Chemical Physics Letters 553 (2012) 11–16
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
e 1 Rþ Þ: A model system
Gas-phase rotational spectroscopy of AlCCH ð X
for organo-aluminum compounds
M. Sun a, D.T. Halfen a,b, J. Min a, D.J. Clouthier c, L.M. Ziurys a,b,⇑
a
Department of Chemistry and Biochemistry, University of Arizona, Tucson, AZ 85721, United States
Department of Astronomy and Steward Observatory, Arizona Radio Observatory, Steward Observatory, University of Arizona, Tucson, AZ 85721, United States
c
Department of Chemistry, University of Kentucky, Lexington, KY 40506, United States
b
a r t i c l e
i n f o
Article history:
Received 25 July 2012
In final form 16 August 2012
Available online 24 August 2012
a b s t r a c t
e 1 Rþ Þ has been measured using
The pure rotational spectrum of AlCCH in its ground electronic state ð X
Fourier transform microwave (FTMW) and mm/sub-mm direct absorption spectroscopy. AlCCH was created in a DC discharge from HCCH and aluminum vapor, either produced by a Broida-type oven, or generated from Al(CH3)3 in a supersonic jet source. Rotational transitions were measured for five
isotopologues of AlCCH, with 13C and deuterium substitutions. From these data, rotational and Al and
D quadrupole parameters were determined, as well as an accurate structure. AlCCH appears to exhibit
an acetylenic arrangement with significant covalent character in the Al–C single bond.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
Aluminum–carbon compounds play a wide range of roles in
chemistry. For example, metal-doped carbide clusters are regarded
as a new class of functional materials for semiconductors, ceramics, and hydrogen storage [1,2]. Aluminum carbide clusters are particular important in this regard, as they exhibit non-classical and
non-stoichiometric structures that differentiate them from other
metal analog compounds, which have cubic and layered frameworks [1–4]. In fact, certain AlmCnHx clusters, such as Al2C2H12,
have excellent potential as hydrogen storage materials [1]. Organo-aluminum compounds are also common in synthesis and
catalysis. The Al atom can participate in many aspects of chemical
bonding, such as C–H, C–C and C–O insertions [5], often resulting in
unusual structures for a main group element [6]. Furthermore, certain aluminum-containing reagents show special catalysis capability, such as the famous Ziegler–Natta catalyst [7]. In addition,
simple aluminum-bearing molecules, such as AlNC and AlOH, have
been identified in the interstellar medium [8,9].
One particular system that has attracted attention from both
experimental and theoretical chemists is the Al-acetylene complex
[10–14]. Matrix isolation experiments initially suggested a vinyllike structure for the adduct, AlHCCH [11], while theory predicted
the vinylidene form, AlCCH2 [12,14]. Mechanisms for aluminuminduced acetylene–vinylidene rearrangement have been the subject of various computational studies, as well [15]. More recent
⇑ Corresponding author at: Department of Chemistry and Biochemistry,
University of Arizona, Tucson, AZ 85721, United States. Fax: +1 520 621 5554.
E-mail address: [email protected] (L.M. Ziurys).
0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cplett.2012.08.034
computational work has included several species in the AlCn series
and AlCCH [16,17].
In spite of their obvious chemical significance, there are few
experimental studies of small Al-acetylene species such as AlCCH,
AlC2, or AlCCH2, perhaps because of their explosive, as well as elusive, behavior. The need for spectroscopic characterization of simple aluminum acetylides would seem to be imperative in order to
understand their bulk function. The few past investigations were
typically carried out using matrix-isolation methods combined
with theoretical calculations. In 1990, Knight and coworkers successfully produced AlC, AlC2 and their 13C isotopologues in neon
and argon matrices at 4 K, and studied these species by electron
spin resonance (ESR) [18]. In 1993, Burkholder & Andrews subjected a mixture of aluminum atoms and C2H2 frozen in argon at
12 K to intense UV–Vis irradiation. These authors identified one
main photolysis product as AlCCH by assignment of three new IR
bands to the C„C stretching, H–C„C bending, and C–Al stretching
modes [19]. With a similar approach, Taylor and coworkers in 1994
also created AlCCH, as well as AlC2, in an argon matrix and identified the molecules by IR spectroscopy [20]. The first gas-phase
work on these species occurred in 2007, when Maier and coworkers synthesized AlCCH and AlC2 in a supersonic jet by laser ablation
techniques, and characterized them by their electronic spectra
[21–23]. These authors established the basic geometries of these
two molecules for the first time: a linear structure for AlCCH and
a T-shape for AlC2, as previously predicted by ab initio methods
[21–23]. Following that work, Gharaibeh and Clouthier measured
additional electronic spectra of AlCCH [24].
