Experimentally Established ¹⁵N NMR Absolute Shielding Scale for Theoretical Calculations

Abstract

The nitrogen atom of 2,6-di-tert-butyl-N,N-diethylpyridin-4amine (DEAP) is not available for non-covalent interactions. This molecule has been used to define the reference 15 N NMR absolute chemical shielding (σ ref ) required to convert between the chemical shift scale used in experiments and the absolute shielding scale used in theoretical calculations. The accuracy of the obtained σ ref was tested for solid samples of acetanilide-15 N, the protonated homodimer of pyridine-15 N, and poly(4vinylpyridine-15 N). Experimental 15 N NMR chemical shift tensors were compared to 15 N NMR shielding tensors calculated using the TPSSh, B3LYP, and ωB97XD functionals and the polarizable continuum model approximation. General recommendations are given for the smallest reliable basis set size. The reported structure of DEAP can be used to calculate σ ref for any other calculation method.

The Scope of the Applicability of Non‐relativistic DFT Calculations of NMR Chemical Shifts in Pyridine‐Metal Complexes for Applied Applications

“…The isotropic chemical shift values of these tensors, δ iso , reported in Table 1, were obtained from the original isotropic absolute shielding values, σ iso , as δ iso =σ ref –σ iso . The value of the absolute shielding of the reference compound 15 NO 2 CH 3 in the ωB97XD/def2tzvp approximation is σ ref (15 N)=−143±3 ppm [23] . It is expected that the error in estimating the δ iso in this approximation should not exceed 3 ppm and does not change when using a larger basis set [23] .…”

“…[a] δ iso =σ ref –σ iso , σ ref ( 15 N)=−143±3 ppm [23] . [b] The standard deviation (SD(CN)) for N−C bonds in this structure is 0.002 Å.…”

“…The value of the absolute shielding of the reference compound 15 NO 2 CH 3 in the ωB97XD/def2tzvp approximation is σ ref (15 N) = À 143 � 3 ppm. [23] It is expected that the error in estimating the δ iso in this approximation should not exceed 3 ppm and does not change when using a larger basis set. [23] The value calculated for MeBiPy_DFT falls within this range.…”

“…[23] It is expected that the error in estimating the δ iso in this approximation should not exceed 3 ppm and does not change when using a larger basis set. [23] The value calculated for MeBiPy_DFT falls within this range. For NOFZUD and NOF-ZUD01 the error is larger regardless of the basis set for the ωB97XD functional but falls within the expected range for the TPSSh and B3LYP functionals, Table S4.…”

“…[a] δ iso = σ ref -σ iso , σ ref ( 15 N) = À 143 � 3 ppm. [23] [b] The standard deviation (SD(CN)) for NÀ C bonds in this structure is 0.002 Å. complexes was intentionally refrained from because the geometric parameters of these systems in the solid state and the isolated state are expected to differ, even without corrections for relativistic effects. In contrast, the influence of the environment on the geometry of bipyridine itself should be significantly less.…”