Dramatic effect of solvent on the NMR spectral resolution of hyperbranched polyethers

NMR spectroscopy has served as a powerful tool to discriminate between the various types of subunits present in hyperbranched polymers. This discrimination is critical for the determination of degree of branching (DB), which is an important structural parameter that distinguishes hyperbranched structures from dendrimers. However, often times spectral signatures due to the various subunits, namely linear (L), dendritic (D) and terminal (T), are not well‐resolved. We demonstrate here that a dramatic enhancement in the peak resolution could be achieved in a series of hyperbranched polyethers, by recording the proton NMR spectra in an aromatic solvent, such as benzene, instead of the usual CDCl₃. The assignment of the various peaks was confirmed by comparison of the longitudinal relaxation times (T₁) of different protons belonging to the individual subunits. Interestingly, the chemical shift differences between the different types of subunits persist even when fairly long spacer segments separate the branch‐points in these hyperbranched polyethers. This permitted the determination of the DB of an entire series of hyperbranched polymers of varying branching densities without the need for comparison with model compounds. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1474–1480, 2007     

Temperature and solvent dependent NMR studies on mangiferin and complete NMR spectral assignments of its acyl and methyl derivatives

By employing concerted 1 and 2D NMR techniques, exact NMR spectral assignments have been made of the acyl (2-7) and methyl (8 and 9) derivatives of mangiferin (1) isolated from the leaves of Bombax ceiba. Derivatives 2, 8 and 9 have been reported in literature, while 3-7 represent new compounds. The acetates 2 and 3 were found to be unstable and were converted into the same penta-acetate 4 at room temperature. Extensive NMR studies on mangiferin (1) and its derivatives showed that H-4 exchanges with deuterium of the solvent molecule more easily. This exchange under acidic conditions occurred at that position (C-4) where electrophilic substitution reactions can easily take place. This is the first report describing the exchange of C-4 proton of mangiferin (1), or any other xanthone, with deuterium of solvent molecules.     

1H detection in MAS solid-state NMR spectroscopy of biomacromolecules employing pulsed field gradients for residual solvent suppression

In this communication, we demonstrate the feasibility of 1H detection in MAS solid-state NMR for a microcrystalline, uniformly 2H,15N-labeled sample of a SH3 domain of chicken alpha-spectrin, using pulsed field gradients for suppression of water magnetization. Today, B0 gradients are employed routinely in solution-state NMR for coherence order selection and solvent suppression. We suggest to use gradients to purge water magnetization which cannot be suppressed using conventional water suppression schemes. The achievable gain in sensitivity for 1H detection is in the order of 5 compared to the 15N detected version of the experiment (at a MAS rotation frequency of 13.5 kHz). We expect that this labeling concept which achieves high sensitivity due to 1H detection, in combination with the possibility to measure long range 1H-1H distances as we have shown previously, to be a useful tool for the determination of protein structures in the solid state.     

Linear solvation energy relationships VIII—solvent effects on NMR spectral shifts and coupling constants

The solvatochromic comparison method is used to unravel solvent polarity and hydrogen bonding effects on a variety of NMR spectral shifts and coupling constants. Solvent effects are rationalized in terms of the solvatochromic parameters π*, δ, α and β. Properties analyzed include ¹⁹F shifts of 5-fluoroindole, ¹H shifts of fluorodinitromethane, tert-butanol, phenol, 2-methylbut-1-en-3-yne, and thioacetamide, ¹H and ¹³C shifts and J(¹³C¹H) coupling constants of chloroform, ¹³C shifts of acetone, ¹⁵N shifts of pyridine, ¹⁵N and ²⁹Si shifts of 1-methylsilatrane, and some J(¹¹⁹Sn,C,¹⁹F) coupling constants of polyalkyltin compounds.     

Effects of substituent and solvent on the structure and spectral properties of maleimide derivatives

The maximum absorption wavelength , emission wavelength (λ_em) and the related oscillator strength (f) of the maleimides in the ground and first excited states were calculated by using the DFT, CIS and the time-dependent density functional theory (TD-DFT) methods, where the molecular structures were optimized by DFT/B3LYP/6-31G^* calculation. Solvent effects on the maleimides were examined using the PCM simulation at DFT/B3LYP level with the 6-31G^* basis set. For N-substituted maleimide, the substituent gives only a slight influence on the maleimide chromophore, while planar conformation of PhMLH leads to the improvement in π-delocalization from substituent to maleimide unit. For 3,4-substituted maleimide, the steric repulsion between substituent and maleimide chromophore influences the extent of π-delocalization and the molecular conformation. The calculated and λ_em of maleimides are in good agreement with the experimental data. In the gas phase, both absorption and emission peaks are red-shift as compared to the non-substituted maleimide. Under solvent environment, the more planar conformation of PhMLH shows a blue-shift in the calculated and λ_em as compared with other N-substituted maleimides. For 3,4-substituted maleimides, the effect of substitution produces the most significant spectral red-shift as compared to other maleimides.     

