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Methyl internal rotation, dipole moment, and nuclear quadrupole coupling of 2-bromopropane
JOURNAL
OF MOLECULAR
SPECTROSCOPY
(1992)
151,243-259
Methyl Internal Rotation, Dipole Moment, and Nuclear Quadrupole Coupling of 2-Bromopropane MICHAELMEYER,WOLFGANGSTAHL,ANDHELMUTDREIZLER Abteilung Chemische Physik, Institut ftir Physikalische Chemie der Universitiit Kiel, Olshausenstr. 40-60, D-2300 Kiel, Germany
Themicrowave spectrum of 2-bromopropane has been investigated to determine nuclear quadrupole and spin-rotation coupling constants of bromine for the ‘%r and s’Br species by microwave Fourier transform spectroscopy. The dipole moment was found to be 2.194 D. From excited torsional states the parameters V, and V ‘,*of the torsional potential function were obtained. 0 1992 Academic Press. Inc.
INTRODUCTION
The microwave spectrum of 2-bromopropane, (CHs)&ZHBr, was assigned for the first time by Schwendeman and Tobiason ( I). They determined the heavy-atom geometry and the diagonal elements of the nuclear quadrupole coupling tensor with respect to the principal axes of inertia. Ikeda et al. (2) carried on the hyperIme structure analysis in order to calculate the elements of the coupling tensor in their principal axis system and presented a complete substitution structure. We reinvestigated the spectrum in the torsional ground state to determine the dipole moment. This required more accurate hype&e parameters. The accuracy of the microwave Fourier transform (MWFI) technique also made possible the determination of the diagonal elements of the spin-rotation coupling tensor. Our interest in the problem of internal rotation led us to the assignment of the rotational transitions in excited torsional states since it was not possible to resolve the torsional fine structure of the ground state transitions. The parameters V, and V’,z , of the torsional potential function were determined from these measurements.
EXPERIMENTAL
DETAILS
The sample of 2-bromopropane was obtained from EGA-Chemie, Steinheim. For the measurements waveguide MWFT spectrometers (3) were used from 5 to 36 GHz. The sample pressure was kept in the order of 1 mTorr (0.13 Pa) and the temperature ranged from 220 to 250 K. In the case of narrow multiplets caused by nuclear quadrupole hyperline structure and additional torsional fine structure a refinement (4) of the measured frequencies was necessary. The dipole moment was determined utilizing an improved (5) molecular beam MWFT spectrometer of the Balle and Flygare (6) type, supplemented with Stark
243
0022-2852192 $3.00 Copyright 0 1992 by Academic Press. Inc. All rights of reproduction in any form reserved.
244
MEYER,
STAHL,
AND DREIZLER
TABLE I Rotational Transitions in the Torsional Ground Stateof 2-Bromopropane (CH,)2CH79Br J
Km
1
0
2
0
2
1
2
1
3
0
3
1
3
1
3
2
3
2
a)
K+ -
difference
3’
K1
K;
ZF
ZF’
5
3 3 3 5 3 1 5 3 1 5 3 1 5 3 1 5 3 1 3 1 7 5 3 1 7 5 3 7 5 3 1 7 5 3 7 5 3 1 7 5 3 7 5 3 1 7 5 3 7 5 3 1 3
3 1 7 5 3 5 3 1 1
5 3 5 3 1 7 5 3 3 1 9 7 5 3 7 5 3 9 7 5 3 7 5 3 9 7 5 3 7 5 3 9 7 5 3 7 5 3 9 7 5 3 3 between
observed
obs.
/HHz
(CH3)2CHB’Br
6/kHz'
5189.503 5309.613 5093.765 10363.071 10361.923 10494.283 10482.039
-3 4 0 3 -4 8 9
10374.672 11019.305 11139.917 10996.112 11086.204 11092.790 10930.435 9775.493 9896.426 9739.280 9858.353 9686.177 15423.398 15421.393 15451.063 15453.616 15540.347 15367.568 15334.014 14661.239 14690.348 14689.827 14659.739 14744.908 14651.758 14606.645 16525.107 16554.649 16557.168 16527.700 16621.557 16510.057 16462.035 15814.681 15937.268 15850.400 15729.533 15818.918 15934.676 15849.054 15605.463 15725.577 15640.627 15519.941 15641.548
2 3 10 1 6 -7 -8 8 -6 13 12 4 1 -2 -3 -8 10 -2 -5 3 1 2 -1 -2 0 -1 -2 -4 0 -7 8 0 -3 -6 2 -3 10 3 7 -1 -6 -7 0 2 -6
and calculated
obs./NHz
b/kHz
5155.630 5255.966 5075.599 10290.921 10289.964 10400.377 10390.301 10220.021 10300.544 10939.819 11040.504
0 5 0 4 -5 0 1 6 7 1 -2
10995.617 11001.122 10865.377 9713.524 9814.446 9683.069 9782.595 9638.630 15317.635 15316.027 15340.878 15342.879 15415.395 15270.912
7 0 2 -1 -7 -16 0 5 2 9 5 -2 -7 -7
14564.391 14588.794 14588.347 14563.270 14634.268 14556.497 14518.810 16401.945 16426.683 16428.761 16404.197 16482.477 16389.387 16349.255 15700.854 15803.171 15730.627 15629.608 15704.200 15801.115
2 2 -4 -3 -7 4 -3 0 0 -5 0 2 6 -2 -5 -1 6 -1 -10 9
15497.848 15598.182 15527.146 15426.338 15527.821
-1 -2 -2 -2 -2
frequencies.
THE
MlCROWAVE
SPECTRUM
245
OF Z-BROMOPROPANE
TABLE I-Continued tCHx)zCH7*Br J
K-
K+
- J'
KI
K;
2F
2F'
11 9 7 5 11 9 7 5 11 9 7 5 13 11 9 7 13 11 9 7 15 13 11 9 15 13 11 9 15 13 11 9 17 15 13 11 17 15 13 11 11 15 13 11 11 9 7 5 15 13 11 9 17 15 13 11
9
obs./NHz
(CH3)2CH*'Br
b/kHz'
obs./HHz
4 -5 0 -6 -3 -4 -1 2 -2 -3 6 2 -3 -2 0 5 -3 -2 -2 0 2 2 12 -3 6 3 3 5 1 2 6 6 -12 -10 -9 -2
20203.052 20201.075 20212.973 20214.789 19373.024 19382.906 19387.337 19378.241 21810.772 21819.884 21825.967 21816.103 24936.736 24935.067 24941.895 24944.133 24140.016 24144.223 24148.533 24143.581 32398.721 32400.441 32404.286 32402.484
-2 1 3 2 0 1 1 0 0 0 3 5 1 -6 4 -4 -1 0 -1 0 3 3 -1 5
34073.608 34072.334 34075.580 34076.803 33551.664 33552.907 33555.453 33554.136 6113.133 6122.992 6119.647 6108.899
0 0 -2 -2 -1 1 1 -1 4 -13 -5 -12
6/kHz
4
0
4
3
0
3
4
1
4
3
1
3
4
1
3
3
1
2
5
0
5
4
0
4
5
1
5
4
1
4
6
1
5
5
1
4
6
2
5
5
2
4
6
1
6
5
1
5
2
6
6
2
5
0
7
6
0
6
1
7
6
1
6
1
3
4
1
4
1
5
6
1
6
1
6
7
1
7
7 5 3 9 7 5 3 9 7 5 3 11 9 7 5 11 9 7 5 13 11 9 7 13 11 9 7 13 11 9 7 15 13 11 9 15 13 11 9 15 13 11 9 11 3 7 5 15 13 11 9 17 15 13 11
20338.025 20335.542 20349.198 20351.984 19502.072 19513.894 19519.183 19508.332 21974.342 21985.215 21992.505 21980.713 25097.482 25095.532 25103.617 25106.445 24299.713 24304.728 24309.880 24303,965 32633.668 32635.675 32640.300 32638.155 31032.753 31046.204 31046.055 31033.370 29054.558 29057.257 29061.186 29058.292 36076.650 36084.352 36085.638 36077.567
33769.556 33771.027 33774.073 33772.503 6199.937 6211.755 6207.140 6194.850 12831.643 12842.638 12840.084 12828.809 16850.026 16859.460 16857.949 16847.354
2 2 3 3 5 6 2 -4 3 9 -3 -4 -8 9 -1 -3
MEYER,
246
STAHL,
AND DREIZLER
TABLE I-Continued (CH3)zCH79Br J
K-
K+
7
2
5
- J' 7
KL
K;
2F
2F'
2
6
17 15 13 11 25 23 21 19 27 25 23 21 27 25 23 21 29 27 25 23 33 31 29 27 33 31 29 27 35 33 31 29 37 35 33 31 41 39 37 35 41 39 37 35 45 43 41 39 45 43 41 39 45 43 41 39
17 15 13 11 25 23 21 19 27 25 23 21 27 25 23 21 29 27 25 23 33 31 29 27 33 31 29 27 35 33 31 29 37 35 33 31 41 39 37 35 41 39 37 35 45 43 41 39 45 43 41 39 45 43 41 39
11
3
a
11
3
9
12
2
10
12
2
11
12
3
9
12
3
10
13
3
10
13
3
11
15
4
11
15
4
12
15
3
12
If
3
13
16
4
12
16
4
13
17
4
13
17
4
14
19
5
14
19
5
15
19
4
15
19
4
16
21
5
16
21
5
17
21
5
17
21
4
17
21
4
17
21
4
18
obs./HHz
b/kHza
5400.715 5478.559 5467.393 6322.460 6331.484 6330.336 6321.212 25195.616 25205.921 25204.705 25194.334
1 3 3 -3 1 3 2 0 -7 1 0 7
13160.085 13171.094 13169.913 13158.768 6599.908 6607.309 6606.609 6599.152 22504.050 22515.306 22514.205 22502.939
1 3 1 -3 2 0 -2 0 1 -2 -5 -6
13675.502 13685.275 13684.490 13674.557 6494.513 6500.569 6500.114 6494.039 23605.339 23616.117 23615.272 23604.509 13559.438 13567.829 13567.491 13558.624 19960.645 19948.005 19948.899 19961.522 35501.232 35511.366 35510.611 35500.562
