Two-dimensional Exchange NMR Spectroscopy Research Articles

Ex Situ 23Na Solid-State NMR Reveals the Local Na-Ion Distribution in Carbon-Coated Na2FePO4F during Electrochemical Cycling

The potential Na-ion cathode material Na2FePO4F is investigated here by ex situ 23Na solid-state nuclear magnetic resonance (ssNMR) in order to characterize the structure and ion mobility as a function of electrochemical cycling. The use of fast magic angle spinning (MAS) speeds of 65 kHz allows for the collection of high-resolution 23Na NMR spectra that reveal two unique peaks at +450 and −175 ppm, corresponding to the two crystallographically unique Na sites in the material of interest. Two-dimensional NMR exchange spectroscopy results reveal that chemical exchange between the Na ions residing in distinct environments has a maximum hopping rate of ∼200 Hz. The collection of one-dimensional NMR spectra as a function of electrochemical cycling reveals the reproducible formation of a new peak at +320 ppm in the 23Na NMR spectrum at all intermediate states of charge. The appearance of this resonance at +320 ppm is attributed to the fully oxidized (NaFePO4F) phase that is present even upon initial electroche...

Application of selective two-dimensional exchange NMR spectroscopy to the study of molecular dynamic processes

A vector model of the multiplet-selective excitation of spin systems by a three-pulse sequence is considered, and, on this basis, a new two-dimensional exchange NMR spectroscopy technique is suggested. This technique provides means to investigate intramolecular and intermolecular chemical and spin exchanges by observing the dynamic behavior of the multiplet pattern of a nucleus that is scalarwise coupled with the unobservable nucleus undergoing exchange.

Correlation between bowl-inversion energy and bowl depth in substituted sumanenes

AbstractThe correlation between the bowl-inversion energy and the bowl depth for sumanenes monosubstituted with an iodo, formyl, or nitro group was investigated experimentally and by theoretical calculations. The bowl-inversion energies of the substituted sumanenes were determined experimentally by two-dimensional NMR exchange spectroscopy measurements. Various density functional theory methods were examined for the calculation of the structure and the bowl-inversion energy of sumanene, and it was found that PBE0, ωB97XD, and M06-2X gave better fits of the experimental value than did B3LYP. The experimental value was well reproduced at these levels of theory. The bowl structures and bowl-inversion energies of monosubstituted sumanenes were therefore calculated at the ωB97XD/6-311+G(d,p) level of theory. In both the experiments and the calculations, the correlation followed the equation ΔE=acos4θ, whereais a coefficient, ΔEis the bowl-inversion energy, and cos θ is the normalized bowl depth, indicating that the bowl inversion follows a double-well potential energy diagram.

Specialized Hsp70 Chaperone (HscA) Binds Preferentially to the Disordered Form, whereas J-protein (HscB) Binds Preferentially to the Structured Form of the Iron-Sulfur Cluster Scaffold Protein (IscU)

The Escherichia coli protein IscU serves as the scaffold for Fe-S cluster assembly and the vehicle for Fe-S cluster transfer to acceptor proteins, such as apoferredoxin. IscU populates two conformational states in solution, a structured conformation (S) that resembles the conformation of the holoprotein IscU-[2Fe-2S] and a dynamically disordered conformation (D) that does not bind metal ions. NMR spectroscopic results presented here show that the specialized Hsp70 chaperone (HscA), alone or as the HscA-ADP complex, preferentially binds to and stabilizes the D-state of IscU. IscU is released when HscA binds ATP. By contrast, the J-protein HscB binds preferentially to the S-state of IscU. Consistent with these findings, we propose a mechanism in which cluster transfer is coupled to hydrolysis of ATP bound to HscA, conversion of IscU to the D-state, and release of HscB.

Dramatic Enhancement of Hyperpolarized Xenon-129 2D-NMR Exchange Cross-Peak Signals in Nanotubes by Interruption of the Gas Flow

Recently we demonstrated how the exchange dynamics of gas atoms localized near the openings of self-assembled l-alanyl-l-valine (AV) nanotubes can be studied by continuous-flow hyperpolarized xenon-129 two-dimensional NMR exchange spectroscopy (2D-EXSY). We noted that the finite residence time of the hyperpolarized gas inside the sample space suppresses the cross-peak signals associated with any exchange processes involving the free gas. Here we demonstrate that the gas-phase diagonal-peak and exchange cross-peak signals in the hyperpolarized 2D-EXSY spectrum of Xe in AV are dramatically enhanced by interrupting the gas flow during the EXSY pulse sequence. This simple but highly effective modification of the standard continuous flow hyperpolarized 2D-EXSY method is expected to broaden the range of exchange and diffusion processes that can be interrogated in nanoporous materials.

