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+).