The Astrophysical Journal, 501:L151–L153, 1998 July 1
q 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A.
THE SUBMILLIMETER-WAVE SPECTRUM OF KCCH (X1S)
J. Xin and L. M. Ziurys
Department of Astronomy, Department of Chemistry, and Steward Observatory, University of Arizona,
933 North Cherry Avenue, Tucson, AZ 85721-0065
Received 1998 February 17; accepted 1998 May 5; published 1998 June 18
ABSTRACT
The pure rotational spectrum of KCCH (X1S) has been observed in the laboratory for the first time using
millimeter/submillimeter direct absorption techniques. Thirty-seven rotational transitions of this molecule were
recorded in the range 225–490 GHz, and 15 transitions were recorded for its deuterium isotopomer, KCCD. Both
species were created by the reaction of potassium metal and acetylene or deuterated acetylene under DC discharge
conditions. The transition frequencies were analyzed to produce precise spectroscopic constants for KCCH and
KCCD. This study is the first experimental information available for potassium monoacetylide and indicates that
it is a linear species, with some, but not total, ionic character to its bonding. Rest frequencies are also now
available for KCCH for astronomical searches.
Subject headings: ISM: molecules — line: identification — molecular data
quencies and spectroscopic parameters of these molecules are
presented.
1. INTRODUCTION
Acetylene-derivative species have for many years played a
prominent role in interstellar chemistry. Although acetylene
itself has no permanent dipole moment and hence cannot be
detected via a pure rotational spectrum, a whole series of alkynyl compounds have been observed by radio astronomy that
are composed of HCCH fragments. These molecules have been
detected both in molecular clouds and in the envelopes of latetype stars. Among the species observed include the ethynyltype radicals CCH, C3H, C4H, C5H, and C6H (e.g., see Thaddeus
1994 for a review), as well as the long-chain cyanopolyacetylides HC3N, HC5N, HC7N, etc. Also significant among the
carbon-chain molecules are those containing methyl groups
such as CH3CCH and CH3CCCN.
Because of the prominence of acetylene-type species in interstellar gas, we have been carrying out a systematic study in
the laboratory of metal-acetylide derivatives of the general formula MCCH. Past spectroscopic investigations of such species
have been rare, and few have been carried out at high enough
spectral resolution to warrant radio astronomical searches. Most
of these studies were conducted by the Bernath group, who
measured optical spectra of both CaCCH and SrCCH using
laser-induced fluorescence (Bopegedera, Brazier, & Bernath
1987, 1988). Our first work also concerned the alkaline earth
monoacetylide radicals, including the measurement of the pure
rotational spectra of MgCCH (Anderson & Ziurys 1995a),
CaCCH (Anderson & Ziurys 1995b), and SrCCH (Nuccio, Apponi, & Ziurys 1995), using millimeter/submillimeter direct
absorption techniques. More recently, we have expanded our
millimeter studies to include the alkali metal monoacetylides
NaCCH (Li & Ziurys 1997; Brewster, Apponi, & Ziurys 1998)
and LiCCH (Apponi, Brewster, & Ziurys 1998). Previous spectroscopic measurements of these two molecules did not exist.
Our studies provided the first rotational rest frequencies for
metal monoacetylides with sufficient accuracy for astronomical
observations.
In this Letter, we present the first laboratory detection of
KCCH and measurement of its pure rotational spectrum. A
wide range of transition frequencies were recorded for this
molecule, which has a 1S ground electronic state. Measurements were also carried out for KCCD. Rotational constants
of both isotopomers have been determined. Here the rest fre-
2. EXPERIMENTAL
The millimeter-wave spectrum of KCCH and its deuterium
isotopomer were recorded using one of the spectrometers of
the Ziurys group, which is described in detail in Ziurys et al.
(1994). The instrument is a quasi-optical system utilizing a
Gunn oscillator/varactor multiplier frequency source, a double
pass reaction chamber, and an InSb detector. The source is FM
modulated, and phase-sensitive detection is employed; signals
are recorded at twice the modulation frequency, resulting in
second-derivative spectra.
KCCH was created in a DC discharge of acetylene and potassium vapor. The metal vapor was produced in a Broida-type
oven, which is about 4 inches (10.2 cm) in diameter and is
attached to the spectrometer. As the metal was vaporized, it
was mixed with about 8 mtorr of acetylene, added through the
bottom of the oven, and approximately 20 mtorr of argon carrier
gas. This mixture was flowed into the spectrometer and discharged. The discharge voltage used was 220 V at about 80
mA. The discharge caused the otherwise colorless gases to glow
blue-gray in color. For KCCD, DCCD was used as the precursor
gas instead of HCCH and a slightly lower pressure (∼6 mtorr)
was needed. Potassium metal and its residual products were
found to be highly explosive, and extreme care had to be taken
when cleaning the spectrometer after use.
