THE ASTROPHYSICAL JOURNAL, 460 : L77–L80, 1996 March 20
q 1996. The American Astronomical Society. All rights reserved. Printed in U.S.A.
THE MILLIMETER AND SUBMILLIMETER ROTATIONAL SPECTRUM OF CaCH 3 (X̃ 2 A1 )
M. A. ANDERSON
AND
L. M. ZIURYS
Department of Chemistry, Arizona State University, Box 871604, Tempe, AZ 85287-1604
Received 1995 November 6; accepted 1995 December 27
ABSTRACT
The pure rotational spectrum of the calcium monomethyl radical, CaCH 3 (X̃ 2 A1 ), has been measured in the
laboratory using millimeter/submillimeter direct absorption techniques. CaCH 3 was produced by the reaction of
calcium vapor and an organometallic methyl compound. Seventeen rotational transitions in the frequency range
136 –377 GHz were recorded for this free radical, which is a symmetric top species with one unpaired electron.
In every transition, several K-components were observed, as well as fine-structure splittings. The data were
analyzed using a 2 A1 Hamiltonian, and rotational and spin-rotation parameters were determined. These
high-resolution measurements for CaCH 3 will now enable a search for this molecule in the interstellar medium.
Subject headings: ISM: molecules — line: identification — methods: laboratory — molecular data
1994b). To summarize, this instrument consists of a tunable
source of millimeter/submillimeter radiation, gas cell, and a
detector. The source consists of phase-locked Gunn oscillators
and Schottky diode multipliers. The radiation is propagated
quasi-optically through the cell, which is a double-pass system.
The detector is a helium-cooled InSb bolometer.
CaCH 3 was produced in a DC glow discharge by the
reaction of calcium vapor and an organometallic methyl
compound. The metal vapor was created in a Broida-type
oven, which is attached to the cell, and entrained in approximately 10 mtorr of argon carrier gas. This mixture was reacted
with about 7 mtorr of either dimethyl mercury [Hg(CH 3 ) 2 ] or
tetramethyl tin [Sn(CH 3 ) 4 ]. The discharge current used was
typically 450 mA at 1 kV. Once the discharge started, the
argon pressure could be reduced to 0 mtorr. The discharge
mixture glowed a bright orange-red color when CaCH 3 was
being produced. Dimethyl mercury was found to produce
somewhat stronger signals than Sn(CH 3 ) 4 .
Spectra were recorded over the range 136 –377 GHz. Line
widths over this interval were 250 – 800 kHz. The center
frequency was established by fitting each feature with a
Gaussian line profile. Estimated accuracy of the measurements is H150 kHz.
1. INTRODUCTION
There are two metallic elements that might be expected to
be found in interstellar molecules, but the detection of which
has yet to be accomplished. These elements are iron and
calcium. Cosmically, iron is about a factor of 5 less prevalent
than magnesium, and calcium has an abundance comparable
to sodium and aluminum. While the pure rotational spectra of
several simple calcium compounds of astrophysical interest are
known, including CaNC (Steimle, Saito, & Takano 1993), CaF
(Anderson, Allen, & Ziurys 1994), CaH (Barclay, Anderson,
& Ziurys 1993), and CaCCH (Anderson & Ziurys 1995a), data
for iron-bearing molecules are scarcer. An unexpected set of
metal compounds has been found in interstellar gas, namely,
AlCl, AlF, NaCl, KCl, MgCN, MgNC, and NaCN (see Ziurys
et al. 1994a for a review). Therefore, the lack of identification
of calcium and iron-containing molecules may be due to
random factors. There are many possible molecular carriers of
these elements in the interstellar medium, which at present do
not have known rest frequencies and hence cannot be observed.
