THE ASTROPHYSICAL JOURNAL, 530 : 323È328, 2000 February 10
( 2000. The American Astronomical Society. All rights reserved. Printed in U.S.A.
THE PURE ROTATIONAL SPECTRUM OF GAS-PHASE NaNH (X3 1A )
2
1
J. XIN, M. A. BREWSTER, AND L. M. ZIURYS
Department of Astronomy, Department of Chemistry, and Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721
Received 1999 August 9 ; accepted 1999 August 31
ABSTRACT
The pure rotational spectrum of NaNH (X3 1A ) has been recorded using millimeter/submillimeter
2
1
direct absorption techniques. This study is the Ðrst gas-phase detection of this molecule in the laboratory. NaNH was generated by the reaction of sodium vapor and NH in the presence of a DC dis2
3
charge. The K \ 0, 1, 2, 3, 4, and 5 components in 10 separate rotational transitions were measured,
a
identifying the molecule as a planar, near-prolate asymmetric top with C symmetry. The data were
2v
analyzed using an S-reduced Hamiltonian, and rotational constants A, B, and C were determined, as well
as various centrifugal distortion parameters. An r structure was additionally calculated for the molecule.
0
NaNH may be produced in circumstellar or interstellar gas via the radiative association reaction
2
Na` ] NH ] NaNH` ] hl, followed by dissociative electron recombination. Theoretical calculations
3
3
by Petrie suggest this pathway may be sufficiently fast to synthesize detectable concentrations of
NaNH .
2
Subject headings : circumstellar matter È ISM : molecules È line : identiÐcation È
methods : laboratory È molecular data
1.
INTRODUCTION
Petrie concludes that the reaction of Na` ] NH ]
NaNH ] hl is sufficiently fast to produce NaNH` in 3cir3
3
cumstellar
gas, which then reacts with free electrons
to
create NaNH in the following process :
2
NaNH` ] e~ ] NaNH ] H .
(2)
3
2
Given the abundance of NH in IRC ] 10216, Petrie concludes that NaNH , sodium 3amide, may be a candidate for
2
detection in this object.
While NaNH is known in solution phase as an organic
2 present there have been no gas-phase
reagent, up to the
spectroscopic data available for the species which would
allow an astronomical study. Optical spectroscopy has been
carried out for several alkaline earth amides, including
CaNH and SrNH (Wormsbecher, Penn, & Harris 1983 ;
2 et al. 1990).
2 Recently, the pure rotational specWhitham
trum of CaNH has been recorded as well (Brewster &
2
Ziurys 1999). These
studies concluded that the alkaline
earth amides are planar or quasi-planar with 2A ground
1
electronic states.
In this paper, we present the Ðrst laboratory detection of
gas-phase NaNH and measurements of its pure rotational
2
spectrum in the millimeter-wave
region 300È550 GHz. Ten
rotational transitions were recorded, from which rotational
and centrifugal distortion constants have been determined.
NaNH appears to be planar molecule with C symmetry
2v the rest
and an 21A ground electronic state. Here we present
1
frequencies and spectroscopic analysis for this interesting
new species.
To date, seven metal-bearing molecules have been
detected toward the late-type carbon star IRC ]10216,
where they are distributed in various regions throughout
the circumstellar envelope. For example, the metal halides
NaCl, AlCl, KCl, and AlF are found close to the star in the
inner shell (e.g., Guelin, Lucas, & Neri 1997), while MgNC
and MgCN appear to arise exclusively from the outer
envelope (Guelin, Lucas, & Cernicharo 1993 ; Ziurys et al.
1995). The presence of metal-containing species in the outer
shell, where the gas kinetic temperature is around 25 K,
leads to some interesting questions on how such molecules
form. Neutral-neutral reactions are likely too slow to
synthesize chemical compounds in an expanding stellar
envelope, and many ion-molecule processes involving
ionized metal atoms M` as reactants are endothermic.
