Solvent Dependence of H-F Coupling Parameters in Fluorobenzene

Solvent dependence of NMR spectral parameters has been the object of considerable interest in recent years (see Ref. 1 for a brief summary). More recently attention has been devoted to the solvent dependence of NMR parameters in fluorine-containing compounds. In their investigation of fluorobenzene, Mohanty and Venkateswarlu observed the effect of solvents on the proton and fluorine chemical shifts, but did not report any dependence of the H-K or H-F couplings on the solvent medium.² For 1,2-dichlorofluoroethylene Bell and Danyluk found a linear correlation between the directly bonded ¹³C-H and ¹³C-F couplings and the H-F couplings.³ These results, along with linear correlations between the ¹³C-H (or ¹³C-F) coupling and the proton (or fluorine) chemical shift, led to the inference that the medium effect acts to change the coupling parameters and shieldings by a similar mechanism.     

Measurement of the proton NMR field shift in aqueous solutions containing Cu2+, Cr3+, Fe3+ and Mn2+ ions

There are presented measurements of the NMR field shifts for aqueous protons in aqueous solutions containing paramagnetic inorganic compounds of Cu²⁺, Cr³⁺, Fe³⁺ and Mn²⁺ ions. The measurements have been performed on samples in the form of transversally magnetized long cylinders using both the internal and external NMR standards. The experimentally determined shifts are related to the NMR field position of protons in pure water. The results for demagnetizing shifts are compared with the data which were computed from the magnetic susceptibility values (measured by magnetostatic method), the chemical shifts are compared with the results of other authors. Results of measurements indicate a small chemical shift of internal standards in some solutions.     

Hydroxyl Substituent Chemical Shift (SCS) Effects in Alcohols: The 17O NMR of Diols

The ¹⁷O NMR spectra for a series of saturated diols were investigated. From these studies both hydroxyl induced substituent chemical shift (SCS) effects of hydroxyl oxygen ¹⁷O NMR chemical shifts were determined. In addition, linear correlations between the ¹⁷O chemical shift of the hydroxyl oxygen (ROH) and the ¹³C chemical shift for the methyl group in the corresponding hydrocarbon (RCH₃) were obtained.

     

取代苯甲酰卤羰基17O-NMR化学位移的研究

按照苯甲酰氯、苯甲酰氟和苯甲酰溴三类化合物,提出了一个计算这三类化合物羰基¹⁷O-NMR化学位移的公式：δ_cal=δ_0n+C×(Δ_o+Δ_m+Δ_p),通过线性回归法确定了13种取代基参数,经回归检验表明该公式的置信度为99.5%,与实验值的偏差Δδ在5.0以内的羰基¹⁷O-NMR化学位移计算值在90%以上.     

Systematic trends in the237neptunium Mössbauer isomer shift: Overlap of IV,V and VI neptunium oxidation states and correlation between isomer shift and crystal structure

The influence of the neptunium ion environment on the²³⁷Np Mössbauer isomer shifts has been studied in various metal coordination complexes: fluorides, oxides, oxide fluorides and polycarboxylates. A linear dependence between the isomer shift and the mean neptunium-ligand distance in a series of hexavalent Np compounds has been evidenced and the feasibility of overlapped isomer shift areas, namely Np(IV), Np(V) and Np(VI) has been established.     

125Te NMR chemical-shift trends in PbTe–GeTe and PbTe–SnTe alloys

Complex tellurides, such as doped PbTe, GeTe, and their alloys, are among the best thermoelectric materials. Knowledge of the change in ¹²⁵Te NMR chemical shift due to bonding to dopant or “solute” atoms is useful for determination of phase composition, peak assignment, and analysis of local bonding. We have measured the ¹²⁵Te NMR chemical shifts in PbTe-based alloys, Pb_1−xGe_xTe and Pb_1−xSn_xTe, which have a rocksalt-like structure, and analyzed their trends. For low x, several peaks are resolved in the 22-kHz MAS ¹²⁵Te NMR spectra. A simple linear trend in chemical shifts with the number of Pb neighbors is observed. No evidence of a proposed ferroelectric displacement of Ge atoms in a cubic PbTe matrix is detected at low Ge concentrations. The observed chemical shift trends are compared with the results of DFT calculations, which confirm the linear dependence on the composition of the first-neighbor shell. The data enable determination of the composition of various phases in multiphase telluride materials. They also provide estimates of the ¹²⁵Te chemical shifts of GeTe and SnTe (+970 and +400±150 ppm, respectively, from PbTe), which are otherwise difficult to access due to Knight shifts of many hundreds of ppm in neat GeTe and SnTe.     

