Submillimeter-wave spectrum of the AsH2 radical in the 2B1 ground electronic state

Article abstract of DOI:10.1063/1.477153

The pure rotational spectrum of the AsH₂ radical in its ²B₁ ground electronic state was observed for the first time by microwave spectroscopy. The AsH₂ radical was generated in a free-space cell by dc-glow discharge of a mixture of H₂ and O₂ gases over arsenic powder. Fifty-five fine and hyperfine components of six rotational transitions were measured in the frequency region of 304-374 GHz, and were analyzed by least-squares methods. Molecular constants, including the rotational constants, the centrifugal distortion constants, the spin-rotation coupling constant incorporating the centrifugal distortion term, and the hyperfine coupling constants associated with the arsenic and hydrogen nuclei, were precisely determined. The bonding in AsH₂ was discussed on the basis of the hyperfine coupling constants, first determined in the present study.

Full text of DOI:10.1063/1.477153

Submillimeter-wave spectrum of the AsH 2 radical in the 2 B 1 ground electronic state
Hideo Fujiwara, Kaori Kobayashi, Hiroyuki Ozeki, and Shuji Saito
Citation: The Journal of Chemical Physics 109, 5351 (1998); doi: 10.1063/1.477153
View online: http://dx.doi.org/10.1063/1.477153
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 109, NUMBER 13
1 OCTOBER 1998
Submillimeter-wave spectrum of the AsH2 radical
2
in the B1 ground electronic state
Hideo Fujiwara, Kaori Kobayashi, Hiroyuki Ozeki, and Shuji Saitoa)
Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan
͑Received 22 April 1998; accepted 1 July 1998͒
The pure rotational spectrum of the AsH2 radical in its 2B1 ground electronic state was observed for
the first time by microwave spectroscopy. The AsH2 radical was generated in a free-space cell by
dc-glow discharge of a mixture of H2 and O2 gases over arsenic powder. Fifty-five fine and
hyperfine components of six rotational transitions were measured in the frequency region of 304–
374 GHz, and were analyzed by least-squares methods. Molecular constants, including the rotational
constants, the centrifugal distortion constants, the spin-rotation coupling constant incorporating the
centrifugal distortion term, and the hyperfine coupling constants associated with the arsenic and
hydrogen nuclei, were precisely determined. The bonding in AsH2 was discussed on the basis of the
hyperfine coupling constants, first determined in the present study. © 1998 American Institute of
Physics. ͓S0021-9606͑98͒03337-6͔
I. INTRODUCTION
wave radiation was supplied by a tripler or a quadrupler
driven by millimeter-wave klystrons. Production of AsH2
was similar to that of AsH.11 The radical was produced by
dc-glow discharge ͑130–150 mA͒ of a mixture of H2 gas ͑25
mTorr͒ and O2 gas ͑1 mTorr͒ over arsenic powder at a cell
temperature of Ϫ120 to Ϫ100 °C. The addition of a small
amount of oxygen gas was essential to the production of
gaseous AsH2. dc-glow discharge in AsH3 was also em-
ployed for the production of the radical, but there was no
significant change in the intensity of the signal, and so this
method was discarded.
Dihydrides of the group V elements furnish a series of
fundamental bent triatomic free radicals in molecular spec-
troscopy. The NH2 ͑Ref. 1͒ and PH2 ͑Ref. 2͒ radicals have
been extensively studied and well characterized by various
high-resolution molecular spectroscopic methods in the re-
gion from optical to microwave, whereas the dihydride of the
third row element As, AsH2, has been studied so far only in
the optical region. The first spectroscopic investigation of
AsH2 and AsD2 in the gas phase was carried out in 1966 by
3
2
2
˜
˜
Dixon and his collaborators who observed A A1ϪX B1
transitions of both species in the visible region using flash
photolysis of arsine. Later they determined the rotational
constants, centrifugal distortion constants, and spin-rotation
coupling constants of AsH2 in both electronic states.4 The
same bands generated by ArF laser photolysis were also
studied in emission, and the vibrational structure of both
electronic states was analyzed in 1986.5 Jackel and Gordy
studied the ESR spectrum of AsH2, but did not report hyper-
fine coupling constants.6 Spectroscopic constants of AsH2,
including its equilibrium bond length, vibrational frequency,
and dipole moment, were predicted by several quantum
chemical calculations.7–9
The hyperfine coupling constants provide information as
to the nature of the chemical bonding, which has not been
systematically available for the arsenic bearing hydrides.
