Single vibronic level emission spectroscopic studies of the ground state energy levels and molecular structures of jet-cooled HGeBr, DGeBr, HGeI, and DGeI

Article abstract of DOI:10.1063/1.2355496

Single vibronic level dispersed fluorescence spectra of jet-cooled HGeBr, DGeBr, HGeI, and DGeI have been obtained by laser excitation of selected bands of the A¹ A″-X ¹ A′ electronic transition. The measured ground state vibrational intervals were assigned and fitted to anharmonicity expressions, which allowed the harmonic frequencies to be determined for both isotopomers. In some cases, lack of a suitable range of emission data necessitated that some of the anharmonicity constants and vibrational frequencies be estimated from those of HGeCl/DGeCl and the corresponding silylenes (HSiX). Harmonic force fields were obtained for both molecules, although only four of the six force constants could be determined. The ground state effective rotational constants and force field data were combined to calculate average (r_z) and approximate equilibrium (r^z_e) structures. For HGeBr r_e^z(GeH) = 1.593(9) A, r_e^z(GeBr)=2.325(21) A, and the bond angle was fixed at our CCSD(T)/aug-cc-pVTZ ab initio value of 93.6°. For HGeI we obtained r_e^z(GeH) = 1.589(1) A, r _e^z(GeI)=2.525(5) A, and bond angle=93.2°. Franck-Condon simulations of the emission spectra using ab initio Cartesian displacement coordinates reproduce the observed intensity distributions satisfactorily. The trends in structural parameters in the halogermylenes and halosilylenes can be readily understood based on the electronegativity of the halogen substituent.

Full text of DOI:10.1063/1.2355496

THE JOURNAL OF CHEMICAL PHYSICS 125, 114301 ͑2006͒
Single vibronic level emission spectroscopic studies
of the ground state energy levels and molecular structures
of jet-cooled HGeBr, DGeBr, HGeI, and DGeI
Brandon S. Tackett, Yunjing Li, and Dennis J. Clouthiera͒
Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0055
Kezia L. Pacheco and G. Alan Schick
Department of Chemistry, Eastern Kentucky University, Richmond, Kentucky 40475-3102
Richard H. Judge
Department of Chemistry, University of Wisconsin-Parkside, Kenosha, Wisconsin 53141-2000
͑Received 21 April 2006; accepted 22 August 2006; published online 15 September 2006͒
Single vibronic level dispersed fluorescence spectra of jet-cooled HGeBr, DGeBr, HGeI, and DGeI
1
1
˜
˜
have been obtained by laser excitation of selected bands of the A AЉ-X AЈ electronic transition.
The measured ground state vibrational intervals were assigned and fitted to anharmonicity
expressions, which allowed the harmonic frequencies to be determined for both isotopomers. In
some cases, lack of a suitable range of emission data necessitated that some of the anharmonicity
constants and vibrational frequencies be estimated from those of HGeCl/DGeCl and the
corresponding silylenes ͑HSiX͒. Harmonic force fields were obtained for both molecules, although
only four of the six force constants could be determined. The ground state effective rotational
constants and force field data were combined to calculate average ͑rz͒ and approximate equilibrium
͑rze͒ structures. For HGeBr rez͑GeH͒=1.593͑9͒ Å, rze͑GeBr͒=2.325͑21͒ Å, and the bond angle was
fixed at our CCSD͑T͒/aug-cc-pVTZ ab initio value of 93.6°. For HGeI we obtained rze͑GeH͒
=1.589͑1͒ Å, rze͑GeI͒=2.525͑5͒ Å, and bond angle=93.2°. Franck-Condon simulations of the
emission spectra using ab initio Cartesian displacement coordinates reproduce the observed
intensity distributions satisfactorily. The trends in structural parameters in the halogermylenes and
halosilylenes can be readily understood based on the electronegativity of the halogen substituent.
© 2006 American Institute of Physics. ͓DOI: 10.1063/1.2355496͔
I. INTRODUCTION
͑GeH4͒ with bromine atoms and were able to resolve the K
structure in the low-lying bands, which yielded approximate
The germylenes ͑XYGe:͒ are thought to play an impor-
tant role as intermediates in semiconductor growth processes
for the production of germanium thin films, so the develop-
ment of spectroscopic methods for detecting and quantifying
them in the gas phase has many practical applications. The
study of these reactive molecules is also of fundamental in-
terest, as the carbenes, silylenes, and germylenes are in-
volved in a wide variety of chemical reactions and their
structures, chemical kinetics, energetics, and physical prop-
erties have been extensively studied in the last few decades.
For several years, we have been engaged in studies of
the electronic spectra of the carbenes and their heavier ana-
logs in our laboratory. The monohalogermylenes have been
of particular interest, as very little was known about their
spectra prior to our work. HGeBr was first reported by Isabel
and Gillory,1 as a product of the photolysis of H3GeBr in an
argon matrix. The authors observed all of the ground state
vibrational fundamentals of HGeBr and DGeBr by infrared
absorption spectroscopy and determined a simple valence
force field. Ito et al.2 studied the laser-induced fluorescence
spectra of HGeBr produced by the reaction of germane
¯
values for A-B and DK for the combining states. By transfer-
ring the Ge–H and Ge–Br bond lengths from other mol-
ecules, the authors derived ground and excited state bond
angles from their data. No experimental data were available
for HGeI or DGeI at the beginning of our studies.
The monohalogermylenes have been the subject of a
very small number of ab initio studies.3–5 For spectroscopic
purposes, the most useful are the multireference singles and
doubles configuration interaction ͑MRSDCI͒ calculations of
Benavides-Garcia and Balasubramaniam3 which predicted
the geometries, term values, and dipole moments of the three
lowest lying electronic states of HGeX ͑X=Cl,Br,I͒.
