Spectroscopic detection and characterization of iodogermylene (HGel)

Article abstract of DOI:10.1063/1.477591

Monoiodogermylene has been detected for the first time using pulsed discharge and laserinduced fluorescence techniques. HGeI and DGeI were produced by an electric discharge through argon seeded with H₃GeI or D₃GeI. Although the vibronic structure in the spectra was very limited, all three excited state vibrational frequencies have been obtained for both isotopomers. Analysis of the partially resolved rotational structure of the 0₀⁰ bands gave the following approximate r₀ structures, with the bond angles constrained to previous ab initio values: r₀^″(Ge-I)=2.525(10)A, r₀^″(H-Ge)=1.593(15)A, θ₀^″(HGeI)=93.5°, r₀^′(Ge-I)=2.515(10)A, r₀^′(H-Ge)=1.618(15)A, and θ₀^′(HGeI)=116.2°. The fluorescence lifetime of monoiodogermylene in the lowest rovibronic levels is 1.515±0.004μs and shows significant variations on deuteration and with rotational and vibrational level.

Full text of DOI:10.1063/1.477591

Spectroscopic detection and characterization of iodogermylene (HGel)
Warren W. Harper, Chad M. Klusek, and Dennis J. Clouthier
Citation: The Journal of Chemical Physics 109, 9300 (1998); doi: 10.1063/1.477591
View online: http://dx.doi.org/10.1063/1.477591
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 109, NUMBER 21
1 DECEMBER 1998
Spectroscopic detection and characterization of iodogermylene „HGel…
a)
Warren W. Harper, Chad M. Klusek, and Dennis J. Clouthier
Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0055
͑
Received 13 July 1998; accepted 25 August 1998͒
Monoiodogermylene has been detected for the first time using pulsed discharge and laser-
induced fluorescence techniques. HGeI and DGeI were produced by an electric discharge
through argon seeded with H GeI or D GeI. Although the vibronic structure in the spectra was very
3
3
limited, all three excited state vibrational frequencies have been obtained for both isoto-
0
0
pomers. Analysis of the partially resolved rotational structure of the 0 bands gave the follow-
ing approximate r0 structures, with the bond angles constrained to previous ab initio
values: r0Љ(Ge–I)ϭ2.525(10) Å, r0Љ(H–Ge)ϭ1.593(15) Å, ␪0Љ(HGeI)ϭ93.5°, rЈ0(Ge–I)
ϭ2.515(10) Å, rЈ(H–Ge)ϭ1.618(15) Å, and ␪Ј(HGeI)ϭ116.2°. The fluorescence lifetime of
0
0
monoiodogermylene in the lowest rovibronic levels is 1.515Ϯ0.004 ␮s and shows significant
variations on deuteration and with rotational and vibrational level. © 1998 American Institute of
Physics. ͓S0021-9606͑98͒00445-0͔
dichlorogermylene.16 In 1993, Saito and Obi reported the
6
I. INTRODUCTION
first observation of the electronic spectra of GeH and GeD ,
obtained by photolysis of phenylgermane in a supersonic ex-
2
2
Spectroscopic studies of the germylenes ͑XYGe:͒ have
lagged behind similar studies of the carbenes and silylenes.
1
pansion. They observed five vibronic bands for each species
For example, the electronic spectra of methylene (CH ) and
2
n
0
2
(2 , nϭ0–4) and showed that each band consisted of only
silylene (SiH ) have been known since 1959 and 1968, re-
spectively, and high-resolution infrared spectra have been
2
3
–5
1–3 rotational lines, due to an upper state rotation-induced
predissociation. Contemporaneously, we observed the spec-
obtained for both species, while the electronic spectrum of
6
trum of GeH using the pulsed discharge jet technique with
2
GeH was first reported only five years ago and the gas-
2
GeH as a precursor, and were able to obtain sufficient rota-
4
phase infrared spectrum is as yet unknown. The major rea-
sons for this seeming lack of interest in the germylenes are
the spectroscopic complications inherent in the existence of
five isotopes of germanium in natural abundance and the lack
of readily available germylene precursors. Despite these dif-
ficulties, significant progress has been made in recent years.