Reactions that may be responsible for producing metalcontaining species in circumstellar gas are radiative association processes involving M`. In general, few such
reactions of this type have been studied experimentally,
with the exception of those involving Na`. For example,
Smith et al. (1983) have measured termolecular association
rates of Na` with H , N , O , CO, CH , and H O, and
2 (1989)
2 2 have examined
4
2
Passarella & Castleman
sodium
ion
reacting with NH and CO , in the presence of argon. The
3 that the2rate k of the reaction
latter authors found
Na` ] NH ] NaNH` ] hl
(1)
3
3
exhibited an interesting temperature dependence of
k P T ~2.5, in the presence of a third body. At room temperature, the rate was k D 10~13 cm3 s~1. If these properties apply to the analogous two-body process, then the rate
of reaction (1) at 20 K would be k D 10~10 cm3 s~1 which is
about an order of magnitude slower than the Langevin rate.
Moreover, Petrie (1996) carried out an ab initio calculation
of this rate constant and obtained k D 6 ] 10~10 cm3 s~1,
in excellent agreement with the extrapolated experimental
value. As noted by Petrie, the rate of reaction (1) is 5 orders
of magnitude larger than extrapolated rate constants of
Na` reacting with H , N , O , CO, H O, or CH , at
2 by2 Smith
2 et al. (1983).
2
4
T \ 20 K, as determined
Therefore,
2.
EXPERIMENTAL
The measurements were carried out with one of the
quasi-optical, millimeter-wave direct absorption spectrometers of the Ziurys group, which is described in detail in
Ziurys et al. (1994). The instrument consists of a Gunn
oscillator/varactor multiplier source, a reaction chamber
containing a Broida-type oven, and a helium-cooled InSb
detector.
NaNH was created in a DC discharge by the reaction of
NH with2 sodium vapor, generated by the oven. Approx3 10È15 mtorr of ammonia was mixed with 30 mtorr
imately
323
324
XIN, BREWSTER, & ZIURYS
of argon gas and added to the cell through the bottom of
the oven, entraining the sodium vapor in the NH /Ar
mixture, which was then discharged using a current of3220
mA at 20È40 V. The discharged gases glowed bright yelloworange, partly due to atomic sodium emission.
Because no accurate estimates of the rotational constants
of NaNH were available, a large frequency range (D60
2
GHz) was initially searched to establish the presence of
NaNH . Scans 100 MHz in range were used for this
2
purpose. Several hundred lines were recorded in this initial
search, and harmonic relationships were established
between most of the features, which enabled the rotational
assignments to be made. Once approximate rotational constants were determined, additional transitions were readily
found. Individual line measurements were then carried out
from scans 5 MHz in range, using an average of two scans,
one in increasing and the other in decreasing frequency.
Line widths ranged from 700 to 1000 kHz. Frequency accuracy is estimated to be ^100 kHz.
3.
RESULTS AND ANALYSIS
NaNH was found to be an asymmetric top molecule
2 ground electronic state. Its rotational energy
with an 1A
1
levels are therefore
described by quantum numbers J, K ,
a
and K . The C symmetry of the species results in a dipole
c
2v
moment only along the molecule-Ðxed a-axis, which is also
the symmetry axis. Consequently, selection rules for the
strong electric dipoleÈallowed transitions are a-type,
namely *K \ 0 and *K \ ^1, for *J \ 0, ^ 1, where J
a
is the quantum
numberc indicating rotational angular
momentum. Its projections are K and K , and K , K ¹ J.
a
c
a c
The value of K cannot change ; consequently, each rotaa
tional transition consists of ““ K -components.ÏÏ Because
a
K ] K \ J or J ] 1, the K -components follow a somea
c
a
what regular pattern for each rotational transition. There is
only one transition J ] J ] 1 possible for K \ 0, but two
transitions for each K D 0 component. The a““ doublets ÏÏ of
a
K \ 1 lines are usually split the widest in frequency, and
a
the separation decreases steadily with K so that the coma
ponents eventually collapse into single lines. Moreover,
there is an intensity alternation of the K components
a
because of fermion statistics for C symmetry
species
2v 1
arising from the two proton spins (I \ ) of NaNH . For
2
2
the ground vibrational state, odd-numbered K coma
ponents are statistically a factor of 3 stronger than evennumbered K lines.
