THE JOURNAL OF CHEMICAL PHYSICS 131, 224317 共2009兲
The Fourier transform microwave spectrum of the arsenic dicarbide
radical „CCAs: X̃ 2⌸1/2… and its 13C isotopologues
M. Sun,1 D. J. Clouthier,2 and L. M. Ziurys1,a兲
1
Department of Chemistry and Department of Astronomy, Arizona Radio Observatory, and Steward
Observatory, University of Arizona, Tucson, Arizona 85721, USA
2
Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506, USA
共Received 28 September 2009; accepted 4 November 2009; published online 11 December 2009兲
The pure rotational spectrum of the CCAs radical in its ground electronic and spin state, X̃ 2⌸1/2, has
been measured using Fourier transform microwave techniques in the frequency range of
12– 40 GHz. This species was created in a supersonic expansion from a reaction mixture of AsCl3
and C2H2 or CH4 diluted in high pressure argon, using a pulsed nozzle containing a dc discharge
source. Three rotational transitions were measured for the main isotopologue, 12C 12CAs, in the ⍀ = 21
ladder; both lambda-doubling and arsenic 共I = 3 / 2兲 hyperfine interactions were observed in these
spectra. In addition, two to four rotational transitions were recorded for the 13C 13CAs, 13C 12CAs,
and 12C 13CAs species. In these three isotopologues, hyperfine splittings were also resolved arising
from the 13C nuclei 共I = 21 兲, creating complex spectral patterns. The CCAs spectra were analyzed with
a case 共a兲 Hamiltonian, and effective rotational, lambda-doubling, and arsenic and carbon-13
hyperfine constants were determined for the ⍀ = 21 ladder. From the effective rotational constants of
the four isotopologues, an rm共1兲 structure has been derived with rC–C = 1.287 Å and rC–As
= 1.745 Å. These bond lengths indicate that the predominant structure for arsenic dicarbide is
C v C v As·, with some contributing C w C and C w As triple bond characters. The hyperfine
constants established in this work indicate that about 2 / 3 of the unpaired electron density lies on the
arsenic atom, with the remaining percentage on the terminal carbon. The value of the arsenic
quadrupole coupling constant 共eqQ = −202 MHz兲 suggests that the As–C bond has a mixture of
covalent and ionic characters, consistent with theoretical predictions that both ␲ backbonding and
electron transfer play a role in creating a linear, as opposed to a cyclic, structure for certain
heteroatom dicarbides. © 2009 American Institute of Physics. 关doi:10.1063/1.3267483兴
I. INTRODUCTION
Compared to nitrogen and phosphorus-containing molecules, arsenic-bearing species have not attracted as much
attention from spectroscopists.1 In the field of pure rotational
spectroscopy, for example, only a limited number of molecules have been characterized, such as AsF3,2–4 AsCl3,5
AsBr3,6 AsH,7 AsH2,8,9 AsH3,10,11 CH3CAs,12 and AsP,13 using far-infrared, millimeter-wave, or microwave techniques.
However, due to a rising interest in functional materials
made of As-doped carbon clusters, as well as the reactivity of
arsenic ylides in organic synthesis, examining the basic properties of As-bearing compounds has acquired a renewed importance. For example, calculations on numerous arseniccontaining organic species have been carried out at various
levels of theory to achieve an understanding of As–C bonding, and RAsv CF2-type molecules have been synthesized
and their reactivity experimentally investigated.14–17
Very recently, a novel arsenic-containing molecule has
been produced in the gas phase and studied using electronic
spectroscopy: arsenic dicarbide 共CCAs兲, the smallest Asdoped carbon cluster. The 2⌬r − X̃ 2⌸r band system of this
free radical was investigated by Wei et al.,18 who were able
a兲
Electronic mail: [email protected].
0021-9606/2009/131共22兲/224317/10/$25.00
to establish estimates of rotational constants for both the 12C
and 13C isotopologues in the ⍀ = 21 ladder of the ground electronic state, X̃ 2⌸r. These authors also determined that the
molecule is linear, as predicted theoretically.18
CCAs is only the tenth main group dicarbide that has
been studied by gas-phase spectroscopy and certainly warrants additional investigation. In the present paper, we
present the first pure rotational study of this free radical using Fourier transform microwave 共FTMW兲 techniques. Spectra of four isotopologues of arsenic dicarbide, CCAs, 13C2As,
13
CCAs, and C 13CAs, have been recorded in their X̃ 2⌸1/2
electronic states; from the resulting rotational constants, the
ground state geometry has been refined. In addition, hyperfine structures arising from As and 13C nuclear spins were
also resolved in the spectra, providing insight into the bonding in this radical. Here we present our data and analysis and
a comparison of these results with the properties of other
group V dicarbides.
II. EXPERIMENTAL
Measurements of the pure rotational spectra of the four
CCAs isotopologues were conducted in the 12– 40 GHz
range using the FTMW spectrometer of the Ziurys group.
This Balle–Flygare-type narrow-band spectrometer consists
131, 224317-1
© 2009 American Institute of Physics
224317-2
J. Chem. Phys. 131, 224317 共2009兲
Sun, Clouthier, and Ziurys
TABLE I. Measured rotational transitions of CCAs 共X̃ 2⌸1/2兲 in megahertz.
