Carbonyl Tilting and Bending Potential Energy Surface of Carbon Monoxyhemes
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The CO tilting (τ) and bending (β) potential energy surface of carbon monoxyheme has been investigated with local density functional calculations. The calculations indicate that τ and β are strongly coupled and that simultaneous, in-phase displacements along these coordinates represent a low-energy pathway across the surface. Calculations on two small model complexes indicate that strong τ−β coupling also occurs in these systems. Accordingly, this feature appears to be a general characteristic of the Fe−CO unit in a square-planar coordination geometry. In-phase τ−β CO deformations (τ + β) of as much as 25° can take place with the expenditure of relatively small amounts of energy (2 kcal/mol or less). However, very large distortions of 45−60° are energetically demanding and unreasonable. The calculations also rule out the possibility that the coordination geometry of the proximal histidine is the major determinant of the CO orientation. Both a full vibrational analysis on a small model complex and a limite...