Photoinduced reactions of methyl radical in solid parahydrogen

Article abstract of DOI:10.1063/1.477276

Photolysis of methyl iodide in solid parahydrogen (p-H₂) at about 5 K is studied with ultraviolet light at 253.7 and 184.9 nm. It is found that the light at 253.7 nm produces only methyl radical, whereas the light at 184.9 nm yields both methyl radical and methane. The mechanism of the formation of the photoproducts is elucidated by analyzing the temporal behavior of the observed vibrational absorption. It is concluded that methyl radical in the ground state does not react with p-H₂ molecules appreciably but that the radical in the electronic excited state of B(²A₁^′), accessible by reabsorption of 184.9 nm photons by the radical, decomposes to a singlet methylene CH₂ a(¹A₁) and a hydrogen atom (²S) and that the singlet methylene reacts with a p-H₂ molecule to give methane.

Full text of DOI:10.1063/1.477276

Photoinduced reactions of methyl radical in solid parahydrogen
Mizuho Fushitani, Norihito Sogoshi, Tomonari Wakabayashi, Takamasa Momose, and Tadamasa Shida
Citation: The Journal of Chemical Physics 109, 6346 (1998); doi: 10.1063/1.477276
View online: http://dx.doi.org/10.1063/1.477276
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 109, NUMBER 15
15 OCTOBER 1998
Photoinduced reactions of methyl radical in solid parahydrogen
Mizuho Fushitani, Norihito Sogoshi, Tomonari Wakabayashi, Takamasa Momose,
and Tadamasa Shida
Division of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
͑Received 27 April 1998; accepted 13 July 1998͒
Photolysis of methyl iodide in solid parahydrogen (p-H2) at about 5 K is studied with ultraviolet
light at 253.7 and 184.9 nm. It is found that the light at 253.7 nm produces only methyl radical,
whereas the light at 184.9 nm yields both methyl radical and methane. The mechanism of the
formation of the photoproducts is elucidated by analyzing the temporal behavior of the observed
vibrational absorption. It is concluded that methyl radical in the ground state does not react with
2
˜
p-H2 molecules appreciably but that the radical in the electronic excited state of B( A1Ј), accessible
1
˜
by reabsorption of 184.9 nm photons by the radical, decomposes to a singlet methylene CH2 a( A1)
and a hydrogen atom (2S) and that the singlet methylene reacts with a p-H2 molecule to give
methane. © 1998 American Institute of Physics. ͓S0021-9606͑98͒00139-1͔
I. INTRODUCTION
II. EXPERIMENT
The experimental procedure is the same as in our previ-
ous work7,8 and is elaborated in a review article.3 Briefly,
normal hydrogen gas was converted to p-H2 which con-
tained about 0.01% residual orthohydrogen. The converted
gas was mixed with ϳ0.02% CH3I at room temperature. The
mixed gas was introduced into a cylindrical copper cell of
dimensions 5 to 10 cm long and 2 cm in diameter attached
with BaF2 windows at both ends of the cylinder. The tem-
perature of the cell was kept at 8.1Ϯ0.2 K during the intro-
duction of the sample gas for some 1.5 h. The temperature
during the crystal growth was slightly lower than the previ-
ous work7,8 in order to avoid the formation of iodide cluster
in solid p-H2 as much as possible.6,7 Then, the temperature
was slowly lowered to 4.5Ϯ0.1 K and kept at the same tem-
perature throughout the experiment. A low pressure 20 W
mercury lamp was used for the photolysis, which emitted
UV light at 253.7 and 184.9 nm and weak lines in the visible
region. To effect the selective photolysis at 253.7 nm, a cut-
off filter Toshiba UV-25 was used. Since the filter transmit-
ted wavelengths longer than 200, light at 184.9 was com-
pletely cut while light at 253.7 nm was transmitted by about
60%. For the exclusive photolysis at 184.9 nm, a narrow-
band filter Acton Research Corporation 185-N-ID was used
which transmitted wavelengths between 169 and 220 nm.
