A density functional theory benchmark of the formation enthalpy and first CO dissociation enthalpy of hexacarbonyl complexes of chromium, molybdenum, and tungsten

Abstract

Reaction energies have been calculated for the hexacarbonyl metal complexes of chromium, molybdenum, and tungsten using six different Density Functional Theory (DFT) methods. Two gas phase reactions have been utilized to benchmark the computed results. The reactions are the heat of formation of the hexacarbonyl complex from its metal dioxide, dicarbon and dioxygen; and the first dissociation enthalpy of the carbonyl ligand. It was found that all DFT methods agreed well with the available experimental data. For the formation reaction, all methods underestimate the formation enthalpy, while non-hybrid methods do better than the methods involving functional hybridization. Including a diffuse function in the basis set decreases the magnitude of the formation enthalpy. For the CO dissociation reaction the LYP methods underestimate the bond enthalpy consistently for all the metals. There was not much difference between using a hybrid and a non-hybrid functionals, except when comparing the PWP91 and B3PW91 methods, but it was found that the level of exchange and correlation affects the result, especially when using the LYP-based methods. A comparison of calculated geometries and frequencies is also made with available experimental and computational data, confirming that the ground state of WO 2 is a singlet.