J⬘
F⬘
J⬙
F⬙
Parity
vobs
vo-c
1.5
2
3
2
2
3
1
0
0
1
2
0.5
2
2
1
2
2
1
1
1
1
1
f
e
e
e
f
e
f
e
f
f
12 742.164
13 183.708
13 283.391
13 466.271
13 484.937
13 552.714
13 639.583
13 710.418
13 889.089
14 369.309
0.000
−0.001
0.000
−0.001
−0.001
−0.002
0.003
0.001
−0.005
0.001
2.5
3
2
4
3
2
1
1
4
1
3
2
3
1
2
1.5
3
2
3
2
1
0
1
3
1
3
2
2
0
1
e
e
f
f
f
f
e
e
f
f
f
e
e
e
21 849.122
22 177.334
22 213.081
22 220.538
22 239.456
22 307.744
22 394.597
22 450.278
22 465.450
22 503.102
22 508.777
22 591.895
22 644.102
22 657.554
0.002
−0.004
−0.002
0.000
0.004
−0.002
0.005
−0.002
0.003
0.001
0.000
0.002
−0.004
0.002
3.5
4
3
5
4
3
2
2
5
2
4
4
2
3
3
2.5
4
3
4
3
2
2
1
4
2
3
4
1
3
2
f
f
e
e
e
f
e
f
e
f
e
f
e
f
30 863.179
31 074.015
31 187.792
31 188.780
31 199.177
31 218.476
31 235.826
31 403.691
31 461.820
31 464.341
31 478.796
31 481.432
31 487.414
31 488.578
0.001
−0.005
−0.001
0.000
0.001
0.002
0.001
0.001
0.000
0.002
−0.001
−0.003
−0.001
0.002
of a vacuum chamber 共background pressure of ⬃10−8 torr
maintained by a cryopump兲 which contains a Fabry–Pérottype cavity constructed from two spherical aluminum mirrors
in a near-confocal arrangement. Antennas are embedded in
each mirror for injecting and detecting microwave radiation.
A supersonic jet expansion is used to introduce the sample
gas, produced by a pulsed-valve nozzle 共General Valve兲 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 mirror axis.
More details regarding the instrumentation can be found in
Ref. 19.
The 12C 12CAs radical was generated in the gas phase
using the precursors AsCl3 and unpurified acetylene. Argon
at a pressure of 20 psi, seeded with 0.3% acetylene, was
passed over liquid AsCl3 共Aldrich, 99%兲 contained in a
Pyrex U-tube,18 and the resultant gas mixture delivered
through the pulsed discharge nozzle 共0.8 mm orifice兲 at a
repetition rate of 12 Hz. The gas pulse duration was set to
500 ␮s, which resulted in a 20– 30 SCCM 共SCCM denotes
cubic centimeter per minute at STP兲 mass flow. CCAs production was maximized with a discharge of 1000 V at
50 mA. To produce 13C 13CAs, 0.3% H 13C 13CH 共Cambridge
Isotopes, 99% enrichment兲 in argon was used under the same
sample conditions, while a mixture of 0.2% CH4 and 0.2%
13
CH4 共Cambridge Isotopes, 99% enrichment兲, also in argon,
was employed to create C 13CAs and 13CCAs. The backing
pressure was increased from 20 to 25 psi to optimize the
C 13CAs and 13CCAs signals. Normally, 1000 shots per scan
were taken for the CCAs and 13C 13CAs spectral measurements, while 2000 shots per scan were used for C 13CAs and
13
CCAs.
Within a single gas pulse, three 150 ␮s free induction
decay signals were recorded. The Fourier transform of the
time domain signals produced spectra with a 600 kHz bandwidth with 2 kHz resolution. Because of the beam orienta-
224317-3
Fourier transform microwave spectrum of CCAs
CCAs(X2Пr): = 1/2
F = 5→4
J = 3.5 → 2.5
e
f
F = 5→4
F = 4→3
Image
Image
31187.2
31188.8
Frequency (MHz)
31403.2
31404.8
FIG. 1. Spectrum of the J = 3.5→ 2.5 transition of 12C 12CAs in its electronic
ground state, X̃ 2⌸1/2, near 31 GHz, composed of lambda doublets, indicated
by e and f, as well as the hyperfine structure arising from the nuclear spin of
As共I = 3 / 2兲, labeled by the F quantum number. The Doppler doublets are
indicated for each component, and there is a frequency break in the data.
The spectrum was created by combining 20 successive scans, with 1000
shots per scan and 20 psi 共138 kPa兲 backing pressure with 20 SCCM gas
flow.
tion to the cavity axis, every measured transition appears as a
Doppler doublet with a full width at half maximum of about
5 kHz. Transition frequencies are simply taken as the average of the two Doppler components.
III. RESULTS
The search for the pure rotational spectrum of CCAs in
its X̃ 2⌸1/2 state was based on the optical work of Wei et
al.,18 who provided estimates of the rotational constant B,
spin-orbit parameter A, and lambda-doubling constant p for
two CCAs isotopologues 共12C 12CAs and 13C 13CAs兲. It was
assumed that the magnitude of the magnetic hyperfine splittings in CCAs resembled those of CCP, whose pure rotational spectra had been recently recorded by our group.20
Furthermore, because arsenic 共I = 3 / 2兲 has an electric quadrupole moment,21 unlike phosphorus 共I = 21 兲, the possibility of
additional hyperfine interactions had to be taken into consideration.
Frequency predictions were made based on these assumptions, and the region from 31 110 to 31 430 MHz was
searched to locate the J = 3.5→ 2.5 transition of the CCAs
main isotopologue 共⍀ = 21 spin-orbit ladder兲. The main hyperfine components in both lambda doublets in this 2⌸1/2 fine
structure level were readily found within the predicted range,
with a splitting of about 200 MHz. 共The 2⌸3/2 component is
875 cm−1 higher in energy and is not expected to be populated in the free jet expansion兲. However, because arsenic has
a nuclear spin of I = 3 / 2, each lambda doublet should consist
of a cluster of four strong 共⌬F = −1兲 hyperfine components,
where F = J + I共As兲. After additional searching, 14 hyperfine
components were measured for the J = 3.5→ 2.5 transition, as
well as for the J = 2.5→ 1.5 transition near 22 GHz, including
J. Chem. Phys. 131, 224317 共2009兲
many weaker ⌬F = 0 transitions, as shown in Table I. In addition, ten hyperfine lines of the J = 1.5→ 0.5 transition were
also measured, a total of 38 individual features.
