Journal of Molecular Spectroscopy 269 (2011) 231–235
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Journal of Molecular Spectroscopy
journal homepage: www.elsevier.com/locate/jms
Fourier transform microwave spectroscopy of LiCCH, NaCCH, and KCCH: Quadrupole
hyperfine interactions in alkali monoacetylides
P.M. Sheridan a,⇑, M.K.L. Binns a, M. Sun b, J. Min b, M.P. Bucchino b, D.T. Halfen b, L.M. Ziurys b
a
b
Department of Chemistry and Biochemistry, Canisius College, Buffalo, NY 14208, United States
Department of Chemistry, Department of Astronomy and Steward Observatory, University of Arizona, Tucson, AZ 85721, United States
a r t i c l e
i n f o
Article history:
Received 13 June 2011
In revised form 21 July 2011
Available online 2 August 2011
Keywords:
Fourier transform microwave spectroscopy
Metal acetylides
Quadrupole coupling constants
Laser ablation
Discharge assisted
a b s t r a c t
The alkali metal monoacetylides LiCCH, NaCCH, and KCCH and their deuterium isotopologues have been
investigated using Fourier transform microwave (FTMW) spectroscopy in the frequency range 5–37 GHz.
The molecules were synthesized in a supersonic expansion by the reaction of metal vapor, produced by
laser ablation, with acetylene or DCCD. Use of target rods of the pure metal and a DC discharge immediately following the laser interaction region were significant factors in molecule production. Multiple rotational transitions were recorded for all species, except where only the J = 1 ? 0 line was accessible (Li
species). Quadrupole hyperfine interactions arising from the metal nuclei were resolved in each molecule,
as well as those from the deuterium nucleus in the deuterated isotopologues. From a combined analysis
with previous millimeter-wave data, refined rotational constants were determined for these species, as
well as 7Li, 23Na, 39K, and D eQq parameters. The values of the metal quadrupole coupling constants
are comparable to those of the alkali halides and hydroxides, indicating a similar degree of ionic character
in the metal–ligand bond.
Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction
Molecular hyperfine parameters can provide insight into chemical bonding [1–3]. For example, quadrupole coupling constants
have been used to gauge the ionic/covalent character of alkali
metal-containing compounds, such as fluorides [4–6], chlorides
[7–9], hydroxides [10–12] and borohydrides [13]. Typically, Fourier transform microwave (FTMW) and molecular beam resonance
spectroscopic techniques have been employed to measure the
hyperfine parameters of such species, where they have been generated in the gas phase by heating or ablating the solid salt. This gasphase production method, however, limits the type of ligand that
can be attached. As a result, hyperfine parameters for a number
of alkali metal-containing species such as the amides [14,15],
monomethyls [16] and hydrosulfides [17,18] have not yet been
measured, despite the existence of extensive millimeter-wave
measurements.
Another class of compound for which hyperfine parameters have
not yet been established is the alkali monoacetylides: LiCCH, NaCCH
and KCCH. Their rotational spectra have been recorded at millimeter
wavelengths [19–22], and these studies clearly indicate linear
geometries for these molecules. Because these measurements
⇑ Corresponding author. Address: Department of Chemistry and Biochemistry,
Canisius College, 2001 Main Street, Buffalo, NY 14208, United States. Fax: +1 716
888 3112.
E-mail address: [email protected] (P.M. Sheridan).
0022-2852/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jms.2011.07.008
occurred at high frequencies, however, hyperfine structure, which
would chiefly arise from quadrupole coupling, was not observed.
Information regarding the degree of ionic/covalent bonding character between the metal and the CCH group could only be speculated
from the structural parameters.
Recently, the first experimental observation of monomeric copper acetylide, CuCCH, has been reported [23]. The pure rotational
spectra of several isotopologues of this molecule were recorded
using the FTMW and millimeter-wave spectrometers of the Ziurys
group. CuCCH was successfully synthesized in the supersonic nozzle of the FTMW instrument, using the newly developed technique
of discharge-assisted laser ablation. Building on the success of this
study, we have employed discharge-assisted laser ablation to produce LiCCH, NaCCH, KCCH and their deuterium isotopologues, and
to measure their microwave spectra. To conduct this study,
specially-designed alkali metal ‘‘rods’’ were fabricated, which, to
the best of our knowledge, is a novel approach. Quadrupole hyperfine structure was resolved in the spectra arising from the metals
and the deuterium nuclei. In this paper, we present these data,
analysis of the spectra, and an interpretation of the quadrupole
constants.