In our laboratory, we have begun to measure the pure
rotational spectra of metal monomethyl radicals of the structure MOCH 3 . Our work has been aided by high-resolution
optical spectroscopy of CaCH 3 (Brazier & Bernath 1987,
1989) and MgCH 3 (Rubino, Williamson, & Miller 1995). We
recently have recorded the first pure rotational spectra of
MgCH 3 (Anderson & Ziurys 1995b), SrCH 3 (Anderson, Robinson, & Ziurys 1996), and BaCH 3 (Robinson et al. 1996),
including data from several excited vibrational modes (Anderson, Brazier, & Ziurys 1996), which are complicated because
of the presence of Jahn-Teller interactions.
Here we present the first measurements of the pure rotational spectrum of CaCH 3 in its ground vibrational and
electronic state X̃ 2 A1 . We have recorded several K-components, ranging from K 5 0 to K 5 8, in 17 rotational transitions
in the frequency range 136 –377 GHz. Fine-structure splittings
have also been resolved. In this Letter we present our data and
rotational analysis.
3. RESULTS
CaCH 3 is a symmetric top molecule with one unpaired
electron in a s-type orbital, and hence has a 2 A1 ground
electronic state. This state is best represented in a Hund’s case
b coupling scheme, and therefore the rotational quantum
number used for 2 A1 species is N. In addition, there is
quantization of the rotational angular momentum along the
symmetry axis, labeled by quantum number K. Moreover, the
spin of the unpaired electron couples with the rotation,
resulting in fine structure indicated by quantum number J,
where J 5 N 1 S. The total angular momentum J can, in
addition, couple with any nuclear spin I present to produce
hyperfine interactions characterized by quantum number F,
with F 5 J 1 I.
For CaCH 3 , K-ladder and fine-structure splittings were
resolved in the 17 rotational transitions observed, which are
listed in Table 1. No evidence of hyperfine structure was
observed, however, which could arise from the methyl protons.
2. EXPERIMENTAL
The measurements were made with one of the Arizona
State University millimeter-wave spectrometers (Ziurys et al.
L77
L78
ANDERSON & ZIURYS
TABLE 1
TRANSITION FREQUENCIES
N 3 N9
8 3 9 ...........
K 3 K9
J 3 J9
n obs
(MHz)
n obs 2 n calc
(MHz)
030
7.5 3 8.5
8.5 3 9.5
7.5 3 8.5
8.5 3 9.5
7.5 3 8.5
8.5 3 9.5
8.5 3 9.5
9.5 3 10.5
8.5 3 9.5
9.5 3 10.5
8.5 3 9.5
9.5 3 10.5
8.5 3 9.5
9.5 3 10.5
8.5 3 9.5
9.5 3 10.5
9.5 3 10.5
10.5 3 11.5
9.5 3 10.5
10.5 3 11.5
9.5 3 10.5
10.5 3 11.5
9.5 3 10.5
10.5 3 11.5
9.5 3 10.5
10.5 3 11.5
10.5 3 11.5
11.5 3 12.5
10.5 3 11.5
11.5 3 12.5
10.5 3 11.5
11.5 3 12.5
10.5 3 11.5
11.5 3 12.5
10.5 3 11.5
11.5 3 12.5
11.5 3 12.5
12.5 3 13.5
11.5 3 12.5
12.5 3 13.5
11.5 3 12.5
12.5 3 13.5
11.5 3 12.5
12.5 3 13.5
11.5 3 12.5
12.5 3 13.5
12.5 3 13.5
13.5 3 14.5
12.5 3 13.5
13.5 3 14.5
12.5 3 13.5
