The molecular structures of two aminophosphonium salts (bromide and tetrafluoroborate) have been determined by X-ray analysis. They have similar conformations and hydrogen bond (HB) networks: the N H acid proton is bonded to the anion and, in the case of the fluoroborate, to the oxygen atom of the phosphine oxide, forming a pseudo six-membered ring closed by a weak N H¢ ¢ ¢O intramolecular hydrogen bond (IMHB). These compounds have been studied by multinuclear NMR in solution, including the 15N-labeled derivatives, to determine a complete set of coupling constants. A coupling of j1:5j Hz between the 15N and the 31P nuclei, separated by three bonds, was observed experimentally for the bromide in CDCl3 solution, which appears to be a classical 3JN-P across the covalent bonds and not a 3hJN-P across the IMHB.

We acknowledge the financial support of the DGI/MCyT (projects no.BQU-2000-0252 and 0906). The work in Murcia was generously supported by Fundaci´on S´eneca-CARM (project PB/2/FS/99) and Acedesa (a division of Takasago). One of us (P. L.-L.) also thanks Fundaci´on S´eneca for a fellowship.Wewarmly thank Professor Janet del Bene (Youngstown State University, Dept. of Chemistry, youngstown, OH, 44555, USA) for carrying out the MP2 calculations and Drs. R. H. Contreras and J. Peralta for providing the program to carry out the calculation of the coupling constants.

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