International Symposium
on Molecular Spectroscopy
Microwave spectra of
1- and 2-bromobutane
Soohyun Ka, Jihyun Kim, Heesu Jang
and Jung Jin Oh
Research Institute of Global Environment
Sookmyung Women’s University, Korea
22 June 2015
Abstract
•
The rotational spectrum of 1-bromobutane measured by the 480 MHz bandwidth chirpedpulse Fourier transform microwave (CP-FTMW) spectrometer at the Eastern Illinois University.
In this paper, the ab initio calculation and the analysis of rotational spectrum were performed,
and the properties of gas molecule are reported.
•
1-bromobutane have five conformers; aa, ag, ga, gg, gg’. The transitions were assigned to
three different conformers which are most stable forms; aa, ag, ga. The spectra for the
normal isotopic species and 81Br substitution were observed and assigned.
•
The rotational spectrum of 2-bromobutane has been observed in the frequency region 7-18
GHz. 2-bromobutane has the three possible conformers; G+, A, G-. The difference of their
energy is very small, so the spectra of all conformers were found in the full range of our
spectrum.
•
Consequentially, the rotational constants, nuclear quadrupole constants, and centrifugal
distortion constants were determined and the dipole moment of the aa conformer with 79Br
were measured.
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Molecular Spectroscopy
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Previous Study
•
[1-BrBu] F. A. Momany, R. A. Bonham, and W. H. McCoy, Electron diffraction determination of the
molecular structure of the butyl halides. IV. The structure and conformational analysis of n-butyl
bromide in the gas phase, J. Am. Chem. Soc., 85, 3077-3080 (1963)
•
[1-ClBu] E. M. Belloot, JR. and E. Bright Wilson, Free energy of Conformational Changes in Vapors
from Low Resolution Microwave Spectroscopic Intensities, J. Mol. Spec., 66, 41-45 (1977)
•
[1-XBu] W. E. Steinmetz, F. Hickernell, and I. K. Mun, The conformational analysis of 1-Haloalkanes
by low resolution microwave spectroscopy, J. Mol. Spec., 68, 173-182 (1977)
•
[1-BrBu] K. Aarset, K. Hagen, R. Stølevik, and P. C. Sæbø, Molecular structure and conformational
composition of 1-chlorobutane, 1-bromobutane, and 1-iodobutane as determined by gas-phase
electron diffraction and ab initio calculations, Struct. Chem., 6(3), 197-205 (1995)
•
[2-BrBu] K. Aarset, K. Hagen, and R. Stølevik, Molecular structures and conformational
compositions of 2-chlorobutane and 2-bromobutane; an investigation using gas-phase electron
diffraction data and ab initio molecular orbital calculations, J. Mol. Struct., 567-568, 157-165
(2001)
•
[1-BrBu] O. Takahashi, K. Yamasaki, Y. Kohno, K. Ueda, H. Suezawa, and M. Nishio, Origin of the
gauche preference of n-propyl halides and related molecules investigated by ab initio MO
calculations: Importance of the CH/n hydrogen bond, Chem. Phys. Lett., 440, 64-49 (2007)
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::: 1-bromobutane :::
Possible conformers
