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.