The germylenes do have a variety of redeeming features
that make them attractive to spectroscopists. Divalent germa-
nium compounds are more stable than those of carbon or
silicon so that they can be generated in higher concentra-
tions. The larger spin-orbit coupling constant of germanium
would be expected to enhance triplet–singlet transitions, so
that they might be studied in detail—few such carbene or
silylene transitions have been observed. Finally, the ger-
mylenes are thought to be important intermediates in a vari-
ety of semiconductor growth processes for the production of
germanium films, so that development of spectroscopic
methods for the detection, quantification and characterization
of these intermediates is of practical as well as fundamental
interest.
0
0
tional structure in the 0 band to obtain the first ground and
1
7
excited state structures of germylene. In conjunction with
our experimental work, we used the CASSCF method to
characterize those features on the excited state potential en-
ergy surfaces responsible for the dissociation process and
located the transition states for both the direct dissociation
˜
1
1
1
ϩ
g
GeH2 (A B1)→Ge( D)ϩH2( ⌺ ) and the indirect triplet
3
3
1
ϩ
g
state dissociation GeH (
˜
a B )→Ge( P)ϩH ( ⌺ ).
2
1
2
The triatomic halogermylenes ͑HGeX, XϭF, Cl, Br, I͒
have received little attention until very recently. In early
work, Isabel and Guillory1
8,19
photolyzed various haloger-
manes in argon matrices and identified all the vibrational
fundamentals of HGeBr and DGeBr and a single absorption
band of HGeCl. The electronic emission spectra of HGeCl
and DGeCl were studied by Patel and Stewart20 in 1977 by
dispersing the chemiluminescence given off by the reaction
of germane with chlorine. The bending and Ge–Cl stretching
frequencies in the ground and excited states were determined
0
0
and the 0 band was assigned to a feature at 464.31 nm. Ito
In recent years, we have studied the laser-induced fluo-
rescence spectra of several triatomic germylenes in super-
et al. reported the first LIF studies of the electronic spectra of
21
22
HGeCl and HGeBr, produced at ambient temperatures by
the reaction of germane with chlorine or bromine atoms.
sonic expansions. Building on earlier low-resolution high-
temperature studies in the literature,7
–14
we were able to
¯
They were able to determine A-B values for several bands of
˜
1
˜
1
obtain vibrationally resolved spectra of the A B -X A and
a B -X A band systems of difluorogermylene and
1
1
each species and, using estimates of the H–Ge and Ge–X
bond lengths from other molecules, they obtained bond
angles that reproduced the observed rotational constants. In
1998, we reported the first jet-cooled spectra of HGeCl and
3
˜
1
15
˜
1 1
a͒Author to whom correspondence should be addressed.
0021-9606/98/109(21)/9300/6/$15.00
9300
© 1998 American Institute of Physics
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1

J. Chem. Phys., Vol. 109, No. 21, 1 December 1998
Harper, Klusek, and Clouthier
9301
HGeBr,23 obtained using the pulsed discharge technique with
halogermane (H GeX) precursors. The excited state vibra-
3
tional frequencies were determined for the hydrogen and
0
0
deuterium isotopomers of each molecule and the 0 bands
were rotationally analyzed. Molecular structures were deter-
mined for the ground and excited states, although correla-
tions between the parameters necessitated that the bond
angles be constrained to previously reported ab initio
2
4
values.
