PIFF

PIFF (Pi-Force-Field) is one of two Molecular Mechanics programs which are provided with the INTERCHEM software. It was written by Dr. Armin Widmer of Sandoz-Pharma, Basel, Switzerland. It is particularly designed to cope with molecules containing delocalised pi-electrons. A combined pi-SCF-LCAO-MO and force-field method is used to compute molecular configuration and energies (H. J. Lindner, Tetrahedron, 1974, 30, 1127-1132). Because it uses the closed shell SCF method, PIFF is restricted to molecules with an even number of pi-electrons (including the special case of zero).

• Currently PIFF is parameterized for the following atom types:-
  • C (sp3 and sp2) Note that no distinction is made between C(sp2) and C(aromatic)
  • N(sp3, sp2, trigonal planar)
  • O(sp3 and sp2)
  • S (bivalent, including disulphide bonds)
  • Cl
  • H
    
• In structure optimizations PIFF may be supplied with inter atomic distance and torsion angle constraints, derived (for example) from nuclearOverhauser effect experiments and nmr coupling constants.
  
• The treatment of delocalized pi-electrons in force field calculations poses some special problems. Consider the simple cases of benzene and naphthalene. Suitable bond lengths and other parameters might be found to describe the aromatic structure of benzene. If these parametersare then applied to the naphthalene molecule, the calculated bond lengths are essentially all the same, and the same as in benzene. Experimentally, it is found that certain bond lengths in naphthalene are about two thirds of the way between a single and a double bond, while other bond lengths are one-third of the way, as is predictable from the Kekule forms where each bond is either double in two and single in one, or the reverse. The reason for the failure of the benzene parameters is clear enough. In naphthalene, the bond lengths are determined largely by the bond orders. In benzene, the bond orders are all equal and were not included in the parametrization. But to correctly reproduce the structures of conjugated molecules, the effects of bond order variation must be considered.
  
• To solve this difficulty, a combination of the usual molecular mechanics method and a quantum chemical treatment of the pi-system is used. In the approach proposed by H. J. Lindner pi-bond orders are determined through a semi-empirical pi-electron SCF calculation, e.g. of the Pariser-Parr-Pople type. It is then assumed that simple correlations between the resulting pi-bond orders and the following parameters exist: equilibrium bond length, equilibrium bond angle, bond stretching force constant, out of plane bending force constant, and torsional barrier. Force field parameters are then derived from these relations and the results of the SCF calculation, and the equilibrium (minimum energy) conformation of themolecule is determined through the application of standard minimization techniques. As the results of the SCF procedure depend on the molecular geometry, a new application of the SCF method will give a new, slightly different, force field. The combination of SCF and MM is therefore applied iteratively, until both the bond order matrix and the force field energy remain unchanged (i.e. SCF/MM self consistency is reached).

  Revised 27th November 1997