Infrared spectroscopic studies on photolysis of ethyl iodide in solid parahydrogen

Article abstract of DOI:10.1021/jp961911r

The photolysis of ethyl iodide in solid parahydrogen leads to the formation of ethyl radical, ethylene, and ethane upon near-UV illumination at about 5 K which are characterized by vibrational spectroscopy. The mechanism of the formation of the products is elucidated consistently. Two kinds of ethylene are discriminated spectroscopically which show distinctly different temporal behaviors during illumination and standing under dark. One of them is attributed to a complex between ethylene and iodine atom. The present work demonstrates that the cage effect is insignificant in the solid parahydrogen matrix, and a variety of elementary reactions of in situ photolysis can be studied in detail in contrast to conventional rare gas matrices.

Full text of DOI:10.1021/jp961911r

522
J. Phys. Chem. A 1997, 101, 522-527
Infrared Spectroscopic Studies on Photolysis of Ethyl Iodide in Solid Parahydrogen
Norihito Sogoshi, Tomonari Wakabayashi, Takamasa Momose, and Tadamasa Shida*
DiVision of Chemistry, Graduate School of Science, Kyoto UniVersity, Kyoto 606-01, Japan
ReceiVed: June 27, 1996; In Final Form: September 17, 1996X
The photolysis of ethyl iodide in solid parahydrogen leads to the formation of ethyl radical, ethylene, and
ethane upon near-UV illumination at about 5 K which are characterized by vibrational spectroscopy. The
mechanism of the formation of the products is elucidated consistently. Two kinds of ethylene are discriminated
spectroscopically which show distinctly different temporal behaviors during illumination and standing under
dark. One of them is attributed to a complex between ethylene and iodine atom. The present work demonstrates
that the cage effect is insignificant in the solid parahydrogen matrix, and a variety of elementary reactions of
in situ photolysis can be studied in detail in contrast to conventional rare gas matrices.
Introduction
used for beam splitter and optical windows, respectively. A
brief description of the procedure is given below. Parahydrogen
gas containing less than 0.1% orthohydrogen was mixed with
0.008% ethyl iodide at room temperatures and then introduced
to a copper cell attached to the base of a cryostat kept at 8.2-
8.3 K. An optically transparent solid sample was grown in the
cell by continuously flowing the gas into the cell for about 1 h.
The optical measurement was carried out at about 5 K, using
the same Nicolet FTIR spectrometer of a resolution of 0.25 cm-1
and the MCT detector as before.
Photochemistry and photophysics of alkyl iodides have been
subjected to a number of investigations as reviewed in our
previous study on methyl iodide.1 In the previous work we
had shown that solid parahydrogen is a useful matrix for
characterizing photoproducts by infrared absorption spectros-
copy.1,2 The matrix allows photofragments to be separated and
trapped, giving highly resolved vibrational spectra.1,2 This is
in contrast to conventional rare gas and other matrices where
in situ photolysis of the alkyl iodides “all failed undoubtedly
due to the cage effect”.3 Thus, Pacansky et al., e.g., had to use
a special precursor, C2H5COO-OOCC2H5, to obtain ethyl
radical by in situ photolysis in an argon matrix.4-6 Apparently,
the spacer molecule of the CO2 byproduct prevented two ethyl
radicals from recombining.
This difference between the parahydrogen and conventional
matrices can be ascribed to the extreme softness, shown by the
very high isothermal compressibility,7 and to the exceedingly
high thermal conductivity,8 both being special properties of a
quantum solid. The softness originating from the large ampli-
tude of the zero point lattice vibration allows easy separation
of the alkyl radical and iodine atom, while the high thermal
conductivity facilitates rapid cooling to stabilize the separated
photofragments. These properties can be used for characterizing
elementary reactions which is otherwise difficult. An equally
important advantage of the parahydrogen matrix is that the
spectroscopic resolution can be much higher than in other
matrices, as reviewed by Oka9 and as shown in our studies on
molecules such as CH4,10 CD3H,11 and methyl radical.12
In this work we report a photolytic study on ethyl iodide as
a natural extension of the previous work on methyl iodide to
broaden the understanding of reactions occurring in the cryo-
genic matrix at about 5 K. Aided by gas phase studies13-16
three products, ethyl radical, ethylene, and ethane, are identified
by infrared spectroscopy. An unexpected finding is the forma-
tion of two spectroscopically different ethylenes. One of them
shows remarkable spectral changes under illumination and dark
standing. The whole reaction scheme is elucidated consistently.