At the time the present work was undertaken, no experi-
mental studies of either HGeF or HGeI had been reported,
although the silylene analogs were well-known.25,26 Earlier,
we had attempted to detect monofluorogermylene by LIF
techniques, both by the reaction of germane with fluorine
and by using H GeF as a precursor in our pulsed discharge
3
FIG. 1. Portions of the low-resolution A AЉ-X 1AЈ LIF spectra of jet-
˜
1
˜
23
jet, without success. We concluded that HGeF, if it can be
prepared at all, likely does not fluoresce on electronic exci-
tation. We next turned our attention to iodogermylene and in
the present work we report the successful spectroscopic iden-
tification and characterization of HGel.
cooled DGel and HGeI showing the vibrational assignments and partially
resolved rotational structure. An asterisk denotes a feature due to GeH or
2
GeD2.
23
we used for synthesizing H GeCl and H GeBr. Phenylger-
3
3
mane was synthesized by the LiAlH reduction of trichlo-
4
II. EXPERIMENT
rophenylgermane ͑Gelest͒ in ether. A small quantity ͑1–2 g͒
of phenylgermane was transferred in vacuum to a 200 ml
Pyrex reaction vessel equipped with a Teflon vacuum stop-
cock. A slight molar excess of HI was added to the vessel
which was then sealed and allowed to react overnight at a
temperature of Ϫ78 °C. After removing excess HI, the crude
Iodogermylene was synthesized in a pulsed discharge
jet27,28 by seeding the vapor pressure of H GeI at Ϫ30 °C in
3
4
0
0 psi of argon and expanding the gas mixture through the
.8 mm orifice of a pulsed molecular beam valve ͑General
Valve, Series 9͒. The gas pulse traveled downstream through
a 3 mm flow channel drilled into a 26 mm long Delrin cyl-
inder attached to the end of the valve and fitted with a pair of
stainless steel ring electrodes. A high voltage pulse applied
to the electrodes at the appropriate time initiated a discharge
which dissociated the precursor, producing HGeI which was
cooled by further expansion downstream. The pulsed dis-
charge apparatus was mounted vertically in the center of a 20
in. vacuum chamber pumped by a 10 in. diffusion pump. The
H GeI product and benzene byproduct were distilled out of
3
the reaction flask in vacuum and used without further purifi-
cation. D GeCl was similarly prepared by reaction of
3
phenylgermane-d3 with DI. The purity and identity of the
precursors was established by gas-phase infrared
spectroscopy.3
0–32
III. RESULTS AND ANALYSIS
Ϫ6
background pressure was ϳ2ϫ10 Torr, rising to ϳ3
High quality CASSCF and MRSDCI calculations by
Ϫ5
ϫ10 Torr when the pulsed valve was in operation at 10
24
Benavides-Garcia and Balasubramanian predict that HGeI
Hz.
should be a bent molecule of C symmetry with a ground
S
The supersonic expansion was crossed horizontally with
the beam from a tunable dye laser ͑Lumonics, HyperDye
300͒ and the resulting fluorescence imaged through a suit-
able long-wave pass filter onto the photocathode of a photo-
multiplier ͑EMI 9816QB͒. The spectra were calibrated with a
pulsed laser wavemeter ͑Burleigh WA-4500͒ to an accuracy
Ϫ1
of Ϯ0.02 cm . The pulsed fluorescence signals were pro-
cessed with gated integrators and recorded on a computer-
ized data acquisition system of our own design.29
Fluorescence decays over three or more lifetimes were
recorded by digitizing the PMT output on a digital storage
oscilloscope ͑LeCroy 9450A͒ with a temporal resolution of
2.5 ns per data point. The discharge and scattered laser light
backgrounds were subtracted from the decay curve by
slightly detuning the laser wavelength from the feature of
interest. The decay curves were averaged over several hun-
dred laser shots, background subtracted, and transferred to a
computer for analysis.
1
1
FIG. 2. Low-resolution spectra of the 30 and 10 bands of HGeI plotted on
1
the same wave number scale. The 3 band shows resolved germanium iso-
tope splittings and a greater Q Ϫ Q separation due to a larger value of
0
r
p
The iodogermane precursor was synthesized by the reac-
tion of phenylgermane with HI, similar to previous methods
1 1
¯
(A-B) in the excited state.