In this Letter, we present measurements of the pure rotational
e 1 Rþ ground electronic state using both
spectrum of AlCCH in its X
12
M. Sun et al. / Chemical Physics Letters 553 (2012) 11–16
the Fourier transform microwave (FTMW) and millimeter/submillimeter direct absorption methods. Spectra of 13C and D-substituted isotopologues were also recorded, enabling an accurate
structural determination for comparison with theory. Hyperfine
interactions arising from the aluminum and deuterium nuclei were
also resolved. Here we describe our experimental methods, spectral analysis, and an interpretation of these findings in terms of
aluminum–ligand bonding.
2. Experimental
The measurements of the five AlCCH isotopologues were first
conducted using the Fourier transform microwave (FTMW) spectrometer of the Ziurys group, which operates in the 4–60 GHz
range. This Balle–Flygare type narrow-band spectrometer consists
of a vacuum chamber with an unloaded pressure about 108 torr,
maintained by a cryopump, and a Fabry–Perot type cavity constructed from two spherical aluminum mirrors in a near confocal
arrangement. An antenna is imbedded into each mirror for injecting and detecting radiation. A supersonic jet expansion is used to
introduce the sample gas, produced by a pulsed-valve nozzle (General Valve) containing a DC discharge source. In contrast to other
FTMW instruments of this type, the supersonic expansion is injected into the chamber at a 40° angle relative to the optical axis.
The Fourier transform of the time domain signals produces spectra
Table 1
Observed rotational transitions of AlCCH and its
0
M
J
1
?
0
2
?
1
J
3
4
24
25
29
30
31
a
b
?
?
00
2
3
23
24
28
29
30
Values in MHz.
Hyperfine collapsed.
00
M
F
2.5
3.5
1.5
2.5
3.5
2.5
1.5
4.5
3.5
0.5
2.5
1.5
3.5
4.5
1.5
2.5
3.5
0.5
5.5
3.5
4.5
1.5
2.5
5.5
2.5
3.5
1.5
4.5
4.5
6.5
5.5
3.5
2.5
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
2.5
2.5
2.5
1.5
3.5
3.5
1.5
3.5
2.5
1.5
2.5
2.5
4.5
4.5
0.5
1.5
2.5
0.5
4.5
3.5
3.5
1.5
2.5
5.5
1.5
2.5
0.5
4.5
3.5
5.5
4.5
3.5
2.5
b
b
b
b
b
e 1 Rþ Þ.a
C isotopologues ð X
13
0
F
with a 600 kHz bandwidth with 4 kHz resolution. Because of the
beam orientation to the cavity axis, every measured transition appears as a Doppler doublet with a FWHM of about 5 kHz. Transition
frequencies are simply taken as the average of the two Doppler
components. More details regarding the instrumentation can be
found in Ref. [25].
AlCCH was generated in an argon plasma from Al(CH3)3 and
unpurified acetylene. Argon at a pressure of 242 kPa (35 psi),
seeded with 0.3% acetylene, was passed over liquid Al(CH3)3
(Aldrich, 99%) contained in a Pyrex U-tube at room temperature
[26], and the resultant gas mixture delivered through the pulsed
discharge nozzle at a repetition rate of 10 Hz. The gas pulse duration was set to 500 ls, which resulted in a 30–35 sccm mass flow
rate. AlCCH production was maximized at a discharge of 1000 V
at 50 mA. To produce Al13C13CH and AlCCD, 0.3% 13C2H2 (Cambridge
Isotopes, 99% enrichment) and C2D2 (Cambridge Isotopes, 99%
enrichment) in argon was used respectively under the same sample
conditions, while a mixture of 0.2% CH4 and 0.2% 13CH4 (Cambridge
Isotopes, 99% enrichment), also in argon, was employed to create
AlC13CH and Al13CCH. Typically, 1000 shots per scan were averaged
for the AlCCH, Al13C13CH, and AlCCD spectral measurements, while
2000 shots per scan were used for AlC13CH and Al13CCH.