Spectral Assignment
Figures 1-3 demonstrate the spectral regions where changes
were observed upon UV illumination (spectra II-IV), subse-
quent standing under dark (spectrum V), reillumination (spec-
trum VI), and annealing at 8.0 K for 5 min and recooling to
5.0 K (spectrum VII). Spectrum I is for the sample before
illumination. The seven spectra in Figures 1-3 are chosen
representatively, to avoid congestion, from a number of spectra
recorded. Figure 3 shows the three spectral regions to be
ascribed to ethylene. The spectra are expanded to demonstrate
detailed differences between two kinds of ethylene discussed
later.
Figure 4 shows the temporal change of the integrated spectral
intensity of the species designated in the figure. The intensities
are normalized to the strongest absorption for each of the 14
measurements. The lines drawn in the figure are for clarity
only and not fitted to any known curve. The spectra in Figures
1-3 and the plots in Figure 4 are for a typical experiment in
which ethyl iodide was almost exhausted and ethyl radical and
ethylene attained their maxima at the end of the first period of
illumination (Figure 4). If the illumination was continued
beyond this point, the yield of the radical declined. The
scattering of the plots in Figure 4 was due to the spectral overlap
and the difficulty of separation of spectral signals from the
background. The spectral assignment was made by referring
to the literature and by internal consistency among the observed
spectra.
Ethyl Iodide. Spectra I in Figures 1-3 are, of course, due
to the intact ethyl iodide. Reference to the gas phase spectrum
leads to the following correspondence between the present and
the literature values in parentheses,15 all in units of cm-1: Figure
1, 3021.9 (3025), 2988.0 (2989), 2982.4 (2987), 2967.9 (2973);
Figure 2, 1218.3 (1208), 1208.3 (1201); Figure 3, 955.5 (957).
In addition, we detected ethyl iodide peaks at 2926.5 (2932),
Experimental Section
The experimental procedure is described in our previous
work1,2 with the exception that methyl iodide is replaced with
ethyl iodide from Nacalai Tesque, Inc., and KBr and BaF2 are
X Abstract published in AdVance ACS Abstracts, December 15, 1996.
S1089-5639(96)01911-1 CCC: $14.00 © 1997 American Chemical Society

Photolysis of Ethyl Iodide in Solid Parahydrogen
J. Phys. Chem. A, Vol. 101, No. 4, 1997 523
Figure 2. Infrared absorption spectra of ethyl iodide in solid
parahydrogen, 1150-1250 cm-1. See the caption for Figure 1. In
spectrum I, ethyl iodide (1218.3, 1208.3 cm-1). The sharp peak at
1167.3 cm-1 and the huge absorption at ∼1250 cm-1 through ∼1165
cm-1 in spectra I-VII are due to the intrinsic absorption of the matrix
(see the text).
Figure 1. Infrared absorption spectra of ethyl iodide in solid
parahydrogen, 2960-3130 cm-1. The seven spectra are chosen
representatively from the total fourteen spectra. Spectrum I is as
deposited, II is the same as I after 155 min UV illumination, III is
after 270 min UV illumination, and IV is after 420 min UV illumination.
Spectrum V is the same as IV after 6 h standing under dark. Spectrum
VI is the same as V after 180 min UV reillumination. Spectrum VII is
the same as VI after the sample was annealed at 8.0 K for 5 min and
then recooled to 4.99 K. The temperature of spectral measurement
varied from 4.99 to 5.27 K, but the fluctuation within this range did
not affect the spectra quality. The scale of the ordinate is common to
all of the spectra which are shifted arbitrarily for legibility. The spectral
assignment (cm-1) is as follows (see text); ethyl iodide (I), 3021.9,
2988.0, 2982.4, 2967.9; ethyl radical (R), 3122.8, 3032.6, 2984.3; the
high-frequency ethylene (EH), 3101.0, 2984.2; the low-frequency
ethylene (EL), 3099.1, 2984.2; and ethane (E), 2979.3.
tion serves merely as the background of the absorption of ethyl
iodide.
