- Email: [email protected]
The microwave spectrum and centrifugal distortion constants of vinyl cyanide
JOZTRNAL UF MOLECULAR
SPECTKOSCOPT
48, 1-16 (1973)
The Microwave Spectrum and Centrifugal Constants of Vinyl Cyanide
Distortion
M. C. L. GERRY Deparfrirent of Cken&ry,
The C’niversity of British Columbia,
T’amouver 8, B. C.. Canada
GISBERT WINNEWISSER ,~~a~-Planck-Institut Ihision
of Physics,
ftir Radioastronorrzie, Bonn, Germany and National Research Council oj’Canada, Ottawa, Canada
The rotational spectrum of the ground state of vinyl cyanide has been assigned and measured in the frequency regions 8-37 GHz and 90-200 GHz. A total of 140 transitions has been measured and assigned to quantum numbers as high as J = 35 and K = 12. These new measurements have been used to calculate accurate values for all the rotational and quartic centrifugal distortion constants and some sextic distortion constants. The rotational constants are in MHz:
k = 49850.700
zt 0.013;
B = 4971.2141
f
0.0008;
I? = 4513.8289
f
0.0008.
INTRODUCTION ‘There have been more than twenty molecules identified in interstellar space by their microwave spectra (1). Amongst them are complex organic molecules which have been detected in “dark” and “black” clouds notably in the direction of the Orion nebula and the galactic center clouds Sagittarius A and Sagittarius B2. Although the present list of detected interstellar molecules does not give a full account of the chemical conditions in space, it already seems to suggest that many of the interstellar molecules are related in that they are molecular fragments to which successive numbers of hydrogen atoms or radicals have been attached (e.g., CN-HCN-CH&H-CH3NH2 and COH2CO-CHIOH). One of the molecules observed has been cyanoacetylene, HCC-CN (Z), a derivative of acetylene which may well have been formed from the ethynyl radical (-CCH) and the cyanogen radical (-CN) by surface reactions on interstellar dust grains. A more detailed discussion of this formation scheme has been given (3) and will be published elsewhere (3). Similarly vinyl cyanide HzC=CH-CN could be synthesized and may be observable in interstellar space. Although none of the presently known interstellar molecules contains a carboncarbon double bond, there seems to us to be no obvious reason why this class of molecules should be missing in the list of interstellar molecules. Especially since vinyl cyanide shows very strong transitions in the microwave region, a thorough interstellar search seems to be warranted. In order to aid the interstellar search for this molecule in some
Copyright f$3 197.3 by Academic Press, Inc. .UI rights of reproduction in any form rwerved.
2
GERRY AND WINNEWISSER
H, CCH CN J=14 -
13 K.13
& 1
o
2
/
1, I
13;800
13;ooo
II ,I
1
,,
I
13hOO
MHz
i ._m._,.l.
FIG. 1. Millimeter-wave spectrum of the complete J = 14 + 13 u-type rotational transition of vinyl cyanide near 133 GHz. The top line represents the recorder output for some selected R-components. The lower third of the figure shows for each K-component the calculated rigid rotor positions (x) and the measured and calculated line positions (e) taking centrifuga ldistortion into account. The length and the direction of the arrows indicate the effect of centrifugal distortion.
of the molecule-rich dark and black clouds, we have measured part of its microwave and millimeter wave spectrum in the frequency regions 8-37 GHz and 90-200 GHz. More than one hundred ground-state transitions have been selected, assigned and measured. The selection of these transitions has been made such that their analysis would yield a fairly complete set of molecular constants which can be used then to predict, to a high degree of confidence, any molecular transition with quantum numbers J 5 35 and K < 15. These predictions include, of course, some of the potentially interesting astrophysical transitions. There have been two previous studies of the microwave spectrum of vinyl cyanide. In the first study (S), the five K-components of the a-type J = 3 + 2 transition were measured to give approximate values of the rotational constants, the nitrogen nuclear quadrupole coupling constants and the dipole moment. Subsequently, Costain and Stoicheff (6) reported further transitions, for several isotopic species. The J = 2 + 1 and 3 + 2, a-type transitions with their K-components were measured and, more importantly, two low J b-type transitions could be assigned ; they gave improved rotational constants together with some quartic centrifugal distortion constants and the bond lengths and angles. However, there has been no previous assignment of transitions to high values of J, nor have there been any measurements on millimeter-wave transitions. Accordingly, we have measured several a-type transitions with most of their K-components up to values of J as high as 18 and about fifty b-type transitions originating from several different rotational subbands with values of K 6 4. From these new experi-
MICROWAVE
SPECTRUM OF VINYL
CYANIDE
3
TABLE 1. Observed and calculated a-type rotational transitions (MHz) of vinyl cyanide
Transition
Observed Frequency
Calculated Frequency
Centrifugal Distortion Tctal High Order
Deviation
9485.03
-
0.01
0.00
0.07
18966.54
-
0.07
0.00
0.00
16513.08
0.38
0.00
0.01
19427.63
0.16
‘).cc
-0.04
0.23
0.00
0.00
27767.36
0.47
0.00
-0.04
29139.10
0.0x
0.00
-0.13
26456.94
1.81
0.00
0.40
28470.64
1.80
O.O@
0.43
0.00
0.07
9.15
0.00
0.04
-
28440.99
-C.24
41,, - 3!,1
37019.00
37018.93
10i,9
96982.49
96082.45
-
lOL,3 - 9:,,, 9476C.83
94760.80
-
1.76
0.00
0.03
lO;,c - 9:,;
95325.49
-
2.93
0.00
-0.00
94928.63
6.38
0.00
0.08
94941.65
6.34
0.00
-0.07
-
91,:
95325.49
103,; - 9!,.? 94928.71 103,; - 9x,,
94941.58
Ref.
--__
mental data accurate values of the ground-state centrifugal distortion constants have been obtained, constants.
rotational constants and quartic along with values for some sextic
EXPERIMENTAL
The spectrum was observed using three different spectrometers. In the frequency region 8-37 GHz the 100 kHz Stark modulated spectrometer at the University of British Columbia was used ; this has been described previously (7). Accuracy of the measurements using this spectrometer is estimated to be better than 0.1 MHz. Above 90 GHz millimeter wave spectrometers at the National Research Council of Canada (8) and the University of Kiel (9) were used. The millimeter wave frequencies were generated by harmonic multiplication of the frequencies of various reflex klystrons covering the fundamental frequency range 2645 GHz. The accuracy of frequency measurement obtained in the millimeter-wave region by the two spectrometers at LNRC and at Kiel is different. Since the instrument at NRC is conventional, measurement accuracy is estimated to be better than 0.15 MHz. The on-line use of a small
GERRY
AND WINNEWISSER
TABLE 1.
Transition
Observed Frequency
Calculated Frequency
(Continued)
Centrifugal Distortion Total High Order
Deviation
‘O*,, - %,6
94913.16C
94913.14
18.32
0.00
0.02
10, 6 - 9, ,:
94913.16'
94913.26
18.32
0.00
-0.10
lo5
94914.05
94914.00
33.61
0.00
0.05
94925.05
52.26
0.00
0.03
94942.54
74.24
0.00
0.13
103575.41
-10.17
0.00
0.17
- 9,d
10,
- 9,
94925.08
10,
- 9,
94942.67
11o,ll- IO,,,, 103575.5gc 11 1,11- lOI,,, 103637.23
101637.24
- 6.52
0.00
-0.01
11 1,10- lC,,, 106641.39
106641.40
-12.39
0.00
-0.0'
";,I,- lo,,, 104212.58
104212.66
- 3.87
0.00
-0.08
'1, a - lo>,, 104960.66
104960.56
- 5.65
0.00
0.10
'lx,, - lO,,a 104432.77
104432.80
4.94
0.00
-0.03
lo,,7 104453.85
104453.94
4.86
0.00
-0.09
18.08
0.00
0.22
“,,a
-
I',,, - lo,,7 104411.49c
104411.27
ll,,, - 104,s
lo4411.49c
104411.49
18.08
0.00
0.00
11:
- 10,d
lC4408.93
104408.91
34.91
0.00
0.02
'1,
- 10,
104419.33
104419.32
55.42
0.00
0.01
"7
- 10,
104437.49
104437.53
79.60
0.00
-0.04
'1,
- 10,
104461.50
104461.52
lC7.43
0.00
-0.02
1'3
- 109
104490.36
104490.37
138.87
0.00
[email protected]
"I,
- 1010
104523.87'
104523.52
173.86
0.00
0.35
l?ef.