16 0 -4 -2 -2 -5 1 6 0 -4 -4 0 -7 -12 1 13 -4 6 2 -6 3 -1 -4 -10
5469.773
(CH3)zCHelBr obs./HHz
24737.279 24745.940
b/kHz
24744.913 24736.186 9057.251 9065.654 9064.658 9056.196 12729.338 12738.427 12737.430 12728.274 6260.011 6265.973 6265.401 6259.392 21887.013 21896.380 21895.465 21886.090 9284.948 9292.057 9291.434 9284.267 13077.748 13085.761 13085.093 13077.007 6072.608 6077.415 6077.059 6072.235 22751.419 22760.331 22759.636 22750.725 12804.687 12811.629 12811.193 12804.176
6 0 2 1 0 3 -5 7 -5 11 -4 2 2 2 -5 -12 -3 -1 -1 -5 1 -6 4 0 6 8 -1 3 -4 -9 2 7 0 2 0 -4 -7 -14 0 24
34457.393 34465.876 34465.264 34456.805
4 -5 2 -2
THE MICROWAVE
SPECTRUM
247
OF 2-BROMOPROPANE
TABLE I-Continued (C!H3)2CH79Br J
Km
K,
- J'
K'
K;
8
2
7
8
1
7
9
2
8
9
1
8
3
9
11
2
9
11
14
3
12
14
2
12
15
3
13
15
2
13
20
4
17
20
3
17
21
4
18
21
3
18
22
5
18
22
4
18
2F 19 13 21 19 17 15 25 23 21 19 31 29 27 25 33 31 29 27 43 41 39 37 45 43 41 39 47 45 43 41
2F' 19 13 21 19 17 15 25 23 21 19 31 29 27 25 33 31 29 27 43 41 39 37 45 43 41 39 47 45 43 41
obs./HHz
6/kHz'
6865.424 6868.490 5243.916 5231.557 5233.487 5245.955 14241.259 14228.430 14230.792 14249.876 7003.906 6994.197 6995.181 7004.934 5113.094 5105.883 5106.584
;: 0 0 1 2 -3 -5 -5 -6 2
6121.467 6115.286 6115.750 6121.901
2 7 5 -7
16079.057 16068.205 16068.962 16079.772
(CHx)ZCHalBr obs./HHz
6/kHz
-1 -1 -3 -3 4 10
-5 -1 -10 -1
1293.551 7285.263 7286.108 7294.437 5366.305 5360.097 5360.692 5366.921
-4 2 0 1 0 3 0 1
4646.577 4642.636 4642.946 4646.841 16877.092 16867.869 16868.482 16877.712
-7 -5 12 -2 -11 17 -1 -3
plates ( 7). The beam was prepared from a sample containing 2% 2-bromopropane argon. A backing pressure of 250 Torr (33 kPa) was used throughout. TORSIONAL
GROUND
in
STATE
The spectrum of 2-bromopropane shows strong a- and weak c-type transitions. The high sensitivity of the spectrometers enabled us to detect c-type transitions of this molecule for the first time. The measured lines of both species (CHj)$ZH 79Br and ( CHj )2CH *’ Br are listed in Table I. For the evaluation of the spectroscopic parameters we used the program HFS written by Gripp for the investigation of the spectrum of 2-iodopropane (8). The rotational and quartic centrifugal distortion constants based on the Hamiltonian given by van Eijck (9) are listed in Table II together with bromine quadrupole and spin-rotation coupling constants. The representation I’ wasused.The accuracy of the rotational constant A’ was substantially improved by the inclusion of c-type transitions. The derived hyperfme structure parameters in Table III confirm the results of Ikeda et al. (2). The angle between the axes a and z of the inertia and coupling tensors respectively does not change noticeably under the isotopic substitution.
248
MEYER,
STAHL,
AND DREIZLER
TABLE II Spectroscopic Parametersof2-Bromopropaneinthe TorsionalGround State
A'
/MHZ
8039.1110
(7)'
8039.0122
(15)
B'
/MHZ
2917.7666
(2)
2894.4914
(2)
C'
/MHZ
2295.3167
(2)
2280.8920
(2)
DJ'
/kHZ
0.6374
123)
0.6227
(25)
DJK'
/kHz
2.5781
(74)
2.520
(13)
DK'
/kHz
3.657
(76)
3.92
(12)
bJ'
/kHz
-0.15860
(16)
R6j
/kHz
-0.034901(76)
XBB X-
-0.15527
(25)
-0.03416
(12)
/MHz
479.8908
157)
400.9616
(59)
b /NH.?_
-49.9939
(78)
-41.6992
(99)
176.89
(16)
147.93
(19)
X,,
/MHz
Caa
/kHz
2.5
(14)
4.2
(16)
Cbb
/kHz
6.69
(39)
6.29
(42)
ccc
/kHz
6.12
(39)
5.54
(43)
a=
5
/kHz
a)
single
standard
b)
X- = Xt,t,- Xcc.
c)
standard
error
5
in units
of the
last
digit.
deviation.
For the determination of the dipole moment of ( CH3)*CH 7YBr the spacing of the Stark plates was calibrated with OCS using the dipole moment 0.7 15 19 D ( 10). We diagonalized the truncated Stark Hamiltonian matrix and fitted the components 1pa 1 and 1p,J to the measured frequencies of Table IV. This required the consideration of spin-rotation interaction and centrifugal distortion in order to avoid systematic errors. With the spectroscopic parameters of Table II being fixed the components of the dipolemomentweredeterminedtobe 11~~1=2.163(2)Dand 1~~1 =0.370(11)D. The standard deviation of the fit was 3 kHz. The total dipole moment is JJ = 2.194 ( 3) D. A comparison of the dipole moments of related molecules is given in Table V. The decrease of (p, 1 from 2Aluoropropane to 2-bromopropane explains the increasing difficulty of the measurement of c-type transitions.
THE MICROWAVE
SPECTRUM
OF 2-BROMOPROPANE
249
TABLE III Nuclear Quadrupole Coupling Constants and Derived Parameters of 2-Bromopropane in the Torsional Ground State
X aa
/MHZ
479.8908
(57)
400.9616
(59)
Xbe*Xxx
/NHz
-264.9423
(48)
-221.3304
(58)
Xcc
/NH.2
-214.9485
(48)
-179.6312
(58)
176.89
(16)
147.93
(19)
X
-257.3916
(764)
-215.1496
(871)
522.3339
(765)
436.4800
(872)
-0.01446
(15)
-0.01416
(20)
13.492
(11)
13.501
(15)
1.19704
(4)
1.19634
( 60)
1.19670
30)
XXb
/MHZ
-265.10
(41)
-221.81
(38)
XYYb
/MHZ
-258.10
(269)
-214.66
(298)
XZZb
/NH2
523.20
(269)
436.47
(299)
a)
asymmetry
b) Ref.
parameter
q = (Xxx
- x yy)/Xzz.
(~2).
EXCITED
TORSIONAL
STATES
The excited torsional states were assigned for both isotopic species. We started searching in a frequency range aroung the J’ + J = 1 + 2 a-type transitions of the ground state for satellites showing a similar hyperfme pattern, determined an initial set of spectroscopic parameters and proceeded with a search for lines with higher values of J. We could detect only the stronger a-type transitions. Often the measurements were interfered with strong lines nearby. We found that due to the high barrier of internal rotation the torsional fine structure of low J transitions could not be resolved. These lines, compiled in Table VI and VII, were used for the determination of the spectroscopic parameters in Table VIII and IX. A few transitions above J = 18 showed the typical triplet pattern of a two-top molecule with a high barrier for each hyperfme component which confirmed our assignment to the excited torsional states.
250
MEYER, STAHL, AND DREIZLER TABLE IV Stark Effect Measurements of ( CH3)2CH7pBr E/Vcm-1
J
K-
K+
21.79
2
1
2
21.53
- J' 1
K!
K;
1
1
2
1
2
1
1
1
2
1
1
1
1
0
2
1
2
1
1
1
2
1
1
1
1
0
108.97
2
0
2
1
0
1
145.33
2
0
2
1
0
1
211.94
2
0
2
1
0
1
290.59
2
0
2
1
0
1
363.31
2
0
2
1
0
1
36.40
a) difference
between
observed
and
2F 7
7 7 5 5 3 7 7 7 7 7 3 7 7 7 5 5 7 7 3 7 7 7 5 5 7 7 7 5 7 3 3 1 1
2F'
2Mp
5 5 5 3 3 1 5 5 5 5 5 1 5 5 5 3 3 5 5 1 5 5 5 3 3 5 5 5 3 5 1 1 1 1
1 3 5 1 3 1 1 3 1 3 5 1 1 3 5 1 3 1 3 1 1 3 5 1 3 1 3 5 3 3 1 1 1 1
calculated
obs./NHz 9775.589 9775.626 9775.692 9896.613 9896.514 9739.430 9775.671 9775.734 11019.175 11019.098 11018.942 10995.924 9775.783 9775.877 9776.076 9896.946 9896.661 11019.108 11018.980 10995.819 10362.944 10363.074 10363.319 10362.195 10361.736 10362.851 10363.080 10363.518 10361.586 10363.112 10494.220 10494.217 10374.646 10314.703
a/kHza 3 1 5 1 0 2 7 5 3 2 4 -4 3 3 -1 -5 -1 3 7 -5 3 0 3 0 0 1 2 2 2 -1 0 -1 1 -4
frequencies.
TABLE V Dipole Moments from Stark Effect Measurements
(CHx)zCHza
0.083
(CHx)zCHF"
1.880
0.083 0.541
1.958
(C!Hx)zCH'5C!l=
2.099
0.423
2.141
(C!H,)zCH79Brd
2.163
0.370
2.194
a) Ref.
(Is).
b) Ref.
(16).
c) Ref.
(G).
d) this
work.
