THE ASTROPHYSICAL JOURNAL, 482 : L215–L217, 1997 June 20
q 1997. The American Astronomical Society. All rights reserved. Printed in U.S. A.
LABORATORY DETECTION AND SUBMILLIMETER SPECTRUM OF NaCCH (X̃ 1S)
B. Z. LI
AND
L. M. ZIURYS1
Department of Chemistry, Arizona State University, Tempe, AZ 85287-1604
Received 1997 February 10; accepted 1997 April 3
ABSTRACT
Sodium monoacetylide has been observed in the laboratory for the first time using millimeter/submillimeter
direct absorption techniques. The species was produced by the reaction of sodium vapor and acetylene under DC
discharge conditions. Nineteen rotational transitions of NaCCH in its X 1S ground state and 14 lines of the
deuterium isotopomer NaCCD were recorded in the range 270 –510 GHz. From these data, highly accurate
rotational constants were determined. The detection of NaCN and NaCl in the envelope of the evolved carbon
star IRC 110216 suggests that other sodium-bearing species may be present in this object.
Subject headings: ISM: molecules — line: identification — molecular data
interstellar medium. Thus far, no sodium-bearing species has
been found in the outer shell. If a substantial fraction of
sodium enters the interstellar medium in undepleted form,
presumably via mass loss from evolved stars, what becomes of
this element in the outer envelope of IRC 110216?
Several possible interstellar sodium compounds have been
studied in the laboratory at high spectral resolution. These
include NaF (e.g., Muntianu, Guo, & Bernath 1996), NaO
(Yamada, Fujitaki, & Hirota 1989), NaH (Sastry, Herbst, &
DeLucia 1981), and NaOH (e.g., Pearson & Trueblood 1973).
Searches for these species have thus far proved unsuccessful,
but NaCl and NaCN are clearly present in IRC 110216.
One possible carrier of sodium bearing molecules is
NaCCH, sodium monoacetylide. This species is particularly
interesting because of the large abundance of HCCH, CCH,
and other acetylene chains in the envelope of IRC 110216
(e.g., Ridgway et al. 1976; Glassgold 1996). In fact, the carbon
chain radicals C4H and C3H appear to have a similar distribution to MgNC in the outer envelope of this star (Guélin et al.
1993); hence, metal-bearing species and carbon chains exist in
the same gas. NaCCH could readily be produced from the
reaction of sodium or Na1 with photodissociation fragments
such as CCH or HCCH1.
Up to the present time, no laboratory spectroscopy data has
existed for NaCCH at any wavelengths, although very recently
some theoretical calculations have been done for the species
by Woon (private communication). Both optical and millimeter-wave studies had been carried out for various alkaline
earth monoacetylide radicals, such as MgCCH (Anderson &
Ziurys 1995a), CaCCH (Bopegedera, Brazier, & Bernath
1987; Anderson & Ziurys 1995b), and SrCCH (Bopegedera et
al. 1987; Nuccio, Apponi, & Ziurys 1995), but none for the
alkali metal counterparts. Here we present the first laboratory
detection of NaCCH. The pure rotational spectrum of this
molecule has been recorded in its 1S ground state using
millimeter/submillimeter direct absorption techniques, and
multiple transitions of its deuterium isotopomer, NaCCD,
have been observed as well. Highly accurate rotational constants of this molecule have consequently been determined. In
this Letter we present these results.
1. INTRODUCTION
Mass loss from evolved stars is a major source of the heavy
elements in the interstellar medium. Consequently, understanding the physical and chemical processes that occur in the
circumstellar shells of such stars is very important in evaluating
elemental abundances and their depletions in interstellar gas.
The envelope of one late-type star that has been studied
extensively to date is that of the carbon-rich object IRC
110216. In the circumstellar shell of this star, dozens of
molecules have been detected and studied (e.g., see Glassgold
1996), but only recently have species been observed that
contain the heavier, refractory elements. The molecules NaCl,
AlCl, KCl, and AlF (Cernicharo & Guélin 1987), NaCN
(Turner, Steimle, & Meerts 1994), MgNC (Kawaguchi et al.
1993), and MgCN (Ziurys et al. 1995) have been thus far
discovered in IRC 110216, clear evidence that some fraction
of the refractory elements remain in the gas phase in the
stellar envelope as opposed to being incorporated into dust
grains. Moreover, MgNC and probably MgCN appear to arise
almost entirely from the outer envelope (Guélin, Lucas, &
Cernicharo 1993; Ziurys et al. 1995), suggesting that some
metallic elements may actually enter the interstellar medium
in the gas phase.
Sodium is one of the few metallic elements whose interstellar depletion is not thought to be very severe. Measurements
of optical absorption lines suggest that the abundance of this
element is depleted by factors of 3–10 over solar system values
(e.g., Morton 1975; Cardelli, Savage, & Ebbets 1991), as
opposed to over 99% depletion for refractories like calcium.
