Dynamic NMR Techniques Research Articles

Chemistry of 2,14-Dithiacalix[4]arene: Alkylation and Conformational Behavior of Peralkylated Products.

2,14-Dithiacalix[4]arene, prepared on a multigram scale, was alkylated using the reaction conditions well known from the chemistry of classical calixarenes or thiacalixarenes to study the specific conformational preferences and dynamic behavior of the corresponding tetraalkylated derivatives. As proved by the combination of the X-ray crystallography and dynamic NMR techniques, the presence of mixed bridges (-CH2- and -S- groups) within the basic skeleton brings about considerable changes in the mutual ratios of the individual conformers compared to the parent macrocycles. Interestingly, certain conformers, hardly accessible for common calixarenes/thiacalixarenes (e.g., 1,2-alternates) are easily prepared in very good yields in the case of 2,14-dithiacalix[4]arene, which makes this mixed-bridge system attractive as molecular scaffold for supramolecular applications.

Mechanisms and Beyond: Elucidation of Fluxional Dynamics by Exchange NMR Spectroscopy.

Detailed mechanistic information is crucial to our understanding of reaction pathways and selectivity. Dynamic exchange NMR techniques, in particular 2D exchange spectroscopy (EXSY) and its modifications, provide indispensable intricate information on the mechanisms of organic and inorganic reactions and other phenomena, for example, the dynamics of interfacial processes. In this Review, key results from exchange NMR studies of small molecules over the last few decades are systemised and discussed. After a brief introduction to the theory, the key types of dynamic processes are identified and fundamental examples given of intra- and intermolecular reactions, which, in turn, could involve, or not, bond-making and bond-breaking events. Following that logic, internal molecular rotation, intramolecular stereomutation and molecular recognition will first be considered because they do not typically involve bond breaking. Then, rearrangements, substitution-type reactions, cyclisations, additions and other processes affecting chemical bonds will be discussed. Finally, interfacial molecular dynamics and unexpected combinations of different types of fluxional processes will also be highlighted. How exchange NMR spectroscopy helps to identify conformational changes, coordination and molecular recognition processes as well as quantify reaction energy barriers and extract detailed mechanistic information by using reaction rate theory in conjunction with computational techniques will be shown.

Colloid particle transport in a microcapillary: NMR study of particle and suspending fluid dynamics

Precise manipulation of the hydrodynamic interaction between particles is particularly important for operation of microfluidic devices. Shear-induced migration gives rise to dynamical patterns within the flow that have been observed in a range of systems. In this work NMR ‘active’ colloidal particles (a=1.25µm) at volume fraction of 22% in an aqueous phase are flowed through a µ-capillary (R=126µm) and the transport dynamics of the particle and suspending fluid phases are studied using dynamic NMR techniques. Simultaneous interrogation of shear rheology of the suspending fluid and particle phases of colloidal suspensions is presented. The dynamic behavior of the suspending fluid is shown to carry within it information about the structure of the colloidal particle ensembles on the time scales investigated (Δ=25ms→250ms) providing rich experimental data for further investigation and model verification. The importance of determining the particle concentration profile within μ-capillaries is explicitly demonstrated as shear induced migration causes significant concentration gradients to occur at strong flow conditions (i.e. Pep=270).

Role of Coordination Geometry in Dictating the Barrier to Hydride Migration in d6 Square-Pyramidal Iridium and Rhodium Pincer Complexes