All transition frequencies were recorded using an average of
two 5 MHz scans, one in increasing frequency and the other
in decreasing frequency. Typical line widths for KCCH were
around 800 kHz; those for KCCD were about 1000 kHz because
these measurements were done at a higher frequency. Experimental precision is estimated to be 550 kHz.
3. RESULTS
The rotational rest frequencies recorded for KCCH and
KCCD are presented in Table 1. Because both species have 1S
ground electronic states, each transition J 1 1 R J consists of
a single line. Thirty-seven transitions were recorded for KCCH
in the range 225–484 GHz, while 15 were measured for KCCD
in the 412–489 GHz region. Lines of KCCH arising from the
L151
L152
XIN & ZIURYS
Vol. 501
TABLE 1
Transition Frequencies for KCCH and KCCDa
KCCH
J9 R J
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
a
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
37
38
39
40
4l
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
......
KCCD
nobs
nobs 2 ncalc
225402.646
231313.900
237223.408
243131.582
249038.026
254942.752
260845.819
266747.126
272646.683
278544.416
284440.314
290334.393
296226.511
302116.665
308004.914
313891.156
319775.273
325657.423
331537.451
337415.328
323291.023
349164.582
355035.835
360904.895
366771.595
419463.984
425306.098
431145.500
436982.436
442816.562
448648.013
454476.734
460302.678
466125.773
471946.101
477763.614
483578.034
0.017
0.060
20.085
0.031
0.047
0.011
0.016
20.003
20.001
20.020
20.035
0.004
20.012
20.053
20.026
20.001
20.064
20.022
20.000
0.005
20.005
0.046
0.021
0.063
0.037
0.056
0.036
20.07l
0.011
20.031
20.034
20.021
20.009
20.040
20.001
0.089
20.015
nobs
nobs 2 ncalc
412463.917
417903.159
423340.200
428774.880
434207.287
439637.297
445064.946
450490.217
455913.082
461333.500
466751.405
472166.830
477579.815
482990.170
488398.059
0.050
20.016
0.002
20.032
20.007
20.025
20.028
20.011
0.020
0.045
0.019
20.004
0.036
20.030
20.018
Fig. 1.—Laboratory rotational spectra of KCCH and KCCD observed in
this study near 296 and 439 GHz, respectively. Both isotopomers are closedshell linear molecules, and hence each rotational transition consists of a single
line labeled by J 1 1 R J. The two spectra shown both span 80 MHz in
frequency and were taken in a single, 50 s scan.
precision. The term in the Hamiltonian for I0 takes the form
H0 5 I0 J 4 (J 1 1) 4.
In MHz.
first and second quanta of the lowest vibrational mode, the v5
bend, were also identified. Although potassium, hydrogen, and
deuterium have nuclear spins of 3/2, 1/2, and 1, respectively,
no magnetic or nuclear quadrupole splittings were observed in
the data. This result is expected because only high-J transitions
were studied, where such interactions should be negligible for
closed-shell molecules.
Figure 1 presents representative data for both KCCH and
KCCD. The upper spectrum shows the J 5 80 R 79 transition
of KCCD near 439 GHz, and the lower one shows the J 5
50 R 49 line of KCCH near 296 GHz. Each spectrum covers
a total range of 80 MHz and was recorded in a single, 50 s
scan.
The data sets for KCCH and KCCD were analyzed separately
using a simple Hamiltonian which includes only molecular
frame rotation and its centrifugal distortion corrections to second (D0) and third (H0) order. For KCCH, a sufficiently wide
range of transitions were recorded (J 5 38 R 37 through J 5
82 R 81) such that a fourth-order centrifugal distortion correction I0 had to be used to fit the data to near the experimental
(1)
The resulting spectroscopic parameters from this analysis are
given in Table 2. The rms of the data fits were 38 and 27 kHz
for KCCH and KCCD, respectively. The highest residual was
89 kHz, as shown in Table 1. Unfortunately, to our knowledge,
no other data exists for KCCH for comparison, experimental
or theoretical. However, we did estimate the rotational constant
of KCCH by scaling that of NaCCH (Brewster et al. 1998) by
the ratio of B values for MgCCH and CaCCH (Anderson &
Ziurys 1995a, 1995b). Our predicted rotational constant was
TABLE 2
Spectroscopic Constants for KCCH and KCCD
Parameter
(MHz)
KCCH
KCCD
B0 . . . . . . . . . .