A representative spectrum of CCAs measured with the
FTMW system is given in Fig. 1. Here the strongest hyperfine components of the J = 3.5→ 2.5 transition near 31 GHz
of the 2⌸1/2 substate are shown: the F = 5 → 4 lines arising
from the two lambda doublets, indicated by e and f, as well
as the F = 4 → 3 transition in the e doublet. Each feature is
composed of two Doppler components. A few weak, contaminating lines arising from the image bandpass are also
present.19 There is a frequency break in the spectrum in order
to display both lambda doublets.
For the 13C doubly substituted species, 13C 13CAs, the
search was aided by frequency predictions made on the basis
of the data of Wei et al.,18 the As hyperfine constants of the
main isotopologue, and the 13C hyperfine constants of
13 13
C CP. A case a␤J coupling scheme was assumed for this
species: F1 = J + I1共As兲, F2 = F1 + I2共 13C␣兲, and F = F2
+ I3共 13C␤兲 for 13C␣ 13C␤As. A larger interaction from C␣ was
expected than from C␤, based on results for the CCP radical.
The resulting spectral pattern was much more complicated in
this case. Four successive rotational transitions of 13C 13CAs
with a total of 143 hyperfine components 共⌬F = 0 , ⫾ 1兲 were
measured in the range of 12– 38 GHz, from J = 1.5→ 0.5 to
J = 4.5→ 3.5, as shown in Table II.
Figure 2 presents a typical spectrum of 13C 13CAs showing the eight strongest hyperfine components of the e parity
lambda doublet of the J = 3.5→ 2.5 transition near 29 GHz. A
frequency break appears in the spectrum to display the complete pattern. The single F = 5 → 4 and F = 4 → 3 features
from Fig. 1 for the e doublet of CCAs are now each split into
four components, resulting from the hyperfine interactions of
the two 13C nuclei. Doublets are first generated by 13C␣,
labeled by F2, and then each line is further split by C␤ into
two lines, labeled by F. From the figure it is apparent that the
splitting from 13C␣ is about 4 – 5 MHz, much wider than that
generated by the coupling of the 13C␤ nucleus, which is
about 1 MHz. Further experiments with the singly substituted 13C species confirmed these hyperfine assignments.
Additional measurements were conducted for 13CCAs
and C 13CAs in order to establish a more precise geometry.
Here the coupling scheme is F1 = J + I1共As兲 and F = F1
+ I2共 13C␣/␤兲. The hyperfine analysis of 13C 13CAs aided in the
search for these two isotopologues. As shown in Table III,
between 21 and 40 GHz, three rotational transitions of
C 13CAs were recorded, and two were measured for 13CCAs,
each consisting of 10–14 hyperfine components. In total, 34
lines were obtained for C 13CAs, while only 24 features were
recorded for 13CCAs. Line contamination from other unknown molecules, as well as a less efficient synthetic
method, makes the study of these two species more difficult.
Representative spectra of the J = 3.5→ 2.5 transition of
13
CCAs 共lower panel兲 and C 13CAs 共upper panel兲 are displayed in Fig. 3. For both isotopologues, the four strongest
hyperfine components of the e parity lambda doublet are
shown, in contrast to the eight components in Fig. 2. A frequency break is necessary in the case of 13CCAs to show the
four hyperfine features 共see lower panel兲, but not for
224317-4
J. Chem. Phys. 131, 224317 共2009兲
Sun, Clouthier, and Ziurys
TABLE II. Measured rotational transitions of
C 13CAs 共X̃ 2⌸1/2兲 in megahertz.
13
J⬘
F 1⬘
F 2⬘
F⬘
J⬙
F 1⬙
F 2⬙
F⬙
Parity
vobs
vo-c
1.5
3
3
3
3
3
3
3
3
3.5
3.5
2.5
2.5
3.5
3.5
2.5
2.5
4
3
3
2
4
3
3
2
0.5
2
2
2
2
2
2
2
2
2.5
2.5
1.5
1.5
2.5
2.5
1.5
1.5
3
2
2
1
3
2
2
1
e
e
e
e
f
f
f
f
12 372.191
12 378.100
12 388.073
12 394.193
12 666.064
12 671.448
12 675.537
12 681.343
0.001
0.000
−0.003
0.000
0.001
−0.005
−0.005
−0.003
2.5
3
3
3
3
2
2
2
2
4
4
3
4
3
4
3
3
2
2
2
2
1
1
1
1
1
4
4
4
4
1
1
3
3
3
3
2
2
2
2
3
3
3
3
1
2
2
1
2
2
1
3.5
3.5
2.5
2.5
2.5
1.5
2.5
1.5
4.5
4.5
3.5
3.5
3.5
3.5
2.5
2.5
2.5
2.5
1.5
1.5
1.5
1.5
0.5
1.5
1.5
4.5
4.5
3.5
3.5
