Ionization of the three isomeric hydroxybenzoates by free electron transfer: Product distribution depends on the mobility of the phenoxyl group

Article abstract of DOI:10.1039/b409108f

Ionization of 2-, 3- and 4-hydroxy-methylbenzoate by free electron transfer (FET) to n-butyl chloride parent radical cations results in different product patterns. Whereas 3- and 4-hydroxybenzoate directly form comparable amounts of solute radical cations and phenoxyl radicals, from 2-hydroxybenzoate arise only radical cations. We interpret this finding as a molecule dynamic effect connected with the femtosecond rotation of the phenoxyl group which leads to a marked n-electron density shift. For 3- and 4-hydroxybenzoate, the diversity of all solute rotamers transforms in two different kinds of radical cations, unstable ones which promptly dissociate and longer living ones. Because of the H bridge bonding to the ester carbonyl, in 2-hydroxymethylbenzoate the phenoxyl group rotation is practically excluded and this should be the reason that exclusively stable radical cations derived from the planar molecule structure are formed.

Full text of DOI:10.1039/b409108f

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R E S E A R C H P A P E R
Ionization of the three isomeric hydroxybenzoates by free electron
transfer: product distribution depends on the mobility of the
phenoxyl group
a
a
a
ab
Ortwin Brede,* Ralf Hermann, Nikolas Karakostas and Sergej Naumov
a
University of Leipzig, Interdisciplinary Group Time-Resolved Spectroscopy, Permoserstrasse
5, D-04303 Leipzig, Germany. E-mail: brede@mpgag.uni-leipzig.de; Fax: þ49-341-235-2317
1
Leibnitz Institute of Surface Modification, Permoserstrasse 15, D-04303 Leipzig, Germany
b
Received 15th June 2004, Accepted 8th September 2004
First published as an Advance Article on the web 5th October 2004
Ionization of 2-, 3- and 4-hydroxy-methylbenzoate by free electron transfer (FET) to n-butyl chloride parent
radical cations results in different product patterns. Whereas 3- and 4-hydroxybenzoate directly form comparable
amounts of solute radical cations and phenoxyl radicals, from 2-hydroxybenzoate arise only radical cations.
We interpret this finding as a molecule dynamic effect connected with the femtosecond rotation of the phenoxyl
group which leads to a marked n-electron density shift. For 3- and 4-hydroxybenzoate, the diversity of all solute
rotamers transforms in two different kinds of radical cations, unstable ones which promptly dissociate and longer
living ones. Because of the H bridge bonding to the ester carbonyl, in 2-hydroxymethylbenzoate the phenoxyl
group rotation is practically excluded and this should be the reason that exclusively stable radical cations derived
from the planar molecule structure are formed.
complete delocalization over the whole molecule (planar struc-
ture) and a distinct localization on the hetero atom (901 twisted
Introduction
structure), see also Fig. 5 in the discussion chapter. The
diversity of all rotation states can be roughly summarized
under these two borderline conformer structures from which
after ionization two different types of radical cations arise—a
stable planar one with delocalized charge and an unstable one
with a pronounced charge localization at the hetero group.
Depending on the type of the solvent such as different
alkanes or alkyl chlorides (RX), the free energy of the electron
transfer reaction runs from 0.5 to 2.0 eV, i.e. reaction (2) is a
Molecular radical cations of alkanes and alkyl chlorides are
1
–3
known as very efficient electron transfer oxidants . In liquid
state they can easily be generated by radiolysis of the basic
solvents, as indicated by eqn. (1) for the example of n-butyl
4
chloride.
ꢀ
1
solvꢁ
ꢀ
1
n-C H
4 9
Cl
[n-C
4
H
9
Cl þ e
] - n-C
H
4 9
Cl
ꢀ
ꢁ
þ n-C
4
H
9
þ Cl
(1)
Recently it was found that the electron tra/>nsfer ionization
of arenes (Ar) substituted with rotating groups such as –OH,
1
0
very exergonic process.
It should also be taken into account that vibrational excita-
tion of the resulting phenol radical cation by excess energy can
–
SH, SeH, –CH
5
2
SiMe
products, as formulated for phenols in reaction (2).