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1

9302
J. Chem. Phys., Vol. 109, No. 21, 1 December 1998
Harper, Klusek, and Clouthier
TABLE I. Assignments and band origins for the observed vibronic bands in
Ϫ1
the LIF spectrum of HGeI and DGeI ͑in cm ͒.
Band origina
Assignment
HGeI
DGeI
0
0
18 929.294
19 132.7
19 266.4
19 333.4
19 584.6
20 471.8
20 674.1
20 740.0
19 001.759
19 206.8
19 261.5
¯
0
1
0
3
1
0
2
2
0
3
2
0
2
19 490.8
20 146.0
¯
1
0
1
1
0
1
0
1
1
0
3
1
0
1
2 ?
¯
aThe 0 band origins are from high-resolution spectra and are accurate to
0
0
Ϫ1
Ϫ1
Ϯ0.005 cm ; others have an estimated error of Ϯ0.2 cm
.
state structure of r(Ge–I)ϭ2.580 Å, r(Ge–H)ϭ1.575 Å,
and ␪(HGeI)ϭ93.5°. Such a molecule will have 3aЈ vibra-
tional modes labeled ␯ ͑Ge–H stretch͒, ␯ ͑bend͒, and ␯
1
2
3
˜
1
˜
1
͑
Ge–I stretch͒. The allowed A AЉϪX AЈ electronic transi-
Ϫ1
tion is predicted to occur about 1600 cm lower in energy
than that of HGeBr,23 at about 19 060 cm . This transition
is expected to give rise to C-type bands with unrestricted
vibrational quantum number changes on excitation. Germa-
nium has five naturally occurring isotopes with relative
Ϫ1
0
0
FIG. 3. High-resolution spectra of selected branches of the 0 bands of
HGeI and DGeI. The top panel shows the KЈϭ1ϪKЉϭ0 subband of HGeI,
a
a
with resolved germanium isotope splittings in the Q-branch and partially
resolved P- and R-branch structure. The middle panel shows the isotopically
resolved qQ0- and qQ1-branches generated by the axis-switching mecha-
70
72
73
abundances of
Geϭ21.23%,
Geϭ27.66%,
Ge
nism. The bottom panel illustrates the KЈϭ1ϪKЉϭ0 subband of DGeI and
7
4
76
a
a
ϭ7.73%, Geϭ35.94%, and Geϭ7.44%, which would
be expected to complicate the observed rovibronic structure
in the spectra of iodogermylene.
the rotational assignments.
Weak spectra of HGeI and DGeI were found in the
Ϫ1
0
1
8 900–20 500 cm ͑530–490 nm͒ region, with the 00
intensity from the C-type transitions through the axis-
Ϫ1
33
bands at 18 929 and 19 002 cm , respectively. Low wave
number spectra of both HGeI and DGeI are shown in Fig. 1,
illustrating the typical three subband pattern expected for
switching or axis-tilting mechanism. The maximum of the
q
Q0 branch was taken as the band origin for these bands. The
higher bands were only observed at low resolution and did
0
C-type bands under jet-cooled conditions. The 00 band is
not show pronounced axis-switching Q-branches. For them,
Ϫ1
¯
r
p
shifted 72 cm to the blue on deuteration, comparable to the
the value of (A-B)Ј, approximated as ( Q Ϫ Q )/4, was
1 1
Ϫ1
r
shifts observed for HGeCl ͑99.8 cm ͒ and HGeBr ͑86.13
subtracted from the Q -branch maximum to obtain an esti-
0
Ϫ1
cm ͒ and due primarily to the proportionally greater change
mate of the band origin. The results are given in Table I.
