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.     

The 13C NMR spectra of π-arenechromium carbonyl complexes

The ¹³C NMR spectra of π-arenechromium complexes XC₆H₅Cr(CO)₂L (where I, L  CO; II, L  PPh₃; X  aliphatic and aromatic substituents) were investigated. The effect of the nature of the substituent X on the chemical shifts of the carbon atoms of the aromatic ring in these complexes was analyzed.It was established that a correlation exists between δ(¹³C) and σ_R (Taft's constant) in complexes I, not only for C(4) nuclei but also for C(3) metal atoms, the difference being that the slopes of the corresponding curves are opposite.     

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 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.     

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 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.     

19F NMR study of mixed fluorohalide anions of niobium(V) and tantalum(V) in solution

Anions of the type MF_nX^-_6-n (M = Nb^V, Ta^V; X = Cl_-, Br_-), including geometric isomers, have been characterised in solution by ¹⁹F NMR spectroscopy. Thirty eight fluorine-containing complexes of tantalum of the type TaF_xCl_yBr^-_z have been found from the 46 theoretically possible. The relative stability of mixed tantalum fluorochloro and fluorobromo anions of the type TaF_nX^-_6-n has been studied.     

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 substituent chemical shifts (SCS) of 4-substituted bicyclo[2.2.2]oct-1-yl flourides

The results of a linear multiple regression analysis indicate that the ¹⁹F SCS of 4-substituted bicyclo[2.2.2]oct-1-yl fluorides are essentially a manifestation of electric field and electronegativity effects.     

13C NMR and SCCCMO studies on substituted benzophenonetricarbonylchromium complexes

¹³C NMR studies on benzophenonetricarbonylchromium and its p-F, p-Cl and p-OCH₃ derivatives, with the substitutents on the uncomplexed rings, show a small substituent effect on the complexed ring and on the carbons of the Cr(CO)₃ group. The SCCCMO calculations show π-electron donation from the ring to the metal but greater σ-electron back-donation which leaves the ring more negatively charged than before complexation. The chromium atom is more positively charged than it is in benzenetricarbonylchromium. The trends is calculated CO bond orders are in agreement with the trends in CO infrared stretching frequencies.     

29Si and 13C NMR spectra of chloro- and fluoro-substituted linear permethylpolysilanes

²⁹Si and ¹³C NMR spectra are reported for the three halopolysilane series Me(SiMe₂)_nCl, Cl(SiMe₂)_nCl and F(SiMe₂)_nF, where n = 2 to 6. Except for the dihalodisilanes (XSiMe₂)₂, data for all of the compounds fit linear relationships based on substituent constants for chlorine or fluorine atoms in the α, β and γ positions. The effects of halogen substitution on ²⁹Si and ¹³C chemical shifts are rapidly attenuated along the polysilane chain, becoming negligible four atoms away from the halogen. The NMR data provide no evidence for long-range electronic transmission from chlorine or fluorine in halopermethylpolysilanes of the type suggested by other workers [1].     

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.     

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.     

Chemical shift correlations in the 13C, 17O and 31P NMR spectra of some Mo(CO)4((PPh2O)2Y(R)R′) (Y(R) = P(O), Si(Me); R′ = alkyl,haloalkyl, aryl) and [Mo(CO)4(PPh2O)2]2Si complexes

The syntheses and ¹³C, ¹⁷O, ²⁹Si and ³¹P NMR spectra of a series of Mo(CO)₄((PPh₂O)₂Y(R)R′) (Y(R) = P(O), Si(Me); R′=alkyl, haloalkyl, aryl) and [Mo(CO)₄(PPh₂O)₂]₂Si complexes are given. The chemical shift ranges of the cis and trans carbonyl ¹³C and ¹⁷O, phenyl C(1) ¹³C and ³¹P resonances are relatively large and, with the exception of the cis carbonyl ¹⁷O chemical shifts, the correlations between the chemical shifts of the various resonances are excellent. These correlations are consistent with the model of metal carbonyl ¹³C and ¹⁷O chemical shifts proposed by Bodner and Todd. In addition they allow the model to be extended to include the diphenylphosphinite ³¹P chemical shifts in these complexes. The excellent correlations may be due to the presence of the chelate ring which limits the rotation around the molybdenum-phosphorus bond and to the fact that all three groups directly bonded to the phosphorus remains constant.     