computer for signal averaging with the video type spectrometer at Kiel has been shown by Winnewisser (9) to be very suitable for highly accurate and rapid data acquisition in the millimeter-wave region. The averaging process is found not only to increase the sensitivity of the millimeter-wave spectrometer by a factor of more than 100, but also to improve the accuracy of the frequency measurement by a factor of more than 10 over the more conventional frequency measurement technique employed at NRC (8). Thus, some of the millimeter-wave transitions are accurate to about f2 kHz at the fundamental frequency of the klystron. Furthermore, in the millimeter-wave region some measurements were made independently at both laboratories and found to be in agreement to within the quoted error limits. The samples used were obtained commercially and were used without further purification. Spectra&were observed and measured at room temperature, and also with the
MICROWAVE
SPECTRUM
OF VINYL
CYANIDE
TABLE 1. (continued) --
Transition
absorption of lGlO/.L
Observed Frequency
Calculated Frequency
Centrifugal Distortior Total High Order
Deviation
131267.422
131267.482
-19.521
-0.004
-n.060
129219.213
129219.226
-15.006
-8.002
-0.013
135539.974
135539.958
-26.025
-0.002
132524.583
132524.505
-;3.588
-0.902
-9.012
134021.823
134021.835
-18.681
0.083
-0.012
132959.401
132959.429
- 3.220
0.000
-0.028
133030.674
133030.685
- 3.594
0.000
-0.011
132917.752
132917.768
13.537
0.000
-0.016
132918.991
132919.024
13.523
0.001~
-0.033
132900.010c 132900.034
35.022
o.on1
-0.024
132900.010c 132900.046
;5.C?2
0.000
-0.336
1329C5.288
132905.326
61.152
0.000
-0.038
132923.739
132923.792
91.931
0.000
-0.053
132951.274
132951.310
127.341
0.000
-0.034
132985.953
132985.954
167.351
0.000
-0.001
133026.756
133026.756
211.924
3.000
0.046
133073.083
133072.991
261.015
8.000
0.093
cell cooled with dry ice. Measurements
MICROWAVE
were made
Ref.
C.iil6
at sample
pressures
SPECTRUM
The vinyl cyanide molecule is a slightly asymmetric prolate rotor, whose inertial axes are oriented such that a- and b-type transitions occur. It has been found that the three atoms CCN are collinear within experimental accuracy and make an angle of 15’59’ with the u-axis (6). The direction of the dipole moment is essentially in the direction of the CN bond, and since ~~ = 3.68D and pc~b= 1.25D (5) the u-type transitions are considerably stronger than the b-type transitions. a. The a-Type Spectrum
The observed ground state a-type spectrum of vinyl cyanide exhibits the typical features of a slightly asymmetric rotor with R- and Q-branch transitions. The strong R-branch transitions which occur throughout the centimeter- and millimeter-wave region
6
GERRY
AND WINNEWISSER TABLE 1. (continued)
Transition
Centrifugal Oistortion High Order Total
Observed Frequency
Calculated Frequency
140429.48
140429.44
-23.42
-0.01
0.04
138395.16
'38395.154
-18.792
-0.002
0.006
Deviation
145141.487
145141.497
-31.996
-0.003
-0.010
141945.385
141945.382
-18.081
-0.002
0.003
143759.253
143759.250
-24.934
0.004
0.003
0.00
0.05
142472.40
'42472.35
7.40
142572.93c
142572.98
- 8.00
0.00
-0.05
142424.446
142424.459
10.559
0.000
-0.013
142426.490
142426.511
10.543
0.000
-0.021
142399.51
142399.50
33.61
0.00
142401.95
142401.88
61.62
0.00
142419.675
142419.712
94.606
0.000
0.01 0.07 -0.037
142448.00
142447.94
132.54
0.00
'42484.17
'42484.2'
175.41
0.00
-0.04
142527.21
142527.26
223.16
0.00
-0.05
i42576.31
142576.39
275.75
0.00
-0.08
show the expected K < J components; J 6 18 because of the inertial asymmetry
Ref.
0.06
these are split into doublets for K < 4 and of the molecule. The transitions having J < 3, which are found in the centimeter-wave region, are further split by quadrupole coupling of the nitrogen nuclear spin with the molecular rotation. This hyperfine structure confirms the assignments. In the millimeter-wave region the K-structure of each of these R-branch transitions forms two subband heads, the first for K = 2 and the second near K = 5. The latter head is rather pronounced and with increasing K quantum number the lines are degraded from it towards shorter wavelengths. This behaviour, which repeats itself for every millimeter-wave transition, is characteristic for heavy but slightly asymmetric rotors and has already been found in the millimeter-wave spectrum of propynal, HC&HO (10) and recently in the millimeterwave spectrum of acrolein, CHZCHCHO (11). A typical u-type R-branch transition with all its K-components is illustrated in Fig. 1, where some different K components are reproduced in the top line, while the center presents an overview of the entire J transition. The lower part of the figure gives a comparison of the calculated and observed spectrum. The magnitude and sign of the centrifugal distortion contribution to each K component is indicated by the length
MICROWAVE
SPECTRUM
OF VINYL
7
CYASIDE
TABLE 1. (continued)
Transition
Observed Frequency
Calculated Frequency
1%,1~-181,17
183343.630
163343.667
-63.522
-0.016
19:,,i-lE:,,,. 182899.39
182699.39
-61.35
0.91
160534.30
180534.39
-32.85
(1.00
193,!r,-lE1,1; 180858.66
180656.66
-35.64
0.01
0.00
19j,17-18S,1i
Deviation
-0.037 0.00 -0.09
19i,li-lE4,!:
180476.95
180476.99
-10.26
0.00
-0.04
19.,,i-lE&,,,
180487.99
180487.89
-10.35
0.00
0.10
19i,i5-18:,,,
180411.85'
180411.76
19.20
0.00
3.07
L-18g,ij
180411.85'
160411.97
19.19
0.00
-0.12
180395.31
54.82
0.00
0.05
180406.34
96.65
0.00
-0.05
l%,l
19r.
-lE,d
18C395.36
19,
-18,
180406.29
19,
-18~
180434.80
180434.66
144.73
0.00
0.12
-189
180475.23'
180475.56
199.00
0.00
0.33
180526.47
160526.46
259.46
0.00
0.01
193 1910
-181,
1911
-1811
180585.68
180585.99
326.02
0.00
-0.11
-1812
180653.34
180653.28
398.64
0.00
0.06
96C0.07
9600.03
- 3.67
0.00
0.04
7: ,c - 71 ;'
12797.71
12797.71
- 5.61
0.00
0.00
81,7 - EI,,
16450.01
16449.98
- E.25
0.00
0.03
91,8 - 91,9
20555.32
20555.29
-11.78
0.00
0.03
12,,11-121,);
35563.92
35563.96
-29.59
0.00
-0.04
191:
6i,S
- 61 ,c
Ref.
.____.
and direction of the arrows. The J = 14 +- 13 transition located near 133 GHz produces 13 K-components, the four lowest of which (K = 1, - . a, 4) exhibit K-doubling caused mainly by the inertial asymmetry splitting. This splitting, Av, is approximately proportional to the asymmetry of the molecule and decreases strongly with increase of the quantum number K : Av - (bJK where b, is Wang’s asymmetry parameter for the molecule. The splitting of the K = 4 line for the J = 14 +- 13 transition has been measured to be 1.259 f 0.005 MHz and is shown in Fig. 1 to be well resolved. The line with K = 5 and all higher K lines show no observable splitting. Furthermore for values of K > 10 the assignment becomes increasingly more uncertain since these lines, which are weak, overlap with other equally intense or stronger rotational transitions which presumably arise within various vibrationally excited states. One example is given in Fig. 1 where at the predicted position (133180.65 f 0.08 MHz) of the K = 13 line, two lines could be found, at 133181.018 MHz and 133180.473 MHz. Although it seems
GERRY
8
AND WINNEWISSER TABLE 1. (continued)
Transition
Observed Frequency
16?,1~-162,1~
10102.90
10102.91
-
31.90
0.03
-0.01
17,,,s-17;,1t
12642.12
12642.15
- 43.48
0.04
-0.03
182,16-182,17
15584.71
15584.73
- 58.10
0.06
-0.02
19,,,7-192,IH
18953.19
18953.14
- 76.20
0.08
0.05
232,31-232,22
36995.37
36995.36
-192.73
0.22
0.01
8974.73
8974.74
-111.19
0.47
-0.01
27 3+-273,25
Calculated Frequency
Centrifugal Oistortion Total High Order
Ceviation
28x,,,-283,~~
10982.13
10982.14
-143.87
0.64
-0.01
293,26-a
13318.53
13318.53
-183.91
0.85
0.00
303,:7-303,2@
16014.31
16014.31
-232.37
1.12
0.00
353,x-353,33
35827.73
35827.74
-636.73
3.31
-0.01
8164.26'
8164.32
-274.71
3.51
-0.06
9835.X+
9836.06
-344.39
4.54
-0.10
,?7
a
"Unsplit" transition with quadrupole hyperfine structure accounted for. transitions the quadrupole structure was unresolved.
b
Costain and Stoicheff, ref. 6
Ref.