1
0
0
in ␯ on excitation in the hydrogen compound. The 3 and
We had hoped that the 00 bands of HGeI and DGeI
1
1
0
2
bands are readily apparent in the spectra of Fig. 1 and
would not show an appreciable germanium isotope effect
and could be rotationally resolved, as was the case for the
spectra of chloro- and bromogermylene. However, all of the
Q-branches in the high-resolution iodogermylene spectra ex-
hibited resolvable isotope splittings, as illustrated in Fig. 3
and Table II. In general, this meant that the P- and R-
branches consisted of a jumble of unresolved transitions
from the various germanium isotopomers, making a detailed
rotational analysis impossible at our highest resolution of
give values for the Ge–I stretching and HGeI bending ex-
Ϫ1
cited state fundamentals of 203.4 and 337.1 cm , respec-
1
tively. As shown in Fig. 2, the 30 band has a substantial
germanium isotope effect, so that the Q-branch intensity is
distributed over several weak features, whereas transitions
1
involving only the Ge–H stretching mode, such as 1 , or the
0
n
bending mode (2 ) do not exhibit resolved isotope effects.
The smaller Q Ϫ Q separation of the 1 band as shown in
0
r
p
1
0
1
1
Ϫ1
r
Fig. 2 is also diagnostic of transitions involving ␯Ј , as
0.04 cm . Fortunately, as shown in Fig. 3, the P0- and
1
2
5,27,28
r
shown in our previous work on the halosilylenes
and
R0-branches had a few resolved rotational lines, due to a
2
3
halogermylenes. Both the germanium isotope effects and
the Q-branch separations were used as an aid in making the
vibronic assignments collected in Table I.
fortuitous blending of the various isotopic transitions, so that
sufficient information was obtained to derive approximate
molecular structures in the combining states.
¯
The band origins of the observed transitions were deter-
Approximate values of A-B were obtained from the Q-
0
74
mined in one of two ways. High-resolution spectra of the 00
branch maxima of the H GeI isotopomer using the data
q
bands ͑vide infra͒ show pronounced Q bands which gain
given in Table II and the combination relations:
0
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J. Chem. Phys., Vol. 109, No. 21, 1 December 1998
Harper, Klusek, and Clouthier
9303
TABLE II. Q-branch maxima for the 0 bands of the various isotopomers of HGeI and DGeI ͑in cmϪ1͒.
0
0
HGeI
DGeI
70Ge
72Ge
74Ge
76Ge
70Ge
72Ge
74Ge
76Ge
pQ2
pQ1
qQ0
qQ1
rQ0
rQ1
sQ0
rQ2
¯
¯
¯
¯
18 992.032
18 998.066
19 001.412
19 002.159
19 005.559
19 014.512
19 017.872
19 024.892
18 992.209
18 998.244
19 001.597
19 002.344
19 005.720
19 014.675
19 018.035
19 025.040
18 992.377
18 998.405
19 001.759
19 002.491
19 005.890
19 014.830
19 018.174
19 025.179
18 992.530
18 998.567
¯
a
18 922.282
18 928.959
18 930.288
18 936.981
18 954.297
18 960.979
¯
18 922.462
18 929.131
18 930.468
18 937.145
18 954.460
18 961.134
¯
18 922.634
18 929.294
18 930.615
18 937.308
18 954.624
18 961.280
¯
18 922.781
18 929.449
18 930.778
18 937.472
18 954.771
18 961.427
¯
¯
19 006.036
19 014.978
19 018.352
19 025.311
aThe estimated error for these measurements is Ϯ0.005 cmϪ1
.
