Calculation of rz structures from rs structures

Abstract

Experimental bond lengths are determined in a variety of ways. Gas phase electron diffraction experiments give the thermal average value of the internuclear distance ( r g). Two kinds of operational bond lengths ( r 0 and r s) may be derived directly from rotational and rotation-vibrational spectroscopy. The distance between the equilibrium nuclear positions is called r e, and that between the average nuclear positions at zero K is called r z. The relationship derived for diatomic molecules, r s = (r c + r z ) 2 , was used to estimate the r s structures from molecular mechanics (MM3) calculations of r g, and subsequently r z and r e bond lengths, for a group of 23 molecules. This relationship works well for polyatomic molecules. The root-mean-square (RMS) deviation between experimental r s bond lengths and those evaluated by the MM3 force field over this set of molecules is 0.0058 Å, which is similar to the experimental uncertainty ( RMS = 0.0066 A ̊ ).