The transmission at 184.9 nm was approximately 18.3%. The
infrared absorption of samples was observed using a Nicolet
Magna 750 Fourier-transform infrared ͑FTIR͒ spectrometer
with a resolution of 0.25 cmϪ1 in combination with a KBr
beam splitter and a liquid nitrogen cooled mercury cadmium
telluride ͓MCT ͑HgCdTe͔͒ detector. The experimental pro-
cedure for the photolysis of deuterated methyl iodide (CD3I)
was the same as that for CH3I.
Solid parahydrogen (p-H2) is a useful matrix in matrix-
isolation spectroscopy because it allows both high-resolution
rotation–vibration spectroscopy in favorable cases1–6 and ki-
netic studies of photolysis of embedded molecules.3,7–10 In a
series of our work on alkyl iodides in the p-H2 matrix, we
have shown that the iodide is easily photolyzed to alkyl radi-
cals and an iodine atom. This is in remarkable contrast to the
conventional rigid rare gas matrices. In the latter matrices,
the cage effect prevents the photofragments from being
separated.11–14 The advantageous feature of the mitigation of
the cage effect in the p-H2 matrix is attributed to the fact that
solid p-H2 forming a hexagonal close packed crystal is a
quantum solid having the following properties: ͑1͒ a large
lattice constant, ͑2͒ a large amplitude of the zero-point lattice
vibration, and ͑3͒ a large thermal conductivity.3 Due to the
properties of ͑1͒ and ͑2͒, solid p-H2 can be regarded as a
spacious and soft lattice for foreign molecules, and due to
͑3͒, the excess energy imparted to nascent photofragments is
quickly removed and the fragments are rapidly frozen to be
protected from geminate recombination.
On account of these properties, we have been successful
in photolyzing methyl iodide in the p-H2 matrix to obtain
methyl radical for its rotation–vibration spectral analysis.3,7,8
In these studies, using a low pressure mercury lamp, how-
ever, we obtained methane and ethane, in addition to methyl
radical. As for the mechanism of the formation of ethane, a
consistent explanation was put forth by referring to the result
of laser photolysis of methyl iodide in molecular cluster
beams, which invokes the involvement of methyl iodide
dimers.3,7,15–17 However, as to the production of methane,
there remained an ambiguity and we conjectured two
possibilities.8 The present work is intended to settle the ar-
gument on the formation mechanism of methane by examin-
ing the dependence of the formation of methane upon the
wavelength of ultraviolet ͑UV͒ light used for the photolysis.
III. EXPERIMENT RESULTS
Figure 1 shows part of the FTIR spectrum of methyl
iodide in solid p-H2 before ͑lower panel͒ and after UV irra-
diation ͑upper panel͒. The figure is reproduced from our pre-
0021-9606/98/109(15)/6346/5/$15.00
6346
© 1998 American Institute of Physics
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J. Chem. Phys., Vol. 109, No. 15, 15 October 1998
Fushitani et al.
6347
FIG. 1. Infrared absorption spectra of methyl iodide in solid p-H2 in the
spectral region of 2900–3200 cmϪ1. Bottom: the sample as deposited and
recorded before UV irradiation. Top: after a four-hour UV ͑253.7ϩ184.9
nm͒ irradiation without filters. The symbols I, R, M, and E stand for methyl
iodide, methyl radical, methane, and ethane, respectively.
vious paper.7 The sample was irradiated for 240 min with
full light from the low pressure mercury lamp described in
the preceding section. The absorption bands are assigned as
methyl iodide ͑I͒, methyl radical ͑R͒, methane ͑M͒, and
ethane ͑E͒.7,8 It is seen that methyl iodide is photolyzed com-
pletely to yield the three products. The mechanism of the
formation of ethane has been accounted for as a one-photon
concerted process of methyl iodide dimer7 and will not be
repeated in the present work. Instead, we will focus on the
mechanism of the formation of methane which was argued
rather arbitrarily in the previous work.8 It is found that the
absorption of the radical scarcely decays, if at all, in the dark
for a few days at about 4.5 K ͑see below, Fig. 4͒. Therefore,
the reaction of methyl radical in the electronic ground state,
CH3ϩH2→CH4ϩH, cannot be responsible for the remark-
able appearance of methane upon the four-hour irradiation
shown in Fig. 1. Thus, the observed methane must have been
produced by some photochemical process.
