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

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.

Determination of relative tensor orientations by γ-encoded chemical shift anisotropy/heteronuclear dipolar coupling 3D NMR spectroscopy in biological solids

“…The calculated principal components of the magnetic shielding tensor, σ ii (i = 1, 2, or 3), were converted to the principal components of the chemical shift tensors, δ ii , using the relationship δ ii = σ iso (ref) - σ ii , where σ iso (ref) = 244.6 ppm is the absolute shielding for liquid ammonia at 25 °C. 76 …”

Ab Initio Studies of Amide-15N Chemical Shifts in Dipeptides: Applications to Protein NMR Spectroscopy

“…The correlation between the GIAO and DFT results for the alanine residues in the three proteins was very high, R 2 = 0.98, and the shape functions for the φ i -1 and ψ i rotation plots were essentially identical using both methods. We used an absolute shielding of 244.6 ppm for liquid ammonia at ambient temperature to convert calculated shieldings to the IUPAC chemical shift scale …”

Ro‐Vibrational Corrections to NMR Parameters

“…The importance of spin-orbit contributions to the geometry dependence of nuclear shieldings was also nicely illustrated for the series of molecules CX 2 (X=O,S,Se,Te), where it was possible to reproduce the experimentally observed secondary isotope shifts on carbon only when spin-orbit contributions were included in the calculations [63,64]. The field of temperature effects on shieldings in polyatomic molecules in the gas phase was pioneered, experimentally as well as theoretically, by Jameson and coworkers in the early 90s [65][66][67][68]. Considering the crudeness of the theoretical methods used in these early works (both in terms of the basis sets that could be used as well as the fact that most of the calculations were done at the Hartree-Fock level), the agreement between the theoretical and experimental results is very good.…”