Journal of Molecular Spectroscopy 257 (2009) 128–132
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Journal of Molecular Spectroscopy
journal homepage: www.elsevier.com/locate/jms
The sub-millimeter and Fourier transform microwave spectrum of HZnCl (X 1R+)
R.L. Pulliam *, M. Sun, M.A. Flory, L.M. Ziurys *
Department of Chemistry, Department of Astronomy, Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA
a r t i c l e
i n f o
Article history:
Received 1 April 2009
In revised form 30 June 2009
Available online 8 July 2009
Keywords:
Fourier transform microwave spectroscopy
Sub-millimeter
Rotational spectroscopy
HZnCl
a b s t r a c t
The pure rotational spectrum of HZnCl (X 1R+) has been recorded using sub-millimeter direct-absorption
methods in the range of 439–540 GHz and Fourier transform microwave (FTMW) techniques from 9 to
39 GHz. This species was produced by the reaction of zinc vapor and chlorine gas with H2 or D2 in a
d.c. glow discharge for the sub-millimeter studies. In the FTMW measurements, HZnCl was created in
a discharge nozzle from Cl2 and (CH3)2Zn. Between 5 and 10 rotational transitions were measured in
the sub-millimeter regime for four zinc and two chlorine isotopologues; four transitions were recorded
with the FTMW machine for the main isotopologue, each consisting of several chlorine hyperfine components. The data are consistent with a linear molecule and a 1R+ ground electronic state. Rotational and
chlorine quadrupole constants were established from the spectra, as well as an rm(2) structure. The Zn–
Cl and Zn–H bond lengths were determined to be 2.0829 and 1.5050 Å, respectively; in contrast, the
Zn–Cl bond distance in ZnCl is 2.1300 Å, longer by 0.050 Å. The zinc–chlorine bond distance therefore
shortens with the addition of the H atom. The 35Cl electric quadrupole coupling constant of
eQq = 27.429 MHz found for HZnCl suggests that this molecule is primarily an ionic species with some
covalent character for the Zn–Cl bond.
Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction
High resolution gas-phase spectroscopy of small, zinc-containing species has advanced in recent years with the study of such
species as ZnCN [1], ZnF [2], ZnH [3], ZnCl [4] and ZnO [5]. Even
more recently, molecules with the zinc atom inserted into H–C
and H–Cl bonds have been investigated. For example, the rotational spectrum of HZnCN [6] has recently been recorded, as well
as that of HZnCH3 [7]. Using argon matrix methods, Macrea et al.
found that a weakly-bound linear Zn–HCl complex will spontaneously form from the reaction of zinc and HCl, which upon radiation,
creates the linear HZnCl species [8].
Yu et al. recently measured the gas-phase ro-vibrational spectrum of HZnCl using Fourier transform infrared methods [9]. These
authors reported rotational constants for four isotopologues of this
molecule, which was found to be linear with a 1R+ ground electronic state. The acquired spectra were, however, quite dense,
and ab initio calculations were necessary to carry out the assignments. An rs structure was determined using the rotational constants of the four isotopologues, resulting in the bond lengths in
the ranges rZn–H = 1.596–1.789 Å and rZn–Cl = 2.079–2.088 Å. Because the H atom was not isotopically substituted, there was considerable uncertainty in the Zn–H bond length. The work of Yu
* Corresponding authors. Fax: +1 520 621 5554.
E-mail addresses: [email protected] (R.L. Pulliam), [email protected].
edu (L.M. Ziurys).
0022-2852/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jms.2009.07.001
et al. was followed by a theoretical study of HZnCl by Kerkines
et al. [10], who employed CCSD(T) methods with relativistic corrections. These calculations suggested values for rZn–H and rZn–Cl
of 1.499 and 2.079 Å [10], respectively.
Here we report measurements of the pure rotational spectrum
of HZnCl. Data were obtained for seven isotopologues of this species, including DZnCl. Both direct absorption sub-millimeter-wave
and Fourier transform microwave (FTMW) methods were used in
this study. From the data, rotational, centrifugal distortion, and
the 35Cl-nuclear quadrupole coupling constants were determined,
as well as r0, rs, and rm(2) bond lengths. Here we describe our results
and analysis, and present revised experimental bond distances for
HZnCl.