In 1998 we reported the laser-induced fluorescence ͑LIF͒
spectra6,7 of jet-cooled HGeCl, HGeBr, and HGeI, which al-
lowed us to refine our understanding of the ground and first
excited singlet states of these species. In each case, we were
able to obtain rotational constants of modest accuracy and
molecular structures were derived from these constants. Due
to the near coincidence of the a axis with the Ge–X bond
and correlations between the bond angle and Ge–H bond
length, we were forced to constrain the bond angle to an
ab initio value in the structure determinations. These studies
did not give much information about the ground state vibra-
a͒
Author to whom correspondence should be addressed. Electronic mail:
dclaser@uky.edu
0021-9606/2006/125͑11͒/114301/8/$23.00
125, 114301-1
© 2006 American Institute of Physics

114301-2
Tackett et al.
J. Chem. Phys. 125, 114301 ͑2006͒
tional energy levels, although a few hot bands were detected.
Recently, we have reported8 single vibronic level emission
spectra of jet-cooled HGeCl and DGeCl, which allowed us to
determine the ground state harmonic force field and obtain a
good estimate of the equilibrium molecular structure. In the
present work we report similar studies of HGeBr and HGeI
and their deuterated isotopomers.
II. EXPERIMENT
The monohalogermylenes ͑HGeBr, DGeBr, HGeI, or
DGeI͒ were produced by seeding the vapor of the appropri-
ate cooled ͑from −40 °C up to room temperature depending
on the strength of the signal͒ monohalogermane ͑H3GeX or
D3GeX; X=Br or I͒ into 40 psi of argon, injecting pulses of
this gas mixture into a flow channel, and striking an electric
discharge through each gas pulse prior to expansion of the
products into a vacuum chamber. This pulsed discharge
source, described in detail elsewhere,9,10 produced a variety
of products, including substantial quantities of the ger-
mylenes ͑HGeX or DGeX͒ and diatomic GeX species. The
reactive intermediates were rotationally and vibrationally
cooled by free jet expansion into vacuum at the exit of the
pulsed discharge apparatus.
FIG. 1. An example of a dispersed emission spectrum of HGeBr. The A˜-X˜
vibronic transition pumped by the excitation laser is given in bold at the left
of the experimental trace and the lower state vibrational level assignments
are indicated. The wave number scale is displacement from the excitation
laser, giving a direct measure of the ground state vibrational energy. The
lower trace is a Franck-Condon simulation of the emission spectrum in the
harmonic approximation, obtained using ab initio ͑B3LYP/aug-cc-pVTZ͒
geometries and force fields for the two combining states.
III. RESULTS AND ANALYSIS
Single vibronic level ͑SVL͒ emission spectra were ob-
tained by fixing the wavelength of a 355 nm pumped tunable
dye laser ͑Lumonics HD-500͒ or a low-resolution ͑4 cm−1͒
Both HGeBr and HGeI are bent molecules of Cs sym-
metry with three a vibrational modes conventionally labeled
Ј
p
optical parametric oscillator ͑GWU VisIR2-ULD͒ on the Q1
as ␯ ͑Ge–H stretch͒, ␯ ͑bend͒, and ␯ ͑Ge–X stretch͒. The
1
2
3
branch of a band in the LIF spectrum and imaging the result-
ing fluorescence with f /1.5 optics onto the entrance slit of a
0.50 m scanning monochromator ͑Spex 500M͒. The wave-
length resolved fluorescence signals were detected with a
cooled, red-sensitive photomultiplier ͑RCA C31034A͒, am-
plified with a gain of 800, processed using a gated integrator
and recorded digitally. A 1800 lines/mm grating blazed at
400 nm was employed in this work, with a bandpass of
0.2–0.4 nm, depending on the signal intensity. The mono-
chromator scan was calibrated to an estimated accuracy of
2 cm−1 using known emission lines from argon and neon
lamps.
1
1
˜
˜
A AЉ-X AЈ electronic transition involves electron promo-
tion from a doubly occupied, nonbonding ͑␴ ͒ lone pair
nb
orbital on germanium to an out-of-plane germanium p orbital
and results in absorption bands following c-type selection
rules. Although the molecules of interest have several natu-
rally abundant isotopomers, in most cases the isotope split-
tings were not resolved in the LIF or emission spectra, so in
all calculations involving isotopic masses, we have assumed
that we are dealing with the most abundant isotopic species.
Although it makes little difference in the final analysis, we
prefer this approximation over the use of atomic weights,
since we usually excite a Q-branch maximum in the LIF
spectrum which preferentially excites the predominant isoto-
pomer to some extent.
Bromogermane ͑H3GeBr͒ and iodogermane ͑H3GeI͒
were synthesized by condensing a slight molar excess of hy-
drogen bromide or hydrogen iodide onto degassed phe-
nylgermane at liquid nitrogen temperature.6,7 The reaction
mixture was sealed in a Pyrex vessel, cooled with dry ice,
and allowed to warm slowly overnight to room temperature.
The resulting mixture of the halogermane, benzene, and
small quantities of GeH4 was purified by trap-to-trap distil-
lation in vacuum to isolate the desired product, whose purity
was established by gas phase infrared spectroscopy. The deu-
terated halogermanes ͑D3GeBr and D3GeI͒ were prepared in
a similar fashion using phenylgermane-d3 and DBr or DI.
