19F NMR chemical shifts. 1. Aliphatic fluorides

The (19)F NMR shielding for the alkyl fluorides from methyl fluoride to tert-butyl fluoride has been calculated using IGAIM and has been separated into the contribution from each of the molecular orbitals. The relatively large change in fluorine shielding, in contrast to the adjacent carbon, was found to be due to the tensor components normal to the C-F bond axis. As the number of adjacent p-orbitals increases, the lone-pair p orbitals at fluorine become involved with MOs using these orbitals. The increase in the number of occupied orbitals associated with the fluorine leads to increased opportunities for mixing with virtual orbitals and to the increase in paramagnetic deshielding. The same pattern is seen on going from acetylene to 2-tert-butylacetylene and is also seen in the methyl (13)C shielding in the series ethane, propane, isobutane, and neopentane. The fluorine shielding in the series of fluoromethanes also decreases with increasing fluorine substitution. With carbon tetrafluoride, the decreased shielding arises from the highest occupied MO, which is a nonbonding linear combination of pure p functions at the fluorines.     

(Pentafluorophenyl)sulfur fluorides, 19F NMR

The low-temperature ¹⁹F NMR spectra of (pentafluorophenyl)sulfur trifluoride are explained by the trigonal bipyramid structure ( 1a ) with the pentafluorophenyl ring in the basal plane and a relatively high barrier to rotation about the C? S bond. In the presence of a hydrogen fluoride scavenger, there is a high barrier to intramolecular rearrangement of the sulfur fluorines about the sulfur atom. The single basal fluorine couples strongly with one ortho fluorine and weakly with the other, but the two apical fluorines couple equally with both ortho fluorines. A six-bond coupling between S? F and p? F is found in (pentafluorophenyl)sulfur trifluoride and in (pentafluorophenyl)-sulfinyl fluoride, but not in (pentafluorophenyl)sulfonyl fluoride.     

19F NMR chemical shifts and C−F bonds in graphite fluorides

Institute for Inorganic Chemistry, Siberian Branch, USSR Academy of Sciences. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 4, pp. 170–172, July–August, 1989.     

A19F NMR study of the anion mobility in lanthanum oxide fluorides

Conclusion All the oxide fluorides studied show intensive diffusion of the fluoride ions, and this is responsible for their high electrical conductivity. The anisotropy of the magnetic shielding of the fluorine nuclei in the rigid lattice has been determined from the field dependence of the second moment. In the high-temperature range, the anisotropy was measured from the form of the resonance line. The coincidence of the values of the anisotropy at high and low temperatures shows that in stoichiometric lanthanum oxide fluoride below the temperature of the phase transformation into the cubic modification, the fluoride ions move only through the equivalent positions of the fluorine sublattice. Analysis of the chemical shifts and their anisotropy shows that the movement of the fluoride ions has vacancy character. Above T_c, the exchange of positions with oxygen is possible. The behavior of the spectra in the intermediate range indicates high dynamic nonuniformity of the fluoride ions, brought about by the existence of regions with an increased fluorine mobility, which apparently have a structure similar to that of the nonstoichiometric lanthanum oxide fluorides with an increased fluorine concentration. The replacement of oxygen by fluorine at first leads to a sharp increase in the mobility of the fluoride ions (x=0.04), after which it has practically no influence up to x=0.143. It is not excluded that the increase in the mobility is related not to the excess of fluorine but to a rearrangement of the crystal structure. Judging from the behavior of the spectral lines in the intermediate range, nonstoichiometric lanthanum oxide fluorides are dynamically more uniform.L. V. Kirenskii Institute of Physics, Siberian Branch, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 24, No. 5, pp. 44–48, September–October, 1983.     

19F chemical shifts of bridgehead fluorides

The ¹⁹F chemical shifts of a series of bridgehead fluorides are reported. It is found that, contrary to earlier conclusions based on a limited number of substrates, the fluorine shifts occur randomly.     

19F and 1H NMR spectra of halocarbons

19F NMR chemical shifts and coupling constants are reported for 215 compounds. For 77 of these compounds, 1H NMR spectral data are also given. Long-range couplings, including 8J(F,F) and 5J(F,H), are reported. The complexity of halocarbon spectra owing to the presence of rotational isomers, asymmetric centers, long-range couplings, and chlorine isotope effects are illustrated, and the methods used for analyzing such complex spectra are briefly discussed.     

Cluster models and ab initio calculations of (19)F NMR isotropic chemical shifts for inorganic fluorides

