Scaling-down the CSA recoupling in S-CODEX 1D-MAS exchange experiments

We introduce a significant extension to the 1D-MAS exchange experiment CODEX that improves its ability to determine the amplitude and – in favorable cases – the reorientation angles of slow dynamic process in condensed matter. Large chemical-shift anisotropy (CSA) and/or large angle reorientation require small amounts of CSA recoupling in the CODEX experiment. This in turn prompts for sometimes inconveniently high MAS frequencies and leads to the necessity of using different MAS frequencies in the same dataset to determine the topology of the motion. We introduce a simple experimental approach that overcomes these difficulties by scaling-down the CSA recoupling by altering the timing of the NMR pulse sequence. We briefly present theory and calculations and demonstrate the usefulness of the approach for two examples: the slow twofold jump of the dimethylsulfone molecules as well as the slow threefold jump in the stacks of a pyramidic liquid crystal.     

NMR Studies of Restricted Rotation in Aminatophosphorus(1+) Salts

Abstract

A series of aminatophosphorus(1+) salts were synthesized as potential fungicides. Proton NMR of a set of compounds 1 within this series were observed to show selective line broadening of the methylene protons indicating a dynamic process which is slow on the NMR time scale. X-ray crystal structures of two of the compounds in the series showed a relatively planar nitrogen eliminating nitrogen inversion as the source of the observed line broadening. The methylene protons appeared as a sharp doublet (³J_PH = 3.6 Hz) at room temperature when R=OCH₃, a broad singlet when R=OCF₃, and a sharp multiplet consistent with the AB portion of an ABX (X = ³¹P) spin system when R=SCH. Examination of the temperature dependence of the spectra revealed that the observed lineshape and temperature effects were consistent with slow rotation about the N-Ph bond and dependent primarily upon the size of the substituent R. Thus the slow rotation was thought to be due to steric factors and not the influence of electronic effects of the substituent R on the P-N bond. Rotation rates estimated from NMR lineshape analysis and plotted as a function of temperature for 1 when R = SCH₃ and R¹ = Ph gave an calculated energy barrier ΔG₂₉₈ ^? of 17 kcal/mol. Similar studies for a variety of substituents R might be useful as a means of measuring relative steric bulk. At low temperature (ca. -50°C) broadening of the PPh, resonances began to appear indicating a second independent dynamic process thought to be slow rotation about the N-PPh₃ bond on the NMR time scale at that temperature.     

On the Bulk Properties of a Poly(Bicycloacrylate) Carrying Semifluorinated Side Groups

We prepared a novel fluorinated polymer from a bicycloacrylate monomer, (1H,1H,2H,2H)-perfluorododecyl 2-(bicyclo[3.1.0]hex-1-yl)acrylate, by radical ring opening polymerization. The bulk properties of the polymer were investigated by thermal analysis and X-ray diffraction, which proved the existence of a smectic mesophase up to the isotropization temperature of 74 °C. Furthermore, a solid state NMR study was started to characterize domains with different mobility mainly by ¹³C cross-polarization magic angle spinning and T₂ selective experiments. We found that the polymer is a homogeneous sample with the presence of dynamic motions in the kHz regime below the glass transition temperature.     

NMR spectroscopy in coordination supramolecular chemistry: A unique and powerful methodology

Metal-mediated self-assembly is emerging as a very important strategy for the synthesis of supramolecular species. Still, a major challenge in coordination supramolecular chemistry continues to be the characterization of the self-assembled complexes and the investigation of their dynamic behaviour in solution. In this context, NMR spectroscopy appears as a unique and powerful methodology. This practical-oriented review describes the rich variety of NMR techniques which are applied to the investigation of different aspects of the structure and behaviour of supramolecular complexes. “Classic” 1D NMR spectra reflect characteristic chemical shifts due to metal–ligand interactions or encapsulation phenomena, as well as symmetry and chiral properties of host–guest assemblies. Mainstream ¹H, ¹³C, ¹⁹F and ³¹P spectra are eventually complemented by the use of NMR-active metal nuclides. Homo- and heteronuclear 2D correlation experiments are ubiquitous in the literature, providing through-bond and through-space connectivities. Increasingly, diffusion measurements are also gaining popularity in this field, being used to gain information about molecular size, intermolecular interactions and even association constants of supramolecular complexes. Knowledge about the thermodynamic properties and the dynamic behaviour of coordination supramolecular assemblies is essential for the development of their practical applications. The most frequently used NMR methodologies for the calculation of association constants (simple signal integration, NMR titration and diffusion measurements) and for the investigation of dynamic supramolecular equilibria (lineshape analysis, selective inversion recovery experiments and 2D EXSY spectra) are described, together with the use of variable-temperature investigations for the determination of the thermodynamic and activation parameters of self-assembly and encapsulation processes.     