Therefore we studied AsH2 by microwave spectroscopy and
determined detailed molecular constants including the hyper-
fine coupling constants.
Spectral line frequencies of AsH2 were predicted using
the molecular constants reported by the optical spectroscopic
study.4 Several lines which indicated paramagnetism were
detected in the region of 326.1–326.3 GHz and 351.0–351.5
GHz by time-averaging observations of 40 s. They were as-
signed to the NKaKcϭ111-000 Jϭ1.5-0.5 and Jϭ0.5-0.5
transitions, respectively. Successively, five Q branch transi-
tions were detected mainly in the 300 GHz region: 211-202
,
Jϭ2.5-2.5 and Jϭ1.5-1.5, 221-212 , Jϭ2.5-2.5 and J
ϭ1.5-1.5, 321-312 , Jϭ3.5-3.5 and 2.5-2.5, 331-322 , J
ϭ3.5-3.5, and 441-432 , Jϭ4.5-4.5. Figure 1 shows an ex-
ample of the observed spectral lines which are relatively
strong. Most of the observed lines of AsH2 were very weak
and could not be seen on the CRO display. The averaging
time of one observation was 40–320 s. Each line frequency
was determined by averaging five or six pairs of upward and
downward frequency sweep measurements. The observed
line frequencies are listed in Table I.
III. ANALYSIS
II. EXPERIMENT
2
The ground electronic state of the AsH2 radical has B1
Spectral lines of AsH2 were observed by using a
frequency-modulated microwave spectrometer combined
with a 2 m free-space absorption cell.10 The submillimeter-
symmetry. Since AsH2 has two identical hydrogen nuclei(I
ϭ1/2), symmetric rotational levels of (KaKc)
ϭ(even even) or ͑odd odd͒ are combined with the antisym-
metric hydrogen nuclear spin function of I(H)ϭI(H1)
ϩI(H2)ϭ1, whereas the ͑eo͒ or ͑oe͒ levels are combined
a͒
Electronic mail: saito@ims.ac.jp
0021-9606/98/109(13)/5351/5/$15.00
5351
© 1998 American Institute of Physics
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5352
J. Chem. Phys., Vol. 109, No. 13, 1 October 1998
Fujiwara et al.
Unresolved lines were included in the fit by averaging
frequencies of the individual components which had been
weighted in proportion to their relative intensities when fit-
ting the data. The 48 observed lines of AsH2 were used in the
least-squares fit and the corresponding molecular constants
determined are listed in Table II. The standard deviation of
the fit is 28 kHz, which is comparable to the frequency mea-
surement errors, as shown in Table I. The centrifugal distor-
tion constants ⌬N and ␦N were fixed to the values calculated
from centrifugal distortion constants ␶aaaa , ␶bbbb , ␶
,
aabb
and
␶
which were determined by electronic
abab
spectroscopy.4 Centrifugal distortion terms of the spin-
rotation coupling constants ⌬sN , ⌬Ns K , and ⌬Ks were fixed to
the values derived using symmetric top approximation.17 The
other centrifugal distortion terms ␦sN and ␦Ks were fixed at
zero.
FIG. 1. Submillimeter-wave lines of an AsH2 radical: the NKaKc
ϭ331-322 , Jϭ7/2-7/2, F1ϭ5-5, Fϭ5-5, 4-4, and 6-6 transitions observed
by dc-glow discharge ͑130–150 mA͒ of a mixture of H2͑25 mTorr͒ and
O2͑1 mTorr͒ gas over arsenic powder. The integration time was 80 s. Ver-
tical lines represent the calculated frequencies and intensities.
An initial analysis gave residuals of several hundreds of
kHz in the fit. Inclusion of the nuclear spin-rotation coupling
constants for As reduced the standard deviation of the fit
from 553 to 28 kHz. An attempt was made to determine the
nuclear spin-rotation coupling constants of the hydrogen, but
it was found that these values were smaller than their uncer-
tainties in the least-squares fit. Therefore these values were
set to zero in the final analysis.
with the symmetric nuclear spin function of I(H)ϭ0. There-
fore the fine-structure level is split into 12 hyperfine sublev-
els by I(As)ϭ3/2 and I(H)ϭ1 for the symmetric rotational
level, and into four hyperfine sublevels by I(As) for the an-
tisymmetric rotational level.
