The Astrophysical Journal, 508:L109–L112, 1998 November 20
q 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A.
LABORATORY DETECTION AND PURE ROTATIONAL SPECTRUM OF THE NaCH RADICAL (X̃ 3S2)
J. Xin and L. M. Ziurys
Department of Astronomy, Department of Chemistry, and Steward Observatory, 933 North Cherry Avenue,
University of Arizona, Tucson, AZ 85721-0065
Received 1998 July 27; accepted 1998 August 25; published 1998 October 21
ABSTRACT
The sodium methylidyne radical, NaCH, has been detected in the laboratory for the first time using submillimeter
direct absorption spectroscopy. The species was created in the gas phase in a DC discharge by the reaction of
sodium vapor and CH4. Nine rotational transitions of this molecule were recorded in the frequency range 339–520
GHz, and five transitions of its deuterium isotopomer, NaCD. Each transition of sodium methylidyne was found
to consist of triplets resulting from fine structure interactions, identifying the ground state of the molecule to be
X̃ 3S2. The data were analyzed with a 3S2 Hamiltonian in a case b basis, and rotational, spin-spin, and spinrotation constants have been accurately determined. NaCH may be formed in the envelopes of AGB stars from
the reaction of sodium with photodissociation products of methane.
Subject headings: ISM: molecules — line: identification — molecular data
ZrCH (Barnes et al. 1997), all using laser induced fluorescence
methods. Interestingly, these transition metal methylidynes
have proven to be linear molecules, with a triple bond formed
between the metal and carbon atoms.
In this Letter we present the first laboratory observation of
an alkali metal methylidyne, NaCH, and measurement of its
submillimeter spectrum. This work is the first time any metal
methylidyne has been studied in the laboratory at high spectral
resolution. Rotational transitions were recorded for NaCH and
its deuterium isotopomer in the frequency range 330–520 GHz.
Sodium methylidyne was found to be a linear molecule with
a X̃ 3S2 ground electronic state. Here we present our measurements and spectroscopic analysis of this new free radical.
1. INTRODUCTION
One of the most abundant molecules detected thus far toward
the circumstellar shell of the AGB star IRC 110216 is methane,
CH4 (e.g., Glassgold 1996). The prevalence of CH4 is not surprising, given the fact the molecule is simple and closed shell
and this star is carbon-rich. Other small, stable, carboncontaining compounds such as C2H2 and HCN are similarly
abundant; they also all are thought to undergo photodissociation
at the envelope edge and initiate chemical networks that lead
to other molecules. For example, CN and HNC arise from the
photodissociation of HCN, while C2H and C4H are the daughter
species of C2H2 (Glassgold 1996). However, few models have
speculated on the photochemical chain of CH4. Obvious photodissociation products are CH3, CH2, and CH, although these
species have yet to be detected toward IRC 110216.
One group of molecules that might be formed from the photodissociation of methane are the metal methylidynes of the
formula M-CH. Over the past few years, several metal-bearing
species have been detected in the envelope of IRC 110216,
including the halide species NaCl, KCl, AlCl, and AlF (Cernicharo & Guélin 1987), as well as the cyanide/isocyanide
compounds MgNC (Kawaguchi et al. 1993), NaCN (Turner,
Steimle, & Meerts 1994), and MgCN (Ziurys et al. 1995). A
few of the metal-containing molecules (MgNC, MgCN) are
present in the outer part of the circumstellar envelope as well,
where they may be formed in photochemical processes (e.g.,
Guélin, Lucas, & Cernicharo 1993). One photodestruction
product, the CN radical, appears to be linked in the syntheses
of metal compounds; another such product, CH, may be doing
the same.