Ethyl Radical. Guided by the proximity to the literature
frequencies4-6,17 and by inference from our study on methyl
radical which showed a tendency for the radical to disappear
on annealing,1 we obtained the following correspondence
between the present and the literature frequencies (cm-1) in
parentheses:6,17 Figure 1 (spectra II-VI), 3122.8 (3112)6 and
(3114),17 3032.6 (3033)6 and (3036),17 and 2984.3 (2987).6
Among the above three the one at 2984.3 cm-1 seems to
nearly coincide, by accident, with the absorption of ethylene at
1450.2 (1462), 1446.9 and 1445.9 (1447), 1381.4 and 1380.0
(1385), and 1046.6 (1049), but the spectra are not shown to
save space.
2984.2 cm-1
.
Outside the wavenumber region in Figure 1, weak absorption
peaks were detected at 2879.1, 2812.1, and 2716.5 cm-1 whose
temporal behavior is similar to that of the above three peaks as
shown in Figure 4. In ref 6 seven more peaks are reported at
2920, 2842, 1440, 1366, 1175, 1138, and 540 cm-1; similarly,
in ref 17 eight, at 2844, 1442, 1383, 1369, 1185, 1133, 1025,
and 532 cm-1. Since the lowest attainable frequency in the
present work is 910 cm-1, we cannot compare the reported 540 6
and 532 17 cm-1 with the present work. As for the remaining
six6 and seven,17 we failed to detect the corresponding peaks.
A reinvestigation of spectroscopic assignment of the radical is
now under way.18
Ethylene. Ethyl radical in the gas phase photodecomposes
to ethylene and hydrogen atom upon near-UV illumination.13,14
Thus, we expect the formation of ethylene in the present work
also. For reference, we measured the absorption spectrum of
ethylene in parahydrogen (0.004%, 4.90 K). The complete
The intensity of the absorption of ethyl iodide decreased in
an approximately exponential way upon illumination, as seen
from the plots in Figure 4. Only well-separated and relatively
intense peaks were selected for plotting (see caption of Figure
4).
A remark on Figure 2 showing the absorption of not only
ethyl iodide but also parahydrogen may be appropriate at this
moment. Except for the rapidly decaying absorption of ethyl
iodide peaking at 1218.3 and 1208.3 cm-1, the absorption
between about 1250 and about 1165 cm-1 remains constant
through the light and dark periods. The sharp peak at 1167.3
cm-1 is ascribed to the zero-phonon band of the solid para-
hydrogen corresponding to the U-band of the freely rotating
hydrogen molecule, i.e., due to the transition of v′ ) 0, J′ ) 4
r v′′ ) 0, J′′ ) 0. The rest of the absorption is the
accompanying phonon band.9 For the present work the absorp-

524 J. Phys. Chem. A, Vol. 101, No. 4, 1997
Sogoshi et al.
Figure 3. Infrared absorption spectra reproduced for the three regions of ethylene. See the caption for Figure 1 for the spectral numbers I-VI.
Peaks at 3101.0, 1440.6, and 957.3 cm-1 are assigned to the high-frequency ethylene, while those at 3099.1, 1440.0, and 948.6 cm-1 are to the
low-frequency ethylene. The quickly decaying peak at 955.5 cm-1 in spectra I-III is due to ethyl iodide.
isolation of ethylene in the solid parahydrogen matrix was not
yet been attained, but strong peaks were observed at 3098,
2981.8, 1442-1435, and 958 cm-1. These can be compared
with the gaseous frequencies of 3106 (ν9 (b2u) CH2 a-str), 2989
(ν11 (b3u) CH2 s-str), 1444 (ν12 (b3u) CH2 sciss), and 949
Reaction Mechanism
The observed spectral changes were analyzed separately in
the order of the first period of continual illumination, the
subsequent standing under dark, the reillumination, and the final
thermal annealing.