The observed data of AsH2 were fitted to a conventional
Hamiltonian, appropriate for an asymmetric top molecule in
a doublet electronic state with two nuclear spins, of the form
IV. DISCUSSION
The molecular constants determined in the present study
are compared with previously reported values in Table II.
Rotational constants and the spin-rotation coupling constants
of the present study agree with those determined by the op-
tical spectroscopic study.4 However, the precision of molecu-
lar parameters is significantly improved in the present study.
Hyperfine coupling constants of the arsenic and hydro-
gen nuclei of AsH2 were determined for the first time in the
present study. The determined value of the Fermi contact
term of As was far from the value predicted from
aF(NH2)/bF(NH) and aF(PH2)/bF(PH), whereas the deter-
mined values of other magnetic dipole coupling constants
and the electric quadrupole coupling constants of As were
close to the predicted values. The Fermi contact terms of the
group V nuclei in the atom(4S), monohydride(3⌺Ϫ), and
dihydride(2B1) forms are listed in Table III. The atomic
value, A, which is a magnetic dipole interaction constant,
includes only a contribution from the Fermi contact term in
HϭHrotϩHsrϩHhfs As͒ϩH H͒,
͑
͑
hfs
where Hrot is the rotational Hamiltonian with its centrifugal
distortion effect and Hsr is the spin-rotation interaction term
with the centrifugal distortion effect. Hhfs(As) comprises the
magnetic dipole and electric quadrupole hyperfine interac-
tion terms and Hhfs(H) refers to magnetic dipole hyperfine
terms. The matrix elements were calculated by the standard
procedure using the basis function with the coupling scheme
of JϭNϩS, F1ϭJϩI(As), and FϭF1ϩI(H).12,13
In the initial analysis, the rotational constants and the
spin-rotation coupling constants determined by Dixon et al.4
were used. The hyperfine coupling constants were estimated
from those of AsH ͑Ref. 11͒ and the ratios between those of
monohydride and dihydride of other group V elements, N
͑Refs. 14 and 1͒ and P.15,16 The predicted hyperfine coupling
constants were as follows; the Fermi contact term aF(As)
ϭϪ21 MHz, the magnetic dipole coupling constants
Taa(As)ϭTbb(As)ϭ300 MHz, the electric quadrupole cou-
4
the case of spherical symmetry, S. For N and P, the Fermi
contact term of the atom and that of the dihydride are posi-
tive, but for As the atomic value is negative, whereas the
value for AsH2 is positive. As can be seen in Table III, the
values of Fermi contact terms of the respective nuclei vary
linearly with the number of the bonded hydrogens. This is
considered to be due to two factors. The first is the s char-
acter of the unpaired electron, and the second is the spin
polarization of the s orbits due to the 4p unpaired electrons.
According to an optical interference spectroscopic study of
As͑I͒, the spin polarization contribution, a1C0P , was evaluated
to Ϫ36͑6͒ MHz.21 Therefore the spin polarization contribu-
tion is considered to decrease with an increase of the number
of the bonded hydrogens, that is to say, with a decrease the
pling
constants
␹
aa(As)ϭ11 MHz
and
␹bb(As)
ϭϪ125 MHz. These were useful for the assignment of the
observed rotational transitions, but were not helpful to the
assignment of the hyperfine structure of the low J transitions,
N
KaKcϭ111-000 Jϭ1.5-0.5, Jϭ0.5-0.5 and NKaKc
ϭ211-202 Jϭ1.5-1.5. Therefore Taa(As), Tbb(As),
aa(As), and ␹bb(As) were determined using antisymmetric
␹
rotational transitions(Iϭ0) in the first instance. The constant
of aF(As) could not be determined from the observed hyper-
fine structure because this parameter does not affect ⌬F1
ϭ0 transitions of Q branches. The low J transitions, NKaKc
ϭ111-000, Jϭ1.5-0.5 and Jϭ0.5-0.5, could be assigned
with changing aF(As) by trial and error.
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J. Chem. Phys., Vol. 109, No. 13, 1 October 1998
Fujiwara et al.
