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THE ASTROPHYSICAL JOURNAL, 530 : 357È361, 2000 February 10
( 2000. The American Astronomical Society. All rights reserved. Printed in U.S.A.
LABORATORY DETECTION OF FOUR NEW CUMULENE CARBENES : H C , H C , H C , AND D C
2 7
2 8
2 9
2 10
A. J. APPONI,1,2 M. C. MCCARTHY,1,2 C. A. GOTTLIEB,2 AND P. THADDEUS1,2
Received 1999 June 14 ; accepted 1999 September 29
ABSTRACT
Four new cumulene carbenes, H C , H C , H C , and D C , were detected in the laboratory by
2 7
2 8
2 9
2 10
Fourier transform microwave spectroscopy. Like the shorter cumulenes in this series, all four were found
to have singlet electronic ground states and linear carbon chain backbones. Rotational and centrifugal
distortion constants were determined to high accuracy for each molecule, and the entire radio spectrum
of each can be calculated to better than 0.3 km s~1 in equivalent radial velocity up to 100 GHz. Upper
limits of the H C , H C , and H C column densities have been estimated for the circumstellar shell of
2 7 star
2 IRC
8
2 9
the evolved carbon
]10216.
Subject headings : ISM : molecules È line : identiÐcation È molecular data È molecular processes È
radio lines : ISM È stars : individual (IRC ]10216)
1.
INTRODUCTION
used in the present study. The strongest lines of the four
cumulene carbenes were observed in a discharge through a
mixture of 0.5% diacetylene (HC H) in neon. The best
4
source conditions were nearly identical
to those used to
produce H C (McCarthy et al. 1997b) : a 1300È1500 V
2 6 applied in the throat of the supersonic
pulsed discharge
nozzle, a gas pulse 150 ks long, and a nozzle pressure of 2.5
atm. The average Ñow rate of gas was 10 cm3 s~1 at the 6
Hz repetition rate of the pulsed nozzle. The confocal
mirrors of the Fabry-Perot cavity and the Ðrst-stage ampliÐer were cooled with liquid nitrogen to reduce system noise
by nearly a factor of 4 relative to that at room temperature.
To our surprise, lines of H C were 50% stronger than
7
those observed for H C 2under
optimized conditions.
2
6
Otherwise, a modest decrease in intensity of about a factor
of 3 was observed from one cumulene to the next in the
series, as illustrated in Figure 1 for the doubly deuterated
species.
Transition frequencies of the longer cumulene carbenes
beyond H C were estimated by scaling the theoretical
6
rotational 2constants
B and C of Maluendes & McLean
(1992) by a factor of D1.01. In the doubly deuterated isotopic species, 2/3 of the population at the low rotational
temperature of our beam is in the K \ 0 ladderÈi.e., the
population is 16/9 times higher than in the highest populated (K \ 1) ladder of the normal isotopic species. In FTM
spectroscopy, intensities are proportional to the square, not
the Ðrst power, of the number density (Steinfeld 1974), so
double deuteration enhances line intensities in the K \ 0
ladder by a factor of about (16/9)2 \ 3.16 relative to those
of the normal isotopic species. It therefore proved expedient
to search Ðrst for the K \ 0 ladder of the doubly deuterated
species.
Cumulene carbenes represent some of the most unusual
polar molecules known to exist in space. All are fairly energetic isomers of either more stable acetylenic chains
(HC H) or ring chains (C
H ) and typically lie at least
2n above ground. The2n`1
2
0.5 eV
existence
of several metastable
cumulene carbene isomers in the interstellar and circumstellar gas provides a striking example of how far the chemistry
in space departs from thermal equilibrium, thus o†ering an
opportunity to probe the chemical and physical conditions
in speciÐc astronomical sources. Detection of these carbon
chains with radio telescopes is aided by their large dipole
moments. Extensive laboratory work was conducted on the
Ðrst four members of the cumulene series H C , H C ,
2 3 et
2 al.