1.5
1.5
3.5
2.5
3.5
2.5
2.5
2.5
1.5
1.5
2.5
2.5
3.5
3.5
0.5
1.5
1.5
1.5
2.5
2.5
1.5
4
3
3
2
3
2
2
1
5
4
4
4
3
3
3
2
2
3
2
1
1
2
1
1
2
5
4
4
3
1
2
4
3
3
2
3
2
2
1
3
2
4
3
1
2
1
1
2
3
2
1.5
3
3
3
3
2
2
2
2
3
3
2
3
2
3
2
2
1
1
1
1
0
0
0
1
1
3
3
3
3
1
1
3
3
3
3
2
2
2
2
2
2
2
2
0
1
1
0
1
1
0
3.5
3.5
2.5
2.5
2.5
1.5
2.5
1.5
3.5
3.5
2.5
2.5
2.5
2.5
1.5
1.5
1.5
1.5
0.5
0.5
0.5
0.5
0.5
1.5
1.5
3.5
3.5
2.5
2.5
1.5
1.5
3.5
2.5
3.5
2.5
2.5
2.5
1.5
1.5
1.5
1.5
2.5
2.5
0.5
0.5
0.5
0.5
1.5
1.5
0.5
4
3
3
2
3
2
2
1
4
3
3
3
2
2
2
1
1
2
1
0
0
1
1
1
2
4
3
3
2
1
2
4
3
3
2
3
2
2
1
2
1
3
2
1
1
0
0
1
2
1
e
e
e
e
e
e
e
e
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
e
e
e
e
e
e
f
f
f
f
f
f
f
f
f
f
e
e
e
e
e
e
e
e
e
e
e
20 491.546
20 494.117
20501.009
20 503.598
20 820.454
20 820.981
20 822.215
20 822.755
20 866.312
20 868.555
20 872.583
20 872.634
20 873.701
20874.947
20 882.290
20 883.585
20 889.238
20 889.470
20 905.021
20 905.646
20 951.534
20 954.835
20 985.102
21 045.769
21 047.683
21 094.415
21 096.700
21 098.666
21 101.028
21 117.045
21 117.793
21 158.588
21 160.931
21 161.161
21 163.538
21 163.704
21 164.871
21 170.316
21 171.594
21 235.002
21 236.513
21 239.001
21 240.236
21 260.357
21 291.939
21 292.585
21 308.730
21 312.488
21 314.482
21 319.110
−0.006
−0.003
0.004
0.001
−0.004
0.001
−0.001
0.000
0.002
0.004
0.001
−0.001
−0.001
−0.002
−0.001
0.001
0.003
0.001
0.000
−0.006
−0.005
−0.002
−0.006
0.005
0.007
0.000
0.004
−0.002
0.001
0.000
−0.003
0.001
0.000
0.002
−0.002
0.001
0.002
0.001
0.006
0.000
0.000
0.003
0.000
−0.002
0.004
0.001
−0.004
0.000
−0.005
−0.005
3.5
4
4.5
5
2.5
4
4.5
5
f
28 967.882
−0.005
224317-5
J. Chem. Phys. 131, 224317 共2009兲
Fourier transform microwave spectrum of CCAs
TABLE II. 共Continued.兲
J⬘
4.5
F 1⬘
F 2⬘
F⬘
4
4
4
3
3
3
3
5
4
5
4
5
5
4
4
3
3
3
3
2
2
2
2
2
2
5
5
5
5
2
4
4
4
4
2
2
2
2
2
2
3
2
3
4
4
4
3
3
3
3
3
3
4.5
3.5
3.5
3.5
3.5
2.5
2.5
5.5
4.5
5.5
4.5
4.5
4.5
3.5
3.5
3.5
3.5
2.5
2.5
2.5
2.5
2.5
2.5
1.5
1.5
5.5
5.5
4.5
4.5
1.5
3.5
4.5
4.5
3.5
2.5
2.5
1.5
2.5
2.5
1.5
2.5
1.5
2.5
4.5
3.5
4.5
3.5
3.5
3.5
3.5
2.5
2.5
4
4
3
4
3
3
2
6
5
5
4
5
4
4
3
4
3
3
2
3
2
3
2
2
1
6
5
5
4
2
4
5
4
3
3
2
1
3
2
2
3
1
2
5
4
4
4
3
4
3
3
2
5
6
5
6
6
5
6
5
5.5
6.5
5.5
6.5
5.5
4.5
5.5
4.5
6
7
5
6
6
5
5
4
J⬙
3.5
F 1⬙
F 2⬙
F⬙
Parity
vobs
vo-c
4
4
4
3
3
3
3
4
3
4
3
4
4
3
3
2
2
2
2
2
2
1
1
1
1
4
4
4
4
1
3
3
3
3
2
2
1
1
1
1
2
1
2
4
4
4
2
2
3
3
3
3
4.5
3.5
3.5
3.5
3.5
2.5
2.5
4.5
3.5
4.5
3.5
3.5
3.5
2.5
2.5
2.5
2.5
1.5
1.5
2.5
2.5
1.5
1.5
0.5
0.5
4.5
4.5
3.5
3.5
1.5
2.5
3.5
3.5
2.5
2.5
2.5
0.5
1.5
1.5
0.5
1.5
0.5
1.5
4.5
3.5
4.5
2.5
2.5
3.5
3.5
2.5
2.5
4
4
3
4
3
3
2
5
4
4
3
4
3
3
2
3
2
2
1
3
2
2
1
1
0
5
4
4
3
2
3
4
3
2
3
2
1
2
1
1
2
0
1
5
4
4
3
2
4
3
3
2
f
f
f
f
f
f
f
e
e
e
e
e
e
e
e
e
e
e
e
f
f
e
e
e
e
f
f
f
f
f
f
f
f
f
e
e
f
f
f
f
f
f
f
e
e
e
f
f
e
e
e
e
28 969.219
28 973.044
28 974.387
29 178.652
29 179.799
29 180.127
29 181.280
29 303.072
29 303.338
29 304.299
29 304.330
29 306.428
29 307.684
29 308.237
29 309.279
29 312.720
29 313.602
29 319.263
29 320.246
29 325.756
29 326.192
29 348.444
29 349.525
29 354.143
29 355.303
29 509.218
29 510.468
29 511.637
29 512.916
29 535.000
29 570.706
29 570.755
29 571.796
29 571.813
29 576.772
29 577.332
29 586.411
29 592.569
29 592.912
29 593.759
29 594.155
29 594.339
29 595.040
29 595.615
29 596.525
29 596.942
29 597.201
29 597.825
29 603.835
29 604.764
29 607.282
29 608.248
0.000
0.005
0.008
−0.006
−0.004
−0.002
0.000
0.000
0.001
−0.003
−0.005
0.001
0.002
0.006
0.002
0.004
0.004
0.000
0.001
0.001
0.003
0.000
0.001
−0.003
0.000
0.001
−0.002
0.000
0.000
−0.005
0.004
0.005
0.000
0.004
0.001
−0.001
0.000
0.003
−0.001
−0.002
0.004
0.002
0.001
0.001
−0.003
−0.001
0.003
0.001
−0.002
−0.001
−0.004
−0.001
4
5
4
5
5
4
5
4
4.5
5.5
4.5
5.5
4.5
3.5
4.5
3.5
5
6
4
5
5
4
4
3
f
f
f
f
f
f
f
f
37 723.618
37 724.228
37 724.319
37 725.002
37 726.284
37 726.478
37 727.072
37 727.209
0.000
0.006
−0.008
0.001
−0.002
−0.003
−0.003
0.000
224317-6
J. Chem. Phys. 131, 224317 共2009兲