5353
TABLE I. Observed and calculated transition frequencies for an AsH2 radi-
cal͑MHz͒.
TABLE I. ͑Continued.͒
F1Ј-F1
a
obs
b
F-F
␯
⌬␯
a
obs
b
F-F
␯
⌬␯
F1Ј-F1
N
KaKcϭ441-432
Jϭ4.5-4.5
N
KaKcϭ111-000
Jϭ1.5-0.5
4-3
6-6
5-5
4-4
3-3
374 113.946͑13͒
Ϫ0.025
0.005
0.025
3-2
3-2
3-2
2-1
1-1
1-1
0-1
1-1
326 258.257͑29͒
326 243.817͑17͒
326 249.817͑22͒
326 212.995͑13͒
326 207.532͑10͒
326 187.146͑28͒
326 173.002͑20͒
326 168.436͑22͒
326 124.034͑24͒
326 107.382͑25͒
326 104.039͑13͒
Ϫ0.023
Ϫ0.007
0.030
Ϫ0.005
0.030
374 164.208͑29͒
374 217.370͑18͒
374 267.246͑28͒
3-2
2-1
3-2
1-2
2-1
1-1
2-2
3-3
Ϫ0.004
aValues in parentheses indicate standard deviation of the frequency mea-
surement and apply to the last digits of the frequencies.
Ϫ0.003
Ϫ0.001
Ϫ0.028
Ϫ0.086c
0.021
bResiduals in the least-squares fit. ⌬␯ϭ␯obsϪ␯
.
calc
cWeight is 0.1.
dUnresolved line. The calculated frequencies were obtained by averaging the
frequencies of components weighted in proportion to their relative intensi-
ties.
2-2
2-2
2-2
2-2
1-1
Ϫ0.056c
eNot included in the least-squares fit because of the overlap with other lines.
N
KaKcϭ111-000
Jϭ0.5-0.5
3-3
2-2
2-2
351 004.801͑17͒
350 943.787͑18͒
Ϫ0.017
Ϫ0.002
2-2
number of unpaired electrons. If the spin polarization contri-
bution is assumed to decrease linearly with an increase of the
number of bonding hydrogens in the series of XHn (n
ϭ0–2) species, then the spin-polarization corrected term is
calculated to be 69.8 MHz for AsH2. The value corresponds
to 0.48% s character on the unpaired electron orbitals in
AsH2, when compared with that of atomic arsenic.22 The
0.48% s character is about one-third of the corresponding
2-1
2-1
2-1
2-1
2-1
1-2
1-2
2-1
1-0
3-2
1-1
2-2
2-3
1-2
351 101.008͑24͒
351 093.683͑6͒
Ϫ0.084c,d
0.009d
351 082.370͑30͒
351 460.908͑13͒
351 418.764͑28͒
0.156e
Ϫ0.005
0.023
N
KaKcϭ211-202
Jϭ2.5-2.5
5-5
4-4
4-4
4-4
313 170.066͑22͒
313 167.278͑21͒
Ϫ0.230e
Ϫ0.012d
4-4
3-3
TABLE II. Molecular constants of the AsH2(X 2B1) radical ͑MHz͒.a
3-3
3-3
4-4
3-3
313 434.669͑27͒
313 439.393͑22͒
Ϫ0.212
Ϫ0.006
2-2
2-2
2-2
3-3
2-2
1-1
Ϫ0.121d,e
MW
Visibleb
313 550.337͑15͒
313 557.204͑32͒
A
B
C
226 347.254͑94͒
214 738.272͑27͒
108 369.402͑29͒
12.57c
226 301͑387͒
214 723͑405͒
108 423͑270͒
12.57d
0.019
N
N
KaKcϭ211-202
Jϭ1.5-1.5
4-4
3-3
3-3
3-3
1-2
330 360.461͑28͒
330 359.143͑21͒
330 357.053͑19͒
329 692.629͑28͒
329 562.073͑23͒
0.000
Ϫ0.048
0.006
0.013
0.028
⌬
