INFLUENCE OF BENZOANNULATION ON THE MOLECULAR AND ELECTRONIC-STRUCTURES OF TETRACYANOQUINODIMETHANES

Citation
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
Citations number
81
Categorie Soggetti
Chemistry Physical
ISSN journal
00223654
Volume
100
Issue
15
Year of publication
1996
Pages
6138 - 6146
Database
ISI
SICI code
0022-3654(1996)100:15<6138:IOBOTM>2.0.ZU;2-N
Abstract
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.