Molecular relaxation processes in very dilute systems: A 13C NMR study of alcohols in alkanes

¹³C T₁ relaxation times have been determined for n-butanol in C₆D₁₂ in the concentration range 0.001 ? _x ? 1 (using ¹³C labelled alcohol) and for t-butanol in the range 0.01 ? _x ? 1. In the former case, this has allowed us to probe molecular mobility down to the region of monomeric alcohols. Comparison with previous results from measurements of ¹H chemical shifts, dielectric relaxation, and the picosecond dynamics of electron solvation allows us to build up a detailed picture of molecular clustering and liquid structure in alkane—alcohol mixtures. For n-butanol, the local liquid structure is established by x = 0.2 while t-butanol does not appear to form aggregates larger than trimers until x = 0.5.     

Einflüsse von substituenten in 6-stellung auf die 13C-chemischen verschiebungen der kohlenstoffatome des purins

The ¹³C chemical shifts of purines substituted in the 6 position are reported. Signals are assigned on the basis of general chemical shift rules and by proton “off-resonance” decoupling. Substituent effects (Z_6i) of the substituent X in the 6 position of purine on the ¹³C chemical shifts of purine ring carbon atoms are determined. A linear correlation exists between the substituent effects of X on C-6 (Z₆₆) and Pauling's electronegativity values E_x of the substituent X.     

Crystalline aluminium hydroxy fluorides—Suitable reference compounds for F chemical shift trend analysis of related amorphous solids

On the basis of MAS NMR-data for crystalline AlF_x(OH)_3−x·H₂O samples in the pyrochlore structure, ¹⁹F chemical shifts correlate with the average chemical composition of the octahedral environment, given by AlF_xO_6−x in these compounds.The attribution of local structures in sol-gel derived amorphous AlF_x(OX)_3−x·XOH (X = H, R (alkyl)) compounds is of special interest as these or consecutively prepared solids exhibit remarkable features, for example, a high surface (HS) area accompanied by a high Lewis acidity.By transferring this scale of a ¹⁹F chemical shift trend analysis to such compounds a prediction of the chemical nature of the average Al coordination becomes possible.A new synthetic approach to crystalline aluminium hydroxy fluorides involving a sol gel fluorination as the first reaction step and an aluminium alkoxide as precursor compound is presented. Varying the amount of HF leads to different F-OH-ratios in the AlF_x(OH)_3−x compounds.     

Isotope shift in chromium

Thirty-three spectral lines of chromium atom in the blue-violet region (425–465 nm) have been investigated with the method of laser-induced resonance fluorescence on an atomic beam. For all the lines, the isotope shifts for every pair of chromium isotopes have been determined. The lines can be divided into six groups, according to the configuration of the upper and lower levels. Electronic factors of the field shift and the specific mass shift (F_ik and M_ik^SMS, respectively) have been evaluated and the values for each pure configuration involved have been determined. Comparison of the values F_ik and M_ik^SMS to the ab initio calculations results has been performed. The presence of crossed second order (CSO) effects has been observed.     

Comprehensive parameter set for the prediction of the 13C-NMR chemical shifts of sp3-hybridized carbon atoms in organic compounds

A parameter set is described for prediction of the carbon-13 chemical shifts of sp³-hybridized carbon atoms based on a simple linear additivity relationship. It was tested against 88 692 known chemical shift values. Many of the previously reported parameters were amended and new parameters added. With this parameter set, the chemical shifts in 99.7% of the 88 692 cases can be estimated with an overall standard deviation of 5.6 ppm for the predicted values relative the the experimental ones. The additivity parameters reported here can be used per se or in connection with a recent computer program for the estimation of carbon-13 chemical shifts, which automatically selects and applies additivity rules appropriate for the individual carbon atoms of the chemical constitution entered.     