3-3
2-2
2-3
3-2
N
⌬
Ϫ31.366͑54͒
54.050͑41͒
5.59c
Ϫ39.80d
NK
⌬
␦
␦
60.66d
K
N
K
5.59d
2-1
Ϫ2.8638͑20͒
Ϫ33 088.39͑30͒
Ϫ11 858.45͑21͒
89.60͑17͒
Ϫ5.01d
KaKcϭ221-212
Jϭ2.5-2.5
⑀
⑀
⑀
⌬
⌬
⌬
⌬
␦
␦
Ϫ31 500͑2100͒
Ϫ12 000͑2200͒
200͑2400͒
4-4
3-3
2-2
1-1
336 514.314͑15͒
336 733.122͑14͒
336 874.198͑36͒
336 957.464͑18͒
0.037
0.004
Ϫ0.041
Ϫ0.143d
aa
bb
cc
s
2.63e
Ϫ4.93e
Ϫ7.385͑73͒
15.59e
0.0
N
s
N
N
KaKcϭ221-212
Jϭ1.5-1.5
NK
s
3-3
2-2
378 630.565͑23͒
378 101.973͑15͒
0.030
Ϫ0.030
KN
s
K
s
KaKcϭ331-322
Jϭ3.5-3.5
6-6
N
s
5-5
5-5
5-5
4-4
4-4
4-4
3-3
3-3
353 602.002͑16͒
353 599.436͑25͒
353 600.242͑20͒
353 705.173͑9͒
0.005
Ϫ0.002
Ϫ0.042
0.028
0.0
K
5-5
4-4
5-5
4-4
3-3
4-4
3-3
2-2
aF(As)
57.820͑21͒
Ϫ287.694͑73͒
Ϫ322.150͑45͒
31.21͑17͒
Ϫ140.88͑16͒
0.672͑19͒
0.471͑21͒
0.125͑16͒
Ϫ50.811͑53͒
Ϫ0.81͑19͒
Ϫ3.13͑12͒
T
T
␹
aa(As)
bb(As)
aa(As)
bb(As)
aa(As)
bb(As)
cc(As)
aF(H)
aa(H)
bb(H)
353 698.652͑20͒
353 790.296͑28͒
353 779.640͑26͒
353 852.219͑23͒
0.012d
␹
Ϫ0.001
C
C
C
Ϫ0.147d,e
Ϫ0.201e
3-3
2-2
2-2
N
N
KaKcϭ321-312
Jϭ3.5-3.5
T
T
5-5
4-4
3-3
2-2
304 158.137͑12͒
304 298.804͑22͒
304 369.710͑27͒
304 398.436͑25͒
0.011
Ϫ0.025
0.002
aThe numbers in parentheses represent three times the standard deviation in
units of the last digits.
0.012
bReference 4.
KaKcϭ321-312
Jϭ2.5-2.5
cFixed.
4-4
3-3
2-2
1-1
310 624.317͑13͒
310 448.651͑18͒
310 275.658͑19͒
310 140.750͑16͒
0.016
0.009
Ϫ0.001
Ϫ0.023
dDerived from the centrifugal distortion constants ␶aaaa , ␶bbbb , ␶aabb , and
␶
.
abab
eFixed to the values derived from the rotational constants, the spin-rotation
coupling constants, and centrifugal distortion terms. See text.
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5354
J. Chem. Phys., Vol. 109, No. 13, 1 October 1998
Fujiwara et al.
TABLE III. Comparison of the Fermi contact terms of group V element
TABLE IV. Comparison of the magnetic dipole coupling constants of group
V element nuclei ͑MHz͒.
nuclei ͑MHz͒.a
A X(4S)
bF XH(3⌺Ϫ
)
aF XH2(2B1)
X
X
N
P
As
a
b
c
b
c
f
i
d
XH
Spin density
c
Ϫ68.00͑14͒
81.6%
Ϫ476.843͑78͒
Ϫ478.801͑58͒
N
P
10.450 91͑7͒
18.832͑30͒
͓1.04%͔
128.119͑17͒
͓0.96%͔
Ϫ14.097͑25͒
9.9k
͓0.07%͔
28.061͑52͒
86.6%
95.7%
͓1.55%͔
207.25͑13͒
e
g
55.055 691͑24͒
d
e
f
f
XH2 Taa
Tbb
Ϫ43.035͑91͒
Ϫ300.24͑13͒
Ϫ321.86͑29͒
Ϫ287.694͑73͒
Ϫ322.150͑45͒
609.844
͓1.55%͔
d
e
h
j
Ϫ44.63͑12͒
87.67
As
Ϫ66.204͑1͒
57.820͑21͒
Tcc
622.10
84.8%
Ϫ30.2k
69.8k
Spin density
79.0%
91.4%
͓0.48%͔
aReference 14.
