Approximate re structures from experimental rotational constants and ab initio force fields

Vibration-rotation constants calculated from MP2/6-31G(d) ab initio quadratic and cubic force fields were used together with ground state rotational constants to derive approximate r(e) structures for asymmetric rotor molecules with four to 10 atoms. The good agreement with published r(e) (COCl2) and r(m)(o), (COCl2, HCOOH) structures suggests that vibration-rotation constants from low level small basis set ab initio calculations may be sufficiently accurate to derive approximate equilibrium structures for larger molecules. Results are also presented for HCOCl, FCOCl, CF2NCl, CF2NH, CH3CH2PH2, CH3OCOCl and CH3OCOCN. Problems encountered during the r(0)/r(s), structure determinations persisted to some extent for the r(e) structures of the last two molecules in the list. Problems were less severe or nonexistent for the r(e) structures of the other molecules studied. (C) 2001 Elsevier Science B.V. All rights reserved.