Computations of Fe-57-NMR chemical shifts with the SOS-DFPT method

Fe-57 Shielding tensors of substituted iron-carbonyl complexes have been computed employing the density-functional-based SOS-DFPT method (sum-over-states density-functional perturbation theory) with the IGLO (individual gauge for localized orbitals) choice of gauge origins and with large basis sets. The shieldings computed for [Fe(CO)(5)], [Fe(CO)(3)(H2C=CHCH=CH2)], [Fe(CO)(3)(cyclo-C4H4)], [Fe(CO)(4)(H2C=CHOMe)], [Fe(CO)(4)(H2C=CHCN)], [Fe(CO)(3)(H2C=CHCH=O)], and [Fe(CO)(2)(C5H5)R] (R = Me, Bu, i-Pr) correlate with the experimental delta(Fe-57) values. However, the slope of the correlation line is 0.55 instead of 1, i.e., only about one half of the substituent effects on sigma(Fe) is recovered in the calculations. Nearest-neighbor effects appear to be described qualitatively (cf. in the [Fe(CO)(2)(C5H5)R] series, whereas effects of more remote substituents, e.g., for [Fe(CO)(4)(H2C=CHX)] (X = MeO and CN)) are not reproduced. Dissociation energies of these species are discussed because of their relevance to experimental rate constants for substitution processes which are known to correlate with delta(Fe-57). Even though the delta(C-13) and delta(H-1) data of ferrocene (7) are well reproduced theoretically the computed sigma(Fe) shielding of 7 deviates substantially from the sigma(calc.)/delta(expt.) correlation, possibly indicating additional shortcomings in the theoretical description of this molecule.