E. Orti et al., INFLUENCE OF BENZOANNULATION ON THE MOLECULAR AND ELECTRONIC-STRUCTURES OF TETRACYANOQUINODIMETHANES, Journal of physical chemistry, 100(15), 1996, pp. 6138-6146
The molecular and electronic structures of TCNQ and its pi-extended de
rivatives benzo-TCNQ and TCAQ have been investigated using the semiemp
irical PM3 method and ab initio 6-31G calculations. The steric hindra
nce introduced by lateral benzoannulation determines the loss of plana
rity of the TCNQ moiety for benzo-TCNQ and TCAQ. For both molecules, t
he most stable conformation corresponds to a butterfly-type structure,
in which the TCNQ ring adopts a boat conformation and the lateral ben
zene rings remain planar. This structure lies, at the 6-31G level, 33
.45 kcal/mol below the fully planar conformation and 17.50 kcal/mol be
low a second local minimum for TCAQ, while it only lies 3.35 kcal/mol
below the planar structure for benzo-TCNQ. The geometries of the anion
s and dianions have been also optimized at the semiempirical and ab in
itio levels. Although reduction induces the aromatization of the TCNQ
moiety, the anions of benzo-TCNQ and TCAQ remain nonplanar. The stabil
ity of the anions with respect to the neutral systems is thus calculat
ed to decrease along the series TCNQ > benzo-TCNQ > TCAQ, thus explain
ing the decrease of the electron-acceptor properties along this series
. For the dianions, the polycyclic skeleton is fully planar and the C(
CN)(2) groups are twisted out of the molecular plane. While the first
two extra electrons are mostly incorporated into the C(CN)(2) groups,
the third electron added to form the trianion is shown to enter the ce
ntral polyacenic unit for TCAQ. This result explains the formation of
relatively stable trianions and tetraanions for TCAQ. PM3 calculations
have been especially useful in characterizing the stationary structur
es located for each compound and have been used as a guide for 6-31G
calculations. PM3-optimized geometries are in good agreement with expe
rimental X-ray data and 6-31G results.