251
THE MICROWAVE SPECTRUM OF 2-BROMOPROPANE TABLE VI Rotational Transitions of (CHp)$H 79Brin the Excited Torsional States q = 1 and q = 2 with Unresolved Torsional Fine Structure q-2
q-1 J
Km
K+ - J'
2
0
2
K1
K;
2F 7 5
2F' 5 3
2 3
1 0
2 3
7
5
3
1
3
2
1
3
1
2
2
1
3
2
1
2
2
3 4
2 1
2 3
2 3
2 1
4
0
4
3
0
3
4
1
4
3
1
3
5
0
5
4
0
4
5
1
5
4
1
4
6
1
6
5
1
5
7
1
7
6
1
6
7
2
6
6
2
5
11
2
9
11
2
10
15
3
12
15
3
13
18
4
14
18
4
15
19
5
14
19
5
15
23
6
17
23
6
18
24
6
18
24
6
19
9 7 9 3 9 5 3 9 7 9 11 9 7 11 9 7 5 11 9 7 5 13 11 9 7 13 11 9 7 15 13 11 9 17 15 13 11 17 15 13 11 25 23 21 19 31 29 37 35 41 39 37 35 49 47 45 43 51 49 47 45
7 5 7 1 7 3 1 7 5 7 9 7 5 9 7 5 3 9 7 5 3 11 9 7 5 11 9 7 5 13 11 9 7 15 13 11 9 15 13 11 9 25 23 21 19 31 29 37 35 41 39 37 35 49 47 45 43 51 49 47 45
obs./HHz
6/kHz'
10346.433 10345.299 9759.888 15398.815 15396.822 14637.961 14636.518 16498.218 16530.278 16500.812 15788.897 15911.525 15580.279 21938.692 21949.567 21956.850 20305.878 20303.407
7 2 -5 11 -3 5 26 9 -1 -3 1 14 5 6 -5 -6 9 -9
20319.854 19471.204 19483.032 19488.328 19477.474 25058.131 25056.184 25064.268 25067.088 24261.342 24266.367 24271.518 24265.589 29008.788 29011.462 29015.439 29012.526 33716.482 33717.954 33721.004 33719.428 36019.077 36026.789 36028.079 36019.992 20404.288 20415.568 20414.136 20402.699 22642.463 22641.370
-7 7 -1 -5 -7 9 -5 -12 -7 1 2 -12 -21 4 -11 12 -9 16 8 3 -5 3 2 -1 -8 8 6 -4 -10 1 5
6464.474 6470.496 6470.046 6463.997 6110.451 6115.220 6115.048 6110.002 9077.338 9083.493 9083.169 9076.902
-1 -2 5 -4 7 -3 1 -7 -13 18 -23 20
obs./HHz
b/kHz
10343.893 10342.735 9755.811 15393.740 15391.729 14631.518 14629.995 16497.953 16530.046
6 -3 11 4 2 16 -24 0 16
15707.921 15910.560 15577.266 21937.622 21948.504 21955.788 20297.065 20294.566 20308.830 20311.032 19462.081 19473.889 19479.197 19468.336 25044.678 25042.721 25050.802 25053.638 24249.232 24254.260 24259.423 24253.502
-4 2 14 7 -2 2 6 2 -4 -11 6 -3 5 6 10 5 0 -16 -2 0 12 -19
33697.668 33699.138 33702.179 33700.609 36008.032 36015.129 36017.026 36008.959
13 10 -1 -11 1 -6 0 -3
18514.995 18514.098 6613.139 6619.300 6618.832 6612.649 6285.061 6289.987 6289.787 6284.622
4 -11 5 0 0 0 1 2 -8 2
9324.452 9324.171
-3 8
252
MEYER, STAHL, AND DREIZLER TABLE VII Rotational Transitions of (CH,)&Hs’Br in the Excited Torsional States q = 1 and q = 2 with Unresolved Torsional Fine Structure q=2
g-1
J
K-
K,
- J'
K’
K; 1 2
2F 7
9 7 9
9
4
0
4
3
0
3
3 9 7 9 7 11 9 7 5 11 9 1
4
1
3
3
1
2
5
1
5
4
1
4
5
0
5
4
0
4
6
1
5
5
1
4
7
0
7
6
0
6
7
1
7
6
1
6
19
5
14
19
5
15
20
5
15
20
5
16
23
6
17
23
6
18
24
6
18
24
6
19
5 11 9 7 5 11 9 7 13 11 9 7 15 13 11 9 17 15 13 11 17 15 13 11 41 39 37 35 43 41 39 37 49 47 45 43 51 49 47 45
2F' 5 7 5 7 7 1 7 5 7 5 9 7 5 3 9 7 5 3 9 7 5 3 9 7 5 11 9 7 5 13 11 9 7 15 13 11 9 15 13 11 9 41 39 37 35 43 41 39 37 49 47 45 43 51 49 47 45
ohs./HHz
b/kHza
9698.047 15293.218 15291.609 14541.263 16375.250 16377.511 15675.275 15777.623 15472.841 15573.225 19342.345 19352.235 19356.658
6 9 4 -2 4 -6 10 4 6 12 -2 -5 -20
20171.107 20169.134 20181.036 20182.854 21775.385 21784.498 21790.580 21780.705 24106.101 24110.415 24105.451 24897.608 24895.950 24902.770 24905.014 32346.611 32348.343 32352.184 32350.394
7 3 0 -1 9 1 -5 -11 2 -5 -14 9 3 -3 -6 0 1 -13 4
34019.309 34022.547 34023.780 33498.913 33500.161 33502.702 33501.384 6044.506 6049.297 6048.936 6044.121 8985.856 8991.792 8991.343 8985.395 5629.359 5633.107 5632.977 5629.015 8406.676 8411.546 8411.287 8406.287
8 -9 -1 11 11 -2 -8 8 3 5 0 9 1 -20 -8 -8 2 -16 -14 7 22 -9 -15
obs./MHz
b/kHz
15288.025 15286.406 14534.627 16374.905
15 2 7 3 19
15674.114 15776.466 15469.666 15570.039 19332.993 19342.860 19347.303 19338.193 20162.171 20160.166 20172.071 20173.899 21774.182 21783.304 21789.390 21779.542 24093.706 24098.008 24093.073 24883.996 24882.329 24889.141 24891.406 32341.168 32342.873 32346.734 32344.938 33997.823 33996.527 33999.772 34001.008 33479.700 33480.939 33483.479 33482.173 6186.215 6191.106 6190.735 6185.826 9180.351 9186.386 9185.966 9179.887 5793.363 5797.221 5797.069 5793.006 8634.903 8634.534
-4 2 0 12 9 -2 0 -11 10 -2 -6 -11 2 1 1 -10 4 -6 -10 4 1 -9 -11 5 -4 -1 -7 24 -3 -10 -2 10 6 -6 -6 6 -3 -2 2 8 -10 5 -5 -3 9 -11 -1 -2 4
9693.836
253
THE MICROWAVE SPECTRUM OF 2-BROMOPROPANE TABLE VIII SpectroscopicParameters of (CH3)ICH”Br in Excited Torsional States
A’
lMHz
8029.329
B’
/MHZ
2912.965
C’
/MHZ
2291.728
DJ ’
/kHz
0.636
(10)
0.6472
(52)
DJX'
/kHz
2.73
(37)
2.398
(99)
DK'
/kHz
4.8
(31)
2.72
(87)
6.l'
/kHz
-0.1542
(27)
-0.1549
(21)
R.5'
/kHz
-0.0380
(90)
-0.0287
(23)
Xea
/MHz
480.035
(461
479.981
(39)
X_
/MHZ
-49.558
(96)
-50.205
(48)
Xac
/MHZ
176.44
(57)
177.56
(53)
B
/kHz
a)
see
footnotes
10
of Table
8011.480
(18)
(1)
2913.503
(1)
(1)
2290.181
(1)
156)
9
II.
It was not possible to accomplish the further assignment to the individual first excited states q = 1 and q = 2 without ambiguity. In contrast to the molecules measured so
far the torsional splittings for rotational transitions corresponding to different torsional states, here q = 1 and q = 2, are the same within the accuracy of the measurements. The measured splittings are close to the resolution limit. An accurate measurement of relative intensities to distinguish between both excited states was impossible. The nuclear spin statistical weights ( 11) do not provide any help for our assignment. Therefore we had to transfer the q-assignment from 2-chloropropane (12) by a comparison of the rotational constants. The theory for the internal rotation analysis has been outlined previously ( 13). We used a semirigid model with a Hamiltonian set up in the principal axis system. The eigenvalues were calculated using a diagonalization of appropriately truncated matrices. Since the direct approach with a basis set consisting of symmetric rotor functions and symmetry-adapted exponential functions corresponding to the limit of tsvo free internal rotors leads to unmanagable matrix dimensions we carried out prediagonalizations. First the uncoupled single-top internal rotation problem was solved. Then the matrix of all operators being independent of the quantum number J and diagonal in K was set up and diagonalized for each symmetry species and all necessary values of K.
254
MEYER, STAHL, AND DREIZLER TABLE IX $.wztroscopic Parameters of (CH,)FH
*‘Br in Excited Torsional States
¶=I
912
A'
/HHz
8029.159
B'
/MHz
C'
/MHz
DJ'
/kHz
0.6174
(90)
0.6352
(84)
DJK'
/kHz
2.83
(26)
2.03
(25)
DK'
/kHz
5.8
(20)
3.5
(18)
6J'
/kHz
-0.1515
(44)
-0.1495
(41)
R6'
/kHz
-0.0401
(46)
-0.0254
(41)
KS,
/MHZ
401.103
(34)
401.022
(32)
X-
/MHZ
-41.567
(77)
-41.814
(78)
X,,
/MHZ
147.67
(62)
148.20
(54)
(r
/kHz
a)
see
(34)
8011.630
(30)
2889.729
(1)
2890.253
(1)
2277.325
(1)
2215.140
(1)
10
footnotes
of Table
9
II.