Direct Observation of Atoms Entering and Exiting l-Alanyl-l-valine Nanotubes by Hyperpolarized Xenon-129 NMR

The kinetics of gas --> channel and channel --> gas exchange of Xe in self-assembled L-alanyl-L-valine (AV) nanotubes was facilitated by continuous flow hyperpolarized xenon-129 two-dimensional exchange NMR spectroscopy. Cross-peaks due to gas atoms entering or exiting were observed at Xe pressures of 92, 1320, and 3300 mbar, corresponding to Xe fractional occupancies ranging from 0.047 to 0.64. At each pressure, the rate of desorption from the channels was determined by fitting the mixing time dependence of the cross-peak and diagonal-peak signals to a magnetization exchange model, assuming steady-state Langmuir adsorption under hyperpolarized gas flow conditions. The observed rate constant for desorption of Xe from AV nanotubes decreased from 4.5 s-1 to 2.0 s-1 over the occupancy range studied, a finding that is consistent with a decrease in the diffusivity in the channels.

Formation of Porous Aluminophosphate Frameworks Monitored by Hyperpolarized 129Xe NMR Spectroscopy

The formation of two aluminophosphate frameworks, AlPO4-5 and AlPO4-18, has been monitored using hyperpolarized 129Xe NMR as a probe. The reaction progress from an amorphous phase to AlPO4-5 and its subsequent conversion to AlPO4-18 is observed for xenon adsorbed in samples that have been reacted for varying lengths of time. The extreme sensitivity of 129Xe chemical shifts to local environments of the xenon leads to separate NMR peaks for xenon adsorbed in the two different aluminophosphate frameworks present in a single reaction mixture. For xenon adsorbed in AlPO4-5, an anisotropic 129Xe NMR line shape at δiso ≈ 63 ppm is observed, whereas for xenon in AlPO4-18, a peak with a small anisotropy at δiso ≈ 74 ppm is observed. Grand canonical Monte Carlo simulations provide a theoretical description of xenon adsorbed in the two aluminophosphate frameworks, and the resulting simulated 129Xe NMR line shapes are in good agreement with experimental data. Two-dimensional exchange NMR spectroscopy was used to examine the exchange of xenon between AlPO4-5, AlPO4-18, and bulk xenon gas. The results indicate that the domains of AlPO4-5 are in intimate contact with those of AlPO4-18, facilitating a higher intercrystallite exchange than with the bulk xenon gas.

Isomerization dynamics in homo- and heterochiral atropoisomer copper(I) diimine complexes: a 2D EXSY NMR study.

Two-dimensional exchange NMR spectroscopy has been employed to study the isomerization process of copper(I) complexes formed upon complexation of Cu+ with a racemic mixture of the atropoisomer diimine benzimidazole-pyridine ligands 1-3 and evaluate the configurational stability of the pseudotetrahedral complexes [Cu(1-3)2]PF6. Racemization of the heterochiral isomers RSLambda/RSDelta proceeds through an intramolecular ligand rearrangement on a time scale of about 1.9 s(-1) for 1, 4.4 s(-1) for 2, and 0.3 s(-1) for 3 in CD2Cl2 at room temperature. The intramolecular Lambda/Delta isomerizations in the homochiral diastereoisomers RRDelta/SSLambda and RRLambda/SSDelta of [Cu(1)2]PF6 proceed at room temperature on a time scale of about 0.6 s(-1) for the conversion of RRDelta/SSLambda into RRLambda/SSDelta and 13 s(-1) for the conversion of RRLambda/SSDelta into RRDelta/SSLambda. The kinetics of these intramolecular exchange processes were found to be sensitive to the stabilizing interligand pi-stacking interactions that develop within the [Cu(1-3)2]+ structure and to the bulkiness of the benzimidazole aryl substituents. The kinetics of racemization in the heterochiral RSLambda/RSDelta isomers of [Cu(3)2]PF6 with the bulky cumyl-derived ligand were 1 order of magnitude lower than in [Cu(2)2]PF6 with the tolyl-based ligand. Slower intermolecular ligand exchanges between all the isomers have also been shown to occur at ambient temperature in CD2Cl2 through complete ligand dissociation. Free energies at 298 K varying between 66.7 and 74.4 kJ.mol(-1) and entropies varying between -26.4 and 28.3 J.K(-1).mol(-1) were determined for the intramolecular Lambda/Delta isomerizations. For the intermolecular ligand exchanges free energies at 298 K varying between 55.6 and 62.5 kJ.mol(-1) and entropies varying between -97.9 and -74.5 J.K(-1).mol(-1) were measured.

