Project #3: Vibrational frequencies in transition metal – dioxygen adducts

 

*    Background

*    Research Goals and Projects

*    Reading

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Background:

Comparison of vibrational frequencies obtained from experiment and theoretical calculations can be used to identify the mode of dioxygen coordination in 1:1 transition metal – O2 adducts.  This approach has particular merits in those cases when the most generally applied theoretical procedure of computing energies for different geometries and assigning the experimental complex as the lowest energy optimized structure proves problematic.  In the case of 1:1 adducts of CuI with dioxygen, singlet states can exhibit high degrees of multideterminantal character, leading to potentially unreliable results from density functional theory (DFT) and necessitating the use of expensive higher-order methods (e.g. multireference second-order perturbation theory – CASPT2) to obtain accurate energies.  In other cases, the relative energy of η1 and η2 metal/O2 complexes may be computed to be within the range of error expected for DFT calculations (~3 kcal/mol with the B3LYP functional).  A common experimental approach of testing for splitting in ν(O–O) values for mixed-label isotopomers has recently been shown also to be highly problematic for assigning the O2 binding mode.  Specifically, the lack of such splitting was shown not to be diagnostic for side-on versus end-on dioxygen coordination.

However, in order for comparison of experimentally and theoretically determined vibrational frequencies to be meaningful, computed vibrational frequencies must be both inexpensive and highly accurate.  DFT methods would seem to fit this prescription, as they can efficiently handle large models and generate highly reliable optimized geometries.  However, a body of evidence exists which suggests that ν(O–O) and ν(M–O) in transition metal complexes computed by DFT methods are of insufficient quality for the task at hand.  For example, computed O–O stretch frequencies (even when scaled) differ from experimental values by 50 cm-1 in the side-on 1:1 Cu-O2 adducts supported by β-diketiminate and anilido-imine ligands when B3LYP with a triple-zeta polarized basis set is used.  For other metal-O2 1:1 adducts, errors can routinely be in the range of 20-50 cm-1.  The inaccuracy of computed frequencies in 1:1 M-O2 adducts is highlighted by their poor correlation with experimental values (see Figure).  Known vibrational frequency scaling factors for particular combinations of functionals and basis sets may not be applicable to these cases since they were derived from studies on molecules not containing transition metals.

 

 

 

 

 

 

 

 

 

 

 

 

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Research Goals and Projects:

This facet of research will therefore focus on determining an optimal DFT-based method for computing ν(O–O) and ν(M–O) in transition metal complexes.  Combination(s) of density functionals, basis sets, and scaling factors which lead to minimal error when measured against experimental data will be ascertained.  Pure, hybrid, and meta-GGA density functionals will be considered, as will double- and triple-zeta basis sets with and without polarization and diffuse functions.  A database of experimental ν(O–O) and ν(M–O) in 1:1 M-O2 adducts will be assembled in order to assess the accuracy of the DFT methods.  Isotopologue splitting Δν(18O2) can also be included in experimental/theoretical comparisons for cases where such experimental data are available.

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Reading:

(1)        Bauschlicher, C. W.; Partridge, H. J. Chem. Phys. 1995, 103, 1788-1791.

(2)        Cramer, C. J.; Tolman, W. B.; Theopold, K. H.; Rheingold, A. L. Proc. Natl. Acad. Sci. U. S. A. 2003, 100, 3635-3640.

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