FIG. 2. Infrared absorption spectra of methyl iodide in solid p-H2 at about
4.5 K in the spectral region of 2900–3200 cmϪ1. Spectrum ͑a͒: the sample
as deposited and recorded before UV irradiation. Spectrum ͑b͒: after a 120
min UV ͑253.7 nm͒ irradiation through a cutoff filter, UV-25. Spectrum ͑c͒:
after a 240 min UV ͑253.7 nm͒ irradiation with the same filter as ͑b͒.
Spectrum ͑d͒: after the 240 min UV ͑253.7 nm͒ irradiation and a 90 min UV
͑184.9 nm͒ irradiation through a narrow-band filter, 185-N-D. Spectrum ͑e͒:
after the 240 min UV ͑253.7 nm͒ irradiation and a 170 min UV ͑184.9 nm͒
irradiation. The symbols I, M, and R stand for methyl iodide, methane, and
methyl radical, respectively.
light, while methane appeared retardedly at later stages of the
irradiation. Figure 3 shows the integrated absorption intensi-
ties of the degenerate C–H bending vibration (␯4) of methyl
iodide, methyl radical, and methane as a function of time of
In order to uncover the photochemical process, photoly-
sis with selective wavelengths was carried out. Traces ͑a͒–
͑c͒ in Fig. 2 show the spectra of CH3I in solid p-H2 upon the
irradiation with a filter UV-25, which cuts 184.9 light but
transmits 253.7 nm light, while traces ͑d͒ and ͑e͒ show the
spectral change with an additional irradiation through the
narrow-band filter, which transmits only 184.9 nm light. Al-
though the spectral data in Fig. 2 are rather noisy due to the
attenuation of the photon flux and the absorption of atmo-
spheric water, it is evident that in traces ͑a͒–͑c͒ the absorp-
tion of methyl iodide at approximately 2960–2975 cmϪ1 di-
minishes continuously and that the absorption of methyl
radical at around 3171 cmϪ1 appears, but that the absorption
of methane at about 3026 cmϪ1 does not. However, as is
seen in traces ͑d͒ and ͑e͒, the succeeding irradiation with
184.9 nm light induces the appearance of methane at about
3026 cmϪ1
.
We carried out another series of experiments in which a
CH3I/p-H2 system was irradiated exclusively by 184.9 nm
light by use of a narrow-band filter. As a result, it was found
that methyl radical was formed from the beginning of the
irradiation as in the case of the irradiation with 253.7 nm
FIG. 3. Temporal behavior of the degenerate C–H bending vibration (␯4) of
methyl iodide at 1244.6–1251.2 ͑circles͒, methyl radical at 1401.4–1403.0
͑triangles͒, and methane at 1306.7–1308.6 cmϪ1 ͑squares͒ upon irradiation
of 184.9 nm photons. The curves are the theoretical intensities I1(CH3I),
I2(CH3), and I3(CH4) in the text.
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6348
J. Chem. Phys., Vol. 109, No. 15, 15 October 1998
Fushitani et al.
the irradiation with 184.9 nm light. The scale of the ordinate
is normalized to the intensity of the iodide at tϭ0 and the
relative intensities of methyl radical and methane are ampli-
fied by a factor of ten for better legibility. It is seen that the
curve for methyl radical is slightly convexed while the curve
for methane concaved. The analysis of the curves in Fig. 3
will be made in the next section.
d CH I
͓
͔
3
ϭϪk1 CH I ,
͑1͒
͑2͒
͑3͒
͓
͔
3
dt
d CH
͓
͔
͔
3
4
ϭk1 CH I Ϫk CH ,
͓
͔
͓
͔
3
2
3
dt
d CH
͓
ϭk2 CH .