a
A typical pattern is shown in Figure 1, which is a stick
diagram indicating the positions and relative intensities of
the K -components observed for the J \ 15 ] 16 transition. a Here the K \ 1 lines [J(K , K ) \ 15(1, 15) ]
c
16(1, 16) and 15(1, 14)a ] 16(1, 15)] are aseparated
by over 5
GHz, and the rest of the K -components fall within these
a
two features. The single component
for K \ 0 lies roughly
in between the two K \ 1 lines, and theatwo components
a of this feature and are separated by
for K \ 2 are to the left
a
about D150 MHz. The K \ 3, 4, and 5 components
a
appear at lower frequencies relative
to K \ 0 and are cola The alternating
lapsed (or nearly collapsed) into single lines.
intensity pattern results in the K \ 1 and 3 features being
a weaker lines were also
the strongest in the transition. Some
FIG. 1.ÈStick Ðgure showing the progression of lines observed for the J \ 15 ] 16 rotational transition of NaNH (X3 1A ) near 354 GHz in terms of K
2 by over
1 5 GHz, are situated at eithera
components. The K \ 0 line is a single feature appearing near the middle of the pattern, while the K \ 1 doublets, split
a
a
end. The K \ 2 components, separated by only a few hundred MHz, lie o†set from the pattern center, as well as the other K doublets, which are virtually
a single features. The intensity pattern alternates with K value because of proton spin statistics.
a
collapsed into
a
TABLE 1
OBSERVED TRANSITION FREQUENCIES OF NaNH (X3 1A )a
2
1
J@@
14
14
14
14
14
14
14
14
14
14
14
15
15
15
15
15
15
15
15
15
15
15
16
16
16
16
16
16
16
16
16
16
16
17
17
17
17
17
17
17
17
17
17
17
18
18
18
18
18
18
18
18
18
18
18
19
19
19
19
19
19
19
19
19
19
K@@
a
1
5
5
4
4
3
3
2
2
0
1
1
5
5
4
4
3
3
2
2
0
1
1
5
5
4
4
3
3
2
0
2
1
1
5
5
4
4
3
3
2
0
2
1
1
5
5
4
4
3
3
2
0
2
1
1
5
5
4
4
3
3
2
0
2
K@@
c
14
10
9
11
10
12
11
13
12
14
13
15
11
10
12
11
13
12
14
13
15
14
16
11
12
13
12
14
13
15
16
14
15
17
13
12
14
13
15
14
16
17
15
16
18
13
14
15
14
16
15
17
18
16
17
19
14
15
16
15
17
16
18
19
17
]
J@
15
15
15
15
15
15
15
15
15
15
15
16
16
16
16
16
16
16
16
16
16
16
17
17
17
17
17
17
17
17
17
17
17
18
18
18
18
18
18
18
18
18
18
18
19
19
19
19
19
19
19
19
19
19
19
20
20
20
20
20
20
20
20
20
20
K@
a
1
5
5
4
4
3
3
2
2
0
1
1
5
5
4
4
3
3
2
2
0
1
1
5
5
4
4
3
3
2
0
2
1
1
5
5
4
4
3
3
2
0
2
1
1
5
5
4
4
3
3
2
0
2
1
1
5
5
4
4
3
3
2
0
2
K@
c
15
11
10
12
11
13
12
14
13
15
14
16
12
11
13
12
14
13
15
14
16
15
17
12
13
14
13
15
14
16
17
15
16
18
14
13
15
14
16
15
17
18
16
17
19
14
15
16
15
17
16
18
19
17
18
20
15
16
17
16
18
17
19
20
18
l
obs
329550.183
330306.051
330306.051
331011.281
331011.281
331532.873
331533.410
331857.984
331977.641
332034.730
334565.702
351469.877
352282.523
352282.523
353034.063
353034.063
353590.701
