13C-19F couplings and 19F NMR shifts of cycloalkyl,bicycloalkyl and steroid monofluoro compounds1

The stereospecifity of ¹³C-¹⁹F couplings is investigated with 20 alicyclic compounds. One bond couplings, ranging from 168 to 214 Hz, can be represented as a function of the corresponding ¹³C-H coupling constants. For comparison ¹³C-¹H couplings are determined for norboraane and adamantane and indicate considerable s character at the bridgehead C-H bond of the latter compound. One bond and geminal couplings (ranging from 18 to 24 Hz) are found to depend not significantly on the steric environment. With vicinal couplings a strong dependence is established on torsional angles, which is fitted to a Karplus function. Typical values for CCCF trans arrangements are around 10 Hz, for gauche angles less than 1.5 Hz. Vicinal couplings are substantially altered by electronegative substituents and by hybridization changes of participating carbon atoms. The ³J values observed with cycloalkylfluorides are interpreted on the basis of model geometries for the corresponding hydrocarbons. The influence of solvent and temperature changes is restricted to one bond couplings. ¹⁹F shifts in cyclohexane derivatives are constantly at higher field for axial fluorine (by ~20 ppm), but otherwise there is no significant relation to the orientation of neighbouring bonds, nor to ¹³C shifts of the Cα-F carbon atoms or to the corresponding one bond couplings.     

基于竞争配位的Ca2+特异性响应1H/19F磁共振成像分子探针

合成了一种含~(19)F的Mn~(2+)配合物3,12-二(2-氧代-2-((2,2,2-三氟乙基)氨基)乙基)-6,9-二氧杂-3,12-二氮杂十四烷酸锰(Ⅱ)(Mn(Ⅱ)-L,1),可实现对Ca~(2+)特异响应的~1H/~(19)F磁共振成像分析。~(19)F核在近距离的顺磁性Mn~(2+)影响下产生了顺磁弛豫增强作用,使~(19)F的横向弛豫时间T_2急剧缩短而磁共振信号锐减。当有Ca~(2+)存在时,与配体L的竞争配位使得~(19)F远离Mn~(2+)离子,从而~(19)F磁共振信号得到恢复。同时,由于Mn~(2+)离子从配合态变为游离态,水配位数增加使得其对~1H的纵向弛豫时间T_1弛豫性能增加,从而~1H磁共振成像信号也增强。相关实验的结果证实了该配合物是一种能对Ca~(2+)特异性响应的~1H/~(19)F磁共振成像探针。     

Transmission mechanisms of four-bond 19F19F coupling constants in fluoropropenes: A theoritical and experimental study

Four-bond FF couplings were measured in cis- and trans-isomers of 1,3-diflouro- and 1,3,3,3-tetraflouro-propene. These couplings show a pronounced depedence on the member of flourines attached to the sp³ carbon atom. This dependence is much larger for the cis-isomers than for the trans ones. A theoretical analysis of transmission mechanisms of the Fermi contact, orbital and dipolar interacting terms is carried out using inner projections of the polarization propagator at the INDO level of approximation. For each interacting term three different coupling pathways are considered, namely, through-space, σ- and π-electron contributions. Trends for each mechanism are discussed and results are compared with the experimental values.     

19F Chemical shift tensor in fluorobenzene compounds

Fluorine-19 NMR multiple-pulse experiments have been applied to a series of meta-, para- and ortho-substituted fluorobenzene compounds in the solid state. The principal elements of the ¹⁹F chemical shift tensor (σ₁₁, σ₂₂, σ< ₃₃) were determined and the orientation of the tensor axes was inferrred from secondary information like molecular motion, related compounds and liquid crystal studies. Comparison with anisotropies obtained from molecules dissolved in liquid crystals in the nematic phase is discussed where data are available. Using the Gicrke-Flygarc approach we were able to exctract the spin-rotation interaction tensor elements C_??, C_tcy;tcy; and C_zz of ¹⁹F in sever; fluorobenzene compounds.     

