EPR spectra and rotation of CH₃, CH₂D, CHD₂, and CD₃ radicals in solid H₂
EPR spectra of CH₃, CH₂D, CHD₂, and CD₃ radicals have been observed in H₂ matrix in the temperature range 1.6-4.2 K. The radicals were obtained by condensation on a cold substrate of two gas flows: deuterium mixed with 2 mol % methane passed through a discharge and pure hydrogen avoiding the dischar...
Збережено в:
| Дата: | 2003 |
|---|---|
| Автори: | , |
| Формат: | Стаття |
| Мова: | English |
| Опубліковано: |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
2003
|
| Назва видання: | Физика низких температур |
| Теми: | |
| Онлайн доступ: | http://dspace.nbuv.gov.ua/handle/123456789/128863 |
| Теги: |
Додати тег
Немає тегів, Будьте першим, хто поставить тег для цього запису!
|
| Назва журналу: | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| Цитувати: | EPR spectra and rotation of CH₃, CH₂D, CHD₂, and CD₃ radicals in solid H₂ / Yu.A. Dmitriev R.A. Zhitnikov // Физика низких температур. — 2003. — Т. 29, № 6. — С. 695-698. — Бібліогр.: 12 назв. — англ. |
Репозитарії
Digital Library of Periodicals of National Academy of Sciences of Ukraine| id |
irk-123456789-128863 |
|---|---|
| record_format |
dspace |
| spelling |
irk-123456789-1288632018-01-15T03:02:59Z EPR spectra and rotation of CH₃, CH₂D, CHD₂, and CD₃ radicals in solid H₂ Dmitriev, Yu.A. Zhitnikov, R.A. 3-й Международный семинар по физике низких температур в условиях микрогравитации EPR spectra of CH₃, CH₂D, CHD₂, and CD₃ radicals have been observed in H₂ matrix in the temperature range 1.6-4.2 K. The radicals were obtained by condensation on a cold substrate of two gas flows: deuterium mixed with 2 mol % methane passed through a discharge and pure hydrogen avoiding the discharge. The CD₃ and CHD₂ spectra were found to be a superposition of two spectra: high-temperature and low-temperature. A transformation of the shape of CD₃ and CHD₂ spectrum with decreasing sample temperature was observed. This is attributed to a change in the populations of the lowest rotational states of the radicals. Compared to known results for deuterated methyl radicals in Ar, the present observations suggest an existence of a hindering barrier for the radical rotation in solid H₂. 2003 Article EPR spectra and rotation of CH₃, CH₂D, CHD₂, and CD₃ radicals in solid H₂ / Yu.A. Dmitriev R.A. Zhitnikov // Физика низких температур. — 2003. — Т. 29, № 6. — С. 695-698. — Бібліогр.: 12 назв. — англ. 0132-6414 PACS: 32.30.-r, 76.30.-v http://dspace.nbuv.gov.ua/handle/123456789/128863 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| language |
English |
| topic |
3-й Международный семинар по физике низких температур в условиях микрогравитации 3-й Международный семинар по физике низких температур в условиях микрогравитации |
| spellingShingle |
3-й Международный семинар по физике низких температур в условиях микрогравитации 3-й Международный семинар по физике низких температур в условиях микрогравитации Dmitriev, Yu.A. Zhitnikov, R.A. EPR spectra and rotation of CH₃, CH₂D, CHD₂, and CD₃ radicals in solid H₂ Физика низких температур |
| description |
EPR spectra of CH₃, CH₂D, CHD₂, and CD₃ radicals have been observed in H₂ matrix in the temperature range 1.6-4.2 K. The radicals were obtained by condensation on a cold substrate of two gas flows: deuterium mixed with 2 mol % methane passed through a discharge and pure hydrogen avoiding the discharge. The CD₃ and CHD₂ spectra were found to be a superposition of two spectra: high-temperature and low-temperature. A transformation of the shape of CD₃ and CHD₂ spectrum with decreasing sample temperature was observed. This is attributed to a change in the populations of the lowest rotational states of the radicals. Compared to known results for deuterated methyl radicals in Ar, the present observations suggest an existence of a hindering barrier for the radical rotation in solid H₂. |