353591.307
353932.044
354077.258
354104.525
356816.325
373379.881
374250.454
374250.454
375048.161
375048.161
375639.885
375640.612
375996.373
376161.622
376170.529
379056.546
395279.532
396209.343
396209.343
397053.033
397053.033
397679.655
397680.936
398050.525
398205.199
398257.088
401285.647
417168.284
418158.646
418158.646
419048.184
419048.184
419710.001
419711.587
420093.802
420234.547
420336.627
423503.041
439045.501
440097.823
440097.823
441032.928
441032.928
441729.887
441731.900
442125.586
442248.839
442408.626
l [l
obs
calc
0.002
[0.012
[0.012
0.000
[0.001
[0.069
[0.013
[0.015
[0.126
[0.013
[0.004
[0.009
0.003
0.003
0.033
0.032
0.028
[0.031
0.035
[0.063
[0.004
0.012
0.009
[0.006
[0.006
0.038
0.036
0.106
[0.068
0.001
0.016
[0.061
0.039
[0.005
[0.001
[0.001
0.015
0.013
[0.060
0.021
0.038
0.006
[0.078
0.012
0.005
0.010
0.010
0.015
0.012
0.066
0.079
0.051
0.035
[0.014
[0.005
0.001
0.025
0.025
[0.104
[0.109
[0.004
[0.026
0.023
0.051
0.022
326
XIN, BREWSTER, & ZIURYS
Vol. 530
TABLE 1ÈContinued
J@@
19
20
20
20
20
20
20
20
20
20
20
20
21
21
21
21
21
21
21
21
21
21
21
22
22
22
22
22
22
22
22
22
22
22
23
23
23
23
23
23
23
23
23
23
23
K@@
a
1
1
5
5
4
4
3
3
2
0
2
1
1
5
5
4
4
3
3
2
0
2
1
1
5
5
4
4
3
3
2
0
2
1
1
5
5
4
4
3
3
2
0
2
1
K@@
c
18
20
16
15
17
16
18
17
19
20
18
19
21
17
16
18
17
19
18
20
21
19
20
22
18
17
19
18
20
19
21
22
20
21
23
19
18
20
19
21
20
22
23
21
22
]
J@
20
21
21
21
21
21
21
21
21
21
21
21
22
22
22
22
22
22
22
22
22
22
22
23
23
23
23
23
23
23
23
23
23
23
24
24
24
24
24
24
24
24
24
24
24
K@
a
1
1
5
5
4
4
3
3
2
0
2
1
1
5
5
4
4
3
3
2
0
2
1
1
5
5
4
4
3
3
2
0
2
1
1
5
5
4
4
3
3
2
0
2
1
K@
c
19
21
17
16
18
17
19
18
20
21
19
20
22
18
17
19
18
20
19
21
22
20
21
23
19
18
20
19
21
20
22
23
21
22
24
20
19
21
20
22
21
23
24
22
23
l
obs
445708.075
460910.603
462026.259
462026.259
463007.087
463007.087
463739.062
463741.609
464145.335
464247.240
464472.654
467900.080
482762.950
483943.563
483943.563
484969.737
484969.737
485736.831
485740.067
486152.477
486229.169
486528.375
490078.413
504602.021
505849.108
505849.108
506920.429
506920.429
507722.692
507726.762
508146.300
508193.723
508575.321
512242.418
526427.136
527742.381
527742.381
528858.679
528858.679
529696.066
529701.143
530126.206
530140.147
530613.023
534391.392
l [l
obs
calc
[0.011
0.003
[0.032
[0.032
0.026
0.020
0.026
[0.024
0.015
[0.012
0.015
[0.022
[0.032
[0.013
[0.013
0.028
0.018
0.011
[0.029
0.054
0.031
0.049
[0.027
[0.027
[0.004
[0.004
[0.002
[0.015
[0.001
[0.020
0.032
0.039
0.081
[0.029
[0.069
0.024
0.024
0.001
[0.016
[0.035
[0.015
[0.051
0.012
0.071
[0.076
a In MHz.
observed in this frequency region but are not shown in the
Ðgure ; they arise primarily from the v excited vibrational
4
state, the out-of-plane puckering motion.