Phenylgermane and phenylgermane-d3 were prepared by the
lithium aluminum hydride ͑or deuteride͒ reduction of phe-
nyltrichlorogermane ͑Gelest͒.11 The hydrogen bromide
͑Matheson͒ and deuterium bromide ͑Cambridge Isotope
Labs͒ were obtained commercially. Hydrogen iodide and
deuterium iodide were synthesized by the reaction of H2O or
D2O with a mixture of red phosphorus and iodine.12
A. HGeBr and DGeBr
We were able to obtain quite extensive SVL emission
spectra of the monobromogermylenes, following excitation
of the 000, 210, 301, and 101 bands of HGeBr and the 000, 201, 110,
110310, and 201310 bands of DGeBr. Interferences from GeBr
A-X emission13 excited simultaneously by the laser were
somewhat of a problem, especially for the higher energy LIF
bands. As a typical example of the emission spectra, we
show in Fig. 1 the spectrum of HGeBr obtained by laser
excitation of the 210 band. The first bending level of the ex-
cited state emits primarily down to the bending levels in the
ground state, although there is a node at 23, indicative of a
mismatch in the phase of the vibrational wave functions
which causes a minimum in the Franck-Condon factors. In a
similar fashion, excitation of the 310 band leads to strong

114301-3
Spectroscopic studies of HGeBr
J. Chem. Phys. 125, 114301 ͑2006͒
emission down to 31 and combinations of ␯Љ and ␯Љ. The
2
3
levels of the Ge–H stretching mode ͑␯Љ͒ are first evident on
1
excitation of the 110 band. The gas phase fundamentals of
1835, 694, and 291 cm−1 are in accord with the matrix
values1 of 1858, 701, and 283 cm−1, respectively.
The emission spectra of DGeBr were generally similar to
those of HGeBr, with prominent bending progressions ob-
served after 000 and 201 band excitations and activity in ␯Љ and
1
␯Љ after 110 excitation. Unfortunately, the 12 level could not be
2
observed in this case so the x11 anharmonicity constant was
not directly determinable. The 310 band proved to be too weak
to obtain emission spectra, although pumping other levels
involving ␯Ј gave some information on the Ge–Br stretching
3
levels in the ground state. Our gas phase values for the fun-
damentals ͑1322, 498, and 290 cm−1͒ are in excellent agree-
ment with the previous matrix values1 of 1336, 502, and
281 cm−1, respectively.
FIG. 2. An example of a dispersed emission spectrum of DGeI, obtained by
single vibronic level excitation of the 000 band, with ground state vibrational
assignments. The wave number scale is displacement from the excitation
laser, giving a direct measure of the ground state vibrational energy. The
lower trace is a Franck-Condon simulation of the emission spectrum in the
harmonic approximation, obtained using ab initio ͑B3LYP/aug-cc-pVTZ͒
geometries and force fields for the two combining states.
B. HGeI and DGeI
Obtaining good SVL emission spectra of HGeI and
DGeI proved to be much more difficult, because of the lim-
7
ited number of LIF bands observed in the A˜-˜X spectrum and
strong interferences from the A-X bands of GeI which occur
in the same region.14 Excitation of the 000 bands of HGeI and
DGeI gave strong, well-resolved emission spectra down to
the ground state bending levels, as illustrated by the DGeI
spectrum in Fig. 2. The 112n levels were observed in the 101
band emission spectrum of HGeI, although the 12 level was
not found, despite considerable effort. The 310 band of DGeI
was sufficiently intense and free of overlap to obtain mem-
bers of the 312n progression. Attempts to pump other bands
of HGeI and DGeI met with limited success, as they were
severely contaminated by GeI emission.
transferred or estimated from the HSiX and HGeCl species.
The resulting constants are summarized in Table II. The har-
monic frequencies ͑␻ ͒ were determined from the data in
i
Table II using the relationship15
1
␻ = ␻0 − x0 −
x0ij,
͑2͒
͚
i
i
ii
2
jꢀi
and are given in Table III.
With the harmonic frequencies of the hydrogen and deu-
terium isotopomers in hand, it is possible to obtain the
ground state harmonic force fields of HGeBr and HGeI. In
the latter case, the many assumptions made about the vibra-
tional constants constrains the normal coordinate analysis to
the determination of an approximate set of force constants.
In each case, we have used the previously derived r0
structure6,7 in the analysis and varied the force constants to
minimize the sum of the squares of the deviations between
the observed and calculated harmonic frequencies. The force
constants in internal coordinates ͑fij͒ were refined using the
ASYM20PC program of Hedberg and Mills.16 The lack of cen-
trifugal distortion constants or other data to help with the
refinement limited the number of determinable force con-
stants, so that f12 and f13 were constrained to 0.0 for both
molecules. The resulting refined force constants, which re-
produce the harmonic frequencies very satisfactorily to
within a few tenths of cm−1 in each case, are presented in
Table III.
The final aspect of the data analysis was to attempt to
obtain reliable estimates of the equilibrium structures of
HGeBr and HGeI in the ground state. The first issue was to
decide how to treat the bond angle, which, as previously
mentioned, is highly correlated with the Ge–H bond length.
In previous work, we had fixed the bond angle at the
ab initio MRSDCI value of Benavides-Garcia and
Balasubramaniam,3 but in this work we elected to do our
C. Data analysis
The measured ground state vibrational energy levels of
HGeBr, DGeBr, HGeI, and DGeI are summarized in Table I.
In each case, the reported values are taken from those bands
in the emission spectra which were as free of overlap as
possible and had the highest signal-to-noise ratio. A total of
37 levels were measured for HGeBr and 45 for DGeBr, and
these were all found to fit the usual anharmonic expression
3
3
3
0
G ͑ ͒ =
␻
+
i vi
x0
ijvivj
,
͑1͒
v
͚
͚ ͚
0
i=1
i=1 jജi
to within the estimated experimental accuracy ͑ 2 cm−1͒.