2. Experimental
The FTMW spectrometer of the Ziurys group [24] was used to
measure the microwave spectra of LiCCH, NaCCH, KCCH and their
232
P.M. Sheridan et al. / Journal of Molecular Spectroscopy 269 (2011) 231–235
deuterium isotopologues in the range 5–37 GHz. The instrument
has been described in detail elsewhere. Briefly, the Balle–Flygare
type instrument [25] consists of a typical Fabry–Perot cavity that
contains two spherical aluminum mirrors in a near confocal
arrangement, housed in a cryo-pumped vacuum chamber. Antennas are embedded in opposite mirrors for the injection and detection of microwave radiation. A supersonic jet is introduced into the
cavity by a pulsed valve, aligned 40° relative to the optical axis.
Time domain signals are recorded over a 400 kHz frequency range,
from which spectra with 2 kHz resolution are created via a fast
Fourier transform. Transitions appear as Doppler doublets with a
full width at half maximum of 5 kHz (resolved features) and transition frequencies were taken as the doublet average.
In previous microwave experiments for alkali metal-containing
species a sufficient gas-phase quantity of the molecule was produced by either heating the solid salt such as LiCl [7] or by laser
ablating the solid salt such as NaCl [11]. In this work, the synthetic
scheme employed laser ablation of the pure metal. Producing a target rod of an alkali metal is problematic, so instead a 3 cm notch
was machined in an aluminum rod, with a diameter 2 mm smaller
than the rod itself. A thin piece of the solid metal was then tightly
pressed into the notch. For better adhesion, the lithium piece was
mounted with superglue, but not for potassium or sodium, as contact with the glue produced an explosion, even under an argon
atmosphere. Once inserted in the ablation housing, only the alkali
metal-coated portion of the rod was exposed to the laser beam as
the rod was rotated and translated.
To produce the alkali metal acetylides, discharge assisted laser
ablation was employed. The pure alkali metal vapor was ablated
in the presence of a 0.3% mixture of acetylene in argon. A gas backing pressure of 40 psi was used and the pulsed valve was opened for
500 ls. The second harmonic (532 nm) of a Nd:YAG laser (200 mJ
per pulse, 10 Hz repetition rate) was used to ablate the alkali metal
surface 990 ls after the pulse valve was opened. A DC discharge of
1000 V was applied for a duration of 1000 ls after the opening of
the valve. Typically, 250–1000 shots were averaged per frequency
step. It should be noted that the alkali metal acetylides could be
produced without the dc discharge; however, the resulting spectra
were considerably weaker, by at least a factor of 10. The mechanism
by which the discharge increases molecular production has not yet
been definitively established; it has been speculated that the discharge may be breaking up clusters, or further activating the metal
atoms from the ablation source. The deuterium isotopologues of
each alkali metal acetylide were synthesized using the same
conditions, with DCCD (99% enrichment, Cambridge Isotope Labs)
substituted for acetylene.
3. Results
The ground electronic state of the alkali metal acetylides is 1R+.
Their spectra are therefore quite simple, except for the possible
presence of quadrupole coupling and nuclear spin-rotation interactions of the alkali metal nuclei (I = 3/2 for Li, Na and K). Such hyperfine interactions were predicted for the acetylides on the basis of
the alkali fluorides. Using these predictions, a 10 MHz frequency
region was initially searched, centered on a given rotational transition. Only the J = 1 ? 0 transition could be observed for the lighter
species LiCCH with this instrument; for NaCCH and KCCH the
search focused on the J = 3 ? 2 rotational transition, because of
greater instrument sensitivity at these frequencies (27.1 GHz and
17.8 GHz, respectively). After the most intense hyperfine components of these data were assigned, predictions were made for other
rotational transitions and the spectra recorded. A similar process
was conducted for the deuterium isotopologues.