Reduced depletion might be expected for sodium because it is
in the alkali group and hence is a softer metal with a low
melting point; therefore, some nonnegligible fraction of the
sodium produced in late-type stars may enter the interstellar
medium in the gas phase. As mentioned, to date two sodiumcontaining species, NaCl and NaCN, have been detected in the
circumstellar shell of IRC 110216, supporting such a hypothesis. However, both molecules have been observed to arise
almost exclusively from the inner envelope of this star, as
Plateau de Bure interferometer maps have demonstrated
(Guélin, Lucas, & Nevi 1996), where temperatures and densities are much higher than what is typically found in the
2. EXPERIMENTAL
The spectra of NaCCH and its deuterium isotopomer were
recorded using one of the spectrometer systems of the Ziurys
Current address: Departments of Astronomy and Chemistry, and Steward
Observatory, University of Arizona, Tucson, AZ 85721.
1
L215
L216
LI & ZIURYS
Vol. 482
TABLE 1
OBSERVED TRANSITION FREQUENCIES
FOR
NaCCH
AND
NaCCD (X 1S)
NaCCH
NaCCD
vobs
vobs 2 vcalc
vobs
vobs 2 vcalc
J 3 J9
(MHz)
(MHz)
(MHz)
(MHz)
31 3 32 . . . . . . . . . . . . .
32 3 33 . . . . . . . . . . . . .
33 3 34 . . . . . . . . . . . . .
34 3 35 . . . . . . . . . . . . .
35 3 36 . . . . . . . . . . . . .
36 3 37 . . . . . . . . . . . . .
37 3 38 . . . . . . . . . . . . .
38 3 39 . . . . . . . . . . . . .
39 3 40 . . . . . . . . . . . . .
40 3 41 . . . . . . . . . . . . .
41 3 42 . . . . . . . . . . . . .
42 3 43 . . . . . . . . . . . . .
43 3 44 . . . . . . . . . . . . .
44 3 45 . . . . . . . . . . . . .
48 3 49 . . . . . . . . . . . . .
49 3 50 . . . . . . . . . . . . .
50 3 51 . . . . . . . . . . . . .
51 3 52 . . . . . . . . . . . . .
52 3 53 . . . . . . . . . . . . .
53 3 54 . . . . . . . . . . . . .
54 3 55 . . . . . . . . . . . . .
55 3 56 . . . . . . . . . . . . .
291514.237
300595.148
309673.377
318748.988
327821.720
336891.592
345958.496
355022.424
364083.208
373140.874
382195.251
...
...
...
445478.981
454504.750
463526.666
472544.590
481558.511
490568.289
499573.916
508575.282
0.022
0.017
20.026
0.034
0.012
0.002
20.026
0.008
20.034
20.006
20.020
...
...
...
0.028
0.000
0.015
0.002
0.021
20.001
20.002
20.026
272560.602
281057.126
289551.657
298044.237
306534.704
315023.036
323509.151
331992.986
340474.476
348953.600
357430.194
365904.271
374375.761
382844.554
...
...
...
...
...
...
...
...
0.027
0.014
20.033
20.009
20.013
20.004
20.000
0.000
20.004
0.030
0.005
20.002
0.006
20.015
...
...
...
...
...
...
...
...
group (described in Ziurys et al. 1994). Very briefly, the
instrument consists of a phase-locked Gunn/multiplier source,
a double-pass gas cell, and an InSb detector. The radiation is
quasi-optically propagated through the system using a series of
Teflon lenses and a wire grid and a rooftop reflector. Data are
recorded using phase-sensitive detection, and the spectrometer is operated under computer control.
The sodium monoacetylide molecule was created in the gas
phase in a DC discharge of acetylene gas and sodium vapor.
The metal vapor was produced using a Broida-type oven
attached to the cell chamber, which was operating at about
1008C. The sodium vapor was introduced into the chamber
from the oven together with about 20 mtorr of argon carrier
gas, and this mixture reacted with acetylene (5– 8 mtorr), also
added through the oven. The DC discharge was operated at
200 V and 60 – 80 mA. The reaction mixture glowed bright
yellow when discharged, presumably from the atomic sodium
D lines. To synthesize NaCCD, all the other experimental
conditions were kept the same except that acetylene gas was
replaced with DCCD and the gas pressure was kept a little
lower (13– 6 mtorr) because the deuterated species is more
expensive. The frequencies of the absorption signals were
determined by fitting the line profiles with Gaussian curves.
The typical line widths were 500 – 800 kHz, varying with
frequency (270 –510 GHz) during the experiment.
3. RESULTS AND DISCUSSION
The rotational frequencies recorded for NaCCH and
NaCCD in their X 1S ground states are presented in Table 1.
