Adsorption of N2
Diatomic nitrogen adsorbed on the two surfaces is pictured in Figure 6e
and 6j. This end-on, top position corresponds to the lowest energy
optimization achieved with B97-D3. The N-N bond makes an angle of 89⁰
with the (111) and (100) surfaces. The B3LYP-D3 N-N bonds make angles of
30⁰ and 7⁰ with the (111) and (100) surfaces respectively. The spin
density per atom before and after adsorption of N2 can
be found in Supplementary Data Figure S1. With B3LYP-D3, the spin
density profile is consistent before and after adsorption, with a slight
reduction in spin on atom 5 for the (111) surface because the N-N bond
is angled 30⁰ towards that atom, providing context for slight charge
transfer between the N and Ir atom. With B97-D3, the (111) surface sees
a similar effect. Notably, there is a greater variance in B97-D3 ground
state spin density before and after bonding than with B3LYP-D3. On the
(100) surface, the spin density drop is particularly steep
(N2 oriented 89⁰). This suggests bonding of the inner N
atom to Ir atom number 5. This bond results from an interaction between
the Ir 5d-states and π-bonds polarized in the direction of Ir, thus
weakening the N-N bond and laying ground for N-Ir electron pairing.
Adsorbate energy is still relatively small due to σ-bond repulsion along
the axis of the bond.43 Covalent bond formation is
also evidenced by the bond lengths in Table 2a, where N2molecules that adsorbed preferentially at the top position,
perpendicular to the catalyst (B97-D3), adsorbed greater than 1 nm
closer than those adsorbed at an angled or horizontal position
(B3LYP-D3). Bond energies are all far from the experimentally determined
9.8 eV, but similar to theoretical predictions of -0.54 and -0.21
eV.38
Vibrational analysis of N2 as a free molecule is broken
down by method in Table 2b.38,40,41 B97-D3 predicts
the stretching mode of N2 to within
10cm-1, while both B3LYP methods largely overestimate
it. After adsorption, B3LYP-D3 predicts a 10cm-1decrease in the frequency of the stretching mode, while B97-D3 predicts
an ~170 cm-1 decrease in the
frequency. Experiment gives the adsorbed frequency to be 2185/2210/2223
cm-1, which is right on par with B97-D3
predictions.38,41 From this we can conclude B97-D3
correctly predicts the lowest energy position on the surface is at the
top position, with the N2 molecule perpendicular to the
surface.