Finally the matrices of the total Hamiltonian were diagonalized with a subsequent calculation of transition frequencies and linestrengths. For the analysis the moment of inertia 1, and the angles between the symmetry axis i and principal axes of inertia were calculated from the substitution structure (2). We used the same parameters for both species since the angle L b, i is invariant under isotopic substitution in the czc-plane and from our hyperhne structure analysis, Table III, we conclude that practically no rotation of the principal axis system with respect to the molecule occurs. The moments of inertia I,, Ib, and 1, were fixed to the ground state values. The small variation of the moments of inertia with the torsional state has a negligible influence on the internal rotation parameters. The measured internal rotation splittings relative to the A4 component of Table X and XI were averaged for the hyperflne structure of each rotational transition separately. On the basis of this treatment for each isotopic species a quite reasonable fit of the parameters V, and I/ ‘r2of the torsional potential function Eq. ( 1) to the averaged splittings in Table XII is possible: V(al, (Ye)= ;T/3( 1 - cos ICY,)+ &( + &(
1 - cos 3~2) + &(
1 - cos 6a,) 1 - cos 6~~2)
+ Vi*(cos 3aicos 3CQ- 1) + V;*sin 3crlsin 3az
( 1)
255
THE MICROWAVE SPECTRUM OF 2-BROMOPROPANE TABLE X Transitions of (CH,)ZCH’~B~ in Excited Torsional States with Torsional Fine Structure q-1 J
K-
19
4
K,
15
-
J'
K’
19
4
K;
2F
2F'
16
41
41
obs. /HHz
Ga AA EE
23
30
32
34
a)
5
7
a
a
symmetry
18
23
24
26
23
30
32
34
species
5
7
a
a
19
24
25
21
of
39
39
37
31
35
35
49
49
41
41
45
45
43
43
63
63
61
61
59
59
51
57
61
67
65
65
63
63
61
61
11
71
69
69
61
67
65
65
group
CT
EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE
X C~V
(II).
23147.992
23748.043 23748.093 23758.104 23758.151 23758.199 23757.418 23757.468 23157.519 23741.371 23741.426 23747.474 16655.137 16655.190 16655.246 16662.719 16662.768 16662.822 16662.287 16662.340 16662.389 16654.814 16654.867 16654.920 7566.090 7566.121 7566.153
7565.860 7565.891 1565.922
q-2 obs./HHz 23805.96d 23806.011 23806.052 23816.793 23816.834 23816.874 23815.947 23815.989 23816.032 23805.142 23805.179 23805.219 23413.661 23413.711 23413.758 23423.864 23423.913 23423.962 23423.168 23423.216 23423.266 23413.045 23413.092 23413.142 17106.041 17106.102 17106.157
17105.741 17105.790 17105.847 7853.788 7853.819 7853.851 7858.124 7858.157 7858.191 7857.957 7857.989 7858.020 7853.533 7853.565 1053.599
15353.776 15353.831 15353.886 15860.614 15860.669 15860.726 15860.389 15860.445 15860.498 15353.240 15353.296 15353.349
256
MEYER, STAHL, AND DREIZLER TABLE XI Rotational Transitions of ( CH3)2CH *‘Br in the Excited Torsional States q = 1 and q = 2 with Torsional Fine Structure g-1 J
K-
K+
- J'
19
4
15
19
-
-
KL
K;
2F
2F'
4
16
41
41
39
39
37
37
G' AA EE
EA,AE AA EE
-
obs./NHz 22680.895 22680.936 22680.916
23
32
5
8
18
24
23
32
5
8
19
25
35
35
49
49
47
41
45
45
43
43
61
61
65
65
63
63
61
61
EE
34
8
26
34
8
27
69
61
69
61
EA,AE AA EE EA,AE AA
EE EA,AE a)
see
footnote
of Table
obs./HHz 22952.396 22952.433 22952.472
22961.341 22961.384 22961.421 22960.641
EA,AE
AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA
9-2
22960.681
22680.202 22680.242 22680.281 22653.864 22853.910 22853.951
22661.599
22661.645 22661.692 22653.340 22653.389 22653.438 6797.313 6191.403 6197.431 6800.605 6800.631 6800.662 6800.470 6800.493 6800.525
6197.221 6797.244 6197.218 13990.320
22680.118 22951.692 22951.734 22951.114 23014.068 23014.115 23014.162 23022.435 23022.483 23022.529 23021.878
23021.922 23021.968 23013.541 23013.593 23013.640 7061.749 1061.778 7061.808 7065.103 1065.128 7065.155 1064.951 1064.979 1065.008 1061.577 1061.606 1061.636
13990.311 13990.422 13990.689 13990.140 13990.791
X.
The results of the analysis are given in Table XIII. The remaining parameters V, and V,, of Eq. ( 1) could not be determined. DISCUSSION OF THE INTERNAL ROTATION
The limited experimental information for the internal rotation analysis of 2-bromopropane forced us to introduce some assumptions. The internal rotation parameters
257
THE MICROWAVE SPECTRUM OF 2-BROMOPROPANE TABLE XII Averaged Internal Rotation Splittings Referred to the AA-Species of 2-Bromopropane in Excited Torsional States
J 19
K_
K,
- J'
KI
K;
q
Ga
4
15
19
4
15
1
EE EA,AE EE EA,AE EE EA,AE EE EA,AE EE EA,AE EE EA,AE EE EA,AE EE EA,AE EE EA,AE EE EA,AE
1
23
5
la
23
5
19
30
7
23
30
7
24
32
a
24
32
a
25
34
a
a)
see
26
34
footnote
a
2 2 1 1 2 2 1 1 2 2 1 1 2 2 1 1 2 2
27
of Table
(CH,)zCH79Br obs./kHz 6/kHz
-41 -82 -49 -99 -49 -98 -52 -105 -52 -108 -31 -63 -32 -65 -56 -110 -56 -111
-2 -5 -1 -2 4 7 1 1 2 0 -3 -7 -1 -3 -4 -6 0 1
(CHIj2CH81Br obs./kHz 6/kHz -41 -80 -41 -17
-41 -95 -46 -93
-4 -5 0 4 -2 -4 3 6
-21
2
-51
0
-28 -51 -51 -102
2 3 -2 -3
X
determined are consistent with those of related molecules given in Table XIV. The nearly equal torsional fine structure of 2-bromopropane in both excited states is due to the very high barrier leading to small splittings compared to those of other molecules. The coupling terms removing the degeneracy between the first two excited states are small compared to those of related molecules. As expected the barrier parameter I’, increases from propane to 2-bromopropane. Especially the difference between V3 of 2-chloropropane and 2-fluoropropane is large. For 2-iodopropane I’, = 17.28 kJ / mol determined by FIR spectroscopy ( 14) is of similar magnitude as for 2-bromopropane. No excited torsional states have been assigned so far in the microwave spectrum of 2-iodopropane. An estimation of the splittings for the first excited states based on the barrier and the molecular structure (2) shows that the splittings are even smaller than those of 2-bromopropane and it is unlikely to find resolvable transitions. The potential coupling with Vi, has a maximum in the case of 2-fluoropropane. It is possible to determine this parameter from the first excited torsional states q = 1 and q = 2 but not from the ground state spectra q = 0. The coupling parameter I’,, has been fixed to zero since its determination requires the investigation of higher torsional states. From the harmonic approximation of the potential function Eq. ( 1 ),
the high correlation of V3, Y12,and V6 is obvious because these parameters enter in the same factor. Therefore V3obtained from spectra of low torsional states should be
258
MEYER, STAHL, AND DREIZLER TABLE XIII Internal Rotation Parameters of 2-Bromopropane (CHs) zCH79Br
(CH3)zCHerBr
/O
64.34]a
64.341
Lb,i
/O
34.481
34.481
Lc,i
/o
68.611
68.611
I.
/amu
[3.166]
[3.166]
V3
/kJ
mol-1
V12’
/kJ
mol-'
FC
/GHz
F'=
/GHz
L
a,i
A2 b
a)
/kHz fixed
b)lJ=
17.28
(3)
-0.410
(4)
-0.410
(4)
1163.5)
f-0.1531
[-0.1641
117.0
117.8
(2)
4
parameters 4.184
c) derived
(3)
[163.5]
SC
0
17.19
in squared
(2)
4 brackets.
cal.
parameters.
TABLE XIV Internal Rotation Parameters Obtained by Microwave Spectroscopy q Vp/kJ
mol-1
Vlz'/kJ
mol-1
F/GHz
F'/GHz
I./amu
A2
185.9
-22.68
3.214
186.8
-24.02
3.2146
167.6
-4.62
3.1826
0
13.23
1,2
13.45
0
13.91
1,2
13.87
-1.356
167.6
-4.62
3.1826
1,2
16.44
-0.773
162.1
-1.23
3.222d
1,2
16.61
-0.837
163.8
-1.24
3.186d
(CHX)2CH79BrC
1,2
17.19
-0.410
163.5
-0.15
3.166d
(CHI)2CHelBrC
1,2
17.28
-0.410
163.5
-0.16
3.1666
a) Ref.
b) Ref.
(CH3)zCHza
(CH,)zCHFa
(CH,)2CH'*Clb
(13).
(12).
-0.665
c) this
work.
d)
assumed.
THE
MICROWAVE
SPECTRUM
OF 2-BROMOPROPANE
259
considered as an effective value. The kinetic coupling term F’ is determinable from the microwave spectra since it depends on the same structural parameters as the internal rotation constant F. In favorable cases such as propane the kinetic coefficients F and F’ can be derived directly from the microwave spectra; in other cases an estimation of these parameters from the molecular structure is possible with reasonable accuracy. ACKNOWLEDGMENTS Financial support by the Deutsche ForschungsgemeinschatI, the Fonds der Chemie and the Land SchleswigHolstein is gratefully acknowledged. J.-U. Grabow assisted us in the Stark effect measurements. The calculations were carried out at the computer center of the University of Kiel.
