Dependence Of NMR Chemical Shifts Research Articles

Negative thermal expansion of a disordered native protein

Despite immense interest in thermal expansion of solid materials, negative thermal expansion in particular, expansivity of proteins near ambient temperature is minimally reported. One reason for the paucity of protein expansion studies could be the susceptibility of proteins to conformational changes often within subdenaturing temperatures, given that the expansivity phenomenon should include only vibrational fluctuations in atom positions in the absence of a conformational change. This study encounters a disordered plant protein called At PP16-1 ( Arabidopsis thaliana phloem protein type 16-1) which shows negative thermal expansion in the 292–309(±1) K range of temperature at no expense of conformational transition. Temperature dependence of NMR chemical shifts of backbone atoms, global tumbling time derived from 15 N relaxations, and volume compressibility measurements yield a negative linear expansion coefficient 〈α〉 ∼ −5.3 × 10 −3 K −1 averaged over a sizable set of hydrogen bonds, and a decrease in the root mean square volume fluctuation by ∼36%. A frequency spectrum of normal modes in which one or a fewer low frequency vibrational modes of large fluctuation amplitudes are excited at the lower end, and many high-frequency vibrational modes of lower fluctuation amplitudes are excited at the higher end of the temperature range can explain negative thermal expansion.

Dependence of NMR chemical shifts upon CH bond lengths of a methyl group involved in a tetrel bond

Four different Lewis acids that might participate in a tetrel bond with a nucleophile (SEt2Me+, NMe4+, SMe2, NMe3) are examined. The NMR chemical shifts of the methyl C and H atoms are calculated as the CH bond lengths are systematically stretched and contracted, in the absence of a base. The C shielding diminishes by roughly 2 ppm for a stretch of 0.01 Å, while that of H drops by only 0.3 ppm. The deshieldings caused purely by the bond length changes are far too small to account for the amounts that are computed when the nucleophile is actually present.

NMR and TRLFS studies of Ln(iii) and An(iii) C5-BPP complexes.

C5-BPP is a highly efficient N-donor ligand for the separation of trivalent actinides, An(iii), from trivalent lanthanides, Ln(iii). The molecular origin of the selectivity of C5-BPP and many other N-donor ligands of the BTP-type is still not entirely understood. We present here the first NMR studies on C5-BPP Ln(iii) and An(iii) complexes. C5-BPP is synthesized with 10% 15N labeling and characterized by NMR and LIFDI-MS methods. 15N NMR spectroscopy gives a detailed insight into the bonding of C5-BPP with lanthanides and Am(iii) as a representative for trivalent actinide cations, revealing significant differences in 15N chemical shift for coordinating nitrogen atoms compared to Ln(iii) complexes. The temperature dependence of NMR chemical shifts observed for the Am(iii) complex indicates a weak paramagnetism. This as well as the observed large chemical shift for coordinating nitrogen atoms show that metal-ligand bonding in Am(C5-BPP)3 has a larger share of covalence than in lanthanide complexes, confirming earlier studies. The Am(C5-BPP)3 NMR sample is furthermore spiked with Cm(iii) and characterized by time-resolved laser fluorescence spectroscopy (TRLFS), yielding important information on the speciation of trace amounts of minor complex species.

Evidence for covalence in a N-donor complex of americium(iii)

The molecular origin of the selectivity of N-donor ligands, such as alkylated bis-triazinyl pyridines (BTPs), for actinide complexation in the presence of lanthanides is still largely unclear. NMR investigations of an Am(nPrBTP)3(3+) complex with a (15)N labelled ligand showed that it exhibits large differences in (15)N chemical shift for coordinating N-atoms in comparison to both lanthanide(III) complexes and the free ligand. The temperature dependence of NMR chemical shifts observed for this complex indicates a weak paramagnetism. This fact and the observed large chemical shift for bound nitrogen atoms allow us to conclude that metal-ligand bonding in the reported Am(III) N-donor complex has a larger share of covalence than in lanthanide complexes. This may account for the observed selectivity.

