Radical Yields in the Radiolysis of Branched Hydrocarbons: Tertiary C-H Bond Rupture in 2,3-Dimethylbutane, 2,4-Dimethylpentane, and 3-Ethylpentane

Article abstract of DOI:10.1021/jp030658+

Gel permeation chromatography has been applied to iodine scavenging studies of the distribution of radicals produced in the radiolysis of symmetrically branched hydrocarbons 2,3-dimethylbutane, 2,4-dimethylpentane, and 3-ethylpentane. The principal iodides observed are those expected as a result of simple bond rupture. In the case of 2,3-dimethylbutane all five expected iodides are readily resolvable and it is shown that the loss of H from a tertiary position is favored over loss from a primary position by a factor of ～10. A similar ratio is also observed for 2,4-dimethylpentane. The higher ratio of 15 observed for 3-ethylpentane indicates a dependence on the number of tertiary sites on the alkane. The relative yield of ～3.3 for the loss of secondary and primary H atoms from 2,4-dimethylpentane and 3-ethylpentane is similar to that for normal alkanes, indicating a negligible effect of the adjacent tertiary carbon. In all three cases the rupture of terminal C-C bonds is relatively infrequent with C-C rupture occurring preferentially at the bonds adjacent to the tertiary carbon.

Full text of DOI:10.1021/jp030658+

9240
J. Phys. Chem. A 2003, 107, 9240-9247
Radical Yields in the Radiolysis of Branched Hydrocarbons: Tertiary C-H Bond Rupture
in 2,3-Dimethylbutane, 2,4-Dimethylpentane, and 3-Ethylpentane
Robert H. Schuler*,† and Laszlo Wojnarovits‡
Radiation Laboratory, UniVersity of Notre Dame, Notre Dame, Indiana 46530-0579, and Institute of Isotope
and Surface Chemistry, Chemical Research Center, Hungarian Academy of Sciences, Budapest, Hungary
ReceiVed: May 27, 2003
Gel permeation chromatography has been applied to iodine scavenging studies of the distribution of radicals
produced in the radiolysis of symmetrically branched hydrocarbons 2,3-dimethylbutane, 2,4-dimethylpentane,
and 3-ethylpentane. The principal iodides observed are those expected as a result of simple bond rupture. In
the case of 2,3-dimethylbutane all five expected iodides are readily resolvable and it is shown that the loss
of H from a tertiary position is favored over loss from a primary position by a factor of ∼10. A similar ratio
is also observed for 2,4-dimethylpentane. The higher ratio of 15 observed for 3-ethylpentane indicates a
dependence on the number of tertiary sites on the alkane. The relative yield of ∼3.3 for the loss of secondary
and primary H atoms from 2,4-dimethylpentane and 3-ethylpentane is similar to that for normal alkanes,
indicating a negligible effect of the adjacent tertiary carbon. In all three cases the rupture of terminal C-C
bonds is relatively infrequent with C-C rupture occurring preferentially at the bonds adjacent to the tertiary
carbon.
Introduction
to the square of the number of C-C bonds and is strongly
affected by the presence of additional methyl groups on the
adjacent carbon atom. Only a few iodine scavenging studies
have been devoted to a more general determination of the
distribution of radicals produced in the radiolysis of branched
alkanes.8-11 Of these, the gas chromatographic study of Castello,
Grandi, and Munari10 on 2,3-dimethylbutane is the most relevant
to the present study. In addition to the expected iodides, they
found small yields of iodides other than those that would result
from simple bond rupture. However, because of the relatively
high temperatures required for the separation of the tertiary
iodides and their fragility, gas chromatography is generally
unsuitable for the quantitative determination of their yields.
As indicated previously,1 gel permeation chromatography
(GPC) provides a particularly attractive alternative to the
approaches used in previous studies because many of the iodides
of interest can be separated at room temperature. In the present
study, we have used GPC to examine the distribution of iodides
produced in the radiolysis of 2,3-dimethylbutane, 2,4-dimeth-
ylpentane, and 3-ethylpentane containing millimolar iodine as
a radical scavenger. These hydrocarbons have been chosen as
examples because the number of radicals expected from simple
bond rupture is limited by their symmetry. The approaches used
are essentially the same as those used in the previous study on
normal alkanes.1 The application of 3-D spectrophotometric
detection provides information that is valuable to the positive
identification of the individual iodides. Chromatograms recorded
spectrophotometrically or radiochemically provide the quantita-
tive information needed for the determination of the individual
radical yields.
In a previous study of the radical yields in the radiolysis of
normal alkanes, it was shown that secondary C-H bonds are
ruptured ∼3 times more frequently than primary bonds.1 To
provide for a comprehensive discussion of the radiolysis of
branched alkanes, information is also needed on the relative
frequency of rupture of tertiary C-H bonds. The radical
sampling experiments of Holroyd2,3 have indicated that in the
cases of isopentane, 2,4-dimethylpentane, and 2,2,4-trimethyl-
pentane the loss of H from a tertiary position is, respectively,
8.4, 5.5, and 10.1 times greater than from a primary position.4
However, the application of radical sampling is complicated in
that it requires information on the disproportionation/combina-
tion ratio of the reactions between the radicals. Also, one has
to carry out such studies under high dose rate conditions to
minimize additional complications from radical transfer reac-
tions. Whereas Holroyd’s studies show that the loss of tertiary
H atoms is of considerable importance in the radiolysis of
branched alkanes, quantitative information available from such
radical sampling experiments is very limited.