3
etc. yields two characteristic direct
–7
5
happen. Therefore, instead of reaction channel (2) another
reaction path is imaginable where an excited radical cation
decays in a competition between vibrational relaxation and a
promt deprotonation, as formulated in eqns. (3a,b).
ð2a; bÞ
These findings can be understood if two main assumptions are
fulfilled: (i) the electron transfer occurs non-adiabatically with-
out any complex formation and the reaction pattern is deter-
mined by the solvent rotamer distribution. Furthermore, it is
known that the electron transfer from any solutes to solvent
parent radical cations derived from alkenes and alkyl chlorides
is an energetically favoured and non-hindered process, i.e., the
electron jumps from the solute molecule to the solvent cation in
the first approach of the reactants, independent of their
encounter geometry. For typifying this process the term free
ð3a; bÞ
In such a situation the decay modes (3) should essentially
depend on the energy excess (*). In other words, the product
pattern should be independent of the rotation of the substitu-
ents around the C– –OH axis. In a former paper we have
demonstrated that by use of different solvents (resulting in
different excess energies) such assumption is not confirmed by
the observed product ratio of reaction (2).
In this paper, we analyze the free electron transfer ionization
of the three isomer hydroxybenzoates. In this particular case,
the 3- and 4-hydroxybenzoates (3-BOH, 4-BOH) exhibit mo-
bile phenoxyl groups. In contrast, the phenoxyl group of the
2-hydroxybenzoate (2-BOH) is strongly rotation-hindered be-
cause of a hydrogen bond bridging to the carboxyl group, see
subsequent structure formulae.
1
0
8
,9
electron transfer (FET) has been introduced. (ii) On the
other hand, the molecule dynamics of the solute molecules
must be taken into account. For phenols (ArOH) and other
chalcogenols, the rotation of the substituents around the C–O
axis in times of about 100 fs is connected with strong changes
of the electron distribution in the highest occupied molecular
5
orbitals. Quantum-chemical calculations indicate that the
density of the n-electrons of the hetero atom varies between
5
184
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 1 8 4 – 5 1 8 8
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4

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Experimental
Pulse radiolysis: The samples were irradiated with a high
energy (1 MeV) electron beam of 12 ns pulse duration gener-
ated by a pulse transformer electron accelerator ELIT (Insti-
tute of Nuclear Physics, Novosibirsk, Russia). The dose per
pulse was measured using the absorbance of the solvated
electron in slightly alkaline solution. The experiments were
Fig. 1 Absorption spectra obtained in the pulse radiolysis of a N
purged solution of 5 ꢂ 10
2
ꢁ3
ꢁ3
mol dm
3-hydroxymethylbenzoate
(3-BOH) in n-butyl chloride: taken (’) 100 ns and (K) 1.2 ms after
the pulse, (J) difference spectrum between (’) and (K). The inset
ꢀ
1
shows a characteristic time profile: spike caused by n-C H Cl , decay
4
9
ꢀ
1
caused by 3-BOH , background according to 3-BO .
ꢀ
ꢁ
5
ꢁ3
done at 100 Gy, which generates about 10 mol dm of
ꢀ
1
BuCl as primary oxidants. Transient species formed were
detected by the optical absorption technique, in our case
consisting of a pulsed xenon lamp XBO 450 W (Osram), a
SpectraPro-500 monochromator (Acton Research Corpora-
tion), a R955 photomutiplier (Hamamatsu Photonics) and a
1
details of the instrumental setup are given elsewhere.
GHz digitizing oscilloscope TDS 640 (Tektronix). Further
1
0,11
All
experiments were performed at room temperature using freshly
prepared solutions in 1-BuCl purged with purest grade N for
5 min prior to the pulse radiolysis measurements. The solu-
2
1
tions were continuously passed through the sample cell with an
optical path length of 1 cm.
Chemicals: n-Butyl chloride has been purified by chromato-
graphy using molecular sieve treatment (A4, X13) and distilla-
tion under nitrogen. The solvents from VWR were of liquid
chromatography grade. The solvents purity was proofed by
UV-spectroscopy before use. The solutes methylsalicylate
(
2-BOH, 99%, Aldrich), methyl-3-hydroxybenzoate and
Fig. 2 Absorption spectra obtained in the pulse radiolysis of a N
purged solution of 10 mol dm 4-hydroxymethylbenzoate (4-BOH)
2
ꢁ2
ꢁ3
methyl 4-hydroxybenzoate (3-BOH or 4-BOH, both 99%,
Aldrich) were applied as received.