aa
ag
ga
gg’
gg
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Molecular Spectroscopy
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::: 1-bromobutane :::
Summary of Previous Study
Previous Study
This Study
Conformer
B+C
(MHz)
Ratio
(%)
Ratio
(%)
ΔG
(kJ/mol)
ΔE
(kJ/mol)
ΔE
(kJ/mol)
B+C
(MHz)
B+C
(MHz)
ΔE
(kJ/mol)
aa
1799.8(0.9)
36
22.9
0
0
0
1798.7973
1799.9456
0.079
ga
2307(24)
24
37.7
0.5
1.3
2.9
2314.9399
2321.3071
0.096
ag
1994.8(1.2)
24
21.3
1.93
4.2
2.9
-
2004.3014
2.561
gg
-
16
17.7
2.39
4.6
3.8
2816.7008
2893.6109
0
gg’
-
-
0.42
11.68
14.0
-
-
2979.0515
10.599
Method
LRMW
(b)
ED at RT
(a)
MP2/
6-311G(d,p)
(d)
MP2/
6-311G(d,p)
(d)
HF/
6-31G*
(c)
EDat 18℃
(c)
FTMW
MP2/
6-311+G
(2d,2p)
MP2/
6-311+G
(2d,2p)
(a)
(b)
(c)
(d)
F. A. Momany et al., J. Am. Chem. Soc., 85, 3077-3080 (1963)
W. E. Steinmetz et al., J. Mol. Spec., 68, 173-182 (1977)
K. Aarset et al., Struct. Chem., 6(3), 197-205 (1995)
O. Takahashi et al., Chem. Phys. Lett., 440, 64-49 (2007)
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Molecular Spectroscopy
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::: 1-bromobutane :::
Hyperfine Transitions
-
+
I = 3/2
+
-
Nuclear quadrupole moments
ΔF = +1
J=6
ΔF = -1
ΔF = 0
F=15/2
F=15/2
F=15/2
F=13/2
F=13/2
F=13/2
F=11/2
F=11/2
F=11/2
F= 9/2
F= 9/2
F= 9/2
F=13/2
F=13/2
F=13/2
F=11/2
F=11/2
F=11/2
F= 9/2
F= 9/2
F= 9/2
F= 7/2
F= 7/2
F= 7/2
ΔJ=+1
J=5
•
•
2I+1 components  4 transitions
Very strong intensity
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Molecular Spectroscopy
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2I components  3 transitions
•
•
6
2I-1 components  2
transitions
Very weak intensity
::: 1-bromobutane :::
Hyperfine Splitting
F→F
F→F-1
ΔF = +1
F=15/2
F=13/2
J=6
F=11/2
F= 9/2
F=13/2
F=11/2
J=5
International Symposium on
Molecular Spectroscopy
F=13/2
F=13/2
F=11/2
F=11/2
F= 9/2
F= 9/2
F=13/2
F=13/2
F=11/2
F=11/2
F= 9/2
F= 7/2
F= 7/2
2I+1 components  4 possibilities
Very strong intensity
F=15/2
F= 9/24.7 MHz
F= 9/2
•
•
F=15/2
•
•
F→F
2I components  3
7
F= 7/2
•
•
•
F→F-1
2I-1 components  2
Very weak intensity
::: 1-bromobutane :::
Spectrum assignment
(a) ga conformer
81Br
79Br
Br isotopic ratio 81Br: 49.463%
79Br: 50.537%
(b) aa conformer
(c) gg conformer
(d) Experimental (7.5 ~ 18 GHz)
by JB95
(a) ga conformer
(b) aa conformer
(c) gg conformer
μa=1.621 D, μb=1.257 D, μc=0.216 D
μa=2.179 D, μb=0.744 D, μc=0.000 D
μa=1.705 D, μb=1.041 D, μc=0.478 D
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a-type & b-type transition assigned
only a-type transition assigned
only a-type transition assigned
::: 1-bromobutane :::
Rotational constants
ga
79Br
calc: ab initio at MP2/6-311+G(2d,2p)
aa
81Br
79Br
gg
81Br
79Br
81Br
A (MHz, calc)
8271.3308
16054.1124
6126.6379
B (MHz, calc)
1210.3063
915.1317
1508.2537
C (MHz, calc)
1111.0008
884.6139
1385.3572
A (MHz)
8196.3730(7)
8183.7297(6)
15770.66(9)
15760.27(1)
6240.618(2)
6235.48(8)
B (MHz)
1207.26281(1)
1197.95218(9)
914.89072(6)
907.47581(6)