Unfortunately, almost nothing is known experimentally
about metal methylidyne molecules, although they clearly are
of fundamental interest for organometallic chemistry since they
involve a metal bonded to the simplest of alkane groups. Only
very recently has any progress been made in the study of such
species. Within the past few years, Merer and collaborators
have begun investigating transition metal methylidynes using
optical spectroscopy (Barnes et al. 1995; Barnes, Merer, &
Metha 1997). This group has recorded spectra of the
3
D–X˜ 3D and 2 P–X˜ 2 S transitions of VCH and TiCH, respectively, and have additional measurements of NbCH, TaCH, and
2. EXPERIMENTAL
The transition frequencies of NaCH and NaCD were recorded using one of the millimeter/submillimeter spectrometers
of the Ziurys group, described in detail elsewhere (Ziurys et
al. 1994). To summarize, the instrument consists of a phaselocked Gunn oscillator/varacter multiplier source, a reaction
chamber, and an InSb detector. The radiation is propagated from
the source, through the cell (a double-pass system), and to the
detector quasi-optically using a series of lenses and a wire grid.
The Gunn oscillator is FM-modulated for phase-sensitive detection, and all signals are recorded at 2f.
The NaCH radical was created in a DC discharge by the
reaction of sodium vapor and methane. The vapor was generated in a Broida-type oven attached to the reaction chamber.
Approximately 20–25 mtorr of argon was mixed with about 8
mtorr of CH4 and the two gases added to the metal vapor
through the bottom of the oven. This complete mixture was
then discharged using a current of 500–700 mA at 200 V, which
caused it to glow bright yellow-orange, likely due to the sodium
atomic D-line emission. NaCD was made by an almost identical
method except CD4 was used instead of CH4 and at a slightly
lower pressure (∼6 mtorr).
The actual measurements were made from an average of two
scans, 5 MHz in total scan width, one in increasing and the
other in decreasing frequency. Gaussians were fitted to the line
profiles to determine their center. Average line widths were
L109
L110
XIN & ZIURYS
Vol. 508
3/2. However, such interactions are likely to be negligible considering the higher N transitions studied here.
The transition frequencies measured for NaCH and NaCD
are listed in Table 1. For NaCH, nine transitions were recorded,
each consisting of three fine structure components; in the case
of NaCD, only five transitions were observed, again each composed of a triplet. As can be deduced from the table, the fine
structure components in general are closely spaced in frequency, being separated by no more than 30–45 MHz. This
splitting increases as N decreases; at very low N, it will become
even larger and one component (J 5 N 2 1) will deviate from
the other two as the coupling scheme reverts to Hund’s
case a.
The data were analyzed in a case b basis dNJSS using a 3S
Hamiltonian, which consists of rotation, spin-spin, and spinrotation terms, including their centrifugal distortion corrections
Fig. 1.—Qualitative diagram showing the rotational energy levels of a 3S
molecule in a case b coupling scheme. The rotational levels are indicated by
quantum number N, and the fine structure levels by J 5 N 2 1 (F3), N (F2),
and N 1 1 (F1). The fine structure splittings give rise to a triplet pattern for
each rotational transition, as shown.
∼900 kHz, and experimental accuracy is estimated to be
5100 kHz.
3. RESULTS AND ANALYSIS
The ground state for NaCH was found to be 3 S, with the
coupling scheme best approximated by a Hund’s case b basis.
A molecule with a 3S ground state has two unpaired electrons
such that the total spin angular momentum is S 5 1 . In a case
b basis, the spin couples with the rotational angular momentum,
indicated by quantum number N, giving rise to fine structure
levels labeled by quantum number J, where J 5 N 1 S. For
S 5 1, J takes on the values N 2 1, N, N 1 1, and hence every
rotational level is split into three sublevels, indicated by F1
(N 1 1), F2 (N), and F3 (N 2 1). The strongest electric dipoleallowed lines are for DN 5 DJ 5 51, and therefore every rotational transition consists of triplets, as shown in energy level
diagram in Figure 1.