1H,2H,19F and 31P NMR and ESR investigations of aryl-t-butyl nitroxides

Aspects of translocation of spin to fluorine and phosphorus are discussed. (2p–3d)_π overlap in phosphorus is insignificant. Spin polarization and (2p–3p)_π overlap are opposing spin transfer mechanisms for phosphorus. The³¹P NMR spectrum of the nitroxide from 7a shows some features which are not fully understood. Some gain in resolution of the NMR spectra can be achieved by deuteration, but it is considerably lower than predicted.     

Trends in 19F-19F and 29Si-19F coupling constants in organosilicon derivatives containing SiF2 and Si2F4 units

Consideration of ¹⁹F-¹⁹F and ²⁹Si-¹⁹F coupling constants in a series of organosilicon derivatives containing SiF₂ and Si₂F₄ units reveals a number of trends which are useful for structural and stereochemical assignments. For example the vicinal ¹⁹F-¹⁹F coupling constants in a number of CSiF₂SiF₂C- derivatives (including straight chain compounds, disilacyclobutanes and disilacyclohexanes) show an apparent linear dependence on dihedral angle, varying in magnitude from near zero for small values of φ up to ca. 19 Hz for φ ～ 180°. This is particularly useful for stereochemical assignments [1,2]. In addition ²⁹Si-¹⁹F coupling constants appear to fall in quite distinct ranges (¹J_SiF > 300 Hz, 29 Hz < ²J_SiF < 55 Hz, ³J_SiF < 10 Hz). This is quite useful for structural assignments [1,6]. Reaction of SiF₂ with 1,3-cyclohexadiene gives two new silicon fluorine compounds: a disilabicyclo[2,2,2]octene and an HSi₂F₅-substituted cyclohexadiene.     

A 19F endor study of copper-doped CdF2

The anisotropy of the first-shell fluorine ENDOR is utilized to elucidate the nature of the paramagnetic species in copper- doped CdF₂. The coordination of the paramagnetic center is eight-fold cubic. The unusual orientation of its electronic g-tensor is explained.     

19F NMR chemical shifts of n-F-alkyl compounds

The examination of ¹⁹F chemical shifts for ca. 650 F-alkylated compounds of general formula CF₃(CF₂)_nCF₂X led to the following conclusions: the CF₂ groups α to X are very sensitive to the nature of X, and are spread over a range of 85 ppm. The effect of the length of the F-alkyl chain decreases rapidly, so that δCF₂(α) can already be considered as characteristic of X for n = 1 or 2 for most practical purposes. Solvent effects (in 9 different solvents having ε = 1.8 to 52.1) were found to be rather small except for the F-alkyl iodides. A chart which indicates the domain in which the CF₂(α) resonance signal is to be expected is given for 42 different series of F-alkylated compounds; it is expected to provide the synthetic chemist with a useful tool for the identification and characterization of such compounds.     

19F chemical shifts and mutual influence of ligands

By comparing the positions of ¹⁹F shifts in pentafluoro complexes of d^o transition metals and non-transition elements in higher oxidation state complexes MF₅L and EF₅L with the positions of corresponding MF₆ and EF₆, the spectra of all the pentafluoro complexes can be divided into a few types which depend on the nature of L. Correlation of chemical shifts with structural data and force constants implies that the change in the bond strength of the fluorine and the central ion has opposite effects in the ¹⁹F NMR spectra of the complexes of transition and non-transition elements.     

The mechanism of serine fluorodehydroxylation: 13C and 19F nmr studies

Competetive reaction pathwys responsible for the incomplete flourination of serine by SF₄ elucidated and inhibited.     

New designs for MRI contrast agents

New designs for Magnetic Resonance Imaging contrast agents are presented. Essentially, they all are host–guest inclusion complexes between -cyclodextrins and polyazamacrocycles of gadolinium (III) ion. Substitutions have been made to the host to optimise the host–guest association. Molecular mechanics calculations have been performed, using the UFF force field for metals, to decide on the suitability of the substitutions, and to evaluate the host–guest energies of association. Interesting general conclusions have been obtained, concerning the improvement of Magnetic Resonance Imaging contrast agents; namely, a set of rational methodologies have been deduced to improve the association between the gadolinium (III) chelates and the cyclodextrins, and their efficiency is demonstrated with a large set of substituted complexes, opening new doors to increase the diagnostic capabilities of Magnetic Resonance Imaging.     