Table 1 lists the rotational transitions recorded for the alkali
metal acetylides. As the table shows, each transition consists of
multiple hyperfine components, labeled by quantum number F,
where F = J + I. For LiCCH, three individual hyperfine components
Table 1
e 1 Rþ .a
Observed rotational transitions (in MHz) for MCCH X
LiCCH
F0 ? F00
mobs
mobs mcalc
1 0
1.5
2.5
0.5
1.5
1.5
1.5
21088.214
21088.121
21088.043
0.001
0.002
0.000
2 1
1.5
2.5
1.5
0.5
3.5
2.5
1.5
0.5
3 2
4 3
a
NaCCH
J0 ? J00
mobs
KCCH
mobs mcalc
mobs
mobs mcalc
9018.782
9020.601
9022.052
0.001
0.002
0.001
5940.287
5942.000
5943.363
0.004
0.003
0.005
0.5
2.5
2.5
0.5
2.5
1.5
1.5
1.5
18038.587
18038.740
18040.037
18040.403
18040.559
18040.559
18041.854
18043.671
0.000
0.002
0.003
0.000
0.001
0.001
0.002
0.001
11881.554
11881.702
11882.925
11883.269
11883.417
11883.417
11884.641
11886.354
0.001
0.003
0.001
0.003
0.004
0.004
0.003
0.003
3.5
2.5
2.5
1.5
4.5
3.5
2.5
1.5
1.5
3.5
3.5
1.5
0.5
3.5
2.5
2.5
1.5
2.5
27058.706
0.001
27060.073
27060.073
27060.524
27060.524
27061.369
27061.886
0.002
0.002
0.003
0.003
0.001
0.001
17823.165
17823.962
17824.455
17824.455
17824.880
17824.880
17825.680
17826.167
17827.392
0.004
0.001
0.004
0.004
0.005
0.005
0.005
0.002
0.003
4.5
3.5
2.5
4.5
5.5
3.5
2.5
4.5
2.5
1.5
3.5
4.5
3.5
2.5
36078.498
36080.109
36080.109
36080.321
36080.321
0.004
0.002
0.002
0.003
0.003
36081.928
0.005
23764.538
23766.053
23766.053
23766.251
23766.251
23766.852
23767.766
0.006
0.006
0.006
0.005
0.005
0.005
0.005
Complete line list including millimeter-wave data available as Supplemental material.
P.M. Sheridan et al. / Journal of Molecular Spectroscopy 269 (2011) 231–235
were measured in the J = 1 ? 0 transition, and four transitions
were recorded for NaCCH and KCCH (J = 1 ? 0 through 4 ? 3),
totaling 19 and 21 hyperfine components, respectively. For LiCCD,
seven individual hyperfine components were measured, but only
in the J = 1 ? 0 transition. The J = 1 ? 0 through 4 ? 3 transitions
were recorded for NaCCD and KCCD, with a total of 16 and 30
hyperfine components observed, respectively. The transition frequencies for the deuterated species are found in the Supplemental
Section of the journal.
A spectrum of the J = 1 ? 0 transition of LiCCH near 21 GHz is
shown in Fig. 1. The Doppler doublets for each hyperfine component are labeled by brackets. Three hyperfine features are visible,
but one Doppler component of the F = 0.5 ? 1.5 line is blended
with the F = 2.5 ? 1.5 transition. Fig. 2 shows the J = 1 ? 0 transition of NaCCH near 9 GHz; there are two frequency gaps in the figure to display all three hyperfine components. The J = 2 ? 1
transition near 12 GHz of KCCH is displayed in Fig. 3. Eight hyperfine transitions are visible in these data, with the strongest component consisting of two blended features (F = 3.5 ? 2.5 and
F = 2.5 ? 1.5).
Fig. 4 shows the J = 1 ? 0 transition of LiCCD. Again, the hyperfine components appear as Doppler doublets and are indicated by
brackets. The spectrum is clearly more complex than that of LiCCH
(Fig. 1), because of the deuterium nucleus. Hyperfine components
are therefore labeled by F1 and F, where F1 = J + I1 (I1 refers to the
alkali metal) and F = F1 + I2 (I2 refers to the deuterium).
The spectra of the alkali-metal acetylides were fit to a 1R+
effective Hamiltonian of the following form [3]
b eff ¼ H
b rot þ H
b eQq
H
233
Fig. 2. Spectrum of the three hyperfine components of the J = 1 ? 0 rotational
transition of NaCCH near 9 GHz, measured in this work. The Doppler doublets are
indicated by brackets and the hyperfine components are labeled by the F quantum
number. Unlike the J = 1 ? 0 transition of LiCCH, each individual hyperfine
component is clearly resolved and two frequency breaks are needed to display all
three features. This spectrum is a composite of three, 400 kHz wide scans, with 250
shots averaged per scan.
