Download effect of an uniform electric field on charge transfer processes. a

JOSÉ L. ANDRÉS
AGUSTÍ LLEDÓS
MIQUEL DURAN
JUAN BERTRÁN
Departament de Química,
IIniversitat Autônoma de Barcelona
08193 Bellaterra, Catalonia,
SPAIN
EFFECT OF AN UNIFORM
ELECTRIC FIELD
ON CHARGE
TRANSFER PROCESSES.
A THEORETICAL STUDY (*"
The influence of an external uniform electric field on the proton
transfer in the (H3 0 2 ) — system and on the methyl transfer in the
(FCH3 F) — system is analyzed theoretically. The electric field effect
is incorporated in the one-electron part of the Fock • matrix.
Reaction profiles are dramatically modified with the increase in
intensity of the applied field. The electric field is found to intervene
in the reaction coordinate. In this way, strong fields emerge as a
new kind of catalysts in chemical reactions.
(*) Presented at the meeting «Química Teórica para a Biotecnologia em Portugal», Estalagem da Boega (Vila Nova
de Cerveira), 19-22 July 1987.
Rev. Port. Quím., 30, 1 (1988)
New techniques like Field Ionization Spectroscopy and Field Ionization Mass Spectroscopy have allowed to reach electric fields of
the range 10 9-10 11 V/m [ 1]. It is well known
that the field-free characteristics of molecules
differ dramatically from those which characterize them under such strong fields [2]. One
can also think that external electric fields will
influence the reactivity of chemical systems.
In particular, it is obvious that charge-transfer processes will be very influenced by the
presence of strong fields.
Processes like proton transfer or Walden
inversion reactions allow to define clearly
their reaction coordinate. That is the reason
why modifications in their reaction profiles
due to the presence of strong electric fields
have been studied theoretically. In a number
of cases the electric fields have been originated by the presence of an ion [3-8]. To our
knowledge, however, only two theoretical
works have introduced an uniform electric
field, in both cases on proton transfer reactions. In a pioneer paper, Parker [9] studied
the modifications in the tunnelling effect in
the proton transfer between DNA bases,
where the electric field was introduced in an
empirical way. In a recent paper, Zundel [ 101
computed the first-order interaction between
the electric field and the dipole moment, and
found that double-well shapes were substantially modified with the increase in intensity
of the electric field. Our ultimate goal is to
study both proton-transfer and Walden inversion reactions under the influence of an uniform electric field. The latter is introduced
by proper changes in the one-electron part of
the Fock matrices in the Hartree-Fock SCF
method.
In the (H 3 O 2 ) — system, previous theoretical
calculations with the 4-31G basis set showed a
double-well minimum with a barrier height
for the proton transfer of 0.20 kcal/mol [ 111.
When electric fields of the order 10' V/m were
applied along the H-O-H axis, the barrier
height was substantially lowered. For an
intensity of 7.2 10' V/m, the barrier no longer
1
JOSÉ L. ANDRFS, AGUSTÍ LLEDÓS, MIQUEL DURAN e JUAN BERTRÁN
exists. Likewise, in the (FCH 3 F) — system, use
of the 3-21G basis set showed that when no
field was applied, a double-well minimum is
obtained with a barrier to the methyl transfer
of 12.2 kcal/mol referred to the minima [8].
When electric fields of the order 10 8 V/m are
applied along the F-C-F axis, the barrier
heights are dramatically modified. For an
intensity of 1.4 10 9 V/m, the barrier does
not exist anymore.
Analysis of the changes occurred in electronic
distribution in the minima of the above fieldfree processes due to the presence of an
electric field may help us to understand the
reasons why the systems evolve spontaneously towards the products. Presence of
such electric fields causes by itself an electronic transfer in the same direction of the
transfer process, that is, makes the reacion to
advance. Furthermore, electron densities at
the forming and breaking bond critical points
are increased and diminished respectively due
to the effect of the external electric field.
A similar effect could be produced by a shorthening of the first bond and lengthening of
the second one. These changes obviously
belong to the internal reaction coordinate.
Since the electric field causes similar modifications in the electronic distribution, one can
say that the external electric field belongs also
to the internal reaction coordinate.
To understand the reasons of the spontaneous
evolution of the system towards products, one
must look at the forces acting on nuclei. In
the field-free minimum, no forces act obviously. When an electric field is applied, there is
an electron density reorganization, so' that
forces are induced in the nuclei, and the system is no longer in equilibrium. Nuclei move
along the forces and reach the geometry optimal under the electric field. Changes produced in electron density in the direction of the
field induce the nuclei to move in this same
direction. On the contrary, the electric field
acts in the opposite direction on the nuclei,
which are positively charged. Depending on
whether the electron density around a nucleus is larger or smaller than the nuclear
2
charge, the force acting on it will follow the
direction of the electric field or go against it.
In the two processes considered, either the
F— or the OH — are negatively charged, and
either the H' in the (H 3 0 2 ) — system or the
methyl in the (FCH 3 F) — system are positively
charged. Thus, forces will appear in the direction of the field at the negatively-charged
groups, whereas forces against the field will
arise at the positively-charged groups, so the
systems will evolve in such a way that initial
bonds will break and new bonds will be formed. The proton or methyl transfer, respectively, will thus be produced.
An interesting conclusion of this work is that
the mere presence of an external electric field
may produce an acceleration in the reaction
rate without the presence of any catalyst.
Clearly, if the electric field has to help in the
charge transfer, it must be oriented with respect to the reacting system. This cannot be
achieved in isotropic media, but it is conceivable in anisotropic media. In particular, it
can be achieved in electrode interfaces and
membranes, where it is well known that presence of electric fields accelerates several
chemical processes. Finally, it is worth noting
that changes in enzymatic conformations previous to enzymatic reactions allow the reaction field to be oriented adequately with
respect to the chemical process in such a way
that enzymatic catalysis may be explained in
some cases by a suitable orientation of the
electric field.
ACKNOWLEDGMENT
This work has been supported by the Spanish
«Dirección General de Investigación Científica
y Técnica» under Project N.° PB86-0529.
(Received, 24th December 1987)
REFERENCES
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Rev. Port. Quím., 30, 1 (1988)
EFFECT OF AN UNIFORM ELECTRIC FIELD ON CHARGE TRANSFER PROCESSES. A THEORETICAL STUDY
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ANDR E S,
M.
Efeito de um campo eléctrico uniforme em processos de
transferência de carga. Um estudo teórico.
Faz-se o estudo teórico da influência de um campo eléctrico
externo sobre a transferência do protão no sistema (H 3 0 z)
e a transferência do metilo no sistema (FCH3 F) . O efeito
do campo eléctrico é incorporado na parte mono-electrónica
da matriz de Fock. Os perfis de reacção são modificados
dramaticamente com o aumento de intensidade do campo
aplicado. Verifica-se que o campo eléctrico intervém na
coordenação de reacção. Desta maneira, os campos intensos aparecem como um novo tipo de catalisador de reacções
químicas.
-
—
BERTRÁN,
I
RESUMO
DURAN,
A.
LLED6S,
Chem. Phys. Lett., 1986, 124, 177.
Rev. Port. Quím., 30, 1 (1988)
J.
BERTRAN,
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