Nematic reorientation in electric and magnetic fields

Homogeneous reorientation processes of two nematic liquid crystals in electric and magnetic fields have been observed using proton nuclear magnetic resonance spectroscopy (NMR). Using a recently developed experimental set-up, it is possible to study reorientation processes in liquid crystals by means of NMR experiments in a very flexible way. The time constant τ describing these processes has been determined as a function of the applied electric field. It emerges that the electric field cannot only be used to increase the reorientation time but also to slow the director reorientation by approximately one order of magnitude. Experimental data for 5CB and a fluorinated liquid crystal (BCH-5 FFF) are presented. The reorientation time measured as a function of the electric field can be used to calculate the rotational viscosity γ ₁. By repeating these experiments at different temperatures it was possible to investigate the temperature behaviour of γ ₁.     

Study of liquid–liquid interfaces by an easily implemented localized NMR sequence

To selectively extract heavy metals from solutions containing fission products, it is essential to optimize the liquid–liquid extraction processes. Such an objective requires improving the fundamental knowledge of the different mechanisms that are involved in these processes. In that respect, we propose a localized NMR sequence named LOCSY to assess the concentration profiles of different species involved in these processes. One of the goals of this sequence is to study the products as close as possible to the liquid–liquid interface with the help of a standard NMR spectrometer of chemistry labs. The one‐dimensional spatial localization along the NMR tube is obtained by a discrete stepping of the frequency‐selective excitation pulses under a pulsed field gradient. Specific data processing has been developed to obtain the 1D NMR spectra as a function of the vertical position in the NMR tube. The LOCSY sequence has been tested and evaluated on three different systems: (i) a cylindrical phantom inserted in the NMR tube containing 4‐methylsalicylic acid solution, (ii) D₂O/olive oil biphasic system, and (iii) the dissolution of solid saccharose in D₂O. These examples illustrate potential applications of the LOCSY sequence, particularly the possibility to measure concentration profiles and to study phenomena such as diffusion, provided the dynamic range is compatible with NMR timescale and sensitivity. Copyright © 2010 John Wiley & Sons, Ltd.     

Synthesis and conformational studies of chiral meso-(α,β-unsaturated)-porphyrins

We describe a method to improve the yield and the kinetics of the difficult syntheses of α,β-unsaturated porphyrins, which enabled us to obtain a new chiral porphyrin derived from (S)-(−)-perillaldehyde in a 6% yield. Variable temperature NMR experiments on the free base, the zinc(II) and the nickel(II) complexes showed that two distinct and consecutive dynamic processes linked with the meso substituents rotation occurred. These processes can be analyzed as an evolution of the conformer composition upon temperature change. Higher values are found for the free energies of rotation of the substituent (measured by variable temperature ¹H NMR) compared to those of other equivalent porphyrins like meso-tetraphenyl porphyrin or meso-tetracyclohexyl porphyrin.     

Structure and conformational processes of bis(o-cumyl)sulfide, sulfoxide and sulfone

The NMR solution spectra of the title sulfide and sulfone show decoalescence of the geminal methyl signals of the isopropyl groups at low temperature (−178 °C for the ¹³C signal of sulfide at 150.8 MHz and −147 °C for the ¹H signal of sulfone at 600 MHz). The barriers for the related dynamic processes were measured (4.3 and 7.0 kcal mol⁻¹ for the sulfide and sulfone, respectively). The preferred conformer of sulfide has a propeller shape with a C₁ symmetry, as suggested by Molecular Mechanics (MM) calculations. In the case of sulfone the preferred conformer has a propeller shape with a C₂-anti symmetry, as indicated by calculations and supported by X-ray crystallographic determination. The computed contour map of the potential energy shows that in both cases the dynamic processes take place via correlated rotations (cogwheel mechanism) of the two aromatic substituents about the Ar-S bonds. Dynamic processes could not be observed by NMR in the title sulfoxide, which was also found to adopt a propeller shaped conformation, as indicated by MM calculations and X-ray diffraction.     

