TEMPERATURE DEPENDENCE OF J(FF) IN 2-FLUORO-ALPHA-CHLORO-ALPHA, ALPHA-DIFLUORO-TOLUENE AND 2-FLUORO-ALPHA, ALPHA-DICHLORO-ALPHA-FLUORO-TOLUENE.
ILLINOIS UNIV URBANA NOYES CHEMICAL LAB
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The F-F coupling constants JFF between the fluorine atom at the 2-position and the fluorines in the CFCl2 and CF2Cl groups were measured and found to have an appreciable temperature dependence in 2-fluoro-alpha, alpha-dichloro-alpha-fluoro-toluene and 2-fluoro-alpha-chloro-alpha,alpha-difluoro-toluene. The effect of temperature on the fluorine chemical shift of the CFCl2 and CF2Cl groups was also studied in these compounds. The observed temperature dependences of JFF are consistent with the existence of several conformers differing in the rotational angle of the CFCl2 and CF2Cl groups. The relative stabilities of the rotamers depend upon the repulsive non-bonded interactions of the atoms in the CFCl2 and CF2Cl groups with the 2-F and 6-H atoms. The potential functions for rotation of the CFCl2 and CF2Cl groups were estimated by means of both the simple van der Waals repulsive potential and the modified Buckingham potential. The approximate dependence of JFF upon the dihedral angle between the C-F bond in the CFX2 group and that of the 2-fluorine atom was obtained from an analysis of the high and low temperature limits for JFF in several compounds, including results reported earlier for some benzotrifluorides. It is found that JFF increases sharply at F-F internuclear separations of 2.5 to 2A, which supports the view that a non-bonded interaction makes a major through space comtribution to coupling of the fluorines. Author
- Organic Chemistry
- Atomic and Molecular Physics and Spectroscopy