The vibrational frequencies and anharmonicities of HGeBr
were determined without constraints, whereas x011 of DGeBr
was transferred from the experimentally determined value
for DGeCl.8 Examination of the experimentally determined
anharmonicity constants for the HSiX and DSiX molecules
shows that they are remarkably consistent through the series,
as they are for HGeCl and HGeBr. Consequently, transfer-
ring the constants from known germylenes should lead to
little error. In an analogous fashion the energy levels of HGeI
and DGeI were fitted to Eq. ͑1͒, but many of the anharmo-
nicity constants and some of the vibrational frequencies were

114301-4
Tackett et al.
J. Chem. Phys. 125, 114301 ͑2006͒
TABLE I. Vibrational assignments, ground state vibrational energies, and residuals ͑in cm−1͒ from the single vibronic level emission spectra of HGeBr,
DGeBr, HGeI, and DGeI.
HGeBr
DGeBr
HGeI
DGeI
Assignment
Energy
Assignment
Energy
Assignment
Energy
Assignment
Energy
11
12
1835͑0.4͒
3601͑−0.1͒
694͑−0.3͒
1384͑−0.5͒
2069͑−0.3͒
2751͑0.2͒
3428͑0.2͒
4101͑1.1͒
4768͑0.5͒
5429͑−1.4͒
291͑0.3͒
579͑−0.6͒
2514͑−0.3͒
3189͑−0.5͒
3860͑0.2͒
4527͑0.4͒
5188͑−0.1͒
5846͑0.2͒
6499͑0.1͒
4927͑0.6͒
983͑−0.4͒
1671͑−0.4͒
2354͑−0.3͒
3034͑0.1͒
3709͑0.2͒
4379͑−0.1͒
5045͑0.7͒
1270͑0.3͒
1956͑0.4͒
2638͑0.6͒
3315͑−0.1͒
3987͑−0.5͒
2126͑0.8͒
2803͑−0.4͒
3477͑0.0͒
4145͑−0.1͒
4810͑−0.4͒
11
21
1322͑0.0͒
498͑−0.8͒
994͑−0.5͒
1489͑−0.3͒
1982͑0.2͒
2471͑0.1͒
2959͑0.2͒
3445͑−0.2͒
3929͑0.2͒
4410͑−0.3͒
4889͑−0.3͒
290͑−0.4͒
1813͑−0.5͒
2303͑−0.6͒
2790͑−0.1͒
3275͑0.1͒
3758͑−0.2͒
4239͑−0.2͒
4718͑0.4͒
5194͑−0.5͒
5670͑1.2͒
6142͑1.4͒
6610͑−0.9͒
1614͑0.1͒
787͑−0.6͒
1283͑0.4͒
1775͑0.0͒
2266͑0.2͒
2754͑0.6͒
3240͑0.1͒
3724͑0.3͒
4205͑−0.8͒
4686͑0.3͒
1566͑−2.3͒
2060͑0.9͒
2549͑1.0͒
3036͑1.2͒
3518͑−0.8͒
2104͑0.5͒
2592͑0.9͒
3078͑1.1͒
3560͑−0.6͒
4040͑−1.2͒
4521͑0.3͒
4996͑−1.1͒
11
21
1839͑0.5͒
661͑1.1͒
21
22
473͑0.7͒
943͑0.1͒
21
22
22
1316͑0.8͒
1965͑−0.8͒
2613͑0.3͒
3255͑−0.6͒
3895͑0.2͒
2483͑−0.2͒
3124͑−0.5͒
3761͑−0.0͒
4394͑−0.1͒
5023͑0.4͒
23
1411͑0.1͒
1877͑−0.7͒
2341͑−1.0͒
2805͑0.2͒
3265͑−0.1͒
3724͑0.8͒
4180͑−0.2͒
4635͑0.1͒
239͑−0.1͒
710͑−0.5͒
1178͑−0.6͒
1646͑0.5͒
2110͑0.7͒
2573͑0.6͒
3035͑1.2͒
3491͑−1.8͒
22
23
23
24
23
24
24
25
24
25
25
26
25
26
26
27
26
27
1121
1122
1123
1124
1125
28
27
28
29
28
29
210
31
31
210
32
31
2131
2231
2331
2431
2531
2631
2731
1121
1122
1123
1124
1125
1126
1127
1222
2131
2231
2331
2431
2531
2631
2731
2132
2232
2332
2432
2532
1131
112131
112231
112331
112431
1121
1122
1123
1124
1125
1126
1127
1128
1129
11210
11211
1131
2131
2231
2331
2431
2531
2631
2731
2831
2931
2232
2332
2432
2532
2632
112131
112231
112331
112431
112531
112631
112731
own theoretical predictions using the GAUSSIAN 03 suite of
programs.17 Calculations using both density functional
theory ͑B3LYP͒ ͑Refs. 18 and 19͒ and coupled cluster
singles and doubles with triple excitations included perturba-
tively ͓CCSD͑T͔͒ theory with Dunning’s correlation consis-
tent triple-zeta basis sets augmented by diffuse functions20
͑aug-cc-pVTZ͒ consistently gave bond angles of 0.3°–0.5°
smaller than the MRSDCI predictions, as shown in Table IV.
We chose to use the CCSD͑T͒/aug-cc-pVTZ bond angles in
all subsequent structure refinements.
