Intramolecular Chemical Exchange Research Articles

Ion clustering, aggregation and diffusion in amide based deep eutectic solvents: A microstructural investigation using various NMR spectroscopic techniques and molecular dynamics simulation

A thorough understanding of the structure and dynamics of deep eutectic solvents (DESs) is necessary to optimize the uses of this new class of solvents for many potential applications. Herein, we have studied ion clustering, aggregation and diffusion mechanism in DESs composed of acetamide and sodium/potassium thiocyanate using 1D NMR, pulsed field gradient stimulated echo (PFGSE) NMR and 2D nuclear Overhauser effect spectroscopy (2D NOESY) techniques, and molecular dynamics simulation. We have varied the fraction of salts to investigate the composition dependent microscopic structural insight. A great enhancement in diffusion coefficient (D) of acetamide with increasing the fraction of KSCN (fKSCN) has been observed in PFGSE measurements. The experimental diffusion activation energy (Ea) decreases significantly with increasing fKSCN, which is in excellent agreement with the Ea values obtained from simulated mean squared displacements (MSD). The calculated radial distribution functions (RDF) indicate that the Na+ ions strongly coordinate with acetamide molecules and form ion-clusters and aggregated structures in the medium. However, as Na+ ions are gradually replaced by K+ ions, ion-clustering and aggregation decrease due to relatively weak coordination ability of larger cation, which supports the enhancement of experimental D with increasing fKSCN. In the 2D NOESY study, the intensity of the cross peaks between methyl and amide protons of acetamide decreases relative to the diagonal peak intensity as the fKSCN increases in the medium. This study indicates that the extent of intermolecular NOE reduces as the structural flexibility increases due to less efficient ion-clustering with increasing K+ ion concentration, thus strongly corroborates the results from PFGSE experiment and simulation. Furthermore, 1D and 2D NMR data exhibit an intramolecular chemical exchange between two amide protons in acetamide, the extent of which increases with increasing temperature as well as fKSCN in the DES system. The present study provides a multifaceted picture of composition-dependent distinct microstructures in DES.

In situ simultaneous electrochemical ESR study of radicals generated from 2,2-dinitroethene-1,1-diamine (FOX-7). Intramolecular chemical exchange resulting in an alternation line-width effect

The molecule of 2,2-dinitroethene-1,1-diamine (FOX-7) is one of the most interesting molecules with multiple redox centres stabilized by push-pull effect. To reveal the detailed mechanism of its electrochemical process radical intermediates formed in the course of its electrochemical reduction in organic aprotic media have been studied by in situ simultaneous electrochemical ESR measurements (SEESR). The radical generated on the second reduction step possesses an alternating line-width (AL) effect in the ESR spectra as a result of intramolecular dynamic processes in the timescale of ESR splitting constants. The spectra measured at different temperatures (230–335 K) were analysed with the help of a fitting program which includes a molecular dynamics. Observed dynamics describes well an asymmetric 2-site exchange model for the whole temperature range. With help of the optimized parameters and quantum chemical calculations this radical has been identified as 2,2-dinitroethane-1-amine-1-imine radical dianion, [(H2N)(HN)C=C(NO2)2]2−. The dynamic process responsible for the AL effect consists of mutual turning (changing of dihedral angle) of the both nitro groups, resulting in an intramolecular spin-density (electron) transfer. The dynamic parameters of the process have been established.

Suppression Mechanism for Enol–Enol Isomerization of 2‐Substituted Dimedones

Abstract2‐Substituted dimedones, described as having a simple chemical structure, are a type of cyclic β‐diketone with enol–enol interconversion. The influence of the enol‐enol isomerism interferes with structure analysis of many dimedones in solution by NMR spectroscopy. It was demonstrated that the isomerism of the 2‐substituted dimedones depends on solvent, time, solute concentration, and temperature. Negative activation energy of the isomerization was determined by the NMR line shape and spin‐spin relaxation analyses, which is an extraordinary phenomenon found for the first time in the molecular dynamics of NMR. Suppression of the isomerization by heating, dilution of the solute, and time control in several low‐polarity solvents enabled analysis of the enol form in detail. It was clarified that deceleration of the intramolecular chemical exchange rate in the isomerization was responsible for the decrease in the intermolecular hydrogen‐bonding effect in the suppression mechanism.

Kinetics of intramolecular chemical exchange by initial growth rates of spin saturation transfer difference experiments (SSTD NMR).

We report here the Initial Growth Rates SSTD NMR method, as a new powerful tool to obtain the kinetic parameters of intramolecular chemical exchange in challenging small organic and organometallic molecules.