| format |
Article |
| author |
Dmitriev, Yu.A. Zhitnikov, R.A. |
| author_facet |
Dmitriev, Yu.A. Zhitnikov, R.A. |
| author_sort |
Dmitriev, Yu.A. |
| title |
EPR spectra and rotation of CH₃, CH₂D, CHD₂, and CD₃ radicals in solid H₂ |
| title_short |
EPR spectra and rotation of CH₃, CH₂D, CHD₂, and CD₃ radicals in solid H₂ |
| title_full |
EPR spectra and rotation of CH₃, CH₂D, CHD₂, and CD₃ radicals in solid H₂ |
| title_fullStr |
EPR spectra and rotation of CH₃, CH₂D, CHD₂, and CD₃ radicals in solid H₂ |
| title_full_unstemmed |
EPR spectra and rotation of CH₃, CH₂D, CHD₂, and CD₃ radicals in solid H₂ |
| title_sort |
epr spectra and rotation of ch₃, ch₂d, chd₂, and cd₃ radicals in solid h₂ |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| publishDate |
2003 |
| topic_facet |
3-й Международный семинар по физике низких температур в условиях микрогравитации |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/128863 |
| citation_txt |
EPR spectra and rotation of CH₃, CH₂D, CHD₂, and CD₃ radicals in solid H₂ / Yu.A. Dmitriev R.A. Zhitnikov // Физика низких температур. — 2003. — Т. 29, № 6. — С. 695-698. — Бібліогр.: 12 назв. — англ. |
| series |
Физика низких температур |
| work_keys_str_mv |
AT dmitrievyua eprspectraandrotationofch3ch2dchd2andcd3radicalsinsolidh2 AT zhitnikovra eprspectraandrotationofch3ch2dchd2andcd3radicalsinsolidh2 |
| first_indexed |
2025-07-09T10:07:17Z |
| last_indexed |
2025-07-09T10:07:17Z |
| _version_ |
1837163500549439488 |
| fulltext |
Fizika Nizkikh Temperatur, 2003, v. 29, No. 6, p. 695–698
EPR spectra and rotation of CH3, CH2D, CHD2,
and CD3 radicals in solid H2
Yu.A. Dmitriev and R.A. Zhitnikov
A.F. Ioffe Physico-Technical Institute, 26 Politekhnicheskaya Str, St. Petersburg 194021, Russia
E-mail: dmitriev.mares@pop.ioffe.rssi.ru
Received December 19, 2002
EPR spectra of CH3, CH2D, CHD2, and CD3 radicals have been observed in H2 matrix in the
temperature range 1.6–4.2 K. The radicals were obtained by condensation on a cold substrate of
two gas flows: deuterium mixed with 2 mol % methane passed through a discharge and pure hydro-
gen avoiding the discharge. The CD3 and CHD2 spectra were found to be a superposition of two
spectra: high-temperature and low-temperature. A transformation of the shape of CD3 and CHD2
spectrum with decreasing sample temperature was observed. This is attributed to a change in the
populations of the lowest rotational states of the radicals. Compared to known results for
deuterated methyl radicals in Ar, the present observations suggest an existence of a hindering bar-
rier for the radical rotation in solid H2.
PACS: 32.30.–r, 76.30.–v
Introduction
Methyl radical (CH3) isolated in various matrices
has been extensively studied by EPR since the late
50’s [1–6]. At low temperatures near 4.2 K, the EPR
spectrum of the radical is well-known to consist of for
lines with equal intensity 1:1:1:1, instead of the bino-
mial intensity distribution 1:3:3:1. This effect has
been first explained by McConnel [7]. The equal in-
tensity of four lines was attributed to the symmetry
requirements of the wavefunctions. According to
McConnel and Freed [8], three protons are considered
to move about the C3 molecular axis in the matrix in a
threefold potential well with a finite barrier height.