1463.8266(4)
1452.1707(4)
C (MHz)
1107.67707(8)
1099.61264(8)
883.90660(7)
876.95290(6)
1352.8742(4)
1342.6930(4)
DJ (kHz)
0.3171(8)
0.3116(6)
0.0634(3)
0.05829(3)
1.5084(3)
1.4786(2)
DJK (kHz)
-5.639(1)
-5.675(9)
-3.091(1)
-3.004(9)
-12.775(2)
-13.00(1)
d1 (kHz)
0.0511(5)
0.0442(5)
0.278(4)
0.281(3)
χaa (MHz)
87.165(9)
73.833(9)
363.04(3)
303.67(3)
157.831(1)
132.69(2)
χbb-χcc (MHz)
420.2412(6)
350.2016(6)
184.904(1)
153.212(1)
141.976(5)
117.952(7)
χab (MHz)
394.70(4)
329.92(4)
-337.810(1)
-282.005(2)
-336.105(3)
-280.1(4)
χac (MHz)
-72.26(2)
-59.81(2)
221.615(3)
186.5(6)
χbc (MHz)
-87.377(2)
-74.04(1)
-172.68(8)
-144.24(7)
N
99(26)
105(28)
86(23)
98(26)
47(14)
35(9)
Δνrms (kHz)
3.58
5.12
6.08
5.10
3.90
5.71
κ (Kappa)
-0.972
-0.972
-0.996
-0.996
-0.955
-0.955
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::: 1-bromobutane :::
Dipole moments of aa conformer
• Observed (point) and calculated
(continuous lines) frequency shifts
for some of the observed
ΔMF=0 Stark components
in CH3CH2CH2CH279Br aa conformer
• As the aa conformer has symmetry plane,
it is expected the aa conformer would
possess non-zero μa and μb dipole
moment components.
aaobs
aacalc
Δobs-calc
μa (D)
2.229(13)
2.179
0.050
μb (D)
0.603(60)
0.744
-0.141
μc (D)
0.000
0.000
0.000
μtot (D)
2.309
2.303
0.006
N
49
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Molecular Spectroscopy
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::: 2-bromobutane :::
Structure & ab initio calculation
ΔE (kJ/mol)
A
G-
G+
2.67
2.08
0.00
Dihedral angle
Calculated angle
G+
+60°
+63.80°
A
180°
174.57°
G-
-60°
-67.03°
MP2/6-311+G(2d,2p)
 2-bromobutane ab initio calculation result with MP2/6-311+G(2d,2p)
Conformer
Previous study
ΔE (kJ/mol)
This study
ΔE (kJ/mol)
G+
0.00
0.00
A
1.77
2.08
G-
3.52
2.67
Method
HF/6-311+G(d,p) -(a)
MP2/6-311+G(2d,2p)
(a) K. Aarset, et al., J. Mol. Struct., 567-568, 157-165 (2001)
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::: 2-bromobutane :::
Spectrum assignment
Br isotopic ratio 81Br: 49.463%
79Br: 50.537%
(a) G+ conformer
(b) A conformer
(c) G- conformer
(d) Experimental (7.5 ~ 18 GHz)
(a) G+ conformer
μa=1.910 D μb=1.010 D μc=0.280 D
a-type & b-type transition assigned
(b) A conformer
μa=2.250 D μb=0.440 D μc=0.260 D
only a-type transition assigned
(c) G- conformer
μa=2.010 D μb=0.780 D μc=0.520 D
only a-type transition assigned
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::: 2-bromobutane :::
Rotational constants
G+
79Br
calc: ab initio at MP2/6-311+G(2d,2p)
A
81Br
79Br
G81Br
79Br
81Br
A (MHz, calc)
3882.291333
6534.080423
4595.988187
B (MHz, calc)
2277.22846
1553.409642
1993.058953
C (MHz, calc)
1526.229396
1319.320593
1630.532496
A (MHz)
3857.83420(2)
3853.30685(2)
6412.915(3)
6409.136(2)
4532.801(1)
4530.476(1)
B (MHz)
2272.32632(2)
2253.63508(2)
1555.2157(4)
1542.28983(3)
1998.3289(1)
1981.7974(9)
C (MHz)
1520.60969(1)
1511.52569(1)
1316.55295(3)
1307.12676(3)
1626.3843(6)
1615.1494(6)
ΔJ (kHz)
0.709(4)
0.697(4)
0.1470(3)