Syntheses of the olefin hydride complexes [(POCOP)M(H)(olefin)][BAr(f)(4)] (6a-M, M = Ir or Rh, olefin = C(2)H(4); 6b-M, M = Ir or Rh, olefin = C(3)H(6); POCOP = 2,6-bis(di-tert-butylphosphinito)benzene; BAr(f) = tetrakis(3,5-trifluoromethylphenyl)borate) are reported. A single-crystal X-ray structure determination of 6b-Ir shows a square-pyramidal coordination geometry for Ir, with the hydride ligand occupying the apical position. Dynamic NMR techniques were used to characterize these complexes. The rates of site exchange between the hydride and the olefinic hydrogens yielded ΔG(++) = 15.6 (6a-Ir), 16.8 (6b-Ir), 12.0 (6a-Rh), and 13.7 (6b-Rh) kcal/mol. The NMR exchange data also established that hydride migration in the propylene complexes yields exclusively the primary alkyl intermediate arising from 1,2-insertion. Unexpectedly, no averaging of the top and bottom faces of the square-pyramidal complexes is observed in the NMR spectra at high temperatures, indicating that the barrier for facial equilibration is >20 kcal/mol for both the Ir and Rh complexes. A DFT computational study was used to characterize the free energy surface for the hydride migration reactions. The classical terminal hydride complexes, [M(POCOP)(olefin)H](+), are calculated to be the global minima for both Rh and Ir, in accord with experimental results. In both the Rh ethylene and propylene complexes, the transition state for hydride migration (TS1) to form the agostic species is higher on the energy surface than the transition state for in-place rotation of the coordinated C-H bond (TS2), while for Ir, TS2 is the high point on the energy surface. Therefore, only for the case of the Rh complexes is the NMR exchange rate a direct measure of the hydride migration barrier. The trends in the experimental barriers as a function of M and olefin are in good agreement with the trends in the calculated exchange barriers. The calculated barriers for the hydride migration reaction in the Rh complexes are ∼2 kcal/mol higher than for the Ir complexes, despite the fact that the energy difference between the olefin hydride ground state and the agostic alkyl structure is ∼4 kcal/mol larger for Ir than for Rh. This feature, together with the high barrier for interchange of the top and bottom faces of the complexes, is proposed to arise from the unique coordination geometry of the agostic complexes and the strong preference for a cis-divacant octahedral geometry in four-coordinate intermediates.

Tritopic phenanthroline and pyridine tail-tied aza-scorpiands

The synthesis of two new tritopic double-scorpiand receptors in which two equivalent 5-(2-aminoethyl)-2,5,8-triaza[9]-(2,6)-pyridinophane moieties have been linked with 2,6-dimethylpyridine (L1) or 2,9-dimethylphenanthroline (L2) units is reported for the first time. Their acid-base behaviour and Zn(2+) coordination chemistry have been studied by pH-metric titrations, molecular dynamic calculations, NMR, UV-Vis and steady-state fluorescence techniques. L1 and L2 behave, respectively, as hexaprotic and heptaprotic bases in the experimental conditions used (298.1 +/- 0.1 K, 0.15 mol dm(-3) NaCl, pH range under study 2.0-11.0). These ligands are able to form mono-, bi- and trinuclear Zn(2+) complexes depending on the Zn(2+)-receptor molar ratio. Interaction of L1 and L2 with pyrophosphate (PPi), tripolyphosphate (TPP) and adenosine 5'-triphosphate (ATP) has been followed by pH-metric titrations, (1)H and (31)P NMR techniques and molecular dynamic analysis. Finally, formation of mixed complexes Zn(2+)-L-PPi, Zn(2+)-L-TPP and Zn(2+)-L-ATP has been studied for both receptors by potentiometric titrations.

NMR application for a study of reversible photochemical kinetics: A multisite case

AbstractExpressions describing the NMR signal shape for photoinduced chemical exchange in a multi‐site system without spin‐spin splitting are obtained by dynamic NMR techniques. The methods provide a framework for planning experimental strategies aimed at studies of photoinduced chemical exchange in dissolved substances. © 2007 Wiley Periodicals, Inc. Concepts Magn Reson Part A 30A: 140–146, 2007

The Molecular Behavior of 1,3,5-Trioxane in a Cyclophosphazene Inclusion Compound: A2H NMR Study

Dynamic 2H NMR techniques were employed to examine the ordering behavior and dynamics of perdeuterated 1,3,5-trioxane-d6 in the inclusion compound with tris-(1,2-dioxyphenyl)-cyclotriphosphazene. T...