First Illumination Period. The reactant ethyl iodide decays
exponentially whereas the radical grows approximately in an
exponential fashion, as shown in Figure 4. This suggests
reaction 1.
(ν7 (b1u) CH2 wag) cm-1 19
.
With these reference data and with the consideration that the
absorption of the stable ethylene should not vanish as easily as
radicals on annealing, we ascribe the following observed
absorption to ethylene: Figure 1 (spectra II-VI), ∼3103 to
∼3096 and 2984.2; Figure 3 (spectra II-VI), ∼3103 to ∼3096
peaking at 3101.0 (EH), 3099.1 (EL), ∼1441 to ∼1439 peaking
at 1440.6 (EH) and 1440.0 (EL), ∼960 to ∼945 peaking at 957.3
(EH) and 948.6 (EL).
C2H5I + hν f ‚C2H5 + ‚I
(1)
Ethyl radical is known to absorb near-UV light,20 and
“conclusive evidence” for the methyl CsH bond cleavage of
the radical is reported for the photolysis with 248 nm photons.14
Therefore, the following reaction will take place while reaction
1 is in progress.
The suffixes H and L stand for “high” and “low” frequencies
and represent two families of absorption peaks to be ascribed
in the next section to two complexes of ethylene. Despite the
closeness of the frequencies of the two ethylenes their temporal
behavior is distinctly different, as shown in Figure 4. This
apparent peculiarity will also be discussed in the next section.
‚C2H5 + hν f ‚H + CH2dCH2
(2)
Because of the absorption of two photons in succession, the
production of the ethylene from reactions 1 and 2 should be
slow at the beginning. Comparison of the lower two panels in
Figure 4 immediately suggests that the ethylene classified as
the low-frequency ethylene is identified as such ethylene. It is
noted that this ethylene should accompany the iodine atom
produced by reaction 1.
Before the discussion on the reaction mechanism is continued,
a preparatory pause is placed here for further discussion: The
spectral line shape of ethyl iodide in spectrum I (Figures 1-3)
suggests incomplete isolation of the iodide in the matrix. In
particular, the absorption peaking at 955.5 cm-1 in Figure 3
shows a “tail” toward lower frequencies which indicates the
presence of clusters of the iodide. The size of the clusters is
likely small, as the absorption of crystalline ethyl iodide in the
p-H2 matrix is significantly red-shifted and broad with a typical
Ethane. Figure 1 shows that the broad absorption at about
2980 cm-1 continues to grow in spectra II-VII, indicating that
the absorption is due to a stable photoproduct. Since the most
probable candidate is ethane, the spectrum of ethane in
parahydrogen was measured to obtain a matching peak at about
2980 cm-1. In this spectral range of 2980 ( 10 cm-1 we have
already assigned peaks to ethyl iodide (2988.0, 2982.4 cm-1),
ethyl radical (2984.3 cm-1), and ethylene (2984.2 cm-1).
However, since the absorption due to ethyl iodide decays quickly
as shown in Figure 4 and since the absorption of ethyl radical
and ethylene is sharp compared with the growing broad
absorption attributed to ethane, the temporal change of the
growing absorption at 2979.3 cm-1 is easily recognized and
plotted in Figure 4.

Photolysis of Ethyl Iodide in Solid Parahydrogen
J. Phys. Chem. A, Vol. 101, No. 4, 1997 525
previous study on methyl iodide in solid parahydrogen in
conformity with reaction 3 which suggests the presence of the
dimer in the parahydrogen matrix.1,2
Thus, a simple analogy from the molecular beam experiment
would predict the formation of ethyl radical (•R) and butane
(R2) in the ethyl iodide/p-H2 system of the present study.
However, no absorption due to butane21 was detected despite a
thorough survey of the spectrum. Instead, the absorption of
ethane and the high-frequency ethylene increased monotonously
(see Figures 1 and 4). Therefore, in the ethyl iodide/p-H2
system, disproportionation rather than dimerization seems to be
favored; that is, the product corresponding to R2 in reaction 3
is equal to the products of reaction 4.