Radical scavenging experiments5 provide a more direct
approach. Of the scavengers that have been used to date,
molecular iodine appears to be the simplest. Because alkyl
radicals react with iodine at diffusion-controlled rates,6 scaveng-
ing occurs at millimolar iodine concentrations on the micro-
second time scale and is more rapid than radical transfer
processes. As a result, the radical yields are represented by the
yields of the iodides produced. The yields of methyl radicals
produced in the radiolysis of a number of branched alkanes have,
for example, been determined in radiochromatographic studies
using iodine as the scavenger.7 That study demonstrated that
the rupture of a given CH3-C bond is inversely proportional
Although the present results on 2,3-dimethylbutane differ in
detail with those reported by Castello et al.,10 they demonstrate
that, as they concluded, the loss of H occurs preferentially at a
tertiary position and that the rupture of the carbon backbone
occurs mainly at the central C-C bond. In the present study,
these conclusions are further generalized by the results on 2,4-
* Corresponding author. E-mail schuler.1@nd.edu.
† University of Notre Dame.
‡ Hungarian Academy of Sciences.
10.1021/jp030658+ CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/02/2003

Radical Yields in Branched Hydrocarbon Radiolysis
J. Phys. Chem. A, Vol. 107, No. 43, 2003 9241
dimethylpentane and 3-ethylpentane, for which scavenging data
do not currently exist in the literature. The results presented
here provide a quantitative basis for future discussions of the
radiation chemistry of branched hydrocarbons.
and 3-iodopentane), spectra extracted at the chromatographic
peaks and the elution times provided positive identification. A
reference sample of 2-iodo-3-methylbutane was prepared by
iodine trapping of the radicals produced in the radiolytic
reduction of 2-bromo-3-methylbutane. Because tertiary iodides
absorb at somewhat longer wavelengths (∼267 nm) than the
primary (∼256 nm) and secondary iodides (∼261 nm), spectra
extracted from the 3-D data provide identification where
reference samples were not available.
Experimental Section
The experimental arrangements and spectrophotometric and
radiochemical methods used in this study have been described
previously in considerable detail.1
Most chromatograms were displayed at 260 nm where all
saturated alkyl iodides exhibit significant absorption.1,12-14 These
chromatograms provided quantitative data for the determination
of the iodide yields. The areas under the chromatographic peaks
were determined with Waters Millennium or Origin 6.1 software
as described previously.1 Where chromatographic peaks were
only partially resolved, the various contributions were deter-
mined using the Gaussian analysis program in Origin 6.1.
Quantitative analysis of the spectrophotometric recordings
requires information on the sensitivities of the various iodides
such as that given in Table S1 of ref 1. Most normal iodoalkanes
have similar extinction coefficients of ∼470 M-1 cm-1 at 260
nm. Methyl and ethyl iodide, however, have extinction coef-
ficients of only 341 and 423 M-1 cm-1, respectively, and
sensitivities that are correspondingly lower.12 The extinction
coefficients of the secondary iodides at 260 nm are 500-540
M-1 cm-1, so their sensitivities are ∼10% higher than those of
the primary iodides.12 Where extinction coefficients were
available, the relative yields of the different iodides were
determined using their corresponding sensitivities. In the other
cases, approximate yields were determined using sensitivities
of similar iodoalkanes. In the cases of the tertiary iodides, the
sensitivities were assumed to be that of tert-butyl iodide, which
at 260 nm is comparable to that of isopropyl iodide (i.e., 7%
higher than that of the primary iodides). A comparison of the
approximate relative yields determined with this approach with
the results obtained in radiochemical measurements shows that
the yields determined spectrophotometrically are accurate to
better than 10%.
Radiochemical Studies. Analogous radiochemical studies
were carried out with 125I2 labeled iodine as the scavenger. The
chromatography was essentially the same as in the spectropho-
tometric studies. The resolution was similar except that the peaks
were somewhat broadened by the loop in the radioactivity
detector. Activity in the eluting stream was recorded in ASCII
format, and areas under the chromatographic peaks were
determined by Gaussian analysis. Elution was also followed
spectrophotometrically at 265 nm. The determination of the
relative yields of the different iodides from their areas in the
radiochromatograms is quite straightforward because the radio-
chemical sensitivities of all monoiodides are identical. The yields
of the individual iodides were determined assuming a total yield
of 5.6. The Discussion section that follows is based on the yields
determined in these radiochemical studies. Normalization of the
spectrophotometric signals to those determined radiochemically
provides information on the extinction coefficients and corre-
sponding sensitivities of the iodides where samples are not
available for direct measurement.
Hydrocarbons. Samples of 2,3-dimethylbutane were obtained
from Aldrich, Phillips, and Merck, AG; 2,4-dimethylpentane,
from Aldrich and Phillips; and 3-ethylpentane, from TCI and
Merck, AG. As received, all exhibited significant absorption
below 300 nm, indicating the presence of olefinic impurities.
After passing them through columns of silica gel and silver
nitrate on alumina, the absorbance at 220 nm was less than 0.1.
A chromatographic analysis of the resultant samples did not
indicate the presence of any impurities that would interfere with
the chromatographic analysis.
Sample Preparation and Irradiation. For the spectropho-
tometric studies, the samples were outgassed using freeze-thaw
methods after the addition of ∼1 mM I2. The irradiation cells
had a quartz cuvette attached that permitted the iodine concen-
tration to be monitored. Irradiations were at room temperature
in a 60Co irradiator at a dose rate of ∼200 krads/h. Doses were
50 to 300 krads. At these doses, 15 to 50% of the iodine was
consumed. In the radiochemical experiments, the dose rate was
∼100 krads/h. Freshly prepared samples containing 1 mM 125I2
were purged of oxygen as described previously1 and analyzed
immediately after the irradiations. In the following text, radiation
chemical yields are given as values of G in units of molecules
per 100 eV of absorbed energy.