Mobility Studies on Siloxane-Based Inorganic-Organic Hybrid Polymers by Dynamic Solid State, Suspended State, and Deuterium NMR Spectroscopy: Supported Organometallic Complexes. XXVI [1]Dedicated to Professor Joachim Strähle on the Occasion of his 65th Birthday

Organic modified siloxanes of the type RSi(OMe2)2(CH2)3C6D4(CH2)3(OMe2)2SiR [R = Me (4), R = OMe (5)] were sol-gel processed employing solvents of different polarity (MeOH and THF) to yield the corresponding inorganic-organic hybrid polymers X4a — X5b with different physical properties. These polymers were investigated by multinuclear (2H, 13C, and 29Si) solid state and 1H suspension state NMR spectroscopy, including dynamic NMR techniques, in order to correlate the mobilities of these xerogels with physical properties, especially with the cross-linkage.

Conformational flexibility of inositol phosphates: influence of structural characteristics

Dynamic NMR and molecular modeling techniques were employed to investigate the influence of structural characteristics on the ring inversion processes of inositol phosphates. The results indicate that inositol phosphates that contain the syn-1,3,5-triaxial trisphosphate arrangement show great proclivity to adopt the sterically hindered conformation. These results suggest that the ability to form chelation cages with counter-ions is a major factor in the ring inversion processes of inositol phosphates.

NMR Investigations of Restricted Rotations and Metallotropic Shifts in Rhenium(I) Tricarbonyl Halide Complexes of Pyridyl Carboxamide and Thioamide Ligands

A series of rhenium(I) tricarbonyl halide complexes of pyridyl mono- and di-carboxamides and -thioamides has been studied by dynamic NMR techniques. Oxygen coordination to ReI of the carboxamide ligand reduced the energy barrier to C–N rotation by 2–13 kJ mol–1 whereas sulfur–ReI coordination of the thioamide ligands led to a reduction of 28–33 kJ mol–1. In the pyridyl dicarboxamide and -dithioamide metal chelate complexes metallotropic shifts occur between the three donor atoms, O, N, O or S, N, S, energies [ΔG‡ (298.15 K)] being in the range 78–89 kJ mol–1; these high values arise from the strong electron donating properties of the -NR2 (R = Me, Et) groups.

Trimethylplatinum(IV) complexes of dithiocarbamato ligands: an experimental NMR study on the barrier to C–N bond rotation

The trimethylplatinum(IV) complexes of a number of dithiocarbamato ligands have been prepared. The complexes are dimeric in the solid-state and in solution, with the ligands acting in both a bridging and chelating fashion. Restricted rotation about the ligand C-N bonds in solution leads to the formation of four rotomers. The kinetics of the restricted rotation were measured by a variety of dynamic NMR techniques in the slow and intermediate exchange regimes. Two distinct processes are observed, namely the independent rotation about each C-N bond and correlated rotation about both C-N bonds. The free energies of activation, which are strongly dependent on the nature of the ligand substituents, are in the range 65 – 88 kJ mol-1 at 298 K. The origins of the barrier to rotation and the effects of the N substituents are discussed. The X-ray structures of fac-[PtMe3(Me2NCS2)]2 (2) and fac-[PtMe3(Ph(H)NCS2)]2 , (6) are reported.