GFT NMR,a new approach to rapidly obtain precise high-dimensional NMR spectral information

Widely used higher-dimensional Fourier transform (FT) NMR spectroscopy suffers from two major drawbacks: (i) The minimal measurement time of an N-dimensional FT NMR experiment, which is constrained by the need to sample N - 1 indirect dimensions, may exceed by far the measurement time required to achieve sufficient signal-to-noise ratios. (ii) The low resolution in the indirect dimensions severely limits the precision of the indirect chemical shift measurements. To relax on constraints arising from these drawbacks, we present here an acquisition scheme which is based on the phase-sensitive joint sampling of the indirect dimensions spanning a subspace of a conventional NMR experiment. This allows one to very rapidly obtain high-dimensional NMR spectral information. Because the phase-sensitive joint sampling yields subspectra containing "chemical shift multiplets", alternative data processing is required for editing the components of the multiplets. The subspectra are linearly combined using a so-called "G-matrix" and subsequently Fourier-transformed. The chemical shifts are multiply encoded in the resonance lines constituting the shift multiplets. This corresponds to performing statistically independent multiple measurements, and the chemical shifts can thus be obtained with high precision. To indicate that a combined G-matrix and FT is employed, we named the new approach "GFT NMR spectroscopy". GFT NMR opens new avenues to establish high-throughput protein structure determination, to investigate systems with a higher degree of chemical shift degeneracy, and to study dynamic phenomena such as slow folding of biological macromolecules in greater detail.     

Polymer‐solvent interactions during phase transition in poly(vinyl methyl eiher)/D2O solutions as studied by NMR methods

The structural‐dynamic changes and polymer‐solvent interactions during temperature‐induced phase transition in poly(vinyl methyl ether) (PVME)/D₂O solutions in a broad range of concentrations (0.1‐30 wt.‐%) were studied by ¹H NMR methods. In the whole concentration range the phase transition is manifested by line broadening (linewidth 350‐500 Hz) of a major part of PVME units, evidently due to the formation of globular‐like structures. Above the LCST transition, the fraction of phase‐separated PVME segments is equal to 0.8±0.1, independent of polymer concentration. While at low concentrations the transition is virtually discontinuous, at high concentrations the transition region is ∼ 3 K broad. Measurements of nonselective and selective ¹H spin‐lattice relaxation times T₁ of solvent (HDO) molecules evidenced that at elevated temperatures, where most PVME forms globular structures, a part of solvent molecules is bound to PVME forming a complex; the lifetime of the bound water (HDO) molecules is ≤2 s.     

NMR spectral properties of the tetramantanes – nanometer‐sized diamondoids

Tetramantanes, and all diamondoid hydrocarbons, possess carbon frameworks that are superimposable upon the cubic diamond lattice. This characteristic is invaluable in assigning their ¹H and ¹³C NMR spectra because it translates into repeating structural features, such as diamond‐cage isobutyl moieties with distinctively complex methine to methylene signatures in COSY and HMBC data, connected to variable, but systematic linkages of methine and quaternary carbons. In all tetramantane C₂₂H₂₈ isomers, diamond‐lattice structures result in long‐range ⁴J_HH, W‐coupling in COSY data, except where negated by symmetry; there are two highly symmetrical and one chiral tetramantane (showing seven ⁴J_HH). Isobutyl‐cage methines of lower diamondoids and tetramantanes are the most shielded resonances in their ¹³C spectra (<29.5 ppm). The isobutyl methylenes are bonded to additional methines and at least one quaternary carbon in the tetramantanes. W‐couplings between these methines and methylenes clarify spin‐network interconnections and detailed surface hydrogen stereochemistry. Vicinal couplings of the isobutyl methylenes reveal positions of the quaternary carbons: HMBC data then tie the more remote spin systems together. Diamondoid ¹³C NMR chemical shifts are largely determined by α and β effects, however γ‐shielding effects are important in [123]tetramantane. ¹H NMR chemical shifts generally correlate with numbers of 1,3‐diaxial H–H interactions. Tight van der Waals contacts within [123]tetramantane's molecular groove, however, form improper hydrogen bonds, deshielding hydrogen nuclei inside the groove, while shielding those outside, indicated by Δδ of 1.47 ppm for geminal hydrogens bonded to C‐3,21 . These findings should be valuable in future NMR studies of diamondoids/nanodiamonds of increasing size. Copyright © 2015 John Wiley & Sons, Ltd.     