ð1Þ
b rot contains rotational and centrifugal distortion paramewhere H
b eQq describes the quadrupole interactions. The nuclear
ters and H
spin-rotation coupling was also initially considered in the analysis,
but the corresponding constant, CI, could not be reliably determined
within a 3r uncertainty. For each species, a combined least squares
fit of the FTMW and the previously measured millimeter-wave data,
weighted by uncertainties of 5 kHz and 50 kHz, respectively, was
performed using SPFIT [26]. For the deuterated molecules, an additional term was included to account for the deuterium quadrupole
coupling.
Spectroscopic parameters for each monoacetylide species are
listed in Tables 2 and 3. Rotational constants from the previous
Fig. 3. Spectra of the J = 2 ? 1 rotational transition of KCCH near 12 GHz. Eight
hyperfine components, arising from the 39K nucleus and labeled by the F quantum
number, are visible in the data. Doppler doublets are indicated by brackets. Six of
the hyperfine components appear as single features but the F = 3.5 ? 2.5 and
F = 2.5 ? 1.5 components are blended. The spectrum is a composite of five scans,
each 400 kHz wide, with 250–1600 shots averaged per scan.
millimeter-wave work are also included in the tables for comparison. For each acetylide, the rotational constants determined in this
analysis are in good agreement with those of the previous millimeter studies. In some cases, such as NaCCD, higher-order centrifugal
distortion constants could also be established. The quadrupole coupling constants, eQq, were determined for the first time for the metal and deuterium nuclei, as well. The rms of each combined fit is
consistent with the accuracy of the FTMW and millimeter-wave
instruments. Residuals for the millimeter-wave transitions are
available as Supplemental material.
4. Discussion
Fig. 1. Spectrum of the J = 1 ? 0 rotational transition of LiCCH measured in this
work near 21 GHz. Three closely-spaced hyperfine components, arising from the 7Li
nucleus and labeled by the F quantum number, are visible in the data. Doppler
doublets are indicated by brackets. This spectrum is a single, 400 kHz wide scan,
created from a 1500 shot average.
Table 4 lists the metal quadrupole coupling constants for several
alkali-metal containing molecules, namely the fluorides, chlorides,
hydroxides and borohydrides. The metal quadrupole coupling
234
P.M. Sheridan et al. / Journal of Molecular Spectroscopy 269 (2011) 231–235
Table 4
Quadrupole coupling constants for alkali species (in MHz).
7
Species
a
c
d
e
f
g
h
i
j
Fig. 4. Spectra of the J = 1 ? 0 rotational transition of LiCCD near 19 GHz. In these
data, a total of 15 hyperfine components, labeled by the F1 and F quantum numbers,
respectively, are present, arising from the 7Li and D nuclei. Doppler doublets are
indicated by brackets. The spectrum was generated by an average of 10 000 shots in
a single 400 kHz wide scan.
constant is small in magnitude for each species, which is consistent
with an ionic description of the alkali metal–ligand bond [1–3]. For
the acetylides, the magnitude of the eQq constant for a particular
metal is very similar to those listed for the other ligands in Table
4. This comparison suggests that the acetylides possess a high degree of ionic metal–ligand bonding character similar to the halides,
hydroxides and borohydrides.
Covalent bonding character can often be examined using the
electronegativity difference between two atoms in a bond [3]. This
quantity is smaller between each alkali metal and carbon than
between each alkali metal and fluorine or chlorine [3]. Because a
decreasing electronegativity difference is associated with an
increase in covalent bonding, the alkali metal–acetylide bond most
39
Na
a
MF
M35Cl
MOH
MBH4
MCCH
MCCD
b
23
Li
K
b
0.41590 (12)
0.24993(50)d
0.2958(15)g
8.4401(15)
5.6698(60)e
7.584(52)h
3.385(31)i
7.264(20)j
7.442(47) j
0.378(47)j
0.272(37)j
7.932397(10)c
5.66583(3)f
7.454(52)h
4.256(24)i
6.856(18)j
6.873(14)j
From Ref. [4].
From Ref. [5].
From Ref. [6].
From Ref. [7].
From Ref. [8].
From Ref. [9].
From Ref. [10].
From Ref. [11].
From Ref. [13].
