Half-lives and fluorine atom shifts of stabilized 1-fluoro-1-lithioethenes bearing hydrogen, fluorine, phenyl, and/or dimethylphenylsilyl groups in the β-positions have been determined by a low-temperature 19F NMR spectroscopy. Some 1-fluoro-1-lithioethenes displayed an exceptionally low value of the trans-3JFF coupling constant. Stereoselectivity of carbenoid formation, as well as an effect of configuration on the stability is discussed. 相似文献
19F and 13C NMR spectra of perfluorinated compounds (i.e., tetrafluorophthalic anhydride, its hydroxyl- and amino-derivatives, N-pentafluorophenyltetrafluorophthalimide, and hexafluoroindan-1,3-dione) were analysed. Different signals in NMR spectra were assigned based on the analysis of spin-spin coupling constants. All assignments made were further confirmed by density functional theory (DFT) calculations of 13C chemical shifts and JC,F coupling constants. 相似文献
Abstract Long-range proton-carbon coupling constants are useful in the assignment of 13C NMR spectra and in stereochemical analysis. The measurement of vicinal coupling constants, 3JC-H, and their interpretation based on appropriate Karplus-type relationships (e.g. for 3JH-C-C-C or 3JH-C-O-C)1–3 provide valuable information in conformational studies of carbohydrates. Nevertheless, the use of 3JC-H in carbohydrate studies is rather rare because their measurement is time consuming and analysis of 1H-coupled 13C NMR spectra is complicated. However, 2D NMR methods 4–6 that allow precise measurement of long-range couplings in a reasonable time have become available recently. 相似文献
The 19F n.m.r. spectrum of hexafluoroquinazoline has been analyzed giving the signs and magnitudes of all but two of the coupling constants, and supporting the analyses of the 19F n.m.r. spectra of heptafluoroquinoline and heptafluoroisoquinoline. Long range F? F inter-ring coupling constants are a guide for assessing π-electron delocalization pathways. In addition, nitrogen in the β position enhances the distant F-5, F-7 meta-coupling constant, implying a π-contribution to J(FF)meta. A rationalization of changes in peri-coupling constants is also presented. The observed changes appear to be the sum of two effects: (i) ring contraction by the insertion of shorter C? N bonds into the aromatic ring and (ii) a secondary effect which is dependent on the existence of a β-nitrogen, adjacent to the peri nuclei. 相似文献
The 19F NMR spectra of the cis- ( 1 ) and the trans-isomer ( 2 ) of the 1,2-dichlorotetrafluorocyclopropane and that of the 1,1-dichlorotetrafluorocyclopropane ( 3 ) have been investigated at different temperatures and in several solvents. From chemical shift calculations the two geminal fluorines in the cis-isomer ( 1 ) could be assigned and on this basis the two vicinal coupling constants of 1 , Jtrans (ca. 140°) and Jcis (ca. 0°), were unequivocally distinguished. By frequency sweep double resonance Jtrans has been shown to be of opposite sign to Jgem, whereas for Jcis the situation has been found to be reversed. Therefore Jtrans is presumably negative and Jcis positive. Only the N(Jcis + Jtrans) value could be extracted from the vicinal coupling constant in the fragment ? CFCl? CFCl? could be evaluated. It has been noted that Jcis is more sensitive to changes in temperature than is Jtrans. The variations of Jcis and Jtrans induced by solvents are, on the contrary, small and irregular and no correlation with the dielectric constant of the medium has been noted. The different temperature dependence of Jcis and Jtrans can be useful for assigning the vicinal F? F coupling constants in cyclopropane derivatives and also for defining their signs. This method was applied to the coupling constants extracted from the 13C satellite spectrum of isomer 3 . The coupling constants results were compared with some literature data already known, and some rationalisation and correlations from the trends was attempted. 相似文献
The 35Cl and 19F nuclear relaxation data for liquid CCl3F are analyzed in terms of the extended rotational diffusion model for symmetric top molecules. The J-diffusion limit is shown to be in accordance with the relaxation data, but the M-diffusion limit is incompatible with the data. The 19F spin-rotation coupling constants obtained from the analysis are consistent with predictions based on chemical shift. 相似文献
A simple to use nuclear magnetic resonance analysis method has been tested on complex 1H, 19F, and 13C multiplets. This open-source line-shape analysis method analysis of total lineshape (ANATOLIA)1 provides some significant advantages over traditional assign-iterate methods of NMR spectral analysis by avoiding false minima and progressing optimisation to the global minimum. The target molecules are 1-perfluorotol-4-yl-2-perfluorotol-4-yl-oxymethyl-1H-benzimidazole (molecule-I) and 1-tetrafluoropyrid-4-yl-2-tetrafluoropyrid-4-yl-thio-1H-benzimidazole (molecule-II) which were produced as part of a family of fluorinated drug scaffolds prepared for anticancer and antiparasitic screening. Spectra display significant second-order effects with 1H Δδ = 3.68 and 4.67 Hz for the aromatic hydrogen “triplets”, with 19F 4JAA', 4JBB', 4JXX', and 4JYY' coupling constants range from +4.8 to −14.0 Hz and for 13C-isotopomers 19F Δδ of up to 111.56 Hz. A spin-system of six coupling nuclei (HaHbHcHd FYFY') was analysed in 12 s, a spin-system of nine coupling fluorine nuclei (AA'BB'CCC-YY') was analysed within 2 min, and 10 coupling nuclei (XX'YY'ZZZ-BB'-Hd) was optimised in 6 min using a laptop computer. ANATOLIA was also robust enough to be able to yield accurate spectral values from inaccurate input values. In both compounds, a fluorine–fluorine coupling constant was identified between the two fluoro-aromatic rings (FBB' and FYY') of +4.05 and +4.67 Hz and attributed to a through-space interaction. Ab initio structure optimisations and coupling constant calculations provided useful input data for spectral analysis. A modern 19F nuclear magnetic resonance spectrum of perfluorotoluene (octafluorotoluene) and analysis from 1975 was used as a test data set to assess ANATOLIA. 相似文献
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 CSiF2SiF2C- 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 (1JSiF > 300 Hz, 29 Hz < 2JSiF < 55 Hz, 3JSiF < 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. 相似文献
High-precision line measurements and tickling were shown to yield information, often not accessible, on the assignment and signs of spin coupling constants. For pentafluorothiophenol-d, this information was obtained for the three meta-JFF's. Relative to C6F5SH, the 19F resonances in C6F5SD experienced an upfield ‘isotope’ shift. 相似文献