectroscopy ◽  
Line Broadening ◽  
Barrier Crossing ◽  
Nuclear Magnetic

ABSTRACT Chemical exchange line broadening is an important phenomenon in nuclear magnetic resonance (NMR) spectroscopy, in which a nuclear spin experiences more than one magnetic environment as a result of chemical or conformational changes of a molecule. The dynamic process of chemical exchange strongly affects the sensitivity and resolution of NMR experiments and increasingly provides a powerful probe of the interconversion between chemical and conformational states of proteins, nucleic acids, and other biologic macromolecules. A simple and often used theoretic description of chemical exchange in NMR spectroscopy is based on an idealized 2-state jump model (the random phase or telegraph signal). However, chemical exchange can also be represented as a barrier crossing event that can be modeled by using chemical reaction rate theory. The timescale of crossing is determined by the barrier height, the temperature, and the dissipation modeled as collisional or frictional damping. This tutorial explores the connection between the NMR theory of chemical exchange line broadening and strong collision models for chemical kinetics in statistical mechanics. Theoretic modeling and numeric simulation are used to map the rate of barrier crossing dynamics of a particle on a potential energy surface to the chemical exchange relaxation rate constant. By developing explicit models for the exchange dynamics, the tutorial aims to elucidate the underlying dynamical processes that give rise to the rich phenomenology of chemical exchange observed in NMR spectroscopy. Software for generating and analyzing the numeric simulations is provided in the form of Python and Fortran source codes.

Muscle 31P-NMR in humans: estimate of bias and qualitative assessment of ATPase activity

1991 ◽  
Vol 71 (5) ◽  
pp. 1700-1704 ◽  
Author(s):  
T. Binzoni ◽  
P. Cerretelli
Keyword(s):  
Nmr Spectroscopy ◽  
Nmr Spectra ◽  
31P Nmr ◽  
Chemical Exchange ◽  
Relaxation Times ◽  
Lattice Relaxation ◽  
Spin Lattice Relaxation ◽  
Qualitative Assessment ◽  
Longitudinal Magnetization ◽  
31P Nmr Spectra

A mathematical model is developed whereby the longitudinal magnetization of phosphocreatine (PC), ATP, Pi, and total phosphate (PT) can be calculated on the basis of assumed chemical rate constants (kappa i) and spin lattice relaxation times of the muscle PC in equilibrium ATP in equilibrium Pi exchange system. By means of this model, some unexplained 31P nuclear magnetic resonance (NMR) spectroscopy results from the literature (e.g., a decrease of PT in a closed system) could be explained simply on the basis of the physiological variability of kappa i. Moreover, appropriate model simulations indicate that 1) 31P-NMR spectra obtained with short relaxation delays may be influenced to various extents by the metabolic and physicochemical status of the muscle; 2) the assessment of kappa i by standard NMR spectroscopy techniques may be extremely critical; 3) delta PC/delta Pi, as obtained from conventional 31P-NMR spectra, may represent a sensible index of kappa 2 (the pseudo first-order chemical exchange rate constant of the adenosinetriphosphatase reaction); 4) delta PC/delta Pi changes as detected from sequential (short relaxation delays) 31P-NMR spectra obtained in humans during metabolic transients (e.g., during transition from rest to work and vice versa) may represent an index of transient changes of kappa 2.

scholarly journals Towards resolving the complex paramagnetic NMR spectrum of small laccase: Assignments of resonances to residue specific nuclei

2020 ◽  
Author(s):  
Rubin Dasgupta ◽  
Karthick B. S. S. Gupta ◽  
Huub J. M. de Groot ◽  
Marcellus Ubbink
Keyword(s):  
Active Site ◽  
Nmr Spectra ◽  
Chemical Exchange ◽  
Streptomyces Coelicolor ◽  
Paramagnetic Nmr ◽  
Line Broadening ◽  
Active Site Residues ◽  
Nmr Spectrum ◽  
Exchange Processes ◽  
Copper Ligands

