Interpretation of the Chemical Shift of C1F

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Abstract
The chemical shift of 19F in gaseous ClF, about 160 ppm above the estimated shift for the free ion F−, represents a striking discrepancy with simple fluorine chemical-shift theories, which generally predict fluorine resonance shifts to be paramagnetic relative to F−. This result can be accounted for satisfactorily in terms of a simple p-bonded model with the use of a relatively uncomplicated equation for shielding. In the second-order or excitation term, it is essential (1) to consider separate contributions from different excitations, (2) to include terms on both atoms for some matrix elements, and (3) to recognize the role of the π-bonding coefficient even though the π shell is filled. The anomaly for ClF arises from the valence shell π*→σ* excitation, which corresponds to electron circulation in opposite senses on the two atoms. No such effect occurs for F2, on account of symmetry restrictions, or for HF, because of the absence of π electrons on the H atom.