Assignment of the spectrum of NaCH as arising from a 3S
ground state was thus based on the appearance of triplet lines
at regularly spaced intervals. This pattern is clearly visible in
the representative data displayed in Figure 2. In this figure, the
N 5 20 r 21 transition of NaCH near 474.5 GHz and the
N 5 23 r 24 line of NaCD near 478.9 GHz are shown. The
triplet sets in each spectrum are indicated by the labeling F1,
F2, and F3. No evidence of hyperfine splittings was found in
the data, which would originate from the sodium spin of I 5
Fig. 2.—Spectra of the N 5 21 R 20 rotational transition of NaCH near
474.5 GHz and the N 5 24 R 23 lines of NaCD near 478.9 GHz in their X˜
3 2
S ground states observed in this work. The triplet pattern within each transition, labeled by F1, F2, and F3, results from fine structure interactions. Each
spectrum covers 80–100 MHz in frequency with a scan duration of about
50 s.
No. 1, 1998
DETECTION AND SPECTRUM OF NaCH RADICAL
(e.g., Hirota 1985):
ˆ 5H
ˆ 1H
ˆ 1H
ˆ .
H
ef f
rot
ss
sr
(1)
The individual spin-spin and spin-rotation Hamiltonians are
Ĥss 5
2
l(r)(3Sz2 2 S 2 ),
3
L111
this result is expected when evaluating data not involving crossspin component transitions (e.g., see Hirota 1985 or Brown,
Davies, & Johnson 1986). On the other hand, all parameters
appear to scale appropriately with mass from NaCH to NaCD,
and the rms of the two data fits are 47 and 25 kHz (see Table
2). Moreover, residuals from both fits are less than 90 kHz.
with one exception (see Table 1).
(2)
4. DISCUSSION
Ĥsr 5 g(r)(N · S),
(3)
where the spin-spin constant l and spin-rotation parameter g
are defined as functions of r because centrifugal distortion corrections may be applied to them. In a case b basis, the spinrotation matrix consists of only diagonal elements, but the spinspin interaction takes on a more complicated form because Sz
is not defined in this coupling scheme.
The resulting spectroscopic parameters derived for NaCH
and NaCD using this formulation are listed in Table 2. As the
table shows, five constants were found necessary to fit the data,
namely, the rotational term B, its centrifugal distortion correction D, g, l, and lD, the centrifugal distortion term for the
spin-spin interaction. (The corresponding correction to g, gD,
was not needed for the fit.) Although the error on l is large,
This study has resulted in the detection of a new molecule,
NaCH, in the laboratory. Its identification is confirmed by
measurements of its deuterium isotopomer, NaCD. This radical
was found to be linear, with a 3S ground state. The carbon
atom for this molecule must be sp hybridized, with the unpaired
electrons residing in the two remaining p (or nonbonding p)
orbitals, which are perpendicular to each other. Such bonding
is primarily covalent.
Without the spectroscopic evidence presented here, it is not
clear whether NaCH would have a triplet ground state or even
be linear. The p2 electron configuration, applicable to linear
NaCH, results in 1S1, 3S2, and 1D terms. In fact, the assignment
of a S2 state is inferred from these implied terms, not from
any spectroscopic information. Linear NaCH could have any
one of these three terms as a ground state, although triplet
states usually lie lower than singlets. Alternatively, the carbon
TABLE 1
Observed Transition Frequencies of NaCH and NaCD (X̃ 3S2)a
NaCH
N RN
0
00
15 R 14 . . . . . .
16 R 15 . . . . . .
17 R 16 . . . . . .
18 R 17 . . . . . .
19 R 18 . . . . . .
20 R 19 . . . . . .
21 R 20 . . . . . .
22 R 21 . . . . . .
23 R 22 . . . . . .
24 R 23 . . . . . .
25 R 24 . . . . . .
26 R 25 . . . . . .
a
00
J RJ
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
15
14
13
16
15
14
17
16
15
18
17
16
19
18
17
20
19
18
21
20
19
22
21
20
23
22
21
24
23
22
25
24
23
26
25
24
0
16
15
14
17
16
15
18
17
16
19
18
17
20
19
18
21
20
19
22
21
20
23
22
21
24
23
22
25
24
23
26
25
24
27
26
25
In units of megahertz.
