Teófilo R. F., Kiralj R., Ferreira M. M. C., Kubota L. T., "Quantitative Structure-Property Relationship (QSPR) study of phenolic passivation at the platinum electrode". Lappeenranta, Finland, 11-15/07/2007: 10th Scandinavian Symposium on Chemometrics (SSC10), Book of Abstracts, (2007) 37. Oral O19.

10th Scandinavian Symposium on ChemometricsO 19

Quantitative Structure-Property Relationship  (QSPR)  study  of  phenolic
passivation at the platinum electrode

Reinaldo F. Teófilo, Rudolf Kiralj, Márcia M. C. Ferreira, Lauro T. Kubota
Laboratório  de  Quimiometria  Teórica  e  Aplicada,    Instituto   de   Química,    Universidade  Estadual
de Campinas, Campinas SP, 13084-971, Brazil.

E-mail: marcia@iqm.unicamp.br

Keywords: electrochemical passivation, QSPR, chronoamperometry

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Oxidation  of  phenolic compounds  (PCs)  at solid electrodes produces phenoxy radicals,  which couple to
form a passivation  polymeric film  on  the  electrode surfaces via mechanism  of  OH-mediated oxydation
[1,2]. The current literature is concerned with the structure of  the passivation  products  (polymeric films)
and not with the structure  of  the  reactants,  which  might  contain  useful  information  for  prediction of
passivation mechanisms and polymer yields from particular PCs.

This  work  consists  of  chronoamperometry  coupled  with  Quantitative Structure-Property  Relationship
(QSPR) with the aim  to  build a regression model  for  prediction  of  phenolic electrochemical passivation
rates.  The  passivation on the platinum electrode  was  monitored  for  20 PCs at 50 mV  above  the  PCs
oxidation peaks (5.0x10-4 mol/L PC, 0.05 mol/L  phosphate buffer, pH 6.5, 90 s measuring in each 0.2 s):
catechol, chloroguaiacol, dopamine,  guaiacol,  hydroquinone,  L-dopa,  o-aminophenol,  o-nitrophenol, p-
aminophenol, paracetamol,  phenol, resorcinol, serotonine,  5-hydroxyindole,  o-cresol,  p-chloro-m-cresol,
m-cresol,  p-cresol,  o-chlorophenol  and L-tyrosine.  The difference between the current densities after 15
and  90 s  of  oxidation, normalized by  the  eletric current mean  and  transformed into negative logarithm,
was defined as  the  passivation rate  pDj.  Molecules of PCs were modeled and geometry-optimized at the
B3LYP 6-31G**  level.  Various molecular descriptors  were  calculated, autoscalled  and  correlated with
pDj in systematic variable selection  (leave-one-out crossvalidation).  Obtained PLS (Partial Least Squares)
regression model was used to predict pDj.

The parsimonious PLS model had one latent variable with  58%  total variance, SEV=0.114,  SEP=0.098,
Q=0.845 and R=0.898. Only three compounds had pDj  predicted with errors above 10% but below 16%.
Five  descriptors  were  used:  NPA  atomic  charge  of  the  carbon chemically bound  to  the  OH  group
(Qcnpa),  Julg's aromaticity index (Ar),  HOMO orbital energy  (H), the number of acidic hydrogen atoms
divided  by  the  number  of  non-hydrogen  atoms  (HBD/N)  and  an  unweighted  3D MoRSe signal  06
descriptor (Mor06u). The regression vector shows  that pronounced electropassivation  by PCs  is related
to:  i) formation  of more compact polymers  (smaller PCs with less acidid hydrogen atoms), ii) formation
of more stable phenoxide ions (from more stable ground states),  iii)  easier scission of  the O-H bond due
to higher C-O bond polarization and its weakened electron delocalization with the benzene ring.
 

Acknowledgement: CNPq, FAPESP.
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References
[1] Gatrell M., Kirk D. W., Habibi M. J. Electrochem. Soc. 1993, 140, 903-911.
[2] Gatrell M., Kirk D. W., Habibi M. J. Electrochem. Soc. 1993, 140, 1534-1540.
 

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