This work.
likely possess a greater degree of covalent character. Unfortunately, quantifying such character from alkali metal quadrupole
coupling constants is difficult [27]. However, if it is assumed that
there is some degree of sp hybridization on the metal atom in
the acetylides, and that the hybridization forms the greatest contribution to the value of eQq for each metal, then the Townes–Dailey
model can be used to quantitatively examine the covalent character of the metal–ligand bond. In the Townes–Dailey model, the metal quadrupole coupling constants can be expressed in terms of
eQqn10, the coupling generated by an electron in a p orbital on
the metal atom [1,27,28]:
1
eQq ¼ eQqn10 nnpr nnpp
2
ð2Þ
The values of eQqn10 for the Li, Na, and K atoms are 0.29 MHz,
4.77 MHz and 4.79 MHz, respectively [29]. It is reasonable to assume that npp is zero for an alkali metal and hence npr is calculated
Table 2
e 1 Rþ .a
Spectroscopic constants (in MHz) for MCCH X
a
b
c
d
e
Parameter
LiCCHb
LiCCHc
NaCCHb
NaCCHd
KCCHb
KCCHe
B
D
H
L
eQq (M)
rms
10544.0915(32)
0.011375(11)
2.78(99) 108
10544.0909(46)
0.011373(14)
2.7(1.3) 108
4510.12329(86)
0.00282733(64)
4.12(14) 109
4510.116(10)
0.0028240(48)
3.63(70) 109
2970.8168(31)
0.0017560(13)
1.310(22) 108
2.73(13) 1013
0.378(47)
0.010
0.078
7.264(20)
0.032
0.083
2970.83066(77)
0.00176168(43)
1.403(10) 108
3.257(76) 1013
6.856(18)
0.077
0.038
Values in parenthesis are 3r standard deviations.
This work.
From Ref. [19]. The rms reflects a combined fit of ground vibrational state and m5 vibrational state rotational transitions.
From Ref. [20]. The rms reflects a combined fit of ground vibrational state and m5 vibrational state rotational transitions.
From Ref. [21]. The L constant is given as I in Ref. [20].
Table 3
e 1 Rþ .a
Spectroscopic constants (in MHz) for MCCD X
a
b
c
d
e
Parameter
LiCCDb
LiCCDc
NaCCDb
NaCCDd
KCCDb
KCCDe
B
D
H
L
eQq (M)
eQq (D)
rms
9622.8794(21)
0.0086090(18)
9622.8736(92)
0.0086047(69)
4181.19005(91)
0.00228463(95)
2.88(18) 109
4181.0949(59)
0.00225585(88)
2765.21740(58)
0.0014454(19)
9.78(36) 109
1.77(20) 1013
6.873(14)
0.157(20)
0.013
2764.999(14)
0.0013966(21)
4.97(11) 109
0.272(37)
0.152(33)
0.027
0.024
7.442(47)
0.193(48)
0.009
0.055
Values in parenthesis are 3r standard deviations.
This work.
From Ref. [19]. The rms reflects a combined fit of ground vibrational state and m5 vibrational state rotational transitions.
From Ref. [20].
From Ref. [21].
0.027
P.M. Sheridan et al. / Journal of Molecular Spectroscopy 269 (2011) 231–235
to be 1.3, 1.5 and 1.4 for LiCCH, NaCCH and KCCH, respectively.
Since npr represents the number of electrons in the pr orbital on
the metal atom, and the maximum value is 1, these values are not
physical. Nevertheless, they suggest that hybridization is not the
dominant contribution to eQq, and that the covalent character of
the metal–ligand bond, even in the acetylides, is minimal. Polarization of the core electrons on the metal by the ligand most likely
makes the dominant contribution to the value of eQq.
The values of the deuterium quadrupole coupling constants for
LiCCD, NaCCD and KCCD are 0.152(33) MHz, 0.193(48) MHz, and
0.157(20) MHz, respectively, which are essentially the same, within the reported uncertainties. For CuCCH, eQq(D) was determined
to be 0.214(23) MHz [23], which overall is slightly larger in value.
This small but significant variation likely results from increased
metal–ligand covalent bonding character in CuCCH, which influences the electronic environment at the deuterium nucleus.
Direct ablation of the pure metal appears to be a promising way
of synthesizing alkali containing compounds in the gas phase. As
mentioned previously, alkali metal quadrupole coupling constants
have not yet been measured for the amides, monomethyls and
hydrosulfides, and this method may offer an avenue to their synthesis and study. Surprisingly, the alkali eQq constants have not
yet been measured for the deuterated alkali metal hydroxides
either, a situation where use of the salt may have been the limiting
factor.
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jms.2011.07.008.
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