Proton shielding tensors in potassium hydrogen maleate

The proton NMR in single crystals of potassium hydrogen maleate has been sttudied by means of multiple-pulse line-narrowing techniques. The magnetic shielding tensors of all magnetically inequivalent protons in the unit cell could be determined independently. Two of these protons are carboxylic, forming hydrogen bonds. The orientations of the shift tensors are consistent with the position of the hydrogens at the midpoints of the 0–0 intervals. The range of anisotropy of 32 ppm, found for the shift tensor of the caboxylic hydrogen, is larger than that found for hydrogen bonds in acids and seems to be characteristics of acidic salts.The other protons in the unit cell are olefinic. Two features distinguish this type of protons from those studied so far: (1) The magnetic shielding tensor is not even approximately axially symmetric, the principal values being ?2.4, ?5.1, ?7.3 ± 05 ppm (from adamantane); and (2) the principal directions reflect all characteristic directions of the carboncarbon double bond (while the CH direction is of no importance). The principal value in the direction perpendicular to the sp² system is the least shielded one.     

Proton magnetic shielding and susceptibility effects in single crystals of ferrocene

The proton NMR in single crystals of ferrocene has been studied by multiple pulse techniques at room temperature. In crystals of natural shape with plane faces the angular dependence of the resonance frequency due to the anisotropy of the shielding and due to the bulk susceptibility was found to be of about equal size, making analysis of the data practically impossible. By using a single crystal sphere the shape dependent part of the susceptibility contribution could be eliminated. In addition, the resolution obtained by the multiple pulse technique is considerably higher for spheres than for non-spherical crystals. As the ferrocene molecules rotate rapidly about their fivefold axes at room temperature, the shielding tensor σ must be axially symmetric. Having this in mind, the data could be analyzed to yield both the shielding tensor with Δσ = σ₆ - σ_⊥ = ?6.5 ± 0.1 ppm, σ_iso = ?4.2 ± 0.5 ppm from a spherical sample of TMS and the anisotropy of the susceptibility Δ _x = 30 × 10^?6 cgs units.     

Magnetic shielding tensor components in a highly strained phosphorus molecule: 1,2,3-triphenylphosphirene

Using the magic-angle spinning (MAS) technique the principal values of the ³¹P NMR magnetic shielding tensor were measured in 1,2,3-triphenylphosphirene: σ₁₁ = −58; σ₂₂ = 23 and σ₃₃ = 636 ppm. The anisotropy is quite large for a three-coordinate phosphorus compound. The high-field component σ₃₃ corresponds to the direction perpendicular to the ring, and is responsible for the high-field resonance in the isotropie phase.     

Orbital Analysis of Carbon‐13 Chemical Shift Tensors Reveals Patterns to Distinguish Fischer and Schrock Carbenes

Fischer and Schrock carbenes display highly deshielded carbon chemical shifts (>250 ppm), in particular Fischer carbenes (>300 ppm). Orbital analysis of the principal components of the chemical shift tensors determined by solid‐state NMR spectroscopy and calculated by a 2‐component DFT method shows specific patterns that act as fingerprints for each type of complex. The calculations highlight the role of the paramagnetic term in the shielding tensor especially in the two most deshielded components (σ₁₁ and σ₂₂). The paramagnetic term of σ₁₁ is dominated by coupling σ(M=C) with π*(M=C) through the angular momentum operator perpendicular to the σ and π M=C bonds. The highly deshielded carbon of Fischer carbenes results from the particularly low‐lying π*(M=C) associated with the CO ligand. A contribution of the coupling of π(M=C) with σ*(M=C) is found for Schrock and Ru‐based carbenes, indicating similarities between them, despite their different electronic configurations (d⁰ vs. d⁶).     