͓
͔
3
dt
Under the initial condition of CH
ϭ0, and CH
͓ ͔
4 tϭ0
͓
͔
3
tϭ0
IV. DISCUSSION OF THE REACTION MECHANISM
A. Mechanism consistent with experiment
ϭ0, the above equations can be solved immediately. As a
result, the time dependence of the intensities I(X) of the
three species X are obtained as in Eqs. ͑4͒–͑6͒, where I1 , I2 ,
and I3 are the constants proportional to the oscillator
strengths of CH3I, CH3 , and CH4 , respectively,
The results in the preceding section indicate that with the
253.7 nm irradiation only methyl radical is formed while
with the 184.9 nm irradiation both methyl radical and meth-
ane are produced. We propose the following mechanism of
reactions ͑I͒–͑III͒ to be consistent with the experiment.
UV light at both 253.7 and 184.9 nm produces methyl
I CH I͒ϭI eϪk t
,
͑4͒
1
͑
3
1
k1
k2Ϫk1
eϪk1tϪeϪk t͒,
͑
͑5͒
2
I CH ͒ϭI
͑
3
2
#
radical with an excess energy (CH3 ), but the radical is de-
activated to its ground state as in reaction ͑I͒. Since methyl
k2
k1
I CH ͒ϭI 1Ϫ
eϪk1tϩ
eϪk t
.
͑6͒
2
radical absorbs at wavelengths shorter than 216 nm, to be
͑
ͩ
ͪ
4
3
k2Ϫk1
k2Ϫk1
18,19
2
˜
exited to a 3s Rydberg state (B AЈ1),
the deactivated
The three curves in Fig. 3 are drawn by the least-squares
procedure with the best fitting parameters of I1ϭ1.000
Ϯ0.002, I2ϭ0.255Ϯ0.012, I3ϭ0.220Ϯ0.003, k1ϭ(2.30
radical reabsorbs light at 184.9 but not at 253.7 nm as in
˜
reaction ͑II͒. The excited radical in the B state dissociates
1
2
˜
into a singlet methylene (a A1) and a hydrogen atom ( S),
the former executing the insertion reaction with a hydrogen
molecule as in reaction ͑III͒,
Ϯ0.03)ϫ10Ϫ4 minϪ1, and k2ϭ(5.47Ϯ0.69)ϫ10Ϫ4 minϪ1
.
We may go a little further with the parameters obtained
above: Since the parameters I1ϪI3 are proportional to the
oscillator strengths f(X) of the observed band of the three
species, we have the relation I1 :I2 :I3Хf(CH3I,␯4):
f(CH3 ,␯4):f(CH4 ,␯4)ϭ1.00:0.26:0.22. The apparent rate
constants k1 and k2 should be proportional to the product of
the absorption coefficient ␧ and the quantum yield ␩ of pho-
tolysis in reactions ͑I͒ and ͑II͒ at 184.9 nm. Thus, we have
␧(CH3I͒␩͑CH3I͒:␧͑CH3͒␩͑CH3)ϭ2.3:5.5.
#
CH Iϩh␯ 253.7 nm, 184.9 nm͒→CH ϩI→CH ϩI, ͑I͒
͑
3
3
3
2
1
˜
˜
͑
2
*
CH ϩh␯ 184.9 nm͒→CH B AЈ͒→CH a A ͒ϩH,
͑
͑
3
3
1
1
͑II͒
1
˜
CH a A ͒ϩH →CH .
͑III͒
͑
2
1
2
4
The second step of reaction ͑II͒ is put forth because a
˜
subpicosecond dynamics study of the radical in the B state
20
1
In order to strengthen the credence of the above mecha-
nism we will examine other conceivable mechanisms ͑1͒–͑5͒
in the next section, all of which will be disproved in the end.
˜
has disclosed a rapid dissociation into CH2(a A1) and H.
