Rapid Chemical Exchange Research Articles

109Ag NMR-Study of the Selective Solvation of the Ag+ Ion in Solvent Mixtures of Water and the Organic Solvents: Pyridine, Acetonitrile, and Dimethyl Sulfoxide

In mixtures of water (W) and one of the organic solvents pyridine, acetonitrile, and dimethyl sulfoxide (O), the silver ion forms the following solvate complexes: AgW2, AgWO, and Ag02. The chemical shift of 109Ag is strongly affected by the ligating solvent molecules, and replacing the ligand W by one of the three organic ligands yields a higher Larmor frequency. In solvent mixtures, only a single resonance line has been observed because of rapid chemical exchange. The measured chemical shifts in the range up to 400 ppm are mean values of the chemical shifts of the different solvate species in a given mixture, weighted with their relative concentrations. The 109Ag chemical shifts were determined for 0.05 to 0.15 molal solutions of AgNO3, as functions of the mole fractions of the solvent components. Using a Gaussian least squares fitting routine, the individual chemical shifts of the Ag+ solvate complexes and the corresponding equilibrium constants were determined. This fit was successful for the whole mole fraction range of DMSO, while in the solvent systems with acetonitrile and with pyridine at higher concentrations of the organic component the chemical shift is influenced by more than two solvent molecules. In these cases equilibrium constants were calculated from chemical shift data for solutions of low mole fraction of acetonitrile and pyridine.

Further analysis of the possible effects of isotope-selective photodissociation on interstellar carbon monoxide

view Abstract Citations (42) References (10) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Further analysis of the possible effects of isotope-selective photodissociation on interstellar carbon monoxide Chu, Y. -H. ; Watson, W. D. Abstract The possible influence of isotope-selective photodissociation on the abundance ratios of the various isotopic forms of the CO molecule in the interstellar medium is reinvestigated. In the study here, chemical fractionation is included in the reaction network. Though isotope-selective photodissociation can in principle alter the 16O to 18O ratios under representative conditions, it seems to have negligible impact on the 13CO/12CO ratios. In addition, the ratio 13C16O/13C18O is found to remain essentially equal to 12C16O/12C18O in the presence of isotope selective photodissociation due also to the rapid chemical exchange of the carbon isotopes. Publication: The Astrophysical Journal Pub Date: April 1983 DOI: 10.1086/160852 Bibcode: 1983ApJ...267..151C Keywords: Carbon Isotopes; Carbon Monoxide; Interstellar Chemistry; Interstellar Gas; Photodissociation; Abundance; Chemical Fractionation; Nuclear Fusion; Reaction Kinetics; Stellar Evolution; Astrophysics full text sources ADS |

The metal NMR spectra of thiolate and phenylselenolate complexes of zinc(II) and mercury(II)