Two kinds of structures were determined. First, the
average ͑rz͒ structures were derived by adding the harmonic
contributions to the vibration-rotation constants ͑␣͒ obtained
from the normal coordinate analysis to the previously deter-
mined =0 rotational constants6,7 ͑B0͒ to obtain the average
v
Љ
͑Bz͒ rotational constants. The structural parameters were then
refined by fitting the six average rotational constants, with
the bond angles constrained to our ab initio values. The

114301-5
Spectroscopic studies of HGeBr
J. Chem. Phys. 125, 114301 ͑2006͒
3
2
rze = rz − a͗u2͘ + K,
͑5͒
TABLE II. The ground state vibrational constants of HGeBr, DGeBr, HGeI,
and DGeI ͑in cm−1͒.
where a is defined in Eq. ͑4͒ and the other quantities are
defined above. The results are summarized in Table IV. All
the evidence suggests that rez structures derived in this fash-
ion are good approximations to the equilibrium structure. For
example, direct comparisons of the re and rze structures of
HSiF show excellent agreement,27 as do the experimentally
derived rze structure of HSiCl and the best estimated re struc-
ture, obtained from experimental B0 constants and ab initio
rotation-vibration ͑␣͒ parameters.28,29
HGeBr
DGeBr
HGeI
DGeI
a
0
1
0
2
0
3
x011
x022
x033
x012
x013
x023
␻
␻
␻
1868.20͑60͒
696.50͑11͒
291.81͑47͒
−33.82͑30͒
−2.208͑16͒
−1.10͑23͒
−14.68͑8͒
0.47͑40͒
1339.31͑44͒
499.60͑11͒
291.34͑64͒
−16.9
1872.36͑53͒
661.54͑26͒
240.0
1341.5
473.23͑16͒
239.86͑54͒
−18.0
−34.0
−1.067͑12͒
−0.71͑32͒
−7.09͑7͒
0.69͑49͒
−1.648͑69͒
−2.074͑52͒
−0.5
−0.971͑19͒
−0.5
−14.3
−7.0
0.0
0.0
−1.901͑55͒
−2.0
−1.67͑13͒
IV. DISCUSSION
aStandard errors of 1␴ are given in parentheses. Values without errors are
transferred from similar molecules and fixed in the least squares analysis.
In this work we have obtained SVL emission spectra
1
˜
from the A AЉ electronic states of HGeBr, DGeBr, HGeI,
and DGeI and used the data to refine the ground state har-
monic force fields and molecular structures. We have used
Franck-Condon simulations of the emission spectra as a
check on the validity of the assignments. Ideally, these would
be generated from the force fields of the ground and excited
states, as we have done previously for the HSiX molecules,28
but insufficient data were available from the LIF spectra of
the HGeX species to determine the excited state harmonic
frequencies, so we have resorted to ab initio theory. Using
the B3LYP/aug-cc-pVTZ method and basis set, we calcu-
lated the ground and excited state molecular structures, vi-
brational frequencies, and mass-weighted Cartesian displace-
ment coordinates, which were used as input to the simulation
program originally developed by Yang et al.30 and locally
modified for the calculation of SVL emission spectra. The
Franck-Condon factors were calculated in the harmonic ap-
average structure differs from the equilibrium structure
solely because of the anharmonicity of the vibrations. In the
structure determination, allowance was made for a 0.0035 Å
Laurie contraction ͑␦rz͒ of the Ge–H bond upon deuteration,
determined using21–23
3
2
␦rz = a␦͗u2͘ − ␦K,
͑3͒
where the change ͑␦͒ in the root-mean-square amplitude
͑͗u2͒͘ on deuteration and the change in the perpendicular
amplitude correction ͑K͒ were obtained from the force field.
The Morse anharmonicity parameter ͑a͒ ͑Ref. 24͒ was ob-
tained from the equation
6B2e
2
a = ␲␻
3
c␮
2hB3e
4
proximation and the appropriate frequency factors ͑Iϰ␯ ͒
ͱ
␣ +
,
͑4͒
ͩ ͪ
e
e
␻
e
were applied prior to convolution over a suitable line shape
function with a full width half maximum of 10 cm−1. Al-
though theory reproduced the ␯Ј and ␯Ј frequencies quite
where the ␻ and Be values are the ground state spectro-
e
2
3
scopic constants of the GeH diatomic molecule.25
well, ␯Ј was always substantially larger than experiment,
1
The ground state equilibrium bond lengths ͑rze͒ can be
estimated from the rz structure using anharmonicity data
from the appropriate diatomic molecules14,25,26 ͑GeI, GeH,
and GeBr͒ and the results of the normal coordinate analysis.
The bond lengths are given by the relationship most often
used in the analysis of microwave data,21–23
suggesting that the true potential is very anharmonic along
the Q1 normal coordinate. This anomaly had little effect on
the Franck-Condon simulations as the Ge–H stretch is inac-
tive in most of the experimental emission spectra.
Figure 1 shows the Franck-Condon simulation for the 201
band emission spectrum of HGeBr and the comparison with
TABLE III. The ground state harmonic force fields of HGeBr and HGeI.
HGeBr
DGeBr
HGeI
DGeI
Parameter
Obs.
Calc.
Obs.
Calc.
Obs.
Calc.
Obs.
Calc.
a
␻
␻
␻
͑cm−1
͑cm−1
͑cm−1
͒
͒
͒
1909.1͑1.0͒
707.0͑1.0͒
293.6͑1.0͒
1909.0
706.8
293.2
1359.4͑1.0͒
505.0͑1.0͒
292.5͑1.0͒
1359.5
505.3
292.9
1913.6͑1.0͒
671.7͑1.0͒
241.5͑1.0͒
1913.7
671.7
241.4
1363.0͑1.0͒
478.5͑1.0͒
241.2͑1.0͒
1362.8
478.6
241.3
1
2
3
b,c
f11 ͑aJ Å−2
f22 ͑aJ͒
͒
2.1349͑9͒
2.1454͑4͒
0.7418͑18͒
1.967͑20͒
0.154͑60͒
0.6698͑8͒
1.676͑38͒
0.220͑60͒
f33 ͑aJ Å−2
f23 ͑aJ Å−2
͒
͒
aEstimated uncertainties used in the force field refinement are given in parentheses.
bStandard errors of 1␴ are given in parentheses.
cThe internal coordinates are R1 =⌬r͑H–Ge͒, R2 =⌬␪͑H–Ge–X͒, and R3 =⌬r͑Ge–X͒.

