Atomic Environments in N‑Containing Graphitic Carbon Probed by First-Principles Calculations and Solid-State Nuclear Magnetic Resonance

To understand the nature and structure of N-doping centers in carbon materials, we combine two-dimensional (2D) solid-state NMR experiments and chemical shift calculations for 15N, 13C, and 1H nuclei from density functional theory (DFT). Comparisons of predicted chemical shifts with experimental 2D 13C–15N spectra show good agreement and the calculations explain the spectral broadening seen in the experiments. The major differences between the chemical shifts of graphitic/pyridinic/pyrrolic N-moieties are understood by comparing the electronegativities of the various environments. Moreover, the signal broadening is explained using four different factors: (1) the standalone N/C geometry, (2) the effect of a second N atom nearby, (3) the first or second neighbor C atom difference, and (4) the influence of residual water, which is important to understand the electrocatalytic environment. An intuitive correlation between the charge of the probed atom and the chemical shift is validated: the smaller the charge, i.e., higher electron density, the more shielded the nucleus is, and hence the smaller the associated chemical shift. These results can improve the understanding of the nature of heteroatom sites in nitrogen–carbon materials and contribute to the rational design of these materials with desired electronic properties and improved electrochemical performance.