The Endo-Exo Isomerization of N-Phenyl-5-norbornene-2,3-dicarboximide

ENDO isomers of N-phenyl-5-norbornene-2,3-dicarboximide was converted to exo-isomers below retro-Diels-Alder temperature under various reaction conditions. The exo-endo yield ratio was studied by NMR and the ¹H and ¹³C NMR chemical shifts of the isomers are reported. While no intermediate could be isolated using several different radical trapping agents, the results show that 7,5 proton chemical shift is a possible isomerization mechanism.     

Natural Abundance 17O NMR Study of Substituted α,α,α-Trifluoromethoxybenzenes

Natural abundance ¹⁷O NMR chemical shift data for meta- and para-substitued α,α,α-trifluoromethoxybenzenes recorded in acetonitrile at 75° C are reported. The ¹⁷O NMR signals for the trifluoromethoxy compounds are deshielded by greater than 65 ppm compared to analogous methoxy compounds. A quantitative relationship between ¹⁷O NMR chemical shifts for the trifluoromethoxy and methoxy benzenes is reported.     

1H NMR Study of Some Thiosalicylic Acid Complexes

Five thiosalicylic acid (TSA) complexes were prepared. The ¹H NMR of these complexes were recorded and analysed. It was found that molybdenum, arsenic, selenium, silver and tellurium metals coordinates through the sulfur atom. The ¹H NMR chemical shift of ring protons of As, Se and Te shift to low field upon complexation, while it shifts to high field when TSA complex with Ag and Mo     

17O DOR and other solid-state NMR studies concerning the basic properties of zeolites LSX

We demonstrate complementary ¹H, ¹⁷O, ²⁷Al and ²⁹Si measurements for basic low-silica-X zeolites, which were unloaded and pyrrole and formic acid-loaded. It was found that the acid–base-system is not stabile, if the loading exceeds one pyrrole molecule or two formic acid molecules per supercage.¹⁷O DOR NMR spectra exhibit at least four lines, which are broadened by a distribution of chemical shifts in a similar extend as the ²⁹Si MAS NMR spectra are broadened by distribution of Si–O–Al angles. A strong cation influence upon ¹⁷O shifts was observed. But there was no strong influence of the acid molecules on the mean value of the ¹⁷O shift of the spectra.     

Isomer-shift analogue in neutron resonances

For the first time, the recently predicted chemical shift of neutron resonances, to be regarded as an analogue to the Mössbauer isomer shift, has been experimentally observed studying the 6.67 eV resonance of ²³⁸U. The experimental shifts were determined by a chi-square fitting technique from the time-of-flight transmission spectra of metallic uranium and four uranium compounds measured at the Dubna IBR-30 pulsed reactor. A computational method has been applied to estimate, and compensate for, the influence of the crystal-lattice vibrations on the experimental values thus obtained. The electron density differences at the nucleus have been calculated for the various sample pairs using available data on chemical X-ray shifts in uranium compounds, on Mössbauer isomer shifts in isovalent neptunium compounds and on free-ion electron densities. The resonance shift results lead to the conclusion that the mean-square charge radius of ²³⁸U diminishes by 1.7_?0.8^+1.2 fm² upon capturing the resonance neutron.     

Controversy about the reference compound for correct chemical shift determination of 195Pt nuclei

Several groups exploring the ¹⁹⁵Pt NMR in solids, including metallic and magnetic materials, use different standards for chemical shift (Knight shift) determination. Commonly applied H₂PtCl₆ and Na₂PtCl₆ (IUPAC δ scale) lead to considerable underestimation of the shifts since H₂PtCl₆ has considerable own ¹⁹⁵Pt NMR shift due to its Van Vleck paramagnetism. In this Letter new results on ¹⁹⁵Pt NMR in heavy fermion system CeInPt₄ are presented and rationalized scale for the Knight shift determination is discussed.     