bReference 15.
cReference 11.
dReference 1.
aValues in square brackets indicate s character of the unpaired electron.
bReference 18.
cReference 14.
dReference 1.
eReference 16.
fThe present study.
eReference 19.
fReference 15.
gReference 16.
hReference 20.
iReference 11.
where i is the ionic character of each ␴ bond when As has
␴
jThe present study.
kSpin-polarization corrected Fermi contact terms.
a negative charge. The orbital population nz is cited from the
spin density. The electric quadrupole coupling constant is
represented as follows
␹
ϭ„n Ϫ n ϩn ͒/2…eQq / 1ϩc ⑀͒,
͑
͑
Ј
␣␣
␣
␤
␥
410
values of NH2 and PH2 as shown in Table III. This suggests
a degree of sp hybridization of the orbital including the un-
paired electron correlated with that of the bonding orbitals.
Table IV shows a comparison of the magnetic dipole
coupling constants of the group V element nuclei and the
spin densities calculated from the parameters by comparing
them with the corresponding atomic values.22 The spin den-
sities of the dihydrides are slightly smaller than those of the
monohydrides. This is consistent with the fact that the s char-
acters of the unpaired electron of the dihydrides are slightly
larger than those of the monohydrides, as shown in Table III.
The tendency of the spin densities to increase from NH2 to
AsH2 is very similar to that of the corresponding monohy-
drides.
where c is the negative charge on the As, i.e., 2i . Using
Ј
␴
values of the quadrupole coupling constant due to one atomic
4p electron, eQq410ϭϪ497 MHz, obtained from 1/r3
͗
͘
ϭ9.102 as reported by Morton and Preston22 for As, the
charge screening correction ͓⑀ϭ0.15 ͑Ref. 23͔͒ and the ob-
served quadrupole coupling constants ͓␹aaϭ31.21(17)MHz
and ␹bbϭϪ140.88(16)MHz͔, i and a2s were calculated to
␴
be 0.020 and 0.118, respectively. The derived i is close to
␴
the theoretical value of 0.015 calculated from Mulliken
population analysis of an ab initio calculation on AsH2.8
Similarly in the case of AsH2, the s characters of the ␴ bond
of the AsH radical and AsH3 were calculated to be 0.149 and
0.114, respectively, using values of the observed eQq of AsH
and AsH3,11,24 the spin density of AsH ͓0.957 ͑Ref. 11͔͒, and
The s character of the valence orbital and the ionic char-
acter of the bonding orbital can be evaluated from the elec-
tric quadrupole coupling constants. Assuming that the va-
lence orbitals of arsenic in the AsH2 radical are sp2-like,
they can be expressed as23
the ionic character i cited from Mulliken population
␴
analysis.25,26 The electric quadrupole coupling constants, the
s character, and the ionic character of AsHn (nϭ1–3) are
shown in Table V. From the above result, the As–H bond
character is considered to be nearly the same regardless of
the number of bonded hydrogens. The fact that the ionic
2
s
2
s
1/2
1/2
␺ ϭ 1Ϫ2a ͒ ␺ ϩ 2a ͒
␺
,
py
͑
͑
1
s
2
1/2
1/2
1/2
␺ ϭa ␺ Ϫ„ 1Ϫ2a ͒/2… ␺ ϩ 1/2͒
␺
,
,
͑
͑
2
s
s
px
py
py
s
TABLE V. Comparison of the electric quadrupole coupling constants, the s
character of the valence orbital, and the ionic character of bonding orbital of
AsHn (nϭ1–3).