Sun, Clouthier, and Ziurys
TABLE II. 共Continued.兲
J⬘
F 1⬘
F 2⬘
F⬘
4
4
4
4
3
3
3
3
6
6
6
6
5
5
5
5
3
3
3
3
4
4
4
4
4.5
4.5
3.5
3.5
3.5
3.5
2.5
2.5
6.5
6.5
5.5
5.5
5.5
4.5
5.5
4.5
3.5
3.5
2.5
2.5
3.5
4.5
3.5
4.5
5
4
4
3
4
3
3
2
7
6
6
5
6
5
5
4
4
3
3
2
4
5
3
4
J⬙
F 1⬙
F 2⬙
F⬙
Parity
vobs
vo-c
3
3
3
3
2
2
2
2
5
5
5
5
4
4
4
4
2
2
2
2
3
3
3
3
3.5
3.5
2.5
2.5
2.5
2.5
1.5
1.5
5.5
5.5
4.5
4.5
4.5
3.5
4.5
3.5
2.5
2.5
1.5
1.5
2.5
3.5
2.5
3.5
4
3
3
2
3
2
2
1
6
5
5
4
5
4
4
3
3
2
2
1
3
4
2
3
f
f
f
f
f
f
f
f
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
37 729.725
37 730.439
37 733.227
37 733.972
37 752.353
37 753.183
37 755.345
37 756.192
37 920.271
37 921.056
37 921.833
37 922.635
37 954.466
37 954.972
37 955.198
37 955.729
37 964.636
37 965.316
37 965.924
37 966.648
37 967.594
37 967.949
37 968.295
37 968.585
0.000
0.005
−0.004
0.006
−0.001
−0.004
0.007
0.001
0.000
−0.007
−0.001
−0.001
−0.003
−0.003
−0.003
0.001
0.003
0.003
0.003
−0.003
0.001
0.001
0.003
−0.003
C 13CAs. 共Neglecting the break, both figures have the same
number of MHz/in. to facilitate comparisons of the hyperfine
splittings.兲 The outer 13C nucleus, C␣, clearly has a stronger
interaction with the unpaired electron in this radical, generating a splitting about a factor of 2 larger than the middle C␤
carbon, a situation also found for the CCP radical.20 It is
obvious that the S / N ratio of the two 13C singly substituted
13
C13CAs(X2Пr): = 1/2 e
F1 = 5 → 4
F2 = 5.5 → 4.5
F = 6→ 5
J = 3.5 → 2.5
4→ 3
4.5 → 3.5
F1 =
F2 =
F = 4→3
5→ 4
F1 =
F2 = 4.5 → 3.5
F = 5→ 4
F = 4→ 3
F1 = 4 → 3
F2 = 3.5 → 2.5
F = 4→3
F = 3 →2
29302.5
29304.0
IV. ANALYSIS
The data from the four CCAs isotopologues were analyzed by using the nonlinear least squares routine SPFIT
共Ref. 22兲 with the following Hund’s case 共a兲 effective
Hamiltonian:
Ĥeff = Ĥrot + Ĥso + Ĥld + Ĥmhf + ĤeQq + Ĥnsr .
F1 = 5 → 4
F2 = 5.5 → 4.5
F = 5→4
F1 = 4 → 3
F2 = 4.5 → 3.5
F = 5 →4
species is not as good as that of the other two isotopologues.
Production of arsenic dicarbide from methane does not appear to be as favorable as from acetylene.
29307.0
29308.5
Frequency (MHz)
FIG. 2. Spectrum of the lambda doubling e component of the J = 3.5→ 2.5
transition of 13C 13CAs共X̃ 2⌸1/2兲 near 29 GHz. The hyperfine structure in
these data arises from three nuclear spins, as indicated by F1共As兲, F2共 13C␣兲,
and F共 13C␤兲. The Doppler doublets are shown for each transition and there
is a frequency break in the data. The spectrum was created from an aggregate of 25 successive scans, 1000 shots each, and 20 psi 共138 kPa兲 backing
pressure with 20 SCCM gas flow.
共1兲
The terms in the Hamiltonian are nuclear rotation, electron
spin-orbit coupling, lambda-doubling, magnetic and electric
quadrupole hyperfine, and nuclear spin-rotation interactions,
respectively. The individual isotopologues were analyzed
separately.
In order to fit the FTMW data, which only included transitions in the ⍀ = 21 ladder, the spin-orbit constant A was fixed
in all cases to the value of 857.4 cm−1 from Wei et al.18 All
other parameters were allowed to float in the fit. Because
measurements of a single spin-orbit ladder were involved
共⍀ = 21 兲, only the p lambda-doubling constant was determined; it was assumed that the lambda-doubling q constant
was negligible, as in the case of CCP,20 i.e., p + 2q ⬃ p. A
centrifugal distortion correction to p, pD, was also used in the
fit. Furthermore, only the case 共c兲 hyperfine constants h1/2,
h1/2D, and d could be determined for each atom with a
nuclear spin. The parameter h1/2D is the centrifugal distortion
correction to h1/2 = a − 共b + c兲 / 2 and d is the parity-dependent
hyperfine term. Due to data set limitations, the h1/2D parameter for both 13CCAs and C 13CAs was fixed to the value
224317-7
J. Chem. Phys. 131, 224317 共2009兲
Fourier transform microwave spectrum of CCAs
TABLE III. Measured rotational transitions of
12
C 13CAS and
C 12C 共X̃ 2⌸1/2兲 in megahertz.
