We have measured the 15N nuclear magnetic resonance frequencies in 15N-labeled molecules (NNO, NNO, NH3, N2, and HCN) in gas phase samples and also in CH3NO2 as neat liquid. By using the previously determined temperature dependence of samples of the these gases at various densities, we are able to reduce the measured frequencies to the zero-density limit at 300 K, and obtain shielding differences between rovibrationally averaged isolated molecules at this shielding measurements from molecular beam studies to provide an 15N absolute shielding scale based on 15NH3.
Studies of density dependence of 129Xe chemical shift in xenon gas at room temperature have shown that while the chemical shielding does have quadratic and cubic dependence on density over densities up to 250 amagat, σ(ρ, T) = σ0 + σ1(T)ρ + σ2(T)ρ2 + σ3(T)ρ3, the curve is essentially linear up to about 100 amagat. We have now obtained σ1(T), the linear density coefficient of chemical shielding, for pure xenon over the temperature range 240–440 °K. The experimental values of σ1(T) can be fitted by a fourth degree polynomial: σ1(τ) = 0.536 − 0.135 × 10−2τ + 0.132 × 10−4 τ2 − 0.598 × 10−7τ3 + 0.663 × 10−10τ4 (ppm/amagat), where τ = T − 300 °K. Comparison is made with σ1(T) for other nuclei and with σ1(T) predicted by various theoretical models.
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