4
H C , and H C over the past several years (Vrtilek
2
5
2
6
1990 ; Killian et al. 1990 ; McCarthy et al. 1997b). This work
led to the identiÐcation of H C , H C (Cernicharo et al.
2 1991),
3 2 and
4 H C (Langer et
1991a, 1991b ; Kawaguchi et al.
2 6
al. 1997) in the carbon-rich circumstellar envelope
of IRC
]10216, the cold dark cloud Taurus molecular cloud 1
(TMC-1), or both.
By means of Fourier transform microwave (FTM) spectroscopy, we have now detected in a laboratory supersonic
molecular beam the next four members of the cumulene
series, H C , H C , H C , and their doubly deuterated iso2 plus
7 2D 8C .2 Each
9 of these molecules has a linear
topomers,
2 10
carbon chain backbone,
a singlet electronic ground state,
and a rotational spectrum of a nearly prolate rotor with C
2v
symmetry, which gives rise to ortho and para nuclear spin
statistics for the K (hereafter shortened to K) levels. The
detection of D C a was made possible by exploiting these
10 favorably populateÈin a rotationally
spin statistics, 2which
cold sourceÈthe K \ 0 level of this isotopic species by a
factor of (16/9)2 with respect to that of the normal isotopic
species. The decrease in line intensity for successively longer
members in this series was too great to allow the detection
of H C .
2 10
2.
3.
EXPERIMENT
The same FTM spectrometer used to detect other carbon
chain and ring chain molecules (Thaddeus et al. 1998) was
1 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street,
Cambridge, MA 02138.
2 Division of Engineering and Applied Sciences, Harvard University, 29
Oxford Street, Cambridge, MA 02138.
357
RESULTS
The measured rotational transitions of H C , H C , and
2 7are 2listed
8 in
H C and their deuterated isotopic species
2
9
Tables 1, 2, and 3. At least Ðve transitions for each were
measured in both the K \ 0 and 1 ladders, with the exception of D C , where only three K \ 1 transitions were mea9
sured. In2 addition,
six K \ 0 transitions of D C were
measured and are listed in Table 4. As shown in 2the10energy
level diagrams (Fig. 2), the K \ ^1 levels, which lie approximately 20 K above ground in the normal isotopic species,
are metastable owing to the ortho-para spin statistics and
hence are populated in our rotationally cold (T D 3 K)
rot
358
APPONI ET AL.
Vol. 530
TABLE 1
MEASURED ROTATIONAL FREQUENCIES OF H C AND D C
2 7
2 7
H C
2 7
TRANSITION
J@ @ @ ] J
Ka ,Kc
Ka,Kc
Frequency
(MHz)
D C
2 7
O[C
(kHz)
Frequency
(MHz)
5 ] 4 ...........
8499.393
1
...
1,5
1,4
5 ] 4 ...........
8506.016
0
7999.115
0,5
0,4
5 ] 4 ...........
8512.487
0
...
1,4
1,3
6 ] 5 ...........
10199.268
0
9585.105
1,6
1,5
6 ] 5 ...........
10207.216
0
9598.933
0,6
0,5
6 ] 5 ...........
10214.980
[2
9612.578
1,5
1,4
7 ] 6 ...........
11899.142
0
11182.620
1,7
1,6
7 ] 6 ...........
11908.415
0
11198.749
0,7
0,6
7 ] 6 ...........
11917.475
1
11214.673
1,6
1,5
8 ] 7 ...........
13599.014
0
12780.133
1,8
1,7
8 ] 7 ...........
13609.612
0
12798.564
0,8
0,7
8 ] 7 ...........
13619.966
0
12816.763
1,7
1,6
9 ] 8 ...........
15298.883
[1
14377.642
1,9
1,8
9 ] 8 ...........
15310.807
0
14398.378
0,9
0,8
9 ] 8 ...........
15322.456
0
14418.853
1,8
1,7
10
]9
........
...
...