Several rotational transitions of AlCCH were also measured in
the millimeter/submillimeter region using direct absorption spectroscopy. The instrument in this case consists of a radiation source,
a double-pass gas cell, and InSb hot electron bolometer detector,
Al13CCH
AlCCH
AlC13CH
Al13C13CH
mobs
mo–c
mobs
mo–c
mobs
mo–c
mobs
mo–c
9945.376
9954.297
9958.083
19895.316
19896.999
19899.101
19901.358
19905.196
19905.918
19906.800
19908.022
19914.064
29848.998
29848.998
29852.453
29852.965
29855.069
29856.261
29856.798
29857.197
29857.197
29857.902
29859.011
39800.910
39806.485
39806.812
39807.212
39807.802
39807.802
39808.462
39808.714
39810.752
39811.415
238731.258
248667.671
288399.791
298329.190
308257.045
0.001
0.001
0.000
0.000
0.001
0.002
0.000
0.001
0.001
0.001
0.000
0.000
0.020
0.001
0.005
0.003
0.002
0.005
0.000
0.021
0.001
0.001
0.001
0.006
0.002
0.001
0.003
0.022
0.001
0.001
0.001
0.000
0.005
0.050
0.068
0.039
0.008
0.084
9897.660
9906.587
9910.375
19799.884
19801.571
0.000
0.001
0.000
0.002
0.002
9606.350
9615.278
9619.061
19217.270
19218.950
0.001
0.001
0.002
0.000
0.002
9566.069
9575.000
9578.786
19136.706
19138.390
0.001
0.002
0.000
0.002
0.000
19805.932
19809.773
19810.495
19811.379
19812.601
0.000
0.000
0.000
0.000
0.000
19223.316
19227.156
19227.881
19228.762
19229.985
0.001
0.002
0.001
0.001
0.001
19142.754
19146.596
19147.325
19148.203
19149.426
19155.471
0.001
0.002
0.005
0.000
0.001
0.001
28711.105
28714.561
28715.078
28717.180
28718.372
28718.908
0.004
0.004
0.002
0.000
0.004
0.002
28719.308
28720.013
28721.124
0.001
0.000
0.001
38289.316
38289.645
38290.044
0.002
0.000
0.004
38290.635
38291.297
38291.547
0.001
0.000
0.002
M. Sun et al. / Chemical Physics Letters 553 (2012) 11–16
and is described in more detail elsewhere [27]. The source, which
operates from 65 to 850 GHz, is comprised of sets of Gunn oscillators and Schottky-diode multiplier combinations. The steel reaction cell contains a Broida-type oven, and is water cooled. The
radiation is directed through the system from a feedhorn to the
detector using several Teflon lenses, a polarizing grid, and a rooftop
reflector. Frequency modulation and phase-sensitive detection are
utilized to remove background noise, and the system is under computer control.
AlCCH was formed in the millimeter-wave system in a DC discharge from a mixture of metal vapor, produced in the Broida oven,
with acetylene and argon. The Broida oven method was employed
instead of using Al(CH3)3 because it gave less contaminated spectra. About 20 mTorr of Ar and 5 mTorr of HCCH were introduced
into the gas cell from beneath the oven. The acetylene could be
added from over the top of the oven with similar results. To prevent coating of the optics, approximately 10 mTorr of Ar was flowed over the lenses, as well. A DC discharge of 1 A at 50 V was
needed to create this species, and the plasma created glowed a
light purple color from atomic emission of argon. To stabilize the
discharge, the oven had to be operated at high capacity to generate
sufficient metal vapor. However, high vapor production was problematic because the aluminum would often condense over the top
of the oven, forming a crust that inhibited the synthesis of AlCCH.
The millimeter/submillimeter search for AlCCH was greatly
facilitated by transition frequency predictions based on the microwave spectra. However, even with these data, production of AlCCH
in the millimeter system was difficult, and intensities of the signals
obtained were quite weak and required significant signal-averaging. Rest frequencies for AlCCH were determined by recording scan
pairs 5 MHz wide, with one scan taken increasing in frequency, and
the other decreasing in frequency. Typically, 20–60 such scans
were needed to acquire an adequate signal-to-noise ratio. Gaussian
13
line profiles were fit to the observed features to obtain the center
frequency and line widths, which ranged from 570 to 760 kHz from
248 to 308 GHz. The experimental accuracy is estimated to be
±50 kHz.
3. Results
This Letter of the pure rotational spectrum of AlCCH was based
on the optical work [21,24], which provided estimates of the rotational constant B [21,24]. The quadrupole constant for AlCN, eQq
(Al) = 37.22 MHz [28], was used to predict the aluminum hyperfine splittings. (The spin of the 27Al nucleus is I = 5/2.) The initial
search was conducted with the FTMW instrument. A frequency
survey for the J = 2 ? 1 transition of AlCCH was carried out based
on the two estimated B values. A cluster of lines near 19.9 GHz
was found with a pattern that matched the predicted Al hyperfine
structure. Subsequent measurements of the J = 1 ? 0, 3 ? 2, and
4 ? 3 transitions confirmed the detection of AlCCH. As shown in
Table 2
e 1 Rþ Þ.a
Observed rotational transitions of AlCCD ð X
J0
?