Gel Permeation Chromatography. In most experiments, two
Phenomenex 300 mm × 7.8 mm Phenogel 5µ 50-Å columns
were used in series for the separation of the product iodides.
Three columns were used where additional resolution was
desired. After irradiation, a 50- or 200-µL sample was introduced
into the eluent stream using a loop injector. At the flow rate
used (0.5-0.7 cm3/min), these volumes correspond to injection
times of 5 and 20 s. The smaller volume is comparable to the
detector volume and was used where maximum resolution was
desired. Because the peak widths were inherently at least 30 s,
this volume did not broaden the peaks significantly, and the
peak profiles were essentially Gaussian in shape. Experiments
with an injection volume of 20 µL did not improve peak shape
or resolution.The hydrocarbons used for elution were n-hexane
containing ∼15% hexane isomers (Fisher), chromatographic-
grade isopentane (Aldrich), cyclopentane (Aldrich), or cyclo-
hexane (Fisher). The use of saturated alkanes and cycloalkanes
as eluents allowed spectrophotometric recordings down to 200
nm. As shown in the Results and Data Analysis section, the
resolution of components that have similar elution times
critically depends on the eluting system used. In particular, we
find that the addition of isopentane to hexane selectively
increases the elution times of the primary iodides. Appropriate
mixtures were used to optimize the resolution.
Spectrophotometric Studies. Absorption spectra of the
column effluent were recorded every second in 3-D format with
a Waters 996 diode array detector and Waters Millennium
software. These recordings allowed the data to be examined in
the form of contour plots that provide considerable insight into
the identity of the individual peaks and possible contributions
from non-iodine-containing products. Where reference samples
were available (methyl-, ethyl, isopropyl, and isobutyl iodides
Results and Data Analysis
Before proceeding with a detailed analysis of the data, it is
helpful to consider the relationships between structure and
radical yields noted previously. For example, it has been shown
that the methyl radical yields from a wide variety of alkanes
are low and can be correlated by a relation that takes into

9242 J. Phys. Chem. A, Vol. 107, No. 43, 2003
Schuler and Wojnarovits
Figure 1. Contour plot for 2,3-dimethylbutane containing 1 mM I2
irradiated to a dose of 100 krads. Elution was with hexane containing
25% isopentane. Contours are given in increments of 0.001 AU
(absorbance unit). The chromatogram extracted at 260 nm is displayed
below the contour plot. In order of elution, the five most intense peaks
are identified as 2-iodo-2,3-dimethylbutane, 1-iodo-2,3-dimethylbutane,
1-iodo-3-methylbutane, isopropyl iodide, and methyl iodide from their
retention times and spectra (Figure 2).
Figure 2. Absorption spectra at the chromatographic maxima in Figure
1. The peaks at 47.7 and 49.2 min have absorption maxima at 267 and
257 nm that are typical of tertiary and primary iodides. They are
identified as 2-iodo-2,3-dimethylbutane and 1-iodo-2,3-dimethylbutane.
The 261-nm absorption maxima of the peaks at 50.3 (2-iodo-3-
methylbutane) and at 53.7 min (isopropyl iodide) are typical of
secondary iodides. Methyl iodide, observed at 68.1 min in Figure 1,
has an absorption maximum at 256 nm. The latter three iodides have
spectra and elution times identical to those of reference samples.
account the number and type of methyl groups.7 Using the
generalizations given in that study, the predicted methyl radical
yield for 2,3-dimethylbutane is 0.45, for 2,4-dimethyl pentane,
0.31, and for 3-ethylpentane, 0.09. These values compare with
experimental yields of 0.34 from 2,3-dimethylbutane and 0.28
from 2,4-dimethylpentane.7 Because the methyl radical yields
are low, one expects the yields of the complementary radicals
produced by the scission of a terminal CH3-R bond to be
correspondingly low.
The study of the distribution of radicals produced in the
radiolysis of the normal alkanes1 has shown that radical
production is dominated by the loss of H atoms, with the rupture
of a secondary C-H bond being 3.2 times more likely than for
a primary C-H bond. That study also showed that the scission
of the carbon backbone preferentially occurs near the center of
the alkane and is asymmetric, with the yield of the smaller
fragment being somewhat greater than that of the larger
fragment.
2,3-Dimethylbutane. Because 2,3-dimethylbutane contains
only two types of C-H bonds, two types of C-C bonds, and
no secondary H atoms, this branched alkane represents the
simplest of the three examples under consideration. Only five
radicals are expected as the result of simple bond rupture: the
primary and tertiary radicals produced by C-H scission, the
isopropyl radical produced by the rupture of the central C-C
bond, and the methyl radical and its complement, the 3-methyl-
2-butyl radical, by the rupture of a terminal CH3-C bond. In
the contour plot of Figure 1, all five iodides resulting from iodine
scavenging of these radicals are well resolved. As is typical of
GPC, these iodides elute in order of decreasing molecular
weight. The spectra extracted at the chromatographic maxima
are given in Figure 2. The first iodide to elute (at 47.7 min) has
an absorption maximum at 267 nm and also absorbs strongly
at 210 nm, as is characteristic of tertiary iodides.14 It is readily
identified as 2-iodo-2,3-dimethylbutane produced as a result of
scavenging of the radicals resulting from the loss of a tertiary
H atom. The second peak (at 49.2 min) has an absorption
maximum at 256 nm and is assigned as the primary iodide,
1-iodo-2,3-dimethylbutane. These are followed by 2-iodo-3-
Figure 3. Dependence of the retention times of (9) 2-iodo-2,3-
dimethylbutane, (b) iodo-2,3-dimethylbutane, (2) 1-iodo-3-methylbu-
tane, and (1) isopropyl iodide on the composition of hexane-isopentane
eluents. Methyl iodide elutes at 68-72 min and is not shown.