Soluble thiolato-zincates and -mercurates(II) of the general type M(SR) 2− 4 and the phenylselenolato complexes M(SePh) 2− 4 (M = Zn or Hg) have been prepared in situ from the appropriate metal nitrate and excess thiolate or selenolate. The high field metal NMR spectra of these complexes have been measured, at 25.0 and 71.5 MHz for 67Zn and 199Hg respectively, with isotopic enrichment to 89.7 atom% for 67Zn, but at natural abundance for 199Hg. The zinc complexes studied in D 2O solution, were Zn(SR) 2− 4 (R = Me, Pr i, or Ph), Zn??? S S]) 2 2 and Zn(SePh) 2− 4. The variation of δ Zn with ligand was found to be −SePh < −SPh < −Spr i < −SMe š −S(CH 2) 2S −, as has been found earlier for 113Cd also; the shifts for solutions in which [Zn] total = 0.15 M, to higher frequency than the resonance of 2m Zn(ClO 4) 2 at 297 K range from 224 to 418 ppm, and the approximate linewidths from ca. 55 Hz for Zn(SMe) 2− 4 at 368 K to ca. 1200 Hz for Zn(SPr i) 2− 4 at 297 K or Zn??? S S]) 2− 2 at 363 K. The line widths appear to be the major factor limiting the application of 67Zn NMR towards complexes of less symmetrical thiolates; rapid chemical exchange is not a problem, as 1H and 13C NMR studies of the zinc complexes show them to be less labile than their cadmium analogs. Conversely the mercury complexes studied here ▪ ▪(R = H, Me, Et, or CH 2OH), Hg(SR) 4 (R = M=, Et, Pr i, or Ph) and Hg(SePh) 2− 4) are more labile than their cadmium counterparts, and this lability reduces the utility of 199Hg NMR in cases where multiple mercury-binding sites are available. The overall variation of δ Hg with ligand , −SePh < −SPh < −SR (R = alkayl) < chelating vicinal dithiolates, is the same as found in the metal NMR of the corresponding zinc and cadmium complexes, but the (relatively small) effects of alkyl substitution are dissimilar. For ca. 0.1 M solutions in 1:1 (ν:ν) H 2O: D 2O at 297 K, the 199Hg chemical shifts to higher frequency than the signal of 0.1 M Hg(ClO 4) 2 in 5% (ν:ν) HClO 4 range from 1275 ppm in Hg(SePh) 2− 4 to 2234 ppm for ▪ the linewidths are relatively sharp ( e.g. ???4 Hz for Hg(SMe) 2− 4, ???75 Hz for Hg(SPh) 2− 4).

Identification of intercalated species in AsF 5NO 2SbF 6 intercalated graphite using 19F N.m.r.; fluorine exchange between the various species

An intercalated graphite was prepared by reaction of polycrystalline graphite with AsF 5 followed by reaction with NO 2SbF 6. At −73%, four narrow lines are found in the 19F n.m.r. spectrum at +124.1, +112.7, +66.7, and +46.8 ppm (relative to CFCl 3). These lines are attributed to SbF 6 − , SbF 5, AsF 6 − in rapid chemical exchange with AsF 5, and AsF 3, respectively. This demonstrates the use of n.m.r. for the qualitative analysis of the chemical species present in the interlamellar space. As the temperature is increased, the lines broaden (except for that of AsF 3) and subsequently narrow, indicating, further, that chemical exchange of fluorine is occurring among SbF 6 − , SbF 5, AsF 6 − and AsF 5. At 22 °C the exchange rate is ∼ 10 3 s −1 for SbF 6 − and >3 × 10 3 s −1 for the other species. The data are interpreted in terms of a model in which fluorine exchange between SbF 6 − and the fluoro-arsenic species occurs via SbF 5.

109Ag NMR-Studies of the Selective Solvation of the Ag+ -Ion in Water-Ethylamine-Mixtures and of the Coordination of Halide-Anions to the Silver-Ethylamine Solvate Complexes

Abstract 109Ag chemical shift measurements of 0.02 up to 3 molar solutions of AgNO3, AgCl, and AgBr in solvent mixtures of H2O (W) and ethylamine (ea) were performed. The extremely long relaxation times T1 and T2 were determined with new techniques. The Ag+ -ion in solvent mixtures of W and ea shows a strongly selective solvation by ea. The 109Ag chemical shift of the solvate complex [Ag ea2]+ surrounded solely by W is δ2 = (335 ± 2) ppm (referred to the Ag+-ion in W). A further solvation in addition to the inner solvation sphere was determined; this solvation is not or only weakly selective. There is a rapid chemical exchange; the lifetime of the inner solvation sphere is long compared with the Larmor period, whereas the solvation outside this sphere is changed in times shorter than the Larmor period. In contrast to the NO3- -anion, the halide anions Cl- and Br- are partly coordinated to the [Ag ea2]+ complex. The equilibrium constants for this coordination were determined as well as the chemical shifts of the [Ag ea2 · Cl] and the [Ag ea2 · Br] complexes. The bromine ion is coordinated for shorter times than the Larmor period, whereas the time of the coordination of the chlorine ion may be comparable to the Larmor period or shorter.