Ligand Exchange Dynamics in Aluminum Tris-(Quinoline-8-olate): A Solution State NMR Study

The exchange kinetics between the three symmetry-inequivalent ligands in the meridianal isomer of aluminum tris-(quinoline-8-olate) (Alq(3)), a widely used electron-transporting and light-emitting material in the field of organic light emitting diodes, have been studied using two-dimensional exchange NMR spectroscopy in solution. The three inequivalent ligands were found to exchange on a time scale of about 5 s(-)( 1) at room temperature. A simple first-order mechanism based on consecutive 180 degrees flips of the ligands is sufficient to quantitatively explain the experimental data. Activation enthalpies between 83 and 106 kJ mol(-)( 1) were found for the flips of the three inequivalent ligands. The activation entropies are positive, suggesting a highly disordered transition state. These findings elucidate the internally mobile nature of the Alq(3) complex, and may have important implications for the morphology of vapor deposited thin films of Alq(3) as well as for crystallization-assisted device failures.

Heme Ligation and Conformational Plasticity in the Isolatedc Domain of Cytochrome cd1 Nitrite Reductase

The heme ligation in the isolated c domain of Paracoccus pantotrophus cytochrome cd(1) nitrite reductase has been characterized in both oxidation states in solution by NMR spectroscopy. In the reduced form, the heme ligands are His69-Met106, and the tertiary structure around the c heme is similar to that found in reduced crystals of intact cytochrome cd1 nitrite reductase. In the oxidized state, however, the structure of the isolated c domain is different from the structure seen in oxidized crystals of intact cytochrome cd1, where the c heme ligands are His69-His17. An equilibrium mixture of heme ligands is present in isolated oxidized c domain. Two-dimensional exchange NMR spectroscopy shows that the dominant species has His69-Met106 ligation, similar to reduced c domains. This form is in equilibrium with a high-spin form in which Met106 has left the heme iron. Melting studies show that the midpoint of unfolding of the isolated c domain is 320.9 +/- 1.2 K in the oxidized and 357.7 +/- 0.6 K in the reduced form. The thermally denatured forms are high-spin in both oxidation states. The results reveal how redox changes modulate conformational plasticity around the c heme and show the first key steps in the mechanism that lead to ligand switching in the holoenzyme. This process is not solely a function of the properties of the c domain. The role of the d1 heme in guiding His17 to the c heme in the oxidized holoenzyme is discussed.

Diffusion on curved, periodic surfaces.

We present a simulation algorithm for a diffusion on a curved surface given by the equation phi(r)=0. The algorithm is tested against analytical results known for diffusion on a cylinder and a sphere, and applied to the diffusion on the P, D, and G periodic nodal surfaces. It should find application in an interpretation of two-dimensional exchange NMR spectroscopy data of diffusion on biological membranes.

Two-dimensional exchange NMR spectroscopy in fractionally deuterated molecules: separation of exchange and cross-relaxationprocesses by proton dilution

Two-dimensional exchange NMR spectroscopy in fractionally deuterated molecules: separation of exchange and cross-relaxationprocesses by proton dilution

Observation of the exchange of small molecules between phases in a SDS micellar system

In an aqueous solution containing sodium docecylsulfonate, an exchange of benzene molecules between those inside and outside of the micelles was observed by two-dimensional NMR exchange spectroscopy. Exchange rates were evaluated by integrating the EXSY cross peaks.