Correlations in 11B NMR spectra of dihalo derivatives of bis(1,2-dicarbollyl)cobalt(III)

In the ¹¹B NMR spectra of dihalo derivatives of bis(dicarbollyl)cobalt(III), we have identified a correlation between the ¹¹B NMR chemical shifts of substituted boron atoms and boron atoms found in other positions on the carborane skeleton. We have observed an increased shielding effect for fluorine atoms (compared with other halogens), manifested in an upfield shift of the ¹¹B NMR signals for antipodal and trans boron atoms. For the fluorine-containing compound Bu₄N⁺ [8,8′-F₂-3,3′-Co(1,2-C₂B₉H₁₀)₂]⁻, we propose the following sequence of electron density transfer: B(8) → {B(6) and B(10)} → B(4, 7). __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 4, pp. 547–549 (cont.), July–August, 2006.     

Temperature-dependent 29Si NMR resonance shifts in lightly- and heavily-doped Si:P

Careful NMR measurements on a very lightly-doped reference silicon sample provide a convenient highly precise and accurate secondary chemical shift reference standard for ²⁹Si MAS-NMR applicable over a wide temperature range. The linear temperature-dependence of the ²⁹Si chemical shift measured in this sample is used to refine an earlier presentation of the paramagnetic (high-frequency) ²⁹Si resonance shifts in heavily-doped n-type silicon samples near the metal–nonmetal transition. The data show systematic decreases of the local magnetic fields with increasing temperature in the range 100–470 K for all samples in the carrier concentration range from 2×10¹⁸ cm⁻³ to 8×10¹⁹ cm⁻³. This trend is qualitatively similar to that previously observed for the two-orders of magnitude larger ³¹P impurity NMR resonance shifts in the same temperature and concentration ranges. The ²⁹Si and ³¹P resonance shifts are not related by a simple scaling factor, however, indicating that impurity and host nuclei are affected by different subsets of partially-localized extrinsic electrons at all temperatures.     

Molecular structure,experimental and theoretical 1H and 13C NMR chemical shift assignment of cyclic and acyclic α,β‐unsaturated esters

The experimental ¹H and ¹³C NMR spectra of 13 phenyl cinnamates and four 4‐methylcoumarins were investigated and their chemical shifts assigned on the basis of the two‐dimensional spectra. For the unsubstituted cinnamic acid phenyl ester, optimized molecular structures were calculated at a B3LYP/6‐311++G(d,p) level of theory. ¹H and ¹³C NMR chemical shifts were also calculated with the GIAO method at the B3LYP/6‐311 + G(2d,p) level of theory. The comparison between experimental and calculated NMR chemical shift suggests that the experimental spectra are formed from the superposition spectra of the two lowest energy conformers of the compound in solution. The most stable s‐cis configuration found in our studies is also the conformation adopted for a related phenyl cinnamate in solid state. The experimental results were analyzed in terms of the substituent effects. Copyright © 2013 John Wiley & Sons, Ltd.     

NMR chemical shifts in copper (I) chalcogen compounds

NMR chemical shifts of ⁶³Cu have been determined in the three binary Cu(I) chalcogenides and in six ternary Cu(I) chalcopyrites. In all compounds studied, a decrease of chemical shift is observed with decreasing atomic number of the chalcogen. This downshift is discussed, and accounted for by p-d hybridization of the copper chalcogen bonds.     

Synthesis of Fluorinated Graphene Oxide and its Amphiphobic Properties

The richly functionalized basal plane bonded to polar organic moieties makes graphene oxide (GO) innately hydrophilic. Here, a methodology to synthesize fluorinated graphene oxide by oxidizing the basal plane of fluorinated graphite, allowing for tunable hydrophobicity of GO, is reported. Fluorine exists as tertiary alkyl fluorides covalently bonded to graphitic carbons, and using magic‐angle spinning (MAS) ¹³C NMR as a primary tool chemical structures for the two types of synthesized fluorinated graphene oxides (FGOs) with significantly different fluorine contents are proposed. The low surface energy of the C–F bond drastically affects GO's wetting behavior, leading to amphiphobicity in its highly fluorinated form. Ease of solution processing enables the fabrication of inks that are spray‐painted on various porous/non‐porous substrates. These coatings maintain amphiphobicity for solvents with surface tensions down to 59 dyn/cm, thus bypassing existing lithographic means to create similar surfaces. The approach towards fluorinating GO and fabricating graphene‐based surfaces with tunable wettability opens the path towards unique, accessible, carbon‐based amphiphobic coatings.     