13
12
C 13CAs
13
C 12CAs
J⬘
F 1⬘
F⬘
J⬙
F 1⬙
F⬙
Parity
vobs
vo-c
vobs
vo-c
2.5
4
4
3
3
2
4
4
3
3
2
4.5
3.5
3.5
2.5
2.5
4.5
3.5
3.5
2.5
2.5
1.5
3
3
2
2
1
3
3
2
2
1
3.5
2.5
2.5
1.5
1.5
3.5
2.5
2.5
1.5
1.5
f
f
f
f
f
e
e
e
e
e
21 949.866
21 952.201
21 957.819
21 959.083
21 977.250
22 185.329
22 187.759
22 327.275
22 328.730
22 394.466
−0.002
−0.007
−0.006
0.002
−0.001
0.005
−0.002
−0.001
−0.004
0.003
21 094.542
21 100.843
21 100.305
21 110.006
21 116.661
21 324.127
21 328.263
21 468.429
21 464.226
21 542.269
0.000
0.002
−0.008
0.001
0.006
−0.004
−0.004
0.009
0.006
−0.007
3.5
5
4
5
4
3
3
2
5
5
4
4
2
3
3
5.5
4.5
4.5
3.5
3.5
2.5
2.5
5.5
4.5
4.5
3.5
2.5
3.5
2.5
2.5
4
3
4
3
2
2
1
4
4
3
3
1
2
2
4.5
3.5
3.5
2.5
2.5
1.5
1.5
4.5
3.5
3.5
2.5
1.5
2.5
1.5
e
e
e
e
e
e
e
f
f
f
f
f
f
f
30 819.675
30 820.782
30 820.937
30 821.825
30 831.216
30 832.190
30 867.873
31 033.862
31 035.178
31 094.570
31 095.701
31 112.221
31 119.071
31 119.922
0.003
0.007
0.001
0.003
−0.003
0.001
0.010
−0.003
0.003
0.003
0.006
−0.007
−0.006
−0.001
29 622.177
29 622.324
29 625.524
29 627.211
29 631.654
29 638.180
29 667.379
29 829.759
29 832.132
29 891.250
29 891.118
29 912.426
29 917.413
29 914.236
−0.001
−0.004
0.000
−0.001
0.003
0.008
−0.005
0.002
0.001
0.000
−0.001
−0.001
0.004
−0.005
4.5
5
6
5
6
4
6
6
5
5
4
5.5
6.5
4.5
5.5
4.5
6.5
5.5
5.5
4.5
4.5
3.5
4
5
4
5
3
5
5
4
4
3
4.5
5.5
3.5
4.5
3.5
5.5
4.5
4.5
3.5
3.5
f
f
f
f
f
e
e
e
e
e
39 674.286
39 674.433
39 675.024
39 675.234
39 680.930
39 878.874
39 879.695
39 912.713
39 913.482
39 925.484
−0.009
−0.006
0.001
0.004
0.001
0.001
0.005
−0.004
0.000
0.000
derived from the analysis of 13C 13CAs. The As hyperfine
constants of the four isotopologues thus determined are in
excellent agreement with each other. Moreover, the 13C hyperfine constants from 13CCAs and C 13CAs are virtually
identical to those of 13C 13CAs. The nuclear spin-rotation
term C1 could only be determined for the arsenic atom, as
attempts to fit this constant for other nuclei within their 3␴
uncertainties were not successful.
The spectroscopic constants from the analysis are listed
in Table IV. The rms values for the CCAs and 13C 13CAs fits
are 2 – 3 kHz while those for 13C 12CAs and 12C 13CAs are
about 4 kHz. The rotational constants of the two main isotopic species agree with those of Wei et al.18 to within 0.2%;
for the lambda-doubling constant p, there is about a 10%
agreement.
V. DISCUSSION
From the rotational constants established in this work for
the four isotopologues, an improved structure for CCAs has
been derived. The resulting bond lengths are listed in Table
V. Several structures were determined for this linear species:
r0, rs, and rm共1兲. 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 account in part for zero-point vibrational
effects.23 The rm共1兲 bond lengths were derived by the method
developed by Watson24 and are believed to be closer to the
equilibrium structure than the rs or r0 geometries. 共The Watson rm共2兲 structure would be optimal, but could not be calculated because no isotopic substitution is possible for the As
atom.兲 As the table shows, the rm共1兲 C–C bond length is
1.287 Å, almost identical to that in CCP, while the C–As
rm共1兲 distance is 1.745 Å. 共All three structures actually agree
to within 0.5%.兲 The difference between the C–P and the
C–As bond lengths is about 0.12 Å primarily due to the
greater atomic radius of arsenic. The theoretical value of the
C–C bond length in CCAs, rC–C = 1.2933 Å, calculated with
density functional methods at the B3LYP/aug-cc-pVTZ
224317-8
J. Chem. Phys. 131, 224317 共2009兲
Sun, Clouthier, and Ziurys
12
C13CAs(X2Пr): = 1/2 e
F1 = 5 → 4
F = 5.5 → 4.5
J = 3.5 → 2.5
F1 = 5 → 4
F = 4.5 → 3.5
F1 = 4 → 3
F = 3.5 → 2.5
F1 = 4 → 3
F = 4.5 → 3.5
30819
30820
13
F1 = 5 → 4
F = 5.5 → 4.5
30821
30822
C12CAs(X2Пr): = 1/2 e
J = 3.5 → 2.5
F1 = 4 → 3
F = 4.5 → 3.5
F1 = 5 → 4
F = 4.5 → 3.5
F1 = 4 → 3
F = 3.5 → 2.5
as well. We conclude that arsenic dicarbide has three contributing resonance structures, with the first being dominant:
C v C v As·, ·C – C w As and C w C – As·. These conclusions are consistent with the hyperfine constants, as discussed later, and mimic the structure found for CCP.20
There is a significant increase in the lambda-doubling
parameter p in CCAs relative to CCP, 188.9 versus
50.0 MHz, assuming q is negligible. Because p is proportional to the product of A ⫻ B,30 the increase in this parameter for CCAs can be accounted for by the larger A value
共875 versus 140 cm−1兲. In fact, the ratio of p parameters for
these two molecules almost scales directly as the product AB.