K
r
p
¯
¯
lines, since B does not change significantly on electronic
⌬2
FЉ͑J,K͒ϭ Q
Ϫ Q
ϭ4͑AЉϪBЉ͒K,
Kϩ1
͑1͒
͑2͒
͑3͒
KϪ1
74
74
excitation in these molecules. For both H GeI and D GeI,
the R (0) line was calculated to occur near and be over-
lapped by the stronger Q -branch of the
isotopomer—by extrapolation, the first resolved R-branch
line was assigned as R (2). The numbering of the
P -branch was established by calculating combination dif-
r
0
s
r
r
q
q
p
¯
r
76
Q Ϫ Q ϭ Q Ϫ Q ϭ Q Ϫ Q ϭAЉϪBЉ,
0
Ge
1
0
1
0
1
0
r
K
r
p
¯
0
⌬2
FЈ͑J,K͒ϭ Q Ϫ Q ϭ4͑AЈϪBЈ͒K,
r
K
K
0
ferences of the type:
K
s
q
⌬2
FЈ͑J,K͒ϭ Q Ϫ Q ϭ4͑AЈϪBЈ͒K.
͑4͒
K
K
¯
1
2
⌬2
FЈ͑J͒ϭR͑J͒ϪP͑J͒ϭ4BЈ͑Jϩ ͒,
͑6͒
͑7͒
¯
r
In order to calculate B, the numbering of the resolved P -
0
r
and R -branch lines had to be established. First, the position
0
r
1
2
of the R (0) line was calculated from:
¯
0
⌬ FЉ͑J͒ϭR͑JϪ1͒ϪP͑Jϩ1͒ϭ4BЉ͑Jϩ ͒,
2
r
¯
¯
˜
␯͑ R ͑0͒͒ϭT ϩ͑AЈϪBЈ͒ϩ2BЈ,
͑5͒
0
0
¯
with BЈ assumed to be half the R-branch line spacing. The
assignments are given in Table III. Once satisfactory P- and
R-branch assignments were made, combination differences
q
¯
with T taken as the Q -branch maximum and 2BЈ approxi-
0
0
¯
mated from the average spacing of the resolved R-branch
of types ͑6͒ and ͑7͒ were used to obtain average B values in
the ground and excited states. By assuming zero inertial de-
fect, a crude but necessary assumption in this case, approxi-
mate values of the rotational constants A , B , and C were
obtained for the ground and excited states. The derived rota-
tional constants are collected in Table IV.
TABLE III. Rotational line frequencies ͑cmϪ1͒ and assignments for the
0
0
0
r
r
0
R (J)- and P (J)-branches of the 0 bands of HGeI and DGeI.
0
0
0
Assignment
HGeI
DGeI
r
18 937.6023a
18 937.7168
18 937.8148
18 937.9129
18 938.0192
18 938.1254
¯
R (2)
19 006.1430
19 006.2431
19 006.3587
19 006.4589
19 006.5668
19 006.6746
19 006.7748
19 006.8750
19 006.9906
19 007.0908
¯
0
r
R (3)
0
r
R (4)
TABLE IV. Approximate rotational constants derived from the partially
0
r
0
R (5)
resolved rotational structure of the 00 bands of HGeI and DGeI.
0
r
R (6)
0
r
74
74
R (7)
Constant
H GeI
D GeI
0
r
R (8)
0
¯
r
A-B
7.9971
0.0551
8.0522
0.0553
0.0549
4.1035
0.0547
4.1582
0.0551
0.0544
R (9)
¯
0
¯
r
B
R (10)
¯
0
r
˜
1AЉ
A0
R (11)
¯
A
0
r
P (4)
18 936.8340
18 936.6951
18 936.5888
18 936.4580
¯
B0
C0
0
r
P (5)
19 005.2491
19 005.1489
19 005.0256
19 004.9255
19 004.8022
19 004.6789
0
r
P (6)
0
¯
r
A-B
6.6582
0.0558
6.7140
0.0560
0.0556
3.3685
0.0551
3.4236
0.0555
0.0546
P (7)
0
¯
r
B
P (8)
0
r
˜
1AЈ
A0
B0
C0
P (9)
¯
X
0
r
P (10)
¯
0
aThe estimated error for these measurements is Ϯ0.005 cmϪ1
.
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1

9
304
TABLE V. A comparison of experimental and ab initio structural parameters for the ground and excited states of the triatomic monohalogermylenes.