methylbutane at 50.3 min, isopropyl iodide (2-iodopropane) at
53.7 min, and methyl iodide at 68.1 min. The latter three are
identified by a comparison of their spectra and retention times
with authentic samples. As expected from the generalization
discussed above, the contributions of methyl iodide and 2-iodo-
3-methylbutane are low. Other than the minor contribution at
57.0 min, which has an absorption maximum at 257 nm and is
attributable to 1-iodopropane, no other iodide is apparent in the
contour plot. There is no evidence that any unsaturated iodides
are produced.
The dependences of the elution times on the eluent composi-
tion are given in Figure 3. It is seen that with increases in the
isopentane content in the eluent, 1-iodo-2,3-dimethylbutane is
delayed more than the other iodides. As is found in the other
studies reported below, isopentane selectively increases the

Radical Yields in Branched Hydrocarbon Radiolysis
J. Phys. Chem. A, Vol. 107, No. 43, 2003 9243
Figure 5. Comparison of radiochromatographic and spectrophotometric
recordings (at 265 nm) for 2,3-dimethylbutane containing 1 mM 125I2
irradiated to a dose of 100 krads. Elution was with 40:60 hexane-
isopentane. The isopropyl iodide peak at 46.8 min was used to normalize
the chromatograms.
Figure 4. Chromatograms at 260 nm for elution with (A) 75:25 (blue)
and (B) 25:75 (green) mixtures of n-hexane and isopentane. The dotted
curves represent Gaussian fits to the observed chromatograms, and the
red curves, the sum of the Gaussians.
TABLE 1: Alkyl Iodide Yieldsa in the Radiolysis of
2,3-Dimethylbutane
Figure 5 shows that there are no significant yields of iodides
other than those noted in the spectroscopic studies. The yields
of the individual iodides determined from their relative areas
in the radiochromatogram are summarized in column 3 of Table
1. Because reference samples were not available for the hexyl
iodides, their yields determined in the radiochemical experiments
must be regarded as more accurate than those given by the
spectrophotometric studies.
The methyl iodide yield given in Table 1 (0.42) is somewhat
higher than that given in ref 7 (0.34) but is similar to the value
of 0.45 predicted by empirical relation IV in that reference. It
is noted that the yield of the 2-iodo-3-methylbutane, produced
as a complement to methyl iodide, is only half the yield of the
latter. This difference is in accord with the asymmetric patterns
noted previously for the radicals produced in the radiolysis of
the low-molecular-weight normal alkanes.1
spectroscopic radiochemical ref 10
2-iodo-2,3-dimethylbutane
1-iodo-2,3-dimethylbutane
2-iodo-3-methylbutane
isopropyl iodide
1.95
0.91
0.16
2.05
0.45
2.00
1.18
0.19
1.77
0.42
2.66
1.60
0.17
1.74
0.60
0.08
0.11
methyl iodide
(1-iodopropane)
(others)
<0.05
<0.02
<0.10
<0.10
tertiary/primary
10.2
a Radiation chemical yields in units of molecules per 100 eV of
absorbed energy.
elution time of the primary iodide relative to those of the
secondary and tertiary iodides. As indicated in the Figure,
2-iodo-3-methylbutane and 2-iodopropane are not resolved with
isopentane elution. Because of the selective delay of the primary
iodide, one can, to a certain extent, tune chromatographic
resolution by the judicious choice of the eluent. The dependences
of elution times and resolution on eluent composition are very
subtle.
The yields reported by Castello et al.10 are given in column
4 of Table 1. It is seen that whereas the yields that they reported
for the iodohexanes are considerably higher than those deter-
mined in the present study the relative yields in the two studies
are somewhat similar. The principal difference is the relatively
low yield that they give for isopropyl iodide. The overall
agreement is, in fact, far better than might be expected
considering the differences in the experimental methodologies,
the 5-fold higher iodine concentration, and the correspondingly
higher doses used in their experiments.
Chromatograms obtained with hexane and also with a 30:70
mixture of hexane and isopentane as the eluents are given in
Figure 4. The areas of the major components were determined
by Gaussian analysis, as indicated by the dotted curves in the
Figure. The peak areas obtained with the other mixed eluents
are essentially independent of the eluent composition and have
a standard deviation on the order of a few percent. In the case
where isopentane was used as the eluent, the area of the
unresolved composite of 1-iodo-2,3-dimethylbutane and 2-iodo-
3-methylbutane agreed with the sum of their areas determined
in the other experiments. These spectrophotometric studies
demonstrate the excellent reproducibility attainable in these
measurements and provide the spectral information needed to
identify the individual iodides. The yields obtained from the
spectrophotometric experiments based on the assumed sensitivi-
ties of the hexyl iodides are given in column 2 of Table 1.
In Figure 5, a typical radiochromatogram is compared with
one simultaneously obtained spectrophotometrically, demon-
strating that except for the relatively low spectroscopic sensitiv-
ity of methyl iodide the spectroscopic sensitivities of the
different iodides are very similar. The radiochromatogram of
Schrob and Trifunac,15 using time-resolved ESR, identified
the tertiary radicals2,3-dimethyl-2-butylsand the isopropyl
radical as the principal intermediates present 10 µs after pulse
irradiation of 2,3-dimethylbutane at -52 °C. However, the
scavenging studies show that there also is an appreciable yield
of the primary radical and the radicals produced by the rupture
of the C-CH3 bond. These latter radicals are not apparent in
the ESR experiments because their ESR spectra involve coupling
to only three or four protons whereas the spin couples to seven
or six protons in the cases of the tertiary hexyl and isopropyl
radicals. As a result, ESR spectra of the latter radicals extend
over much broader field region and mask the signals of the other
radicals.