Each of the 10 rotational transitions of NaNH recorded
in this study follow the approximate pattern of2 Figure 1,
and frequencies for the K \ 0, 1, 2, 3, 4, and 5 components
a 90 separate lines. The transition
were measuredÈa total of
frequencies for these spectra are listed in Table 1. As is
evident from this table, all K \ 1 components are widely
a
split apart in frequency ([5 GHz)
with the splitting increasing with J, while the separation for the K \ 2 lines ranges
a
from 120 to 500 MHz. The K \ 3 components
are split by
a
only a few MHz, and K \ 4 and 5 lines are totally cola
lapsed.
Figure 2 presents spectra of the K -components of the
a 442 GHz. A freJ \ 19 ] 20 transition of NaNH near
2
quency range of about 750 MHz is covered in this Ðgure.
The K \ 0 transition [19(0, 19) ] 20(0, 20)] and the two
a
transitions
composing the K \ 3 components [19(3, 16)
a
] 20(3, 17) and 19(3, 17) ] 20(3,
18)] are single features,
but the K \ 2 components are doublets. The K \ 3 line is
a intensity than the others, as well.a The three
stronger in
additional features, marked by asterisks, arise from excited
vibrational levels. No quadrupole hyperÐne structure was
observed in the spectra, which would arise from the sodium
(I \ 3/2) and nitrogen (I \ 1) nuclear spins. Such structure
is not expected to be resolved at the high J levels recorded.
The data were analyzed using the nonlinear, least-squares
program SPFIT of H. M. Pickett, who employs the Sreduced Hamiltonian of Watson (1977) in the Ir representation for asymmetric tops. Only some of the centrifugal
distortion parameters of this Hamiltonian were used,
No. 1, 2000
PURE ROTATIONAL SPECTRUM OF NaNH
2
327
FIG. 2.ÈSpectrum of the J \ 19 ] 20 rotational transition of NaNH observed in this work, showing the K \ 0, 2, and 3 components only. The K \ 0
a
\ 19
È20
È20 a ). The K \ 3 component is a single feature
feature is a single line, while the K \ 2 transition consists of doublets (J2
and 19
2,17 by
2,18an asterisk.
2,18 This2,19
a
Ka ,Kc are marked
but results from two transitions. Lines
arising from excited vibrational states
spectrum isa a composite of eight separate 100
MHz scans, each about 1 minute in duration.
resulting in the form
HΠ\ AJ 2 ] BJ 2 ] CJ 2 [ D J4 [ D J2J 2
rot
x
y
z
J
JK
z
] d J2(J 2 ] J 2 ) ] d (J 4 ] J 4 )
1
`
~
2 `
~
] H J4J 2 ] H J2J 4 ] L
J4J 4
JK
z
KJ
z
JJKK
z
]L
J2J 6 ] L
J6J 2
JKKK
z
JJJK
z
TABLE 2
SPECTROSCOPIC CONSTANTS FOR
NaNH (X3 1A )a
2
1
Constant
Value
A ..............
B ..............
C ..............
D .............
J
D ............
JK
d ..............
1
d ..............
2
H ............
JK
H ............
KJ
106L
......
JJKK
L
.........
JKKK
108L
......
JJJK
rms of Ðt . . . . . .
387730 (91)
11250.7326 (74)
10914.8259 (71)
0.0239531 (33)
2.39129 (89)
[0.0008363 (38)
[0.0002011 (15)
0.0000426 (13)
0.000515 (62)
0.625 (28)
[0.0002035 (15)
[0.212 (83)
0.062
a In MHz ; errors are 3 p and apply
to the last quoted places.
(3)
where J and J are the operators for the total rotational
z
angular momentum
and its z-component and J are the
raising and lowering operators, where J \ J ^BJ . A, B,
B thex rest y of the
and C are the rotational constants, and
parameters are centrifugal distortion corrections. The equation gives all of the distortion constants that were needed to
successfully analyze the data set ; use of these parameters is
expected, since levels with both high J and K , K values
a c
were part of the data set.
The resulting spectroscopic parameters are listed in Table
2, as well as the rms of the data Ðt, which is 62 kHz. All
three rotational constants were determined in the analysis,
although A has a much larger error associated with it. Distortion constants D , D , H , H , and Ðve other terms
J JK
KJ deÐned, as indicated
were needed for the data
Ðt ; allJKare well
by their uncertainties.
