Teófilo R. F., Kiralj R., Kubota L. T., Ferreira M. M. C., "Study of the electrochemical passivation by phenolics compounds using QSPR". Águas de Lindóia, SP, Brazil, 10-15/09/2006: 10th International Conference on Chemometrics in Analytical Chemistry (CAC-2006, CAC-X), Book of Abstracts (2006) P010. Poster 010.

10th International Conference on Chemometrics in Analytical Chemistry P010

Study of the electrochemical passivation by phenolics
compounds using QSPR

Reinaldo F. Teófilo*, Rudolf Kiralj, Lauro T. Kubota, Márcia M. C. Ferreira
teofilo@iqm.unicamp.br

Instituto de Química, Universidade Estadual de Campinas

Keywords: electrochemical passivation, QSPR, amperometry
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       It is well known that the oxidation of  phenolic compounds  (PCs)  at  solid electrodes produces phenoxy
radicals, which couiple to from a passivation polymeric film  on  the electrode surfaces.¹  During the oxidation
the radical is formed causing a polymerization  and  consequently the surface fouling,  which is a problem  in
electroanalysis and electrooxidation of phenols.   Gatrell  and  Kirk²  assume that the  OH  species react with
adsorberd phenol slowly oxidizing  it and  releasing further sites for OH  electrosorption  and  thus accelerate
the  chemical  reaction.    This  assumption  is  based  on  the   concept  of  a  chemical  reaction  of  organic
compounds with reversibly deposited  OH  at open  Pt  metal sites.
       Although  the majority of  the  studies  prioritizes   the analysis of  the  obtained polymeric films, this work
presents a proposal  studying the  PCs  molecular   structures  relating  to   the  electrochemical  passivation.
Quantitative Structure-Property Relationship   (QSPR)   model  was  built   to  relate  the  phenolic  molecular
descriptors and electrochemical passivation properties.
       The  electrochemical  passivation    was   monitored  using   the  amperometric   method   with   platinum
electrode. The potential was fixed 50 mV  more  positive than   the  PC  oxidation  peak.  The  oxidation  was
carried out in a concentration of 5.0x10^-4 mol L^-1 PC in 0.05 mol L^-1  phosphate buffer solution at  pH  6.5, for
90 s  measuring in each  0.2 s.   The investigated PCs were:  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 density after 15 s  and  90 s of oxidation was considered as a parameter
of  passivation  measurement  (Dj).  The measures were  carried  out  in  triplicate and the mean values  was
used. The averageof  the  mean  triplicate results  was  of  33.6 mA,  presenting  as  minimum  and  maximum
values,  10.3  and  53.8  mA, respectively  and  standard  deviation of 14.3 mA.  The negative logarithm of the
means was used as dependent variable (pDj).
       Molecular  structures  of  20  compounds  in  neutral  state  were  modeled  according  to their or  similar
structures from the Cambridge Structural Database. Geometry optimization was performed at  ab initio  level
(DFT with B3LYP functioinal and 6-31G** basis set) and various molecular descriptors were calculated. The
partial least square (PLS) regression model was built with  autoscalled  data  and  cross-validated  by  leave-
one-out method.   Systematic  variable selection resulted in four descritors, i.e.  NPA  atomic charge  of  the
carbon  chemically  bound  to  the  OH  group  (Qcnpa);  Julg's  aromaticity  index  (Ar);  sum  of  squares  of
the O atomic  orbital  (s, px, py, pz)  coefficients in the  LUMO+1  (Clumo+1)  and  bending frequency for the
angle H-O-C (fCOH). The mean relative error was of 2.53%, and the  Q  and rmsecv was of  0.76  and  0.14,
respectively. The selected molecular descriptors are moderately  (corr. coeff. 0.42-0.76)  correlated  with the
dependent variable.  The final  PLS  model has satisfactory  statistics  and predicted values  (no compounds
with relative error greater than 7.0% by cross-validation).  The first oxidation is crucial for the whole process.
Molecules   that  are  more  difficult   to  oxidize   (higher  oxidation  potential  E_ox)   have   smaller  values  of
dependent variable (<pDj).
       The descriptors selected suggest that a compound could be easier oxidized at the  OH group  if:   i)  the
electron delocalizaiton between the benzene ring and  OH is weaken by substituents,  so the ring is by itself
highly aromatic (high Ar values); ii) the C-O bond polarizaiton is pronounced,  so  O  is negatively and first C
positively (high Qcnpa values) charged, i.e. O is relatively electron-rich; iii)  O  atom  is electron-rich and can
be easily excicted as it has significant contribution to frontier orbitals like LUMO+1  (high LUMO+1);   iv)  the
existing interactions among O, H and C are weaken,  what  results in more flexibility of this group and higher
bending frequency (higher fCOH). In fact, all descriptors show that if the O  from  OH  can be easily oxidized
because its connection  with the ring is weaken,  the first phenolic radical will be formed and,  in this way,  a
lower oxidation potential will be necessary, and a lower passivation process will be observed.
       QSPR is a promising tool in passivation studies  in  electrochemistry,  contributing  to  the elucidation of
phenolic oxidation mechanism and supplying information to minimize the passivation.

Acknowledgment: To CNPq for the financial support.
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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.