For all other
C
Not used in the fit.
d
Where only one subscript is given for a transition the K-type asymmetry doubling was unresolved.
that the latter transition is the desired K = 13 line there remains an uncertainty in this assignment. This transition and all other similarly uncertain assignments have not been used in the analysis. The stronger u-type Q-branch transitions are located in the centimeter-wave region. Several branches, having J 6 3.5 and K 6 3, were assigned and measured between 8 and 37 GHz. The lines were found to be broad (line widths up to about 0.3 MHz), suggesting the possibility of resolution of hyperfine structure. The transitions resisted all attempts at such resolution, however; probably the large line breadths are due at least partially to the large dipole moment of the molecule. The measured u-type microwave and millimeter-wave transitions together with their assignments are listed in Table 1. For the R-branch transitions having J < 3, which show hyperfine structure, the hypothetical center frequency is listed. b. The b-Type Spectrum It is difficult to obtain from an a-type spectrum alone independent and accurate determinations of the rotational constants A, Dg and, if the data permit, HK, since for
MICROWAVE Table 2.
SPECTRUM
OF VINYL
CYANIDE
Observed and calculated b-type rotational transitions (Mtk) of vinyl cyanide
Transition
Observed Frequency
Calculated Frequency
Centrifugal Distortion Total
Deviation
2 III?- ll,l
-25910.44a
-25910.45
2.43
0.00
c.01
3c,, - 21,,
-15982.54a
-15982.55
1.21
0.00
0.01
60,~ - 51,:
15010.70
15010.68
- 2.73
0.00
0.02
70,7 - Cl,6
25698.45
25698.47
- 5.38
0.00
-0.02
8 0,R - 71,7
36535.2@
36535.15
- 8.71
0.00
c.05
'7,,17-, 161,1,
136855.637
136855.592
-71.19!
0.029
0.045
121,12- 110,11
142759.361
142759.357
7.791
0.000
0.004
231,22- 236,23
132555.533
132555.547
-2C8.68C
0.017
-c.o13
241,23- 240,24
141277.442
141277.452
-247.610
-C.OlO
-0.010
11 I,~"- lC,,?
- 16275.80
-16275.78
- 7.95
0.00
-0.02
8063.89
8063.88
-34.60
0.00
0.01
14 1,13- 13:,],
20508.65
20508.68
-50.88
0.00
-0.03
13 1,13- 12,,,0
-36903.78
-36803.79
11.32
0.00
-0.01
17 1,1,- 16z,1,
,20517.79
-20517.75
15.77
-0.03
-0.04
18 1,1a- 172,15
-17949.31
-17949.32
23.11
-0.04
0.01
191,19
18,,,,
-16037.47
-16037.49
34.06
-0.06
0.02
201,*,
19,,,,
-14803.25
-14803.24
49.3(3
-0.08
0.01
211,;1-
2@2,1,
-14262.95
-14262.94
69.38
-0.11
-0.01
221 ,22-
21,,1,
-14428.13
-14428.18
04.92
-0.14
0.05
13 1,12-
‘22,ll
Ref.
High Order
b
b
these transitions only the AK, = 0 lines are strong. However, b-type transitions involve AK, # 0 energy level separations and hence these constants are best determined from an analysis of these transitions. It is preferable of course to measure transitions from as many different rotational subbands as possible. Costain and Stoicheff (6) reported two transitions of the K, = 1 +-0 subband. We have now extended their measurements to many other transitions of the K, = 1 + 0 subband and we have also observed various branches of the K, = 2 + 1, . . . , 5 + 4 rotational subbands. Virtually all the b-type transitions assigned in this work lie in the centimeter-wave region, between 8 and 37 GHz (though a few were measured at millimeter wave-frequencies). Since the spectrum is rather complex the assignment (along with that of the
GERRY
10
AND WINNEWISSER
TABLE 2 (Continued)
Transition
Observed Frequency
Calculated Frequency
-15305.76
-15305.75
126.52
-0.18
-0.01
24 1,2c- 232,2, -16897.89
-16897.66
164.73
-0.22
-0.03
141741.689
141741.672
-29.215
0.000
-19285.11
-19285.04
-111.33
0.06
-0.07
21 2,20- zo,,,, -35461.62
-35461.62
- 19.57
-0.09
0.00
2%
,23-
72,s 2(32,1fr
222,20
71,7
1g3,17
Centrifugal Distortion Total
Deviation
0.011
23 2,21 22,&O
19053.23
19053.22
-262.40
0.11
0.01
24 2,22- 233,21
32420.50
32420.47
-323.07
0.13
0.03
23 2,22- 22,,,s -20748.66
-20748.69
- 44.61
0.18
0.03
24 2,23- 233,20 -13892.38
-13892.40
- 53.23
-0.25
0.02
9372.06
9372.05
- 39.93
-0.85
0.02
292,2*- 28,,,,
13962.20
13962.19
- 18.95
-1.11
0.01
302,29-
17987.60
17987.62
11.65
-1.43
-0.02
30 3,27- 29,,,,, -14442.14
-14442.16
-442.72
0.83
0.02
282,27- 273,2't
293,26
32 3,2s- 31,,,a
10282.32
10282.32
-665.44
1.30
0.00
343,31- 33,,,,
36330.79
36330.79
-932.64
1.88
0.00
32,,,,- 31,,,, -13752.86
-13752.83
-273.93
-0.80
-0.03
11133.71
-369.24
-1.92
0.01
40 Q,36- 39s,as -10192.63'
-10192.51
-1044.56
3.77
-0.12
404,37-
-20554.15
- 676.53
-1.19
-0.03
35 3,33- 34,,,,
395,N
11133.72
-20554.18'
Ref.
High Order
a"Dnsplit" transition with quadrupole hyperfine structure accounted for.
For all other
transitions the hyperfine structure was unresolved. bCostain and Stoicheff, ref. 6. 'Not used in the fit.
a-type Q-branches) was done by a bootstrap procedure. This has been described prepreviously (IL’), and is similar to the one suggested by Kirchhoff (13). The spectrum was predicted using the earlier rotational constants (6), and after a general search several previously unreported low J transitions were’identified easily from it. These in turn were
MICROWAVE
SPECTRUM
11
OF VINYL CYANIDE
TABLE 3 OBSEXVEDAND CALCULATEDQUADKUPOLEHYPERFINECUAWONENTS UP VINYL CYANIDE (IN MHZ)
Transition ~~__ 1oi-BOO
303-21?
F’-F”
Observed frequency
l-l 2-l e-1 3-2
Quadrupole correction Observed
Calculated8
9484.19 9485.29 9486.89 - 15982.91
-0.91 0.19 1.79 -0.37
- 15982.38
+0.16
-0.93 0.19 1.87 -0.41 0.16 0.22
a Calculated quadrupole corrections were made using the quadrupole coupling constants of Costain and Stoicheff (6).
combined with the earlier transitions and analyzed by the method given below to predict further transition frequencies and their standard deviations. Several of these having low standard deviations were identified and measured, and were then used in a refined analysis; this in turn reduced the standard deviations of other transitions which could then be assigned and measured. The procedure was repeated until the spectrum had been assigned up to J = 35. The frequency fit was the chief criterion for the assignments. Whenever possible, however, these were confirmed using the nitrogen nuclear quadrupole coupling, Stark patterns, and temperature dependence of the line intensities. As a final confirmation of the assignment a double resonance connection was found between the transitions 162,r4 +-- 162,n and 162,14+-- 171,rr When the former transition, at 10102.90 MHz, was pumped with 60 mw of power the latter, at 20517.79 MHz, showed a 25 percent reduction in intensity. A nearby line at 20508.65 MHz, which was very similar in appearance to the line at 20517.79 MHz, and which was later assigned 141,1x+-- 132,12, was unaffected by the pumping radiation. The measured b-type transitions together with their assignments are listed in Table 2. ANALYSIS AND RESULTS a. .\:uclear Quadrupole
Coupling
Although we have measured many transitions having moderately low values of J, and although many lines were rather broad, nitrogen quadrupole hyperfine structure could be resolved for only two transitions newly measured in the present work. Their measured frequencies, along with the observed and calculated quadrupole corrections, are listed in Table 3. The calculated corrections were obtained using the coupling constants of Costain and Stoicheff (6); the observed and calculated corrections agree within experimental error. Nuclear quadrupole hyperfine structure is usually defined in terms of two constants, designated x aa and 7. If the quadrupole moment of the nucleus in question (nitrogen in the present case) is designated eQ, and the components of the field gradient tensor at that nucleus along the principal inertial axes are qaa, qbb, ycc, then X,, = eQqaa and
12
GERRY TABLE 4.
AND WINNEWISSER
Ground state spectroscopicconstants of vinyl cyanide
Parameter
Value
Rotational Constants (MHZ) R
49850.700
i 0.013a
b
4971.2141 i 0.0008
t
4513.8289 r 0.0008
Centrifugal Distortion Constants (MHZ) ( 2.2448 f 0.0021) x 10-3
AJ
(-8.5442 + 0.0018) x 1O-2
"JK
2.7183 f 0.0021
AK 45
(
4.5716 + 0.0037) x lo-"
6K
(
2.4575 f 0.0097) x 1O-2
HJ
(
4.73
f 2.0
) x 10-s
HJK
( 2.30
f 1.7
) x 10-7
"KJ
(-9.03
f 0.60
) x 10-s
HK
(
4.51
+ 0.80
) x lo-"
hJ
(
3.38
f:0.83
)x
1O-g
(-3.25
+ 3.25
)x
lo-'
( 7.49
f 2.2
)x
10-5
hJK hK bclear Ouadrupole Coupling Constantsb
-3.74
xaa (MHz)
0.127
n
Dipole b!omentc ii,(D)
3.68
q, (D)
1.25
U
2.89 + 0.08
(0)
aErrors cited are the standard errors. 'Costain and Stoicheff, ref. 6 'Wilcox, Goldstein and Summons, ref. 5.