19F and 1H NMR and ESR investigations of aryl-t-butyl nitroxides and nitronylnitroxides

¹H and ¹⁹F NMR and ESR spectra of several nitroxides have been recorded. The ¹⁹F couplings are sensitive to steric factors and this shows why the simple linear relationship a_F = O_eff^F· ?_C^π is not adequate. The π and σ spin delocalization mechanisms are demonstrated. The spin transmission via oxygen and sulphur is affected by geometry (efficiency of pπ-nπ overlap), pπ-dπ overlap only plays a role in positively charged S atoms (sulphones). The absolute sign of the proton coupling in an alkyl chain depends upon chain length and substituents. The syntheses of several stable nitroxides are described.     

Nuclear magnetic resonance study of 19F motion in CsPbF3

The ¹⁹F nuclear magnetic relaxation times and line-widths have been measured in polycrystalline CsPbF₃ in the temperature range from ?150 to +300°C. The observed motional narrowing of the NMR line-width at low temperatures and the temperature dependences of the relaxation times are analysed in terms of a high vacancy diffusion of the F^? ions. A phase transition is found at ?93°C from measurements of the NMR line shapes.     

Effects of external electric fields on 1H, 19F,and 14N NMR spectra of 2, 4, 6-trifluoronitrobenzene

When a polar liquid is subjected to an electric field, the dipoles obtain a small average orientation. At sufficiently high field strengths direct spin—spin dipolar and quadrUpolar electric field interactions may modify the NMR spetrum.NMR electric field experiments are reported for the proton, fluorine, and nitrogen NMR spectra of neat 2, 4, 6-trifluoronitrobenzene. The observed alignment values are smaller than those predicted on the basis of Onsager's theory.The ¹⁴N quadrupole coupling constant as been deduced from a comparison of the qUadrupolar and dipolar electric field effects.An upper limit of the absolute value of the anisotropy of the para-¹⁹F chemical shift has been inferred from the absence of a detectable displacement of the resonance on application of an electric field.     

19F shielding anisotropies of 2,4,6-trifluoronitrobenzene from nematic phase NMR

The customary assumption in nematic phase NMR of axial symmetry of the chemical shift tensor along the chemical bond has been shown to be unwarranted for the fluorine shielding tensor in 2,4,6-trifluoronitrobenzene.     

Determination of fluorine in geological materials by fast neutron activation based on the 19F(n,2n)18F reaction

The determination of fluorine in geological materials by fast neutron activation analysis based on the ¹⁹F(n, 2n)¹⁸F reaction is described. Fast neutrons are produced by irradiation of a thick beryllium target with 14.5 MeV deuerons. A rotating smple holder allows simultaneous irradiation of samples and standards. Fluorine-18 is separated by steam distillation of hexafluorosilicic acid or by extraction with triphenylatimony(V) dichloride and the annihilation radiation is measured with γ—γ coincidence equipment. The nuclear interference of recoil protons that induce the ¹⁸O(p,n)¹⁸F reaction is evaluated by means of synthetic samples: for a rock containing 43.4% of oxygen and 0.5% of hydrogen, the interference corresponds to 4.4 μg g^?1 fluorine. The method was applied to USGS and NIMROC reference rocks: for concentrations between 6000 and 50 μg g^?1, the relative standard deviation ranged from 2 to 10%.     

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.     

Temperature dependence of 19F chemical shifts in substituted benzyl fluorides.

The ¹⁹F chemical shifts of substituted benzyl fluorides are shown to be temperature dependent and this is discussed in terms of conformational effects.     

The preparation and 119Sn and 19F NMR spectra of some trifluoromethylphenyltin(IV) compounds

The preparation of a series of new trifluoromethylphenyltin(IV) compounds, Bu_nSn(C₆H₄CF₃-3)_4-n, (C₆H₄CF₃-3)SnCl₃, (C₆H₄CF₃-2)SnCl₃, and some related adducts with 2,2¹-bipyridyl and 1,10-phenanthroline, is described. ¹¹⁹Sn and ¹⁹F chemical shifts have been determined, together with values of J(¹¹⁹Sn=F) and ³J(¹¹⁹Sn=H_itortho), and the possibility of a “through space” tinfluorine coupling mechanism is also discussed.