15998.188
0,10
0,9
10 ] 9
.........
...
...
16020.941
1,9
1,8
11
] 10
......
...
...
17597.998
0,11
0,10
NOTE.ÈEstimated 1 p measurement uncertainty is 2 kHz.
O[C
(kHz)
...
1
...
0
1
0
0
[1
1
1
[1
0
[1
0
0
[1
0
2
are also in close agreement with those expected from
nuclear spin statistics.
Figure 3b shows the relative abundance of the cumulene
carbene series in our molecular beam as a function of chain
length. The abundance of the cumulenes with an even
number of carbon atoms Ñattens out with increasing chain
length, while those with an odd number of carbons beyond
H C decrease monotonically. A similar e†ect is observed
5 cyanopolyyne series (McCarthy et al. 1998) ; the
in2 the
decrease in abundance of the even carbon C H radicals
n
becomes noticeably shallower with increasing chain
length
TABLE 2
FIG. 1.ÈSample FTM spectra of D C , D C , D C , and D C ,
2 7 from
2 8 the2interaction
9
2 the
10
showing the double-peaked line proÐle resulting
of
Mach 2 supersonic molecular beam with the two traveling waves of the
confocal Fabry-Perot mode. Typical integration times for each spectrum
were approximately 5 minutes.
molecular beam. The spectra were analyzed with WatsonÏs
S-reduced Hamiltonian, which reproduces the observed
rotational spectrum of each chain to high accuracy with
only four spectroscopic parameters : the two rotational constants B and C and the two leading centrifugal distortion
constants D and D (Table 5). The A rotational constant,
J not determinable
JK
however, was
from the present data and
has been estimated by constraining the inertial defect (* \
I [ I [ I ) to zero. For D C , only B and D were
C
A
Bbecause we were2unable
10
eff
effmuch
determined,
to measure
the
weaker transitions in the K \ 1 ladders. The rotational
constants for each species are within 1.1% of those calculated by Maluendes & McLean (1992), and the distortion
constants are in good agreement with those of other molecules of similar size and structure. Crucial conÐrmation of
the identiÐcations is provided by the isotopic shifts of the
doubly deuterated isotopic species ; these agree to better
than 0.1% with those calculated from the theoretical structures. The observed relative intensity ratios of the K ladders
MEASURED ROTATIONAL FREQUENCIES OF H C AND D C
2 8
2 8
H C
2 8
D C
2 8
TRANSITION
Frequency
O[C
Frequency
(MHz)
(kHz)
(MHz)
J@ @ @ ] J
Ka ,Kc
Ka,Kc
7 ] 6 ...........
...
...
7605.331
1,7
1,6
7 ] 6 ...........
...
...
7612.909
0,7
0,6
7 ] 6 ...........
...
...
7620.383
1,6
1,5
8 ] 7 ...........
9182.650
0
8691.809
1,8
1,7
8 ] 7 ...........
9187.620
1
8700.464
0,8
0,7
8 ] 7 ...........
9192.468
0
8709.008
1,7
1,6
9 ] 8 ...........
10330.479
0
9778.278
1,9
1,8
9 ] 8 ...........
10336.067
1
9788.020
0,9
0,8
9 ] 8 ...........
10341.525
0
9797.632
1,8
1,7
10
]9
........
11478.307
0
10864.751
1,10
1,9
10
]9
........
11484.518
0
10875.575
0,10
0,9
10 ] 9
.........
11490.580
0
10886.255
1,9
1,8
11
] 10
......
12626.134
0
11951.223
1,11
1,10
11
] 10
......
12632.965
0
11963.128
0,11
0,10
11
] 10 . . . . . . .
12639.633
[1
11974.878
1,10
1,9
12
] 11
......
13773.960
0
...
1,12
1,11
12
] 11
......
13781.412
0
13050.681
0,12
0,11
12
] 11
......
13788.688
0
...
1,11
1,10
13
] 12
......
...
...