2‚C2H5 f CH2dCH2 + C2H6
(4)
For dimerization to occur, two heavy C2 units must approach
each other, whereas for disproportionation only the hydrogen
atom transfer is required which may be preferred by tunneling
in the solid parahydrogen at about 5 K. The ethylene produced
by reaction 4 should accompany ethane and iodine molecules
produced molecularly by reaction corresponding to (3). On this
basis the low- and high-frequency ethylenes are regarded as two
different charge transfer complexes of ethylene with the iodine
atom and molecule, respectively. However, since the absorption
of both ethylenes is close to that in the gas phase19 as well as
in argon22 and xenon23 matrices, the perturbation due to the
accompanying iodine atom for the low-frequency ethylene and
that due to the accompanying iodine and ethane molecules for
the high-frequency ethylene must be small. Nevertheless, we
expect a larger interaction for the low-frequency ethylene in
view of the well-known complexation between π electronic
systems and a halogen atom.24 In other words, the high-
frequency ethylene is more like the free ethylene and the low-
frequency ethylene is more affected by the open-shell iodine
atom. Since the absolute absorbance of both ethylenes will not
be significantly different, the appearance of both of them to
the same order of magnitude (see Figures 1 and 3) implies that
their yields are similar. This, in turn, implies that the precursors
of both ethylenes, that is, monomeric and dimeric ethyl iodide,
are present to the same order of magnitude in the system.
Dark Period. Figure 4 demonstrates that ethyl iodide is
almost exhausted at the beginning of the dark period when the
yield of ethane is still less than about 45% of that at the end of
the annealing period. Since the iodide should be the seminal
source for ethane whatever the intermediate reaction path is,
the C2 unit originally in the iodide must be transferred to ethyl
radical and ethylene also, which would eventually yield the
additional ethane. Unfortunately, the lack of information on
the absolute absorbance of each entity prevents stoichiometric
analysis, but the qualitative features of the marked, moderate,
and very little, if any, decreases of the low-frequency ethylene,
ethyl radical, and the high-frequency ethylene, respectively, are
obvious. In particular, the low-frequency ethylene decays
rapidly in the dark period and recovers quickly in the subsequent
reillumination period. This feature is shown in Figure 3 for
the three spectral regions of ethylene. From Figure 3 it is easy
to see the parallel behavior of the low- and high-frequency
ethylenes among the three spectral regions.
Figure 4. Temporal changes of the absorption intensity for ethyl iodide,
ethane, ethyl radical, the low-frequency ethylene, and the high-frequency
ethylene. The integrated intensities are normalized to the strongest
absorption for each peak. The horizontal arrows atop are common to
all four panels. They indicate the first period of continual illumination,
the subsequent period under dark, the subsequent period of reillumi-
nation, and the period of annealing to 6.0, 7.0, 8.0, and 9.0 K, each for
5 min standing and then recooling to ∼5.0 K. The various symbols for
plotting represent absorption peaks of the five species as follows. Top
panel for ethyl iodide and ethane: Open circles, ethyl iodide at 3021.9
cm-1; open circles with horizontal bar, sum of the ethyl iodide peaks
at 1381.4 and 1380.0 cm-1; open circles with vertical bar, ethyl iodide
at 1218.3 cm-1; open circles with slant bar, ethyl iodide at 955.5 cm-1
;
solid squares, ethane at 2979.3 cm-1. Second panel for ethyl radical:
Open circles, 3122.8 cm-1; open circles with horizontal bar, 3032.6
cm-1; open circles with vertical bar, 2879.2 cm-1. Third panel for the
low-frequency ethylene: Open circles, 3099.1 cm-1; open circles with
horizontal bar, 1440.0 cm-1; open circles with vertical bar, 948.6 cm-1
.