Binuclear Pt–Tl Bonded Complex with Square Pyramidal Coordination around Pt: A Combined Multinuclear NMR, EXAFS, UV–Vis, and DFT/TDDFT Study in Dimethylsulfoxide Solution

The structure and bonding of a new Pt-Tl bonded complex formed in dimethylsulfoxide (dmso), (CN)(4)Pt-Tl(dmso)(5)(+), have been studied by multinuclear NMR and UV-vis spectroscopies, and EXAFS measurements in combination with density functional theory (DFT) and time dependent density functional theory (TDDFT) calculations. This complex is formed following the equilibrium reaction Pt(CN)(4)(2-) + Tl(dmso)(6)(3+) ⇆ (CN)(4)Pt-Tl(dmso)(5)(+) + dmso. The stability constant of the Pt-Tl bonded species, as determined using (13)C NMR spectroscopy, amounts to log K = 2.9 ± 0.2. The (NC)(4)Pt-Tl(dmso)(5)(+) species constitutes the first example of a Pt-Tl bonded cyanide complex in which the sixth coordination position around Pt (in trans with respect to the Tl atom) is not occupied. The spectral parameters confirm the formation of the metal-metal bond, but differ substantially from those measured earlier in aqueous solution for complexes (CN)(5)Pt-Tl(CN)(n)(H(2)O)(x)(n-) (n = 0-3). The (205) Tl NMR chemical shift, δ = 75 ppm, is at extraordinary high field, while spin-spin coupling constant, (1)J(Pt-Tl) = 93 kHz, is the largest measured to date for a Pt-Tl bond in the absence of supporting bridging ligands. The absorption spectrum is dominated by two strong absorption bands in the UV region that are assigned to MMCT (Pt → Tl) and LMCT (dmso → Tl) bands, respectively, on the basis of MO and TDDFT calculations. The solution of the complex has a bright yellow color as a result of a shoulder present on the low energy side of the band at 355 nm. The geometry of the (CN)(4)Pt-Tl core can be elucidated from NMR data, but the particular stoichiometry and structure involving the dmso ligands are established by using Tl and Pt L(III)-edge EXAFS measurements. The Pt-Tl bond distance is 2.67(1) Å, the Tl-O bond distance is 2.282(6) Å, and the Pt-C-N entity is linear with Pt-C and Pt···N distances amounting to 1.969(6) and 3.096(6) Å, respectively. Geometry optimizations on the (CN)(4)Pt-Tl(dmso)(5)(+) system by using DFT calculations (B3LYP model) provide bond distances in excellent agreement with the EXAFS data. The four cyanide ligands are located in a square around the Pt atom, while the Tl atom is coordinated in a distorted octahedral fashion with the metal being located 0.40 Å above the equatorial plane described by four oxygen atoms of dmso ligands. The four equatorial Tl-O bonds and the four cyano ligands around the Pt atom are arranged in an alternate geometry. The coordination environment around Pt may be considered as being square pyramidal, where the apical position is occupied by the Tl atom. The optimized geometry of (CN)(4)Pt-Tl(dmso)(5)(+) is asymmetrical (C(1) point group). This low symmetry might be responsible for the unusually large NMR linewidths observed due to intramolecular chemical exchange processes. The nature of the Pt-Tl bond has been studied by MO analysis. The metal-metal bond formation in (CN)(4)Pt-Tl(dmso)(5)(+) can be simply interpreted as the result of a Pt(5d(z(2)))(2) → Tl(6s)(0) donation. This bonding scheme may rationalize the smaller thermodynamic stability of this adduct compared to the related complexes with (CN)(5)Pt-Tl entity, where the linear C-Pt-Tl unit constitutes a very stable bonding system.

Localized two‐dimensional 1H magnetic resonance exchange spectroscopy: A preliminary evaluation in human muscle

A localized two-dimensional (2D) (1)H MR chemical exchange spectroscopic (L-EXSY) sequence has been implemented on a whole-body 1.5-T MRI/MRS scanner. The second spectroscopic encoding to monitor the chemical exchange was an integral part of the single-volume localization using three slice-selective 90 degrees radiofrequency (RF) pulses, thereby eliminating the need for any additional RF pulses, off-resonance/continuous wave saturation, or selective inversion, which are essential in the one-dimensional (1)H MR exchange spectroscopy. Even though the TM-crusher dephased single- and higher-order multiple-quantum coherences, the zero-quantum coherences were indistinguishable from the longitudinal magnetization leading to J-coupled 2D cross peaks similar to COSY. With TM of 300 ms, two different exchange cross peaks were recorded in human calf muscle: a first peak, between the mobile tissue water and total creatine pools, and a second peak, possibly between the olefinic and magnetically equivalent poly methylene protons of unsaturated lipids. Our preliminary results demonstrate that the intermolecular and intramolecular chemical exchange mechanisms can be monitored noninvasively in human calf muscle using 2D L-EXSY.

