SYSTEMATIC COMPARISON OF EXPERIMENTAL, QUANTUM-MECHANICAL, AND MOLECULAR MECHANICAL BOND LENGTHS FOR ORGANIC-MOLECULES

Ab initio calculations give, with an accuracy depending on the sophist ication of the method, a bond length as an equilibrium value, r(e). Th e experimental bond lengths are always vibrationally averaged and may be expressed in different ways (r(g), r(z), r(a), etc.). Since high-qu ality ab initio calculations now are capable of giving bond lengths th at are approximately of experimental accuracy, it is important to be a ble to interconvert these values. We find that the bond lengths optimi zed at the TZ2P+f CCSD level may be considered as the converged r(e) v alues and that the MM3 and MM4 force fields successfully convert r(g) to r(e) values. We also evaluated the performance of quantum mechanics at the 6-31G MP2 and the 6-31G* B3LYP levels and found that the bond lengths (r(e)) at the 6-31G B3LYP level are better than these at the 6-31G MP2 level for molecules with only first-row atoms. However, th e bond lengths for the bonds involving second-row atoms are too long a t the 6-31G B3LYP level, and for these, the 6-31G* MP2 level is recom mended. An empirical formula is given for the conversion of the theore tical r(e) values calculated at these levels to the r(g) values.