Physical gelation of melamine formaldehyde resin solutions. II. A combined light-scattering and low-resolution relaxation proton NMR study

The kinetics of physical gelation in aqueous melamine formaldehyde (MF) resin solutions were studied with the aid of low-resolution ¹H NMR T₂ relaxation experiments in combination with both static and dynamic light-scattering measurements. The investigations were conducted on a series of MF resins with increasing degrees of condensation. We show that MF aggregates (aided by hydrogen bonds) were immediately formed upon cooling from reaction to room temperature, that is, storage temperature. Surprisingly, the growth of these aggregates, which eventually led to the formation of a physical gel, did not have a major effect on molecular mobility. By means of light-scattering experiments, we were able to monitor the increase of the size of MF aggregates as a function of storage time. The physically gelled MF solutions were subjected to heating and subsequent cooling runs and again studied by light-scattering and nuclear magnetic resonance (NMR) experiments. MF aggregates were destroyed, depending on the degree of condensation, in the temperature range 35–60 °C according to NMR, and 40–75 °C as determined by light scattering. The process of physical gelation was reversible; upon subsequent cooling, the MF aggregates were formed anew. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3307–3318, 1999     

Investigating Chain Dynamics in Highly Crosslinked Polymers using Solid‐State 1H NMR Spectroscopy

Solid state ¹H NMR line‐shape analysis and (double quantum) DQ ¹H NMR experiments have been used to investigate the segmental and polymer chain dynamics as a function of temperature for a series of thermosetting epoxy resins produced using different diamine curing agents. In these thermosets, chemical crosslinks introduce topological constraints leading to residual stresses during curing. Materials containing a unique ferrocene‐based diamine (FcDA) curing agent were evaluated to address the role of the ferrocene fluxional process on the atomic‐level polymer dynamics. At temperatures above the glass transition temperature (T_g), the DQ ¹H NMR experiments provided a measure of the relative effective crosslink and entanglement densities for these materials and revealed significant polymer chain dynamic heterogeneity in the FcDA‐cured thermosets. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1143–1156     

Conformational analysis of (+)-germacrene A by variable-temperature NMR and NOE spectroscopy

(+)-Germacrene A, an important intermediate in sesquiterpene biosynthesis, was isolated in pure form from a genetically engineered yeast and was characterized by chromatographic properties (TLC, GC), MS, optical rotation, UV, IR, ¹H NMR, and ¹³C NMR data. Variable-temperature 500 MHz ¹H NMR spectra in CDCl₃ showed that this flexible cyclodecadiene ring exists as three NMR-distinguishable conformational isomers in a ratio of about 5:3:2 at or below ordinary probe temperature (25 °C). The conformer structures were assigned by ¹H NMR data comparisons, NOE experiments, and vicinal couplings as follows: 1a (52%, UU), 1b (29% UD), and 1c (19%, DU).     

Ambident PCN Heterocycles: N‐ and P‐Phosphanylation of Lithium 1,3‐Benzazaphospholides

Synthetic and structural aspects of the phosphanylation of 1,3‐benzazaphospholides 1_Li , ambident benzofused azaphosphacyclopentadienides, are presented. The unusual properties of phospholyl‐1,3,2‐diazaphospholes inspired us to study the coupling of 1_Li with chlorodiazaphospholene 2 , which led to the N‐substituted product 3 . Reaction of 1_Li with chlorodiphenyl‐ and chlorodicyclohexylphosphane likewise gave N‐phosphanylbenzazaphospholes 4 and 5 , whereas with the more bulky di‐tert‐butyl‐ and di‐1‐adamantylchlorophosphanes, the diphosphanes 6 and 7 are obtained; in the case of 7 they are isolated as a dimeric LiCl(THF) adduct. Structural information was provided by single‐crystal X‐ray diffraction and solution NMR spectroscopy experiments. 2D exchange spectroscopy confirmed the existence of two rotamers of the aminophosphane 5 at room temperature; variable‐temperature NMR spectroscopy studies of 6 revealed two dynamic processes, low‐temperature inversion at ring phosphorus (ΔH^≠=22 kJ mol^?1, ΔS^≠=2 J K^?1 mol^?1) and very low‐temperature rotation of the tBu₂P group. Quantum chemical studies give evidence that 2‐unsubstituted benzazaphospholides prefer N‐phosphanylation, even with bulky chlorophosphanes, and that substituents at the 2‐position of the heterocycle are crucial for the occurrence of P–N rotamers and for switching to alternative P‐substitution, beyond a threshold steric bulk, by both P‐ and 2‐position substituents.     

