Conformational analysis of cis-Delta(1), -Delta(2), -Delta(3) and -Delta(4)- tetrahydrophthalic anhydrides by semiempirical and ab initio molecular orbital calculations
A. Ben Fredj et al., Conformational analysis of cis-Delta(1), -Delta(2), -Delta(3) and -Delta(4)- tetrahydrophthalic anhydrides by semiempirical and ab initio molecular orbital calculations, J MOL ST-TH, 528, 2000, pp. 219-235
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.