J00
F01
?
F001
F0
?
F00
mobs
mo–c
1
?
0
2
?
1
2.5
2.5
2.5
3.5
3.5
3.5
1.5
1.5
1.5
2.5
2.5
3.5
3.5
3.5
1.5
1.5
4.5
4.5
4.5
3.5
3.5
3.5
0.5
0.5
0.5
0.5
2.5
2.5
2.5
6.5
6.5
6.5
5.5
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
1.5
1.5
3.5
3.5
3.5
1.5
1.5
3.5
3.5
3.5
2.5
2.5
2.5
1.5
1.5
1.5
1.5
2.5
2.5
2.5
5.5
5.5
5.5
4.5
2.5
3.5
1.5
2.5
4.5
3.5
0.5
2.5
1.5
2.5
3.5
3.5
4.5
2.5
1.5
2.5
5.5
3.5
4.5
2.5
4.5
3.5
0.5
1.5
1.5
0.5
1.5
3.5
2.5
7.5
5.5
6.5
6.5
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
2.5
3.5
1.5
1.5
3.5
2.5
1.5
3.5
2.5
1.5
2.5
3.5
4.5
2.5
1.5
2.5
4.5
2.5
3.5
1.5
3.5
2.5
1.5
1.5
2.5
0.5
1.5
3.5
2.5
6.5
4.5
5.5
5.5
9160.587
9160.637
9160.657
9169.522
9169.537
9169.573
9173.321
9173.333
9173.346
18325.801
18325.828
18327.488
18327.511
18327.524
18331.830
18331.874
18335.699
18335.699
18335.718
18336.380
18336.413
18336.469
18337.301
18337.301
18337.314
18337.325
18338.511
18338.529
18338.552
36669.563
36669.563
36669.568
36669.812
0.001
0.000
0.002
0.000
0.002
0.002
0.003
0.000
0.003
0.002
0.002
0.005
0.003
0.001
0.001
0.002
0.000
0.000
0.005
0.006
0.002
0.001
0.002
0.002
0.001
0.003
0.003
0.003
0.002
0.000
0.001
0.001
0.001
4
a
?
3
Values in MHz.
e 1 Rþ Þ observed in this
Figure 1. FTMW spectra of the main isotopologue of AlCCH ð X
Letter. The upper panel shows the three hyperfine components of the J = 1 ? 0
transition near 10 GHz, arising from the 27Al spin of I = 5/2 and labeled by quantum
number F. There are two frequency breaks in the spectrum to show all three lines.
The lower panel presents two Al hyperfine components originating in the J = 2 ? 1
transition near 20 GHz. Doppler components are indicated by brackets. The
J = 1 ? 0 spectrum was created from three, 600 kHz wide scans, with 1000 pulse
averages per scan, while the J = 2 ? 1 spectrum consists of two, 600 kHz wide scans,
also 1000 pulses per scan.
14
M. Sun et al. / Chemical Physics Letters 553 (2012) 11–16
Table 1, 33 hyperfine components arising from four rotational transitions were recorded with the FTMW spectrometer for this molecule. In addition, five rotational transitions were subsequently
measured at millimeter wavelengths in the range 238–309 GHz,
for levels with J = 23–31 (see Table 1). The quadrupole splittings
of aluminum at these high J are essentially collapsed, generating
single lines.
For the singly- and doubly-substituted 13C species, the hyperfine patterns were expected to be the same as the AlCCH. The 13C
nucleus has I = 1/2, and was unlikely to introduce resolvable hyperfine splittings in a closed-shell molecule such as AlCCH. After additional searches, all three carbon-13 isotopologues were found with
virtually identical spectral patterns as the main species. As shown
in Table 1, two rotational transitions were recorded for both AlC13CH and Al13CCH, for a total of 10 hyperfine lines. For Al13C13CH,
four transitions were measured with 26 hyperfine components.
In the case of AlCCD, however, the observed pattern became more
complex because quadrupole splittings due to the deuterium nucleus (I = 1) were also resolved. Three rotational transitions were
recorded for AlCCD, as shown in Table 2, consisting of 33 individual
hyperfine lines, generated by both the 27Al and D nuclei.
Representative FTMW spectra of AlCCH are presented in Figure 1. The upper panel shows the three Al hyperfine components
of the J = 1 ? 0 transition near 10 GHz, indicated by quantum
number F. There are two frequency breaks in the spectrum in order
to display all three lines. Two hyperfine components in the
J = 2 ? 1 transition near 20 GHz are displayed in the lower panel.
Each feature in both panels is composed of two Doppler components, indicated by brackets.
