XI Simpósio Brasileiro de Química Teórica
Caxambu, 18 a 21 Novembro de 2001
 
 
 
 

P301

SEMI-EMPIRICAL REACTIVITY STUDY ON LINEAR POLYACENES

Rudolf Kiralj (PQ) and Márcia M. C. Ferreira (PQ), Instituto de Química, Universidade
Estadual de Campinas, Campinas, SP, 13083-970, Brazil. marcia@iqm.unicamp.br
 

Linear  polyacenes,  although  being  the simples aromatic hydrocarbons,  are known  up  to
the  seventh   member  of   the   series   (heptacene).¹   They   are  characterized   by  rapid
increase in  reactivity  and  decrease  in  stability  as  the  chain length increases.   Such  an
aromaticity  is  called  by  Clar²  as  acene  character;   it   includes  meso-rings  in  aromatic
substitution/addition,   and  terminal  rings  in  some  other  reactions  as  the  most  reactive
sites.   The  following  reactivity  of  linear  polyacenes  was  studied  in  this  work  by  semi-
empirical  PM3:   aromatic   meso-bromination,  biochemical   and   artificial   ortho-oxidation,
total  hydrogenation,  meso-additions   (hydrogenation,  photochemical  oxidation,   addition
of bromine and maleic acid anhydride)  and  meso-oxidation  to  quinones.   The geometries
of reactants and products were optimized,  and  heats  of  formation  of  reactions calculated
from heats of formation and total energies of the molecules. Molecular descriptors as graph-
theoretical  and  PM3  resonance  energy,  hardness  and  Dewar's  localization,  para-  and
ortholocalization numbers were calculated.  The heats  of  the reaction  were  analyzed as a
function of chain length,   and  the  reactions  with  similar  energy  profile  were  studied  by
Principal  Component   (PCA)   and   Hierarchical  Cluster  Analysis   (HCA).    Partial  Least
Squares  Regression  (PLS)  related  heats of reaction to the molecular descriptors and  the
chain length.   Energy profiles  for  meso-ring  reactions have  the  same  shape.   They  are
described  well  in  PCA  by  one  Principal  Component  (over  90%  of original information).
Both   PCA   and   HCA   show  that  acene  character  begins  with  anthracene,   with  high
similarity between  molecules  having  the  same  number  of  different  reactive sites.   Both
analyses  distiguish  anti-   from   syn-   and   in-plane   oriented   reactions.    PLS  models
(Q > 0.96, R > 0.98) are superior to linear regression models (R > 0.75).
1 W. J. Bailey, C.-W. Liao, J. Amer. Chem. Soc. 77 (1955) 992-994.
2 E. Clar: The Aromatic Sextet, Wiley&Sons, New York, 1972.

(FAPESP