The source of the small yield of 1-iodopropane in Figure 1
is not obvious. Samples of 2,3-dimethylbutane from different
sources gave similar yields, suggesting that the n-propyl radical

9244 J. Phys. Chem. A, Vol. 107, No. 43, 2003
Schuler and Wojnarovits
TABLE 2: Alkyl Iodide Yieldsa in the Radiolysis of
2,4-Dimethylpentane
spectroscopic radiochemical
type
2-iodo-2,4-dimethylpentane
3-iodo-2,4-dimethylpentane
1-iodo-2,4-dimethylpentane
2-iodo-4-methylpentane
1-iodo-2-methylpropane
isopropyl iodide
1.73
0.60
0.99
0.24
0.86
0.78
0.40
1.71
0.61b
1.01b
0.24b
0.85
0.76
0.42
tertiary
secondary
primary
secondary
primary
secondary
primary
iodomethane
tertiary/primary
secondary/primary
10.2
3.6
a Radiation chemical yields in units of molecules per 100 eV of
absorbed energy. b Total yield of 1.86 distributed among 1-iodo-2,4-
dimethylpentane, 3-iodo-2,4-dimethylpentane, and 2-iodo-4-methyl-
pentane in a ratio of 54:33:13, as indicated by the spectrophotometric
results.
Figure 6. Radiochromatogram (black) for 2,4-dimethyl pentane
containing 1 mM 125I2 irradiated to a dose of 100 krads. The red curves
represent Gaussian fits to the five chromatographic peaks. The tertiary
2-iodo-2,4-dimethylpentane elutes first (at 36.6 min) and is followed
at 38.3 min by a somewhat broader peak that is a composite of the
primary and secondary heptyl iodides and the secondary hexyl iodide
that complements methyl iodide (Figure 8). The peaks at 42.5, 44.5,
and 55.6 min are attributable to isobutyl, isopropyl, and methyl iodides.
may result from the isomerization of a cation initially produced
in the radiation process. However, it is also possible that it is
attributable to an impurity in the sample such as 2-methylpentane
where propyl and isopropyl radicals are the dominant radicals
produced.16 Neither the normal alkanes previously studied1 nor
the other two hydrocarbons studied here give any indication of
significant yields of radicals produced by other than simple bond
rupture.
2,4-Dimethylpentane. Seven iodides are expected to result
from the simple bond rupture of 2,4-dimethylpentane. However,
in this case, the analysis is complicated in that only five peaks
are apparent in the radiochromatogram of Figure 6 where hexane
was the eluent. The first peak is identified as the expected
tertiary heptyl iodide, 2-iodo-2,4-dimethylpentane, from its
absorption maximum at 269 nm in corresponding spectropho-
tometric recordings (see below). The third, fourth, and fifth
peaks in Figure 6 are identified as isobutyl iodide (1-iodo-2-
methylpropane), isopropyl iodide (2-iodopropane), and methyl
iodide by comparing their elution times with those of authentic
samples. The second peak in Figure 6 is 20% broader than the
first and, as shown below, is a composite of contributions from
the three remaining expected iodides: the primary and secondary
heptyl iodides, 1-iodo-2,4-dimethylpentane and 3-iodo-2,4-
dimethylpentane, and the hexyl iodide, 2-iodo-4-methylpentane.
A radiochromatogram recorded with cyclopentane elution shows
an even broader peak of this composite but does not provide
sufficient resolution to allow analysis. No other iodides are
evident in the radiochromatogram.
The yields of the various iodides, determined from the
radiochromatogram of Figure 6, are given in column 3 of Table
2. The area of the composite peak corresponds to 33% of the
iodides produced. The contributions to the composite peak are
given in Table 2 on the assumption that their relative yields are
in the ratios indicated by the spectroscopic analyses described
below.
The contour plot of Figure 7, where elution was with
cyclopentane, shows that, as indicated above, the first peak that
elutes is the expected tertiary heptyl iodide. This peak is
followed by a partially resolved peak at 46.3 min that has a
maximum at 256 nm and one at 46.8 min with a maximum at
Figure 7. Contour plot for 2,4-dimethylpentane containing 1 mM I2
irradiated to a dose of 100 krads. Elution was with cyclopentane
containing 15% isopentane. Contours represent absorbance levels at
intervals of 0.001 AU. The chromatogram extracted at 260 nm is given
below the contour plot. Peak assignments are the same as in Figure 6.
The contours of isobutyl and isopropyl iodide illustrate the typical
differences between the absorption maxima of primary and secondary
iodides.
260 nm. These peaks make up the composite observed in the
radiochromatogram. The first of these peaks has an absorption
maximum at 256 nm and is identified from its spectrum as the
expected primary heptyl iodide, and the second is a composite
of the secondary heptyl and hexyl iodides, both of which are
expected to have similar spectra with absorption maxima at
∼260 nm. The peaks that follow the composite are identified
by their spectra and elution times as isobutyl, isopropyl, and
methyl iodide (not shown in Figure 7). Because the sensitivities
of these three iodides are known, their contributions can be
determined readily.