In Figure 2, a section of the J = 2 ? 1 transition of AlCCD near
18 GHz is shown. In this case, the labels F1 and F indicate the coupling of the Al and D nuclei, respectively. Doppler doublets are
shown by the brackets. Each component generated by the aluminum coupling (F1 = 4.5 ? 3.5 and F1 = 3.5 ? 2.5) is further split
into triplets due to deuterium (I = 1). As shown in the figure, the
coupling arising from aluminum dominates, but the deuterium
interaction is not negligible (c.f. Figures 1 and 2).
Figure 3 presents the millimeter data for AlCCH. Here spectra of
the J = 25
24, J = 29
28, J = 30
29, and J = 31
30 transitions are shown near 248, 288, 298, and 308 GHz, respectively.
At these high J values, the data appear as single features, as
mentioned. The signal-to-noise ratio is much lower at these
frequencies, illustrating the difficulty of Broida oven production
of AlCCH.
4. Analysis
The spectra for each isotopologue were analyzed using the nonlinear least squares routine SPFIT with a 1R effective Hamiltonian
containing rotation, electric quadrupole, and nuclear spin-rotation
terms [29,30], shown below:
Heff ¼ Hrot þ HeQq þ Hnsr
ð1Þ
For AlCCH, the microwave and millimeter-wave data were combined for the analysis. The rotational constants B, D, eQq(Al), and
the nuclear spin-rotation parameter CI(Al) were determined, with
an rms value 22 kHz, reflecting the combined fit. For the three
13
C-substituted species, the same four parameters were established
from the analysis, with rms values of 1–2 kHz, as is typical for
FTMW measurements. In the case of AlCCD, eQq for the deuterium
nucleus was also determined, as well as B, D, eQq(Al), and CI(Al),
with an rms of 2 kHz. The resulting constants from the analyses
are given in Table 3.
~1 +
AlCCH (X
)
J = 25
248.65
248.67
248.69
J = 29
288.38
288.40
298.33
308.24
29
298.35
J = 31
e 1 Rþ Þ
Figure 2. FTMW spectrum of a section of the J = 2 ? 1 transition of AlCCD ð X
near 18 GHz, showing the multiple hyperfine interactions present in this molecule.
The larger splitting, labeled by quantum number F1, is due to the 27Al nuclear spin.
The additional small triplet structure, present in both the F1 = 4.5 ? 3.5 and the
F1 = 3.5 ? 2.5 components, arises from the deuterium nuclear spin (I = 1). The
deuterium splitting is not completely resolved in the F1 = 4.5 ? 3.5 feature. Doppler
components are indicated by brackets. This spectrum is a compilation of two,
600 kHz wide scans, with 1000 pulse-averages per scan.
28
288.42
J = 30
298.31
24
308.26
30
308.28
Frequency (GHz)
Figure 3. Millimeter spectra of the J = 25
24, J = 29
28, J = 30
29, and
e 1 Rþ Þ near 248, 288, 298, and 308 GHz,
J = 31
30 transitions of AlCCH ð X
respectively. The aluminum hyperfine interaction has become negligible at these
higher frequencies, and the transitions therefore appear as single lines. The data are
an average of 20, 16, 24 and 28 scans, respectively, each 110 MHz wide and
acquired in 70 s, and then cropped to display a 60 MHz wide frequency range.
15
M. Sun et al. / Chemical Physics Letters 553 (2012) 11–16
Table 3
e 1 Rþ Þ.a
Spectroscopic constants for AlCCH ð X
Parameter
B
D
CI(Al)
eQq(Al)
eQq(D)
rms
a
b
c
Al13CCH
AlCCH
AlC13CH
Al13C13CH
AlCCD
MW, MMW
Previous work
MW
MW
MW
MW
4976.08610(55)
0.00218502(81)
0.0049(11)
42.393(28)
4942.7b, 4968.7c
4952.2302(59)
0.00218(77)
0.0050(22)
42.424(42)
4806.5744(59)
0.00196(77)
0.0055(22)
42.424(42)
4786.4349(15)
0.002017(58)
0.0052(14)
42.434(31)
0.001
0.001
0.002
4583.7097(13)
0.001743(62)
0.0045(13)
42.398(25)
0.207(28)
0.002
0.022
Values in MHz; errors are 3r in the last quoted decimal places.
Ref. [21].
Ref. [24].