Abstract. Laccases efficiently reduce dioxygen to water in an active site containing a tri-nuclear copper centre (TNC). One reason for its efficiency in catalysis of this complex reaction can be the presence of mobility of active site residues. To probe mobility, NMR spectroscopy is highly suitable. However, several factors complicate the assignment of resonances to active site nuclei in laccases. The paramagnetic nature causes large shifts and line broadening. Furthermore, the presence of slow chemical exchange processes of the imidazole rings of copper ligands result in peak doubling. A third complicating factor is that the enzyme occurs in two states, the native intermediate (NI) and resting oxidized (RO) states, with different paramagnetic properties. The present study aims at resolving the complex paramagnetic NMR spectra of the TNC of Streptomyces coelicolor small laccase (SLAC). With a combination of paramagnetically tailored NMR experiments, all eight His Nδ1 and Hδ1 resonances for the NI state are identified, as well as His Hβ protons for the RO state. With the help of second shell mutagenesis, selective resonances are tentatively assigned to the T2 histidines. This study demonstrates approaches that can be used for sequence specific assignment of the paramagnetic NMR spectra of ligands in the TNC that ultimately may lead to a description of the underlying motions.

Chromatographic NMR spectroscopy: the effect of hollow silica microspheres on magnetic field inhomogeneities and resonance lineshapes

2020 ◽  
Vol 22 (37) ◽  
pp. 21383-21392
Author(s):  
Federico De Biasi ◽  
Federico Moro ◽  
Diego Frezzato ◽  
Federico Rastrelli
Keyword(s):  
Magnetic Field ◽  
Nmr Spectroscopy ◽  
Nmr Spectra ◽  
Line Broadening ◽  
Silica Microspheres ◽  
Hollow Silica

We provide a description of the line broadening phenomenon observed in chromatographic NMR spectra with hollow silica microspheres.

Synthesis of chloro(2-methylimidazole) ruthenium(III) complexes and their aqueous solution chemistry, and the crystal structure of [2-MeImH]2[RuCl52-MeIm]

2001 ◽  
Vol 79 (10) ◽  
pp. 1477-1482 ◽  
Author(s):  
Craig Anderson
Keyword(s):  
Crystal Structure ◽  
Aqueous Solution ◽  
Nmr Spectroscopy ◽  
Nmr Spectra ◽  
Solution Chemistry ◽  
Proton Nmr Spectroscopy ◽  
Line Broadening ◽  
Reaction Conditions ◽  
H Nmr ◽  
Paramagnetic Complexes

2-Methylimidazole (2-MeIm) reacts with RuCl3 in aqueous acidic ethanolic medium to give (2-MeImH)2[RuCl5(2-MeIm)] (1) and (2-MeImH)[RuCl4(2-MeIm)2] (2) (2-MeImH = protonated 2-methylimidazole), the ratio depending on reaction conditions used. Molecule 1 crystallizes in the space group Pnma: a = 14.046(2), b = 17.294(2), and c = 8.2778(12) Å. The 1H NMR spectra of these ruthenium(III) complexes have been measured and show peaks with large isotropic shifts and large line broadening characteristic of such paramagnetic complexes. The aquation of complexes 1 and 2 were followed by proton NMR spectroscopy. 1,2-Dimethyl imidazole (1,2-diMeIm) reacts with RuCl3 in methanolic solution to give [RuCl3(1,2-diMeIm)(H2O)S] (S=H2O (3a) or CH3OH (3b)). The aquation reactions of complexes 3a and 3b were followed by 1H NMR.Key words: ruthenium, paramagnetic, antitumour, NMR.

NMR spectroscopic investigation of p-substituted 2,4,4,6-tetraphenyl-1,4-dihydropyridines and their oxa and thia analogues

1989 ◽  
Vol 54 (7) ◽  
pp. 1854-1869 ◽  
Author(s):  
Marián Schwarz ◽  
Petr Trška ◽  
Josef Kuthan
Keyword(s):  
Nmr Spectroscopy ◽  
Trifluoroacetic Acid ◽  
Spectroscopic Investigation ◽  
Nmr Spectra ◽  
Dynamic Equilibrium

The 1H, 13C and 19F NMR spectra of photochromic p-substituted 2,4,4,6-tetraphenyl-1,4-dihydropyridines IIa-IIg, 1-methyl-2,4,4,6-tetraphenyl-1,4-dihydropyridines IIIa-IIIg, 2,4,4,6-tetraphenyl-4H-pyrans IVa-IVh, and 2,4,4,6-tetraphenyl-4H-thiopyran V were inspected; it was found that compounds IIa-IIg occur in a dynamic equilibrium with their dihydro tautomer VIa-VIg. Also deuteriodeprotonation of IIa and IIIa and their reaction with trifluoroacetic acid were investigated by NMR spectroscopy.

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