Quantitative determination of mebeverine HCl by NMR chemical shift migration

Quantitative ¹H NMR spectroscopic methods are not frequently reported, but current NMR instrumentation allows ready access to such data. Mebeverine HCl is an attractive molecule for NMR spectroscopy teaching purposes as it possesses a variety of simple but significant functional groups; we fully assign its ¹H and ¹³C NMR spectra. Using mebeverine HCl, we show that concentration changes, in water as a solvent, can lead to significant changes in the ¹H chemical shifts of non-exchangeable aromatic protons and to a lesser extent to aromatic methoxy protons. An important observation is that different protons migrate to different extents as the concentration of the solute is varied, and thus the ¹H NMR spectra are concentration-dependent across a useful range. This chemical shift variation of selected protons was therefore analyzed and applied in the quantitative determination of mebeverine HCl in a medicine (Colofac IBS) formulated as a tablet. Self-association phenomena in water could account for these observed chemical shift migration effects as shown by determining the hydrodynamic radius from the modified form of the Stokes-Einstein equation, and thence the apparent hydrodynamic volume, V_H, for mebeverine HCl in D₂O solution which is 10-fold greater than that seen in either CDCl₃ or CD₃OD.     

Analysis of the β-methylene carbon-13 NMR chemical shifts: from vinylidene chloride copolymers to a general analysis of polymers

Analysis of the carbon-13 NMR chemical shifts of β-methylene of vinylidene chloride copolymers has been done. The analysis, based on the empirical additivity rules, proposes chemical shift additive parameters for the monomer units in vinylidene chloride copolymers. Genetic algorithm has been applied for the optimization of additive parameters. To simplify and generalize the analysis, common functional groups present in the pendant groups; Cl, COOCH₃, CH₃, OCOCH₃, CN, CONH₂, C₆H₅ and COOH substituents were modeled and their additive effects were optimized. The additive parameters enabled to predict the carbon-13 NMR chemical shifts of β-methylene of homopolymers and copolymers at tetrad level of compositional sensitivity.     

Parametric investigation of the isotope shift in odd configurations of Ne i

Isotope shift measurements were performed on 33 transitions of Ne i by means of optogalvanic and intermodulated optogalvanic spectroscopy as well as saturation absorption spectroscopy. Using a large number of line isotope shift values obtained from both literature and our measurements, the level isotope shifts were calculated with improved concistency. Fine structure calculations were performed in odd configurations in order to obtain isotope shift parameters by means of the parametric method, 2p ⁵ 3s: g1 = -1077.5(7), z _2p = 15.3(3), 2p⁵5 s: g ₁ = -104(26), z _2p = 14(6), 2p ²4d: g ₁ = -42(6), z _2p = 17(3).     

Some observations on the relationship between charge and 13C chemical shift in cyclohexadienylic anions and cations

Carbon-13 chemical shifts δ^± in a variety of cyclohexadienylic cations and anions have been separated semi-empirically into contributions from (a) the hypothetical neutral framework, (b) charge effects and (c) interactions between ions and between ions and solvent molecules. Carbon-13 shifts for sites in neutral model compounds, δ⁰, approximate contribution (a) the shift differences for the model carbons δ^±_3,5-δ⁰_3,5 are assumed to reflect electric field effects (b). Thus what remains, δ^±_s-δ⁰_s- (δ^±_3,5-δ⁰_3,5) comes from the charge. The summation of these terms for each of the ions comes to ca. 165 ppm. This shows that a changein charge of ±1 of the neutral model, with correction for the electric field, brings about a change in shift of 165 ppm, thus supporting the linearity of charge with shift.     

Isotope shift and hyperfine structure measurements in titanium I

High accuracy measurements of hyperfine structure due to⁴⁷Ti and⁴⁹Ti in the 3d ² 4s ² a ³ F ₂?3d ² 4s4p z ⁵ D ₁ absorption line at σ=18482.772 cm^?1 have been performed by use of a Doppler-free experiment, where a beam of titanium atoms is crossed by a CW single mode tunable dye laser. They have allowed for the determination of isotope shifts between⁴⁶Ti,⁴⁷Ti,⁴⁸Ti,⁴⁹Ti and⁵⁰Ti. By use of accurate values of mean square nuclear charge radii for the even isotopes, it has been possible to separate mass shifts from field shifts and to determine accurate values for the mean square nuclear charge radii of⁴⁷Ti and⁴⁹Ti. The field shift presents a marked odd-even staggering.     

