Molecular Mechanics: The Cross-conjugated Carbonyl Group in Heterocyclic Compounds. 2. Parameterisation (MM2) of the Adjacent C-O and C-N Bonds

Abstract

The definition, in a previous paper, of a new type of carbon atom (type 44) in the MM2 force field, i.e. when it is cross-conjugated, implies the reparameterisation of the bonds adjacent to the carbonyl bond. By means of a statistical study based on data issued from the Cambridge Structural Data System, we propose here new σ dipole moments and new stretching parameters for the C(44)-O(41) and C(44)-N(40) bonds included in heterocyclic compounds.

These stretching parameters, which are π-bond-order dependent, are quite satisfactory (mean of unsigned deviations: 0.023Å) within limited π-bond-order ranges: 0.25 to 0.32 for the C-O bond, 0.37 to 0.53, for the C-N bond. As for the C(44)=O(7) bond the parameters are generally inappropriate for compounds in which both atoms connected to the type 44 carbon are heteroatoms.

The maximum deviation from the experimental bond length, for both bond types can reach ±0.03Å. Here also, part of the dispersion of the results could be attributed to the variation of the effective dielectric constant D from one crystal to another.