NaCD
nobs
nobs 2 ncalc
nobs
nobs 2 ncalc
339,264.598
339,286.033
339,310.762
361,828.480
361,848.725
361,871.620
384,381.298
384,400.637
384,421.894
406,922.395
406,940.848
406,960.817
429,451.093
429,468.733
429,487.864
451,966.669
451,983.638
452,001.991
474,468.543
474,485.057
474,502.566
496,956.056
496,972.038
496,988.919
519,428.601
519,444.040
519,460.524
)
)
)
)
)
)
)
)
)
0.019
20.015
0.055
20.024
20.032
0.005
20.024
0.080
20.047
0.001
0.081
20.112
0.027
0.028
0.021
20.013
20.051
0.031
20.034
0.020
20.002
20.029
20.030
20.043
0.067
20.059
0.083
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
439,155.189
439,169.776
439,185.007
459,033.609
459,047.800
459,062.537
478,900.848
478,914.669
478,928.970
498,756.218
498,769.834
498,783.759
518,599.418
518,612.768
518,626.265
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
0.008
0.013
0.012
20.020
20.036
20.025
0.040
20.014
0.003
20.017
0.016
0.047
20.010
0.017
20.036
L112
XIN & ZIURYS
atom in NaCH could undergo sp2 as opposed to sp hybridization, producing a trigonal planar geometry with one lone
electron pair. Such a molecule would be bent. Evidently this
geometry is not energetically preferred.
Spectroscopy of transition metal metylidynes has shown that
these species tend to be linear as well. Their linearity appears
to arise from a carbon-metal triple bond, M { C (Barnes et
al. 1997). This bonding also results from sp hybridization of
the carbon atom, but the triple bond occurs with the transition
metals through their partly filled d shell, which sodium does
not have. On the other hand, some stabilization of the 3S structure in NaCH may be gained through p back bonding of the
two unpaired electrons into the empty 3p orbitals of sodium.
Another possibility for bonding would be a very ionic molecule with an Na1CH2 configuration. CH2, however, is not a
typical anion. Moreover, the lone electron pair on the CH2
group would likely undergo some delocalization with the orbitals containing the two unpaired electrons, producing at least
a pseudo–sp3-hybridized carbon. The resulting molecule in this
case would probably be bent.
Because one atom was substituted in the triatomic molecule
NaCH, namely, the deuterium for the hydrogen, an r0 structure
can be determined. The r0 bond lengths are rC2H 5 1.073 Å
and rNa2C 5 2.207 Å. In comparison, the r0 geometries of TiCH
and VCH yield carbon-hydrogen bond distances of 1.085 and
1.080 Å, with M { C lengths of 1.7227 Å (Ti { C) and
1.7025 Å (V { C). These metal-carbon bond lengths are con-
Vol. 508
TABLE 2
Spectroscopic Constants for NaCH and NaCD (X̃ 3S2)
Parameter
(MHz)
NaCH
NaCD
B ...............
D ..............
g ...............
l ...............
lD . . . . . . . . . . . . . .
rms of fit . . . . . .
11322.3175 (34)
0.0284078 (41)
210.76 (13)
9120 (460)
0.0013 (17)
0.047
10000.8136 (43)
0.0203355 (36)
29.75 (24)
9000 (2900)
0.0032 (11)
0.025
siderably shorter than that of NaCH. The vanadium atom has
a radius of 1.30 Å, that of titantium is larger (1.54 Å), and the
atomic radius of sodium is 1.86 Å (Oxtoby & Nachtrieb 1996).
Therefore, the 0.5 Å differences in metal-carbon bond lengths
reflect in part the fact that vanadium and titantium form triple
bonds to carbon and sodium creates a single bond, rather than
atom sizes alone. In contrast, the Na-C bond length in NaCCH
is 2.221 Å (Grotjahn et al. 1998), very close in value to that
in NaCH. A single bond between the sodium and the sphybridized carbon atom must certainly exist for NaCCH.
Rest frequencies are now available to conduct astronomical
searches for NaCH. Given the identification of NaCN and NaCl
in the circumstellar shell of IRC 110216, sodium may be in
the form of other molecules as well. The CH moiety seems an
equally likely group to bond with sodium as CN.
This research was supported by NSF grant AST-95-03274
and NASA grant NAG5-3785.
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