Experimental

• Molecular Mechanics calculations were carried out using Allinger's MM2^[22] force field parameter, as implementedin the CAChe Mechanics program^[23]. Molecular geometry optimizations were computed using the conjugate gradient method. Transition state geometry optimizations were computed using first Conjugate Gradient followed by Block-Diagonal Newton Raphson methods. In the transition states the two forming-bonds lengths were locked on the value previously calculated with MOPAC. The augmented MM2 force field was required when calculating with Zn atoms, allowing the missing parameter to be estimated by the program. All the energy terms were taken into account :
  • Bond Stretch.
  • Bond Angle.
  • Dihedral Angle.
  • Improper Torsion.
  • Van der Waals Interaction (Cut-off distance = 9.000 Å).
  • Electrostatics (using Dipoles).
  • Hydrogens bonds.

• Semi-empirical calculations were carried out in the gas phase at the Restricted Hartree-Fock (RHF) level. They were computed on a Silicon Graphics Indy workstation with MOPAC^[24] 93.00. The PM3^[25] model is the most recent and most accurate and threrefore was used for all calculations. Whenever confirmation of the results were needed, another calculation was performed using the older AM1^[26] model. All structures were optimized using the Eigenvector Following algorithm. For each molecule and transition state, the vibrational spectrum was produced by a Force calculation with MOPAC. Both Ground States and Transition States are characterized by the presence of six (for non-linear molecules) eigenvalues which are very small. A transition state is further characterized by one, and exactly one, negative force constant.

• The Cambridge Structural Database^[27] was searched using the Quest program. We require a R -Factor of less than 6% in order to keep only well-defined structures and only organic compounds were taken into account.