RECEIVED:
August
6, 1991 REFERENCES
I. R. H. SCHWENDEMAN AND F. L. TOBIASON,J. Chern. Phys. 43,201-205 ( 1965 ). 2. C. IKEDA, T. INAGUSA,AND M. HAYASHI,J. Mol. Spectrosc. 135334-348 ( 1989). 3. M. KRUGERAND H. DREIZLER,Z. Naturforsch. A: Phys. Phys. Chem. Kosmophys. 45,724-726 ( 1990) and citations therein. 4. J. HAEKELAND H. MKDER,Z. Naturjbrsch. A: Phys. Phys. Chem. Kosmophys. 43,203-206 ( 1988). 5. U. ANDRESEN,H. DREIZLER,J.-U. GRABOW, AND W. STAHL,Rev. Sci. Instrum. 61,3694-3699 ( 1990). 6. T. J. BALLEAND W. H. FLYGARE,Rev. Sci. Instrum. 52, 33-45 ( 1981). 7. N. HEINEKING,W. STAHL,AND C. THOMSEN,J. Mol. Spectrosc. 146,402-408 ( 199 I). 8. J. GRIPPAND H. DREIZLER,Z. Naturforsch. A: Phys. Phys. Chem. Kosmophys. 45, 7 15-723 ( 1990). 9. B. P. VAN EIJCK,J. Mol. Spectrosc. 53,246-249 ( 1974). 10. J. M. L. H. REINARTZAND A. DYMANUS, Chem. Phys. Lett. 24,346-351 ( 1974). 11. H. DREIZLER,Z. Naturforsch. A: Phys. Phys. Chem. Kosmophys. 16, 1354-1367 ( 1961). 11. M. MEYER, J.-U. GRABOW, H. DREIZLER,AND H. D. RUDOLPH, J. Mol. Spectrosc. 151, 217-242 ( 1992). 13. M. MEYERAND H. DREIZLER,J. MO/. Spectrosc. 148, 310-323 ( 1991). 14. K. D. MOLLER,A. R. DE MEO, D. R. SMITH,AND L. H. LONM)N, J. Chem. Phys. 47, 2609-2616 (1967). 15. D. R. LIDE, J. Chem. Phys. 33, 1514-1518 (1960). 16. M. MEYERAND H. DREIZLER,Z. Naturforsch. A: Phys. Phys. Chem. Kosmophys. 43, 138-142 ( 1987).
OF MOLECULAR
SPECTROSCOPY
(1992)
151,243-259
Methyl Internal Rotation, Dipole Moment, and Nuclear Quadrupole Coupling of 2-Bromopropane MICHAELMEYER,WOLFGANGSTAHL,ANDHELMUTDREIZLER Abteilung Chemische Physik, Institut ftir Physikalische Chemie der Universitiit Kiel, Olshausenstr. 40-60, D-2300 Kiel, Germany
Themicrowave spectrum of 2-bromopropane has been investigated to determine nuclear quadrupole and spin-rotation coupling constants of bromine for the ‘%r and s’Br species by microwave Fourier transform spectroscopy. The dipole moment was found to be 2.194 D. From excited torsional states the parameters V, and V ‘,*of the torsional potential function were obtained. 0 1992 Academic Press. Inc.
INTRODUCTION
The microwave spectrum of 2-bromopropane, (CHs)&ZHBr, was assigned for the first time by Schwendeman and Tobiason ( I). They determined the heavy-atom geometry and the diagonal elements of the nuclear quadrupole coupling tensor with respect to the principal axes of inertia. Ikeda et al. (2) carried on the hyperIme structure analysis in order to calculate the elements of the coupling tensor in their principal axis system and presented a complete substitution structure. We reinvestigated the spectrum in the torsional ground state to determine the dipole moment. This required more accurate hype&e parameters. The accuracy of the microwave Fourier transform (MWFI) technique also made possible the determination of the diagonal elements of the spin-rotation coupling tensor. Our interest in the problem of internal rotation led us to the assignment of the rotational transitions in excited torsional states since it was not possible to resolve the torsional fine structure of the ground state transitions. The parameters V, and V’,z , of the torsional potential function were determined from these measurements.
EXPERIMENTAL
DETAILS
The sample of 2-bromopropane was obtained from EGA-Chemie, Steinheim. For the measurements waveguide MWFT spectrometers (3) were used from 5 to 36 GHz. The sample pressure was kept in the order of 1 mTorr (0.13 Pa) and the temperature ranged from 220 to 250 K. In the case of narrow multiplets caused by nuclear quadrupole hyperline structure and additional torsional fine structure a refinement (4) of the measured frequencies was necessary. The dipole moment was determined utilizing an improved (5) molecular beam MWFT spectrometer of the Balle and Flygare (6) type, supplemented with Stark
243
0022-2852192 $3.00 Copyright 0 1992 by Academic Press. Inc. All rights of reproduction in any form reserved.
244
MEYER,
STAHL,
AND DREIZLER
TABLE I Rotational Transitions in the Torsional Ground Stateof 2-Bromopropane (CH,)2CH79Br J
Km
1
0
2
0
2
1
2
1
3
0
3
1
3
1
3
2
3
2
a)
K+ -
difference
3’
K1
K;
ZF
ZF’
5
3 3 3 5 3 1 5 3 1 5 3 1 5 3 1 5 3 1 3 1 7 5 3 1 7 5 3 7 5 3 1 7 5 3 7 5 3 1 7 5 3 7 5 3 1 7 5 3 7 5 3 1 3
3 1 7 5 3 5 3 1 1
5 3 5 3 1 7 5 3 3 1 9 7 5 3 7 5 3 9 7 5 3 7 5 3 9 7 5 3 7 5 3 9 7 5 3 7 5 3 9 7 5 3 3 between
observed
obs.
/HHz
(CH3)2CHB’Br
6/kHz'
5189.503 5309.613 5093.765 10363.071 10361.923 10494.283 10482.039
-3 4 0 3 -4 8 9
10374.672 11019.305 11139.917 10996.112 11086.204 11092.790 10930.435 9775.493 9896.426 9739.280 9858.353 9686.177 15423.398 15421.393 15451.063 15453.616 15540.347 15367.568 15334.014 14661.239 14690.348 14689.827 14659.739 14744.908 14651.758 14606.645 16525.107 16554.649 16557.168 16527.700 16621.557 16510.057 16462.035 15814.681 15937.268 15850.400 15729.533 15818.918 15934.676 15849.054 15605.463 15725.577 15640.627 15519.941 15641.548
2 3 10 1 6 -7 -8 8 -6 13 12 4 1 -2 -3 -8 10 -2 -5 3 1 2 -1 -2 0 -1 -2 -4 0 -7 8 0 -3 -6 2 -3 10 3 7 -1 -6 -7 0 2 -6
and calculated
obs./NHz
b/kHz
5155.630 5255.966 5075.599 10290.921 10289.964 10400.377 10390.301 10220.021 10300.544 10939.819 11040.504
0 5 0 4 -5 0 1 6 7 1 -2
10995.617 11001.122 10865.377 9713.524 9814.446 9683.069 9782.595 9638.630 15317.635 15316.027 15340.878 15342.879 15415.395 15270.912
7 0 2 -1 -7 -16 0 5 2 9 5 -2 -7 -7
14564.391 14588.794 14588.347 14563.270 14634.268 14556.497 14518.810 16401.945 16426.683 16428.761 16404.197 16482.477 16389.387 16349.255 15700.854 15803.171 15730.627 15629.608 15704.200 15801.115
2 2 -4 -3 -7 4 -3 0 0 -5 0 2 6 -2 -5 -1 6 -1 -10 9
15497.848 15598.182 15527.146 15426.338 15527.821
-1 -2 -2 -2 -2
frequencies.
THE
MlCROWAVE
SPECTRUM
245
OF Z-BROMOPROPANE
TABLE I-Continued tCHx)zCH7*Br J
K-
K+
- J'
KI
K;
2F
2F'
11 9 7 5 11 9 7 5 11 9 7 5 13 11 9 7 13 11 9 7 15 13 11 9 15 13 11 9 15 13 11 9 17 15 13 11 17 15 13 11 11 15 13 11 11 9 7 5 15 13 11 9 17 15 13 11
9
obs./NHz
(CH3)2CH*'Br
b/kHz'
obs./HHz
4 -5 0 -6 -3 -4 -1 2 -2 -3 6 2 -3 -2 0 5 -3 -2 -2 0 2 2 12 -3 6 3 3 5 1 2 6 6 -12 -10 -9 -2
20203.052 20201.075 20212.973 20214.789 19373.024 19382.906 19387.337 19378.241 21810.772 21819.884 21825.967 21816.103 24936.736 24935.067 24941.895 24944.133 24140.016 24144.223 24148.533 24143.581 32398.721 32400.441 32404.286 32402.484
-2 1 3 2 0 1 1 0 0 0 3 5 1 -6 4 -4 -1 0 -1 0 3 3 -1 5
34073.608 34072.334 34075.580 34076.803 33551.664 33552.907 33555.453 33554.136 6113.133 6122.992 6119.647 6108.899
0 0 -2 -2 -1 1 1 -1 4 -13 -5 -12
6/kHz
4
0
4
3
0
3
4
1
4
3
1
3
4
1
3
3
1
2
5
0
5
4
0
4
5
1
5
4
1
4
6
1
5
5
1
4
6
2
5
5
2
4
6
1
6
5
1
5
2
6
6
2
5
0
7
6
0
6
1
7
6
1
6
1
3
4
1
4
1
5
6
1
6
1
6
7
1
7
7 5 3 9 7 5 3 9 7 5 3 11 9 7 5 11 9 7 5 13 11 9 7 13 11 9 7 13 11 9 7 15 13 11 9 15 13 11 9 15 13 11 9 11 3 7 5 15 13 11 9 17 15 13 11
20338.025 20335.542 20349.198 20351.984 19502.072 19513.894 19519.183 19508.332 21974.342 21985.215 21992.505 21980.713 25097.482 25095.532 25103.617 25106.445 24299.713 24304.728 24309.880 24303,965 32633.668 32635.675 32640.300 32638.155 31032.753 31046.204 31046.055 31033.370 29054.558 29057.257 29061.186 29058.292 36076.650 36084.352 36085.638 36077.567
33769.556 33771.027 33774.073 33772.503 6199.937 6211.755 6207.140 6194.850 12831.643 12842.638 12840.084 12828.809 16850.026 16859.460 16857.949 16847.354
2 2 3 3 5 6 2 -4 3 9 -3 -4 -8 9 -1 -3
MEYER,
246
STAHL,
AND DREIZLER
TABLE I-Continued (CH3)zCH79Br J
K-
K+
7
2
5
- J' 7
KL
K;
2F
2F'
2
6
17 15 13 11 25 23 21 19 27 25 23 21 27 25 23 21 29 27 25 23 33 31 29 27 33 31 29 27 35 33 31 29 37 35 33 31 41 39 37 35 41 39 37 35 45 43 41 39 45 43 41 39 45 43 41 39