Bottom panel for the high-frequency ethylene: Open circles, 3101.0
cm-1; open circles with horizontal bar, 1440.6 cm-1; open circles with
vertical bar, 957.3 cm-1
.
bandwidth of 10-20 cm-1 at full width at half-maximum
(fwhm, data not shown). Meanwhile, the photolysis of ethyl
iodide dimers in molecular beams was reported by Donaldson
et al.,16 showing that the dimer yields an iodine molecule by a
one photon process, as in the case of methyl iodide cited in our
previous work.1 They proposed a concerted reaction for alkyl
iodide dimers
The rapid decay and the quick recovery of the absorption of
the low-frequency ethylene in the dark and the reillumination
periods, respectively, implies that the ethylene is transformed
into a photosensitive species during the dark period, which
resumes the low-frequency ethylene upon reillumination. Since
we assume that the iodine-atom-complexed low-frequency
ethylene is produced by reactions 1 and 2, the ethylene unit
(RI)2 + hν f 2‚R and/or R2 + I2
(3)
In fact, both methyl radical and ethane were detected in our

526 J. Phys. Chem. A, Vol. 101, No. 4, 1997
Sogoshi et al.
may react, during the dark period, with a hydrogen atom
photogenerated by reaction 2 as follows.
Annealing Period. The continued increase of ethane and
the rapid decay of ethyl radical in Figure 4 may be understood
if reaction 6 is favored at the elevated temperatures, which is
plausible. Since there is no sign of the regeneration of ethyl
iodide and the appearance of absorption of butane,21 neither the
reverse reaction of 1 nor the dimerization of the radical takes
place. As for the two ethylenes in the annealing period,
broadening and distortion of the spectrum prevent meaningful
plotting in Figure 4 so that they are omitted.
CH2dCH2 + ‚H f ‚C2H5
(5)
According to reaction 5 ethyl radical should increase with
the decrease of the low-frequency ethylene. However, Figure
4 shows the decrease of the absorbance of ethyl radical during
the dark period. Figure 4 also demonstrates a continuous
increase of ethane and a slight decrease of the high-frequency
ethylene under dark. In order to elucidate these results the
following mechanism is proposed on the basis of the reported
migration of hydrogen atom in solid parahydrogen at 4.2 K:25
Some of the hydrogen atoms photogenerated by reaction 2 may
escape into the bulk, where some of them recombine to
hydrogen molecule and some encounter ethyl radical to result
in reaction 6.
Concluding Remarks
As in the previous work on methyl iodide, photoreactions
and thermal reactions in the ethyl iodide system are pursued
by observing the vibrational spectroscopic changes. The system
is relatively simple, involving only ethyl iodide, ethyl radical,
ethane, and ethylene. Yet, new findings are obtained such as
the formation of two kinds of ethylene and the reactions
involving the migrating hydrogen atom. These reactions may
have relevance to molecular evolution on hydrogen covered cold
surfaces of dusts in interstellar space. The cryogenic reactions
may also be enticing to theoretical chemists for the study of
tunneling reaction. An important advantage of the solid
parahydrogen matrix is that we are free from the cage effect
inherent in conventional rare gas matrices. This allows the study
of in situ photolytic reactions in detail by means of vibrational
spectroscopy. Utilizing this advantage, we are aiming at
improving the versatility of matrix isolation spectroscopy.
A remaining problem is that no absorption was detected in
the region where hydrogen iodide in several matrices is reported
to absorb, i.e., 2255-2230 cm-1 in argon,27,28 2240-2210 cm-1
in krypton,28 and 2238 cm-1 in nitrogen.28 In this context a
statement is noted that “the absorbance of monomer HI appears
to be significantly weaker than the absorbance of HI that forms
a hydrogen bond to another molecule. It is therefore difficult
to observe monomer HI in a matrix without significant dimer
absorptions.”27
Another comment will be made on reaction 5; Klein and
Scheer did not mention anything about the prototype reaction
5 in their classic work on reactions between various alkenes
and hydrogen atom at temperatures above 77 K, which may be
allusive to the absence of appreciable reaction between ethylene
and hydrogen atom under their experimental conditions.29
However, the environment in the present work is considerably
different from that of the previous workers so that there may
not be serious conflict between the two studies.
Finally, we note the difference of the behavior of methyl
radical in the previously studied methyl iodide/p-H2 system1,2
versus the behavior of ethyl radical in the present system. The
methyl radical in the methyl iodide/p-H2 system did not show
noticeable decay under dark during the experimental time scale,
which is in contrast to the present system. Further comparative
studies on the two systems are now under way.