Supramolecular Method for the Determination of Absolute Configuration of Chiral Compounds: Theoretical Derivatization and a Demonstration for a Phenolic Crown Ether−2-Amino-1-ethanol System

A combination of anisotropic shielding effect and temperature dependence of NMR chemical shifts makes it possible to develop a supramolecular method for determination of absolute configuration of chiral compounds. The concept, theoretical derivatization, and first demonstration of this daedal noncovalent method using a host-guest system is described. This noncovalent method requires only three substrate solutions and can in principle be applied to any host-guest systems that follow the van't Hoff relation and have a clear anisotropic shielding effect based on a well-defined complex structure regardless of the extent of enantiomer selectivity.

NMR 化学シフトの構造依存：個々の分子軌道に基づく解釈とpre-α効果の提案

NMR 化学シフトの構造依存：個々の分子軌道に基づく解釈とpre-α効果の提案

Enantiomeric Separation with Sodium Dodecanoyl-L-amino Acidate Micelles and Poly(sodium (10-undecenoyl)-L-valinate) by Electrokinetic Chromatography

The sodium salts of N-dodecanoylated L-valine, L-alanine, and L-threonine formed micelles which were then used to resolve enantiomeric 3,5-dinitrobenzoylated amino acid isopropyl esters and amines by electrokinetic chromatography (EKC). Hydrophobicity of the micellar core essential for enantiomer separation was observed with the concentration dependence of NMR chemical shift of amide protons in the surfactants. The amide functionality in chiral micelles was found to be shielded from bulk water. This functionality is thus capable of serving as a hydrogen bonding site. The elution order of separated amino acid derivatives, the D enantiomer eluting faster than the corresponding L enantiomer, indicates that the chiral micelle binds to the L enantiomer, having the same configuration as its chiral component to a greater extent than the D counterpart. In the following, this is discussed in terms of differences in the perturbation of the micellar structure produced by enantiomer penetration. The photopolymerization of vinyl group-terminated chiral surfactants, analogous to the above surfactant derived from L-valine, gave rise to micelle-like polymers with constraints imposed by the covalently linked tails of surfactant monomers. This poly(sodium (10-undecenoyl)-L-vainate) showed chromatographic resolution behavior similar to that of chiral micelles in EKC, indicating that chiral recognition is possible through the explicit polymolecular structure independent of the dynamic association-dissociation equilibrium of ordinal surfactants in the bulk water phase.

Gas-to-liquid shifts in NMR and the validity of the second virial coefficient of chemical shielding

NMR spectra are reported for low density gas samples as well as gas-to-liquid shifts for 19F nuclei in several systems. The density dependence of NMR chemical shifts thus obtained gives values of B-A as a measure of higher order terms. (AIP)

Temperature dependence of NMR chemical shifts in cuprous halides

Measurements of NMR chemical shifts of 63Cu, 35Cl, 81Br, and 127I in the cuprous halides are reported from about −150 °C to temperatures above the melting points. From the low temperature data it is deduced by means of a simple tight binding description of chemical bonding that CuBr is the most ionic of these compounds and CuI the most covalent. In all solid and liquid phases the shifts were found to depend linearly on temperature. In the low temperature cubic phases, the cation shifts increase with temperature, while the anion shifts decrease with temperature. This unusual behavior is shown to be caused by an increase of vibrational overlap with temperature. In the high temperature cubic phases and in the melts the observed increasing diamagnetism of the cation shifts with temperature is related to the highly disordered state of the cuprous ions in these phases. Possible influences of nonstoichiometry on chemical shift are discussed in connection with measurements performed on CuI under iodine vapor.

Dependence of NMR chemical shift on position with respect to an aromatic ring

An expression is given for the dependence of the NMR chemical shift due to π electrons on the position of the nucleus relative to an aromatic ring. Calculations based on the expression are compared with those using three other theories, and are found to agree with a less approximate theory for points not too close to the ring.