Experimentally Established 15N NMR Absolute Shielding Scale for Theoretical Calculations

The nitrogen atom of 2,6-di-tert-butyl-N,N-diethylpyridin-4-amine (DEAP) is notavailable for non-covalent interactions. This molecule has been usedto define the reference N-15 NMR absolute chemical shielding(& sigma;(ref)) required to convert between the chemical shiftscale used in experiments and the absolute shielding scale used intheoretical calculations. The accuracy of the obtained & sigma;(ref) was tested for solid samples of acetanilide-N-15, the protonated homodimer of pyridine-N-15, and poly(4-vinylpyridine-N-15). Experimental N-15 NMR chemical shift tensorswere compared to N-15 NMR shielding tensors calculated usingthe TPSSh, B3LYP, and & omega;B97XD functionals and the polarizablecontinuum model approximation. General recommendations are given forthe smallest reliable basis set size. The reported structure of DEAPcan be used to calculate & sigma;(ref) for any other calculationmethod.