Small Rings and Conjugated Systems

The natural bond length varies between 1.503 A and 1.337 A for bond orders between 0 and 1 - these are the values for pure single and double bonds between two sp -carbons. Similarly, the force constant varies between the values used for isolated single and double bonds. The rotational barrier for an isolated double bond is 250kJ/mol, since there are four torsional contributions for a double bond. 

This approach allows a general treatment of all conjugated system, but requires the addition of a second level of iterations in a geometry optimization. At the initial geometry, a PPP calculation is performed, the r-bond orders are calculated and suitable bond parameters (Ro, and are assigned.These parameters are then used 

The other approach, which is somewhat more general, is to perform a simple electronic structure calculation to determine the degree of delocalization within the tt-system. This approach is used in the MM2 and MM3 force fields, often denoted MMP2 and MMP3. The electronic structure calculation is of the Pariser-Pople-Parr (PPP) 

For commonly encountered conjugated systems like butadiene and benzene, the ad hoc assignment of new parameters is usually preferred as it is simpler than the computationaly more demanding PPP method. For less common conjugated systems the PPP approach is more elegant and has the definite advantage that the common user does not need to worry about assigning new parameters. If the system of interest contains 

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Protonation and solvation in strong aqueous acids, 13, 83 Protonation sites in ambident conjugated systems, 11, 267 Pseudorotation in isomerization of pentavalent phosphorus compounds, 9, 25 Pyrolysis, gas-phase, of small-ring hydrocarbons, 4, 147... 

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