13.5 3 14.5
12.5 3 13.5
13.5 3 14.5
12.5 3 13.5
13.5 3 14.5
13.5 3 14.5
14.5 3 15.5
13.5 3 14.5
14.5 3 15.5
13.5 3 14.5
14.5 3 15.5
13.5 3 14.5
14.5 3 15.5
13.5 3 14.5
14.5 3 15.5
14.5 3 15.5
15.5 3 16.5
14.5 3 15.5
15.5 3 16.5
14.5 3 15.5
15.5 3 16.5
136,134.771
136,190.263
136,123.776
136,179.868
136,035.223
136,096.261
151,255.895
151,311.441
151,243.696
151,299.737
151,207.182
151,264.677
151,146.345
151,206.355
151,061.394
151,124.886
166,374.430
166,429.967
166,361.058
166,417.121
166,321.199
166,378.387
166,254.726
166,314.066
166,161.895
166,223.973
181,490.198
181,545.732
181,475.706
181,531.633
181,432.377
181,489.273
181,360.246
181,418.863
181,259.443
181,320.503
196,602.876
196,658.390
196,587.270
196,643.105
196,540.470
196,597.197
196,462.654
196,520.796
196,353.816
196,414.093
211,712.242
211,767.751
211,695.498
211,751.257
211,645.220
211,701.834
211,561.691
211,619.482
211,444.896
211,504.492
226,817.913
226,873.542
226,800.091
226,855.896
226,746.399
226,802.830
226,657.142
226,714.670
226,532.408
226,591.454
241,919.970
241,975.537
241,900.984
241,956.640
241,843.807
241,900.102
20.041
20.042
0.069
20.005
0.211
20.283
20.014
0.039
20.002
0.006
0.092
20.064
0.177
20.150
0.329
20.256
20.025
0.019
20.062
0.067
0.057
20.014
0.115
20.002
0.225
20.214
0.003
0.044
20.019
0.048
0.031
20.033
0.095
20.076
0.145
20.128
,0.001
0.022
0.006
0.039
0.007
0.004
0.080
20.049
0.050
20.088
0.003
0.019
0.014
0.017
20.033
0.035
0.037
20.032
0.027
20.064
20.120
0.016
20.041
0.044
20.068
20.037
20.012
20.012
0.022
20.032
20.030
0.044
0.029
20.004
20.051
20.035
131
333
9 3 10 . . . . . . . . . .
030
131
232
333
434
10 3 11 . . . . . . . . .
030
131
232
333
434
11 3 12 . . . . . . . . .
030
131
232
333
434
12 3 13 . . . . . . . . .
030
131
232
333
434
13 3 14 . . . . . . . . .
030
131
232
333
434
14 3 15 . . . . . . . . .
030
131
232
333
434
15 3 16 . . . . . . . . .
030
131
232
FOR
CaCH 3 : X̃ 2 A1 (n 5 0)
N3N
K 3 K9
J 3 J9
n obs
(MHz)
n obs 2 n calc
(MHz)
333
14.5 3 15.5
15.5 3 16.5
14.5 3 15.5
15.5 3 16.5
15.5 3 16.5
16.5 3 17.5
15.5 3 16.5
16.5 3 17.5
15.5 3 16.5
16.5 3 17.5
15.5 3 16.5
16.5 3 17.5
15.5 3 16.5
16.5 3 17.5
15.5 3 16.5
16.5 3 17.5
15.5 3 16.5
16.5 3 17.5
15.5 3 16.5
16.5 3 17.5
15.5 3 16.5
16.5 3 17.5
16.5 3 17.5
17.5 3 18.5
16.5 3 17.5
17.5 3 18.5
16.5 3 17.5
17.5 3 18.5
16.5 3 17.5
17.5 3 18.5
16.5 3 17.5
17.5 3 18.5
17.5 3 18.5
18.5 3 19.5
17.5 3 18.5
18.5 3 19.5
17.5 3 18.5
18.5 3 19.5
17.5 3 18.5
18.5 3 19.5
17.5 3 18.5
18.5 3 19.5
18.5 3 19.5
19.5 3 20.5
18.5 3 19.5
19.5 3 20.5
18.5 3 19.5
19.5 3 20.5
18.5 3 19.5
19.5 3 20.5
18.5 3 19.5
19.5 3 20.5
19.5 3 20.5
20.5 3 21.5
19.5 3 20.5
20.5 3 21.5
19.5 3 20.5
20.5 3 21.5
19.5 3 20.5
20.5 3 21.5
19.5 3 20.5
20.5 3 21.5
20.5 3 21.5
21.5 3 22.5
20.5 3 21.5
21.5 3 22.5
20.5 3 21.5
21.5 3 22.5