Table 4
Bond lengths of MCCH and related molecules.a
Molecule
r(Al–C)
(Å)
r(C–C)
(Å)
r(C–H)
(Å)
Method
Refs.
e 1 Rþ Þ
AlCCH ð X
1.963(5)
1.210(7)
1.060(3)
r0
This Letter
1.978
1.986(1)
1.202
1.2061(6)
1.060
1.0634(3)
rs
ð1Þ
This Letter
This Letterb
1.968
1.977
1.980
–
1.215
1.233
1.2313(3)
1.064
1.068
1.0508(1)
ð1Þ
[21]
[21]
[17]
[32]
–
1.213(2)
1.058(1)
ð1Þ
–
1.226
1.062
rm
r0
[33]
e 1 Rþ Þ
LiCCH ð X
e
AlCN ð X 1 Rþ Þ
re
[34]
e 1 A1 Þ
AlCH3 ð X
AlCCAl
HCCH
1.980
e 1 Rþ Þ
ZnCCH ð X
e 1 Rþ Þ
CuCCH ð X
a
b
c
2.015
1.980
1.259
1.20241(9)
rm
r0
re, B3LYP
re, CASSCF
rm
[37]
1.090c
r0
[35]
1.0625(1)
re, MP2
re, Infrared, Raman
[1]
[36]
Values in parentheses are 1r uncertainties.
cb = 0.028(8).
Values were estimated in the original references.
For the main isotopologue AlCCH, the rotational constant obtained was B0 = 4976.08610(55) MHz. This value is in reasonable
agreement with those obtained from the optical experiments, also
shown in Table 3. Apetrei et al. found B = 4943(13) MHz and
Gharaibeh and Clouthier derived 4968.7 MHz [21,24], within
7–34 MHz of the pure rotational value.
5. Discussion
From the rotational constants of the five isotopologues established in this Letter, a structure for AlCCH has been derived. The
resulting bond lengths of AlCCH are listed in Table 4. Several
ð1Þ
structures were determined: r0, rs, and rm . The r0 bond lengths were
obtained directly from a least-squares fit to the moments of inertia,
while the rs substitution structure was calculated using
Kraitchman’s equations, which accounts in part for the zero-point
ð1Þ
vibrational effects [30]. The r m bond lengths were derived by the
method developed by Watson and are believed to be closer to the
equilibrium structure than the rs or r0 geometries [31]. (The Watson
ð2Þ
r m structure would be optimal, but could not be calculated
because no isotopic substitution is possible for the Al atom.) As
the table shows, all three structures agree to within 0.3%. For the
ð1Þ
r m geometry, r(Al–C)(Al–C) = 1.986(1) Å, r(C–C) = 1.2061(6) Å,
and r(C–H) = 1.0634(3) Å. These parameters represent the first
complete structure for this molecule based on experimental data.
Apetrei et al. [21] were only able to determine an r0 value of
r(Al–C) = 1.968 Å, which is in reasonable agreement with our
number.
ð1Þ
The r m structure also agrees quite well with the theoretical predictions, conducted at the CASSCF and DFT levels [17,21]. As shown
in Table 4, the DFT values are r(Al–C) = 1.977 Å, r(C–C) = 1.215 Å,
and r(C–H) = 1.064 Å [21]. The differences in bond lengths between
theory and experiment vary by 0.001–0.009 Å for the DFT calculations, and by 0.004–0.027 Å for the CASSCF method.
Table 4 also lists the parameters for other, similar metal-containing species, as well as acetylene [1,32–37]. The C–H and C„C
bond lengths in HC„CH are within 0.004 Å of those in AlCCH. Furthermore, the Al–C bond in AlCCH is comparable to that measured
experimentally for AlCH3 (r = 1.980 Å) to within 0.006 Å, as well as
to that calculated for AlCCAl [1]. The Al–C bond in aluminum
monomethyl is clearly a single bond, and by inference, so is that
in AlCCH. Also, the CCH moiety apparently retains its basic
Table 5
eQq(Al) and eQq(D) for metal-containing species.
a
b
Species
eQq(Al)a
Free Al atom
AlCH3 b
AlCCH
AlCN
AlF
AlCl
AlBr
AlI
AlCCD
LiCCD
NaCCD
KCCD
CuCCD
ZnCCD
37.52
50.34
42.39
37.22
37.49
29.8
27.9
25.9
Values in MHz.
vaa in place of eQq.
eQq(D)a
Refs.
0.207(28)
0.152(33)
0.193(48)
0.157(20)
0.214(23)
0.253(91)
[30]
[38]
This Letter
[28]
[30]
[30]
[30]
[30]
This Letter
[39]
[39]
[39]
[33]
[32]
16
M. Sun et al. / Chemical Physics Letters 553 (2012) 11–16
triple-bond structure and the integrity of the carbon sp hybridization on substitution of a hydrogen atom with aluminum. The electron density is not significantly transferred from the metal to the
C„CH group, which would alter the carbon-carbon bond length.