2. Experimental
The pure rotational spectrum of HZnCl was measured using one
of the quasi-optical millimeter/sub-millimeter direct absorption
spectrometers of the Ziurys group. The details of the spectrometer
can be found in a previous publication [11]. To summarize, a
water-cooled reaction chamber was employed, constructed of
stainless steel with an attached Broida-type oven, and evacuated
by a Roots-type blower pump. The radiation, which originates from
a Gunn oscillator/Schottky diode multiplier source, is passed
through a polarizing grid, a series of mirrors, and into the reaction
cell, which is a double-pass system. The radiation is reflected back
through the optics by a rooftop mirror and from the grid into a
129
0.021
0.028
0.013
0.023
0.017
457 802.233
467 096.427
476 387.763
485 676.013
494 961.272
m
462 394.515
471 781.525
481 165.535
490 546.474
499 924.292
0.002
0.002
0.000
0.002
0.002
mobs–calc
m
0.031
0.011
0.004
474 219.197
483 845.027
493 467.647
Residuals from combined fit of sub-millimeter and FTMW data.
a
0.002
452 852.483
0.000
0.011
439 889.372
449 619.766
459 347.194
469 071.562
478 792.930
488 511.089
498 226.018
507 937.668
517 645.981
527 350.862
0.086
0.072
0.054
0.001
0.036
0.015
0.011
0.036
0.052
0.088
445 323.531
454 958.434
0.014
0.008
0.011
0.017
387 455.312
397 106.709
406 755.528
416 401.678
0.007
0.005
0.000
460 357.404
469 898.971
479 437.469
488 972.882
498 505.114
0.003
0.008
0.006
0.003
0.001
mobs–calc
m
mobs–calc
m
mobs–calc
m
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
mobs–calc
m
472 024.559
481 606.006
491 184.337
mobs–calc
m
354 682.387
363 988.280
373 291.980
382 593.467
391 892.672
401 189.552
410 484.003
419 776.027
mobs–calc
D64Zn35Cl
H66Zn37Cl
H64Zn37Cl
H68Zn35Cl
H67Zn35Cl
H66Zn35Cl
for the main isotopologue, H64Zn35Cl, and between 4 and 9 transitions for the other six species (H64Zn37Cl, H66Zn35Cl, H66Zn37Cl,
H68Zn35Cl, H68Zn37Cl, and D64Zn35Cl). In addition, the four lowest
energy rotational transitions of H64Zn35Cl were measured with
the FTMW instrument, as shown in Table 2. These transitions
are split by quadrupole interactions of the 35Cl nucleus (I = 3/2).
Several hyperfine components were measured for each transition,
and are labeled by quantum number F.
H64Zn35Cla
R+) are listed in Table 1. A total of ten transitions were recorded
J00
The sub-mm rotational frequencies measured for HZnCl (X
1
J0
3. Results and analysis
Table 1
Sub-mm transition frequencies in MHz of HZnCl (X 1R+:v = 0).
He-cooled InSb detector. The frequency source is modulated at
25 kHz and the data are detected at 2f using a lock-in amplifier.
HZnCl was produced from the reaction of zinc metal (99.9%,
Aldrich), vaporized using the Broida-type oven, with Cl2 and H2
gases. Approximately 5 mtorr (7 103 mbar) of Cl2 was added
over the top of the Broida oven and a 1:1 mixture of H2 and argon
carrier gas (30 mtorr or 40 103 mbar each) was introduced
underneath the oven. A d.c. discharge of 200 V and 50 mA was required for the synthesis. The signals from HZnCl were sufficiently
strong to detect four of the five isotopologues of Zn
(64Zn:66Zn:67Zn:68Zn = 49:28:4:19) and both those of chlorine
(35Cl:37Cl = 3:1) in natural abundance. To produce the deuterated
isotopic species, the hydrogen gas was replaced with 30 mtorr
(40 103 mbar) of D2.
To locate the spectrum of this molecule, a search was conducted over approximately a 35 GHz range (445–480 GHz), based
on the rotational constants of Yu et al. This broad search was necessary to locate all isotopologues. The main isotopologue of HZnCl
was readily identified in these data, as the species had the strongest signals. In the course of identifying the spectra of the other
isotopologues, it was found that some of the assignments by Yu
et al. were in fact due to ZnCl. Chlorine and 67Zn hyperfine splitting were not observed in these data, as expected at these higher J
values. To locate the deuterated species, rotational constants
were scaled from the main isotopologue of HZnCl. Scanning was
conducted ±200 MHz around the predicted frequencies, and
DZnCl lines were identified.