114301-6
Tackett et al.
J. Chem. Phys. 125, 114301 ͑2006͒
TABLE IV. Comparison of the experimental and ab initio structural parameters and harmonic vibrational frequencies for the ground state of monobromoge-
rmylene and monoiodogermylene.
Expt.
Ab initio
b
r0
rz
rez
MRSDCIa
B3LYPb
CCSD͑T͒
HGeBr
2.325͑21͒
1.593͑9͒
93.6
c
r͑GeBr͒ Å
r͑GeH͒ Å
2.329͑12͒
2.327͑21͒
1.606͑9͒
93.6
2.388
1.572
93.9
¯
2.370
1.600
2.337
1.582
93.6
c
1.598͑6͒
␪͑HGeBr͒°
93.9c
93.5
d
␻ ͑cm−1
͒
͒
͒
1909.1/1359.4
707.0/505.0
293.6/292.5
1872/1334
693/495
275/275
1953/1391
713/510
293/292
1
d
d
␻ ͑cm−1
¯
2
␻ ͑cm−1
¯
3
HGeI
e
r͑GeI͒ Å
r͑GeH͒ Å
2.525͑10͒
1.593͑15͒
93.5e
2.527͑5͒
1.601͑1͒
93.2
2.525͑5͒
1.589͑1͒
93.2
2.580
1.575
93.5
¯
2.602
1.599
2.590
1.580
e
␪͑HGeI͒°
93.0
93.2
f
␻ ͑cm−1
͒
͒
͒
1913.6/1363.0
671.7/478.5
241.5/241.2
1889/1346
669/477
228/227
1947/1387
692/494
234/233
1
f
f
␻ ͑cm−1
¯
2
␻ ͑cm−1
¯
3
aFrom Ref. 3.
bThis work, using an aug-cc-pVTZ basis set. A 6-311G** basis set was used for the HGeI calculations.
cFrom Ref. 6.
dHGeBr/DGeBr harmonic frequencies.
eFrom Ref. 7.
fHGeI/DGeI harmonic frequencies.
experiment. The simulation reproduces the overall pattern of
activity in the bending mode quite satisfactorily, including
the minimum in intensity for the 23 level, but underestimates
the intensity of the higher members of the 21n progression.
Since the experimental spectrum is uncorrected for the often
substantial effects of the wavelength dependent sensitivity of
the detection system and the lesser time-dependent variations
in the discharge efficiency, the agreement is quite satisfac-
tory. Figure 2 compares the experimental 000 band emission
spectrum of DGeI and the simulation, which again repro-
duces the bending progression with high fidelity. In this case
there is an extraneous 1012n0 progression predicted theoreti-
cally which is not observed experimentally. We attribute this
discrepancy, which is also apparent in Franck-Condon simu-
lations of the HSiX species,27,28 to the substantial vibrational
anharmonicities in the excited state. A proper treatment
would necessitate explicitly accounting for anharmonic ef-
fects in the Franck-Condon calculations. Nevertheless, the
simulations provide convincing evidence that the assign-
ments are reliable.
Ge–H and Ge–D harmonic stretching frequencies ͑Table III͒
are very similar for monobromogermylene and
monoiodogermylene and differ only slightly from the fre-
quencies of HGeCl/DGeCl ͑1906.9/1360.4 cm−1͒ ͑Ref. 8͒
and GeH/GeD ͑1900.382/1353.317 cm−1͒.25 Similarly, the
germanium-halogen stretching frequencies of the germylenes
͑Table III͒ are very similar to ␻Љ=295.4 and 240.0 cm−1 for
e
GeBr ͑Ref. 26͒ and GeI.14 There can be little doubt that our
approximate anharmonicity analysis and the derivation of the
harmonic frequencies must be substantially correct.
In Table V we have collected together a comparison of
the ground state structures and vibrational frequencies of the
monohalogermylenes and germane. Unfortunately, there is
no experimental data for HGeF, as it has never been ob-
served, although we have tried to obtain the LIF spectrum in
the past.6 For completeness, we have calculated the proper-
ties of HGeF using coupled cluster theory and it is these
predictions that are included in Table V. The data suggest
that the Ge–H bond lengths are virtually invariant to halogen
substitution, contrary to the trend in the HSiX species where
there is a slight increase in the SiH bond length with increas-
ing electronegativity of the halogen.32 However, it is likely
that the bond length variations are obscured by fixing the
bond angle in the structure determinations, as the experimen-
As a further test of the derived harmonic frequencies, we
applied the Redlich-Teller product rule31 for HXY molecules
of Cs symmetry, using the rotational constants from our LIF
studies,6,7
*
*
*
3
2
2
tally derived ␻ vibrational frequencies and f11 stretching
1
␻ ␻ ␻
C
C*
M*
mH
1
2
= ͱ
.
