Temperature 19F NMR Research Articles

Tris(trifluoro and difluorobutane-2,4-dionato)aluminum(III) complexes

Eight tris(β-diketonate)aluminum(III) complexes having various fluoro and aryl substituents have been studied by nuclear magnetic resonance spectroscopy. The complexes are all nonrigid (fluxional) and their 19F nuclear magnetic resonance spectra show four resonances in the nonexchanging region due to cis and trans isomers. A variable high temperature 19F NMR study of these complexes was done and activation parameters are calculated.

Pentacoordinate silicon and germanium derivatives: molecular structure

Abstract The crystal structure of o-(Me2NCH2)C6H4GeMePhCl has been determined by single crystal X-ray diffraction study. The germanium atom has a distorted trigonal-bipyramidal (T.B.P.) geometry. The low temperature 19F NMR spectra of o-(Me2NCH2)C6H4SiMeF2 and o-(Me2NCH2)C6H4SiF3 are also consistent with a T.B.P. geometry. The stability of the T.B.P. structure is not determined by the apicophilicity of the fluorine atoms.

Aminofluorsilane: 1.3-Silylgruppenwanderung und gehinderte Si-N-Bindungsrotation / Aminofluorosilanes: 1.3-Migration of Silyl Groups and Si-N-Rotational Barrier

Triaminofluorosilanes with bulky amino groups are prepared by the reaction of bis- (organyl-trimethylsilyl)amino-difluorosilanes with lithiated amines. A 1,3-migration of trimethylsilyl groups from bulky alkylamino substituents to arylamino substituents is observed in these reactions. Structural isomers of the new triaminofluorosilanes were isolated. The silyl group migration depends on steric and electronic effects. Further reactions of the triaminofluorosilanes with butyllithium and trifluoroorganylsilanes lead to the formation of difluorosilyl-substituted triaminofluorosilanes and LiF. The symmetric compounds show AB-systems for the fluorine atoms of the difluorosilyl groups in the low temperature 19F NMR Spectra, due to hindered rotation about the Si-N-bond. The coalescence temperature depends on the bulkiness of the substituents and is observed at or about room temperature.

Dynamic 19F NMR studies The torsional barrier in pentafluorobenzaldehyde, its conjugate acid and protonated pentafluoroacetophenone

Abstract An upper limit of the barrier to internal rotation around the phenyl-carbonyl bond in pentalfluorobenzaldehyde dissolved in a freon mixture has been estimated from low temperature 19 F NMR study. Protonation of this compound increases drastically the free energy of activation ΔG≠. Complete lineshape analysis leads to ΔG≠ (273 K) = 60.4 kJ/mol, comparable to the value obtained for protonated benzaldehyde. This result, as well as those obtained by CNDO/2 calculations support the conclusions that protonated pentafluorobenzaldehyde is planar in the ground state. This is not the case for protonated pentafluoroacetophenone in which the lower barrier height when compared to protonated acetophenon has been related to the steric strain and dipole repulsion.

Geometrical and rotational isomerism in boron-phosphorus compounds. Low temperature 19F NMR investigation of B 4H 8PF 2N(CH 3) 2

Geometrical and rotational isomerism in boron-phosphorus compounds. Low temperature 19F NMR investigation of B 4H 8PF 2N(CH 3) 2

Variable temperature 31P and 19F NMR studies of Fe(CO)3(PF3)2 and Fe(CO)2(PF3)3

31P and 19F NMR spectra for the compounds Fe(CO)3(PF3)2 and Fe(CO)2(PF3)3 were studied in the temperature range +25 °C down to −100 °C. The four spin–spin coupling constants 1JPF, 3JPF′, 2JPP′, and 4JFF′ were evaluated as a function of temperature. Owing to the fast intramolecular exchange of axial with equatorial ligand groups in these compounds, the observed spectra are time-averaged. The magnitude of the observed time-averaged 2JPP′ coupling constant can be accounted for by the experimentally determined mole fractions for the various isomers and the trans, cis and vicinal phosphorus–phosphorus coupling constants 2JtransPP′, 2JcisPP′ and 2JvicinalPP′, respectively. An angular correlation of the phosphorus–phosphorus coupling constant is attempted. These results are compared to variable temperature ir and 13C NMR studies of these compounds. The usefulness of studying ir or Raman spectra in addition to the NMR spectra for certain nonrigid molecules which fall in the ’’very fast exchange’’ category where no temperature coalescence of NMR spectral lines can be observed, is demonstrated. The simultaneous fitting of both the time-averaged NMR spectra on the one hand and the separate ir spectra on the other is the best way to study such molecules. 151

Permutational isomerization of caged polycyclic oxyphosphoranes

Abstract The reaction of 1-phospha-2,8,9-trioxaadamantane with hexafluoroacetone gives a crystal-line caged polycyclic pentaoxyphosphorane. The ambient temperature fourier transform 13C NMR spectrum of the phosphorane in CH2Cl2 solution, with proton noise-decoupling, shows one singlet for three equivalent methine carbons, and one singlet for three equivalent methylene carbons. Hence, the phosphorane undergoes a rapid permutational isomerization, which is confirmed by the observation of equivalency of the three methine protons and of the three groups of methylene protons in the 1H NMR, and of equivalency of the four CF3 groups in the 19F NMR. The variable temperature 19F NMR spectra in a vinyl chloride-CHFCl2 solvent system disclose that the permutational isomerization of this phosphorane cannot be prevented even at − 165°, although there is a progressive line-broadening indicative of a decrease in the exchange rate. The 31P NMR spectrum of the phosphorane has a signal at a higher magnetic field (∂31P = +42·4 ppm) than H3PO4; the shift is nearly identical in CH2Cl2 and in the acidic (CF3)2CHOH. The remarkably low energy barrier (