Center frequencies were determined from an average of two
5 MHz scans, one in increasing and the other in decreasing frequency. The line profiles were then fit to Gaussian profiles. Experimental uncertainties are estimated at ±50 kHz.
Additional measurements were conducted in the range of 9–
38 GHz using the Fourier transform microwave spectrometer
(FTMW) of the Ziurys group. In this case, the cell consists of a
large vacuum chamber with a set of spherical mirrors in a
Fabry–Perot arrangement. The gas sample is pulsed into the cell
at a 40° angle relative to the cavity axis using a supersonic nozzle
with a 0.8 mm orifice. For more details, see Sun et al. [6].
HZnCl was created in the FTMW machine using a 0.1% mixture of
Cl2 and 2% of (CH3)2Zn in argon. A d.c. discharge of 800 V and about
35 mA was applied to the gas immediately beyond the nozzle. Using
a digital oscilloscope, the time-domain spectrum were recorded and
averaged until an adequate signal to noise ratio was obtained,
typically 1000 pulses. Transition frequencies were predicted based
on the sub-mm data; however, a 30 MHz search was necessary to
locate all the electric quadrupole hyperfine components. Because
of the orientation of the jet expansion relative to the cavity, all
spectral signals consist of two Doppler components, which are
averaged together to obtain the transition frequencies. The fullwidth half-maximum of each line is about 5 kHz and the experimental accuracy of the FTMW frequency measurements is estimated to
be ±1 kHz. (The frequency standard is a rubidium crystal.)
0.003
0.007
0.005
0.004
0.001
0.014
0.005
0.002
R.L. Pulliam et al. / Journal of Molecular Spectroscopy 257 (2009) 128–132
Table 2
FTMW transition frequencies in MHz of HZnCl (X 1R+:v = 0).
J0
F0
1
1
1
2
2
2
2
2
2
3
3
3
3
3
3
3
4
4
4
4
0.5
1.5
2.5
1.5
0.5
2.5
3.5
1.5
0.5
3.5
1.5
2.5
3.5
4.5
2.5
1.5
2.5
3.5
4.5
5.5
a
?
J00
F00
m
mobs–cala
0
0
0
1
1
1
1
1
1
2
2
2
2
2
2
2
3
3
3
3
1.5
1.5
1.5
2.5
0.5
1.5
2.5
1.5
1.5
3.5
0.5
1.5
2.5
3.5
2.5
1.5
1.5
2.5
3.5
4.5
9803.888
9791.546
9798.405
19 592.628
19 593.999
19 594.589
19 594.589
19 599.486
19 606.342
29 384.301
29 389.467
29 389.467
29 391.166
29 391.166
29 394.365
29 396.320
39 186.897
39 186.897
39 187.693
39 187.693
0.003
0.001
0.001
0.003
0.000
0.002
0.001
0.001
0.001
0.007
0.000
0.001
0.001
0.001
0.001
0.003
0.001
0.001
0.001
0.001
From combined fit of sub-millimeter and FTMW data.
Representative spectra of HZnCl are displayed in Figs. 1 and 2.
Fig. 1 shows the J = 50
49 transition of H66Zn37Cl near 467 GHz
and the J = 48
47 transition of H64Zn35Cl at 469 GHz, both observed in natural isotopic abundance. Fig. 2 displays the F = 5/
2 ? 3/2 and F = 1/2 ? 3/2 hyperfine components of the J = 1 ? 0
transition of HZnCl near 9.8 GHz. There are frequency breaks in
both figures. Each hyperfine component in the FTMW data consists
of Doppler doublets, as mentioned, indicated on the spectrum.
The data were analyzed using the least squares fitting program
SPFIT [12] with a Hamiltonian consisting of molecular frame rotation and nuclear quadrupole coupling. In the case of the main isotopologue analysis, the sub-mm and microwave data were
evaluated in a combined fit weighted by the estimated experimental uncertainties. The results of these analyses are given in Table 3.