Solid and Solution State NMR Spectra and the Structure of the Gallium Citrate Complex (NH4)3[Ga(C6H5O7)2]·4H2O

A gallium citrate complex (NH4)3[Ga(C6H5O7)2]·4H2O (1) has been isolated from an aqueous mixture of gallium nitrate and citric acid at pH = 9. In the solid state 1 is monomeric and includes the near centrosymmetric ion [Ga(C6H5O7)2]3− in the asymmetric unit, with each tridentate citrate coordinating to gallium through an alkoxide oxygen, an oxygen of the central carboxyl group and an oxygen of a terminal carboxyl group. The second terminal carboxyl group is not bound to the metal centre. The two alkoxide oxygen−gallium distances in the asymmetric unit are significantly shorter than the four carboxyl oxygen−gallium distances, indicating a stronger bond for the former. The number and intensities of the resolved resonances in the solid state 13C MAS/NMR spectrum are consistent with the X-ray structure. In aqueous solution near neutral pH (5.5 to 6.4) the complex partially dissociates to give an equilibrium of the 1:1 and 1:2 gallium/citrate species together with free citrate. Two sets of resonances are observed in both the 1H and 13C NMR spectra of solutions of the complex and these correspond to bound and free citrate; there is no resolution of resonances for the 1:1 and 1:2 species. The relative strength of the alkoxide oxygen−gallium bond in the metal-bound citrate ligand leads to a slow intermolecular chemical exchange situation (rate < ca. 12 s−1) between the bound and free ligand. The carboxylate oxygen−gallium bonds are quite labile and relatively rapid intramolecular chemical exchange occurs (rate > ca. 700 s−1) between the metal-bound and pendant terminal carboxyl groups, averaging their 1H and 13C NMR signals. The diffusion coefficients for the free and complexed citrate measured from the 1H NMR spectra do not appear to be complicated by chemical exchange effects and, as expected, the diffusion coefficient for the complexed ligand is smaller than for the free ligand. In solution in the intermediate pH range (4.8 to 6.4) a new 71Ga signal is observed at ca. δ = 27 (Δν1/2 ≈︁ 10.6 kHz) which is assigned to the gallium citrate complexed species.

Relationship between intramolecular chemical exchange and NMR-observed rate constants

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTRelationship between intramolecular chemical exchange and NMR-observed rate constantsMalcolm L. H. Green, Luet Lok Wong, and Andrea SellaCite this: Organometallics 1992, 11, 7, 2660–2668Publication Date (Print):July 1, 1992Publication History Published online1 May 2002Published inissue 1 July 1992https://doi.org/10.1021/om00043a059RIGHTS & PERMISSIONSArticle Views539Altmetric-Citations95LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (985 KB) Get e-Alerts Get e-Alerts

Application of 2D FT-EPR spectroscopy to study slow intramolecular chemical exchange

Application of 2D FT-EPR spectroscopy to study slow intramolecular chemical exchange

ST-EPR spectra of systems undergoing intramolecular chemical exchange

ST-EPR spectra of systems undergoing intramolecular chemical exchange

Chemical Exchange and Transient Effects in an AB-Type NMR Spectrum

A density matrix formulation is used to investigate the transient NMR effects which occur when the rf field H1 is changed discontinuously for a coupled AB system undergoing intramolecular chemical exchange. It is shown that the problem reduces to the solution of two matrix differential equations. Explicit solutions are obtained and discussed for two special cases: (i) H1 = 0 for t &amp;lt; 0, H1 = weak observing field for t &amp;gt; 0; and (ii) H1 = ∞ for t &amp;lt; 0, H1 = weak observing field for t &amp;gt; 0. For Case (I) it is shown that the Anderson–Weiss theory gives the same results. Also, our formulation is used to determine the conditions under which the absorption intensity can be calculated by assuming it to be proportional to the z component of the corresponding nuclear magnetization. Finally, the effect of inhomogeneities in the static field H0 is treated briefly. For a Lorentzian distribution of inhomogeneities, it is shown that the transient as well as steady-state broadening effects can be accounted for by replacing the linewidth parameter (1 / T2) with (1 / T20) + (1 / T2*), where (1 / T2*) gives the width of the inhomogeneity function.