2
s
1/2
␺ ϭa ␺ Ϫ„ 1Ϫ2a ͒/2… ␺ Ϫ 1/2͒
␺
͑
͑
3
s
s
px
␺4ϭ␺
,
pz
AsH
AsH2
AsH3
where a2s is the amount of s character in each of the equiva-
a
a
lent ␴ As–H bonding orbitals. The equivalent orbitals ␺
31.21͑17͒
Ϫ140.88͑16͒
␹
␹
aa /MHz
bb /MHz
2
and ␺ form the As–H bonds; ␺ has a lone pair of electrons
3
1
b
c
eQq/MHz
Ϫ99.45͑14͒
Ϫ162.681͑54͒
and ␺ has an unpaired electron. The axes, x, y, and z, cor-
4
as2
0.149
0.118
0.020
0.015e
0.114
respond to the principal axes a, b, and c, respectively. From
the orbitals ␺1Ϫ␺4 , the px , py , and pz orbital population
are found to be
i
␴
i
͑theo.͒
0.01d
0.03f
␴
aPresent study.
bReference 11.
cReference 24.
dReference 25.
eReference 8.
fReference 26.
nxϭ1ϩi ,
␴
n ϭ4a2ϩ 1ϩi ͒ 1Ϫ2a2͒,
͑
͑
␴
y
s
s
nzϭ0.914,
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J. Chem. Phys., Vol. 109, No. 13, 1 October 1998
Fujiwara et al.
5355
ACKNOWLEDGMENTS
character of the As–H bond is small is reasonable, consider-
ing that the difference between the electronegativity of
As͑2.0͒ and that of H͑2.1͒ is minimal.
Nuclear spin-rotation coupling constants for the arsenic
nucleus Caa(As) and Cbb(As) were determined to be
0.672͑19͒ MHz and 0.471͑21͒ MHz, respectively. Using
second-order perturbation theory, Cgg is derived from the
following expression:
The authors thank John M. Brown for sending us infor-
mation on FIR-LMR measurements of AsH2 and its produc-
tion prior to publication. The authors are grateful to Imtiaz
K. Ahmad for her critical reading of the manuscript.
͉
Cgg͉ϭ͉⑀gga/Aso͉.
1 M. Tonooka, S. Yamanoto, K. Kobayashi, and S. Saito, J. Chem. Phys.
106, 2563 ͑1997͒, and references cited therein.
Assuming that the nuclear spin-orbit hyperfine coupling con-
stant aϭ(5/4)Tccϭ762.3 MHz and the spin-orbit coupling
constant Asoϭ617 cmϪ1 cited from the value of AsH,27 Caa
and Cbb are calculated to be 1.36 and 0.49 MHz, respec-
tively. These values are within an order-of-magnitude in
agreement with the observed values. The arsenic nuclear
spin-rotation coupling constant of AsH, CI(As), is 0.471͑17͒
MHz,11 and CN(As) and CK(As) of AsH3 are 0.1035͑26͒
and 0.0856͑42͒ MHz, respectively.24 Cgg is given by
2 T. Hirao, S. Hayakashi, S. Yamamoto, and S. Saito, J. Mol. Spectrosc.
187, 153 ͑1998͒, and references cited therein.
3 R. N. Dixon, G. Duxbury, and H. M. Lamberton, Chem. Commun. ͑Lon-
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C
ggϭ4Bgg⌺a͉ 0͉Lg͉n ͉
2/ E ϪE ͒,
͑
͗ ͘
n 0
where Bgg denotes the rotational constant, Lg the g compo-
nent of the orbital angular momentum, En and E0 the elec-
tronic energy of the excited and ground states, respectively,
and ͉0͘ and ͉n͘ the corresponding wave functions. The values
of rotational constants and EnϪE0 of AsH are
12 I. C. Bowater, J. M. Brown, and A. Carrington, Proc. R. Soc. London, Ser.
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215 882.657͑53͒ MHz11 and 29 880.65 cmϪ1 27
A0 , B0 , and
.
EnϪE0 of AsH2 are 226 347.254͑94͒ MHz, 214 738.272͑27͒
MHz, and 19 907 cmϪ1.4 B0 of AsH3 is 112 470.5760͑45͒
MHz24 and the theoretical value of the energy difference
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Ϫ1 26
E(1E)ϪE(X A1) is 44 400 cm
.
The fact that the
1
nuclear spin-rotation coupling constants of AsH3 are smaller
than those of AsH and AsH2 is reasonable, considering the
small rotational constant and the large energy difference.
In conclusion this microwave spectroscopic study has
provided the detailed and precise molecular constants for the
AsH2 radical. From a comparison of the hyperfine coupling
constants of AsHn (nϭ0–3), the character of the As–H ␴
bond is nearly the same regardless of the number of bonded
hydrogens. However, the change in the number of hydrogen
atoms significantly influences the Fermi contact term.
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