As shown, rotational and centrifugal distortion constants were
determined for the seven species, as well as eQq for the 35Cl nucleus of H64Zn35Cl.
The bond lengths for HZnCl were established using all isotopologue data. The r0 bond lengths were calculated with a direct fit to
Fig. 1. Spectra of the J = 48
47 transition of the main isotopologue H64Zn35Cl near
469 GHz and J = 50
49 transition of H66Zn37Cl near 467 GHz, both in the ground
vibrational state (X 1R+), measured in natural abundance. There is a frequency
break in the figure. Each spectrum was obtained in a single, 110 MHz wide scan in
approximately 2 min, and then cropped to obtain the 30 MHz frequency range
shown.
all seven moments of inertia. Using Kraitchman’s equations, an rs
structure was also determined. Since isotopic substitutions for
each atom in HZnCl were obtained, the rm(2) structure could be calculated, as well. The rm(2) method corrects for zero-point vibrations
and is thought to be closest to the equilibrium structure [13]. The
resulting bond lengths are given in Table 4.
4. Discussion
This rotational study confirms the linear structure suggested by
Macrae et al. [8] and Yu et al. [9] and the 1R+ ground state of HZnCl.
This work has also resulted in improved B0 and D0 values, revising
those of Yu et al. It should be noted that these authors suggested
such measurements be undertaken because of their spectral congestion and uncertainty in J assignments. In particular, the spectra
attributed to the (1 0 0) vibrational band of H64ZnCl, H66ZnCl, and
H64Zn37Cl actually are those of the ground vibrational state of
H66ZnCl, H68ZnCl, and H66Zn37Cl. Furthermore, it is likely that ZnCl
was present in the Yu et al. data and was mistaken for HZnCl. For
example, the ground vibrational state of H66Zn35Cl was reported to
have B0 = 4812 MHz, which agrees well with the B0 = 4811.8 MHz
of 68Zn35Cl [4]. Likewise, rotational constants of H64Zn35Cl and
H64Zn37Cl closely match those of 66Zn35Cl and 66Zn37Cl, respectively [4,9]. Our data cannot verify assignments of the (1 0 0) state
of the H68Zn35Cl isotopologue or of the (2 0 0) vibrational state of
H64Zn35Cl.
Revised bond lengths have also been established for HZnCl. The
rm(2) structure yields rZn–H = 1.5050(12) Å and rZn–Cl = 2.0829(15) Å.
These experimental values compare well with those derived from
the highest level of theory by Kerkines et al. [10]. These authors
calculated rZn–H = 1.499 Å and rZn–Cl = 2.079 Å. Yu et al. reported a
Zn–Cl bond distance in the range of 2.079–2.088 Å, in agreement
with our value. However, their Zn–H distance varied between
1.596 and 1.789 Å, considerably larger than that established here.
Using DFT calculations, however, Yu et al. obtained an rZn–H bond
length of 1.52 Å, which compares well with our value.
As shown in Table 4, addition of the H atom to ZnCl decreases
the Zn–Cl bond length by 0.050 Å, as also predicted by theory
[10]. The unpaired electron in ZnCl, a free radical, is thought to exist in a 12r antibonding orbital that is chiefly 4s in character, i.e.
131
R.L. Pulliam et al. / Journal of Molecular Spectroscopy 257 (2009) 128–132
Table 3
Spectroscopic constants in MHz of HZnCl.a
B0
D0
eQq (Cl)
rms of fit
a
b
H64Zn35Clb
H66Zn35Cl
H67Zn35Cl
H68Zn35Cl
H64Zn37Cl
H66Zn37Cl
D64Zn35Cl
4898.52132(76)
0.00268212(18)
27.429(22)
0.035
4851.6208(34)
0.00263412(79)
4829.114(13)
0.0026109(27)
4807.320(12)
0.0025891(32)
4730.343(12)
0.0025055(23)
4683.252(25)
0.0024575(48)
4673.5727(59)
0.0023197(17)
0.015
0.004
0.005
0.002
0.021
0.006
Values in parenthesis are 3r errors.
Combined fit of FTMW and sub-millimeter data.
Table 4
Bond lengths in Angstroms for HZnCl and related species.