The lowest torsional rotation energy level is split into
two: the lower one of symmetry A and the upper, dou-
bly degenerated one of symmetry E. At sufficiently
low temperatures only the lowest spin-rotational A
state with four symmetric nondegenerate nuclear spin
functions is populated. So the EPR spectrum is a
1:1:1:1 quartet. The rotation is considered to be tun-
neling one and may occur even at helium tempera-
tures. In such a tunneling rotation, not only the A
state but also the E nuclear spin states appear by con-
sidering the next higher rotational levels. The EPR
transitions corresponding to these states have been
first observed for isolated methyl radical in Ref. 5. In
contrast to CH3, a few studies were devoted to
deuterated methyl radicals (CD3, CH2D, and CHD2).
A spectrum of seven components with «non-binomial»
distribution has been predicted for the CD3 radical [9]
at low enough temperatures. The septet has been actu-
ally registered in CD4 matrix [10] at 4.2 K and solid
Ar at 13 K [4]. Though these experimental results are
consistent with the above theoretical scheme, another
observation has been published [5] for CD3 in Ar at
4.2 K showing a strong singlet superimposed on a
weak septet. The authors explained their results with
a new model of a three-dimensional, free quantum ro-
tor with no hindering barrier present. They pointed
out that the electronic state has to be included in the
application of the Pauli principle in order to obtain
correct overall exchange symmetry for bosons.
The present study is aimed at studying deuterated
methyl radicals in another matrix in order to clarify
whether the effect found in [5] is common to other
matrices and to obtain new experimental results which
would help to verify the free rotation model [5].
Results and discussion
The solid samples under study are obtained by gas
condensation on the thin-walled bottom of a quartz
finger filled with liquid helium. Located at the center
© Yu.A. Dmitriev and R.A. Zhitnikov, 2003
of the microwave cavity of the EPR spectrometer, the
bottom is used as a substrate. Both the radiofrequency
gas discharge (channel A) and the matrix gas flow
through a separate inlet tube to avoid the gas dis-
charge (channel B) can be cooled down to liquid ni-
trogen temperature. The products of the gas discharge
without intermediate feeding tubes are supplied di-
rectly onto the substrate in vacuum preventing their
decay on the tube walls. Thus, the sample is obtained
directly in the cavity of the EPR spectrometer, allow-
ing an EPR observation of the sample during the con-
densation and a study of short-lived centres (e.g. free
radicals) due to the gas discharge products. A scheme
of the experimental set-up has been presented in previ-
ous papers [11].
In the presents experiments, molecular deuterium,
D2, mixed with 2 mol % methane, CH4, was prepared
in a glass vessel and passed through the channel A
with discharge on. Simultaneously, the H2 was fed
through the channel B. The latter flow was much
larger than the discharge flow, thus providing an ad-
mixture of D2 in H2 matrix as small as about 1:30. A
pulsed discharge has been employed with the off-duty
factor of 10. The substrate temperature during the de-
position was 4.2 K. Figure 1 shows the EPR spectrum
of a sample of solid H2 with trapped radicals. The ex-
perimental spectrum reveals seven strong lines of the
CD3 radical and weak lines for CH3, CH2D, and
CHD2 radicals. Superimposed is a record of the
high-field lines with a higher gain. The outermost
peak is a high-field component of the CH3 spectrum
which is composed of four lines of equal intensity
and was studied in H2 matrix earlier [6]. Three
equal-spaced lines constitute a part of the CH2D spec-
trum being a triple triplet due to the hyperfine (HF)
splittings of two hydrogen nuclei, major triplet, and
one deuterium, minor triplets. One of the peaks to the
left-side of the high-gained spectrum is a purely CHD2
component while its right neighbor is composed of the
outermost left CD3 component, CHD2 line, and the
CH3 transition at mF = – 1/2. At high enough tem-
peratures, the CHD2 spectrum is a double quintet.
Thus the spectrum in Fig. 2 is a superposition of se-
veral spectra.