Density functional theory study of 13C NMR chemical shift of chlorinated compounds

The use of the standard density functional theory (DFT) leads to an overestimation of the paramagnetic contribution and underestimation of the shielding constants, especially for chlorinated carbon nuclei. For that reason, the predictions of chlorinated compounds often yield too high chemical shift values. In this study, the WC04 functional is shown to be capable of reducing the overestimation of the chemical shift of Cl‐bonded carbons in standard DFT functionals and to show a good performance in the prediction of ¹³C NMR chemical shifts of chlorinated organic compounds. The capability is attributed to the minimization of the contributions that intensively increase the chemical shift in the WC04. Extensive computations and analyses were performed to search for the optimal procedure for WC04. The B3LYP and mPW1PW91 standard functionals were also used to evaluate the performance. Through detailed comparisons between the basis set effects and the solvent effects on the results, the gas‐phase GIAO/WC04/6‐311+G(2d,p)//B3LYP/6‐31+G(d,p) was found to be specifically suitable for the prediction of ¹³C NMR chemical shifts of chlorides in both chlorinated and non‐chlorinated carbons. Further tests with eight molecules in the probe set sufficiently confirmed that WC04 was undoubtedly effective for accurately predicting ¹³C NMR chemical shifts of chlorinated organic compounds. Copyright © 2012 John Wiley & Sons, Ltd.     

Analysis of F and C NMR spectra of tetrafluorophthalic anhydride and its derivatives

¹⁹F and ¹³C NMR spectra of perfluorinated compounds (i.e., tetrafluorophthalic anhydride, its hydroxyl- and amino-derivatives, N-pentafluorophenyltetrafluorophthalimide, and hexafluoroindan-1,3-dione) were analysed. Different signals in NMR spectra were assigned based on the analysis of spin-spin coupling constants. All assignments made were further confirmed by density functional theory (DFT) calculations of ¹³C chemical shifts and J_C,F coupling constants.     

On the hyperfine structure and isotope shift of radium

The hyperfine structure and isotope shift of the heaviest known alkaline earth element radium (Z=88) have been studied in both the atomic Ra I and ionic Ra II spectra. The measurements, carried out by on-line collinear fast-beam laser spectroscopy, yield the hyperfine constantsA andB of the 7s and 7p _1/2 states in Ra II, of all states of the excited 7s 7p configuration and the 7s 7d ³ D ₃ state in Ra I. The data allow a consistent evaluation of the nuclear moments for eight odd-A radium isotopes. In particular, a complete analysis of the hyperfine structure of thesp configuration in the two-electron system provides a stringent test of the validity of the semi-empirical modified Breit-Wills theory. It is shown that the effective operator formalism is equivalent, if relativistic correction functions are used to reduce the number of parameters. The semi-empirical hyperfine fields are evaluated and found to agree generally with those from ab-initio calculations. The isotope shifts in thes?p,s ²?sp,sp?sd transitions are analysed semi-empirically and compared with ab-initio calculations. The consistency of the different analyses proves their validity and classifies the spectrum of Ra I as a model case for a simple and clean two-electron spectrum.     

Saturated amine oxides: part 10. hydroacridines: part 32. linear 17O/13C and 13C/13C chemical shift correlations between saturated azaheterocyclic N‐methylamine N‐oxides,and the methiodides of their parent amines

Two kinds of good linear correlations were found between the chemical shifts of saturated six‐membered azaheterocyclic N‐methylamine N‐oxides and the chemical shifts of the methiodides of their parent amines. One of the correlations occurs between the ¹⁷O chemical shift of the N⁺―O^– oxygen in the N‐oxides and the ¹³C chemical shift of the N⁺―CH₃ methyl group analogously situated in the appropriate methiodide (r = 0.9778). This correlation enables unambiguous configuration assignment of the N⁺―O^– bond, even if the experimentally observed ¹⁷O chemical shift of only one N‐epimer is available, provided the ¹³C chemical shifts of both N⁺―CH₃ groups in the methiodide are known and assigned; furthermore, it can be used also for the estimation of ¹⁷O chemical shifts of the N⁺―O^– oxygens in N‐epimeric pairs of N‐oxides, for which observed ¹⁷O data hardly become available. The second correlation is observed between the ¹³C chemical shift of the N⁺―CH₃ methyl group in the N‐oxides and the ¹³C chemical shift of the N⁺―CH₃ methyl group analogously situated in the appropriate methiodide (r = 0.9785). It can be used for safe configuration assignment of the N⁺―CH₃ group and, indirectly, also of the N⁺―O^– bond in an amine N‐oxide, even if no ¹⁷O NMR data, and the ¹³C chemical shift of only one N‐epimer is available. Copyright © 2015 John Wiley & Sons, Ltd.     