‚C2H5 + ‚H f C2H6
(6)
The hydrogen atom may also encounter the high-frequency
ethylene produced by reaction 4 to cause the same reaction as
5. However, reaction 6 would proceed more easily than reaction
5 because the former is a barrier free reaction. If the absolute
absorbance of ethyl radical is large, the decrease of observed
absorbance of ethyl radical due to reaction 6 may surmount the
increase due to reaction 5 which proceeds with a barrier. Since
the hydrogen atom produced by reactions 1 and 2 will find near-
lying ethylene of reaction 2 more easily than far-lying ethylene
of reaction 4, the low-frequency ethylene will decrease more
easily than the high-frequency ethylene. The fast and slow
decreases of the low- and high-frequency ethylenes during the
dark period may reflect this difference of the reaction efficiency.
Reillumination Period. The quick recovery of the low-
frequency ethylene shown in Figure 4 (see also Figure 3) is
attributed to reaction 2 coupled with reaction 1, resulting in the
complex between ethylene and iodine atom. After the con-
sumption of ethyl iodide the absorption of 253.7 nm photons
by the ethyl radical in reaction 2 becomes efficient, which causes
the excitation of the radical to a 3s Rydberg state.20 The
recovery of the low-frequency ethylene should be related to the
noticeably accelerated decrease of ethyl radical (compare the
second and third panels of Figure 4). The continued increase
of ethane in the top panel of Figure 4 is attributable to reaction
6, which should proceed irrespective of the light and dark
periods.
As for the high-frequency ethylene, which has survived until
the end of the dark period, it starts to decrease appreciably as
is seen from Figure 4 (see also Figure 3). Simultaneously, the
low-frequency ethylene continues to increase overpassing the
maximum in spectrum IV of Figure 3. Since the low-frequency
ethylene originating from monomeric ethyl iodide in reaction
1 is not available limitlessly, the continued increase of the low-
frequency ethylene from spectrum V to VI of Figure 3 must be
associated with other remaining C2 carriers, that is, the high-
frequency ethylene. With the depletion of ethyl iodide and ethyl
radical, both absorbing 253.7 nm photons, the absorption by
iodine molecule produced from the molecular reaction corre-
sponding to 3 will become more and more significant. The
absorption of iodine molecule is well-known to cause photolysis
into two iodine atoms.26 The iodine atom will find the nearby
ethylene from reaction 4, i.e., the high-frequency ethylene, and
will form a complex between the ethylene and the iodine atom,
which is nothing other than the low-frequency ethylene. This
mechanism explains the increase of the low-frequency ethylene
as shown in spectrum VI of Figure 3.
In the present work, dedicated to Professor Saburo Nagakura,
we put emphasis on dynamical aspects of electronically excited
ethyl iodide. There remain, however, several points worthy of
further investigation on the present system from the viewpoint
of vibrational spectroscopy. These include (1) the elucidation
of the aforementioned incomplete agreement of the vibrational
spectrum of ethyl radical between the present work and the
literature,6,17 (2) the broad radical absorption at 3122.8 in
contrast to the very sharp ones at 3032.6, 2984.3, 2879.1, 2812.1,
and 2716.5 cm-1 as well as the broadness of ethane absorption,
and (3) the conspicuously large spectral shift between the low-
and the high-frequency ethylenes at 948.6 vs 957.3 cm-1
assigned to an in-plane C-H scissoring mode of the ethylenic
unit in the complex, to name a few.

Photolysis of Ethyl Iodide in Solid Parahydrogen
J. Phys. Chem. A, Vol. 101, No. 4, 1997 527
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Acknowledgment. The authors are most grateful to Professor
Eizi Hirota for his encouragement throughout the present work.
The work was supported by the Grants-in-Aid for Scientific
Research on Priority Areas No. 05237105 and the Grant (A)
No. 07404034 of the Ministry of Education, Science, Culture,
and Sports, Japan.
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References and Notes
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To be published.