In the limit of the pure precession approximation, this result
would suggest that the nearby perturbing ⌺ state lies at similar energies above the 2⌸ ground state in both molecules.18,31
The values of the magnetic hyperfine constants vary
from nucleus to nucleus in CCAs, following the same pattern
as in CCP. Both h1/2 and d are considerably larger for the
arsenic nucleus, as opposed to the two 13C nuclei, although
the nuclear spin g factors are gN共 75As兲 = 0.960 and gN共 13C兲
= 1.404, respectively. For example, d共As兲 ⬇ 673 MHz,
d共 13C␣兲 ⬇ 97 MHz, and d共 13C␤兲 ⬇ 6 MHz, considering all
isotopologues. The h1/2 constant follows the same trend. The
d parameter can be used to evaluate the average electron spin
density at the three nuclei by comparing it with the atomic
value gs␮BgN␮N具r−3典 and using the expression20,32,33
3
d = g s␮ Bg N␮ N 兺
2
i
29622
29623
29626
29627
Frequency (MHz)
FIG. 3. Spectra of the lambda-doubling e component of the J = 3.5→ 2.5
transition of 12C 13CAs 共upper panel兲 and 13C 12CAs 共lower panel兲 near
30– 31 GHz in the X̃ 2⌸1/2 state. Hyperfine components, labeled by F1 and
F, arise from the coupling of two nuclear spins, As共I = 3 / 2兲 and 13C共I
= 1 / 2兲. The Doppler doublets are shown for each transition. There is a
frequency break in the spectrum of 13C 12CAs to display the same hyperfine
components as for 12C 13CAs. Each spectrum was created by combining 15
successive scans, with 2000 shots per scan and 25 psi 共172 kPa兲 backing
pressure with 30 SCCM gas flow.
level,18 is in reasonable agreement, as well as rC–As
= 1.7341 Å, determined from the laser-induced fluorescence
共LIF兲 experiments of Wei et al.18
Table V also summarizes the C–As and C–C bond
lengths of other relevant molecules.15,17,18,20,25–29 The ethylene C–C bond length of 1.339 Å is representative for a
C v C double bond, while the acetylene C–C bond length,
1.202 Å, is typical of a C w C triple bond. Our experimental
C–C bond distance of 1.287 Å for CCAs falls almost midway between the double and triple bond values. Based on
other known molecules 共see Table V兲, C–As single, double,
and triple bond lengths are about 1.98, 1.80, and 1.65 Å,
respectively. Our value of rC–As = 1.745 Å indicates a predominantly double bond but with some triple bond character
冓 冔
sin2 ␪i
r3i
.
共2兲
Here gS is the electron spin g factor and ␮B and ␮N the Bohr
and nuclear magnetons, and the summation is over all unpaired electrons.30 The electron configuration for CCAs is
postulated to be 共core兲12␴25␲1, and thus only one ␲ electron
needs to be considered.18 Using the d constants for 12C 12CAs
and 13C 13CAs, and the expectation value of 具sin2 ␪典 = 4 / 5 for
a p␲ electron,34 comparison of the molecular versus atomic
values35 of gs␮BgN␮N具r−3典 yields the following spin densities
for C␣C␤As: 30.2% on C␣, 1.9% on C␤, and 67.2% on As.
Clearly the bulk of the unpaired electron density is on the
arsenic nucleus, with a significant amount on the terminal
carbon. The middle carbon carries very little of the total
density. These results are consistent with the proposed resonance structures for this molecule.
To date, three group V dicarbides, CCN,32 CCP,20 and
CCAs, have been characterized by microwave spectroscopy.
The relative values of the hyperfine d constant for the heteroatom for all three species are available and can therefore
be compared to examine trends within this group. The d
parameters are listed in Table VI, as well as their associated
spin densities. CCN has 30% of the spin density on the nitrogen nucleus, as compared to 57.5% for phosphorus in
CCP and 67.2% for arsenic in CCAs. It is obvious that from
CCN to CCAs, the unpaired electron density shifts to the
terminal heteroatom, presumably at the expense of the terminal carbon. Note that for CCP, the spin densities on the carbon nuclei are 33.3% for 13C␣ and 0.8% for 13C␤, as opposed
to 30.2% and 1.9% for the arsenic analog. Thus, the contribution of the C v C v X· and C w C – X· structures increases
224317-9
J. Chem. Phys. 131, 224317 共2009兲
Fourier transform microwave spectrum of CCAs
TABLE IV. Spectroscopic constants 共MHz兲 of C␣C␤As 共X̃ 2␲1/2兲. Values in parentheses are 3␴ uncertainties.