0.1576(1)
0.683(1)
0.640(9)
ΔJK (kHz)
-1.369(1)
-1.301(1)
0.47(3)
0.54(3)
-1.76(7)
-1.68(6)
δ1 (kHz)
0.2654(3)
0.2517(3)
0.027(3)
0.0244(2)
0.207(1)
0.197(9)
χaa (MHz)
300.1367(2)
253.8573(2)
446.043(9)
372.949(9)
317.134(1)
266.182(1)
χbb-χcc (MHz)
135.6128(3)
110.2848(3)
7.632(3)
5.884(3)
62.884(3)
51.616(3)
χab (MHz)
321.18(6)
266.64(9)
179.19(4)
149.08(5)
-274.8(6)
-230.8(7)
χac (MHz)
-140.93(2)
-117.00(2)
145.47(9)
121.79(8)
200.6(9)
165.2(1)
χbc (MHz)
-82.16(1)
-68.25(2)
36.62(3)
33.1(3)
-96.21(9)
-78.70(2)
N
135(25)
127(25)
59(15)
66(17)
35(9)
43(11)
Δνrms (kHz)
5.97
6.05
3.99
5.17
3.53
5.33
κ (Kappa)
-0.357
-0.366
-0.906
-0.908
-0.744
-0.748
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::: 2-bromobutane :::
Hyperfine splitting of G+ isomer
G+ conformer
Stable, strong intensity
F → F+1, F → F component
observed & assigned
G+ 79Br
G+ 81Br
Experimental
spectrum
404-303
G+ 79Br
G+ 81Br
5/2-5/2
13569.05095
-0.01
13503.34265
-0.01
7/2-7/2
13615.85930
-0.01
13542.54050
-0.01
11/2-9/2
13641.61370
0.01
13564.32606
0.00
9/2-7/2
13645.14445
0.01
13567.15612
0.00
5/2-3/2
13649.35960
0.00
13570.93795
0.00
7/2-5/2
13652.98260
0.00
13573.83347
0.00
9/2-9/2
13726.48555
-0.01
13635.47560
0.00
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ΔF = 0
ΔF = +1
(Δν = 10 MHz)
ΔF = 0
ΔF = 0
(Δν = 150 MHz)
::: 2-bromobutane :::
Dipole moments of G+ conformer
• Observed (point) and calculated
(continuous lines) frequency shifts
for some of the observed
ΔMF=0 Stark components
in CH3CH79BrCH2CH3 G+ conformer
• N is the number of distinct Stark
measurement
G+obs
G+calc
Δobs-calc
μa (D)
1.771(16)
1.910
-0.139
μb (D)
1.180(29)
1.010
0.170
μc (D)
0.412(17)
0.280
0.132
μtot (D)
2.168
2.180
-0.012
N
52
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CONCLUSION
•
The ab initio calculation and the measurement of the rotational spectra of 1- and
2-bromobutane were performed.
•
From the rotational spectrum analysis, the most stable conformer is determined to
be the ga conformer for the 1-bromobutane in this study.
•
Our experimental results are consistent with one of the previous studies that the
gauche conformer is most stable. The reason is that the distances between the
bromine and the closest hydrogen in the ga, gg and gg’ conformer (2.92~2.94 Å)
are remarkably shorter than van der waals distance (3.05 Å).
•
For the 2-bromobutane, the G+ isomer is the most stable and it is consistent with
ab initio calculation.
•
The measured dipole moments of 1- and 2-bromobutane show good agreement
with the ab initio calculation.
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Acknowledgement
• Dr. Prof. Sean A. Peebles
in Eastern Illinois University
• Dr. Prof. Rebecca A. Peebles
in Eastern Illinois University
• This research was supported by
Basic Science Research Program
funded by the Ministry of Science, ICT
and future Planning.
Photos from Dr. Prof. Sean A. Peebles Lab in EIU
http://www.ux1.eiu.edu/~cfsap/
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