D0 . . . . . . . . .
108H0 . . . . . .
1013I0 . . . . . .
2970.8168(31)a
0.0017560(13)
1.310(22)
2.73(13)
2764.999(14)
0.0013966(21)
0.497(11)
)
a
Our predicted value was 3092 MHz, derived by scaling from rotational constants of MgCCH, CaCCH, and
NaCCH (see text).
No. 1, 1998
SPECTRUM OF KCCH
3092 MHz, which is very close to the measured value of 2970.8
MHz.
4. DISCUSSION
The potassium monoacetylide rotational spectra were fitted
successfully with a 1S Hamiltonian, confirming that the species
has a linear structure. Such a result is no surprise, because both
NaCCH (Li & Ziurys 1997; Brewster et al. 1998) and LiCCH
(Apponi et al. 1998) are linear. The linearity of the alkali monoacetylides indicates that their bonding has some covalent component involving sp hybridization. The alkali monoacetylides
also must have substantial ionic bonding character corresponding to a M1CCH2 structure because the metals involved are
very electropositive. On the other hand, if the bonding were
completely ionic, the acetylides might be bent, in analogy to
the metal monocyanide/isocyanide species such as NaCN (e.g.,
Van Vaals, Meerts, & Dymanus 1984). In this molecule, the
Na1 ion essentially “orbits” the CN2 group to produce a Tshaped structure and nondirectional polytopic bond (e.g., Dorigo, von Scheleyer, & Hobza 1994). Although the CCH2 moiety differs from CN2 because of the presence of the hydrogen
atom, it has a similar p electron cloud from the triple bond
which could attract the K1 ion. While observation of the lowlying v5 bending mode does indicate some “floppiness” in the
molecule, it is not enough to cause a bent structure.
Because only one isotopic substitution was carried out for
KCCH, r0 bond lengths can be calculated for this molecule,
but not an rs structure. One bond length must also be held to
a fixed value. We have chosen to fix the C—H bond distance
to that obtained from an rs structure of LiCCH (Apponi et al.
1998), which is rC—H 5 1.060 Å. Using this bond distance, we
obtain rC—K 5 2.540 Å and rC—C 5 1.233 Å. The carbon-carbon
bond length in this species from the r0 structure compares quite
favorably with those obtained for LiCCH from an rs analysis
and NaCCH from an r0 analysis, which are 1.227 and 1.217
Å, i.e., only a 0.016 Å maximum difference. Good agreement
between rs and r0 bond lengths for the alkali monoacetylides
L153
is not unexpected, since they are thought to be fairly rigid
molecules (e.g., Bopegedera et al. 1988). The metal-carbon
bond length for potassium monoacetylide also appears to be
reasonable, following an increasing trend from LiCCH and
NaCCH, which have rLi—C 5 1.888 Å and rNa—C 5 2.221 Å,
in comparison to rK—C 5 2.540 Å for KCCH. The larger metalcarbon distances arise from the increase in atom size as one
descends the alkali group in the periodic table. A similar trend
is seen in the alkaline earth monoacetylides (e.g., Nuccio et al.
1995). As also might be predicted, the r0 calcium-carbon bond
length in CaCCH is smaller than that of KCCH (rCa—C 5 2.349
Å; Anderson & Ziurys 1995b) because calcium lies to the right
of potassium in the same row of the periodic table. Consistency
among bond lengths in the metal monoacetylides is additional
evidence that the vibrational ground state in KCCH has been
properly identified.
This study is further evidence that the reaction of metal vapor
and acetylene readily produces the species MCCH. The strength
of signals obtained for molecules such as KCCH is substantial
with relatively short integration times and suggests that longer
metal-acetylide chains like KC4H might be created in detectable
concentrations by the same method. Certainly searching for
longer metal-chain species in the laboratory is worth pursuing.
Finally, these measurements result both in accurate rest frequencies for astronomical searches and precise constants from
which additional transition frequencies can be calculated for
KCCH. Although the cosmic abundance of potassium is not
high (K/H ∼ 1027), it is comparable to that of chlorine and
phosphorus, which have both been observed in interstellar species (e.g., Zmuidzinas et al. 1985; Guélin et al. 1990). The
detection of KCl in the circumstellar shell of IRC 110216
(Cernicharo & Guélin 1987) certainly is evidence that potassium will react to form molecules. Acetylene and its derivatives
may be suitable precursors.
This research is supported by NSF Grant AST-95-03247 and
NASA Grant NAG5-3785.
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