Empirical relations between σI, σoR and taft σ* values of alkyl,acyl, benzoyl and substituted phenyl groups

The X(X) values¹ of the halogens (which resemble the Pauling electronegativities) and of some oxa substituents can be interpreted in terms of the inductive and resonance parameters σ_I and σ^o_R according to the regression equation

and η*_R=η(X)?η(R) it is found that for some substituted methyl, phenyl and benzoyl groups [σ*]^X_R=αη^X_R where α equals ?10.6 and ?10.9 for R = Me and R = Ph, CHO and PhCO respectively. Thus [σ*]^X_Rand η^x_r represent Taft σ* and [σ_I(X)?σ_oR(X)] values relative to that of the parent R group. The hydroxyl frequencies of phenol, and benzoic, acrylic, acetic and formic acids measured in dilute carbon tetrachloride solutions correlate with σ_I(X) and σ^o_R(X) according to the equations v(OH) = ?423.0 σ_I(X) + 3654.7 v(OH) = ?270.0 σ⁰_R(X) + 3586.7 where X = Ph, PhCO, CH₂=CHCO, MeCO and HCO. From these results, it is inferred that the σ* values of substituents having an α sp² hybridized carbon atom are proportional to σ⁰_R according to the equation σ*(X) = 3.97 σ⁰_R(X) + 1 New σ_I σ^o_R and σ* values of some acyi, benzoyl and substituted phenyl groups are presented.     

270 MHz multiple pulse study of the olefinic protons in potassium hydrogen maleate

The nuclear magnetic shielding tensors of the protons in potassium hydrogen maleate (KHM) have been determined in single crystals by means of multiple pulse line narrowing techniques at 270 MHz. The increase of resolution in solid state NMR spectra on switching the spectrometer frequency from 90 MHz to 270 MHz is demonstrated. The results for the carboxylic protons in KHM agree fully with those of a previous 90 MHz study. Unlike at 90 MHz separate lines from individual olefinic proton sites could be resolved at 270 MHz allowing a straightforward determination of the corresponding shielding tensors. The principal shielding components found are 0.9, ?2.0, ?3.1 ± 0.2 ppm relative to liquid water. An assignment of the four experimental shielding tensors to the four olefinic sites in the crystal is proposed on the basis of molecular and local symmetry. According to this assignment the most shielded direction of the olefinic protons is in the molecular plane of the maleate anion and perpendicular to the CH bond axis.     

An Investigation of 1:1 Adducts of Gallium Trihalides with Triarylphosphines by Solid‐State 69/71Ga and 31P NMR Spectroscopy

Several 1:1 adducts of gallium trihalides with triarylphosphines, X₃Ga(PR₃) (X=Cl, Br, and I; PR₃=triarylphosphine ligand), were investigated by using solid‐state ^69/71Ga and ³¹P NMR spectroscopy at different magnetic‐field strengths. The ^69/71Ga nuclear quadrupolar coupling parameters, as well as the gallium and phosphorus magnetic shielding tensors, were determined. The magnitude of the ⁷¹Ga quadrupolar coupling constants (C_Q(⁷¹Ga)) range from approximately 0.9 to 11.0 MHz . The spans of the gallium magnetic shielding tensors for these complexes, δ₁₁?δ₃₃, range from approximately 30 to 380 ppm; those determined for phosphorus range from 10 to 40 ppm. For any given phosphine ligand, the gallium nuclei are most shielded for X=I and least shielded for X=Cl, a trend previously observed for In^III–phosphine complexes. This experimental trend, attributed to spin‐orbit effects of the halogen ligands, is reproduced by DFT calculations. The signs of C_Q(^69/71Ga) for some of the adducts were determined from the analysis of the ³¹P NMR spectra acquired with magic angle spinning (MAS). The ¹J(^69/71Ga,³¹P) and ΔJ(^69/71Ga, ³¹P) values, as well as their signs, were also determined; values of ¹J(⁷¹Ga,³¹P) range from approximately 380 to 1590 Hz. Values of ¹J(^69/71Ga,³¹P) and ΔJ(^69/71Ga,³¹P) calculated by using DFT have comparable magnitudes and generally reproduce experimental trends. Both the Fermi‐contact and spin‐dipolar Fermi‐contact mechanisms make important contributions to the ¹J(^69/71Ga,³¹P) tensors. The ³¹P NMR spectra of several adducts in solution, obtained as a function of temperature, are contrasted with those obtained in the solid state. Finally, to complement the analysis of NMR spectra for these adducts, single‐crystal X‐ray diffraction data for Br₃Ga[P(p‐Anis)₃] and I₃Ga[P(p‐Anis)₃] were obtained.     