Synthesis and solvent effects on the electronic absorption and fluorescence spectral properties of substituted zinc phthalocyanines

The synthesis and spectroscopic properties of the following tetra- and octa-substituted aryloxy zinc(II) phthalocyanines are reported for the first time: 1,(4)-(tetrabenzyloxyphenoxyphthalocyaninato) zinc(II) (7); 2,(3)-(tetrabenzyloxyphenoxyphthalocyaninato) zinc(II) (8); 2,3-(octabenzyloxyphenoxyphthalocyaninato) zinc(II) (9). The new compounds have been characterized by elemental analysis, IR, ¹H NMR spectroscopy and electronic spectroscopy. Spectroscopic properties of these compounds were investigated in different solvents. Protonation of non-peripherally substituted complex 7 resulted in the splitting and red-shifting of the Q-band. The peripherally substituted derivatives 8 and 9, did not show the split in the Q-band. Fluorescence spectra of the derivatives show Stokes shifts typical of MPc complexes.     

Theoretical studies of 31P NMR spectral properties of phosphanes and related compounds in solution

Selected theoretical methods, basis sets and solvation models have been tested in their ability to predict (31)P NMR chemical shifts of large phosphorous-containing molecular systems in solution. The most efficient strategy was found to involve NMR shift calculations at the GIAO-MPW1K/6-311++G(2d,2p)//MPW1K/6-31G(d) level in combination with a dual solvation model including the explicit consideration of single solvent molecules and a continuum (PCM) solvation model. For larger systems it has also been established that reliable (31)P shift predictions require Boltzmann averaging over all accessible conformations in solution.     

Comparative study of the solvent effect on the spectral properties of aminopolyenic molecules with different acceptor substituents

The spectral and fluorescent properties of a number of ωω′-bis(aminopolyenyl)ketones (ketocyanines) and ωω′-bis(3-dimethylaminopropen-2-yliden)-alkylidenmalononitriles (dinitriles), being analogues of usual ketocyanines, but containing the dicyanomethylene fragment instead of the carbonyl group, were studied. Specific and nonspecific solvation of dinitriles and corresponding ketocyanines was studied on the basis of the solvatochromic shifts of the absorption spectra. The photophysical data obtained for the different molecular structures were correlated with different empirical parameters, which characterize solvation by organic solvents: the Dimroth-Reichardt ET(30) parameter of the solvating power, the acceptor number AN, the donor number DN, and the constant π* characterizing the solvent polarity and polarizability. A linear dependence of the maximums in the absorption spectra on the acceptor number AN, which characterizes specific solvation, was found for the ketocyanines under study. This indicates that the electrophilic solvation plays a key role in ketocyanine solutions. For dinitriles, the linear dependence of the maximums of the absorption spectra was obtained as f(π*, AN, DN). In the latter case, along with the electrophilic solvation, there is a substantial contribution of nonspecific and nucleophilic solvation.     

Comparison of theoretical and experimental models for characterizing solvent properties using reversed phase liquid chromatography

Theoretical and experimental quantitative structure–retention relationships (QSRR) models are useful for characterizing solvent properties and column selectivity in reversed phase liquid chromatography (RPLC). The chromatographic behavior of a model analyte, the herbicide atrazine, in a system derived from nine organic solvents and three chromatographic columns was used for developing QSRR models. Multiple linear regression (MLR) and partial least squares regression (PLSR) were used as statistical approaches. The similarities and differences between linear solvation energy relationships (LSER), and semi-empirical and theoretical molecular models were demonstrated. QSRR models show high predictive power, and can successfully predict retention factor (log k) for new solvents. The models are useful for solvent optimization and reducing time for method development in RPLC. The herbicide atrazine can be readily analyzed at a low level, and all three columns provided good resolution, high-performance and symmetrical peaks. The method is suitable for analysis of atrazine in water samples.     