20.5 3 21.5
21.5 3 22.5
20.5 3 21.5
21.5 3 22.5
241,748.825
241,806.098
241,616.144
241,674.694
257,017.864
257,073.408
256,997.672
257,053.380
256,937.152
256,993.359
256,836.427
256,893.474
256,695.922
256,753.945
256,515.308
256,575.011
256,295.624
256,357.220
256,037.004
256,100.960
255,740.330
255,806.615
272,111.423
272,167.003
272,090.073
272,145.758
272,026.150
272,082.215
271,919.729
271,976.603
271,771.090
271,829.024
287,200.368
287,255.895
287,177.891
287,233.567
287,110.527
287,166.624
286,998.445
287,055.270
286,841.800
286,899.571
302,284.453
302,339.986
302,260.915
302,316.566
302,190.129
302,246.160
302,072.457
302,129.004
301,907.834
301,965.367
317,363.515
317,419.105
317,338.719
317,394.394
317,264.647
317,320.595
317,141.173
317,197.760
316,968.762
317,026.112
332,437.195
332,492.692
332,411.257
332,466.914
332,333.884
332,389.757
332,204.787
332,261.277
332,024.681
332,081.671
20.008
0.007
0.059
20.012
20.023
0.028
20.026
0.017
20.023
0.005
20.020
20.007
0.191
20.010
20.023
20.063
20.015
20.001
20.126
0.098
20.040
0.065
20.015
0.072
20.036
0.005
20.016
20.047
20.019
0.008
0.006
0.049
20.030
0.004
20.041
0.009
20.054
0.018
20.045
0.093
20.101
0.060
20.059
20.019
0.002
0.042
20.036
0.030
0.035
0.030
20.103
0.060
20.010
0.087
20.078
20.002
20.020
0.015
20.122
0.029
20.184
0.002
0.013
0.017
20.074
20.004
0.050
0.055
20.071
0.084
,0.001
0.004
434
16 3 17 . . . . . . . . .
030
131
232
333
434
535
636
737
838
17 3 18 . . . . . . . . .
030
131
232
333
434
18 3 19 . . . . . . . . .
030
131
232
333
434
19 3 20 . . . . . . . . .
030
131
232
333
434
20 3 21 . . . . . . . . .
030
131
232
333
434
21 3 22 . . . . . . . . .
030
131
232
333
434
MILLIMETER/SUBMILLIMETER SPECTRUM OF CaCH 3 (X̃ 2 A1 )
No. 1, 1996
L79
TABLE 1—Continued
N3N
22 3 23 . . . . . . . . .
K 3 K9
J 3 J9
n obs
(MHz)
n obs 2 n calc
(MHz)
030
21.5 3 22.5
22.5 3 23.5
21.5 3 22.5
22.5 3 23.5
21.5 3 22.5
22.5 3 23.5
21.5 3 22.5
22.5 3 23.5
21.5 3 22.5
22.5 3 23.5
22.5 3 23.5
23.5 3 24.5
22.5 3 23.5
23.5 3 24.5
22.5 3 23.5
23.5 3 24.5
22.5 3 23.5
23.5 3 24.5
22.5 3 23.5
23.5 3 24.5
23.5 3 24.5
24.5 3 25.5
23.5 3 24.5
24.5 3 25.5
23.5 3 24.5
24.5 3 25.5
23.5 3 24.5
24.5 3 25.5
23.5 3 24.5
24.5 3 25.5
347,505.237
347,560.689
347,478.222
347,533.849
347,397.342
347,453.263
347,262.808
347,319.142
347,074.876
347,131.843
362,567.375
362,622.898
362,539.341
362,594.970
362,455.089
362,511.011
362,315.044
362,371.294
362,119.237
362,176.137
377,623.547
377,678.931
377,594.247
377,649.917
377,506.867
377,562.334
377,361.197
377,417.357
377,157.650
377,214.433
0.009
20.032
20.037
0.013
20.069
0.022
20.052
0.035
20.019
0.119
20.033
20.002
0.014
0.074
20.058
0.069
20.005
0.075
20.101
0.106
0.081
20.028
20.033
0.076
0.080
20.217
0.021
0.080
20.113
0.098
131
232
333
434
23 3 24 . . . . . . . . .
