COMPUTATIONS OF FE-57-NMR CHEMICAL-SHIFTS WITH THE SOS-DFPT METHOD

Fe-57 Shielding tensors of substituted iron-carbonyl complexes have be en computed employing the density-functional-based SOS-DFPT method (su m-over-states density-functional perturbation theory) with the IGLO (i ndividual gauge for localized orbitals) choice of gauge origins and wi th 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)], [F e(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 recov ered in the calculations. Nearest-neighbor effects appear to be descri bed qualitatively (cf. in the [Fe(CO)(2)(C5H5)R] series, whereas effec ts of more remote substituents, e.g., for [Fe(CO)(4)(H2C=CHX)] (X = Me O and CN)) are not reproduced. Dissociation energies of these species are discussed because of their relevance to experimental rate constant s for substitution processes which are known to correlate with delta(F e-57). Even though the delta(C-13) and delta(H-1) data of ferrocene (7 ) are well reproduced theoretically the computed sigma(Fe) shielding o f 7 deviates substantially from the sigma(calc.)/delta(expt.) correlat ion, possibly indicating additional shortcomings in the theoretical de scription of this molecule.