We have found that seven CD3 lines are of equal
linewidth, �H = 0.86(6) G, with HF splitting �H =
= 3.59 G. The relative intensity ratio 1:3:6.5:12:6.5:3:1,
except for the central line, is close to the «binomial»
intensity distribution 1:3:6:7:6:3:1. It was observed in
Ref. 5 that at temperatures above 10 K the intensity
696 Fizika Nizkikh Temperatur, 2003, v. 29, No. 6
Yu.A. Dmitriev and R.A. Zhitnikov
Fig. 1. The EPR spectrum of a solid H2 sample with
trapped methyl radicals. The substrate temperature during
deposition Tsub = 4.2 K.
Fig. 2. The central part of temperature-dependent EPR
spectra of CD3 radical in H2 matrix. The substrate tem-
perature during deposition Tsub = 4.2 K.
distribution for CD3 in Ar is practically «binomial»
one originating not only from the population of J = 0
but also of higher rotational levels. With lowering
temperature down to 4.1 K the central line increased
while the other six lines decreased rapidly. As a result,
the intensity of the central peak relative to the neigh-
boring one reached 15. It was shown that the spectrum
corresponding to the J = 0 rotation level is a singlet.
Turning to our study, one can conclude that the spec-
trum of CD3 in H2 at 4.2 K is a superposition of the
high-temperature nearly «binomial» spectrum and the
low-temperature singlet. The above mentioned CH3
spectrum of four equal lines corresponds to J = 0 thus
being a low-temperature one. Such a difference be-
tween the appearances of the CH3 and CD3 spectra is
not surprising because the energy gap between J = 0
and J = 1 rotational states for the free CH3 is twice as
large as the CD3 gap. Therefore, J = 1 state of the
CH3 radical is not populated at low temperatures
close to 4 K. We have also found that the CHD2 spec-
trum is actually a superposition of a high- and
low-temperature spectrum. This will become clear
later when describing temperature effects. We have
failed to draw conclusion about the CH2D spectrum
appearance, because the central CH2D triplet could
not be seen due to the strong CD3 transitions superim-
posed on it. It is seen from Fig. 1 that the CD3 quan-
tity is well above the others. We have estimated the
yields of the deuterated methyl radicals in reference to
the CH3 yield. These were found to be 35:1 for CD3,
2:1 for CHD2, and 1.3:1 for CH2D. Thus, we conclude
that methane in our discharge was almost completely
deuterated through the intermediate products CH2D
and CHD2 to the final CD4 one. The reactions of the
methyl isotopomers (CH3, CH2D, and CHD2) with
excess deuterium atoms have been studied earlier us-
ing discharge flow/mass spectrometry [12].
Figure 2 shows the central part of the CD3 spec-
trum taken at several temperatures. One can readily
see that the central peak with mF = 0 increases rapidly
with decreasing temperature in reference to the neigh-
bor transitions. We have plotted saturation curves,
that is intensities of the central and neighbor lines ver-
sus microwave power, and found that the high-tem-
perature spectrum shows no saturation in the power
range used at both 4.2 K and 1.6 K, whereas the cen-
tral peak starts to saturate at 4.2 K, reaching a promi-
nent saturation at 1.6 K. Such a difference in the satu-
ration behavior between the lines is a further proof
that the central peak is actually a superposition of
transitions due to different states.
Figure 3 shows a change in the shape of the
high-field quintet of the CHD2 spectrum. The outer-
most left transition (mF(D) = – 2 ) of CHD2 is super-
imposed on the outermost right line of the CH2D
high-field triplet. Here D stands for the splitting due
to the deuterium nuclei. The transition at mF(D) = 0
(the third from the left) of CHD2 is superimposed on
the high-field (mF(D) = – 3) CD3 line. The next two
CHD2 peaks to the right are not seen against the
strong CD3 components. A high-temperature CHD2
spectrum exhibits a «binomial» relative intensity ratio
1:2:3:2:1 for the quintet components. It has been
found previously [5] that in argon matrix the quintet
transforms into a triplet with decreasing temperature
from 10 K to 4.2 K. In our experiments, the spectrum
of CHD2 in H2 was still a double quintet at 4.2 K
showing though an intensity distribution significantly
different from the binomial one which was evident
from the fact that the ratio of the line amplitude at
mF(D) = – 1 to the outermost one at mF(D) = – 2 was
well above 2. One can see from Fig. 3 that the CHD2
transition at mF(D) = – 2 disappears, while the line at
mF(D) = 1 becomes more pronounced against de-
creased high-temperature CD3 lines. Thus, in H2 ma-
EPR spectra and rotation of CH3, CH2D, CHD2, and CD3 radicals in solid H2
Fizika Nizkikh Temperatur, 2003, v. 29, No. 6 697
Fig. 3. Temperature-dependent high-field quintet of the
spectrum of CHD2 matrix-isolated in solid H2. The sub-
strate temperature during deposition Tsub = 4.2 K.