The NMR spectra of porphyrins—19: 13C and proton NMR spectra of metal-free porphyrins with the Type-IX substituent orientation,and of their zinc(II) complexes

The ¹³C and proton NMR spectra of six porphyrins bearing the substituent orientation characteristic of the natural “Type-IX” arrangement are reported and assigned. Significant concentration effects in the spectra of the free base porphyrins, together with the broadening of the C_α (and occasionally C_β) carbon resonances due to NH tautomerism caused a significant loss of data in these spectra. However, the spectra of the corresponding zinc(II) porphyrins (with addition of excess pyrrolidine) show that both these extraneous effects are completely removed to give well-resolved spectra with accurately reproducible chemical shifts. These spectra are assigned and an analysis of the chemical shifts allows the deduction of substituent chemical shift (SCS) parameters for the peripheral substituents at the beta and meso carbons. There is no global effect of these beta substituents, the beta carbon SCS being confined to the immediate pyrrole ring, and the meso carbon SCS to the two adjacent pyrrole rings. The SCS parameters are analyzed and it is shown how they can be used to predict the peripheral and meso carbon chemical shifts of any porphyrin bearing the substituents discussed.     

29Si and 13C NMR spectra of permethylpolysilanes

²⁹Si, ¹³C and ¹H NMR spectra are reported for the series of linear permethylpolysilanes Me(SiMe₂)_nMe where n = 1 to 6, for the cyclic permethylpolysilanes (Me₂Si)_n where n = 5 to 8, and for a few related compounds. For linear polysilanes the ²⁹Si and ¹³C chemical shifts can be accurately calculated from simple additivity relationships based on the number of silicon atoms in α, β, γ and δ positions. Adjacent (α) silicon atoms lead to upfield shifts in the ²⁹Si and ¹³C resonances, whereas more remote silicon atoms lead to downfield shifts. The ²⁹Si chemical shifts of the polysilane chains are linearly related to the ¹³C shifts of the carbon atoms attached to the silicon. The ²⁹Si and ¹³C resonances of the cyclic silanes deviate from this relationship. Ring current effects arising from σ delocalization are suggested as an explanation for the deviations. Proton-coupled ²⁹Si NMR spectra are reported for Me₃SiSiMe₃ and for (Me₂Si)_n, n = 5 to 7.     

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.     

Computation and analysis of 19F substituent chemical shifts of some bridgehead‐substituted polycyclic alkyl fluorides

The ¹⁹F NMR shieldings for several remotely substituted rigid polycyclic alkyl fluorides with common sets of substituents covering a wide range of electronic effects were calculated using the DFT‐GIAO theoretical model. The level of theory, B3LYP/6–311+G(2d,p), was chosen based on trial calculations which gave good agreement with experimental values where known. The optimized geometries were used to obtain various molecular parameters (fluorine natural charges, electron occupancies on fluorine of lone pairs and of the C? F bond, and hybridization states) by means of natural bond orbital (NBO) analysis which could help in understanding electronic transmission mechanisms underlying ¹⁹F substituent chemical shifts (SCS) in these systems. Linear regression analysis was employed to explore the relationship between the calculated ¹⁹F SCS and polar substituent constants and also the NBO derived molecular parameters. The ¹⁹F SCS are best described by an electronegativity parameter. The most pertinent molecular parameters appear to be the occupation number of the NBO p‐type fluorine lone pair and the occupation number of the C? F antibonding orbital. This trend suggests that in these types of rigid saturated systems hyperconjugative interactions play a key role in determining the ¹⁹F SCS. Electrostatic field effects appear to be relatively unimportant. Copyright © 2003 John Wiley & Sons, Ltd.     

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 spectra of mesomeric carbanions generated from polyfluorodiarylacetonitriles. The electronic effect of perfluoroalkyl groups

¹⁹F NMR spectra of 1,2- dimethoxyethane solutions of Na- and Li- salts of polyfluorinated carbanions [p - RC₆F₄$\text{C}$(CN)C₆F₄R′-p] Na⁺ (Li⁺) and of their neutral precursors p-RC₆F₄CH(CN)C₆F₄R′-p / R  F or CF₃ and R′= CF₃, CF₂CF₃, CF(CF₃)₂ and C(CF₃)₃/ have been studied. The values of changes in the chemical shifts of fluorine atoms in the ring and the side chain are practically the same when going from the precursor to carbanion with the perfluoroalkyl group being varied. This gave grounds to conclude that the electronic effect of the perfluoroalkyl groups is rather similar. The ¹⁹F NMR method has revealed no differences in the predominant mechanism of the negative charge distribution by these groups.