Gas-phase 13C chemical shifts in the zero-pressure limit: refinements to the absolute shielding scale for 13C

¹³C chemical shifts have been measured relative to ¹³CO in the zero-pressure limit for over twenty molecules for which theoretical calculations of ¹³C nuclear shielding have recently been reported. Rovibrational averaging effects on the spin-rotation constant in ¹³C¹⁶O have been used to find σ_e(¹³C in ¹³C¹⁶O) = 3.0 ± 1.2 ppm and σ₀(¹³C in ¹³C¹⁶O) = 1.0 ± 1.2 ppm. With the latter, the σ₀ values for the ¹³C nuclei in this work have been determined absolutely and compared with calculated values. Agreement is generally good in most cases except where low-lying n → π transitions contribute significantly to the paramagnetic shielding.     

1H NMR spectra part 31: 1H chemical shifts of amides in DMSO solvent

The ¹H chemical shifts of 48 amides in DMSO solvent are assigned and presented. The solvent shifts Δδ (DMSO‐CDCl₃) are large (1–2 ppm) for the NH protons but smaller and negative (?0.1 to ?0.2 ppm) for close range protons. A selection of the observed solvent shifts is compared with calculated shifts from the present model and from GIAO calculations. Those for the NH protons agree with both calculations, but other solvent shifts such as Δδ(CHO) are not well reproduced by the GIAO calculations. The ¹H chemical shifts of the amides in DMSO were analysed using a functional approach for near ( ≤ 3 bonds removed) protons and the electric field, magnetic anisotropy and steric effect of the amide group for more distant protons. The chemical shifts of the NH protons of acetanilide and benzamide vary linearly with the π density on the αN and βC atoms, respectively. The C=O anisotropy and steric effect are in general little changed from the values in CDCl₃. The effects of substituents F, Cl, Me on the NH proton shifts are reproduced. The electric field coefficient for the protons in DMSO is 90% of that in CDCl₃. There is no steric effect of the C=O oxygen on the NH proton in an NH…O=C hydrogen bond. The observed deshielding is due to the electric field effect. The calculated chemical shifts agree well with the observed shifts (RMS error of 0.106 ppm for the data set of 257 entries). Copyright © 2014 John Wiley & Sons, Ltd.     

17O quadrupole coupling and chemical shielding tensors in an H-bonded carboxyl group: alpha-oxalic acid

We have used single crystal (17)O NMR and density functional theory to investigate intermolecular interactions in a strongly H-bonded system. The chemical shielding (CS) and quadrupole coupling (QC) tensors are determined in oxalic acid dihydrate by single crystal methods. With cross polarization from abundant protons, high quality spectra are obtained in 1-2 min from 10 micromol samples. In the crystal lattice, oxalic acid is H-bonded directly to the hydrate with each carboxyl group accepting two H-bonds at C=O and donating one H-bond from COH. The effects of these intermolecular interactions on the experimentally determined QC and CS tensors are modeled by density functional theory with a procedure that accurately calculates, without scaling, the known QC tensors in both gas-phase water and ice. The ice calculation uses a cluster containing 42 waters (in excess of two complete hydration shells). The same procedure applied to a similarly constructed cluster of hydrated oxalic acid gives QC and CS tensors that are within 3-6% of the observed values while isolated molecule tensors are significantly different. Comparison of the isolated molecule tensors with those from either experiment or the cluster calculation shows the magnitude and directionality of intermolecular interactions on the tensors. The isotropic shift of the COH oxygen is deshielded by 29 ppm, and C=O is shielded by 62 ppm while the spans of the CS tensors are increased by 78 ppm and decreased by 73 ppm, respectively. Magnitudes of the quadrupole coupling constants, which are proportional to the electric field gradients at the (17)O sites, decrease by 2.2 and 1.2 MHz at COH and C=O, respectively.     