Lee and his co-workers also investigated photodissociation
of methyl radical at 193.3 nm using photofragment transla-
tional spectroscopy and demonstrated unambiguously that
B. Mechanisms inconsistent with experiment
˜
the methyl radical in the B state dissociates into the singlet
1
2
˜
1. Reaction of kinetic methyl radical with hydrogen
molecule
methylene (a A1) and a hydrogen atom ( S) in conformity
with reaction ͑II͒.21 The barrier height of this dissociation in
the B state is estimated to be 2200 cm
Ϫ1 20
˜
#
.
Since the photon
One may conceive that the kinetic methyl radicals CH3
energy of 184.9 nm light ͑Х54 100 cmϪ1͒ is sufficiently
larger than the dissociation barrier height of 48 500 cmϪ1
measured from the ground state of methyl radical, there
should be an ample energetic allowance for the whole se-
quence of reaction ͑II͒. As for reaction ͑III͒, a theoretical
calculation predicts no barrier against the reaction,22 so that
reaction ͑III͒ is considered to be complete.
in reaction ͑I͒ undergo the collision-induced reaction ͑IV͒,
#
CH3 ϩH2→CH4ϩH.
͑IV͒
However, the failure of the appearance of methane in traces
͑a͒–͑c͒ in Fig. 1 means that 253.7 nm light cannot yield
methane by this mechanism, the reason for which is ac-
counted for as follows.
According to reactions ͑I͒–͑III͒, the temporal change of
the concentration of iodide CH I , methyl radical CH ,
The kinetic ͑translational͒ energy of methyl radical pro-
duced by reaction ͑I͒ is estimated to be crudely 15 000 cmϪ1
for the case of 253.7 nm light.23 Since the activation energy
of the thermal reaction between methyl radical and hydrogen
͓
͔
͓
͔
3
3
and methane CH under the irradiation with 184.9 nm light
͓
͔
4
should conform to the following kinetic equations, where t is
the irradiation time and k1 and k2 are the apparent rate con-
stants of photodissociation of methyl iodide and methyl radi-
cal, respectively. It is presumed that the 184.9 nm light is
absorbed by the iodide and the radical proportionally to their
concentrations,
molecule is known to be about 3700 cm ,
Ϫ1 24 which is by far
smaller than the above kinetic energy of the methyl radical,
the collision-induced reaction ͑IV͒ is energetically allowed.
Therefore, the failure of the formation of methane at 253.7
nm indicates that the conversion from the translational en-
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J. Chem. Phys., Vol. 109, No. 15, 15 October 1998
Fushitani et al.
6349
#
ergy of CH3 to the internal energy for the large-amplitude
motion to accomplish reaction ͑IV͒ is inefficient. The trans-
lational energy of the methyl radical seems, in actuality, to
be transferred to the phonon of solid hydrogen. The pre-
sumed rapid dissipation of the excess energy to phonon is
probably due to the distinctly high thermal conductivity of
solid p-H2 .3
Similarly to the case of the irradiation with 253.7 nm
light, the collisional reaction ͑IV͒ does not occur for 184.9
nm light either: reaction ͑IV͒ would take place in competi-
#
tion with the deactivation of CH3 ͓the second step of reac-
tion ͑I͔͒. If we assume that reaction ͑IV͒ is the major process
to yield methane, the concentrations of methyl radical and
methane should obey Eqs. ͑7͒ and ͑8͒, where ␣ is the effi-
ciency of the deactivation of the kinetic methyl radical,
FIG. 4. Temporal behavior of the integrated intensities of methyl radical and
methane under dark at 4.5 K. The symbols of the plot represent the inten-
sities of methyl radical at 3169.8–3172.2 (␯3) ͑᭝͒ and at 1401.4–1403.0
(␯4) ͑᭡͒, and the intensities of methane at 3024.2–3027.0 (␯3) ͑छ͒ and at
1304.4–1308.7 cmϪ1 (␯4) ͑ࡗ͒. The scale of the ordinate is in arbitary
common units.
d CH
͓
͔
3
ϭ␣k1 CH I ,
͑7͒
͑8͒
͓
͔
3
dt
d CH
͓
͔
4
ϭ 1Ϫ␣͒k CH I .
