Transient absorption spectroscopy of high-mobility ions in decalin

Article abstract of DOI:10.1021/jp960387l

Transient absorption spectroscopy is used to demonstrate that pulse radiolysis of a decalin mixture (63% cis-decalin + 37% trans-decalin) with 15 MeV electrons results in formation of high-mobility solvent holes. These radical cations react with perylene with a rate constant of 8.5 × 10¹⁰ mol^-1 dm³ s^-1. This scavenging is faster by an order of magnitude than ion-molecule reactions of normally diffusing ions (～(5-7.5) × 10⁹ mol^-1 dm³ s^-1). Under conditions of the experiment, the lifetime of the rapidly migrating solvent holes is ～150 ns, which identifies them as the high-mobility cations observed by dc and microwave conductivity.

Full text of DOI:10.1021/jp960387l

6876
J. Phys. Chem. 1996, 100, 6876-6878
Transient Absorption Spectroscopy of High-Mobility Ions in Decalin†
I. A. Shkrob, M. C. Sauer, Jr., J. Yan, and A. D. Trifunac*
Chemistry DiVision, Argonne National Laboratory, Argonne, Illinois 60439
ReceiVed: February 8, 1996; In Final Form: March 15, 1996X
Transient absorption spectroscopy is used to demonstrate that pulse radiolysis of a decalin mixture (63%
cis-decalin + 37% trans-decalin) with 15 MeV electrons results in formation of high-mobility solvent holes.
These radical cations react with perylene with a rate constant of 8.5 × 1010 mol-1 dm3 s-1. This scavenging
is faster by an order of magnitude than ion-molecule reactions of normally diffusing ions (∼(5-7.5) × 109
mol-1 dm3 s-1). Under conditions of the experiment, the lifetime of the rapidly migrating solvent holes is
∼150 ns, which identifies them as the high-mobility cations observed by dc and microwave conductivity.
Introduction
In agreement with this conclusion, the dc conductivity kinetics
observed in decalins and their mixtures can indeed be accounted
for by a single HMI.14 In this letter, we demonstrate that the
transient absorption kinetics in pulse radiolysis of decalin also
implies that the solvent radical cation is highly reactive and
relatively long-lived.
Ionization of cyclohexane,1-5 methylcyclohexane,5-7 cis-
decalin, and trans-decalin1,3,8-11 results in the generation of
cations whose mobility and scavenging rates are greater by an
order of magnitude than those of the normally -diffusing ions.
Many researchers believe that these ions are solvent radical
cations (holes) involved in rapid resonant charge transfer.1 In
this case, the solvent holes must be long-lived: in pure
cyclohexane the conductivity signal from high-mobility ions
(HMI) decays with a first-order rate constant ∼2 × 106 s-1; in
Results and Discussion
Because the neat trans- and cis-decalins of sufficient purity
are costly and the minimization of the effects of radiolytic
products which scavenge HMI requires the use of rapid flow
of the sample without recirculation, we studied the less
expensive decalin mixture available from Aldrich (63:37 cis-
to-trans according to the GC analysis). In ref 14 we demon-
strated that HMI were present in all decalin mixtures. It was
shown that the decay of HMI is first-order, and the mobility of
HMI and scavenging rates change linearly with the mole fraction
of trans-decalin.14 By analysis of dc conductivity kinetics, it
was determined that the mobility of HMI in the 63:37 mixture
is (4.3 ( 0.3) × 10-3 cm2/V s (vs µ+ + µ- ≈4.1 × 10-4 cm2/
V s for normally diffusing ions) and that the scavenging of HMI
by cyclohexene, triethylamine, toluene, and n-propanol proceeds
with rate constants of 6.6, 5.8, 5.6, and 3.0 ((0.2) × 1010 mol-1