Quantum chemical calculations were done with the GAUS-
SIAN 03 program package using the density functional
1
theory (DFT) Hybrid B3LYP
in n-butyl chloride: taken (’) 100 ns and (K) 1.0 ms after the pulse,
(J) difference spectrum between (’) and (K). The inset shows a
characteristic time profile: little spike caused by n-C H Clꢀ , decay
caused by 4-BOH , background according to 4-BO .
1
1
2
4
9
1
ꢀ ꢀ
3–15
method. The frequency
calculations were performed to determine the nature of the
stationary points found by geometry optimisation, and to
obtain thermochemistry parameters such as zero-point energy
and activation energy E (height of rotation barrier).
a
Results and discussion
Pulse radiolysis measurements: In the pulse radiolysis of diluted
solutions of the hydroxybenzoates (2-, 3-, and 4-BOH) in
n-butyl chloride, the primarily formed solvent parent radical
ꢀ
1
cations n-C
4
H
9
Cl (lmax ¼ 500 nm, t ¼ 130 ns) disappear
under the simultaneous formation of solute species. The corre-
sponding absorption spectra are shown in Figs. 1–3 for the
three isomer hydroxybenzoates. The solute transients are
1
0
3
formed with a typical diffusion-controlled rate (k ¼ 10 dm
ꢁ
1
ꢁ1
ꢀ
mol
n-C
s ) which was determined from the decay of the
Cl signal at 500 nm (see also the spike on the solute
1
4
H
9
transient profile at 425 or 430 nm, given as insets in Figs. 2
and 3).
Fig. 3 Absorption spectra obtained in the pulse radiolysis of a N
purged solution of 5 ꢂ 10
2
Analyzing the shape of the absorption spectra of the solute
species, in all three cases a broad band between 380 and 480 nm
was observed. Except for 2-BOH, the band is superimposed
from the very beginning with two relatively narrow and well
structured absorption bands exhibiting a round (left part) and
a spiky (to the right) shape, situated at around 400 nm. Such
ꢁ3
ꢁ3
mol dm
2-hydroxymethylbenzoate
(2-BOH) in n-butyl chloride: taken (’) 100 ns and (K) 1.0 ms after
the pulse, (J) difference spectrum between (’) and (K). The inset
shows a characteristic time profile: spike caused by n-C H Clꢀ , decay
1
4
9
ꢀ
1
caused by 2-BOH , background unidentified, probably Cl-substituted
cyclohexadienyl type radical.
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 1 8 4 – 5 1 8 8
5185

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phenols, here hydroxybenzoates. Furthermore, we can rule out
its formation by FET (2) in non-polar solution.
ꢀ
aqꢁ
ꢀ
H
2
O
OH , e , H
(4a)
(4b)
(4c)
aqꢁ
2
ꢀ
ꢁ
2
,
e
þ N
O - OH þ OH þ N
9
3
ꢁ1 ꢁ1
k ¼ 9 ꢂ 10 dm mol
s
, ref. 17
ꢀ
ꢁ
ꢀ
ꢁ
OH þ N - N þ OH ,
3
0
3
1
3
ꢁ1 ꢁ1
k ¼ 1.2 ꢂ 10 dm mol
s , ref. 18
ꢀ
ꢀ
1
3ꢁ
N
3
þ ArOH - ArOH þ N
,
, ref. 19
9
3
ꢁ1 ꢁ1
k ¼ 7 ꢂ 10 dm mol
s
(4d)
(4e)
ꢀ
1
ꢀ
1
ArOH þ H O - ArO þ H O
2
3
Furthermore, it must be taken into account that the decay of
ꢀ
1
the radical cations BOH proceeds by deprotonation, prob-
ꢁ
ably by reaction with the counter charge Cl which is formed
Fig. 4 Optical absorption spectra of phenoxyl radical generated by
pulse radiolysis of N purged aqueous solutions of 2-BOH (J,
saturated) and of 4-BOH (’, 2 ꢂ 10 mol dm ) taken in the effect
maxima. Left: time profiles taken of 2-BOH sample; the one on the
right is from 4-BOH solution.
by electron capturing (1). Then, in the cases of 3-BOH and
4-BOH, the decay of the radical cations yields a further
fraction of phenoxyl radicals which are formed via reaction
2
ꢁ4
ꢁ3
(
5) in a delayed manner.