Stereochemical rearrangements in tricarbonylrhenium(I) halide complexes of the nonracemic chiral ligand 2-[(4R,5R)-dimethyl-1,3-dioxan-2-yl]pyridine (L): a dynamic NMR study

Tricarbonylrhenium(I) halide complexes of the nonracemic chiral ligand 2-[(4R,5R)-dimethyl-1,3-dioxan-2-yl]pyridine (L), namely fac-[ReX(CO)3L] (X = Cl, Br, or I), have been prepared and their latent fluxionality studied by dynamic NMR techniques in the slow and intermediate exchange regimes. In solution, these complexes give rise to four diastereoisomers, depending on the configuration at the metal and at the acetal-carbon atom, respectively; the relative populations are in the order SR > RR >>; RS > SS. At moderate temperatures, a reversible "acetal ring flip" leads to formal inversion of configuration at the acetal-carbon atom; the free energies of activation are in the range 84-88 kJ mol-1 at 298 K. Above ca. 370 K, reversible ligand dissociation also occurs, leading to an exchange of all four diastereoisomers on the NMR chemical shift time-scale.Key words: dynamic NMR, fluxionality, tricarbonylrhenium(I) complexes, chiral acetal ligands.

Stereochemical rearrangements in tricarbonylrhenium(I) halide complexes of the nonracemic chiral ligand 2-[(4<i>R</i>,5<i>R</i>)-dimethyl-1,3-dioxan-2-yl]pyridine (L): a dynamic NMR study

Tricarbonylrhenium(I) halide complexes of the nonracemic chiral ligand 2-[(4R,5R)-dimethyl-1,3-dioxan-2-yl]pyridine (L), namely fac-[ReX(CO)3L] (X = Cl, Br, or I), have been prepared and their latent fluxionality studied by dynamic NMR techniques in the slow and intermediate exchange regimes. In solution, these complexes give rise to four diastereoisomers, depending on the configuration at the metal and at the acetal-carbon atom, respectively; the relative populations are in the order SR > RR >>; RS > SS. At moderate temperatures, a reversible "acetal ring flip" leads to formal inversion of configuration at the acetal-carbon atom; the free energies of activation are in the range 84-88 kJ mol-1 at 298 K. Above ca. 370 K, reversible ligand dissociation also occurs, leading to an exchange of all four diastereoisomers on the NMR chemical shift time-scale.Key words: dynamic NMR, fluxionality, tricarbonylrhenium(I) complexes, chiral acetal ligands.

Solid State NMR Studies of n-Alkane/urea Inclusion Compounds

The molecular characteristics of n-alkane chains in urea inclusion compounds are examined over a large temperature range (100 K to 300 K) using dynamic 2H NMR techniques. Particular emphasis is given to the guest properties in the vicinity of a solid-solid phase transition which is known to be accompanied by a distortion of the urea lattice. The analysis of the NMR data reveals several intra-and intermolecular motional processes which are responsible for spin relaxation in these systems. Likewise, it can be distinguished between confor-mational and overall chain order. Both the molecular dynamics and chain ordering are found to be strongly affected by the solid-solid phase transition. It is concluded that with respect to the guest components the phase transition can be assigned to a dynamic order-disorder transition.

Synthesis and Characterization of Styrene-Butadiene Block Copolymer/Silicate Hybrid Materials via the Sol-Gel Process I. Four-Arm Star Styrene-Butadiene Block Copolymer/Silica Hybrids

Novel hybrid materials incorporating four-arm star styrene-butadiene block copolymer (SB-4A) with tetraethoxysilane (TEOS) have been successfully prepared by a sol-gel process. The properties, microstructure, and dynamics of the hybrid materials were investigated in detail by means of dynamic mechanical analysis (DMA), thermogravimetry (TG), and NMR techniques. The results indicate that the properties of the hybrids strongly depend on the TOES content and the silica networks prefer to form in the polystyrene (PS) domains. There exists an optimum range of TEOS content, in this work 30–50% by weight, within which the properties of the hybrids obtained were well improved: storage modulus increases, thermal stability raises, and the high-temperature Tg shifts to higher temperature. 29Si NMR spectrum shows that in the hybrids with well improved properties silica networks formed with Q4 in the greatest abundance.

Dynamic 13C NMR analysis of pyruvate and lactate oxidation in the in vivo canine myocardium: evidence of reduced utilization with increased work.