109Ag Nuclear Magnetic Resonance Studies of Organic and Inorganic Silver Complexes

Abstract The NMR lines of 109Ag have been investigated in solutions of several silver salts in acetonitrile, propionitrile, pyridine, and ethylenediamine, and also in aqueous solutions of Na2S2O3 and ethyl-amine. In these solvents the Ag+ -ions form one or several complexes. In any case a single NMR line was to be detected, i. e. a rapid chemical exchange between different complexes in a sample may be assumed. The concentration dependence of the chemical shifts was determined with high accuracy for these solutions. From these results the chemical shift data of some defined complexes were derived. The chemical shifts of mixtures of AgCl and AgBr and also of AgCl and Agl dissolved in 70% aqueous solution of ethylamine are linear functions of the anions mole fraction. In two samples of AgNO3 dissolved in organic solvents, the ratio υ(109Ag)/υ(107Ag) = 1.149 640 (1) was measured in good agreement with the value from silver salts in aqueous solutions; i. e. no primary isotopic effect was to be detected within these limits of error (0.9 ppm).

Investigation of the complex formation of tetrabutylammonium perchlorate with boron halides by the NMR-11B method

1. The complex formation in the systems tetrabutylammonium perchlorate — BX3 (X=F, Cl, Br) in CHC13 solution was investigated by the NMR-11B method. All the boron halides studied form compounds in solution with the general formula (C4H9)4N[X3BOClO3]. The complexes with BF3 and BCl3 are stable in CHCl3 at ∼20°; the bromide complex breaks down irreversibly. 2. The chemical shift of11B relative to BF3·O(C2H5)2 is 0.0–7.7 and ∼2 ppm for the fluoride, chloride, and bromide complexes, respectively. In all the investigated systems, at room temperature a rapid chemical exchange of the free and complex bound boron halide is observed.

Theory of Molecular Reorientation in Liquids: Magnetic Spin Resonance Line Shapes

The Debye model of rotational Brownian motion and the rotational random jump model have been extended to allow for time fluctuations of the rotational diffusion constant and the jump rate constant, respectively. The treatment is applicable to experimental situations which are conventionally described in terms of a distribution of correlation times and to systems where molecular reorientation is complicated, e.g., by rapid chemical exchange processes. Correlation functions for spin relaxation are discussed in some detail and are compared with the corresponding correlation functions for dielectric relaxation. In particular, the ESR (or NMR) line shape is given for a simple example of very slow reorientation where the rotational correlation time is comparable with the spin relaxation time. In this case, the Debye model and the jump model yield different line shapes, and further changes occur if environmental fluctuations are considered. Details are given for fluctuations described by an Uhlenbeck–Ornstein process.

A nuclear magnetic resonance study of sodium ion interaction with erythrocyte membranes

Sodium-23 nuclear magnetic resonance line widths have been used to study sodium ion interaction with erythrocyte membranes and erythrocyte membrane proteins. Detergent solubilization has been effective in freeing sodium and potassium binding sites which are not available for rapid chemical exchange with the whole membrane. Evidence for sodium ion binding has been obtained by showing that the resonance line width increases in the presence of solubilized membrane. Competition by potassium for binding sites is demonstrated by a decrease of the sodium line width in the presence of potassium ion.

Study of Moderately Rapid Chemical Exchange Reactions by Means of Nuclear Magnetic Double Resonance

A nuclear magnetic double-resonance method for the determination of chemical exchange rates has been developed. The method is applicable to systems in which a nuclear spin is reversibly transferred between two nonequivalent sites, A and B. The lifetime (τA) and spin—lattice relaxation time (T1A) in Site A are obtained through the study of the decay to a new equilibrium value of Signal A upon the sudden saturation of Signal B. The converse experiment permits the determination of τB and T1B. A number of data for cross checks are furthermore obtained through the study of the recovery of the signals upon the release of various combinations of saturating rf fields. A simple theory based on the Bloch equations as modified by McConnell to incorporate the effects of chemical exchange is given. Experimental results on the hydroxyl proton exchange in the system salicylaldehyde and 2-hydroxyacetophenone are well described by this simple theory. The present method, which can readily be extended to systems with several sites, offers a complement to the Gutowsky—Saika single-resonance method and is particularly suited to the study of exchange rates slower than those accessible by the single-resonance method.