ArOH þ Clꢁ - ArO þ HCl
ꢀ
1
ꢀ
(5)
band combination is typical for phenoxyl type radicals. After
about 500 ns, the broad and non-structured absorption has
been completely decayed, obviously under the formation of
further phenoxyl radicals, see Figs. 1 and 2. These figures also
show the spectra of the radical cations (open circles) obtained
as the difference between the spectra taken (early) immediately
and about 1 ms after the electron pulse.
For 2-BOH, however, the decay of the radical cations seems to
result in another product which has a probable structure of a
cyclohexadienyl radical formed by addition of the nucleophile
onto the aromatic ring (reaction (6)). This is assumed from the
very broad and unspecific background absorption (taken after
ꢀ
1
the decay of 2-BOH ) and its sensitivity against oxygen,
see Fig. 3.
For 2-BOH as a solute, however, only the broad and rapidly
disappearing radical cation band has been observed (Fig. 3).
As also found for 3-BOH and 4-BOH, this band is not sensitive
against oxygen, but it is quenched by polar additives such as
DMSO, methanol or triethylamine. Band shape, time beha-
viour and scavenger sensitivity are typical for phenol radical
ArOH þ Clꢁ - Cl ArOH
ꢀ
1
ꢀ
(6)
We assume that for all hydroxybenzoate isomers the absorp-
ꢀ
1
tion spectra of the radical cations BOH have a similar shape,
i.e. such a broad and short-living absorption as shown for
1
ꢀ
2-BOH
radical cation spectrum of 2-BOH in relation to the absorp-
(Fig. 3), in a proper state. Then, from the pure
1
1
0–11,16
ꢀ
cations.
In the case of 2-BOH, neither from the begin-
ꢀ
1
ning nor after the decay of the radical cation band, could any
signal of a phenoxyl radical be detected.
To get a reliable identification, we generated the phenoxyl
tion spectra of the sum of the species, e.g., 3-BOH and 3-BO
(cf. Fig. 2), the ratio of the radical cations and phenoxyl
radicals immediately formed in reaction (2) can be estimated.
So, for both isomers a ratio of radical cations to phenoxyl
radicals of about 50 : 50 was derived. This is in good agreement
with the analogous analysis for the electron transfer involving
ꢀ
radicals (BO ) of 2-BOH and 4-BOH independently from
each other. In the pulse radiolysis of N
diluted aqueous solutions of the hydroxybenzoates containing
2
O bubbled very
ꢁ
3
0 mmol dm sodium azide, first the azide radical is formed,
8
2
a great variety of substituted phenols.
as indicated in the reaction sequence (4a–e). As a good oxidant,
it reacts with BOH under the formation of phenol radical
cations which in neutral solution immediately deprotonate
under the formation of phenoxyl radicals. The spectra of the
latter are shown in Fig. 4. They exhibit the typical two-band
behaviour around 400 nm, mentioned above. So for 4-BOH the
phenoxyl radical shows the same shape as the one found also
for the FET (2) in n-butyl chloride solution. The experiments
with 2-BOH in aqueous solution clearly show that its phenoxyl
radical exists and has similar properties as those of the other
Mechanistic analysis of the FET products: As already men-
tioned in the introduction, the observed reaction pattern of the
free electron transfer from phenols to solvent radical cations in
non-polar surroundings can be related to the molecule dy-
namics of the solutes. Whereas in classical chemistry phenols in
bimolecular reactions are generally taken for planar molecules
which is expressed by the picture of mesomerism, for our
particular case of the FET of type (2) the femtosecond nuclear
motions of the molecule must be taken into account. Table 1
offers molecule dynamics data of differently structured phenols
Table 1 Experimentally obtained lifetime t of the cation radicals in BuCl compared with DFT B3LYP/6-31G(d) calculated quantum chemical
is the activation
parameters: frequencies (scale factor f ¼ 0.96) of polar OH-group rotation and valence O–H oscillations; t - times of one motion; E
a
energy of OH-group rotation; DP(O) difference of electron population on oxygen atom between perpendicular and planar structures in cation
radical state
ꢁ
1
rot ꢂ 10ꢁ15/s
ꢁ1
oscil/cm
osci ꢂ 10ꢁ15/s
ꢁ1
ꢀ
1
ArOH : ArO /%
ꢀ
Solute
PhOH
t/ms
n
rot/cm
t
n
t
E
a
/kcal mol
DP(O)
b
r0.2
0.7
0.7
345(123)
361(125)
376(121)
473(121)
736(133)
97
92
89
70
45
3601(38)
3600(44)
3598(57)
3546(320)
3252(311)
9.3
9.3
9.3
9.4
3.0
3.9
0.130
0.130
0.142
0.088
0.047
50 : 50
50 : 50
50 : 50
62 : 38
100 : 0
a
b
b
b
b
3
4
-BOH
-BOH
4.3
7.4
Curcumin
2-BOH
3.3
0.7
10.3
14.5
a
b
Oscillator strength. Taking ZPE into account.