dm3 s-1, respectively.14 Due to the presence of impurity, the
first-order decay rate of the HMI is ∼1.5 × 106 s-1 in the neat
liquid photoionized under the same conditions. To estimate the
rate constants of typical diffusion-limited reactions in the
solvent, we measured the quenching constants of fluorescence
from the S1 states of pyrene and naphthalene (Table 1). The
fastest quenching occurs via charge transfer with rate constants
of (5-7.5) × 109 mol-1 dm3 s-1. These constants are close to
the dimerization constant of perylene•+, (6.1 ( 0.3) × 109 mol-1
dm3 s-1, as found in radiolytic experiments. In our analysis of
the scavenging kinetics, we assumed that the charge-transfer
reactions of normally diffusing ions occur with rate constant of
pure decalins 3 × 105 s-1 3
. Some of us have previously argued
that low chemical stability of the solvent holes at room
temperature contradicts the observed longevity, and we proposed
that HMI are some secondary ions, for instance, the proton
adducts of alkanes.12
Recently, we studied the formation kinetics of aromatic solute
ions in cyclohexane and found that the lifetime of the mobile
radical cation precursor is ∼30 ns.13 In the subsequent study,
we demonstrated that the dc conductivity kinetics observed in
radiolysis and laser photolysis of cyclohexane are incompatible
with the existence of a single HMI. It seems as if two HMIsa
short-lived solvent radical cation (<30 ns) and a long-lived
secondary ion (∼300 ns)sare needed to explain the results.14
Yet it is unclear what mechanism other than resonant charge
transfer could cause ultrafast migration of the ions. To solve
this riddle, we speculated that in cyclohexane the mobile (chair
form) radical cation is in equilibrium with a normally diffusing
(twist-boat form) ion.13,14 The twist conformer (∼10-4 mol
dm-3) which has lower IP rapidly scavenges chair-form radical
cations. Being endothermic, the backward transfer is relatively
slow, and the equilibrium is reached in 20-30 ns. It was this
process that was perceived as the fast decay of the solvent radical
cations in our transient absorption experiments. After the
1
equilibrium is reached, only ∼ /3 of the solvent holes exist in
the chair form capable of rapid motion via resonant charge
transfer. A thorough analysis of dc conductivity data shows
that the observed kinetics does correspond to the postulated
behavior.14
7.5 × 109 mol-1 dm3 s-1
.
Transient absorption kinetics in the pulse radiolysis of N2O-
saturated decalin were observed at 310-650 nm (we used 30
ps pulses from the 15 MeV Argonne linac). The solution was
flowed to minimize the accumulation of products in the
irradiated zone. The absorption spectra (Figure 1a) are due to
two cations: ion I, which absorbs in the visible region, and ion
II, which absorbs in the UV. The shape of the decay kinetics
does not change from 450 to 650 nm. At 400 nm the absorbance
reaches a minimum and then peaks again at 300 nm. In the
300-400 nm region, the decay is much slower than in the 450-
650 nm region and can be simulated as the weighted sum of
In conformationally frozen cycloalkanes, such as decalins and
methylcyclohexane, the radical cations exist in a single form.
Thus, a pattern different from that of cyclohexane must be seen.
† Work performed under the auspices of the Office of Basic Energy
Sciences, Division of Chemical Science, US-DOE, under Contract W-31-
109-ENG-38.
* To whom the correspondence should be addressed.
X Abstract published in AdVance ACS Abstracts, April 15, 1996.
0022-3654/96/20100-6876$12.00/0 © 1996 American Chemical Society