͑
͓
͔
1
3
dt
Therefore, the temporal dependence of the absorption inten-
sities of methyl radical and methane should be given by Eqs.
͑9͒ and ͑10͒, respectively,
3. Hydrogen abstraction from H2 by electronically
excited CH3
The abstraction of a hydrogen atom from a hydrogen
I CH ͒ϭI ␣ 1ϪeϪk t͒,
͑9͒
molecule by the excited radical in the B state is also conceiv-
˜
1
͑
͑
2
3
able for the production of methane as in reaction ͑VI͒,
I CH ͒ϭI 1Ϫ␣͒ 1ϪeϪk t͒.
͑10͒
1
͑
͑
3
͑
4
2
˜
*
CH3 B AЈ͒ϩH →CH ϩH.
͑VI͒
͑
2
4
1
However, Fig. 3 shows that methane behaves differently
from the above prediction. Thus, although the kinetic energy
imparted to the methyl radical upon the irradiation with
184.9 nm light is even higher than in the case of the irradia-
tion with 253.7 nm light, reaction ͑IV͒ does not account for
the experimental result. The excess kinetic energy in both
cases of 253.7 and 184.9 nm light is regarded as to be dissi-
pated to the matrix in vein.
However, the possibility of reaction ͑VI͒ can be ruled out by
a parallel study of the photolysis of a CD3I/p-H2 system: In
this system, if the reaction corresponding to reaction ͑VI͒
took place, the formation of CD3H should be expected, while
if reactions corresponding to reactions ͑II͒ and ͑III͒ are im-
portant in the CD3I/p-H2 system, the formation of CD2H2
should be dominant. As a result of ͑unfiltered͒ UV irradia-
tion of a CD3I/p-H2 system, absorption lines are found at
2979 and 3017 cmϪ1 which are definitely attributable to the
asymmetrical stretching mode (␯3) of CD2H2 ,26 but no ab-
sorption due to CD3H appeared in the region of 2980–3005
2. Direct production of CH2 from CH3I upon UV
irradiation
with a maximum at 2995 cmϪ1 10
Thus, we conclude that in
.
In the literature it is suggested that methyl iodide excited
to the C state,18 which is accessible by the 184.9 nm light,
may decompose as in reaction ͑V͒,25
the photolyzed CD3I/p-H2 system, reactions corresponding
to reactions ͑I͒–͑III͒ take place but the reaction correspond-
ing to reaction ͑VI͒ does not.
1
At this moment we should like to substantiate the state-
ment in Sec. III that the reaction of methyl radical in the
electronic ground state does not react with hydrogen mol-
ecules: Fig. 4 shows the integrated intensities of the degen-
erate C–H stretching (␯3) and bending (␯4) vibrations of
methyl radical and methane recorded intermittently for the
sample kept in the dark at the same temperature of 4.5 K as
the temperature for recording the data in Fig. 3. The ordinate
of Fig. 4 is in arbitrary units but is common to all the four
absorption bands. It is seen from Fig. 4 that the change of the
absorption intensities with the standing time is very little, if
any, and we consider that reaction ͑VII͒ below for the methyl
radical in the electronic ground state does not proceed appre-
ciably under the present experimental conditions,
˜
CH Iϩh␯ 184.9 nm͒→CH a A ͒ϩHI.
͑V͒
͑
͑
3
2
1
If reaction ͑V͒ were important in the present work, the sin-
glet methylene produced by reaction ͑V͒ could give methane
by reaction ͑III͒. However, it is difficult to assume that reac-
tion ͑V͒, which is a minor process in the gas phase
photolysis,25 becomes major in the p-H2 matrix in the
present work. If the direct dissociation of the iodide into the
singlet methylene is the major process to form methane, it is
easily shown that the time dependence of the intensity of
methane should be of the same form as that in Eq. ͑10͒,
which is not in accord with the temporal profile in Fig. 3.