5
186
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 1 8 4 – 5 1 8 8
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4

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erated by ionization. For our argumentation, this is something
like the other side of the coin, i.e. the confirmation of our
hypotheses by finding an opposite example.
Referring to the mechanism of a competition between
relaxation and deprotonation mentioned above (reactions
3
a,b) it should be stated that if this alternative process runs,
Fig. 5 Phenol: quantum-chemically calculated (B3LYP/6-31G(d))
distribution of n-electrons of the oxygen atom in dependence on the
rotation angle of the OH-group.
deprotonation under phenoxyl radical formation should be
observable for all of the BOH isomers including 2-BOH.
Therefore, according to our experimental findings this pathway
can be clearly excluded.
In the following, the ionization behaviour of the majority of
the phenols should be reconsidered. As already reported,
electronic effects did not influence the yield ratio between
radical cations and phenoxyl radicals, i.e. in these cases a
which were obtained by quantum-chemical calculations (see
Experimental section). Special attention should be given to the
rotation of the C– –OH bond which goes with a frequency of
1
3
about 10 Hz, i.e. with a time per rotation of 100 fs, see also
Table 1. This is true as long as the rotation is not markedly
hindered. The calculations also indicate that such rotations are
accompanied with considerable shifts in the electron distribu-
tion of the highly occupied molecular orbitals in the ground
state molecule. This was already analyzed in detail in a former
5
0 : 50 ratio has been observed. This is reflected in Table 1
for the case of phenol itself and is also valid for the ionization
behaviour of 3-BOH and 4-BOH. The situation changes study-
ing ionization of some natural appearing phenols such as
20
sesamol, trolox and curcumin, since in the latter case—
curcumin consists of phenolic units neighboured to methoxyl
substituents—also partial H bond bridging seems to exist. At
least, the rotation of the phenoxyl group is hindered by an
5
paper. Qualitatively seen, the thermodynamical stable planar
(
mesomeric) state exhibits a more or less uniform distribution
of the n-electrons from the oxygen atom (delocalized due to
favourable resonance with the p-system of the molecular ring).
If the rotation angle increases up to 901, these electrons will
more and more shift towards the hetero atom (localized on
oxygen, because the resonance with the ring p-system is
disturbed), as shown in Fig. 5. As a consequence, the ionization
of the diversity of rotational states described by the two
borderline states—the planar and the 901 twisted state—results
in radical cations of different internal charge distribution.
For all phenol radical cations in sufficiently twisted states
summarized under the term twisted conformer cations, the
comparable high charge localization at the oxygen causes a
high instability of the CO 2 H bond. It can be roughly
assumed that this bond dissociates in the time-scale of the first
stretching vibration. Hence, after about 10 fs this type of
radical cations should already be transformed into phenoxyl
radicals. Up to now this rotamer-dependent dissociation be-
haviour seems to us as the only reasonable explanation of the
experimentally observed synchroneous formation of the solute
radical cation as well as phenoxyl radicals, in the electron
transfer process (2). The kinetic details of the FET involving
phenols are formulated subsequently by reaction sequence (7).
