18O-isotope-induced shifts in 15N nuclear magnetic resonance spectroscopy. 1. Isotope shift in nitrite and its application to the study of oxygen exchange of sodium [15N, 18O2]nitrite

Substitution of 18O for 16O results in an upfield shift in the 15N nuclear magnetic resonance signal of the 15N-enriched nitrite and nitrate ions. The magnitude of the shift in the 15N NMR signal depends on the number of 18O atoms covalently bonded to the nitrogen atom. In nitrite the shift is 0.138 ppM/18O. This relatively large isotope shift permits the direct study by nuclear magnetic resonance spectroscopy of oxygen-exchange reactions involving nitrogen species. Acid-catalyzed medium nitrite(oxygen)-water exchange was followed in a continuous assay mode to demonstrate the ease and applicability of the isotope shift to the study of nitrogen(oxygen)-water exchange reactions. It was found that at pH 6.26 and 280C a sequential mode of exchange of the oxygen atoms is observed. This observation agreed with a mechanism postulated earlier on the basis of indirect evidence