The Gaussian analysis, given in Figure 8b, of the composite
observed with cyclopentane elution is particularly important in
that it provides a measure of the yield of the primary heptyl
iodide. Although cyclopentane elution gave the narrowest peaks
and best overall resolution, it did not resolve the contributions
from the secondary heptyl and hexyl iodides. We note that
2-iodo-4-methylpentane complements methyl iodide and should
be produced in a yield less than 0.4. The peak due to the
secondary iodides in Figure 8b must, therefore, be mainly
attributed to the heptyl iodide, 3-iodo-2,4-dimethylpentane. With
isopentane and cyclohexane elution, the contribution from

Radical Yields in Branched Hydrocarbon Radiolysis
J. Phys. Chem. A, Vol. 107, No. 43, 2003 9245
Figure 9. Chromatogram recorded spectrophotometrically at 260 nm
for 3-ethylpentane containing 1 mM I2 irradiated to a dose of 100 krads.
Elution was with hexane. The red curves represent optimized Gaussian
fits to the five chromatographic peaks. The first peak is attributed to
the secondary heptyl iodide, 2-iodo-3-ethylpentane. (See Figure 10 and
text.) The second peak is 20% wider than the first and is the composite
of the tertiary and primary heptyl iodides and 1-iodo-2-ethylbutane that
complements methyl iodide. These are followed by 3-iodopentane at
53.5 min, its complement ethyl iodide at 64.5 min, and methyl iodide
at 72.4 min.
radiochemical experiments. Their yields, determined in the
spectroscopic experiments, are summarized in column 2 of Table
2.
It is expected that the smaller 2-iodo-4-methylpentane should
elute after the heptyl iodides, as is observed with cyclohexane
elution. The question now arises as to why it elutes ahead of
the heptyl iodides with isopentane elution. With this eluent, the
delay of the primary heptyl iodide relative to that of the smaller
hexyl iodide is similar to the delays noted in Figure 4 for the
corresponding peaks in the case of 2,3-dimethylbutane. 3-Iodo-
2,4-dimethylpentane is also delayed because of its high sym-
metry, as is also noted in other cases of highly symmetrical
molecules.16 Apparently, highly symmetrical compounds are
selectively delayed in GPC because they are more compact and
diffuse more deeply into the gel. (See, for example, the delay
of 3-iodo-3-ethylpentane discussed below in the case of 3-eth-
ylpentane.) Although the magnitudes of these delays are small,
they are large enough that the observed chromatographic patterns
are critically dependent on the eluent. Both the molecular shape
of a given component and that of the eluent play significant
roles in determining elution times in GPC.
Figure 8. Chromatograms at 260 nm for 2,4-dimethylpentane contain-
ing 1 mM I2 irradiated to a dose of 100 krads obtained with elution by
(A) isopentane, (B) cyclopentane containing 15% isopentane, and (C)
cyclohexane. In each case, the tertiary heptyl iodide elutes first. Its
peak is fitted very well by a single Gaussian. The solid green and dotted
blue Gaussians in B represent the contributions of the primary and
secondary heptyl iodides, respectively, to the composite. The dark-
blue Gaussians represent a common contribution of 2-iodo-4-methyl
pentane to the three chromatograms.
2-iodo-4-methylpentane becomes apparent. With isopentane
elution (Figure 8a), this contribution appears as a shoulder before
a composite of the primary and secondary heptyl iodides. With
cyclohexane elution (Figure 8c), it appears after that composite.
Gaussian analysis shows that these shoulders have similar areas
that correspond to a yield of about half that of methyl iodide.
Although the primary and secondary heptyl iodides are not
resolved in Figure 8a and c, the contributions of the secondary
heptyl iodide can be determined by subtracting the area of the
primary heptyl iodide observed with cyclopentane elution from
that of the combined peaks or, alternatively, by constraining
the Gaussian analysis of the composite in Figure 8b to that area.
Taking into account the higher sensitivity of the secondary
iodides, both approaches show that the relative yields of 1-iodo-
2,4-dimethylpentane, 3-iodo-2,4-dimethylpentane, and 2-iodo-
4-methyl pentane are in a ratio of 54:33:13 with their total
corresponding to 33% of the iodides produced, as indicated by
The yield of methyl iodide (0.42) is considerably higher than
that reported previously (0.28).7 Except for the tertiary radical,
the radical yields reported by Holroyd3 are ∼50% lower than
those indicated in Table 2. These differences reflect on the
difficulties in interpreting the data from the sampling experi-
ments. There are no other measurements of these yields in the
literature.
3-Ethylpentane. For 3-ethylpentane, seven radicals are also
expected: the primary, secondary, and tertiary heptyl radicals
produced by the loss of an H atom and methyl and ethyl radicals
and their complements produced by C-C rupture. However,
the chromatogram of Figure 9, recorded spectrophotometrically
with hexane elution, manifests only five iodide peaks. The first
peak is identified as the secondary heptyl iodide from its
absorption maximum at 261 nm. The second peak has an
absorption maximum at 264 nm, indicating unresolved contribu-

9246 J. Phys. Chem. A, Vol. 107, No. 43, 2003
Schuler and Wojnarovits
Figure 10. Contour plot for 3-ethylpentane containing 1 mM I2
irradiated to a dose of 100 krads. Elution was with isopentane containing
5% cyclopentane. Contours represent absorbance levels at intervals of
0.001 AU. The chromatogram extracted at 260 nm is given below the
contour plot. The first peak to elute is the secondary heptyl iodide.