HZnCl (1R+)a
r(H–Zn)
r(Zn–Cl)
r(Zn–C)
a
b
c
d
e
f
g
ZnCl (2R+)b
HZnCH3 (1A1)c
ZnCH3 (2A1)d
HZnCN (1R+)e
ZnCN (2R+)f
ZnH (2R+)g
r0
rs
rm(2)
re
re
r0
r0
rm(1)
rm(1)
re
1.519
2.083
1.519
2.082
1.5050(17)
2.08293(21)
1.499
2.079
2.130033(12)
1.52089(11)
1.92813(18)
1.4950(3)
2.001(7)
1.8966(6)
1.593478(2)
1.9496
Values in parenthesis are 3r errors; re value from Ref. [10].
Ref. [4].
Ref. [7].
Ref. [14].
Ref. [6].
Ref. [1].
Ref. [20].
spherically symmetric [4]. With the addition of the hydrogen atom,
4s4pz hybridized orbitals are created on the zinc atom which form
the H–Zn and Zn–Cl bonds [10]. The formation of the 4s4pz orbitals
results in the elongation of the electron density along the molecular axis (z). The unpaired electron density in ZnCl is altered from a
strictly spherical distribution about zinc to a more concentrated
location between the hydrogen and zinc atoms. Therefore, the
chlorine nucleus can move closer to the zinc atom as repulsion is
reduced between the Cl electron cloud and the Zn 4s electron.
Similar trends in bond lengths are seen in the cases of the closed
shell species HZnCH3 [7] and HZnCN [6], when compared to their
counterparts ZnCH3 [14] and ZnCN [1]. In each case, a decrease
in the zinc–ligand bond length of 0.05 Å occurs with the addition
of the hydrogen atom to the radical species (see Table 4). The unpaired electron of zinc again must move to some form of hybridized orbital, enabling the ligand to more closely approach the
metal atom.
The electric quadrupole constant, eQq, provides information
concerning the magnitude of the electron field gradient across,
in this case, the chlorine nucleus [15]. For atomic chlorine, eQq
has been previously determined to be 109.74 MHz [16] and is
a measure of the atom’s valence state 3s23p5 [16]. In the simplest
interpretation, a covalently-bonded molecule will maintain this
valence state and therefore have an eQq value similar to that of
atomic chlorine [15–18]. In fact, molecules with covalent bonding
have eQq values of 80 MHz for the 35Cl nucleus, on average
[15,17]. A good example is ClCN with its respective quadrupole
coupling constant of 83.2 MHz [17]. For comparison, a known
ionic molecule, NaCl, is reported to have an eQq value of less than
1 MHz [17].
For HZnCl, eQq was found to be 27.249 MHz, significantly
smaller than the canonical values of 80 MHz. This difference
suggests that the species is primarily ionic in nature. Using the
Townes and Dailey approximation, the fraction of ionic character
x in this molecule can be estimated from the following equation
[15–17]:
eQqðHZnClÞ
¼ ð1 xÞ
eQqðClÞ
ð1Þ
The quantity of x in this case was found to be 0.75, indeed indicating that the zinc bond to chlorine is about 75% ionic in nature.
A comparison with HCl can also give insight into the effect of
zinc insertion for a given bond. HCl has a quadrupole coupling constant of eQq = 67.61881(15) MHz [19], a value 2.5 times greater
in magnitude than that of HZnCl. Clearly, the insertion of zinc increases the ionic character of the molecule. It would be useful to
compare these values with eQq of ZnCl, as well. However, the
35
Cl quadrupole constant of ZnCl has yet to be measured
experimentally.
5. Conclusions
The pure rotational spectrum of HZnCl has been measured using
a combination of sub-millimeter and FTMW techniques. These data
have enabled a better determination of the structure and bonding
characteristics of HZnCl. The zinc–chlorine bond length was found
to shorten relative to ZnCl, likely from the transfer of the unpaired
electron on zinc from a spherically symmetric antibonding orbital
into a 4s4pz hybridized bonding orbital, reducing electron repulsion between the two atoms. This stabilization occurs as well in
similar zinc species. However, HZnCl still remains primarily an ionic molecule, as indicated by its quadrupole coupling constant. A
mixture of ionic and covalent bonding is apparently important
for this metal halide species.
Acknowledgment
This research was supported by NSF Grant CHE-0718699.
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