It is noteworthy that all the hyperfine structure observed for vinyl (qbb ~cc)/@za. cyanide, in both the present and earlier work (6), is dependent virtually entirely on x,, and independent of r. Costain and Stoicheff (6) estimated 7)using the molecular structure and assuming cylindrical symmetry for the cyanide group. Several transitions, especially the u-type Q-branches which have been observed in the present work, should have hyperrl =
MICROWAVE Table 5.
SPECTRCM
OF VISYI,
13
CYANIDE
Frequency Prediction for Some a-type Rotational Transitions of Vinyl Cyanide (in MHz). Calculated Frequency
Standard Deviation
2c.c
2a440.797a
.a04b
4-3
42.9
28441.02aa
3 - 2
29.E
28441.iaia
3 - 2
25.6
27767.122a
Transition
F' - F'
30,: - 2,,?
2 -1
Relative Intensity __--
31,: - 21,;
2 -
31 : - 21,l
32,: - 22,l
32,1 - 22,u
7e.c
27767.43aa
4-3
42.P
27767.459a
3-2
29.6
29i3a.a6aa
4 - 3
42.9
29139.19ga
2 -1
2o.c
29i39.20aa
3-Z
29.6
28456.001a
4-3
42.0
28457.203a
2 -1
z0.c
2a457.a7ia
3-2
29.6
28469.903a
4-3
42.9
2847l.105~
2 -1
X.0
28471.772a
1
.004h
.0041‘
.004"
.004b
3790a.856
0.006
4 I,? - 3!,:
38847.744
0.005
5c,: - 40,4
47354.657
0.007
5 1,s - 41,L
46266.941
0.007
5 I,',- 41,?
48552.573
0.007
4 0,4 - 30,3
fine structure depending entirely on 17.As we noted earlier, however, these transitions could not be resolved. This agrees with the very small splittings predicted using the earlier value of 7 (6). Our work thus tends to confirm Costain and Stoicheff’s coupling constants. b. Rotational
and Centrifugal
Distortiofl
Constants
With the hyperfine structure removed the spectrum was analy-zed in terms of Watson’s “reduced” Hamiltonian including both quartic and sextic centrifugal distortion constants (14). Since vinyl cyanide is a near symmetric prolate rotor the IV-representation (IS) is appropriate and this Hamiltonian has the following form :
14
GERRY AND WINNEWISSER TMlLE 5 (continued)
Transition
Standard Deviation
0.008
6 I.5 - 51,5
55510.567
0.008
6 15-51,
58252.576
c.007
0.7
-
60,6
66196.35E
0.008
7 1,’
-
61,~
64749.02@
o.ooa
7 1,6
-
61,5
67946.698
0.008
a C,B
-
‘C,’
75565.703
0.009
a i,B
-
71,’
73981.564
0.009
a 1,’
-
'I,,
77633.836
o.ooa
9 o,g - a,,,
84946.012
0,009
9 l,g - al,,
83207.517
0.009
9 1,8 - al,,
87312.830
0.009
100,lO
-
go,,
94276.646
0.009
101,lO
-
91,9
92426.261
0.009
-
‘l,,
o-1
11.1
455.821
2 -1
13.9
456.770
2-2
41.7
457.259
l-l
a.3
457.402
l-2
13.9
457.892
1-o
11.1
458.626
O.OOob
AJP4 - AJKP2Pa2 - AKPa2 - (P*2 - PC”)(6JP2 + 6KPa2) -
x0,
Calculated Freouencv 567a6.941
‘I,,
= -
Relative Intensity
60,~ - 50 5
7
x0
F' _ F"
(6Jp2 + 6KP,2) (PZ - PE2)) [3]
= HJP6 + HJKP~P,~ + HK.~P~P,~ + HKP,~ + (Pb’ - PC”)(~.JP~ + h.rKP2Pa2 + hd’a4) + (h.rP4 + h.rKP2P,2 + hda4)(Pb2
- PC”), [4]
Here the angular momentum P has components P,, Pb, P, along the a-, b-, and c-principal inertial axes. Analysis is carried out in terms of the three rotational constants A”, B, I? [which differ from the usual ones by small centrifugal terms (IO)], and five quartic and seven sextic centrifugal distortion constants. The analysis was carried out using the iterative least-squares procedure described previously (12,16). Briefly this involved fitting the spectrum to the first order expression
MICROWAVE
SPECTRUM
OF VINYL
CYANIDE
15
TABLE 5 (Continued)
Transition 2,,1 -
31,2
-
21,2
3!,3
41,s - 41,,
51,4 - 51,5
F' _ F"
Relative Intensity
Calculated Frequency
l-l
15.c
1371.7@9
3-3
4T.5
1371.794
2-2
23.1
1371.947
2-2
21.2
2743.480
4-4
40.2
2743.535
3-3
28.0
2743.694
3-3
24.3
4572.305
5-5
39.1
4572.347
4-4
30.1
4572.509
4-4
26.2
6857.943
6-6
38.3
6857.976
5
31.1
6858.141
- 5
Standard Deviation C.OOlb
0.003b
0.004b
0.006b
aThese frequencies were obtained by adding the calculated quadrupole corrections to the calculated frequencies gfven in Table 1. b
Standard deviations of the unsplit frequencies.
(16) for the Hamiltonian Cl] in a rigid asymmetric rotor basis. The constants so obtained were then used in the full Hamiltonian to predict the spectrum. The differences between these calculated frequencies and those calculated using the first-order expression represented the higher order contributions. These were subtracted from the measured frequencies and the resulting frequencies were refit using the first order expression. The procedure was repeated until the higher order effects were stabilized. Double precision (16 digits) arithmetic was used in all calculations. The average deviation between observed and calculated frequencies for transitions used in the fit was 0.03 MHz, while the standard deviation of the fit was 0.05 MHz. The rotational and centrifugal distortion constants calculated in the analysis are given in Table 4. From this table it is seen that reliable values for the rotational and quartic distortion constants have been obtained. However, in general the sextic coefficients are less well determined, and can be expected to change with the inclusion of further high J transitions in the analysis. It seems that of the seven coefficients only four, namely HK, HKJ, hJ and to a lesser extent hi can be obtained from the present data. For all other sextic distortion constants the quoted standard deviations are of the order of the value of the constant. These constants have merely been included to obtain better agreement between observed and calculated spectrum. Similar results have been obtained for vinyl fluoride (12) and for propynal (10). Nevertheless, at the end of each of Tables 1 and 2 two transitions have been added involving J = 39 and 40 which were accuratelypredicted using our constants : it seems evident, then, that our derived con-
16
GERRY
AND
WINNEWISSER
stants are precise enough to predict unmeasured transitions at least up to J = 35, and probably up to J = 40 as well. In deriving predictions for other transitions hitherto unmeasured we have paid special attention to some a-type transitions which may have astrophysical significance. The frequencies of these lines are summarized in Table 5 together with the calculated standard deviations of these predictions. It is assumed that the proper line frequency lies within three times the given standard deviation. Where appropriate we have given for some low J transitions the calculated quadrupole hyperfine structure components. The rotational constants are in reasonable agreement with those given previously (6). On the other hand though partial account for centrifugal distortion was made in the earlier work, too few transitions were measured to obtain accurate constants, and comparison with the present work does not seem profitable. ACKNOWLEDGMENTS One of us (M. C. L. G.) thanks the National Research Council of Canada for support in the form of research grants. One of us (G. W.) is grateful to Dr. C. C. Costain and Dr. M. Winnewisser for the hospitality of their laboratories. The work of G. W. was in part supported by the Deutsche Forschungsgemeinschaft; this support is gratefully acknowledged. RECEIVED:
March 5, 1973 REFERENCES
1. 2. 3. 1. 7. 6. 7. 8. 9. 10. 11. 12. 13. II. 15. 16.