Conformational Analysis of Biphenyl-2,2′-Diacetate

Conformational analysis of biphenyl-2,2′-diacetate by dynamic NMR and UV spectra and by plots of enzyme activity vs temperature plots is described. From dynamic NMR spectra of the biphenyl-2,2′-diacetate with a chiral shift reagent, the coalescence temperature (T_c), the Gibbs energy of activation (ΔG^≠), and the rate coefficient (k) of bipbenyl-2,2′-diacetate were ?5 °C, 59.5 kJ/mol, and 13.3 s^?1, respectively. From analysis of the conformational break in the UV spectra and the discontinuity in the plots of enzyme activity vs temperature, the racemerization temperature of bipbenyl-2,2′-diacetate is about 5°C.     

In and F MAS NMR study of (NH4)3InF6 phases

This study presents for the first time an NMR spectroscopic characterization of the room and high temperature phases of (NH₄)₃InF₆ using ¹⁹F and ¹¹⁵In as probe nuclei. The reversible phase transition to the cubic phase at 353 K was followed by MAS NMR in situ. Static NMR experiments of the room temperature phase and MAS NMR experiments of the high temperature phase allowed the determination of the NMR parameters of both nuclei. Finally, the scalar In-F coupling, rarely observed in solid state NMR, is evidenced in both room and high temperature phases of (NH₄)₃InF₆, and measured in the high temperature phase.     

Hindered Internal Rotation about a C-N Bond in Some Trisubstituted 1,3,5-Triazines

We have studied the temperature dependence of the ¹H NMR spectra of some 2,4,6-trisubstituted 1,3,5-triazines having an NHAlk or NAlk₂ group or groups at the position 2 or 2 and 4 of the heterocycle. We have shown that rotation of these groups about the C-N bond is hindered. We have calculated the free energies of activation for the rotation processes. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1236–1239, August, 2005.     

Evaluation of microstructural features of a new polymeric organic stationary phase grafted on silica surface: A paradigm of characterization of HPLC-stationary phases by a combination of suspension-state H NMR and solid-state C-CP/MAS-NMR

Silica-supported poly(octadecylacrylate) (Sil-ODA_n), polymeric octadecylsilyl silica (polymeric ODS), and monomeric octadecylsilyl silica (monomeric ODS) were studied by a combination of suspension-state ¹H NMR and solid-state ¹³C CP/MAS-NMR to probe the mechanisms underlying their functions as stationary phases for RP-HPLC. Sil-ODA_n, with a strong temperature dependent separation behaviour showed correspondent temperature dependent manifestations in both suspension-state ¹H NMR and solid-state ¹³C CP/MAS-NMR experiments. With a gradual increase in temperature, intensity of proton signals (¹H NMR) of octadecyl moieties (mainly methylene groups) rose dramatically. This dramatic rise was at the same temperature of an endothermic peak detectable in its DSC thermogram indicating a relatively complete solid to liquid phase transition. In addition temperature dependencies of the ratio of trans to gauche conformed well to temperature dependencies of the separation factor between naphthacene and triphenylene (as a simple indicator of shape selectivity). Therefore NMR spectra of Sil-ODA_n were used as a reference for ascertaining percentage of octadecyl moieties of liquid type mobility in the two other stationary phases. Using this method we determined percentage of liquid phase in polymeric ODS and monomeric ODS at various temperatures. We suggest a combination of suspension-state ¹H NMR and solid-state ¹³C CP/MAS-NMR for structure-dynamic characterization of various kinds of hydrocarbon chains grafted onto the silica particles.     

