Adsorption of N
Monatomic nitrogen is pictured in Figure 6a and 6f at its preferential positions – HCP (111) and bridge (100). Average spin density before and after adsorption of N on the cluster surface can be found in Supplementary Data Figure S2. All methods show a consistent spin distribution on non-bonding atoms before and after binding on the (111) surface. Atoms 2, 5, and 6 are the clear sites of bonding for the three unpaired electrons of nitrogen because the spin density drops by approximately one at each location. On the (100) surface, bonding is evident on only atoms 1 and 5. The third unpaired electron on N is not clearly bound to any particular Ir atom.
Bond length as described in Table 3a is the shortest distance between the N atom and the catalyst surface. Bond length for B3LYP and its dispersion-corrected form are 1.17/1.16 Å and 1.26 Å on the (111) and (100) surfaces respectively; B97-D3 length is slightly longer – 1.22/1.29 Å. These are close to that computed by Krekelberg – 1.16 Å and 1.11 Å.38 Moving from B3LYP to B3LYP-D3 to B97-D3, bond energy gets progressively stronger.
Table 3b gives the IR and Raman frequencies for various vibrational modes.38 The asymmetric stretch on the (100) surface is very strongly Raman active, while the symmetric stretch is moderately Raman and IR active. All vibrations on the (111) surface are weak.