D. B. G. G.
M. RANK, C. H. TOWNES, AND W. J. WELCH, Science 174, 1083 (1971). E. TURNER, Astrophys. J. 163, L35 (1971). WINNEWISSER,2nd European Microwave Spectroscopy Conference, Bangor, Wales, 1972. WINNEWISSER,P. G. METZGER, AND H. D. BRE~R, “Advances in Astrochemistry,” Springer Verlag, 1973, in press. W. S. WILCOX, J. H. GOLDSTEIN,ANDJ. W. SIMMONS,J. Cheat. Plzys. 22, 516 (1954). C. C. COSTAINANDB. P. STOICHEFF,J. Ckem. Phys. 30, 777 (1959). W. H. HOCKINGAND M. C. L. GERRY, J. Mol. Spectrosc. 42, 547 (1972). G. WINNEWISSER,J. Mol. Spectrosc. 41, 574 (1972). M. WINNEWISSER,Z. Angew. Physik 30, 3.59 (1971). G. WINNEWISSER,J. Mol. Spectrosc. 1973, in press. G. WINNEWISSERAND M. WINNEWISSER,J. Mol. Spectrosc., to be published. M. C. L. GERRY, J. Mol. Spectrosc. 45, 71 (1973). W. H. KIRCHHOFF,J. Mol. Spectrosc. 41, 333 (1972). J. K. G. WATSON,J. Chem. Phys. 46, 1935 (1967). G. W. KING, R. M. HAINER, ANDP. C. CROSS,J. Ckerrz. Pkys. 11, 27 (1943). P. HELMINGER,R. L. COOK, AND F. C. DE LUCIA, J. Mol. Spectrosc. 40, 125 (1971).
SPECTKOSCOPT
48, 1-16 (1973)
The Microwave Spectrum and Centrifugal Constants of Vinyl Cyanide
Distortion
M. C. L. GERRY Deparfrirent of Cken&ry,
The C’niversity of British Columbia,
T’amouver 8, B. C.. Canada
GISBERT WINNEWISSER ,~~a~-Planck-Institut Ihision
of Physics,
ftir Radioastronorrzie, Bonn, Germany and National Research Council oj’Canada, Ottawa, Canada
The rotational spectrum of the ground state of vinyl cyanide has been assigned and measured in the frequency regions 8-37 GHz and 90-200 GHz. A total of 140 transitions has been measured and assigned to quantum numbers as high as J = 35 and K = 12. These new measurements have been used to calculate accurate values for all the rotational and quartic centrifugal distortion constants and some sextic distortion constants. The rotational constants are in MHz:
k = 49850.700
zt 0.013;
B = 4971.2141
f
0.0008;
I? = 4513.8289
f
0.0008.
INTRODUCTION ‘There have been more than twenty molecules identified in interstellar space by their microwave spectra (1). Amongst them are complex organic molecules which have been detected in “dark” and “black” clouds notably in the direction of the Orion nebula and the galactic center clouds Sagittarius A and Sagittarius B2. Although the present list of detected interstellar molecules does not give a full account of the chemical conditions in space, it already seems to suggest that many of the interstellar molecules are related in that they are molecular fragments to which successive numbers of hydrogen atoms or radicals have been attached (e.g., CN-HCN-CH&H-CH3NH2 and COH2CO-CHIOH). One of the molecules observed has been cyanoacetylene, HCC-CN (Z), a derivative of acetylene which may well have been formed from the ethynyl radical (-CCH) and the cyanogen radical (-CN) by surface reactions on interstellar dust grains. A more detailed discussion of this formation scheme has been given (3) and will be published elsewhere (3). Similarly vinyl cyanide HzC=CH-CN could be synthesized and may be observable in interstellar space. Although none of the presently known interstellar molecules contains a carboncarbon double bond, there seems to us to be no obvious reason why this class of molecules should be missing in the list of interstellar molecules. Especially since vinyl cyanide shows very strong transitions in the microwave region, a thorough interstellar search seems to be warranted. In order to aid the interstellar search for this molecule in some
Copyright f$3 197.3 by Academic Press, Inc. .UI rights of reproduction in any form rwerved.
2
GERRY AND WINNEWISSER
H, CCH CN J=14 -
13 K.13
& 1
o
2
/
1, I
13;800
13;ooo
II ,I
1
,,
I
13hOO
MHz
i ._m._,.l.
FIG. 1. Millimeter-wave spectrum of the complete J = 14 + 13 u-type rotational transition of vinyl cyanide near 133 GHz. The top line represents the recorder output for some selected R-components. The lower third of the figure shows for each K-component the calculated rigid rotor positions (x) and the measured and calculated line positions (e) taking centrifuga ldistortion into account. The length and the direction of the arrows indicate the effect of centrifugal distortion.
of the molecule-rich dark and black clouds, we have measured part of its microwave and millimeter wave spectrum in the frequency regions 8-37 GHz and 90-200 GHz. More than one hundred ground-state transitions have been selected, assigned and measured. The selection of these transitions has been made such that their analysis would yield a fairly complete set of molecular constants which can be used then to predict, to a high degree of confidence, any molecular transition with quantum numbers J 5 35 and K < 15. These predictions include, of course, some of the potentially interesting astrophysical transitions. There have been two previous studies of the microwave spectrum of vinyl cyanide. In the first study (S), the five K-components of the a-type J = 3 + 2 transition were measured to give approximate values of the rotational constants, the nitrogen nuclear quadrupole coupling constants and the dipole moment. Subsequently, Costain and Stoicheff (6) reported further transitions, for several isotopic species. The J = 2 + 1 and 3 + 2, a-type transitions with their K-components were measured and, more importantly, two low J b-type transitions could be assigned ; they gave improved rotational constants together with some quartic centrifugal distortion constants and the bond lengths and angles. However, there has been no previous assignment of transitions to high values of J, nor have there been any measurements on millimeter-wave transitions. Accordingly, we have measured several a-type transitions with most of their K-components up to values of J as high as 18 and about fifty b-type transitions originating from several different rotational subbands with values of K 6 4. From these new experi-
MICROWAVE
SPECTRUM OF VINYL
CYANIDE
3
TABLE 1. Observed and calculated a-type rotational transitions (MHz) of vinyl cyanide
Transition
Observed Frequency
Calculated Frequency
Centrifugal Distortion Tctal High Order
Deviation
9485.03
-
0.01
0.00
0.07
18966.54
-
0.07
0.00
0.00
16513.08
0.38
0.00
0.01
19427.63
0.16
‘).cc
-0.04
0.23
0.00
0.00
27767.36
0.47
0.00
-0.04
29139.10
0.0x
0.00
-0.13
26456.94
1.81
0.00
0.40
28470.64
1.80
O.O@
0.43
0.00
0.07
9.15
0.00
0.04
-
28440.99
-C.24
41,, - 3!,1
37019.00
37018.93
10i,9
96982.49
96082.45
-
lOL,3 - 9:,,, 9476C.83
94760.80
-
1.76
0.00
0.03
lO;,c - 9:,;
95325.49
-
2.93
0.00
-0.00
94928.63
6.38
0.00
0.08
94941.65
6.34
0.00
-0.07
-
91,:
95325.49
103,; - 9!,.? 94928.71 103,; - 9x,,
94941.58
Ref.
--__
mental data accurate values of the ground-state centrifugal distortion constants have been obtained, constants.
rotational constants and quartic along with values for some sextic
EXPERIMENTAL
The spectrum was observed using three different spectrometers. In the frequency region 8-37 GHz the 100 kHz Stark modulated spectrometer at the University of British Columbia was used ; this has been described previously (7). Accuracy of the measurements using this spectrometer is estimated to be better than 0.1 MHz. Above 90 GHz millimeter wave spectrometers at the National Research Council of Canada (8) and the University of Kiel (9) were used. The millimeter wave frequencies were generated by harmonic multiplication of the frequencies of various reflex klystrons covering the fundamental frequency range 2645 GHz. The accuracy of frequency measurement obtained in the millimeter-wave region by the two spectrometers at LNRC and at Kiel is different. Since the instrument at NRC is conventional, measurement accuracy is estimated to be better than 0.15 MHz. The on-line use of a small
GERRY
AND WINNEWISSER
TABLE 1.
Transition
Observed Frequency
Calculated Frequency
(Continued)
Centrifugal Distortion Total High Order
Deviation
‘O*,, - %,6
94913.16C
94913.14
18.32
0.00
0.02
10, 6 - 9, ,:
94913.16'
94913.26
18.32
0.00
-0.10
lo5
94914.05
94914.00
33.61
0.00
0.05
94925.05
52.26
0.00
0.03
94942.54
74.24
0.00
0.13
103575.41
-10.17
0.00
0.17
- 9,d
10,
- 9,
94925.08
10,
- 9,
94942.67
11o,ll- IO,,,, 103575.5gc 11 1,11- lOI,,, 103637.23
101637.24
- 6.52
0.00
-0.01
11 1,10- lC,,, 106641.39
106641.40
-12.39
0.00
-0.0'
";,I,- lo,,, 104212.58
104212.66
- 3.87
0.00
-0.08
'1, a - lo>,, 104960.66
104960.56
- 5.65
0.00
0.10
'lx,, - lO,,a 104432.77
104432.80
4.94
0.00
-0.03
lo,,7 104453.85
104453.94
4.86
0.00
-0.09
18.08
0.00
0.22
“,,a
-
I',,, - lo,,7 104411.49c
104411.27
ll,,, - 104,s
lo4411.49c
104411.49
18.08
0.00
0.00
11:
- 10,d
lC4408.93
104408.91
34.91
0.00
0.02
'1,
- 10,
104419.33
104419.32
55.42
0.00
0.01
"7
- 10,
104437.49
104437.53
79.60
0.00
-0.04
'1,
- 10,
104461.50
104461.52
lC7.43
0.00
-0.02
1'3
- 109
104490.36
104490.37
138.87
0.00
[email protected]
"I,
- 1010
104523.87'
104523.52
173.86
0.00
0.35
l?ef.