(19)F NMR isotropic chemical shift (delta(iso)) calculations are performed in crystallized compounds using the GIAO method with the B3LYP hybrid functional at DFT level. Clusters centered on the studied fluorine atoms mimic the crystalline structures. The 6-311+G(d) basis set is chosen for the central fluorine atom, and the LanL2DZ basis set for the others. The metal atoms are described by the 3-21G(2d) basis set or, when not available, by the CRENBL basis set with the corresponding ECP, and augmented with 2d polarization functions when existing. First, for high-symmetry systems (MF, MF(2), and MF(3) compounds), a systematization of the cluster building up from coordination spheres is proposed, generalized to fluoroperovskites and fluoroaluminates KAlF(4) and RbAlF(4). When applied to rather low symmetry systems such as barium fluorometalates BaMgF(4), BaZnF(4), and Ba(2)ZnF(6), the definition of the coordination spheres is far from easy. Then, for structures built up from a MF(6) octahedron network, we may define different "starting clusters": [FM(2)F(8)] for the shared fluorine atoms, [FMF(4)] for the unshared ones, and [FBa(4)](7+) for the "free" ones. Analogous "starting clusters" are then tested on compounds from the NaF-AlF(3), BaF(2)-AlF(3), and CaF(2)-AlF(3) binary systems and for alpha-BaCaAlF(7) that are also built up from a MF(6) octahedron network. For each of these corresponding fluorine sites, delta(iso) values are calculated with the "starting clusters" and several larger clusters and compared to the experimental delta(iso) values. For the barium-containing clusters, the RMS deviation is equal to 51 ppm. It is suggested that this result may be related to the poor quality of the barium basis sets for which no polarization functions are available for the moment. In total, chemical shifts were calculated for 122 fluorine sites, in a various range of compounds. For the clusters without barium, the ab initio method leads to a RMS equal to 22 ppm, which is a quite nice result keeping in mind that the (19)F chemical shift range is larger than 200 ppm.     

19F, 13C NMR analysis of an oxygen carrier,perfluorotributylamine, and perfluoropentanoic acid

NMR relaxation measurement of perfluorocarbons (PFCs), such as perfluorotributylamine (FTBA), is a convenient method for the determination of oxygen concentrations in tissues and tumors. Previous relaxation studies of FTBA used different ¹⁹F NMR assignments causing some confusion. Fluorine‐detected ¹⁹F, ¹³C HMQC and HMBC and selectively ¹⁹F‐decoupled ¹³C NMR provided unequivocal ¹⁹F and ¹³C assignments for FTBA and perfluoropentanoic acid (FPA). Based on those assignments, ¹³C spin–lattice relaxation time constants (T₁) and effective correlation times for FTBA and FPA are reported and discussed. Copyright © 2003 John Wiley & Sons, Ltd.     

Ab Initio calculation of 19F NMR chemical shielding for alkaline‐earth‐metal fluorides

Gauge‐independent atomic orbital (GIAO) method at Hartree‐Fock (HF) and density functional theory (DFT) levels, respectively, was employed to calculate ¹⁹F NMR chemical shieldings of solid state alkaline‐earth‐metal fluorides MF₂ (M = Mg, Ca, Sr, Ba). The results show that, although the calculated ¹⁹F chemical shieldings tend to be larger than the experimental values, they have a fairly good linear relationship with the observed ones. The calculated results based on different combinations of basis sets show that the B3LYP (a hybrid of DFT with HF) predictions are greatly superior to the HF predictions. When a basis set of metal atom with effective core potential (ECP) has well representation of valence wavefunction, especially wavefunction of d component, and proper definition of core electron number, it can be applied to obtain ¹⁹F chemical shielding which is close to that of all‐electron calculation. The variation of ¹⁹F chemical shielding of alkaline‐earth‐metal fluorides correlates well with the lattice factor A/R².     

19F fast magic-angle spinning NMR studies of perfluoroalkanoic acid self-assembled monolayers

The bonding and dynamic properties of perfluoroalkanoic acid self-assembled monolayers (SAMs) on zirconia and titania powders were characterized by Fourier transform infrared and solid-state 19F magic-angle spinning NMR spectroscopy. The perfluoro fatty acids investigated included C(n)F(2n+1)CO2H, n = 7, 13, 15 and 17. The acids bind to both metal oxides via ionic carboxylate bonds, but complete monolayers are only formed on the zirconia. The shift of the CF3 group from -83 ppm in the bulk state to -85 ppm for the adsorbed monolayers is assigned to CF3 groups located at the air/monolayer interface. With the exception of the perfluorooctanoic acid, 19F spin lattice relaxation measurements indicate that the fluorocarbon chains of the adsorbed acids, even in the case of densely packed monolayers, are significantly more mobile than those in the bulk state. The motions associated with the enhanced mobility of the adsorbed acids are proposed to involve reorientations along the long chain axes. No evidence for chain melting in the fluorocarbon SAMs is found for temperatures well above the melting temperature of the bulk acids.     

19F DOSY NMR analysis for spin systems with nJFF couplings

NMR is a powerful method for identification and quantification of drug components and contaminations. These problems present themselves as mixtures, and here, one of the most powerful tools is DOSY. DOSY works best when there is no spectral overlap between components, so drugs containing fluorine substituents are well‐suited for DOSY analysis as ¹⁹F spectra are typically very sparse. Here, we demonstrate the use of a modified ¹⁹F DOSY experiment (on the basis of the Oneshot sequences) for various fluorinated benzenes. For compounds with significant ⁿJ_FF coupling constants, as is common, the undesirable J‐modulation can be efficiently suppressed using the Oneshot45 pulse sequence. This investigation highlights ¹⁹F DOSY as a valuable and robust method for analysis of molecular systems containing fluorine atoms even where there are large fluorine–fluorine couplings. Copyright © 2014 John Wiley & Sons, Ltd.