͑6͒
ͩ ͪͩ ͪ ͩ ͪ
force constants do increase monotonically from HGeCl to
HGeI. The germanium-halogen bond lengths are comparable
to the available diatomic ground state values35–37 ͑GeF
=1.750 Å, GeCl=2.144 Å, and GeBr=2.308 Å͒ and similar
to the bond lengths of the dihalogermylenes38–41 ͑GeF2
=1.745 Å, GeCl2=2.183 Å, GeBr2=2.359 Å, and GeI2
=2.540 Å͒. The dihalogermylene symmetric stretching
frequencies42–45 ͓GeF2=692 cm−1, GeCl2=391.2 cm−1,
␻ ␻ ␻
1
M
mD
2
3
The experimental harmonic ratios ͑0.5067 for HGeBr and
0.5068 for HGeI͒ are in reasonable agreement with the the-
oretical ratios ͑0.5085 and 0.5075, respectively͒, suggesting
that the transfer of frequencies and anharmonicities from re-
lated molecules does not lead to significant errors in deter-
mining the harmonic frequencies. We also note that the

114301-7
Spectroscopic studies of HGeBr
J. Chem. Phys. 125, 114301 ͑2006͒
TABLE V. A comparison of the ground state structural parameters and vibrational frequencies of the
germylenes.
a
GeH2
HGeFb
HGeClc
HGeBrd
HGeId
r͑GeH͒ Å
r͑GeX͒ Å
1.588͑1͒
¯
1.587b
1.754
1.586͑1͒
2.171͑2͒
93.9
1.593͑9͒
2.325͑21͒
93.6
1.589͑1͒
2.525͑5͒
93.2
␪͑HGeX͒°
91.22͑4͒
1856/1335
916/657
1866/1352f
94.3
e
␻ ͑cm−1
͒
͒
͒
1939/1381
782/568
669/672
1906.9/1360.4
735.3/528.3
404.9/402.4
1909.1/1359.4
707.0/505.0
293.6/292.5
1913.6/1363.0
671.7/478.5
241.5/241.2
1
e
e
␻ ͑cm−1
2
␻ ͑cm−1
3
aReference 33.
bStructure and frequencies from CCSD͑T͒/aug-cc-pVTZ calculations of this work.
cReference 8.
dThis work.
eGeH2 /GeD2 or HGeX/DGeX frequencies.
fHarmonic frequencies from a fitted ab initio potential surface, Ref. 34.
GeBr2=286 cm−1 ͑Ar matrix͒, and GeI2=230 cm−1 ͑Ar ma-
ACKNOWLEDGMENTS
trix͔͒ are also very similar to the ␻ values in Table V. The
3
One of the authors ͑B.S.T.͒ acknowledges the support of
a National Science Foundation Graduate Research Fellow-
ship and two of the authors ͑G.A.S. and K.L.P.͒ acknowledge
funding from the Kentucky NSF EPSCoR program, spon-
sored by both the Commonwealth of Kentucky and NSF
͑EPSCoR Award No. 0132295͒. Another author ͑R.H.J.͒ ac-
knowledges the support of the NSF Research at Undergradu-
ate Institutions program. This research was supported by the
U.S. National Science Foundation, Grant No. CHE-0513495.
data indicate that the derived HGeX bond lengths and
stretching frequencies are entirely consistent with those of
similar molecules and with trends observed for the HSiX
species.
The HGeX bond angles decrease with increasing size of
the substituted halogen, a trend which is somewhat counter-
intuitive, but is exactly the same as was found for the
monohalosilylenes,32 so it can hardly be accidental. As ar-
gued previously,28 the electronegativity of the halogen sub-
stituent can be used to account for this effect. In the HGeX
series, we note that the calculated charge on the central ger-
manium atom increases with increasing substitutent elec-
tronegativity ͑HGeI=0.28, HGeBr=0.28, HGeCl=0.51, and
HGeF=0.70͒, consistent with the conclusion from theoretical
studies on the silylenes46 that electronegative substituents X
1 R. J. Isabel and W. A. Guillory, J. Chem. Phys. 57, 1116 ͑1972͒.
2 H. Ito, E. Hirota, and K. Kuchitsu, Chem. Phys. Lett. 177, 235 ͑1991͒.
3 M. Benavides-Garcia and K. Balasubramanian, J. Chem. Phys. 97, 7537
͑1992͒.
4 M. Su and S. Chu, Tetrahedron Lett. 40, 4371 ͑1999͒.
5 M. Su and S. Chu, J. Am. Chem. Soc. 121, 4229 ͑1999͒.
6 W. W. Harper and D. J. Clouthier, J. Chem. Phys. 108, 416 ͑1998͒.
7 W. W. Harper, C. M. Klusek, and D. J. Clouthier, J. Chem. Phys. 109,
9300 ͑1998͒.
induce a transfer of charge from the lone pair ␴ orbital on
nb
8 B. S. Tackett, D. J. Clouthier, K. L. Pacheco, and G. Alan Schick, J.
Chem. Phys. 124, 124320 ͑2006͒.
Si to the Si–X bond. Thus, in the present case, more electron
withdrawing substituents decrease the spatial extent of the
lone pair orbital, allowing the HGeX bond angle to increase.
The bond angle in GeH2 is 2°–3° smaller, as there is no
unbalanced inductive effect.
9 H. Harjanto, W. W. Harper, and D. J. Clouthier, J. Chem. Phys. 105,
10189 ͑1996͒.
10 W. W. Harper and D. J. Clouthier, J. Chem. Phys. 106, 9461 ͑1997͒.
11 J. M. Meyer and A. L. Allred, J. Phys. Chem. 72, 3043 ͑1968͒.
12 K. Clusius and G. Wolf, Z. Naturforsch. 2A, 495 ͑1947͒.
13 K. B. Rao and P. B. V. Haranath, J. Phys. B 2, 1385 ͑1969͒.
14 A. Chatalic, P. Deschamps, and G. Pannetier, C. R. Seances Acad. Sci.,
Ser. D 269, 1167 ͑1969͒.