This is followed by the tertiary and primary heptyl iodides, 3-iodo-
pentane and ethyl iodide. Methyl iodide is observed as a weak peak at
89 min and is not shown.
tions from the primary and tertiary heptyl iodides. The remaining
three peaks are, in order of elution, identified by comparing
their elution times and spectra with authentic samples such as
3-iodopentane, ethyl iodide, and methyl iodide. Because their
extinction coefficients are known,1 their contributions are readily
determinable. The hexyl iodide produced as a result of the
scission of a terminal CH3-C bond is expected to be produced
in very low yield and to elute with and be masked by the other
more abundant iodides. It is not apparent in the chromatographic
data.
Figure 11. Analysis of the chromatograms (upper) extracted from
Figure 10 at 260 nm and (lower) recorded in an analogous radiochemical
experiment. The five Gaussians represent (blue) the secondary heptyl
iodide, (red) the tertiary heptyl iodide, (green) the primary heptyl iodide,
(orange) the primary hexyl iodide that complements methyl iodide, and
(cyan) 3-iodopentane.
Partial resolution of the heptyl iodides, illustrated in the
contour plot of Figure 10, was achieved using isopentane
containing 5% cyclopentane as the eluent. As with hexane
elution, the component at 54.9 min is assigned as the secondary
heptyl iodide, 2-iodo-3-ethylpentane. This peak is followed by
one at 56.5 min having an absorption maximum at 267 nm that
identifies it as the tertiary iodide, 3-iodo-3-ethylpentane, and
by one at 57.8 min that has an absorption maximum at 257
nm, showing it to be the primary iodide, 1-iodo-3-ethylpentane.
The peak at 58.9 min in Figure 10 is due to 3-iodopentane that
is only poorly resolved from the primary heptyl iodide with
cyclopentane elution but well-resolved with hexane elution
(Figure 9).
TABLE 3: Alkyl Iodide Yieldsa in the Radiolysis of
3-Ethylpentane
spectroscopic radiochemical
type
3-iodo-3-ethylpentane
2-iodo-3-ethylpentane
1-iodo-3-ethylpentane
1-iodo-2-methylbutane
3-iodopentane
ethyl iodide
methyl iodide
1.33
1.64
1.34
1.71
tertiary
secondary
primary
primary
secondary
primary
primary
0.80
0.81
(0.07)b
0.80
(0.07)b
0.78
0.85
0.11
0.80
0.09
tertiary/primary
secondary/primary
14.9
3.2
Figure 11 compares the chromatogram of Figure 10 with a
radiochromatogram recorded under similar conditions. The
spectrophotometric data given in Figure 11a exhibit four peaks
that can be resolved by the indicated Gaussian analysis. For
completeness, an approximate contribution from the 2-iodo-4-
methylpentane that complements methyl iodide is included in
this analysis, but its inclusion has only a minor effect on the
Gaussian fits. The radiochromatogram given in Figure 11b is
considerably noisier than the chromatogram obtained spectro-
scopically but with appropriate constraints can be analyzed
similarly. A comparison of these two chromatograms shows that,
as expected, the extinction coefficient of the primary heptyl
iodide is relatively less than those of the other iodides. From
the areas of the peaks in the radiochromatogram, the yields of
the secondary, tertiary, and primary iodides and 3-iodopentane
are given in column 3 of Table 3. The agreement between the
a Radiation chemical yields in units of molecules per 100 eV of
absorbed energy. b Estimated from ref 7.
two sets of results indicates that the sensitivities assumed in
the spectroscopic experiments are reasonably accurate.
In most cases, tertiary iodides elute before their isomers, so
it is quite surprising that in Figure 10 the secondary iodide is
observed to elute first. In this case, the elution of 3-iodo-3-
ethylpentane is delayed because of its high symmetry and
compact structure that allows it to penetrate more deeply into
the gel. As a result, it appears after the secondary iodide. It is
another manifestation of the delay noted above in the case of
the symmetric secondary heptyl iodide, 3-iodo-2,4-dimethyl-
pentane, produced from 2,4-dimethylpentane. These observa-
tions demonstrate that the resolution in GPC critically depends

Radical Yields in Branched Hydrocarbon Radiolysis
J. Phys. Chem. A, Vol. 107, No. 43, 2003 9247
on the structure of the molecules under consideration and the
eluent being used. We note here that the detailed spectroscopic
information available from the 3-D recordings is essential to
identify the chromatographic peaks properly. Without this
information, it is likely, because tertiary iodides usually elute
before their isomers, that one would assume the first peak in
the radiochromatogram of Figure 9 to be the tertiary iodide.
C-C bonds adjacent to the tertiary site.19,20 Because C-C
rupture occurs only as the result of primary processes, the high
yields of the radicals produced radiolytically by the rupture of
the C-C bonds adjacent to the tertiary site demonstrate that
the energy deposited by ionizing radiation is similarly localized
at tertiary sites.
We especially note here that 2,4-dimethylpentane is a special
case in that the yield of the isobutyl radical is 12% higher than
that of its complement, the isopropyl radical (cf. Figure 6). This
contrasts with the similar yields of ethyl and 3-pentyl radicals
observed in the case of 3-ethylpentane and the relatively low
yield of the complement to the methyl radical produced from
2,3-dimethylbutane. For the normal alkanes, the smaller frag-
ments are observed in slightly greater abundance than their
complements.1 Although one expects the yields of complemen-
tary fragments to be similar factors, other than simple bond
rupture, such as secondary reactions of the ions initially
produced, they play some role in determining the yields of
radicals that can be scavenged at microsecond times.
In summary, this study shows that in the radiolysis of
branched alkanes the loss of H from tertiary positions occurs
an order of magnitude more frequently than from primary
positions but that competitive effects play a role when there
are several tertiary H atoms present. The relative frequency for
the loss from secondary and primary positions (3.2) is similar
to that observed for normal alkanes. These observations provide
the basis for a general understanding of bond rupture in the
radiolysis of saturated branched hydrocarbons.