Letters
J. Phys. Chem., Vol. 100, No. 17, 1996 6877
TABLE 1: Rate Constants kq of Fluorescence Quenching in
Decalina at 25 °C (109 mol-1 dm3 s-1
)
D*
A
kq
typeb
pyrene
CH2I2
nitrobenzene
CH2I2
nitrobenzene
triethylamine
tributylamine
5.1 ( 0.1
5.45 ( 0.05
7.5 ( 0.1
6.5 ( 0.2
1.3 ( 0.03
2.1 ( 0.02
ET
ET
ET
ET
CF
CF
naphthalene
a 63% cis-decalin, 37% trans-decalin. b ET ) electron transfer, CF
) complex formation.
Figure 2. Formation of perylene•+ in the N2O-saturated decalin solution
(63 cis-:37 trans-decalin mixture) with no (a) and 0.1 mol dm-3 toluene
(b). Concentrations of perylene are given in the units of 10-4 mol dm-3
:
for (a) (i) 5.5, (ii) 2.8, (iii) 1.25, (iv) 0.44; for (b) (i) 5.7, (ii) 2.5, (iii)
1.1. Short time scale kinetics in (a) (solid dots) were obtained with 30
ps electron pulses, all other kinetics were obtained with 4 ns pulses
(60-80 kGy/pulse). Solid lines in (a) are a Monte Carlo simulation
for a HMI that lives 150 ns (see text). Solid lines in (b) represent a
simulation using the continuum-diffusion model (compare with the
dashed curves calculated with the Monte Carlo model). The discrepancy
at t > 0.5 µs is due to homogeneous recombination, which was not
included in the calculation.
following constants: µ+ ) 1.3 × 10-4 cm2/V s, µ- ) 2.8 ×
10-4 cm2/V s (for O•- and N2O•-),1 and ꢀ ) 2.17518 (calculated
for an ideal solution). We used an r2 Gaussian distribution of
electrons with bG ≈ 5.84 nm, which corresponds to a free ion
yield of 0.12-0.13 molecule/100 eV.1,18 For t < 0.5 µs, the
agreement between the experimental and theoretical curves is
good. At later times, the agreement is less satisfactory due
to second-order neutralization which is not included in the
model.
Figure 1. (a) Transient absorption spectra observed in radiolysis of
the N2O-saturated 63 cis-:37 trans-decalin mixture with 30 ps electron
pulses (15 MeV, 60 Gy/pulse): (i) 0.2-1 ns, (ii) 1-2 ns, (iii) 2-4 ns,
(iv) 4-8 ns, (v) 10-20 ns, and (vi) 20-50 ns. (b) Dots: normalized
decay kinetics observed at (i) 500 nm (ion I) and (ii) 350 nm (ion II).
The decay kinetics were simulated using a Monte Carlo model (dashed
lines) and convoluted with the response function of the detection system
(solid lines). The mobilities of ion I and ion II were taken as 4.2 ×
10-3 cm2/V s and 1.3 × 10-4 cm2/V s, respectively; the mobility of
anion was 2.8 × 10-4 cm2/V s.
The effect of perylene concentration with no toluene is shown
in Figure 2a. Without toluene the formation of Pe•+ is much
faster and more Pe•+ is formed, which indicates rapid scavenging
of the precursor radical cation. To fit these kinetics we used a
Monte Carlo program.16 The curves shown in Figure 2 are not
corrected for the response function of the photodiode, as the
curves in Figure 1b were. The kinetics were fit assuming that
the solvent radical cation transfers charge to perylene with a
rate constant of 8.5 × 1010 mol-1 dm3 s-1 (consistent with
scavenging constants observed for HMI by dc conductivity) and
decays with rate constant ∼ (6-7) × 106 s-1 in other reactions.
The latter constant is 4-5 higher than that observed for the
first-order decay of HMI in photoconductivity experiments.
However, at our dose (∼0.1 kGy/pulse) the lifetime of HMI
must be reduced by recombination with free anions (∼10-6 mol
dm-3) which proceeds with rate constant ∼2.3 × 1012 mol-1
dm3 s-1 (estimated from the Debye equation1). Additionally,
the lifetime of HMI is shortened by reactions with radiolytic
products.9 In oxygen-saturated cis-decalin, the yield of octalins
and decalin dimers is 0.86 and 0.24 molecule/100 eV, respec-
tively.19 Assuming that the overall yield of these scavengers
is ∼1 molecule/100 eV and that the scavenging constant is
∼(6-7) × 1010 mol-1 dm3 s-1 we obtain the first-order constant
∼(8-9) × 105 s-1. In sum, these reactions account for the
decay constant ∼4.5 × 106 s-1; the actual lifetime of HMI must
be shorter since some geminate ions in overlapping spurs can
also recombine.
the fast kinetics observed at λ 450 nm and the slow kinetics
observed at ≈313 nm. We believe that ion I is the high-mobility
solvent radical cation, while ion II is a normally diffusing olefin
ion(s), whose fast formation in radiolysis of decalins was
observed by fluorescence-detected magnetic resonance.15 Simu-
lations shown in Figure 1b suggest that the fast kinetics observed
at >400 nm is due to geminate decay of HMI; the slow kinetics
is from geminate recombination (and, possibly, slow formation)
of olefin ions.
The absorption spectrum of Pe•+ exhibits a peak at 550 nm.
3Pe, Pe2•+, and PeH+ do not absorb at 530-570 nm, the signal
from Pe•- (centered at 580 nm) was eliminated because the
solution was saturated with electron scavenger, N2O.13 Fol-
lowing refs 13 and 16, we removed the absorptions from decalin
(and toluene) ions by subtracting a half-sum of the traces
observed at 530 and 570 nm from the trace observed at 550
nm (Figure 2). To obtain the kinetics of scavenging of normally
diffusing ions, we added 0.1 mol dm-3 of toluene. Electron
pulses of 4 ns duration were used in these experiments. At 0.1
mol dm-3 toluene the lifetime of ion-I is < 0.2 ns,8,14,17 so the
slow kinetics observed at t > 10 ns is from scavenging of
toluene•+ by perylene. These kinetics are shown in Figure 2b.
The lines drawn through the points are the kinetics simulated
using the continuum-diffusion approach considered in ref 13.
This model does not include reactions in multiple pair spurs
and homogeneous recombination of ions. We assumed the

6878 J. Phys. Chem., Vol. 100, No. 17, 1996
Letters
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JP960387L