computer for signal averaging with the video type spectrometer at Kiel has been shown by Winnewisser (9) to be very suitable for highly accurate and rapid data acquisition in the millimeter-wave region. The averaging process is found not only to increase the sensitivity of the millimeter-wave spectrometer by a factor of more than 100, but also to improve the accuracy of the frequency measurement by a factor of more than 10 over the more conventional frequency measurement technique employed at NRC (8). Thus, some of the millimeter-wave transitions are accurate to about f2 kHz at the fundamental frequency of the klystron. Furthermore, in the millimeter-wave region some measurements were made independently at both laboratories and found to be in agreement to within the quoted error limits. The samples used were obtained commercially and were used without further purification. Spectra&were observed and measured at room temperature, and also with the
MICROWAVE
SPECTRUM
OF VINYL
CYANIDE
TABLE 1. (continued) --
Transition
absorption of lGlO/.L
Observed Frequency
Calculated Frequency
Centrifugal Distortior Total High Order
Deviation
131267.422
131267.482
-19.521
-0.004
-n.060
129219.213
129219.226
-15.006
-8.002
-0.013
135539.974
135539.958
-26.025
-0.002
132524.583
132524.505
-;3.588
-0.902
-9.012
134021.823
134021.835
-18.681
0.083
-0.012
132959.401
132959.429
- 3.220
0.000
-0.028
133030.674
133030.685
- 3.594
0.000
-0.011
132917.752
132917.768
13.537
0.000
-0.016
132918.991
132919.024
13.523
0.001~
-0.033
132900.010c 132900.034
35.022
o.on1
-0.024
132900.010c 132900.046
;5.C?2
0.000
-0.336
1329C5.288
132905.326
61.152
0.000
-0.038
132923.739
132923.792
91.931
0.000
-0.053
132951.274
132951.310
127.341
0.000
-0.034
132985.953
132985.954
167.351
0.000
-0.001
133026.756
133026.756
211.924
3.000
0.046
133073.083
133072.991
261.015
8.000
0.093
cell cooled with dry ice. Measurements
MICROWAVE
were made
Ref.
C.iil6
at sample
pressures
SPECTRUM
The vinyl cyanide molecule is a slightly asymmetric prolate rotor, whose inertial axes are oriented such that a- and b-type transitions occur. It has been found that the three atoms CCN are collinear within experimental accuracy and make an angle of 15’59’ with the u-axis (6). The direction of the dipole moment is essentially in the direction of the CN bond, and since ~~ = 3.68D and pc~b= 1.25D (5) the u-type transitions are considerably stronger than the b-type transitions. a. The a-Type Spectrum
The observed ground state a-type spectrum of vinyl cyanide exhibits the typical features of a slightly asymmetric rotor with R- and Q-branch transitions. The strong R-branch transitions which occur throughout the centimeter- and millimeter-wave region
6
GERRY
AND WINNEWISSER TABLE 1. (continued)
Transition
Centrifugal Oistortion High Order Total
Observed Frequency
Calculated Frequency
140429.48
140429.44
-23.42
-0.01
0.04
138395.16
'38395.154
-18.792
-0.002
0.006
Deviation
145141.487
145141.497
-31.996
-0.003
-0.010
141945.385
141945.382
-18.081
-0.002
0.003
143759.253
143759.250
-24.934
0.004
0.003
0.00
0.05
142472.40
'42472.35
7.40
142572.93c
142572.98
- 8.00
0.00
-0.05
142424.446
142424.459
10.559
0.000
-0.013
142426.490
142426.511
10.543
0.000
-0.021
142399.51
142399.50
33.61
0.00
142401.95
142401.88
61.62
0.00
142419.675
142419.712
94.606
0.000
0.01 0.07 -0.037
142448.00
142447.94
132.54
0.00
'42484.17
'42484.2'
175.41
0.00
-0.04
142527.21
142527.26
223.16
0.00
-0.05
i42576.31
142576.39
275.75
0.00
-0.08
show the expected K < J components; J 6 18 because of the inertial asymmetry
Ref.
0.06
these are split into doublets for K < 4 and of the molecule. The transitions having J < 3, which are found in the centimeter-wave region, are further split by quadrupole coupling of the nitrogen nuclear spin with the molecular rotation. This hyperfine structure confirms the assignments. In the millimeter-wave region the K-structure of each of these R-branch transitions forms two subband heads, the first for K = 2 and the second near K = 5. The latter head is rather pronounced and with increasing K quantum number the lines are degraded from it towards shorter wavelengths. This behaviour, which repeats itself for every millimeter-wave transition, is characteristic for heavy but slightly asymmetric rotors and has already been found in the millimeter-wave spectrum of propynal, HC&HO (10) and recently in the millimeterwave spectrum of acrolein, CHZCHCHO (11). A typical u-type R-branch transition with all its K-components is illustrated in Fig. 1, where some different K components are reproduced in the top line, while the center presents an overview of the entire J transition. The lower part of the figure gives a comparison of the calculated and observed spectrum. The magnitude and sign of the centrifugal distortion contribution to each K component is indicated by the length
MICROWAVE
SPECTRUM
OF VINYL
7
CYASIDE
TABLE 1. (continued)
Transition
Observed Frequency
Calculated Frequency
1%,1~-181,17
183343.630
163343.667
-63.522
-0.016
19:,,i-lE:,,,. 182899.39
182699.39
-61.35
0.91
160534.30
180534.39
-32.85
(1.00
193,!r,-lE1,1; 180858.66
180656.66
-35.64
0.01
0.00
19j,17-18S,1i
Deviation
-0.037 0.00 -0.09
19i,li-lE4,!:
180476.95
180476.99
-10.26
0.00
-0.04
19.,,i-lE&,,,
180487.99
180487.89
-10.35
0.00
0.10
19i,i5-18:,,,
180411.85'
180411.76
19.20
0.00
3.07
L-18g,ij
180411.85'
160411.97
19.19
0.00
-0.12
180395.31
54.82
0.00
0.05
180406.34
96.65
0.00
-0.05
l%,l
19r.
-lE,d
18C395.36
19,
-18,
180406.29
19,
-18~
180434.80
180434.66
144.73
0.00
0.12
-189
180475.23'
180475.56
199.00
0.00
0.33
180526.47
160526.46
259.46
0.00
0.01
193 1910
-181,
1911
-1811
180585.68
180585.99
326.02
0.00
-0.11
-1812
180653.34
180653.28
398.64
0.00
0.06
96C0.07
9600.03
- 3.67
0.00
0.04
7: ,c - 71 ;'
12797.71
12797.71
- 5.61
0.00
0.00
81,7 - EI,,
16450.01
16449.98
- E.25
0.00
0.03
91,8 - 91,9
20555.32
20555.29
-11.78
0.00
0.03
12,,11-121,);
35563.92
35563.96
-29.59
0.00
-0.04
191:
6i,S
- 61 ,c
Ref.
.____.
and direction of the arrows. The J = 14 +- 13 transition located near 133 GHz produces 13 K-components, the four lowest of which (K = 1, - . a, 4) exhibit K-doubling caused mainly by the inertial asymmetry splitting. This splitting, Av, is approximately proportional to the asymmetry of the molecule and decreases strongly with increase of the quantum number K : Av - (bJK where b, is Wang’s asymmetry parameter for the molecule. The splitting of the K = 4 line for the J = 14 +- 13 transition has been measured to be 1.259 f 0.005 MHz and is shown in Fig. 1 to be well resolved. The line with K = 5 and all higher K lines show no observable splitting. Furthermore for values of K > 10 the assignment becomes increasingly more uncertain since these lines, which are weak, overlap with other equally intense or stronger rotational transitions which presumably arise within various vibrationally excited states. One example is given in Fig. 1 where at the predicted position (133180.65 f 0.08 MHz) of the K = 13 line, two lines could be found, at 133181.018 MHz and 133180.473 MHz. Although it seems
GERRY
8
AND WINNEWISSER TABLE 1. (continued)
Transition
Observed Frequency
16?,1~-162,1~
10102.90
10102.91
-
31.90
0.03
-0.01
17,,,s-17;,1t
12642.12
12642.15
- 43.48
0.04
-0.03
182,16-182,17
15584.71
15584.73
- 58.10
0.06
-0.02
19,,,7-192,IH
18953.19
18953.14
- 76.20
0.08
0.05
232,31-232,22
36995.37
36995.36
-192.73
0.22
0.01
8974.73
8974.74
-111.19
0.47
-0.01
27 3+-273,25
Calculated Frequency
Centrifugal Oistortion Total High Order
Ceviation
28x,,,-283,~~
10982.13
10982.14
-143.87
0.64
-0.01
293,26-a
13318.53
13318.53
-183.91
0.85
0.00
303,:7-303,2@
16014.31
16014.31
-232.37
1.12
0.00
353,x-353,33
35827.73
35827.74
-636.73
3.31
-0.01
8164.26'
8164.32
-274.71
3.51
-0.06
9835.X+
9836.06
-344.39
4.54
-0.10
,?7
a
"Unsplit" transition with quadrupole hyperfine structure accounted for. transitions the quadrupole structure was unresolved.
b
Costain and Stoicheff, ref. 6
Ref.
