Adsorption of NH
Figure 6b and 6g shows NH adsorbed at its most preferential site – HCP
(111) and bridge (100). Supplementary Data Figure S3 shows the average
spin density before and after adsorption of NH on the cluster surface.
On the (111) surface, the nitrogen atom is adsorbed between atoms 2, 5,
and 6, just as lone N is. The spin density per atom takes on an almost
identical shape with B3LYP, B3LYP-D3, and B97-D3 (at a lower average
spin density for the latter). For the (100) surface, B3LYP-D3 converges
to only an erroneous edge-effect hollow position. For the other two
methods, the two unpaired e- of NH form bonds with
bridging Ir atoms 5 and 8. The spin structure of B97-D3 is noticeably
more similar to B3LYP after adsorption than before adsorption.
Bond length in Table 4a is consistent between the three methods for
(111) - 1.23/1.24 Å. On the (100) surface, B97-D3 is 0.07 Å shorter. As
with each of the other molecules, bond energy follows the trend
EB3LYP > EB3LYP-D3> EB97-D3.
Table 4b shows NH frequencies for gas phase and Ir-adsorbed states and
how these frequencies compare to benchmark experimental
work.44,45 The N-H stretch perpendicular to the
catalyst surface is strongly Raman active on both surfaces. On the (100)
surface, the wagging mode is strongly Raman active as well, according
B97-D3 calculations. This same mode under the B3LYP method is only
moderately Raman active with a stronger IR activity. All other modes are
weak.