A slight lengthening of this bond is observed in the radical species
ZnCCH, and in CuCCH and LiCCH (see Table 4). Note that the Al–C
bond distance in AlCN is somewhat longer at 2.015 Å.
The electric quadrupole coupling constant of the Al nucleus for
AlCCH was found to be eQq(Al) = 42.393(28) MHz, and did not
substantially vary with isotopic substitution (see Table 3). This value lies in between those for AlCN (37.22 MHz) and AlCH3
(50.34 MHz [38]), as shown also in Table 5. AlF and AlCN have almost identical quadrupole moments, and those for the halides AlCl,
AlBr, and AlI are 29.8, 27.9, and 25.9 MHz, respectively. These
numbers suggest that AlCCH is not as ionic at the halide species,
with some covalent character to the Al–C bond, as indicated by
the derived structure. AlCH3 would appear to be the most covalent
molecule of the series.
The quadrupole constant of AlCCH is also larger than that of the
free Al atom, which is eQq310(Al) = 37.52 MHz (see Table 5). The
predominant contribution to the field gradient at the Al nucleus
in this molecule must arise from the bonding 3pr orbital. If this
were the only contribution, then it might be expected that eQq(AlCCH) eQq(Al). The difference in these two values implies that
polarization of other core orbitals (2p) must be contributing to the
quadrupole constant in AlCCH.
The deuterium quadrupole coupling parameter in AlCCD is very
small (eQq = 0.207(28) MHz) relative to that of aluminum. This result is in part due to the difference in the respective quadrupole
moments, 0.15 barns for the Al nucleus, as opposed to 0.0028 barns
for the D nucleus [39]. The ratio of the quadrupole moments of
50, however, is smaller than the ratio of the constants, which is
near 200. Therefore, the environment seen by the deuterium nucleus varies from that of aluminum, as expected given their respective orbital contributions. The environment around the D nucleus
in MCCD species, on the other hand, does not appear to change significantly with metal substitution. As shown in Table 5, eQq(D) is
nearly the same for the known metal acetylide species, CuCCD,
ZnCCD, AlCCD, and NaCCD, within experimental error (see Table 5).
The exceptions are KCCD and LiCCD, which have noticeably smaller
quadrupole constants near 0.15 MHz, as opposed to 0.2 MHz.
Within the experimental uncertainty, NaCCD could be following
the same trend. This variation likely results from increased
metal–ligand covalent bonding character in AlCCH and the transition-metal acetylides, which influences the electronic environment
at the deuterium nucleus [40].
6. Conclusion
AlCCH and its isotopologues have been characterized in the gas
phase by FTMW and millimeter-wave spectroscopy. An accurate
structure has been determined for this unstable species, and insight gained into the bonding. As found for the transition-metal
monoacetylides CuCCH and ZnCCH, AlCCH appears to have a true
acetylene-like structure with a C–C triple bond and a single
metal–carbon bond. The Al quadrupole coupling constant of this
molecule suggests that there is significant covalent bonding character in the metal–carbon bond, as compared to aluminum halides.
The measurements of AlCCH also demonstrate that organometallic
precursors can be excellent sources of metal vapor in supersonic
nozzles. Finally, this Letter has provided a benchmark system for
future investigations of organo-aluminum compounds.
Acknowledgments
This research is supported by NSF Grant CHE-1057924.
Note added in proof
We acknowledge the work of Cabezas et al. (J.Mol.Spec. 278, 31,
2012), who measured spectra of AlCCH using a laser ablation
source and FTMW spectroscopy. This work was done after our
study was completed.
References
[1] F. Dong, S. Heinbuch, Y. Xie, J.J. Rocca, E.R. Bernstein, Phys. Chem. Chem. Phys.
12 (2010) 2569.
[2] K.L. Knappenberger Jr., C.E. Jones Jr., M.A. Sobny, I. Iordanov, J. Sofo, A.W.
Castleman Jr., J. Phys. Chem. A 110 (2006) 12814.
[3] A.I. Boldyrev, J. Simons, X. Li, L.S. Wang, J. Am. Chem. Soc. 121 (1999) 10193.
[4] V. Dryza, M.A. Addicoat, J.R. Gascooke, M.A. Buntine, G.F. Metha, J. Phys. Chem.
A 109 (2005) 11180.
[5] J.A. Howard, H.A. Joly, B. Mile, J. Chem. Soc., Faraday Trans. 86 (1990) 219.
[6] H.J. Himmel, Organometallics 22 (2003) 2679.