Discussion and Summary
It has previously been shown1 that in the radiolysis of the
normal alkanes the relative yields the for production of
secondary and primary radicals are proportional to the number
of secondary H atoms in the alkane (i.e., that the relative
frequency for the loss of H from a secondary position is
independent of the size of the alkane and a factor of 3.24 greater
than for loss from a primary position). The yields given in Tables
2 and 3 correspond to similar relative frequencies of 3.6 and
3.1 for the cases of 2,4-dimethylpentane and 3-ethylpentane,
respectively. The relative frequency for the production of
secondary and primary radicals is clearly not strongly dependent
on the structure of the alkane.
The yields of tertiary radicals from the three hydrocarbons
studied are high, as expected from the relatively low energy of
a tertiary C-H bond.17 For 3-ethylpentane, the yields of the
tertiary and primary radicals are in the ratio of 1.65 so that,
taking into account the 9-fold greater number of primary H
atoms, the relative frequency for the loss of a tertiary H is 14.9
times that for the loss of a primary H. For 2,3-dimethylbutane
and 2,4-dimethylpentane, this relative frequency is somewhat
lowers10.2 for each of the two tertiary H atomssindicating
competition between the tertiary positions. Hydrogen atoms
should, of course, mainly abstract at the tertiary position and
will contribute to the observed selectively. However, because
the H-atom yield is estimated to be only ∼1.5,18 H abstraction
alone cannot explain the observed selectivity in the yields of
C-H rupture. It is clear that there is considerable selectivity in
the initial bond rupture processes.
Acknowledgment. The research described herein was sup-
ported by the Office of Basic Energy Sciences of the U.S.
Department of Energy. This is contribution NDRL-4452 from
the Notre Dame Radiation Laboratory. We also acknowledge
the support of the Hungarian Research Fund (OTKA T 037294).
References and Notes
(1) Wojnarovits, L.; Schuler, R. H. J. Phys. Chem. A. 2000, 104, 1346.
(2) Holroyd, R. A. J. Phys. Chem. 1966, 70, 1341.
The yields for radicals produced by the rupture of the central
C-C bonds are similar: 1.74 for 2,3-dimethylbutane, 1.61 for
2,4-dimethylpentane, 1.58 for 3-ethylpentane, and somewhat
higher than the total yield of 1.3 for C-C scission in the normal
alkanes.1 The rupture of a central C-C bond is only marginally
affected by groups that are remote from that bond. For 2,3-
dimethylbutane and 2,4-dimethylpentane, the yields for the
rupture of a terminal C-C bond are high, as expected from the
previous study that showed that methyl radical production was
strongly enhanced by the presence of adjacent methyl groups.5
As a result, fragmentation accounts for 40% of the radicals as
compared to 25% in the case of the normal alkanes. For
3-ethylpentane, the methyl radical production is low, as expected
from the previous study. In this case, the yield of ethyl radicals
is 60% greater than the yield of 0.5 expected from its yield
from n-heptane.1 There appears to be a modest effect of the
adjacent ethyl groups analogous to the effect of methyl
substitution noted in ref 7.
(3) Holroyd, R. A. J. Am. Chem. Soc. 1966, 88, 5381.
(4) All ratios of bond rupture given here have been normalized for the
number of equivalent bonds in the hydrocarbon.
(5) Schuler, R. H. J. Phys. Chem. 1958, 62, 37.
(6) Schuler, R. H.; Foldiak, G. J. Phys. Chem. 1978, 82, 2756.
(7) Schuler, R. H.; Kuntz, R. R. J. Phys. Chem. 1963, 67, 1004.
(8) Dewhurst, H. A. J. Am. Chem. Soc. 1958, 80, 5607.
(9) Castello, G.; Grandi, F.; Munari, S. Radiat. Res. 1971, 45, 399.
(10) Castello, G.; Grandi, F.; Munari, S. Radiat. Res. 1974, 58, 176.
(11) Castello, G.; Grandi, F.; Munari, S. Radiat. Res. 1975, 62, 323.
(12) Ref 1, Supporting Information.
(13) Kimura, K.; Nagakura, S. Spectrochim. Acta 1961, 17, 166.
(14) Scoggins, M. W.; Miller, J. W. Anal. Chem. 1966, 38, 612.
(15) Shkrob, I. A.; Trifunac, A. D. Radiat. Phys. Chem. 1995, 43, 83.
(16) Wojnarovits, L.; Schuler, R. H. To be submitted for publication.
(17) The primary, secondary, and tertiary bond energies are ap-
proximately 98, 94, and 92 kcal mol-1. Benson, S. W. Thermochemical
Kinetics; John Wiley and Sons: New York, 1976.
(18) Radiolysis of Hydrocarbons; Foldiak, G., Ed.; Elsevier: Amsterdam,
1981.
(19) Ausloos, P.; Lias, S. G. Far Ultraviolet Photochemistry of Organic
Compounds. In Chemical Spectroscopy and Photochemistry in the Vacuum
UltraViolet; Sandorfy, C., Ausloos, P., Robin, M. B., Eds.; NATO Advanced
Study Institutes Series C; Reidel: Dordrecht, The Netherlands, 1974; p 465.
(20) Rothman, W.; Hirayama, F.; Lipsky, S. J. Chem. Phys. 1973, 58,
1300.
In general, photochemical studies have shown that that excess
energy in excited alkanes is highly localized around tertiary
carbon atoms and leads to the preferential rupture of C-H and