Theory of Linewidths in Electron Spin Resonance Spectra

A general theory of the linewidths in the electron spin resonance spectra of dilute solutions of free radicals has been developed in terms of the relaxation-matrix theory of Bloch, Redfield, and Ayant. In contrast to previous theories, it is shown that a composite line arising from a set of degenerate nuclear-spin states should, in general, consist of a sum of superimposed lines of Lorentzian shape with different widths rather than a single line with an over-all Lorentzian shape. A single Lorentzian line is still obtained, however, as a limiting case when the variation of the widths of the different components of a composite line is small compared to the average width. Although the non-Lorentzian shape of a composite line is often difficult to observe experimentally, a number of other observable properties are predicted by the present development that are outside the scope of the previous theories. For example, linewidth effects resulting from differences in the widths of the separate components of a composite line are predicted that explain the alternation in the linewidths from one hyperfine line to another recently observed in the ESR spectra in certain free radicals. The detailed form of the relaxation matrix is presented for intramolecular anisotropic and isotropic electron—nuclear dipolar interactions, quadrupole interactions, and g-tensor relaxations. Modulations of the spin density and hyperfine splittings are included, as are internal motions, and a number of cross terms between the different relaxation mechanisms arise. In general the relaxation matrix of a composite line contains significant off-diagonal elements, and the determination of the linewidths requires the evaluation of the eigenvalues of the matrix. Problems involving rapid chemical exchange, or modulation by jumps to a small number of sites, can be treated by the relaxation-matrix theory and, under special restrictions, by either the modified Bloch equations or the Anderson theory of motional narrowing. When applicable, these latter procedures can be used over the entire range of exchange rates, while the relaxation-matrix theory is limited to fast rates only.

A New Method for the Study of Moderately Rapid Chemical Exchange Rates Employing Nuclear Magnetic Double Resonance.

A New Method for the Study of Moderately Rapid Chemical Exchange Rates Employing Nuclear Magnetic Double Resonance.

Potential Energy Barrier Determinations for Some Alkyl Nitrites by Nuclear Magnetic Resonance

A technique for studying rapid chemical exchange and reorientation reactions by nuclear magnetic resonance techniques is discussed. A treatment for the general case relating the changes in the proton line width with changes in the frequency of rotation about the O–N bond in the alkyl nitrites is developed. Such a treatent gave fairly accurate potential energy barriers to rotation for methyl, ethyl, n-propyl, and isopropyl nitrite. A temperature controlling device for low temperatures applicable to proton resonance techniques is discussed.

Nuclear Magnetic Interactions in the HF Molecule

Magnetic relaxation times of hydrogen and fluorine in anhydrous hydrofluoric acid cannot be accounted for on the assumption of a pure dipole-dipole interaction between the hydrogen and the fluorine nuclei in the same molecule. However, the introduction, in the Hamiltonian, of a scalar term AI·S resulting from the indirect electron-coupled interaction between the two nuclei removes all the discrepancies between the calculated and observed decay times. Although the splitting due to this scalar term is smeared out by the rapid chemical exchange of the protons with the traces of water present in the acid, the nuclear Overhauser effect provides the extra parameter required to separate the dipole-dipole interaction from the scalar interaction and to calculate separately the scalar splitting and the exchange rate of the protons. The value obtained for the splitting is A/h=615 cps. The same method has been applied to investigate the structure of the HF molecule in solutions. The experimental results can be explained qualitatively by the following picture: (a) The fluorine nuclei form long chains that move more slowly than the Larmor frequency of fluorine. (b) The chemical exchange with the relatively large quantity of water present makes the motion of the hydrogen nuclei fast with respect to the hydrogen Larmor frequency. The results are: Long relaxation times for hydrogen with T1 and T2 equal, relatively short relaxation times for fluorine resonance, with T2 shorter than T1. Solutions of salts of hydrofluoric acid (KF) show the same character as solutions of the acid.