Variation of chemical shielding with intermolecular interaction and rovibrational motion. V. 15N in N2

Abstract

The 15N resonance frequency in 15N2 gas has been measured as a function of density and temperature. The effect of intermolecular interactions, which is observed as a density dependence, is about the same magnitude as that for 13C in two interacting CO molecules, but smaller than previously observed for 15N nuclei in other molecules. The temperature dependence in the zero-pressure limit [σ0(T)−σ0(300) =−(0.85±0.10)×10−3 (T−300) ppm] originates from centrifugal distortion. The effect of vibrational averaging contributes negligibly to the temperature dependence in the range 220–380 K. Comparison with previous theoretical calculations of 15N shielding in N2 as a function of internuclear separation shows reasonably good agreement with the observed temperature dependence in the zero-pressure limit. The results for the N2 system are compared with the isoelectronic CO molecule for which more precise 13C NMR measurements are also reported here.