For all other
C
Not used in the fit.
d
Where only one subscript is given for a transition the K-type asymmetry doubling was unresolved.
that the latter transition is the desired K = 13 line there remains an uncertainty in this assignment. This transition and all other similarly uncertain assignments have not been used in the analysis. The stronger u-type Q-branch transitions are located in the centimeter-wave region. Several branches, having J 6 3.5 and K 6 3, were assigned and measured between 8 and 37 GHz. The lines were found to be broad (line widths up to about 0.3 MHz), suggesting the possibility of resolution of hyperfine structure. The transitions resisted all attempts at such resolution, however; probably the large line breadths are due at least partially to the large dipole moment of the molecule. The measured u-type microwave and millimeter-wave transitions together with their assignments are listed in Table 1. For the R-branch transitions having J < 3, which show hyperfine structure, the hypothetical center frequency is listed. b. The b-Type Spectrum It is difficult to obtain from an a-type spectrum alone independent and accurate determinations of the rotational constants A, Dg and, if the data permit, HK, since for
MICROWAVE Table 2.
SPECTRUM
OF VINYL
CYANIDE
Observed and calculated b-type rotational transitions (Mtk) of vinyl cyanide
Transition
Observed Frequency
Calculated Frequency
Centrifugal Distortion Total
Deviation
2 III?- ll,l
-25910.44a
-25910.45
2.43
0.00
c.01
3c,, - 21,,
-15982.54a
-15982.55
1.21
0.00
0.01
60,~ - 51,:
15010.70
15010.68
- 2.73
0.00
0.02
70,7 - Cl,6
25698.45
25698.47
- 5.38
0.00
-0.02
8 0,R - 71,7
36535.2@
36535.15
- 8.71
0.00
c.05
'7,,17-, 161,1,
136855.637
136855.592
-71.19!
0.029
0.045
121,12- 110,11
142759.361
142759.357
7.791
0.000
0.004
231,22- 236,23
132555.533
132555.547
-2C8.68C
0.017
-c.o13
241,23- 240,24
141277.442
141277.452
-247.610
-C.OlO
-0.010
11 I,~"- lC,,?
- 16275.80
-16275.78
- 7.95
0.00
-0.02
8063.89
8063.88
-34.60
0.00
0.01
14 1,13- 13:,],
20508.65
20508.68
-50.88
0.00
-0.03
13 1,13- 12,,,0
-36903.78
-36803.79
11.32
0.00
-0.01
17 1,1,- 16z,1,
,20517.79
-20517.75
15.77
-0.03
-0.04
18 1,1a- 172,15
-17949.31
-17949.32
23.11
-0.04
0.01
191,19
18,,,,
-16037.47
-16037.49
34.06
-0.06
0.02
201,*,
19,,,,
-14803.25
-14803.24
49.3(3
-0.08
0.01
211,;1-
2@2,1,
-14262.95
-14262.94
69.38
-0.11
-0.01
221 ,22-
21,,1,
-14428.13
-14428.18
04.92
-0.14
0.05
13 1,12-
‘22,ll
Ref.
High Order
b
b
these transitions only the AK, = 0 lines are strong. However, b-type transitions involve AK, # 0 energy level separations and hence these constants are best determined from an analysis of these transitions. It is preferable of course to measure transitions from as many different rotational subbands as possible. Costain and Stoicheff (6) reported two transitions of the K, = 1 +-0 subband. We have now extended their measurements to many other transitions of the K, = 1 + 0 subband and we have also observed various branches of the K, = 2 + 1, . . . , 5 + 4 rotational subbands. Virtually all the b-type transitions assigned in this work lie in the centimeter-wave region, between 8 and 37 GHz (though a few were measured at millimeter wave-frequencies). Since the spectrum is rather complex the assignment (along with that of the
GERRY
10
AND WINNEWISSER
TABLE 2 (Continued)
Transition
Observed Frequency
Calculated Frequency
-15305.76
-15305.75
126.52
-0.18
-0.01
24 1,2c- 232,2, -16897.89
-16897.66
164.73
-0.22
-0.03
141741.689
141741.672
-29.215
0.000
-19285.11
-19285.04
-111.33
0.06
-0.07
21 2,20- zo,,,, -35461.62
-35461.62
- 19.57
-0.09
0.00
2%
,23-
72,s 2(32,1fr
222,20
71,7
1g3,17
Centrifugal Distortion Total
Deviation
0.011
23 2,21 22,&O
19053.23
19053.22
-262.40
0.11
0.01
24 2,22- 233,21
32420.50
32420.47
-323.07
0.13
0.03
23 2,22- 22,,,s -20748.66
-20748.69
- 44.61
0.18
0.03
24 2,23- 233,20 -13892.38
-13892.40
- 53.23
-0.25
0.02
9372.06
9372.05
- 39.93
-0.85
0.02
292,2*- 28,,,,
13962.20
13962.19
- 18.95
-1.11
0.01
302,29-
17987.60
17987.62
11.65
-1.43
-0.02
30 3,27- 29,,,,, -14442.14
-14442.16
-442.72
0.83
0.02
282,27- 273,2't
293,26
32 3,2s- 31,,,a
10282.32
10282.32
-665.44
1.30
0.00
343,31- 33,,,,
36330.79
36330.79
-932.64
1.88
0.00
32,,,,- 31,,,, -13752.86
-13752.83
-273.93
-0.80
-0.03
11133.71
-369.24
-1.92
0.01
40 Q,36- 39s,as -10192.63'
-10192.51
-1044.56
3.77
-0.12
404,37-
-20554.15
- 676.53
-1.19
-0.03
35 3,33- 34,,,,
395,N
11133.72
-20554.18'
Ref.
High Order
a"Dnsplit" transition with quadrupole hyperfine structure accounted for.
For all other
transitions the hyperfine structure was unresolved. bCostain and Stoicheff, ref. 6. 'Not used in the fit.
a-type Q-branches) was done by a bootstrap procedure. This has been described prepreviously (IL’), and is similar to the one suggested by Kirchhoff (13). The spectrum was predicted using the earlier rotational constants (6), and after a general search several previously unreported low J transitions were’identified easily from it. These in turn were
MICROWAVE
SPECTRUM
11
OF VINYL CYANIDE
TABLE 3 OBSEXVEDAND CALCULATEDQUADKUPOLEHYPERFINECUAWONENTS UP VINYL CYANIDE (IN MHZ)
Transition ~~__ 1oi-BOO
303-21?
F’-F”
Observed frequency
l-l 2-l e-1 3-2
Quadrupole correction Observed
Calculated8
9484.19 9485.29 9486.89 - 15982.91
-0.91 0.19 1.79 -0.37
- 15982.38
+0.16
-0.93 0.19 1.87 -0.41 0.16 0.22
a Calculated quadrupole corrections were made using the quadrupole coupling constants of Costain and Stoicheff (6).
combined with the earlier transitions and analyzed by the method given below to predict further transition frequencies and their standard deviations. Several of these having low standard deviations were identified and measured, and were then used in a refined analysis; this in turn reduced the standard deviations of other transitions which could then be assigned and measured. The procedure was repeated until the spectrum had been assigned up to J = 35. The frequency fit was the chief criterion for the assignments. Whenever possible, however, these were confirmed using the nitrogen nuclear quadrupole coupling, Stark patterns, and temperature dependence of the line intensities. As a final confirmation of the assignment a double resonance connection was found between the transitions 162,r4 +-- 162,n and 162,14+-- 171,rr When the former transition, at 10102.90 MHz, was pumped with 60 mw of power the latter, at 20517.79 MHz, showed a 25 percent reduction in intensity. A nearby line at 20508.65 MHz, which was very similar in appearance to the line at 20517.79 MHz, and which was later assigned 141,1x+-- 132,12, was unaffected by the pumping radiation. The measured b-type transitions together with their assignments are listed in Table 2. ANALYSIS AND RESULTS a. .\:uclear Quadrupole
Coupling
Although we have measured many transitions having moderately low values of J, and although many lines were rather broad, nitrogen quadrupole hyperfine structure could be resolved for only two transitions newly measured in the present work. Their measured frequencies, along with the observed and calculated quadrupole corrections, are listed in Table 3. The calculated corrections were obtained using the coupling constants of Costain and Stoicheff (6); the observed and calculated corrections agree within experimental error. Nuclear quadrupole hyperfine structure is usually defined in terms of two constants, designated x aa and 7. If the quadrupole moment of the nucleus in question (nitrogen in the present case) is designated eQ, and the components of the field gradient tensor at that nucleus along the principal inertial axes are qaa, qbb, ycc, then X,, = eQqaa and
12
GERRY TABLE 4.
