Methods
All calculations were implemented in Gaussian 09 using default
convergence limits.27 The computational schemes
compared in this study are B3LYP, B3LYP-D3, and B97-D3. B3LYP is the
Becke-style 3-Parameter functional.28,29 The
dispersion corrected version, B3LYP-D3, uses Grimme’s dispersion and the
Beck-Johnson damping parameter.30 B97-D3 is Grimme’s
functional with Becke-Johnson damped dispersion as
well.29–31 While B3LYP is a hybrid functional, often
used to achieve bond lengths and vibrational frequencies close to
experimental values,32 B97 is a semi-empirical pure
functional which delivers speedier results on larger clusters of atoms.
For consistency between methods and with previous calculations on
platinum,23 we will use the LANL2DZ basis set for the
iridium atoms and 6-311++g** basis sets for nitrogen and
hydrogen.23
The (111) and (100) surfaces of the face-centered cubic iridium catalyst
are both modeled in this study. As a default in Gaussian 09, the most
abundant isotope (Ir-193) is used for calculations.33The clusters built for these calculations are kept at frozen coordinates
during optimization, with an atomic Ir-Ir spacing of 0.3839
nm.34 All adsorbates are unfrozen. Frequency
calculations are conducted at the minimum energy geometries obtained
from optimization. Bending modes with an IR intensity less than
20×10-40 esu2cm2or Raman activity less than 20 A4/AMU are considered
difficult to observe against the cluster vibrations and are therefore
not all recorded in this paper. For general comparisons, this paper
classifies vibrations as weak (activity ≤ 40), moderate (activity=
40-100), strong (activity = 100-200), and very strong (activity= 200+).