14138.232
0,13
0,12
NOTE.ÈEstimated 1 p measurement uncertainty is 2 kHz.
O[C
(kHz)
[1
1
[1
3
0
[1
[1
[1
0
[1
0
0
0
[1
1
...
0
...
0
No. 1, 2000
LABORATORY DETECTION OF NEW CUMULENE CARBENES
359
TABLE 3
TABLE 4
MEASURED ROTATIONAL FREQUENCIES OF H C AND D C
2 9
2 9
MEASURED ROTATIONAL FREQUENCIES OF D C
2 10
H C
2 9
TRANSITION
J@ @ @ ] J
Ka ,Kc
Ka,Kc
Frequency
(MHz)
D C
2 9
O[C
(kHz)
Frequency
(MHz)
11
] 10
......
8898.322
0
...
1,11
1,10
11
] 10
......
8901.632
0
8474.877
0,11
0,10
11
] 10 . . . . . . .
8904.852
[2
...
1,10
1,9
12
] 11
......
9707.258
1
...
1,12
1,11
12
] 11
......
9710.870
0
9245.317
0,12
0,11
12
] 11
......
9714.384
0
...
1,11
1,10
13
] 12
......
10516.194
0
10008.791
1,13
1,12
13
] 12
......
10520.108
1
10015.758
0,13
0,12
13
] 12
......
10523.912
[2
10022.618
1,12
1,11
14
] 13
......
11325.130
0
10778.697
1,14
1,13
14
] 13
......
11329.343
0
10786.199
0,14
0,13
14
] 13
......
11333.442
0
10793.590
1,13
1,12
15
] 14
......
12134.066
1
11548.599
1,15
1,14
15
] 14
......
12138.580
1
11556.638
0,15
0,14
15
] 14
......
12142.973
1
11564.558
1,14
1,13
16
] 15
......
12942.998
[1
...
1,16
1,15
16
] 15
......
12947.813
[1
12327.077
0,16
0,15
16
] 15
......
12952.500
0
...
1,15
1,14
17
] 16
......
13751.932
[1
...
1,17
1,16
17
] 16
......
13757.048
0
...
0,17
0,16
17
] 16
......
13762.028
1
...
1,16
1,15
NOTE.ÈEstimated 1 p measurement uncertainty is 2 kHz.
D C
2 10
O[C
(kHz)
TRANSITION
J@ @ @ ] J
Ka ,Kc
Ka,Kc
15
] 14
......
0,15
0,14
16
] 15
......
0,16
0,15
17
] 16
......
0,17
0,16
18
] 17
......
0,18
0,17
19
] 18
......
0,19
0,18
20
] 19
......
0,20
0,19
NOTE.ÈEstimated 1 p
tainty is 2 kHz.
...
2
...
...
[1
...
[1
[1
[3
2
0
1
0
0
1
...
[1
...
...
...
...
4.
(Gottlieb et al. 1998b). For the shorter chains with ¹7
carbon atoms, however, the odd cumulenes generally have
stronger lines and are more abundant than the even ones.
This preference may indicate that C addition is the dominant mechanism for the production2 of the longer carbon
chains or, alternately, that the abundances of the odd chains
are distributed over more low-lying isomers than those of
the even chains.