ꢁ1
enlarged activation energy of 7.4 kcal mol , and the radical
cation to phenoxyl radical ratio increases to about 62 : 38.
Referring to the ionization of 2-BOH, it can be concluded that
the mobility of the phenoxyl group is the deciding factor of
influence for the free electron transfer reaction phenomenon,
rather than any electronic or steric components.
For a more substantial analysis, the quantum-chemical data
given in Table 1 should be reconsidered. It can be seen that the
vibrational period of C–O 2 H is nearly constantly about 10
fs for all analyzed molecules. This is rapid, compared to the
rotation of the phenoxyl groups. This fact has mechanistic
importance since after ionization of the twisted molecule state,
the newly formed phenol radical cation can deprotonate within
the first vibration. Looking on the rotation of C– –OH, the
calculated frequencies for phenol, 3-BOH and 4-BOH are
nearly constant and correspond to a rotation time of about
1
00 fs. This value is well compatible with the activation energy
ꢁ
1
of the rotation which amounts to E B 4 kcal mol in these
a
cases. For the H bridge bonded phenol derivatives such as
curcumin and in particular 2-BOH, however, the rotation
frequencies are much higher indicating more rigid bonds. That
is reflected also by the activation energies of rotation which
ꢁ
1
now amount to 7.4 kcal mol (partly hindered rotation) and
ꢁ
ð7Þ
1
4.5 kcal mol (completely hindered rotation). These activa-
1
The gross reaction (7) consists of the diffusional approach of
the reactants (slow and rate determining process), the actual
electron jump to the different rotational conformers (extremely
rapid, probably 1 fs) followed by internal relaxation (2a) or
deprotonation (2b, B10 fs) of the different cation types and the
diffusional separation of the products.
tion energies of rotation are also consistent with the calculated
geometrical parameters of the studied molecules. Whereas in
the case of curcumin the H-bond is a part of a 5-membered ring
with a length of 0.2077 nm, in the case of 2-BOH the H-bond
(as part of a 6-membered ring) is shorter and amounts to
0.1738 nm. So it follows that in the case of 2-BOH, H-bonding
is stronger, which also reasons the larger activation energy of
rotation. So, for the FET products, the ratio between radical
cations and phenoxyl radicals changes from 50 : 50 for the non-
rotation hindered cases in favour of phenol radical cations
(curcumin) to radical cations only (2-BOH).
The above discussion concerns the free electron transfer
ionization of 3-BOH and 4-BOH, and, in general, the majority
5
of phenols and other chalcogenols. For 2-BOH, however, the
situation is different since the hydrogen bridge prevents an C–
–
OH rotation completely. The calculation indicates an activa-
ꢁ
1
tion energy for the rotation of about 15 kcal mol which is
extremely high in comparison to that of the other phenols, see
Table 1. The 2-BOH molecule has only planar structure and,
Finally, it should be mentioned that the yield ratio between
ꢀ
1
ArOH and ArO (cf. Table 1) was found to be independent
ꢀ
5
of the type of the non-polar solvent, i.e. using alkanes or
cycloalkanes instead of n-butyl chloride. In rigid matrices,
ꢀ
1
therefore as expected, only radical cations 2-BOH are gen-
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 1 8 4 – 5 1 8 8
5187

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2
R. Mehnert, in Radical Ionic Systems: Properties in condensed
Phases, ed. A. Lund and M. Shiotani, Kluwer, Dordrecht, The
Netherlands, 1991, p. 231.
however, no concomitant radical formation has been observed
2
for similar scavenger systems.
2
3
4
5
6
7
O. Brede, D. Beckert, C. Windolph and H. A. Go
Chem., 1998, 102, 1457.
R. Mehnert, O. Brede and W. Naumann, Ber. Bunsen-Ges. Phys.
Chem., 1982, 86, 525.
O. Brede, R. Hermann, W. Naumann and S. Naumov, J. Phys.
Chem. A, 2002, 106, 1398.
O. Brede, R. Hermann, S. Naumov, G. P. Perdikomatis, A. K.
Zarkadis and M. G. Siskos, Chem. Phys. Lett., 2003, 376, 370.
O. Brede, R. Hermann, S. Naumov, G. P. Perdikomatis, A. K.
Zarkadis and M. G. Siskos, Phys. Chem. Chem. Phys, 2004, 6,
¨
ttinger, J. Phys.