1H NMR spectral properties and conformational preferences of some 1,2,9,10,17,18,25,26-octathia[2.2.2.2]-metacyclophanes

The stereochemistry in solution of three octathia[2.2.2.2]metacyclophanes has been investigated by means of ¹H NMR spectroscopy. The results lead to the conclusion that the saddle or crown conformation is preferred for these compounds, depending on the substitution pattern of the constituent aromatic rings.     

A new phase modulated binomial‐like selective‐inversion sequence for solvent signal suppression in NMR

A new 8 ‐pulse P hase M odulated binomial‐like selective inversion pulse sequence, dubbed ‘8PM’, was developed by optimizing the nutation and phase angles of the constituent radio‐frequency pulses so that the inversion profile resembled a target profile. Suppression profiles were obtained for both the 8PM and W5 based excitation sculpting sequences with equal inter‐pulse delays. Significant distortions were observed in both profiles because of the offset effect of the radio frequency pulses. These distortions were successfully reduced by adjusting the inter‐pulse delays. With adjusted inter‐pulse delays, the 8PM and W5 based excitation sculpting sequences were tested on an aqueous lysozyme solution. The 8 PM based sequence provided higher suppression selectivity than the W5 based sequence. Two‐dimensional nuclear Overhauser effect spectroscopy experiments were also performed on the lysozyme sample with 8PM and W5 based water signal suppression. The 8PM based suppression provided a spectrum with significantly increased (~ doubled) cross‐peak intensity around the suppressed water resonance compared to the W5 based suppression. Copyright © 2016 John Wiley & Sons, Ltd.     

Substituent effect to prevent autoxidation and improve spectral stability in blue light-emitting polyfluorenes

A group of fluorene-based polymers, PF-1SOR and PF-2SOR, were synthesized and characterized as blue light-emitting materials. PF-1SOR and PF-2SOR displayed nematic liquid crystalline mesophase in films cast from solution. Compared with conventional polyfluorene, PF-1SOR and PF-2SOR display blue-shifted UV absorption and structureless blue fluorescence. The photoluminescence spectra of PF-1SOR and PF-2SOR were found insensitive against thermal treatment in air up to 200 degrees C and the blue electroluminescence in their light-emitting devices was independent of the driving voltage. Compared to the conventional polyfluorenes, the improved spectral stability of these polymers is attributed to the anti-oxidization effect of (3,5-di(tert-butyl)phenoxy)sulfonyl side groups attached to the backbone.     

Effect of the solvent composition in methanol-pentane and methanol-acetonitrile mixtures on the spectral and luminescent properties of 1,2-dihydroquinolines

The dependence of absorption and fluorescence spectra, quantum yields, and lifetimes of fluorescence on the solvent composition in the MeOH-C₅H₁₂ and MeOH-MeCN mixtures was studied for 2,2,4,6-tetramethyl-1,2-dihydroquinoline (TMDHQ). The variations in the parameters of deconvolution of the absorption and fluorescence spectra by the Gaussian functions in the MeOH-C₅H₁₂ mixtures of various compositions indicate the specificity of methanol clustering in saturated hydrocarbons and hydrogen bonding between TMDHQ and the methanol clusters of different compositions. At low MeOH concentrations (∼0.2 vol %), TMDHQ molecules are practically completely bound with the MeOH molecules by hydrogen bonds. In the MeOH-MeCN mixtures, the changes in the absorption and fluorescence spectra are observed at a substantially higher MeOH concentration (≥10 vol %) and monotonically change at the further increase in the MeOH concentration that is caused by the peculiarities of MeOH clustering in acetonitrile and the distribution of the TMDHQ molecules between the solvent components. At 50–95 vol % of MeOH in the mixture with MeCN, the fluorescence decay kinetics is described by the biexponential curve with the lifetime of the major component (τ₁) decreasing from 7.5 to 1.1 ns in pure MeCN and MeOH, respectively, and the lifetime of the minor component τ₂ ≈ 4 ns corresponding to the fluorescence lifetime in the solution containing 50 vol % MeOH. This indicates the existence of the free TMDHQ molecules, which are not bound with MeOH molecules or their clusters.     

Chemical modification of pectin to improve its sorption properties

The sorption properties of natural polysaccharide (apple pectin) with respect to heavy metals in a Ringer salt solution were studied as influenced by the procedures and process parameters of its chemical modification.