The combination of deconvolution and density functional theory for the mid-infrared vibrational spectra of stable and unstable rhodium carbonyl clusters

Geometrical optimization and frequency calculations on [HRh(CO)4] were carried out using density functional theory methods with the B3LYP, PBE and B3PW91 functionals in conjunction with the LanL2DZ and DGDZVP basis sets. The accuracy of each calculation was verified by comparing the predicted and the experimentally obtained deconvoluted mid-infrared experimental CO stretching frequencies. The use of the density functional PBE with DGDZVP as the basis set was found to be the most accurate. The same method was then applied to [(C2H5)CORh(CO)4], [Rh2(CO)6(μ-CO)2], [Rh4(CO)9(μ-CO)3] and [Rh6(CO)12(μ3-CO)4]. Again, vibrational spectral patterns and relative band intensities were in very good agreement with those experimentally observed after BTEM deconvolution. The only inconsistency was a constant shift in the predicted wavenumber assignments of ca. 1.5% for the terminal carbonyl stretching modes. In addition, the optimized geometries were also in good agreement with available X-ray structures of isolatable [Rh4(CO)9(μ-CO)3] and [Rh6(CO)12(μ3-CO)4]. DFT not only proved to be a valuable tool in validating and confirming the structure of the reactive and unstable species but it also allowed better assignment of the observed spectra especially when vibrational modes were overlapping. The combination of advanced multi-component deconvolution, like band-target entropy minimization (BTEM), with DFT spectral prediction appears to have considerable potential for exploratory in situ studies of reactive rhodium carbonyl systems.