Moreover, there was detected no significant absorption at-
tributable to HI to be produced by reaction ͑V͒. Therefore,
we conclude that reaction ͑V͒ should not be the main reac-
tion in solid p-H2 .
2
˜
CH X AЉ͒ϩH →CH ϩH.
͑VII͒
͑
3
2
4
2
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6350
J. Chem. Phys., Vol. 109, No. 15, 15 October 1998
Fushitani et al.
In connection with reaction ͑VII͒, we should like to di-
Although the present work is concerned with photochemistry
in solid p-H2 at cryogenetic temperatures, the work contrib-
utes, hopefully, to the understanding of the photochemistry
of methyl radical in hydrogenous systems in the gas phase as
well.
gress a little further and to draw attention to our separate
paper dealing with the dark reactions of CD3 with p-H2 and
of CH3 with p-H2 molecules at about 5 K.10 To our surprise,
CD3 in the dark does react definitely with H2 to produce
CD3H absorbing at 2980–3005 with a maximum at 2995
cmϪ1, which is diametrically different from CH3 in reaction
͑VII͒. This seemingly puzzling result, however, has been
consistently elucidated10 and it does not conflict with the
present work. A brief elucidation for the difference of the
dark reactions of CH3 and CD3 will be reproduced below.
The dark reaction ͑VIII͒ in the photolyzed CD3I/p-H2 is
found to be slightly exothermic, whereas the dark reaction
͑VII͒ in the CH3I/p-H2 system is barely endothermic when
the zero-point vibrational energies of the reactants and the
products are taken into account,10
ACKNOWLEDGMENTS
The authors are grateful to Professor Tetsuo Miyazaki
for his renting them a narrow-band filter. They are indebted
to Professor Nobuaki Washida for his constructive comments
on the manuscript. The present work is partially supported
by the Grants-in-Aid for Scientific Research of the Ministry
of Education, Science, Culture, and Sports of Japan. T.M.
acknowledges supports from the Inamori Foundation and the
Asahi Glass Foundation.
CD3ϩH2→CD3HϩH,
͑VIII͒
This subtle difference between the two systems accounts
consistently for the observed result. At about 5 K the tunnel-
ing chemical reaction in reaction ͑VIII͒ continues to proceed
whereas reaction ͑VII͒ does not appreciably.
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4. Reaction of triplet CH2 with hydrogen molecules
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Shida, J. Chem. Phys. 107, 7717 ͑1997͒.
If the nascent methylene produced by reaction ͑II͒ were
trapped metastably in the p-H2 matrix before reaction ͑III͒
completes, the singlet methylene would relax to its ground
triplet state as in reaction ͑IX͒,
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1
3
˜
͑
˜
CH a A ͒→CH X A ͒.
͑IX͒
͑
2
1
2
1
10 T. Momose, H. Hoshina, N. Sogoshi, H. Katsuki, T. Wakabayashi, and T.
Shida, J. Chem. Phys. 108, 7334 ͑1998͒.
The triplet methylene thus produced might also react with a
hydrogen molecule. In this case, however, the conservation
of the electron spin quantum number demands that the ab-
straction in reaction ͑X͒ should be the major process but that
no methane should be produced,
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3
˜
CH X A ͒ϩH →CH ϩH.
͑X͒
͑
2
1
2
3
Thus, the singlet methylene in reaction ͑II͒ is considered to
react with hydrogen molecules to form methane prior to its
relaxation to the ground state triplet methylene. Furthermore,
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͑III͒, it would relax to the ground state triplet methylene,
which should exhibit the well-known absorption of the triplet
methylene at 963.10 cmϪ1 reported for the gas phase.27–31
We did not observe such a transition at all.
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5. Commitment of migratory H atoms
The stability of the absorption of the polyatomic photo-
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hydrogen atom produced by reaction ͑II͒ may migrate in the
bulk. If it encounters methyl radical, methane would be
formed. However, the stability of the radical mentioned
above implies that such a process cannot be dominant.
By ruling out the possibilities ͑1͒–͑5͒ above, the cre-
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