Frequency
(MHz)
O[C
(kHz)
8487.545
9053.379
9619.211
10185.050
10750.880
11316.713
1
[1
[2
3
0
0
measurement uncer-
DISCUSSION
There are a number of isomers of H C , H C , H C ,
2 7 2 8 2 9
and D C in addition to the cumulenic structures here. A
2
10
recent ab initio calculation on the isomers of C H , for
7 2most
example, concluded that the cumulene was one of the
energetic of the eight studied, lying D0.9 eV above the
ground-state ring chain (Aoki & Ikuta 1994). Of these eight
isomers, only the ring chain has previously been detected
and characterized spectroscopically in the gas phase
(McCarthy et al. 1997a). Very recently, a third low-lying
isomer, the nonpolar linear triplet HC H, has been
7
observed in this laboratory by cavity ring-down
laser
absorption spectroscopy (Ball, McCarthy, & Thaddeus
1999). Although without much additional information it is
impossible to obtain isomeric energies from line intensities,
it is plausible that line intensities scale approximately with
isomeric energy (Gottlieb et al. 1998a). The lines of the
cumulene H C , corrected for dipole moment, are roughly
2 7than those of the corresponding ring chain
4 times weaker
in our molecular beam. This result is somewhat surprising
for an isomer lying almost 1 eV higher in energy. By
analogy with the four C H isomers studied by the same
5 2
TABLE 5
ROTATIONAL AND CENTRIFUGAL DISTORTION CONSTANTS (IN MHz)a
Constant
Ac . . . . . . . . . . . . . . . . . . .
B ....................
C....................
D ] 106 . . . . . . . . . .
J
D ] 103 . . . . . . . . .
JK
H C
D C
H C
D C
H C
2 7
2 7
2 8
2 8
2 9
Asymmetric Top Rotational and Centrifugal Distortion Constants b
276259
851.9116(2)
849.2926(2)
10.1(9)
7.72(8)
139740
802.2014(2)
797.6225(1)
8.0(6)
7.78(6)
268668
574.8403(1)
573.6130(1)
4.0(4)
3.70(5)
137513
544.8547(1)
542.7044(1)
4.2(4)
3.57(6)
275711
404.9170(8)
404.3232(8)
1.7(1)
2.02(3)
D C
2 9
D C
2 10
139496
385.7540(1)
384.6902(1)
1.8(3)
2.02(4)
...
...
...
...
...
E†ective Rotational and Centrifugal Distortion Constantsd
B
..............
849.9399(4)
798.7595(3)
573.9161(4)
543.2384(4)
404.4696(1)
384.9541(4)
...
K/1L
D
] 106 . . . . . .
11.9(9)
9.9(2)
3.9(8)
4.4(9)
1.7(2)
1.8(9)
...
K/1L
B
...............
850.6021(4)
799.9119(2)
574.2267(4)
543.7796(2)
404.6201(1)
385.2221(2)
282.9186(1)
K/0
D
] 106 . . . . . . .
10.1(9)
12.2(7)
4.3(8)
4.2(5)
1.8(2)
1.8(3)
0.9(1)
K/0
B
..............
851.2491(4)
801.0490(3)
574.5298(2)
544.3134(2)
404.7664(1)
385.4855(4)
...
K/1U
D
] 106 . . . . . .
9.7(9)
9.2(3)
3.9(8)
3.1(9)
1.4(2)
0.5(9)
...
K/1U
a Uncertainties in parentheses are 1 p in the units of the last signiÐcant digit.
b Derived from a least-squares Ðt of WatsonÏs S-reduced Hamiltonian.
c Derived assuming a planar structure (i.e., 1/C [ 1/A [ 1/B \ 0).
d The entire radio spectrum in the K \ 0 and ^1 rotational ladders can be calculated with the expressions l
\ 2B (J ] 1)
effC)/2 ^
[ 4D (J ] 1)3, where the indices 1L and 1U refer to the lower and upper frequency K \ ^1 componentsJ`1?J
; B \ (B ]
eff
eff
1/4(B [ C)d [ D K2, where d
is a Kronecker delta ; and D B D ] c(B [ C)2/[4(A [ B1 )], where B1 \ (B ] C)/2, c \ 1/8 for
K,1 for JK
K, 1
eff
J
K \ 0, and 1/32
K \ ^1.
360
APPONI ET AL.
FIG. 2.ÈLowest rotational energy levels of the cumulene carbenes
H C , H C , H C , and D C , showing the transitions detected in the
2 7 isotopic
2 8 2species
9
2 10 and the doubly deuterated species (dots).
normal
(arrows)
Owing to the ortho-para nuclear spin statistics, the K \ ^1 levels are
metastable.
method, the relative intensities of the two C H isomers
7 to
2 ground
here suggest that the cumulene may be closer
than estimated by Aoki & Ikuta (1994). Nonetheless, the
laboratory detection of the remaining Ðve isomers of H C
2 7
may still be possible with present techniques.