Both species are closed-shell molecules, and hence have no net
electron spin or angular momentum. The quantum number
describing their rotation is therefore J. As Table 1 shows, 19
lines of NaCCH and 14 lines of NaCCD were recorded over
the range 270 –510 GHz. Representative spectra of both
isotopomers are shown in Figure 1, which presents the
J 5 41 3 42 transition of NaCCH and the J 5 43 3 44 line
of NaCCD near 382 and 374 GHz, respectively. As expected,
FIG. 1.—Spectra of the J 5 41 3 42 rotational transition of NaCCH near
382 GHz and the J 5 43 3 44 transition of NaCCD near 374 GHz. These
spectra were each recorded in a single 50 s scan, 110 MHz in coverage. A
baseline has been removed from each spectrum.
the transitions of both species appear as single lines. (The data
appear in emission because of the phase used in the modulation detection scheme.)
The analysis of the data presented in Table 1 was carried out
using a 1S Hamiltonian which is expressed as
Heff 5 B0 J~ J 1 1! 2 D0 J 2~ J 1 1! 2 1 H0 J 3~ J 1 1! 3 . (1)
In this equation, B0 is the rotational constant and D0 and H0 the
second- and third-order centrifugal distortion constants. The
H0 term was needed to fit the data successfully. The results of
this analysis are given in Table 2, which lists the spectroscopic
constants for both NaCCH and NaCCD. All three rotational
parameters are well determined for both molecules, and the
rms values for the individual fits are 19 and 16 kHz, respecTABLE 2
SPECTROSCOPIC CONSTANTS
FOR
NaCCH
AND
NaCCD (X 1S) a
Parameters
NaCCH
NaCCD
B0 . . . . . . . . . . . . . . . . . .
D0 . . . . . . . . . . . . . . . . . .
H0 3 108 . . . . . . . . . . .
rms of fit . . . . . . . . . .
4561.8526 (29)
0.0034103 (16)
1.315 (25)
0.019
4263.7255 (76)
0.0024060 (51)
21.24 (11)
0.016
a
In MHz; errors quoted are 3 s in the units of the last quoted
decimal places.
No. 2, 1997
SUBMILLIMETER SPECTRUM OF NaCCH (X̃ 1S)
tively. The constants reproduce all measured frequencies with
residuals of vobs 2 vcalc # 34 kHz, as shown in Table 1.
From the rotational constants, r0 bond lengths for NaCCH
can be estimated. If the C'C bond length is assumed to be
that of acetylene (r0 5 1.204 Å; Handbook of Chemistry and
Physics 1982), and it is also assumed that the COH and COD
bond lengths are the same, then rNaOC 5 2.23 Å and
rCOH 5 0.88 Å. For comparison, the re bond lengths derived
theoretically (Woon, private communication) are rNaOC 5 2.23
Å, rCOH 5 1.07 Å, and rCOC 5 1.23 Å using a RCCSD(T) level
of calculation (restricted coupled cluster with single and
double excitations and perturbation triples). Hence while the
experimental NaOC bond length agrees well with the theory
value, the COH distance is significantly smaller. It also is
different from rCOH found in acetylene, which is 1.06 Å. The
shorter COH distance may arise from a somewhat floppy
molecule. Curiously, the sodium-carbon bond is longer than
that found for the MgOC bond for MgCCH under similar
assumptions, which is 2.04 Å (Anderson & Ziurys 1995). The
shortening of metal-carbon bond distances has been also
found going from left to right in the same row of the periodic
table for the metal monomethyl series NaCH3, MgCH3, and
AlCH3 (e.g., Robinson & Ziurys 1996; Li & Ziurys 1997).
Sodium acetylide is likely to be a very ionic molecule with an
Na1CCH2 structure, as has been postulated for the alkaline
L217
earth monoacetylides (e.g., Anderson & Ziurys 1995). In fact,
NaCCH should be more ionic than MgCCH because it has a
closed-shell electron configuration. A more ionic bond could
result in a shortening of the metal-carbon bond length relative
to a less ionic bond. This effect does not occur, likely because
the ionic character does not vary drastically from NaCCH to
MgCCH. The net cause of the larger bond distance for
NaCCH is probably because sodium has a larger atomic radius
of 1.9 Å, compared with 1.6 Å for magnesium. The dipole
moment of NaCCH is estimated to be 8.2 D (Woon private
communication).
This work will now enable astronomical searches for
NaCCH in IRC 110216 and other objects. In fact, we have
conducted a search for this species toward IRC 110216 via its
J 5 12 3 11 transition at 109,460.9 MHz, using the NRAO
12 m telescope. A peak-to-peak noise level of about 4 mK was
achieved at this frequency, in terms of main-beam brightness
temperature. This measurement implies an upper limit to the
column density of NaCCH of about 9.3 3 1010 cm22, assuming
a rotational temperature of 15 K. Searches for this species
using larger telescopes may be more successful.
This research was supported by NSF grants AST-95-03274
and CHE-95-31244 and NASA grant NAGW 2989.
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