EFFECT OF NITROGEN ISOTOPES ON THE PERPENDICULAR BENDING MODE OF NITRATE ION

Abstract

The Infrared absorption of Nujol mulls of $KNO_{3}$ and of $Ba(NO_{3})_{2}$ has been studied in the vicinity of 12. Solid solutions of varying $N^{14}$ - $N^{15}$ ratio were prepared by dissolving mixtures of $KNO_{3}$ containing a nominal 62% $N^{15}$ with ordinary $KNO_{3}$ in a minimum amount of water. The salt was recrystallized by sudden addition of a large excess of chilled ethanol. At least two components of the absorption have been identified in both the $N^{14}$ region (825 to $835 cm^{-1}$) and in the $N^{15}$ region (806 to $811 cm^{-1}$). In $Ba(NO_{3})_{2}$, on the contrary, only a single peak appears in each region. Potassium nitrate exists in the aragonite structure at room temperature; barium nitrate is similar crystallographically to fluorite. Since the $KNO_{3}$ structure is favorable for the existence of a relatively large dipole-dipole interaction between nearest $NO_{3}^{-}$ neighbors whereas the $Ba(NO_{3})_{2}$ structure is not, the results are interpreted as a dipole-dipole coupling. These results also indicate that isotopic abundance can be measured much more conveniently using barium nitrate.