Isotope shift measurements in zirconium by Doppler-free laser polarization spectroscopy

Isotope shifts in 17 transitions of ZrI have been measured by Doppler-free laser polarization spectroscopy in a hollow cathode discharge. The results were combined with other known isotope shift data of ZrI and tested for consistency in a multidimensional King plot. By including values ofδ〈r ²〉 deduced from optical isotope shifts mass and field shifts were separated, and improved values ofδ〈r ²〉 could be determined from the King plot. The results for the isotope pairs A, A′ are as follows; 90, 92: 0.244(43) fm²; 92, 94: 0.176(20) fm²; 94, 96: 0.126(23) fm².     

15N nuclear magnetic resonance spectroscopy. Natural abundance 15N NMR of monosubstituted indoles

¹⁵N chemical shifts of twenty-four substituted indoles have been determined in natural abundance (in organic solvents) using Fourier transform NMR. The overall chemical shift range is 27 ppm, with groups in the 2-, 3- and 5-ring positions showing the largest substituent effects. Substituents capable of resonance interaction with the indole nitrogen give shifts in the expected directions but they cannot be correlated with known substituent parameters. Compounds measured in DMSO give 0·2 to 10·2 ppm downfield shifts with respect to the same compound measured in CDCl₃. ¹³C NMR data for previously unreported compounds are also reported.     

Structural,magnetic and electrical properties of Fe substituted GdCrO3

Single phase samples of bulk GdCr_1-xFe_xO₃ were prepared for x = 0 to 0.50. The lattice parameters obtained from Rietveld refinement based on Pbnm space group show that they increase systematically with Fe concentration, i.e. with a = 5.3145 Å, b = 5.5249 Å and c = 7.6068 Å for x = 0 to a = 5.3330 Å, b = 5.5670 Å, and c = 7.6382 Å for x = 0.50. Magnetization measurement shows that all samples exhibit antiferromagnetic transition. Their Neel temperature (T_N) gradually decreases upto x = 0.20 and beyond that it increases quite sharply due to considerable concentration of Fe³⁺ – O²⁻ – Fe³⁺ networks. The magnetization reversal observed in the parent compound (GdCrO₃) is found to be suppressed upon Fe substitution and however for x = 0.40, magnetic compensation is observed at T_comp = 125 K. They are explained by considering the variation in the magnitude of weak ferromagnetic moment for different Fe concentrations. Complex impedance spectra measured at different temperatures above room temperature show the thermally activated relaxation of charge carriers with contribution from both grains and grain boundaries. The relaxation frequency of charge carriers and dc conductivity follow the Arrhenius law with comparable activation energy values.     

Cis-trans isomerization and 13C-NMR chemical shift of polyphenylacetylene

Before and after cis-trans isomerization, the observed ¹³C-NMR chemical shifts of poly(phenylacetylene) (PPA) in the solid state were investigated on the basis of ¹³C-NMR chemical shift calculations within AM1 for the cis-transoidal and deflected trans-transoidal forms. Two ¹³C-resonance peaks in the observed CP/MAS ¹³C-spectrum were assigned theoretically by the ¹³C chemical shifts of the main and side chains. After thermal isomerization, the ¹³C peak of the main chain for PPA shifted upfield by 3.5 ppm, in contrast to the downfield shift of the ¹³C peak for polyacetylene. This upfield shift of trans-PPA largely was attributed to the increases of the excitation energy from the ground state to the lowest φ_π–π* state in the paramagnetic terms of ¹³C chemical shift on the main chain carbons with the increase in deflected angle τ of 0 to 80°. The ±80° deflected conformation of the trans-transoidal chain due to the cis-trans isomerization was confirmed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1657–1664, 1999     

NMR study (1H, 13C and 29Si) of the methylallylsilanes

¹H, ¹³C and ²⁹Si NMR data for the compounds (CH₃)_xSi(CH₂CHCH₂)_4-x are reported. The ¹H resonances from the π system are indicative of the electron-supplying inductive effect (+I) of the (CH₃SiCH₂, moiety but the corresponding ¹³C_π chemical shifts seem to be influenced by a sterically induced polarization of the C-H bonds. The ¹³C_All, ¹³C_Me and ²⁹Si chemical shift data reveal an important neighbour anisotropy contribution originating from the π system. Ultraviolet study of the compounds mentioned above gives indication of a σ—π conjugation in accordance with PES and ab initio results [1—5]. The trend observed in the various coupling constants is too small to be Interpreted.