Temperature dependence of the 15N and 1H nuclear magnetic shielding in NH3

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Abstract
The temperature and density dependence of the 15N and the 1H nuclear resonance in 15NH3 gas have been observed. The density dependence which is a measure of the effect of intermolecular interactions on the nuclear shielding is linear, with slopes of −0.041±0.002 ppm/amagat for the 15N nucleus and −0.0032±0.0001 ppm/amagat for the 1H nucleus. The shielding in the limit of zero pressure σ0 varies with temperature due to rovibrational motion. This is of special interest for the 15N shielding in NH3 because of the previously reported exceptional temperature dependence of 31P in PH3. It is found that 15N in NH3 is also an exceptional case and, in fact, near 300 K 15N in NH3 and 31P in PH3 have nearly identical values of dσ0/dT(+0.00651±0.00082 ppm/deg for 15N in NH3 from 320 to 380 K). All other cases, involving a variety of nuclei in a wide variety of molecular types, exhibit dσ0/dT<0.

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