This work completes our series of studies of the SVL
emission spectra of the jet-cooled halosilylenes and ger-
mylenes. A consistent picture emerges from the derived
ground state structures and vibrational frequencies of both
families of molecules. As a general rule, the halogen substi-
tuted silylenes and germylenes can be thought of as minor
modifications of the parent silylene ͑SiH2͒ or germylene
͑GeH2͒, with slight increases in the bond angle with increas-
ing halogen electronegativity. The SiH and GeH bond
lengths of HSiX and HGeX are very similar to those of the
parent molecules, but are slightly elongated in the fluorinated
species, due to inductive effects. The Si–X and Ge–X bond
lengths and vibrational frequencies can be also be predicted
with high fidelity from those of the corresponding diatomic
molecules. In the absence of extensive microwave or infrared
data, our derived rze structures are probably the best approxi-
mations to the equilibrium structures of these molecules that
can be experimentally obtained at this time.
15 G. Herzberg, Electronic Spectra and Electronic Structure of Polyatomic
Molecules, Molecular Spectra and Molecular Structure Vol. III ͑Van Nos-
trand, New York, 1966͒, p. 143.
16 L. Hedberg and I. M. Mills, J. Mol. Spectrosc. 160, 117 ͑1993͒.
17 M. J. Frisch, G. W. Trucks, H. B. Schlegel et al., GAUSSIAN 03, Revision
C.02, Gaussian, Inc., Wallingford, CT, 2004.
18 A. D. Becke, J. Chem. Phys. 98, 5648 ͑1993͒.
19 C. Lee, W. Yang, and R. G. Parr, Phys. Rev. B 37, 785 ͑1988͒.
20 T. H. Dunning, Jr., J. Chem. Phys. 90, 1007 ͑1989͒.
21 K. Kuchitsu, J. Chem. Phys. 49, 4456 ͑1968͒.
22 K. Kuchitsu, T. Fukuyama, and Y. Morino, J. Mol. Struct. 1, 463 ͑1968͒.
23 K. Kuchitsu, T. Fukuyama, and Y. Morino, J. Mol. Struct. 4, 41 ͑1969͒.
24 K. Kuchitsu and Y. Morino, Bull. Chem. Soc. Jpn. 38, 805 ͑1965͒.
25 J. P. Towle and J. M. Brown, Mol. Phys. 78, 249 ͑1993͒.
26 J. Borkowska-Burnecka, W. Zyrnicki, and N. Badowski, J. Mol. Spec-
trosc. 156, 245 ͑1992͒ and references therein.
27 W. W. Harper, D. A. Hostutler, and D. J. Clouthier, J. Chem. Phys. 106,
4367 ͑1997͒.
28 D. A. Hostutler, N. Ndiege, D. J. Clouthier, and S. W. Pauls, J. Chem.
Phys. 115, 5485 ͑2001͒.

114301-8
Tackett et al.
J. Chem. Phys. 125, 114301 ͑2006͒
29 J. Vázquez and J. F. Stanton, J. Phys. Chem. A 106, 4429 ͑2002͒.
30 D.-S. Yang, M. Z. Zgierski, A. Bérces et al., J. Chem. Phys. 105, 10663
͑1996͒.
39 G. Schultz, J. Tremmel, I. Hargittai, I. Berecz, S. Bohatka, N. D. Kagra-
manov, A. K. Maltsev, and O. M. Nefedov, J. Mol. Struct. 55, 207
͑1979͒.
31 G. Herzberg, Infrared and Raman Spectra of Polyatomic Molecules, Mo-
lecular Spectra and Molecular Structure Vol. II ͑Van Nostrand, New York,
1945͒, p. 230.
40 G. Schultz, M. Kolonits, and M. Hargittai, Struct. Chem. 11, 161 ͑2000͒.
41 N. I. Giricheva, G. V. Girichev, S. A. Shlykov, V. A. Titov, and T. P.
Chusova, J. Mol. Struct. 344, 127 ͑1995͒.
32 B. S. Tackett and D. J. Clouthier, J. Chem. Phys. 118, 2612 ͑2003͒.
33 T. C. Smith, D. J. Clouthier, W. Sha, and A. G. Adam, J. Chem. Phys.
113, 9567 ͑2000͒.
42 J. W. Hastie, R. Hauge, and J. L. Margrave, J. Phys. Chem. 72, 4492
͑1968͒.
43 J. Karolczak, Q. Zhuo, D. J. Clouthier, W. M. Davis, and J. D. Goddard,
J. Chem. Phys. 98, 60 ͑1993͒.
34 P. R. Bunker, R. A. Phillips, and R. J. Buenker, Chem. Phys. Lett. 110,
351 ͑1984͒.
44 R. J. Isabel, G. R. Smith, R. K. McGraw, and W. A. Guillory, J. Chem.
Phys. 58, 818 ͑1973͒.
35 M. Tanimoto, S. Saito, Y. Endo, and E. Hirota, J. Mol. Spectrosc. 116,
499 ͑1986͒.
45 S. A. Zaitsev, S. B. Osin, V. A. Koryazhkin, and V. F. Shevel’kov, Vestn.
Mosk. Univ., Ser. 2: Khim. 31, 128 ͑1990͒.
36 D. A. Hostutler, D. J. Clouthier, and G. Wannous, J. Mol. Spectrosc. 215,
66 ͑2002͒.
46 B. T. Luke, J. A. Pople, M. Krogh-Jespersen, Y. Apeloig, M. Karni, J.
Chandrasekhar, and P. V. R. Schleyer, J. Am. Chem. Soc. 108, 270
͑1986͒.
37 Calculated from the data in Ref. 26.
38 H. Takeo, R. F. Curl, and P. W. Wilson, J. Mol. Spectrosc. 38, 464
͑1971͒.