AND WINNEWISSER
Ground state spectroscopicconstants of vinyl cyanide
Parameter
Value
Rotational Constants (MHZ) R
49850.700
i 0.013a
b
4971.2141 i 0.0008
t
4513.8289 r 0.0008
Centrifugal Distortion Constants (MHZ) ( 2.2448 f 0.0021) x 10-3
AJ
(-8.5442 + 0.0018) x 1O-2
"JK
2.7183 f 0.0021
AK 45
(
4.5716 + 0.0037) x lo-"
6K
(
2.4575 f 0.0097) x 1O-2
HJ
(
4.73
f 2.0
) x 10-s
HJK
( 2.30
f 1.7
) x 10-7
"KJ
(-9.03
f 0.60
) x 10-s
HK
(
4.51
+ 0.80
) x lo-"
hJ
(
3.38
f:0.83
)x
1O-g
(-3.25
+ 3.25
)x
lo-'
( 7.49
f 2.2
)x
10-5
hJK hK bclear Ouadrupole Coupling Constantsb
-3.74
xaa (MHz)
0.127
n
Dipole b!omentc ii,(D)
3.68
q, (D)
1.25
U
2.89 + 0.08
(0)
aErrors cited are the standard errors. 'Costain and Stoicheff, ref. 6 'Wilcox, Goldstein and Summons, ref. 5.
It is noteworthy that all the hyperfine structure observed for vinyl (qbb ~cc)/@za. cyanide, in both the present and earlier work (6), is dependent virtually entirely on x,, and independent of r. Costain and Stoicheff (6) estimated 7)using the molecular structure and assuming cylindrical symmetry for the cyanide group. Several transitions, especially the u-type Q-branches which have been observed in the present work, should have hyperrl =
MICROWAVE Table 5.
SPECTRCM
OF VISYI,
13
CYANIDE
Frequency Prediction for Some a-type Rotational Transitions of Vinyl Cyanide (in MHz). Calculated Frequency
Standard Deviation
2c.c
2a440.797a
.a04b
4-3
42.9
28441.02aa
3 - 2
29.E
28441.iaia
3 - 2
25.6
27767.122a
Transition
F' - F'
30,: - 2,,?
2 -1
Relative Intensity __--
31,: - 21,;
2 -
31 : - 21,l
32,: - 22,l
32,1 - 22,u
7e.c
27767.43aa
4-3
42.P
27767.459a
3-2
29.6
29i3a.a6aa
4 - 3
42.9
29139.19ga
2 -1
2o.c
29i39.20aa
3-Z
29.6
28456.001a
4-3
42.0
28457.203a
2 -1
z0.c
2a457.a7ia
3-2
29.6
28469.903a
4-3
42.9
2847l.105~
2 -1
X.0
28471.772a
1
.004h
.0041‘
.004"
.004b
3790a.856
0.006
4 I,? - 3!,:
38847.744
0.005
5c,: - 40,4
47354.657
0.007
5 1,s - 41,L
46266.941
0.007
5 I,',- 41,?
48552.573
0.007
4 0,4 - 30,3
fine structure depending entirely on 17.As we noted earlier, however, these transitions could not be resolved. This agrees with the very small splittings predicted using the earlier value of 7 (6). Our work thus tends to confirm Costain and Stoicheff’s coupling constants. b. Rotational
and Centrifugal
Distortiofl
Constants
With the hyperfine structure removed the spectrum was analy-zed in terms of Watson’s “reduced” Hamiltonian including both quartic and sextic centrifugal distortion constants (14). Since vinyl cyanide is a near symmetric prolate rotor the IV-representation (IS) is appropriate and this Hamiltonian has the following form :
14
GERRY AND WINNEWISSER TMlLE 5 (continued)
Transition
Standard Deviation
0.008
6 I.5 - 51,5
55510.567
0.008
6 15-51,
58252.576
c.007
0.7
-
60,6
66196.35E
0.008
7 1,’
-
61,~
64749.02@
o.ooa
7 1,6
-
61,5
67946.698
0.008
a C,B
-
‘C,’
75565.703
0.009
a i,B
-
71,’
73981.564
0.009
a 1,’
-
'I,,
77633.836
o.ooa
9 o,g - a,,,
84946.012
0,009
9 l,g - al,,
83207.517
0.009
9 1,8 - al,,
87312.830
0.009
100,lO
-
go,,
94276.646
0.009
101,lO
-
91,9
92426.261
0.009
-
‘l,,
o-1
11.1
455.821
2 -1
13.9
456.770
2-2
41.7
457.259
l-l
a.3
457.402
l-2
13.9
457.892
1-o
11.1
458.626
O.OOob
AJP4 - AJKP2Pa2 - AKPa2 - (P*2 - PC”)(6JP2 + 6KPa2) -
x0,
Calculated Freouencv 567a6.941
‘I,,
= -
Relative Intensity
60,~ - 50 5
7
x0
F' _ F"
(6Jp2 + 6KP,2) (PZ - PE2)) [3]
= HJP6 + HJKP~P,~ + HK.~P~P,~ + HKP,~ + (Pb’ - PC”)(~.JP~ + h.rKP2Pa2 + hd’a4) + (h.rP4 + h.rKP2P,2 + hda4)(Pb2
- PC”), [4]
Here the angular momentum P has components P,, Pb, P, along the a-, b-, and c-principal inertial axes. Analysis is carried out in terms of the three rotational constants A”, B, I? [which differ from the usual ones by small centrifugal terms (IO)], and five quartic and seven sextic centrifugal distortion constants. The analysis was carried out using the iterative least-squares procedure described previously (12,16). Briefly this involved fitting the spectrum to the first order expression
MICROWAVE
SPECTRUM
OF VINYL
CYANIDE
15
TABLE 5 (Continued)
Transition 2,,1 -
31,2
-
21,2
3!,3
41,s - 41,,
51,4 - 51,5
F' _ F"
Relative Intensity
Calculated Frequency
l-l
15.c
1371.7@9
3-3
4T.5
1371.794
2-2
23.1
1371.947
2-2
21.2
2743.480
4-4
40.2
2743.535
3-3
28.0
2743.694
3-3
24.3
4572.305
5-5
39.1
4572.347
4-4
30.1
4572.509
4-4
26.2
6857.943
6-6
38.3
6857.976
5
31.1
6858.141
- 5
Standard Deviation C.OOlb
0.003b
0.004b
0.006b
aThese frequencies were obtained by adding the calculated quadrupole corrections to the calculated frequencies gfven in Table 1. b
Standard deviations of the unsplit frequencies.
(16) for the Hamiltonian Cl] in a rigid asymmetric rotor basis. The constants so obtained were then used in the full Hamiltonian to predict the spectrum. The differences between these calculated frequencies and those calculated using the first-order expression represented the higher order contributions. These were subtracted from the measured frequencies and the resulting frequencies were refit using the first order expression. The procedure was repeated until the higher order effects were stabilized. Double precision (16 digits) arithmetic was used in all calculations. The average deviation between observed and calculated frequencies for transitions used in the fit was 0.03 MHz, while the standard deviation of the fit was 0.05 MHz. The rotational and centrifugal distortion constants calculated in the analysis are given in Table 4. From this table it is seen that reliable values for the rotational and quartic distortion constants have been obtained. However, in general the sextic coefficients are less well determined, and can be expected to change with the inclusion of further high J transitions in the analysis. It seems that of the seven coefficients only four, namely HK, HKJ, hJ and to a lesser extent hi can be obtained from the present data. For all other sextic distortion constants the quoted standard deviations are of the order of the value of the constant. These constants have merely been included to obtain better agreement between observed and calculated spectrum. Similar results have been obtained for vinyl fluoride (12) and for propynal (10). Nevertheless, at the end of each of Tables 1 and 2 two transitions have been added involving J = 39 and 40 which were accuratelypredicted using our constants : it seems evident, then, that our derived con-
16
GERRY
AND
WINNEWISSER
stants are precise enough to predict unmeasured transitions at least up to J = 35, and probably up to J = 40 as well. In deriving predictions for other transitions hitherto unmeasured we have paid special attention to some a-type transitions which may have astrophysical significance. The frequencies of these lines are summarized in Table 5 together with the calculated standard deviations of these predictions. It is assumed that the proper line frequency lies within three times the given standard deviation. Where appropriate we have given for some low J transitions the calculated quadrupole hyperfine structure components. The rotational constants are in reasonable agreement with those given previously (6). On the other hand though partial account for centrifugal distortion was made in the earlier work, too few transitions were measured to obtain accurate constants, and comparison with the present work does not seem profitable. ACKNOWLEDGMENTS One of us (M. C. L. G.) thanks the National Research Council of Canada for support in the form of research grants. One of us (G. W.) is grateful to Dr. C. C. Costain and Dr. M. Winnewisser for the hospitality of their laboratories. The work of G. W. was in part supported by the Deutsche Forschungsgemeinschaft; this support is gratefully acknowledged. RECEIVED:
March 5, 1973 REFERENCES
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