scholarly journals Demystifying fluorine chemical shifts: electronic structure calculations address origins of seemingly anomalous 19F-NMR spectra of fluorohistidine isomers and analogues

2015 ◽  
Vol 17 (45) ◽  
pp. 30606-30612 ◽  
Author(s):  
Chandana Kasireddy ◽  
James G. Bann ◽  
Katie R. Mitchell-Koch
Keyword(s):  
Electronic Structure ◽  
Chemical Shift ◽  
Electron Density ◽  
Nmr Spectra ◽  
Chemical Shifts ◽  
Electronic Structure Calculations ◽  
19F Nmr ◽  
19F Nmr Spectra ◽  
Structure Calculations

Understanding localization/delocalization of fluorine electron density is shown to be critical for predicting and interpreting fluorine chemical shift.

Studies of structure and dynamics in a nominally symmetric twisted amide by NMR and electronic structure calculations

2006 ◽  
Vol 84 (3) ◽  
pp. 421-428 ◽  
Author(s):  
Alex D Bain ◽  
Hao Chen ◽  
Paul H.M Harrison
Keyword(s):  
Electronic Structure ◽  
Nmr Spectra ◽  
Chemical Exchange ◽  
Electronic Structure Calculations ◽  
Amide Bond ◽  
Electronic Effects ◽  
H Nmr ◽  
Line Shape Analysis ◽  
Structure Calculations ◽  
C Nmr

Amides that are twisted around the C—N bond show unusual spectroscopy and reactivity when compared with planar amides. The diacyl derivatives of 3,4,7,8-tetramethyl-2,5-dithioglycoluril are intriguing examples of this class, since the crystal structures show that the two acyl groups are twisted by different amounts on either side of the molecule owing to a combination of steric and electronic effects. However, the 1H NMR spectra in solution at room temperature exhibit only one acyl resonance, so there must be fast interconversion among pairs of equivalent structures of each compound. We have prepared a number of derivatives with different acyl groups, both on the glycoluril framework as well as on its dithio analogue. The chemical exchange in solution was slowed down sufficiently by cooling to see individual sites for only two compounds: the dithiodipivaloyl and the dithiodiadamantyl derivatives. The barriers were estimated at 41 kJ mol–1 for the dipivaloyl derivative and 45 kJ mol–1 for diadamantyl derivative. The results show that rotation around the twisted amide bond is slowed by both the steric size of the acyl group and the presence of the thioureido group vs. the ureido group in the glycoluril core. In the solid-state 13C NMR spectra, there is no evidence for any dynamics, even for the diacetyl derivative at ambient temperature. Electronic structure calculations predict a geometry for the dipivaloyl derivative very close to that observed in the crystal structure. These results indicate that the crystal confines, but does not distort the molecule. A mechanism for the exchange is proposed. The relevance of these results to the mechanism of Claisen-like condensations in diacylglycolurils is also discussed.Key words: 1H and 13C NMR, exchange, dynamics, CP/MAS, solids, line shape analysis, amides, twisted amides, atropisomers, glycoluril.

Calculation of fluorine chemical shift tensors for the interpretation of oriented 19F-NMR spectra of gramicidin A in membranes

2009 ◽  
Vol 11 (32) ◽  
pp. 7048 ◽  
Author(s):  
Ulrich Sternberg ◽  
Marco Klipfel ◽  
Stephan L. Grage ◽  
Raiker Witter ◽  
Anne S. Ulrich
Keyword(s):  
Chemical Shift ◽  
Nmr Spectra ◽  
Gramicidin A ◽  
19F Nmr ◽  
19F Nmr Spectra ◽  
Chemical Shift Tensors

78. Chemical shift anisotropy of 19F NMR spectra for intercalants in graphite: Potential for determination of graphite fiber orientation

Carbon ◽  
1982 ◽  
Vol 20 (2) ◽  
pp. 133
Author(s):  
H.A. Resing ◽  
Gerald Ray Miller ◽  
M.J. Moran ◽  
J.P. Reardon ◽  
D. Dominguez ◽  
...  
Keyword(s):  
Chemical Shift ◽  
Fiber Orientation ◽  
Nmr Spectra ◽  
Chemical Shift Anisotropy ◽  
19F Nmr ◽  
Graphite Fiber ◽  
19F Nmr Spectra

13C, 15N, and 19F NMR spectra of 4-fluoro-(3´ or 4´-substituted) azobenzenes

1984 ◽  
Vol 49 (1) ◽  
pp. 58-64 ◽  
Author(s):  
Antonín Lyčka ◽  
Jaromír Kaválek
Keyword(s):  
Nmr Spectra ◽  
Chemical Shifts ◽  
Coupling Constants ◽  
19F Nmr ◽  
19F Nmr Spectra

The 13C and 19F NMR spectra of fifteen 4-fluoro-(3´ or 4´-substituted (azobenzenes have been measured. The δ(19F) values correlate with the corresponding σm,p constants (δ(19F) = 3.60σm,p-30.78). The 15N NMR spectra have been measured of six compounds, and additivity of the 15N substitution chemical shifts has been proved. The measured coupling constants are 6J(19F15N) = 2.1 Hz and 7J(19F13C) = 0.7 Hz.

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