[7] E.J. Arlman, P. Cossee, J. Catal. 3 (1964) 99.
[8] L.M. Ziurys, C. Savage, J.L. Highberger, A.J. Apponi, M. Guélin, J. Cernicharo,
Astrophys. J. 564 (2002) L45.
[9] E.D. Tenenbaum, L.M. Ziurys, Astrophys. J. 712 (2010) L93.
[10] S. Petrie, J. Phys. Chem. A 106 (2002) 5188.
[11] P.H. Kasai, D. McLeod, T. Watanabe, J. Am. Chem. Soc. 99 (1977) 3521.
[12] M. Trenary, M.E. Casida, B.R. Brooks, H.F. Schaefer III, J. Am. Chem. Soc. 101
(1979) 1638.
[13] Y.M. Xie, H.F. Schaefer III, J. Am. Chem. Soc. 112 (1990) 5393.
[14] J.R. Flores, A. Largo, J. Phys. Chem. 96 (1992) 3015.
[15] E.D. Glendening, M.L. Strange, J. Phys. Chem. A 106 (2002) 7338.
[16] X.E. Zheng, Z.Z. Wang, A. Tang, J. Phys. Chem. A 103 (1999) 9275.
[17] Y.J. Liu, Z.X. Zhao, M.X. Song, H.X. Zhang, C.C. Sun, J. Phys. Chem. A 114 (2010)
5035.
[18] L.B. Knight Jr., S.T. Cobranchi, J.O. Herlong, C.A. Arrington, J. Chem. Phys. 92
(1990) 5856.
[19] T.R. Burkholder, L. Andrews, Inorg. Chem. 32 (1993) 2491.
[20] G.V. Chertihin, L. Andrews, P.R. Taylor, J. Am. Chem. Soc. 116 (1994) 3513.
[21] C. Apetrei, H.B. Ding, J.P. Maier, Phys. Chem. Chem. Phys. 9 (2007) 3897.
[22] E. Chasovskikh, E.B. Jochnowitz, E. Kim, J.P. Maier, J. Phys. Chem. A 111 (2007)
11986.
[23] C. Apetrei, A.E.W. Knight, E. Chasovskikh, E.B. Jochnowitz, H. Ding, J.P. Maier, J.
Chem. Phys. 131 (2009) 064305.
[24] M.A. Gharaibeh, D.J. Clouthier, Int. Sym. Mol. Spectrosc. (2011) MF04.
[25] M. Sun, A.J. Apponi, L.M. Ziurys, J. Chem. Phys. 130 (2009) 034309.
[26] J. Wei, R.A. Grimminger, F.X. Sunahori, D.J. Clouthier, J. Chem. Phys. 129 (2008)
134307.
[27] L.M. Ziurys, W.L. Barclay Jr., M.A. Anderson, D.A. Fletcher, J.W. Lamb, Rev. Sci.
Instrum. 65 (1994) 1517.
[28] K.A. Walker, M.C.L. Gerry, Chem. Phys. Lett. 301 (1999) 200.
[29] H.M. Pickett, J. Mol. Spectrosc. 148 (1991) 371.
[30] W. Gordy, R.L. Cook, Microwave Molecular Spectra, Wiley, New York, 1984.
[31] J.K.G. Watson, A. Roytburg, W. Ulrich, J. Mol. Spectrosc. 196 (1999) 102.
[32] J. Min, D.T. Halfen, M. Sun, B. Harris, L.M. Ziurys, J. Chem. Phys. 136 (2012)
244310.
[33] M. Sun, D.T. Halfen, J. Min, B. Harris, D.J. Clouthier, L.M. Ziurys, J. Chem. Phys.
133 (2010) 174301.
[34] B.Y. Ma, Y. Yamaguchi, H.F. Schaefer III, Mol. Phys. 86 (1995) 1331.
[35] J.S. Robinson, L.M. Ziurys, Astrophys. J. 472 (1996) L131.
[36] E. Kostyk, H.L. Welsh, Can. J. Phys. 58 (1980) 912.
[37] D.B. Grotjahn, A.J. Apponi, M.A. Brewster, J. Xin, L.M. Ziurys, Angew. Chem. Int.
Ed. 37 (1998) 2678.
[38] M. Sun, D.T. Halfen, D.J. Clouthier, L.M. Ziurys, in preparation.
[39] C.H. Townes, A.L. Schawlow, Microwave Spectroscopy, Dover, New York, 1975.
[40] P.M. Sheridan, M.K.L. Binns, M. Sun, J. Min, M.P. Bucchino, D.T. Halfen, L.M.
Ziurys, J. Mol. Spectrosc. 269 (2011) 231.