Conclusion
Finally, the time ranges of the single reaction steps which are
relevant for the elucidation of the phenomenon of the free
electron transfer should be considered: The reactants approach
in solution by diffusion. For the used millimolar concentra-
tions, this process needs several tenths of ns. This can be well
analyzed with the time resolution of our pulse radiolysis setup
(
5 ns). Already in the first encounter the electron should jump
2
267.
from the phenol to the parent solvent radical cation. Because of
this rapid jump, the diversity of phenol conformers differing in
the C– –OH group angles should be reflected in the newly
formed radical cation. So, at least two types of radical cations
are formed
8
O. Brede, G. R. Mahalaxmi, S. Naumov, W. Naumann and
R. Hermann, J. Phys. Chem. A, 2001, 105, 3757.
O. Brede, Res. Chem. Intermed., 2001, 27, 709.
G. R. Mahalaxmi, R. Hermann, S. Naumov and O. Brede, Phys.
Chem. Chem. Phys., 2000, 2, 4947.
9
0
1
1
1
O. Brede, H. Orthner, V. Zubarev and R. Hermann, J. Phys.
Chem., 1996, 100, 7097.
(
i) a stable one with charge distribution over the whole
molecule, which relaxes and overcomes up to several hundred ns;
ii) an unstable one with charge preferrably localized at the
12 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, J. A. Montgomery Jr, T. Vreven, K. N.
Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V.
Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Strat-
mann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J.
J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M.
C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Ragha-
vachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul,
S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko,
P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A.
Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M.
W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, J. A.
Pople, GAUSSIAN 03, (Revision B.02), Gaussian, Inc.,
Pittsburgh PA, 2003.
(
oxygen, which deprotonates during the first C–O 2 H bond
oscillation, i.e. in about 10 fs.
Experience has shown that the relaxation of the products by
distributing the excess energy to the surrounding proceeds in
2
1
some picoseconds. However, as shown in this paper, it is
without mechanistic consequence in the investigated case.
The free electron transfer as the experimentally observed
gross reaction (2) is diffusion controlled and, therefore, there is
no chance of improving the time resolution of the investigation
from the nanosecond to a smaller time-scale.
So, as something like an exotic case, a bimolecular electron
transfer reaction has been found which reflects the variety of
5
,10,11
rotamers for a solute molecule.
Studies still in progress
show that this phenomenon is not only limited on phenol-like
compounds. Similar reaction behaviour should be observed in
all cases where intramolecular molecule motions are connected
with electron shifts which lead to different types of radical
cations. In the simplest case, the process can be identified if one
cation is stable and the other one rapidly forms dissociation
products. As a further example, the ionization of benzyl-
13
14
15
A. D. Becke, J. Chem. Phys., 1993, 98, 5648.
A. D. Becke, J. Chem. Phys., 1996, 104, 1040.
Ch. Lee, W. Yang and R. G. Parr, Phys. Rev. B, 1987, 37, 785.
16 G. R. Mahalaxmi, S. Naumov, R. Hermann and O. Brede, Chem.
Phys. Lett., 2001, 337, 335.
1
7
G. V. Buxton, C. L. Greenstock, W. P. Helman and A. Ross,
J. Phys. Chem. Ref. Data, 1988, 17, 513.
Z. B. Alfassi and R. H. Schuler, J. Phys. Chem., 1985, 89, 3359.
Z. B. Alfassi, R. E. Huie, P. Neta and L. C. T. Shoute, J. Phys.
Chem., 1990, 94, 8800.
6
,7
trimethylsilanes has been reported recently.
1
1
8
9
In this paper, as the deciding factor of influence on FET the
rotation ability of the substituent at the aromatic ring has been
demonstrated experimentally.
20 R. Joshi, S. Naumov, S. Kapoor, T. Mukherjee, R. Hermann and
O. Brede, J. Phys. Org. Chem., 2004, 17, 665.
2
1
E. S. Medvedev and V. I. Osherov, Radiationless Transitions in
Polyatomic Molecules, Springer Series in Chemical Physics 57,
Springer, Berlin, 1995, p. 36.
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2
5
188
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 1 8 4 – 5 1 8 8
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4