Proximity effects on lJ(CH) couplings are studied from a theoretical point of view in the following systems:CH4/FH (A) and H*O/HCN (B) which form dimers by hydrogen-bond interactions. 'J(CH) couplings for different intermolecular distances are calculated for the C-H bond facing the atom bearing lone pairs. While in the former system, this coupling is increased owing to the proximity to the F atom; in the latter this coupling is decreased owing to the proximity to the 0 atom. These opposite trends are accompanied by slight shortening and lengthening, respectively, of the corresponding C-H bond lengths. As part of this work, measurements of 'J(CH) couplings in 9 4 1,3-dioxolan-2-y1)-1,2,3,4-tetrafluorotriptycene (I) and l-formyl-2-hydroxy-8-fluoronaphthalene (11) and J ( N H ) in o-fluorobenzamide (III), showing proximity effects as those described above are carried out. These results suggest that J(XH) couplings are adequate probes to distinguish two different types of X-H.-Y hydrogen bonds.

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