17 15 13 11 25 23 21 19 27 25 23 21 27 25 23 21 29 27 25 23 33 31 29 27 33 31 29 27 35 33 31 29 37 35 33 31 41 39 37 35 41 39 37 35 45 43 41 39 45 43 41 39 45 43 41 39
11
3
a
11
3
9
12
2
10
12
2
11
12
3
9
12
3
10
13
3
10
13
3
11
15
4
11
15
4
12
15
3
12
If
3
13
16
4
12
16
4
13
17
4
13
17
4
14
19
5
14
19
5
15
19
4
15
19
4
16
21
5
16
21
5
17
21
5
17
21
4
17
21
4
17
21
4
18
obs./HHz
b/kHza
5400.715 5478.559 5467.393 6322.460 6331.484 6330.336 6321.212 25195.616 25205.921 25204.705 25194.334
1 3 3 -3 1 3 2 0 -7 1 0 7
13160.085 13171.094 13169.913 13158.768 6599.908 6607.309 6606.609 6599.152 22504.050 22515.306 22514.205 22502.939
1 3 1 -3 2 0 -2 0 1 -2 -5 -6
13675.502 13685.275 13684.490 13674.557 6494.513 6500.569 6500.114 6494.039 23605.339 23616.117 23615.272 23604.509 13559.438 13567.829 13567.491 13558.624 19960.645 19948.005 19948.899 19961.522 35501.232 35511.366 35510.611 35500.562
16 0 -4 -2 -2 -5 1 6 0 -4 -4 0 -7 -12 1 13 -4 6 2 -6 3 -1 -4 -10
5469.773
(CH3)zCHelBr obs./HHz
24737.279 24745.940
b/kHz
24744.913 24736.186 9057.251 9065.654 9064.658 9056.196 12729.338 12738.427 12737.430 12728.274 6260.011 6265.973 6265.401 6259.392 21887.013 21896.380 21895.465 21886.090 9284.948 9292.057 9291.434 9284.267 13077.748 13085.761 13085.093 13077.007 6072.608 6077.415 6077.059 6072.235 22751.419 22760.331 22759.636 22750.725 12804.687 12811.629 12811.193 12804.176
6 0 2 1 0 3 -5 7 -5 11 -4 2 2 2 -5 -12 -3 -1 -1 -5 1 -6 4 0 6 8 -1 3 -4 -9 2 7 0 2 0 -4 -7 -14 0 24
34457.393 34465.876 34465.264 34456.805
4 -5 2 -2
THE MICROWAVE
SPECTRUM
247
OF 2-BROMOPROPANE
TABLE I-Continued (C!H3)2CH79Br J
Km
K,
- J'
K'
K;
8
2
7
8
1
7
9
2
8
9
1
8
3
9
11
2
9
11
14
3
12
14
2
12
15
3
13
15
2
13
20
4
17
20
3
17
21
4
18
21
3
18
22
5
18
22
4
18
2F 19 13 21 19 17 15 25 23 21 19 31 29 27 25 33 31 29 27 43 41 39 37 45 43 41 39 47 45 43 41
2F' 19 13 21 19 17 15 25 23 21 19 31 29 27 25 33 31 29 27 43 41 39 37 45 43 41 39 47 45 43 41
obs./HHz
6/kHz'
6865.424 6868.490 5243.916 5231.557 5233.487 5245.955 14241.259 14228.430 14230.792 14249.876 7003.906 6994.197 6995.181 7004.934 5113.094 5105.883 5106.584
;: 0 0 1 2 -3 -5 -5 -6 2
6121.467 6115.286 6115.750 6121.901
2 7 5 -7
16079.057 16068.205 16068.962 16079.772
(CHx)ZCHalBr obs./HHz
6/kHz
-1 -1 -3 -3 4 10
-5 -1 -10 -1
1293.551 7285.263 7286.108 7294.437 5366.305 5360.097 5360.692 5366.921
-4 2 0 1 0 3 0 1
4646.577 4642.636 4642.946 4646.841 16877.092 16867.869 16868.482 16877.712
-7 -5 12 -2 -11 17 -1 -3
plates ( 7). The beam was prepared from a sample containing 2% 2-bromopropane argon. A backing pressure of 250 Torr (33 kPa) was used throughout. TORSIONAL
GROUND
in
STATE
The spectrum of 2-bromopropane shows strong a- and weak c-type transitions. The high sensitivity of the spectrometers enabled us to detect c-type transitions of this molecule for the first time. The measured lines of both species (CHj)$ZH 79Br and ( CHj )2CH *’ Br are listed in Table I. For the evaluation of the spectroscopic parameters we used the program HFS written by Gripp for the investigation of the spectrum of 2-iodopropane (8). The rotational and quartic centrifugal distortion constants based on the Hamiltonian given by van Eijck (9) are listed in Table II together with bromine quadrupole and spin-rotation coupling constants. The representation I’ wasused.The accuracy of the rotational constant A’ was substantially improved by the inclusion of c-type transitions. The derived hyperfme structure parameters in Table III confirm the results of Ikeda et al. (2). The angle between the axes a and z of the inertia and coupling tensors respectively does not change noticeably under the isotopic substitution.
248
MEYER,
STAHL,
AND DREIZLER
TABLE II Spectroscopic Parametersof2-Bromopropaneinthe TorsionalGround State
A'
/MHZ
8039.1110
(7)'
8039.0122
(15)
B'
/MHZ
2917.7666
(2)
2894.4914
(2)
C'
/MHZ
2295.3167
(2)
2280.8920
(2)
DJ'
/kHZ
0.6374
123)
0.6227
(25)
DJK'
/kHz
2.5781
(74)
2.520
(13)
DK'
/kHz
3.657
(76)
3.92
(12)
bJ'
/kHz
-0.15860
(16)
R6j
/kHz
-0.034901(76)
XBB X-
-0.15527
(25)
-0.03416
(12)
/MHz
479.8908
157)
400.9616
(59)
b /NH.?_
-49.9939
(78)
-41.6992
(99)
176.89
(16)
147.93
(19)
X,,
/MHz
Caa
/kHz
2.5
(14)
4.2
(16)
Cbb
/kHz
6.69
(39)
6.29
(42)
ccc
/kHz
6.12
(39)
5.54
(43)
a=
5
/kHz
a)
single
standard
b)
X- = Xt,t,- Xcc.
c)
standard
error
5
in units
of the
last
digit.
deviation.
For the determination of the dipole moment of ( CH3)*CH 7YBr the spacing of the Stark plates was calibrated with OCS using the dipole moment 0.7 15 19 D ( 10). We diagonalized the truncated Stark Hamiltonian matrix and fitted the components 1pa 1 and 1p,J to the measured frequencies of Table IV. This required the consideration of spin-rotation interaction and centrifugal distortion in order to avoid systematic errors. With the spectroscopic parameters of Table II being fixed the components of the dipolemomentweredeterminedtobe 11~~1=2.163(2)Dand 1~~1 =0.370(11)D. The standard deviation of the fit was 3 kHz. The total dipole moment is JJ = 2.194 ( 3) D. A comparison of the dipole moments of related molecules is given in Table V. The decrease of (p, 1 from 2Aluoropropane to 2-bromopropane explains the increasing difficulty of the measurement of c-type transitions.
THE MICROWAVE
SPECTRUM
OF 2-BROMOPROPANE
249
TABLE III Nuclear Quadrupole Coupling Constants and Derived Parameters of 2-Bromopropane in the Torsional Ground State
X aa
/MHZ
479.8908
(57)
400.9616
(59)
Xbe*Xxx
/NHz
-264.9423
(48)
-221.3304
(58)
Xcc
/NH.2
-214.9485
(48)
-179.6312
(58)
176.89
(16)
147.93
(19)
X
-257.3916
(764)
-215.1496
(871)
522.3339
(765)
436.4800
(872)
-0.01446
(15)
-0.01416
(20)
13.492
(11)
13.501
(15)
1.19704
(4)
1.19634
( 60)
1.19670
30)
XXb
/MHZ
-265.10
(41)
-221.81
(38)
XYYb
/MHZ
-258.10
(269)
-214.66
(298)
XZZb
/NH2
523.20
(269)
436.47
(299)
a)
asymmetry
b) Ref.
parameter
q = (Xxx
- x yy)/Xzz.
(~2).
EXCITED
TORSIONAL
STATES
The excited torsional states were assigned for both isotopic species. We started searching in a frequency range aroung the J’ + J = 1 + 2 a-type transitions of the ground state for satellites showing a similar hyperfme pattern, determined an initial set of spectroscopic parameters and proceeded with a search for lines with higher values of J. We could detect only the stronger a-type transitions. Often the measurements were interfered with strong lines nearby. We found that due to the high barrier of internal rotation the torsional fine structure of low J transitions could not be resolved. These lines, compiled in Table VI and VII, were used for the determination of the spectroscopic parameters in Table VIII and IX. A few transitions above J = 18 showed the typical triplet pattern of a two-top molecule with a high barrier for each hyperfme component which confirmed our assignment to the excited torsional states.
250
MEYER, STAHL, AND DREIZLER TABLE IV Stark Effect Measurements of ( CH3)2CH7pBr E/Vcm-1
J
K-
K+
21.79
2
1
2
21.53
- J' 1
K!
K;
1
1
2
1
2
1
1
1
2
1
1
1
1
0
2
1
2
1
1
1
2
1
1
1
1
0
108.97
2
0
2
1
0
1
145.33
2
0
2
1
0
1
211.94
2
0
2
1
0
1
290.59
2
0
2
1
0
1
363.31
2
0
2
1
0
1
36.40
a) difference
between
observed
and
2F 7
7 7 5 5 3 7 7 7 7 7 3 7 7 7 5 5 7 7 3 7 7 7 5 5 7 7 7 5 7 3 3 1 1
2F'
2Mp
5 5 5 3 3 1 5 5 5 5 5 1 5 5 5 3 3 5 5 1 5 5 5 3 3 5 5 5 3 5 1 1 1 1
1 3 5 1 3 1 1 3 1 3 5 1 1 3 5 1 3 1 3 1 1 3 5 1 3 1 3 5 3 3 1 1 1 1
calculated
obs./NHz 9775.589 9775.626 9775.692 9896.613 9896.514 9739.430 9775.671 9775.734 11019.175 11019.098 11018.942 10995.924 9775.783 9775.877 9776.076 9896.946 9896.661 11019.108 11018.980 10995.819 10362.944 10363.074 10363.319 10362.195 10361.736 10362.851 10363.080 10363.518 10361.586 10363.112 10494.220 10494.217 10374.646 10314.703
a/kHza 3 1 5 1 0 2 7 5 3 2 4 -4 3 3 -1 -5 -1 3 7 -5 3 0 3 0 0 1 2 2 2 -1 0 -1 1 -4
frequencies.
