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.