NMR spectroscopy and relaxation studies on liquid-crystalline side-chain polysiloxanes

Two polysiloxanes with mesogenic side chains were investigated with polarization microscopy, DSC, ¹H and ¹³C NMR. From proton NMR the global order parameter of one sample (the other does not orient itself in the magnetic field of 2.1 T) and some information about phase transitions was obtained. ¹³C NMR yielded selective order parameters with respect to different atomic positions. The order parameter varies slightly also within the mesogenic unit, but much stronger within the spacer (odd-even effect). With the help of T_1ρ experiments some molecular motions had been detected and characterized (main-chain motion, over-head rotation of the mesogenic unit, methyl rotation, side-chain rotation).     

Nuclear magnetic relaxation and molecular motion in poly(vinylidine fluoride)

Pulsed NMR T₁, T₂, and T_1ρ measurements are reported for poly(vinylidine fluoride) (PVF₂). The results demonstrate clearly the presence of four relaxation processes, three amorphous and one crystalline. The α relaxation is undoubtedly a crystalline one, while β and γ are both amorphous, in agreement with earlier conclusions from dielectric and dynamic mechanical measurements. The fourth relaxation (β′) observed initially in the mechanical measurements of Kakutani, but undetected in dielectric experiments, has been confirmed in our results and the process is described by an activation energy of 15.1 kcl/mole. Motion of folds on the surface of crystal lamellae is deemed to be the responsible mechanism for the β′ relaxation. Two models have been considered in the interpretation of the α process; rotation of crystalline chains in the vicinity of defects and rotational oscillation of restricted amplitude of all crystalline chains about the main chain axes. Rotation of amorphous chains is a possible mechanism for the γ process while motions of a general nature are responsible for the β relaxation. Our experimental results again indicate that spin diffusion plays an important role in the overall NMR response of the polymer.     

Nuclear magnetic relaxation and diffusion study of the ionic liquids 1-ethyl- and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide confined in porous glass

The molecular dynamics of the room-temperature ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (Bmim Tf2N) confined in porous glass is studied by nuclear magnetic resonance (NMR) relaxometry and diffusometry and is compared with the bulk dynamics over a wide temperature range. The molecular reorientation processes for anions and cations alike are found to be significantly affected by the presence of the glass interface at high temperatures. In this respect, the ionic liquid behaves similarly to polar liquids where proton NMR relaxation is governed by reorientations mediated by translational displacements (RMTDs). This process becomes less significant towards lower temperatures when the characteristic translational correlation times of the ions approach a timescale comparable with those of the RMTD process, and the relaxation dispersions in bulk and in confinement become similar below a temperature corresponding to about 1.2T_g, a value where the onset of dynamic heterogeneity has been observed before. The self-diffusion coefficient, on the other hand, is found to be strongly reduced than the bulk within the accessible temperature range of 248 K and above and is significantly slower than expected from the tortuosity effect, suggesting that ion–surface interactions affect the macroscopic properties.     

Studies on the configuration of nitrogenous stereogenic centres in adducts of rhodium(II) tetraacylates with chiral amines: the application of 1H and 13C NMR spectroscopy

The ¹H and ¹³C NMR spectra of enantiomerically pure amines (S)-N,N-dimethyl-1-phenylethylamine, (S)-N-methyl-1-phenylethylamine, (S)-N-ethyl-1-phenylethylamine and (S)-N-ethyl-N-methyl-1-phenylethylamine in the presence of a twofold molar excess of dirhodium(II) tetratrifluoroacetate and dirhodium(II) Mosher’s acid derivatives [(4S) and (4R)] were measured in CDCl₃ as a solvent. The amines having various substituents at the nitrogen atom (H, CH₃ and CH₂CH₃) formed in such conditions as an equilibrium mixture of C_SN_R and C_SN_S 1:1 adducts. The signals of both diastereoisomers were observed in NMR spectra at either room temperature (303 K) or moderately decreased temperatures (263–273 K). The rates of mutual diastereoisomer conversion were estimated by selective inversion recovery experiments and varied from less than 0.1 to ca. 10 s^?1, depending on the ligand and temperature. Analysis of ¹³C NMR data and NOE experimental data resulted in the unambiguous determination of the configuration at the nitrogen atom with respect to the carbon stereogenic centre.Modelling of adduct structures and calculations of molecular energy and NMR parameters (GIAO) using Density Functional Theory (DFT) were performed in order to support the experimental findings. The calculations were carried out using 3-21G//B3LYP (structure optimizing) and 311G(2d,p)/LanL2DZ//B3LYP theory levels (molecular energy and NMR shielding).