TABLE V Dipole Moments from Stark Effect Measurements
(CHx)zCHza
0.083
(CHx)zCHF"
1.880
0.083 0.541
1.958
(C!Hx)zCH'5C!l=
2.099
0.423
2.141
(C!H,)zCH79Brd
2.163
0.370
2.194
a) Ref.
(Is).
b) Ref.
(16).
c) Ref.
(G).
d) this
work.
251
THE MICROWAVE SPECTRUM OF 2-BROMOPROPANE TABLE VI Rotational Transitions of (CHp)$H 79Brin the Excited Torsional States q = 1 and q = 2 with Unresolved Torsional Fine Structure q-2
q-1 J
Km
K+ - J'
2
0
2
K1
K;
2F 7 5
2F' 5 3
2 3
1 0
2 3
7
5
3
1
3
2
1
3
1
2
2
1
3
2
1
2
2
3 4
2 1
2 3
2 3
2 1
4
0
4
3
0
3
4
1
4
3
1
3
5
0
5
4
0
4
5
1
5
4
1
4
6
1
6
5
1
5
7
1
7
6
1
6
7
2
6
6
2
5
11
2
9
11
2
10
15
3
12
15
3
13
18
4
14
18
4
15
19
5
14
19
5
15
23
6
17
23
6
18
24
6
18
24
6
19
9 7 9 3 9 5 3 9 7 9 11 9 7 11 9 7 5 11 9 7 5 13 11 9 7 13 11 9 7 15 13 11 9 17 15 13 11 17 15 13 11 25 23 21 19 31 29 37 35 41 39 37 35 49 47 45 43 51 49 47 45
7 5 7 1 7 3 1 7 5 7 9 7 5 9 7 5 3 9 7 5 3 11 9 7 5 11 9 7 5 13 11 9 7 15 13 11 9 15 13 11 9 25 23 21 19 31 29 37 35 41 39 37 35 49 47 45 43 51 49 47 45
obs./HHz
6/kHz'
10346.433 10345.299 9759.888 15398.815 15396.822 14637.961 14636.518 16498.218 16530.278 16500.812 15788.897 15911.525 15580.279 21938.692 21949.567 21956.850 20305.878 20303.407
7 2 -5 11 -3 5 26 9 -1 -3 1 14 5 6 -5 -6 9 -9
20319.854 19471.204 19483.032 19488.328 19477.474 25058.131 25056.184 25064.268 25067.088 24261.342 24266.367 24271.518 24265.589 29008.788 29011.462 29015.439 29012.526 33716.482 33717.954 33721.004 33719.428 36019.077 36026.789 36028.079 36019.992 20404.288 20415.568 20414.136 20402.699 22642.463 22641.370
-7 7 -1 -5 -7 9 -5 -12 -7 1 2 -12 -21 4 -11 12 -9 16 8 3 -5 3 2 -1 -8 8 6 -4 -10 1 5
6464.474 6470.496 6470.046 6463.997 6110.451 6115.220 6115.048 6110.002 9077.338 9083.493 9083.169 9076.902
-1 -2 5 -4 7 -3 1 -7 -13 18 -23 20
obs./HHz
b/kHz
10343.893 10342.735 9755.811 15393.740 15391.729 14631.518 14629.995 16497.953 16530.046
6 -3 11 4 2 16 -24 0 16
15707.921 15910.560 15577.266 21937.622 21948.504 21955.788 20297.065 20294.566 20308.830 20311.032 19462.081 19473.889 19479.197 19468.336 25044.678 25042.721 25050.802 25053.638 24249.232 24254.260 24259.423 24253.502
-4 2 14 7 -2 2 6 2 -4 -11 6 -3 5 6 10 5 0 -16 -2 0 12 -19
33697.668 33699.138 33702.179 33700.609 36008.032 36015.129 36017.026 36008.959
13 10 -1 -11 1 -6 0 -3
18514.995 18514.098 6613.139 6619.300 6618.832 6612.649 6285.061 6289.987 6289.787 6284.622
4 -11 5 0 0 0 1 2 -8 2
9324.452 9324.171
-3 8
252
MEYER, STAHL, AND DREIZLER TABLE VII Rotational Transitions of (CH,)&Hs’Br in the Excited Torsional States q = 1 and q = 2 with Unresolved Torsional Fine Structure q=2
g-1
J
K-
K,
- J'
K’
K; 1 2
2F 7
9 7 9
9
4
0
4
3
0
3
3 9 7 9 7 11 9 7 5 11 9 1
4
1
3
3
1
2
5
1
5
4
1
4
5
0
5
4
0
4
6
1
5
5
1
4
7
0
7
6
0
6
7
1
7
6
1
6
19
5
14
19
5
15
20
5
15
20
5
16
23
6
17
23
6
18
24
6
18
24
6
19
5 11 9 7 5 11 9 7 13 11 9 7 15 13 11 9 17 15 13 11 17 15 13 11 41 39 37 35 43 41 39 37 49 47 45 43 51 49 47 45
2F' 5 7 5 7 7 1 7 5 7 5 9 7 5 3 9 7 5 3 9 7 5 3 9 7 5 11 9 7 5 13 11 9 7 15 13 11 9 15 13 11 9 41 39 37 35 43 41 39 37 49 47 45 43 51 49 47 45
ohs./HHz
b/kHza
9698.047 15293.218 15291.609 14541.263 16375.250 16377.511 15675.275 15777.623 15472.841 15573.225 19342.345 19352.235 19356.658
6 9 4 -2 4 -6 10 4 6 12 -2 -5 -20
20171.107 20169.134 20181.036 20182.854 21775.385 21784.498 21790.580 21780.705 24106.101 24110.415 24105.451 24897.608 24895.950 24902.770 24905.014 32346.611 32348.343 32352.184 32350.394
7 3 0 -1 9 1 -5 -11 2 -5 -14 9 3 -3 -6 0 1 -13 4
34019.309 34022.547 34023.780 33498.913 33500.161 33502.702 33501.384 6044.506 6049.297 6048.936 6044.121 8985.856 8991.792 8991.343 8985.395 5629.359 5633.107 5632.977 5629.015 8406.676 8411.546 8411.287 8406.287
8 -9 -1 11 11 -2 -8 8 3 5 0 9 1 -20 -8 -8 2 -16 -14 7 22 -9 -15
obs./MHz
b/kHz
15288.025 15286.406 14534.627 16374.905
15 2 7 3 19
15674.114 15776.466 15469.666 15570.039 19332.993 19342.860 19347.303 19338.193 20162.171 20160.166 20172.071 20173.899 21774.182 21783.304 21789.390 21779.542 24093.706 24098.008 24093.073 24883.996 24882.329 24889.141 24891.406 32341.168 32342.873 32346.734 32344.938 33997.823 33996.527 33999.772 34001.008 33479.700 33480.939 33483.479 33482.173 6186.215 6191.106 6190.735 6185.826 9180.351 9186.386 9185.966 9179.887 5793.363 5797.221 5797.069 5793.006 8634.903 8634.534
-4 2 0 12 9 -2 0 -11 10 -2 -6 -11 2 1 1 -10 4 -6 -10 4 1 -9 -11 5 -4 -1 -7 24 -3 -10 -2 10 6 -6 -6 6 -3 -2 2 8 -10 5 -5 -3 9 -11 -1 -2 4
9693.836
253
THE MICROWAVE SPECTRUM OF 2-BROMOPROPANE TABLE VIII SpectroscopicParameters of (CH3)ICH”Br in Excited Torsional States
A’
lMHz
8029.329
B’
/MHZ
2912.965
C’
/MHZ
2291.728
DJ ’
/kHz
0.636
(10)
0.6472
(52)
DJX'
/kHz
2.73
(37)
2.398
(99)
DK'
/kHz
4.8
(31)
2.72
(87)
6.l'
/kHz
-0.1542
(27)
-0.1549
(21)
R.5'
/kHz
-0.0380
(90)
-0.0287
(23)
Xea
/MHz
480.035
(461
479.981
(39)
X_
/MHZ
-49.558
(96)
-50.205
(48)
Xac
/MHZ
176.44
(57)
177.56
(53)
B
/kHz
a)
see
footnotes
10
of Table
8011.480
(18)
(1)
2913.503
(1)
(1)
2290.181
(1)
156)
9
II.
It was not possible to accomplish the further assignment to the individual first excited states q = 1 and q = 2 without ambiguity. In contrast to the molecules measured so
far the torsional splittings for rotational transitions corresponding to different torsional states, here q = 1 and q = 2, are the same within the accuracy of the measurements. The measured splittings are close to the resolution limit. An accurate measurement of relative intensities to distinguish between both excited states was impossible. The nuclear spin statistical weights ( 11) do not provide any help for our assignment. Therefore we had to transfer the q-assignment from 2-chloropropane (12) by a comparison of the rotational constants. The theory for the internal rotation analysis has been outlined previously ( 13). We used a semirigid model with a Hamiltonian set up in the principal axis system. The eigenvalues were calculated using a diagonalization of appropriately truncated matrices. Since the direct approach with a basis set consisting of symmetric rotor functions and symmetry-adapted exponential functions corresponding to the limit of tsvo free internal rotors leads to unmanagable matrix dimensions we carried out prediagonalizations. First the uncoupled single-top internal rotation problem was solved. Then the matrix of all operators being independent of the quantum number J and diagonal in K was set up and diagonalized for each symmetry species and all necessary values of K.
254
MEYER, STAHL, AND DREIZLER TABLE IX $.wztroscopic Parameters of (CH,)FH
*‘Br in Excited Torsional States
¶=I
912
A'
/HHz
8029.159
B'
/MHz
C'
/MHz
DJ'
/kHz
0.6174
(90)
0.6352
(84)
DJK'
/kHz
2.83
(26)
2.03
(25)
DK'
/kHz
5.8
(20)
3.5
(18)
6J'
/kHz
-0.1515
(44)
-0.1495
(41)
R6'
/kHz
-0.0401
(46)
-0.0254
(41)
KS,
/MHZ
401.103
(34)
401.022
(32)
X-
/MHZ
-41.567
(77)
-41.814
(78)
X,,
/MHZ
147.67
(62)
148.20
(54)
(r
/kHz
a)
see
(34)
8011.630
(30)
2889.729
(1)
2890.253
(1)
2277.325
(1)
2215.140
(1)
10
footnotes
of Table
9
II.