Binuclear metal carbonyl dab complexes : VII. A 13C NMR investigation of MM′(CO)6(DAB) complexes (M = M ′ = Fe,Ru; M = Mn,Re and M′ = Co; DAB = 1,4-diazabutadiene). Dynamic behaviour of σ2-N, σ2-N′, η2-C=N coordinated dab in MnCo(CO)6(DAB) and local scrambling of the carbonyl groups

The ¹³C NMR spectra of MM′(CO)₆(DAB) complexes (M = M′ = Fe, Ru; M = Mn, Re and M′ = Co; DAB = 1,4-diazabutadiene) show very characteristic features which are directly related with the bonding mode of the DAB ligand to the binuclear metal carbonyl fragment. In these complexes the DAB ligand is σ²-N, μ²-N′, η²-C=N or σ²-N, σ²-N′, η²-C=N coordinated. Chemical shifts of about 175 ppm are observed for the σ-coordinated imine fragments and about 60 or 80 ppm for the η²-C=N coordinated imine fragments.In MnCo(CO)₆[diacetylbis(cyclopropylimine)] the DAB ligand is fluxional, and the changes in the spectra when recorded at various temperatures can be interpreted in terms of an exchange between the σ- and π-coordinated part of the DAB ligand.The homodinuclear M₂(CO)₆(DAB) complexes (M = Fe or Ru) contain M(CO)₃ fragments on which the carbonyl groups are involved in a local scrambling process with very different activation parameters (T_c = ?50°C and +85°C).MCo(CO)₆(DAB) complexes (M = Mn, Re), which contain a semi-bridging carbonyl group according to the crystal structure, show rapid interchange of this carbonyl group with the terminal carbonyl groups on cobalt. The electronic balance is kept in equilibrium by an internal compensation within the DAB ligand.     

BN-Substituted fullerenes C60−2x (BN) x : a computational 11B and 15N NMR study

Density functional theory has been used to investigate the ¹⁵N and ¹¹B NMR parameters of heterofullerenes C_60?2x (BN) _x (x = 1, 2, 3, 6, 9, 12, 15, 18, 21, and 24). Geometry structures of all the BN-substituted fullerenes have been optimized at the B3LYP/6-31+G* level of theory. Afterward, ¹¹B and ¹⁵N chemical shielding isotropy and anisotropy (CSI, CSA) parameters have been calculated at the same level. The obtained results illustrate the electrostatic environment divisions of the nuclei into few layers, which have been then confirmed by calculating natural charges at B and N sites. A good correlation has been seen between the layers of CSI and CSA values and three local structures around boron and nitrogen atoms. The effects of curvature of fullerene structure on chemical shielding (CS) parameters of heterofullerenes have also been investigated by computing CS tensors for curved and relaxed structures of a set of small fragments separated from the heterofullerenes, suggesting high sensitivity of CS parameters to the curvature of fullerene structure.     

1H NMR spectra. Part 29§: proton chemical shifts and couplings in esters—the conformational analysis of methyl γ‐butyrolactones