H C , H C , and H C are all good candidates for
2 7
2 detection.
8
2 9
astronomical
Assuming
a rotational temperature
of 10È25 K, their strongest lines should lie at centimeter
wavelengths and are therefore readily accessible to existing
large radio telescopes. Each of these longer chains has
intense microwave spectra owing to its high polarity ; as
shown in Figure 3a, the dipole moments (in D) are roughly
equal to the number of carbon atoms in the chain. To aid
astronomical searches, the e†ective rotational constants for
each species are listed in Table 5, from which frequencies to
100 GHz can be calculated to an accuracy of D0.3 km s~1
in equivalent radial velocity. The e†ective B and D for each
K ladder are related to the asymmetric top constants, as
shown in Table 5 (see footnote d). Using the calculated
frequencies, we have attempted to Ðnd the three new cumu-
Vol. 530
FIG. 3.È(a) Dipole moment vs. chain length for the cumulene carbenes
(McCarthy et al. 1997b and references therein). (b) Relative abundances of
the cumulene carbenes here (H C ) to one another and the four shorter
2 n beam plotted as a function of chain
ones in our supersonic molecular
length. The intensities of the strongest lines were converted to relative
abundance by accounting for di†erences in dipole moment and the rotational partition function. Filled circles correspond to the relative abundances of the even members of the cumulene series, while open circles
correspond to those of the odd members. As the Ðgure shows, the odd
chains decrease in abundance monotonically, but there appears to be a
signiÐcantly shallower decrease in abundance for the even chains.
lene carbenes in existing surveys of the best molecular
sources, but without success. In the survey of IRC ]10216
by Kawaguchi et al. (1995), for example, 3 p upper limits of
10 mK can be set for the strongest lines of H C , H C , and
2 7 and
2 a8rotaH C . On the assumption of optically thin lines
2
9
tional temperature of 20 K, we Ðnd upper limits to the
column densities of H C , H C , and H C of 9 ] 1011,
2 7cm~2,
2 8respectively.
2 9 These limits
1 ] 1012, and 8 ] 1011
could probably be improved by a factor of 3 by astronomical searches at speciÐc frequencies.
With the recent assignment of C~ and other carbon
7 10% of the di†use
chain anions as the carriers of nearly
interstellar bands (DIBs ; Tulej et al. 1998), there is reason to
believe that other carbon chains with similar structure,
perhaps cumulenes like these here, may also be carriers of
di†use bands. H C is the only cumulene for which a
2 3
matrix-isolation spectrum
is presently available ; although it
exhibits a strong absorption feature near 259 nm (Stanton
et al. 1997), which lies outside the wavelength range of
present DIB assignments, the electronic spectra of the
longer members are expected to shift to longer wavelengths.
If the wavelength of the origin bands increases D50 nm
with each C addition (Fulara et al. 1995), strong electronic
2
No. 1, 2000
LABORATORY DETECTION OF NEW CUMULENE CARBENES
transitions of H C may lie near 400 nm, i.e., barely in the
2 9
visible. We estimate the concentration of H C to be
9
5 ] 109 molecules per gas pulse in our pulsed 2nozzle
discharge on the basis of calibration with 1% carbonyl sulÐde
in Ne, indicating that the electronic spectra of all the cumu-
361
lenes up to nine carbon atoms should be detectable by standard laser spectroscopy techniques.
We are grateful to John Stanton for helpful discussions
on the structures of the cumulene carbenes.
REFERENCES
Aoki, K., & Ikuta, S. 1994, J. Mol. Struct., 310, 229
Maluendes, S. A., & McLean, A. D. 1992, Chem. Phys. Lett., 200, 511
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