trix as well, the CHD2 quintet changes to the triplet
with decreasing temperature. This change from
high-temperature to the low-temperature spectrum for
CHD2 corresponds very well to the case of CD3 radi-
cal in H2.
Conclusion
The present results not only verify the effect of
temperature on the shape of the CD3 and CHD2 spec-
trum first observed in Ar [5] but give new information
when comparing the temperature ranges for these
spectrum transformations in Ar and H2: 10–4.2 K and
4.2–1.6 K, respectively. Since the spectrum changes
we discuss are due mainly to changes in the popula-
tions of the lowest J = 0 and J = 1 states of trapped
radicals, the difference in the range suggests that the
energy interval between the above rotational states is
larger in Ar then in H2. In turn, a decrease of this in-
terval for a trapped molecule in comparison with a
free one is due to hindering of the rotation of the mole-
cule in the matrix. Because the interaction energy
CD3–H2 is lower then that of CD3–Ar, one may ex-
pect a more free rotation of CD3 in H2, i.e. a smaller
effect on the radical rotation than in Ar. Indeed, solid
H2 is well know to have a small effect on the parame-
ters of various radicals in comparison with many ma-
trices. However, the present result suggests the
phonon-rotation coupling for CHD2 and CD3 mole-
cules in H2 to be surprisingly high. This unexpected
conclusion requires further theoretical and experimen-
tal study.
1. R.L. Morehouse, J.J. Christiansen, and W. Gordy, J.
Chem. Phys. 45, 1751 (1966).
2. G.S. Jackel and W. Gordy, Phys. Rev. 176, 443
(1968).
3. C.K. Jen, S.N. Foner, E.L. Cochran, and V.A.
Bowers, Phys. Rev. 112, 1169 (1958).
4. E.Ya. Misochko, V.A. Benderskii, A.U. Goldschleger,
A.V. Akimov, A.V. Benderskii, and C.A. Wight, J.
Chem. Phys. 106, 3146 (1997).
5. T. Yamada, K. Komaguchi, M. Shiotani, N.P. Benetis,
and A.R. Sornes, J. Phys. Chem. A103, 4823 (1999).
6. Yu.A. Dmitriev and R.A. Zhitnikov, J. Low. Temp.
Phys. 122, 163 (2001).
7. H.M. McConnel, J. Chem. Phys. 29, 1422 (1958).
8. J.H. Freed, J. Chem. Phys. 43, 1710 (1965).
9. Formation and Trapping of Free Radicals, A.M. Bass
and H.P. Broida (eds.), Academic Press, New York
and London (1960).
10. K. Toriyama, M. Iwasaki, and K. Nunome, J. Chem.
Phys. 71, 1698 (1979).
11. R.A. Zhitnikov and Yu.A. Dmitriev, Astron.
Astrophys. 386, 129 (2002).
12. P.W. Seakins, S.H. Robertson, M.J. Pilling, D.M.
Wardlaw, F.L. Nesbitt, R.P. Thorn, W.A. Payne, and
L.J. Stief, J. Phys. Chem. A101, 9974 (1997).
698 Fizika Nizkikh Temperatur, 2003, v. 29, No. 6
Yu.A. Dmitriev and R.A. Zhitnikov
|