A 19F NMR study of electronic effects in iminophosphoranes and in their methyl iodide salts

¹⁹F NMR shifts have been obtained for the iminophosphoranes(FC₆H₄)_n(C₆H₅)_3?nPNC₆H₄X(n = 1, 3) and (XC₆H₄)₃ PNC₆H₄F and for their methyl iodide salts. The data for the iminophosphoranes indicate the presence of a polar PN bond, considerable electron delocalization from nitrogen to the attached aryl ring, and a modest degree of arly-phosphorus conjugative interaction. In the salts the delocalization of the electron pair of nitrogen is largely curtailed.     

A 19F NMR study of the transmission of electronic effects in triarylantimony and triarylbismuth compounds. A comparison with the nitrogen- and carbon-containing analogues

A ¹⁹F NMR study of the transmission of electronic effects has been made for the systems Ar₂EC₆H₄F-4 (E = Sb, Bi, CH, N). The fluorine chemical shifts obtained are correlated with the polar constants (Σσ^o and Σσ) of the substituents, suggesting that electronic effects are transmitted through the SbC_ar, BiC_ar and CC_ar bonds predominantly by an inductive mechanism, whereas the transmission through the NC_ar bonds is contributed to significantly by classical resonance effects due to competitive conjugation of the lone pair with the aromatic rings, and the substituents therein. A dual parameter correlation of the fluorine chemical shifts with the inductive (σ_I) and resonance (σ^o_R and σ_R) parameters of the substituents in the aromatic rings has led to similar conclusions. The inductive transmission through the bridging Sb and Bi atoms has been assigned to the absence of conjugation of lone pair and vacant d-orbitals of the metals with π-electron systems of the aromatic rings. On the basis of the values of the ? coefficients for the correlation equations obtained it has been established that the transmitting ability of the BiC_ar bonds is close to that of the C_alC_ar bonds and considerably lower than the transmitting ability of the NC_ar bonds.     

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.     

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.     

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.     

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.     

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.     

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 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.     

Metalloidalmethyl substituent effects by 13C and 19F NMR : electron release in the neutral ground state.

¹³C and ¹⁹F chemical shift studies of a series of CH₂M(CH₃)₃ and CH₂M(C₆H₅)₃ (M  Si, Ge, Sn, Pb) - substituted aryl derivatives (phenyl; 1-naphthyl; 2-naphthyl) have established unambiguously that the order of hyperconjugative electron release in the neutral ground state is Pb>Sn>Ge>Si. This order is clearly at variance with the commonly accepted order(Pb>Sn>Ge>Si) based on studies of electron deficient substrates. The phenomenon is discussed in terms of current theories on σ-π interactions. In addition, substituent parameters (σ_I and σ_R^o) for the PB(CH₃)₃ group have been derived utilizing new data from the fluorophenyl tag. These new constants are compared with those previously reported.     

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.     

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.     

Optically active perfluoro-2-propoxypropionic acid: A new chiral reagent for 19F NMR study

Perfluoro-2-propoxypropionic acid, which was prepared by the anionic dimerization of hexafluoro-1,2-epoxypropane, was optically resolved via diastereomeric salt formation with chiral amines. The optically pure (+) and (?) perfluoro-2-propoxypropionic acids thus obtained were found to be a convenient chiral reagent for determining enantiomeric compositions of α-amino acids by means of ¹⁹F nmr analysis.     

13C and 19F NMR substituent chemical shifts (SCS) of some bridgehead-substituted phenyl- and fluorophenylbicyclo [2.2.2] octyltricarbonylchromium(O) derivatives: nature of aryl 19F NMR polar field effects in the Cr(CO)3-complexed benzene ring system

A number of Cr(CO)₃ complexes of bridgehead-substituted phenylbicyclo[2.2.2]octanes and (m- and p-)fluoropheylbicyclo[2.2.2]octanes have been synthesized and their ¹³C and ¹⁹F NMR spectra have been recorded, respectively. The substituent chemical shifts (SCS) of these stereochemically well-defined model systems permit an unambiguous evaluation of polar factors governing ¹³C and ¹⁹F SCS in arene-Cr(CO)₃ complexes. The dual nature of ¹⁹F NMR polar field effects is reaffirmed and the coefficient (A) of the Buckingham equation for linear electric field effects on C(sp²)F bonds in fluoroarene-Cr(CO)₃ complexes has been calculated. A re-examination and re-interpretation of the ¹⁹F chemical shifts of m- and p-substituted fluorophenyltricarbonylchromium derivatives is also reported. New substituent parameters (σ_I and σ^o_R) for C₆H₅ · Cr(CO)₃ as a substituent in the neutral ground state arepresented.     

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.     

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.     

Polar field-inductive parameters of some group IVB metalloidal substitutents: a 13C and 19F NMR study in the phenylbicyclo[2.2.2]octane ring system

A number of bridgehead metalloidal-substituted phenylbicyclo[2.2.2]octyl and (m- and p-)fluorophenylbicyclo[2.2.2]octyl derivatives have been synthesized and their ¹³C and ¹⁹F NMR spectra have been recorded. The appropriate ¹³C and ¹⁹F substituent chemical shifts of these stereochemically well-defined model systems, together with the known polar susceptibility parameters, provide a definitive scale of polar field-inductive parameters (σ₁ values) for a representative array of metalloidal substituents attached to an sp³ hybridized carbon center. The implication of these results with respect to the physical interpretation of σ₁ parameters is discussed. In addition, the previously reported results for alkyl groups in these systems have been re-evaluated in terms of possible through-bond effects involving orbitals of π symmetry of the bicyclo[2.2.2]- octane ring system. Factors determining α, β, γ and δ effects are briefly alluded to.     

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.