Conformational analysis of cis-#DELTA#~1,-#DELTA#~2,-#DELTA#~3 and -#DELTA#~4-tetrahydrophthalic anhydrides by semiempirical and ab initio molecular orbital calculations

摘要

Semiempirical and ab initio molecular orbital calculations have been used to study the conformational geometries and the relative stability of the conformers of cis-#DELTA# 1, -#DELTA# 2, -#DELTA# 3 and -#DELTA# 4-tetrahydrophthalic anhydrides (THP A). The total energy and the structure of the transition states for the interconversion of conformers were calculated with PM3 semiempirical method. It is found that PM3, AM I and MNDO semiempirical schemes underestimate the stability of cis-#DELTA# 1- THPA, contrary to experi- mental findings. The ab initio SCF calculations performed at the Restricted Hanree-Fock (RHF) and MP2levels using effective core pseudo-potential functions with basis sets contracted to 31 split valence (PS-3IG) and (PS-3IG}, are in good agreement with the available experimental evidence, e.g. the cis-#DELTA# 1. THPA is, thermodynamically. the most stable of all the cis-isomeric anhydrides. The ab initio results indicate that the energy between the conformers changes as a function of the AO basis set. The ground states conformations 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- THP #DELTA#. the folded conformation is calculated to be favored over the open conformation in agreement with the results of NMR analysis of cis-#DELTA# 4-tetrahydrophthalic anhydride. The preference of this conformation is also consistent with the crystal structure recently determined by X-ray diffraction technique. The stnlcture analysis of the different conformations at different levels of ab initio calculation, as well as with semiempirical methods, shows that the cyclohexene ring adopts a boat form in cis-A 4- THPA, a half-chair form in cis-#DELTA#~1 - THPA and deformed sofa forms i" both cis-A3- THPA and cis-#DELTA#~2 -THPA; the anhydride ring being flattened in ci.t-A I.THPA andcis-A4-THPA and more twisted in cis-#DELTA#~3 - THPA and cis-#DELTA#~2_THPA. Comparison is made with structures of related c6mpounds.