Transmission of polar substituent effects across the cubane ring system: 19F substituent chemical shifts (SCS) of 4‐substituted (X) cub‐1‐yl fluorides revisited

¹⁹F NMR shieldings of 4‐substituted (X) cub‐1‐yl fluorides ( 4 ) for a set of substituents (X?H, NO₂, CN, NC, CF₃, COOH, F, Cl, HO, NH₂, CH₃, Si(CH₃)₃, Li, O^? and NH) covering a wide range of electronic substituent effects were calculated using the DFT‐GIAO theoretical model. The level of theory, B3LYP/6‐311+G(2d,p), provided ¹⁹F substituent chemical shifts (SCS) in good agreement with experimental values where known. By means of NBO analysis, various molecular parameters were obtained from the optimized geometries. Linear regression analysis was employed to explore the relationship between the calculated ¹⁹F SCS and polar field, resonance and group electronegativity substituent constants (σ_F, σ_R and σ_x, respectively) and also the NBO derived molecular parameters (fluorine natural charges (Q_n), electron occupancies on fluorine of lone pairs (n_F) and occupation number of the C? F antibonding orbital (σ_CF*)). The key determining parameters appear to be n_F and σ_CF*(occup). Both factors are a function of the electrostatic field influence of the substituent (σ_F effect) but are counteractive in their influence on the shifts. No evidence for a significant resonance effect influence on the shifts could be identified. Copyright © 2009 John Wiley & Sons, Ltd.     

Evaluation of various DFT protocols for computing 1H and 13C chemical shifts to distinguish stereoisomers: diastereomeric 2‐, 3‐, and 4‐methylcyclohexanols as a test set

¹H and ¹³C NMR chemical shifts were measured for a set of six isomers—the cis and trans 2‐, 3‐, and 4‐methylcyclohexanols. ¹H and ¹³C NMR chemical shifts were computed at the B3LYP, WP04, WC04, and PBE1 density functional levels for the same compounds, taking into account the Boltzmann distribution among conformational isomers (chair–chair forms and hydroxyl rotamers). The experimental versus computed chemical shift values for proton and carbon were compared and evaluated (using linear correlation (r²), total absolute error (|Δδ|_T), and mean unsigned error (MUE) criteria) with respect to the relative ability of each method to distinguish between cis and trans stereoisomers for each of the three constitutional isomers. For ¹³C shift data, results from the B3LYP and PBE1 density functionals were not sufficiently accurate to distinguish all three pairs of stereoisomers, while results using the WC04 functional did do so. For ¹H shift data, each of the WP04, B3LYP, and PBE1 methods was sufficiently accurate to make the proper stereochemical distinction for each of the three pairs. Applying a linear correction to the computed data improved both the absolute accuracy and the degree of discrimination for most of the methods. The nature of the cavity definition used for continuum solvation had little effect. Overall, use of proton chemical shift data was more discriminating than use of carbon data. Copyright © 2007 John Wiley & Sons, Ltd.     

GIAO Calculations of Chemical Shifts in Ethene‐1,1,2,2‐tetrayltetramethylene tetrathiocyanate

Geometric optimization and gauge including atomic orbital (GIAO). ¹H and ¹³C NMR chemical shift calculations with Hartree–Fock (HF) method and density functional method (B3LYP), using the 6‐31G(d) and 6‐31+G(d) basis sets, are proposed as a tool to be applied in the structural characterization of ethene‐1,1,2,2‐tetrayltetramethylene tetrathiocyanate, thus providing useful support in the interpretation of experimental NMR data. Parameters related to linear correlation plot of computed versus experimental ¹³C NMR chemical shifts in DMSO‐d₆ are provided.