12
C 12CAs
Parameter
B
D
A
p
pD
CI共As兲
h1/2共As兲
h1/2D共As兲
d共As兲
eQq共As兲
h1/2共C␣兲
h1/2D共C␣兲
d共C␣兲
h1/2共C␤兲
h1/2D共C␤兲
d共C␤兲
rms
12
C 13CAs
4 474.593 1共16兲
0.001 121共67兲
26 243 832a
188.851共11兲
−0.001 54共41兲
−0.201 8共99兲
547.262共26兲
0.536共22兲
672.542 3共75兲
−201.790共20兲
C 12CAs
4 421.937 1共16兲
0.001 093共40兲
26 243 832a
187.161共23兲
−0.001 63共55兲
−0.207共56兲
547.65共19兲
0.54共13兲
672.82共16兲
−202.07共23兲
13
C 13CAs
4 250.413 9共37兲
0.001 08共13兲
26 243 832a
179.502共35兲
−0.001 4共12兲
−0.149共83兲
547.61共26兲
0.42共19兲
672.77共22兲
−201.98共24兲
84.54共18兲
0.018 4b
97.304共98兲
36.27共15兲
−0.0120b
6.14共12兲
0.004
0.002
A is fixed to 875.4 cm−1 共Ref. 18兲.
h1/2D共C␣ / C␤兲 is fixed to the fitting results of
13
4 204.779 77共63兲
0.000 978共18兲
26 243 832a
178.077 2共65兲
−0.001 34共15兲
−0.171 8共60兲
547.795共20兲
0.468共14兲
672.907 2共52兲
−201.937共14兲
84.733共40兲
0.018 4共40兲
97.519共20兲
36.132共38兲
−0.012 0共40兲
6.164共22兲
0.003
0.004
a
b
13
C 13CAs.
down the Periodic Table, while nitrogen prefers ·C – C w N
with the electron on the terminal carbon. The effect probably
arises from the fact that the valence orbitals are more diffuse
on phosphorus and even more so for arsenic; nitrogen forms
bonds that are more directional and can make a true triple
bond. Furthermore, nitrogen is substantially more electronegative than the other two atoms, favoring a closed valence
shell.
The quadrupole coupling constants eQq for the isotopologues of arsenic dicarbide are uniformly near −202 MHz.
This constant can be compared to eQq410 of atomic arsenic
关−433 MHz 共Ref. 13兲兴 to estimate the degree of ionic character using the Townes Dailey model,30 namely,
eQq共CCAs兲/eQq共As兲 = 共1 − x兲,
where x is the percent ionic character. Use of this equation
suggests that CCAs is about 53% ionic in its bonding. In the
case of CCN, eQq was determined to be −4.8 MHz, while
eQq210 is −10 MHz 共Ref. 23兲 for the free atom, yielding an
almost identical degree of ionic character.
The relative ionic/covalent bonding contributions for
CCAs and CCN are consistent with theoretical predictions of
dicarbide structures. Largo et al.36 have suggested that the
main group dicarbides form a T-shaped structure if they are
highly ionic. This geometry is a result of charge transfer
from the electropositive heteroatom 共e.g., Na, Al, Mg, Si兲 to
TABLE V. Bond lengths of CCAs and related molecules. Values in parentheses are 1␴ uncertainties.
r共C – C兲
共Å兲
r共C – X兲a
共Å兲
CCAs
1.2884共48兲
1.2851共4兲
1.2872共3兲
1.2933
1.7362共33兲
1.7427共5兲
1.7455共5兲
1.734共4兲
CCP
1.291共2兲
1.288共3兲
1.289共1兲
共CH3兲3As
CH3AsH2
1.615共2兲
1.619共3兲
1.621共1兲
1.968共3兲
1.980
R1Asv CR2R3
CH2 v As
1.865b
1.784
C6H5C w As
CH3C w As
CH2 v CH2
HC w CH
1.651共5兲
1.661共1兲
Molecule
a
1.3391共13兲
1.20241共9兲
X = As or P.
Averaged value for different R1, R2, and R3 groups.
b
共3兲
Method
r0
rs
rm共1兲
Theory/LIF
B3LYP/aug-cc-pVTZ
r0
rs
rm共1兲
rz, Electron diffraction
re, ab initio
MP2/6-31G共d,p兲
rz, x-ray
Theory
B3LYP/aug-cc-pVTZ
rz, x-ray
r0, microwave
rz, microwave
re, infrared, raman
Ref.
This work
This work
This work
18
20
20
20
25
26
17
18
27
15
28
29
224317-10
J. Chem. Phys. 131, 224317 共2009兲
Sun, Clouthier, and Ziurys
TABLE VI. Comparison of the hyperfine constants, nuclear g-factors, and
spin densities for CCX 共X = N, P, and As兲. Data from the main isotopologue
are used.
CCX
h1/2共X兲d
d共X兲d
g Ne
Spin density共X兲
X = Na
36.0
46.8
0.4038
30.0%
X = Pb
484.2
632.5
2.2632
57.5%
X = Asc
547.3
672.5
0.9596
67.2%
a
Reference 32.
Reference 20.
c
This work.
d
In megahertz.
e
Reference 21.
b
the C–C group. The linear structure is preferred for more
electronegative elements such as N, P, and As. In these cases,
backbonding from the 1␲u orbital of the C2 moiety to the
atomic unfilled 4p orbital of the heteroatom occurs, relevant
to the C w C – X· structure. In addition, charge transfer from
the partly occupied 4p orbital 共C v C v X ·兲to the 1␲g antibonding orbital in C2 can also occur. Such structures suggest
a mix of covalent and ionic bonding in CCX linear species,
as indicated by the quadrupole constant.
VI. CONCLUSION
Determining the geometries and electronic properties of
heteroatom dicarbide species remains a challenge for spectroscopists and theoreticians alike. In this work we have better characterized the CCAs molecule using pure rotational
spectroscopy. This species was found to be similar in structure and bonding to CCP, but both molecules differ considerably from CCN. Backbonding and charge transfer involving the ␲ orbitals of the CC moiety and the p orbital of the
electronegative heteroatom appear to dictate the linear structures in CCP and CCAs. The linear geometry in CCN is
more likely a result of a strong triple bond between the C and
N atoms. Studies of additional heteroatom dicarbides would
be quite enlightening in establishing the nature of the bonding in these model carbon cluster systems.
ACKNOWLEDGMENTS
This research is supported by NSF Grant No. CHE0718699. D.J.C. acknowledges support from NSF Grant No.
CHE-0804661 and thanks the Ziurys group for their hospitality during his visit to their laboratory.
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