4.
DISCUSSION
The measurements will now allow for a deÐnitive search
for NaNH . Although lower frequency transitions were not
2 data set obtained is certainly extensive enough
recorded, the
to establish precise spectroscopic constants. From these
parameters, other transitions can be predicted with an accuracy of better than 1 MHz. Of interest to astronomers
would be the K \ 0 components of the J \ 4 ] 3, 5 ] 4,
a 88,654 MHz, 110,811 MHz, and 132,965
and 6 ] 5 lines at
MHz. Detection of the K \ 0 transitions should be suffia
cient to ensure a correct identiÐcation
in circumstellar gas.
(The intrinsic line strengths S for each rotational transition
ij
328
XIN, BREWSTER, & ZIURYS
are very close to 1 : 3 for K \ 0, 2, 4, . . . components versus
a
K \ 1, 3, 5, . . . because of nuclear spin statistics. However,
a
at a rotational temperature of 10 K, the K \ 0 lines are
a
stronger in intensity than the other K components by a
a
factor of at least 2. Exact line strengths can be found in
Appendix V of Townes & Schawlow 1975.) Because of the
relatively high abundance of NH in interstellar clouds,
3
NaNH may also be a good candidate for searches in these
2
objects as well.
Due to the low ionization potential of sodium, the
bonding in NaNH is likely to be predominantly ionic, i.e.,
2
Na`NH~. Such ionic character should result in a rigid
2
planar molecule. Covalent bonding would produce a pyramidal structure like NH . The spectra obtained for
3
NaNH indicate that this molecule does appear to be quite
2
planar. There was no evidence of two inversion states, for
example, and other observed lines were considerably
weaker than the ground-state lines and had signiÐcantly
di†erent rotational constants. Therefore, the additional
lines arise from vibrationally excited states.
Further evidence for a planar structure can be found by
calculating the inertial defect * \ I [ I [ I . For planar
c number
a
b ; if not, then
geometry, * should be a small, positive
nonplanarity or quasi-planar behavior is indicated. Using
the values of A, B, and C from the data Ðt, * was found to
be equal to 0.079 amu AŽ 2Èa little larger but certainly comparable to that found for H CO, which is 0.05767 amu AŽ 2
2 H CO is certainly a planar
(e.g., Clouthier & Ramsay 1983).
molecule, and, by analogy, NaNH 2is also.
2
An r structure can be derived from the three rotational
0
constants determined for NaNH if one geometric param2
eter is assumed (see Gordy & Cook 1984). In this case the
NwH bond length was held constant. Using the value
determined for NH~ of r
\ 1.041 AŽ (Tack et al. 1986),
2
N~H
the sodium-nitrogen bond length was found to be r
\
Na~N
2.051 AŽ , and the HwNwH angle to be h \ 101¡.2. If r
N~H
for NH is used instead (1.025 AŽ : Davies et al. 1977),
2
then r
\ 2.055 AŽ and h
\ 103¡.4. Hence, the
Na~N
H~N~H
HwNwH bond angle in general is much smaller than it
would be for a trigonal planar species (120¡) and is also less
in value than that of H CO, another species with C symmetry (116¡.8 : Clouthier2 & Ramsay 1983). However, 2v
it quite
close to the angle found in NH~(102¡.1) and in NH (103¡.1)
2
2
and is just slightly larger than the range found for CaNH ,
2
(h \ 100¡.5È102¡.9), using the same assumptions (e.g.,
Bernath 1997). Finally, the sodium-nitrogen bond length for
NaNH of r
\ 2.05È2.06 AŽ is shorter than the calcium2
Na~N
nitrogen bond distance in CaNH , which is r
\
2
2.12È2.14 AŽ . This e†ect can be interpreted
in terms ofCa~N
di†erences in covalent radii for sodium and calcium (1.86 AŽ vs.
1.97 AŽ , respectively) and argues for some covalent character
to the bonding in NaNH .
2
This research was supported by NASA grant NAG
5-3785 and NSF grant AST 98-20576.
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