HGeI HGeBr HGeCl
J. Chem. Phys., Vol. 109, No. 21, 1 December 1998
Harper, Klusek, and Clouthier
This work
Ab initioa
Expt.b
Ab initioa
Expt.b
Ab initioa
c
c
r ͑Ge–X͒ Å
r ͑Ge–H͒ Å
2.515͑10͒
1.618͑15͒
116.2
2.591
1.625
2.308͑1͒
1.615͑1͒
2.368
1.594
2.146͑15͒
1.613͑2͒
2.185
1.575
˜
1AЉ
1AЈ
A
␪
͑HGeX͒°
116.2
116.3
116.3
114.5
114.5
c
c
r ͑Ge–X͒ Å
r ͑Ge–H͒ Å
2.525͑10͒
1.593͑15͒
93.5
2.580
1.575
93.5
2.329͑12͒
1.598͑6͒
2.388
1.572
93.9
2.171͑2͒
1.592͑1͒
2.201
1.589
94.3
˜
X
␪
͑HGeX͒°
93.9
94.3
a
From Ref. 24.
From Ref. 23.
b
c
Numbers in parentheses are estimated uncertainties in units of the last significant figure.
In principle, molecular structures for the ground and ex-
cited states of iodogermylene can be calculated from the data
in Table IV. As in previous work on HGeCl and HGeBr, we
were unable to determine all three parameters simultaneously
due to large correlations between the H–Ge bond length and
the HGeX angle. We elected to fix the bond angle at the ab
manium isotope effects became greater at higher energies, so
that the Q-branch intensities were spread over five resolved
transitions, making them difficult to identify. Finally, the
spectra were complicated by overlapping transitions of ger-
mylene and a variety of other unidentified impurities which
often obscured weak iodogermylene bands.
Despite these difficulties, we were fortunate to be able to
identify all three excited state vibrational frequencies of
HGeI and DGeI. The excited state vibrational frequencies of
all the known monohalogermylenes are summarized in Table
2
4
initio MRSDCI values to obtain approximate structures.
For the ground state, the two bond lengths were varied in a
74
least squares fit of the rotational constants of both H GeI
7
4
and D GeI, assuming a 0.003 Å contraction of the H–Ge
bond length on deuteration. A good fit was obtained for all
six rotational constants and the resulting structure is reported
in Table V. The excited state data did not fit as well, as the
VII. The ␯Ј stretching frequency, which is always the most
1
difficult to assign, is consistent with our previous analyses.
In the silylenes, the frequency of the ␯Ј fundamental in-
1
Ϫ1
Ϫ1
A constants of HGeI and DGeI were inconsistent with each
creases from 1747 cm for HSiCl to 1853 cm for HSiI
and a similar monotonic trend from 1263 cm for HGeCl to
1543 cm for HGeI is found in the germylenes. The H–
0
Ϫ1
other for a fixed bond angle of 116.2°. Either the excited
state ab initio bond angle is unreliable, or the assumption of
zero inertial defect is particularly bad in this case. We finally
Ϫ1
Ge–X bending and Ge–X stretching frequencies show a
regular decrease with increasing mass of the halogen atom
and are all lower than those of their silicon analogs.
74
resorted to fitting only the D GeI rotational constants which
gave the excited state structure reported in Table V.
We have explored the vibrational and rotational level
dependence of the fluorescence lifetimes of iodogermylene,
primarily to see if there is any evidence of excited state pre-
dissociation processes. For each vibronic band we excited
The derived structures of HGeI, although crude due to
limitations in the data, are reasonable when compared with
the other germylenes as shown in Table V. In the ground
state, the experimental Ge–X bond lengths show a regular
increase with the size of the halogen atom and are all con-
sistently overestimated by ab initio theory.24 The lower state
Ge–H bond lengths are very similar in all three germylenes,
precisely as is found in the monohalosilylenes. The ground
state ab initio bond angles24 do not change significantly for
the strong Q-branches, preferentially populating K ϭ0, 1, or
a
2
levels in the excited state. In all cases, a plot of the natural
logarithm of the fluorescence decay versus time was found to
be linear over three lifetimes. The results are given in Table
VI.