The ¹H NMR spectra of 35 cyclic and acyclic esters are analysed to give the ¹H chemical shifts and couplings. The substituent chemical shifts of the ester group were analysed using three‐bond (γ) effects for near protons and the electric field, magnetic anisotropy and steric effect of the ester group for more distant protons. The electric field is calculated from the partial atomic charges on the O?C = O atoms, and the asymmetric magnetic anisotropy of the carbonyl group acts at the midpoint of the C = O bond. The values of the anisotropies Δχ_parl and Δχ_perp were for the aliphatic esters 10.35 and ?18.84 and for the conjugated esters 7.33 and ?15.75 (×10^?6 ų/molecule). The oxygen steric coefficients found were 104.4 (aliphatic C = O), 45.5 (aromatic C = O) and 16.0 (C–O) (×10^?6 Å⁶/molecule). After parameterisation, the overall RMS error for the data set of 280 entries was 0.079 ppm. The strongly coupled ¹H NMR spectra of the 2‐methyl, 3‐methyl and 4‐methyl γ‐butyrolactones were analysed and the methyl conformational equilibrium obtained from the observed couplings. The observed versus calculated density functional theory (DFT) ΔG(ax‐eq) was 1.0 (1.01), 0.34 (0.54) and 0.65 (0.71) kcal/mol res. The shielding effect of a methyl cis to a proton in the five‐membered lactone rings is ?0.40 ±0.05 ppm and deshielding trans effect 0.12 ±0.05 ppm, which is common to both five and six membered rings. The cis/trans isomerism in the vinyl esters methyl acrylate, crotonate and methacrylate and methyl furoate was examined using the ¹H chemical shifts. The calculated shifts of both the cis and trans isomers were in good agreement with the observed shifts. Copyright © 2012 John Wiley & Sons, Ltd.     

The 19F shielding anisotropy of p-nitrofluorobenzene from nematic phase NMR

NMR experiments on p-nitrofluorobenzene dissolved in a nematic solvent are reported. It is shown that the ¹⁹F chemical shift tensor does not possess axial symmetry around the CF bond. For σ_aa ? σ_bb a value of 81 ppm, for σ_cc ? σ_bb a value of 156 ppm is determined.     

Single crystal NQR and NMR study of (NH4)2InBr6·H2O

The ⁸¹Br NQR tensors of (NH₄)₂InBr₅·H₂O have been studied on single crystals by combining a 4π Zeeman goniometer with Fourier transform NQR. AT T=293K the coupling constants are 136.27 e²φ_ZZQh^?(su81Br)/Mhz ≦ 146.14 and the asymmetry parameter 0.013 ≦ η (⁸¹Br) ≦ 0.133. The EFG axes φ_zz(⁸¹Br) are almost parallel to the bond directions InBr. The strong changes of φ_yy(¹¹⁵In) and φ_zz(¹¹⁵In) with temperature can be explained by assuming small changes of the In(OH₂) bond distance in the complex anion (InBr₅·H₂Cl^2?. The D₂O molecule within the deuterated complexes flips freely in the range studied (100 ≦ T/K ≦ 298). Small EFG's at the ¹⁴N site in the cation ND^?₄ are due to the EFG of the surrounding lattice charges.     

Ligand effects in the substitution chemistry of cis-bis(piperidine)tetracarbonylmolybdenum(O). A molybdenum-95 NMR study

Molybdenum-95 NMR chemical shifts are reported for a series of Mo(O) compounds of the type Mo(CO)₄(pip)_2−nL_n(n = 1,2; L = substituted pyridine ligands). The σ(⁹⁵Mo) values correlate well with the pK_a values for the substituted pyridines; for the n = 1 series, σ (⁹⁵Mo) ranges from − 1053 ppm (pK_a = 1.86 for 4-CN) to − 1120 ppm (pK_a = 9.61 for 4-NMe₂). The effects of solvent polarity and some in situ reactivity studies are described and the nature of the MoL bond compared to that with piperidine and some other ligands is discussed.     

High-resoluttin 29Si NMR in solid silicates: Correlations between shielding tensor and Si-O bond length

²⁹Si NMR spectra of polycrystalline Ca₆[Si₂O₇¦(OH)₆] and [(CH₃)₄N] ₈Si₈O₂₀·69H₂O were measured using the cross polarization double-resonance technique. Observed shielding tensors are related to the known Si-O bond systems. The arrangement of the four Si-O bonds in the SiO₄ tetrahedra is reflected by the ²⁹Si shielding tensor. The most shielded direction corresponds to the shortest Si-O bond.