Complexes of trimethylplatinum(IV) with butane-2,3-dione monoxime

The dimeric complex of butane-2,3-dione monoxime (Hbdm) with trimethylplatinum(IV), [{PtMe3(bdm)}2]1, and its pyridine (py) and 2,2′-bipyridyl (bipy) adducts [PtMe3(bdm)(py)] and [PtMe3(bdm)(bipy)], show three different configurations for the ionised monoxime ligand. Dimer 1 forms a fluxional solvated monomer in co-ordinating solvents. Its solution-state stereodynamics in CD3OD have been measured by two-dimensional exchange NMR spectroscopy and a ‘windscreen-wiper’ mechanism proposed which is novel in trimethylplatinum(IV) chemistry.

Determination of Monomer Conformations in Noncrystalline Solid Polymers by Two-Dimensional NMR Exchange Spectroscopy

We propose and demonstrate a new approach to the determination of the conformations of molecules or monomers in noncrystalline solids. The method makes use of two-dimensional NMR exchange spectroscopy in combination with specific isotopic labeling. We present experimental results on specifically labeled poly(methyl methacrylate), poly(methyl acrylate), and poly(ethyl methacrylate). Through comparisons between the experimental and simulated two-dimensional spectra, we determine that the ester side groups of poly(methyl methacrylate) and poly(methyl acrylate) have planar, trans conformations and that the side groups of poly(ethyl methacrylate) are also predominantly in the planar, trans conformation in the noncrystalline solid state

Halogenotrimethylplatinum(IV) complexes of 2,6-bis(p-tolylthiomethyl)pyridine (L1): nuclear magnetic resonance studies of their solution state stereodynamics and the crystal structure of fac-[PtBrMe3L1

Under mild conditions 2,6-bis(p-tolylthiomethyl) pyridine (L1) reacted with halogenotrimethylplatinum(IV) to afford complexes of the type fac-[PtXMe3L1](X = Cl or Br). This potentially terdentate ligand acts here in a five-membered S/N chelate bidentate fashion. Solution dynamic NMR studies revealed the presence of three fluxional processes: pyramidal inversion of the co-ordinated sulfur atom, S-S switching with correlated pyramidal inversion, and S-S switching. The mechanism for the S-S exchange process was elucidated by two-dimensional NMR exchange spectroscopy. The crystal structure of fac-[PtBrMe3L1] confirms the bidentate chelate nature of the ligand in the solid state, with a N-Pt-S(1) angle of 81.5°. IR data for the Pt-C, Pt-N and Pt-X stretching modes in these complexes are also presented.

Molecular orientational dynamics in K3C60 probed by two-dimensional nuclear magnetic resonance.

We characterize the orientational dynamics of C 60 -3 anions in K 3 C 60 , using two-dimensional NMR exchange spectroscopy. We find an activated temperature dependence for the reorientation rate between 195 and 220 K, with E a =460±60 meV. Molecular reorientation proceeds via small (<44 o ) rotational jumps, implying that K 3 C 60 is more highly orientationally disordered than previously proposed

Exchange processes in diselenium and selenium-sulfur dihalides, Se2X2 and SeSX2 (X = Br, Cl). A selenium-77 2D-EXSY study

The application of two-dimensional NMR exchange spectroscopy (2D-EXSY) to the study of 77 Se exchange kinetics in mixtures of S 2 Cl 2 and Se 2 Br 2 , shows that 77 Se magnetization transfer takes place predominantly by an exchange process, in which only SeX bonds are broken. Direct exchange via mechanisms, involving ChX radicals, four-chalcogen center transition states and mono- and trichalcogen dihalide species are not consistent with the cross peaks observed in the 2D spectra.

Selective two-dimensional exchange NMR spectroscopy and its applications to the study of molecular dynamic processes. 1. Multiplet-selective excitation

Selective two-dimensional exchange NMR spectroscopy and its applications to the study of molecular dynamic processes. 1. Multiplet-selective excitation

Proton chemical shift anisotropy: Detection of cross-correlation with dipole-dipole interactions by double-quantum filtered two-dimensional NMR exchange spectroscopy

Cross-correlation between proton chemical shift anisotropy and proton-proton dipolar interactions is shown to be an important relaxation mechanism. Cross-correlation leads to a partial conversion of Zeeman order into longitudinal two-spin order. The build-up of longitudinal two-spin order can be observed selectively by means of a novel experiment that combines double-quantum filtration with two-dimensional nuclear Overhauser effect spectroscopy (DQF-NOESY).