030
131
232
333
434
24 3 25 . . . . . . . . .
030
131
232
333
434
As shown in Table 1, for 16 of the 17 transitions studied,
frequencies for the K 5 0, 1, 2, 3, and 4 components were
measured. Higher K-components could be observed in these
transitions but were not recorded, except for the N 5 16 3 17
line. Here a total of nine components were measured, from
K 5 0 to K 5 8. For the remaining transition, N 5 8 3 9, only
K 5 0, 1, and 3 components were recorded. Each K-component was, in addition, split into doublets as a result of
spin-rotation interactions. The average spin-rotation splitting
in the K 5 0 lines was 55.5 MHz, but it increased with K to as
high as 66.3 MHz in K 5 8 of the N 5 16 3 17 transition. The
frequency spacing between the centroids of the K-components
was found to be 1:3:5:7 for the K 5 0, 1, 2, 3, and 4 components, as expected. Also, the K 5 3n lines were found to be
about a factor of 2 stronger in intensity than those with
K Þ 3n, where n 5 1, 2 · · · , due to ortho/para statistics of the
three protons in the methyl group.
Figure 1 shows a spectrum of the K 5 0 and K 5 1 components of the N 5 24 3 25 rotational transition of CaCH 3 near
378 GHz. The spin-rotation doublets of each component are
clearly resolved in the spectrum and have a larger separation
than the splitting between the centroids of the K-components,
which is about 12 MHz.
The data from Table 1 were analyzed using the following
2
A1 Hamiltonian (Brown 1971; Hougen 1980; Endo, Saito, &
Hirota 1984):
Ĥ 5 Ĥ rot 1 Ĥ cd 1 Ĥ sr 1 Ĥ srcd ,
(1)
where Ĥ rot describes rotation, Ĥ cd the centrifugal distortion
correction to the rotation, Ĥ sr the spin-rotation interactions,
and Ĥ srcd its centrifugal distortion correction. Equations for
several of these terms are given in Anderson & Ziurys (1995b).
From the Ĥ rot and Ĥ cd parts, only the effective rotational
FIG. 1.—Spectrum of the K 5 0 and K 5 1 components of the N 5 24 3 25
rotational transition of CaCH 3 near 378 GHz observed in this work. As the
figure shows, each K-component is additionally split into doublets separated by
about 55 MHz due to spin-rotation interactions, which are labeled by quantum
number J. The spectrum covers 100 MHz in frequency and represents one, 4
minute scan.
constant B v and centrifugal distortion corrections D NK and D N
could be established because no transitions crossing K-ladders
were recorded. Two second-order corrections to the centrifugal distortion, H NK and H KN , were also found necessary for a
reasonable fit. The Ĥ sr term of the Hamiltonian consists of two
parts. One depends only on quantum number N and, in the
notation of Herzberg (1996), is characterized by the constant
m 5 (« bb 1 « cc )/ 2. The other part is described by the Herzberg
constant k, which equals
k 5 « aa 2
~« bb 1 « cc !
2
(2)
and depends on both N and K. The terms « aa , « bb , and « cc
describe the diagonal part of the spin-rotation tensor. For the
K 5 0 components, the spin-rotation splitting is given by
m 5 (« bb 1 « cc )/ 2. This splitting increases with K, and hence
the k term is necessary to fit the spin-rotation interactions for
K Þ 0 lines. In principle, both « aa and (« bb 1 « cc )/ 2 can be
determined, provided high-K and low-N transitions are observed. We attempted to fit our data using both constants but
were unsuccessful. Hence, the data were fitted setting « aa 5 0.
(An upper limit to « aa is about 1 MHz.) The final part of
the total Hamiltonian, Ĥ srcd , consists of four terms, only one
of which was found necessary for the analysis, D NKS .
