Medium Effects in NMR. III. Fluorine Resonance in Gases

Abstract

The fluorine resonance of CF4, SiF4, SF6, and CHF3, and in a variety of gas mixtures have been measured at pressures up to 50 atm at room temperature. The pressure dependence of the chemical shift is much more pronounced than for protons [W. T. Raynes, A. D. Buckingham, and H. J. Bernstein, J. Chem. Phys. 36, 3481 (1962)]. The results are interpreted in the same manner as for the proton work (see reference above) in terms of a field and field square contribution to the medium shift. The field square contribution is primarily due to dispersive forces and is 15 to 44 times as large as for hydrogen in CH bonds. The polar contribution (first power of the field) for CF bonds is also larger than for CH bonds but of opposite sign, being responsible for high‐field medium shifts for CF bonds.