A Density Functional Study of the Rotational Barrier of Tricarbonyl(η ⁴ -norbornadiene)iron. Effect of the Torsional Angle on the Carbonyl Stretching Spectra

According to density functional calculations, the rotational barrier of the Fe(CO)3 unit in tricarbonyl(η4-norbornadiene)iron (1) is 0.5 kcal/mol, consistent with the value deduced from dynamic IR spectroscopy and, thus, with a very fast CO site exchange process. An alternative explanation of the observed coalescence of the carbonyl stretching bands through an intrawell mechanism is not supported since the computed ν(CO) frequencies are almost independent of the relevant torsional angle.