The spectroscopic parameters determined from the analysis
are listed in Table 2. These constants reproduce the observations to n obs 2 n calc , 150 kHz (i.e., to within the estimated
experimental accuracy) for almost all of the 174 lines measured. Ten lines, however, had n obs 2 n calc , 150 –300 kHz, and
one feature had n obs 2 n calc 5 329 kHz, which was the largest
deviation. There were some systematic effects in the residual
errors, as well. Most of the larger n obs 2 n calc values occurred
for the lower N, high-K transitions. They typically were present
L80
ANDERSON & ZIURYS
TABLE 2
MOLECULAR CONSTANTS
FOR
CaCH 3 : 2 A1 (v 5 0)
Constant
Millimeter-Wave a
(MHz)
Optical b
(MHz)
B ...................
D NK . . . . . . . . . . . . . . . .
DN . . . . . . . . . . . . . . . . . .
10 4 H KN . . . . . . . . . . . .
10 5 H NK . . . . . . . . . . . .
(« bb 1 « cc )/2. . . . . . . .
D NKS . . . . . . . . . . . . . . . .
7566.3082(20)
0.59818(25)
0.0106272(23)
1.051(25)
1.206(27)
55.493(43)
20.0422(61)
7566.00(20)
0.5883(47) c
0.010607(37) c
0.181(76)
1.082(83)
54.62(40)
···
a
Values in parentheses are 3 standard deviations.
From Brazier & Bernath 1989; originally reported in
cm 21 .
c
D K fixed to 2.10 MHz.
b
in the K 5 4 doublets, where one spin component had
n obs 2 n calc . 0 and the other n obs 2 n calc , 0. These effects
likely arise in the spin-rotation part of the analysis and are
probably due to setting « aa 5 0.
Also included in Table 2 are the constants determined for
CaCH 3 from the optical work of Brazier & Bernath (1989).
There is good agreement between the two sets of parameters,
with the exception of H KN , for which there is about 1 order of
magnitude difference between the two values. This discrepancy is likely due to the fact that in the Brazier & Bernath
analysis, D K was set equal to 2.10 MHz, while here it was fixed
at 0.
4. DISCUSSION
As discussed by Cerny et al. (1993), the electronic structure
of a 2 ( metal hydride species, MH, may be similar to that of a
monomethyl compound, MCH 3 , because both the hydrogen
atom and methyl group have one unpaired electron. This
possibility can be examined by comparing the spin-rotation
parameters of both compounds, normalized by the rotational
constants. For example, Cerny et al. (1993) found g/B 5 0.039
for ZnH and (« bb 1 « cc )/ 2/B 5 0.038 for ZnCH 3 , suggesting
very similar electronic bonding.
The identical comparison can be made for CaH(X 2 ( 1 ) and
CaCH 3 (X̃ 2 A1 ). Using the constants of CaH determined by
Barclay et al. (1993), g/B 5 0.0103. In contrast, for CaCH 3 ,
(« bb 1 « cc )/ 2/B 5 0.0073. There is considerable difference between these two values, unlike the zinc analogs, suggesting that
the electronic structure in the two calcium radicals varies.
Obviously, the presence of the 3d electrons influences the
bonding in the zinc compounds, perhaps due to a shielding
effect. CaCH 3 is thought to have primarily ionic bonding, with
a Ca 1 CH 32 -type configuration (e.g., Brazier & Bernath 1989).
The bonding in CaH must have a fair degree of covalent
character, because the Fermi contact term of the proton is
larger than the spin-spin dipolar constant (b F 5 157.4 MHz
versus c 5 4.7 MHz). The relative degree of ionic versus
covalent bonding may account for the differences in spinrotation constants between CaH and CaH 3 .
Although the carriers of interstellar calcium have yet to be
discovered, CaCH 3 may be one possibility. The measurements
presented here will at least enable this free radical to be
searched for in interstellar gas.
This research was supported by NSF grants AST-92-53682,
AST-95-03274, and NASA grant NAGW 2989. The authors
thank Chris Brazier for use of his Hamiltonian code and Peter
Bernath for suggesting the appropriate methyl precursors.
REFERENCES
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