Finally the matrices of the total Hamiltonian were diagonalized with a subsequent calculation of transition frequencies and linestrengths. For the analysis the moment of inertia 1, and the angles between the symmetry axis i and principal axes of inertia were calculated from the substitution structure (2). We used the same parameters for both species since the angle L b, i is invariant under isotopic substitution in the czc-plane and from our hyperhne structure analysis, Table III, we conclude that practically no rotation of the principal axis system with respect to the molecule occurs. The moments of inertia I,, Ib, and 1, were fixed to the ground state values. The small variation of the moments of inertia with the torsional state has a negligible influence on the internal rotation parameters. The measured internal rotation splittings relative to the A4 component of Table X and XI were averaged for the hyperflne structure of each rotational transition separately. On the basis of this treatment for each isotopic species a quite reasonable fit of the parameters V, and I/ ‘r2of the torsional potential function Eq. ( 1) to the averaged splittings in Table XII is possible: V(al, (Ye)= ;T/3( 1 - cos ICY,)+ &( + &(
1 - cos 3~2) + &(
1 - cos 6a,) 1 - cos 6~~2)
+ Vi*(cos 3aicos 3CQ- 1) + V;*sin 3crlsin 3az
( 1)
255
THE MICROWAVE SPECTRUM OF 2-BROMOPROPANE TABLE X Transitions of (CH,)ZCH’~B~ in Excited Torsional States with Torsional Fine Structure q-1 J
K-
19
4
K,
15
-
J'
K’
19
4
K;
2F
2F'
16
41
41
obs. /HHz
Ga AA EE
23
30
32
34
a)
5
7
a
a
symmetry
18
23
24
26
23
30
32
34
species
5
7
a
a
19
24
25
21
of
39
39
37
31
35
35
49
49
41
41
45
45
43
43
63
63
61
61
59
59
51
57
61
67
65
65
63
63
61
61
11
71
69
69
61
67
65
65
group
CT
EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE
X C~V
(II).
23147.992
23748.043 23748.093 23758.104 23758.151 23758.199 23757.418 23757.468 23157.519 23741.371 23741.426 23747.474 16655.137 16655.190 16655.246 16662.719 16662.768 16662.822 16662.287 16662.340 16662.389 16654.814 16654.867 16654.920 7566.090 7566.121 7566.153
7565.860 7565.891 1565.922
q-2 obs./HHz 23805.96d 23806.011 23806.052 23816.793 23816.834 23816.874 23815.947 23815.989 23816.032 23805.142 23805.179 23805.219 23413.661 23413.711 23413.758 23423.864 23423.913 23423.962 23423.168 23423.216 23423.266 23413.045 23413.092 23413.142 17106.041 17106.102 17106.157
17105.741 17105.790 17105.847 7853.788 7853.819 7853.851 7858.124 7858.157 7858.191 7857.957 7857.989 7858.020 7853.533 7853.565 1053.599
15353.776 15353.831 15353.886 15860.614 15860.669 15860.726 15860.389 15860.445 15860.498 15353.240 15353.296 15353.349
256
MEYER, STAHL, AND DREIZLER TABLE XI Rotational Transitions of ( CH3)2CH *‘Br in the Excited Torsional States q = 1 and q = 2 with Torsional Fine Structure g-1 J
K-
K+
- J'
19
4
15
19
-
-
KL
K;
2F
2F'
4
16
41
41
39
39
37
37
G' AA EE
EA,AE AA EE
-
obs./NHz 22680.895 22680.936 22680.916
23
32
5
8
18
24
23
32
5
8
19
25
35
35
49
49
47
41
45
45
43
43
61
61
65
65
63
63
61
61
EE
34
8
26
34
8
27
69
61
69
61
EA,AE AA EE EA,AE AA
EE EA,AE a)
see
footnote
of Table
obs./HHz 22952.396 22952.433 22952.472
22961.341 22961.384 22961.421 22960.641
EA,AE
AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA EE EA,AE AA
9-2
22960.681
22680.202 22680.242 22680.281 22653.864 22853.910 22853.951
22661.599
22661.645 22661.692 22653.340 22653.389 22653.438 6797.313 6191.403 6197.431 6800.605 6800.631 6800.662 6800.470 6800.493 6800.525
6197.221 6797.244 6197.218 13990.320
22680.118 22951.692 22951.734 22951.114 23014.068 23014.115 23014.162 23022.435 23022.483 23022.529 23021.878
23021.922 23021.968 23013.541 23013.593 23013.640 7061.749 1061.778 7061.808 7065.103 1065.128 7065.155 1064.951 1064.979 1065.008 1061.577 1061.606 1061.636
13990.311 13990.422 13990.689 13990.140 13990.791
X.
The results of the analysis are given in Table XIII. The remaining parameters V, and V,, of Eq. ( 1) could not be determined. DISCUSSION OF THE INTERNAL ROTATION
The limited experimental information for the internal rotation analysis of 2-bromopropane forced us to introduce some assumptions. The internal rotation parameters
257
THE MICROWAVE SPECTRUM OF 2-BROMOPROPANE TABLE XII Averaged Internal Rotation Splittings Referred to the AA-Species of 2-Bromopropane in Excited Torsional States
J 19
K_
K,
- J'
KI
K;
q
Ga
4
15
19
4
15
1
EE EA,AE EE EA,AE EE EA,AE EE EA,AE EE EA,AE EE EA,AE EE EA,AE EE EA,AE EE EA,AE EE EA,AE
1
23
5
la
23
5
19
30
7
23
30
7
24
32
a
24
32
a
25
34
a
a)
see
26
34
footnote
a
2 2 1 1 2 2 1 1 2 2 1 1 2 2 1 1 2 2
27
of Table
(CH,)zCH79Br obs./kHz 6/kHz
-41 -82 -49 -99 -49 -98 -52 -105 -52 -108 -31 -63 -32 -65 -56 -110 -56 -111
-2 -5 -1 -2 4 7 1 1 2 0 -3 -7 -1 -3 -4 -6 0 1
(CHIj2CH81Br obs./kHz 6/kHz -41 -80 -41 -17
-41 -95 -46 -93
-4 -5 0 4 -2 -4 3 6
-21
2
-51
0
-28 -51 -51 -102
2 3 -2 -3
X
determined are consistent with those of related molecules given in Table XIV. The nearly equal torsional fine structure of 2-bromopropane in both excited states is due to the very high barrier leading to small splittings compared to those of other molecules. The coupling terms removing the degeneracy between the first two excited states are small compared to those of related molecules. As expected the barrier parameter I’, increases from propane to 2-bromopropane. Especially the difference between V3 of 2-chloropropane and 2-fluoropropane is large. For 2-iodopropane I’, = 17.28 kJ / mol determined by FIR spectroscopy ( 14) is of similar magnitude as for 2-bromopropane. No excited torsional states have been assigned so far in the microwave spectrum of 2-iodopropane. An estimation of the splittings for the first excited states based on the barrier and the molecular structure (2) shows that the splittings are even smaller than those of 2-bromopropane and it is unlikely to find resolvable transitions. The potential coupling with Vi, has a maximum in the case of 2-fluoropropane. It is possible to determine this parameter from the first excited torsional states q = 1 and q = 2 but not from the ground state spectra q = 0. The coupling parameter I’,, has been fixed to zero since its determination requires the investigation of higher torsional states. From the harmonic approximation of the potential function Eq. ( 1 ),
the high correlation of V3, Y12,and V6 is obvious because these parameters enter in the same factor. Therefore V3obtained from spectra of low torsional states should be
258
MEYER, STAHL, AND DREIZLER TABLE XIII Internal Rotation Parameters of 2-Bromopropane (CHs) zCH79Br
(CH3)zCHerBr
/O
64.34]a
64.341
Lb,i
/O
34.481
34.481
Lc,i
/o
68.611
68.611
I.
/amu
[3.166]
[3.166]
V3
/kJ
mol-1
V12’
/kJ
mol-'
FC
/GHz
F'=
/GHz
L
a,i
A2 b
a)
/kHz fixed
b)lJ=
17.28
(3)
-0.410
(4)
-0.410
(4)
1163.5)
f-0.1531
[-0.1641
117.0
117.8
(2)
4
parameters 4.184
c) derived
(3)
[163.5]
SC
0
17.19
in squared
(2)
4 brackets.
cal.
parameters.
TABLE XIV Internal Rotation Parameters Obtained by Microwave Spectroscopy q Vp/kJ
mol-1
Vlz'/kJ
mol-1
F/GHz
F'/GHz
I./amu
A2
185.9
-22.68
3.214
186.8
-24.02
3.2146
167.6
-4.62
3.1826
0
13.23
1,2
13.45
0
13.91
1,2
13.87
-1.356
167.6
-4.62
3.1826
1,2
16.44
-0.773
162.1
-1.23
3.222d
1,2
16.61
-0.837
163.8
-1.24
3.186d
(CHX)2CH79BrC
1,2
17.19
-0.410
163.5
-0.15
3.166d
(CHI)2CHelBrC
1,2
17.28
-0.410
163.5
-0.16
3.1666
a) Ref.
b) Ref.
(CH3)zCHza
(CH,)zCHFa
(CH,)2CH'*Clb
(13).
(12).
-0.665
c) this
work.
d)
assumed.
THE
MICROWAVE
SPECTRUM
OF 2-BROMOPROPANE
259
considered as an effective value. The kinetic coupling term F’ is determinable from the microwave spectra since it depends on the same structural parameters as the internal rotation constant F. In favorable cases such as propane the kinetic coefficients F and F’ can be derived directly from the microwave spectra; in other cases an estimation of these parameters from the molecular structure is possible with reasonable accuracy. ACKNOWLEDGMENTS Financial support by the Deutsche ForschungsgemeinschatI, the Fonds der Chemie and the Land SchleswigHolstein is gratefully acknowledged. J.-U. Grabow assisted us in the Stark effect measurements. The calculations were carried out at the computer center of the University of Kiel.
RECEIVED:
August
6, 1991 REFERENCES
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