IV. DISCUSSION
TABLE VI. Fluorescence lifetimes ͑␮s͒ of HGeI and DGeI as a function of
excited rovibronic state.
The spectra of iodogermylene showed very few identifi-
able vibronic bands, in contrast to the spectra of chloro- and
Rovibronic State
HGeI
DGeI
0
bromogermylene. In all three molecules the 0 bands are the
0
00
KЈϭ0
1.515Ϯ0.004 ͑2͒a
1.752Ϯ0.041 ͑11͒
1.520Ϯ0.007 ͑2͒
1.658Ϯ0.048 ͑10͒
1.830Ϯ0.060 ͑5͒
2.538Ϯ0.049 ͑10͒
2.310Ϯ0.036 ͑5͒
1.522Ϯ0.033 ͑5͒
1.569Ϯ0.035 ͑10͒
1.522Ϯ0.052 ͑5͒
1.682Ϯ0.050 ͑10͒
¯
strongest, but in iodogermylene few higher transitions were
observed despite considerable effort. At least part of the dif-
ficulty was experimental—the iodogermane precursors were
particularly difficult to work with in the discharge jet and we
found it impossible to maintain a stable discharge for very
long. The electrodes became coated with a thin layer of ma-
terial, perhaps elemental germanium, which disrupted the
discharge and could only be remedied by dismantling and
cleaning the apparatus. A second difficulty was that the ger-
a
0
0
0
3
2
KЈϭ1
a
0
KЈϭ2
a
1
KЈϭ1
a
1
KЈϭ0
a
1
2
KЈϭ1
1.619Ϯ0.059 ͑10͒
¯
a
1
2
KЈϭ2
a
aThe lifetimes are the average over the number of the number of determi-
nations given in parenthesis. The error limits are three times the standard
deviation of the number of measurements.
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1

J. Chem. Phys., Vol. 109, No. 21, 1 December 1998
Harper, Klusek, and Clouthier
9305
TABLE VII. Excited state vibrational frequenciesa and electronic origins for the monohalogermylenes ͑in
Ϫ1
cm ͒.
7
4
35
b
74
35
b
74
81
b
74
81
b
74
H GeIc
74
D GeIc
H Ge Cl
D Ge Cl
H Ge Br
D Ge Br
␯
␯
␯
Ј
1262.9
431.2
398.8
979.6
321.7
398.9
1380.8
419.3
280.3
1047.6
312.1
278.5
1542.5
337.1
203.4
1144.2
259.7
205.0
1
Ј
2
Ј
3
T0
21 515
21 614
20 660
20 746
18 929
19 002
a
In each case, these are the frequencies of the vibrational fundamentals, uncorrected for anharmonicity. Each is
Ϫ1
accurate to Ϯ0.2 cm
.
b
c
From Ref. 23.
This work.
different halogens, as is also found for the silylenes. On elec-
tronic excitation, the Ge–X bond lengths decrease and the
Ge–H bond lengths increase, in accord with the silylenes.
The bond angles increase substantially on electronic excita-
tion in both the germylenes and silylenes, although the
slightly smaller excited state ab initio value24 for HGeI does
not follow the general trend and may be an anomaly.
ACKNOWLEDGMENTS
The authors wish to thank Eric Ferrall for synthesizing
the phenylgermane used to produce the precursors, Tony
Smith for measuring the fluorescence lifetimes, and Tony
Hostutler for help with the wavelength calibration. This work
was supported by the National Science Foundation.
The fluorescence lifetimes of monoiodogermylene show
interesting variations with rovibronic state and isotopomer.
As expected, the lifetimes are quite long, showing the same
trend as found in the monohalosilylenes of increasing life-
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20
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Ϫ1
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1