In this work, substrate selection was monitored in the left ventricle of the canine myocardium by following pyruvate and lactate oxidation under in vivo conditions at basal and elevated workloads. These studies were conducted in the open chest model using dynamic 13C NMR techniques in the presence and absence of dichloroacetic acid (DCA), a well-known activator of pyruvate dehydrogenase (PDH). Following the infusion of (3-(13)C) pyruvate or (3-(13)C) lactate into the left anterior descending artery, highly variable 13C enrichments of glutamate, alanine, aspartate, and citrate were noted under low (RPP < 14,500 mmHg/min), intermediate (RPP = 15,000-25,000 mmHg/min), and high (RPP > 25,500 mmHg/min) rate pressure products (RPP). At low workloads, the myocardium typically oxidized the infused (3-(13)C) pyruvate or (3-(13)C) lactate and incorporated the labeled carbon into the glutamate pool as expected. However, in a few notable instances (n = 3), 13C-enriched pyruvate and lactate were unable to label the glutamate pool under in vivo conditions even at the lowest RPPs, indicating a lack of selection for these substrates by the tricarboxylic acid (TCA) cycle. Nonetheless, the levels of glutamate C4 enrichment observed at low workloads could usually be enhanced by infusion of DCA. Importantly, 13C NMR extract analysis revealed that (3-(13)C) pyruvate or (3-(13)C) lactate labeling of the glutamate pool was reduced (< 20%) at high workloads in spite of increased DCA concentrations.

Atropisomerism in Hindered Naphthyl Sulfones Investigated by Dynamic NMR and Dynamic HPLC Techniques

Atropisomerism in Hindered Naphthyl Sulfones Investigated by Dynamic NMR and Dynamic HPLC Techniques

Problems of using dynamic NMR techniques in studies of photoinduced molecular dynamics

Problems of using dynamic NMR techniques in studies of photoinduced molecular dynamics

Collective Motions and Viscoelastic Properties in Main Chain Liquid Crystal Polymers. A Deuteron Spin Relaxation Study

AbstractMultipulse dynamic NMR techniques have been employed to investigate collective motions and viscoelastic properties in a main chain liquid crystal polymer and low molar mass analogues in the nematic phase. Transverse deuteron spin relaxation times measured as a function of pulse spacing using a modified Carr‐Purcell‐Meiboom‐Gill pulse sequence are analysed in terms of a simple hydrodynamic model for nematic director fluctuations. Combining these results with those of NMR slow sample spinning experiments allows determination of the effective viscosity and the average elastic constant of the material. Results are compared for compounds with different molecular weights and with the results of analogous proton field‐cycling experiments. The viscosity is found to increase approximately as the square of the molecular weight, Mn, whilst the elastic constant shows a much shallower Mn dependence apparently tending to a plateau value at moderate molecular weights.

Synthese und NMR-Spektroskopie von 2,5-diborylierten 3-Plumbolenen und 3-Stannolenen / Synthesis and NMR Spectroscopy of 2,5-Diborylated 3-Plumbolenes and 3-Stannolenes

The organoboration of bis(alkynyl)plumbanes (1) leads to intermediates 7 in which a triorganolead cation is stabilized by intramolecular side-on coordination to the C=C triple bond of an alkynylborate function. These compounds decompose rapidly at room temperature or rearrange to give 1,4-plumbabora-2,5-cyclohexadienes (10). If the compounds 7 are treated with an excess of trialkylborane (2) they are converted into bis(alkenyl)lead derivatives (8) as the final products (8c, 8ca) or again as intermediates for the 2,5-bis(dialkylboryl)-3-plumbolenes 9. A mechanism suggested for the formation of 9 is based on the product distribution (9ab/9ba = 2:1) obtained from the reaction of a distinct compound 7a with different trialkylboranes (2a, 2b). All compounds have been fully characterized by multinuclear magnetic resonance data (1H, 11B, 13C, 207Pb), dynamic NMR and two-dimensional (2 D) NMR techniques. In addition, these techniques have been applied to some of the analogous tin compounds (e.g. 4b, 5).