Conformational analysis of cis-Delta(1), -Delta(2), -Delta(3) and -Delta(4)- tetrahydrophthalic anhydrides by semiempirical and ab initio molecular orbital calculations

Semiempirical and ab initio molecular orbital calculations have been used t o study the conformational geometries and the relative stability of the con formers of cis-Delta(1), -Delta(2), -Delta(3) and -Delta(4)-tetrahydrophtha lic anhydrides (THPA). The total energy and the structure of the transition states for the interconversion of conformers were calculated with PM3 semi empirical method. It is found that PM3, AM1 and MNDO semiempirical schemes underestimate the stability of cis-Delta(1)-THPA: contrary to experimental findings. The ab initio SCF calculations performed at the Restricted Hartre e-Fock (RHF) and MP2 levels using effective con pseudo-potential functions with basis sets contracted to 31 split valence (PS-31G) and (PS-31G*), are in good agreement with the available experimental evidence, e.g. the cis-De lta(1)-THPA is, thermodynamically, the most stable of all the cis-isomeric anhydrides. The ab initio results indicate that the energy between the conf ormers changes as a function of the AO basis set. The ground states conform ations of cis-Delta(1)-THPA are increasingly preferred in the more extended basis sets including d-polarized functions on carbon and oxygen atoms. For cis-Delta(4)-THPA, the folded conformation is calculated to be favored ove r the open conformation in agreement with the results of NMR analysis of ci s-Delta(4)-tetrahydrophthalic anhydride. The preference of this conformatio n is also consistent with the crystal structure recently determined by X-ra y diffraction technique. The structure analysis of the different conformati ons at different levels of ab initio calculation, as well as with semiempir ical methods, shows that the cyclohexene ring adopts a boat form in cis-Del ta(4)-THPA, a half-chair form in cis-Delta(1)-THPA and deformed sofa forms in both cis-Delta(3)-THPA and cis-Delta(2)-THPA; the anhydride ring being f lattened in cis-Delta(1)-THPA and cis-Delta(4)-THPA and more twisted in cis -Delta(3)-THPA and cis-Delta(2)-THPA. Comparison is made with structures of related compounds. (C) 2000 Elsevier Science B.V. All rights reserved.