1. 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

2. Abstract
The rotational spectrum of 1-bromobutane measured by the 480 MHz bandwidth chirped-pulse 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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3. 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, (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, (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, (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), (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., , (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, (2007)
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4. ::: 1-bromobutane ::: Possible conformersaa ag ga
gg’ gg
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5. ::: 1-bromobutane ::: Summary of Previous Study
This Study
Con-former
B+C
(MHz)
Ratio
(%) ΔG
(kJ/mol) ΔE aa
1799.8(0.9) 36
22.9
0.079 ga
2307(24) 24
37.7
0.5
1.3
2.9
0.096 ag
1994.8(1.2)
21.3
1.93
4.2 -
2.561 gg 16
17.7
2.39
4.6
3.8
gg’
0.42
11.68
14.0
10.599
Method
LRMW
(b)
ED at RT
(a)
MP2/
6-311G(d,p)
(d)
HF/
6-31G*
(c)
ED at 18℃
FTMW
6-311+G
(2d,2p)
F. A. Momany et al., J. Am. Chem. Soc., 85, (1963)
W. E. Steinmetz et al., J. Mol. Spec., 68, (1977)
(c) K. Aarset et al., Struct. Chem., 6(3), (1995)
O. Takahashi et al., Chem. Phys. Lett., 440, (2007)
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6. ::: 1-bromobutane ::: Hyperfine Transitions+ -
Nuclear quadrupole moments
ΔF = +1
ΔF = 0
ΔF = -1
J=5
J=6
F=15/2
F=13/2
F=11/2
F= 9/2
F= 7/2
F=15/2
F=13/2
F=11/2
F= 9/2
F= 7/2
F=15/2
F=13/2
F=11/2
F= 9/2
F= 7/2
ΔJ=+1
2I+1 components  4 transitions
Very strong intensity
2I components  3 transitions
2I-1 components  2 transitions
Very weak intensity
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7. ::: 1-bromobutane ::: Hyperfine Splitting
F→F
F→F-1
ΔF = +1
F=15/2
F=13/2
F=11/2
F= 9/2
F= 7/2
F=15/2
F=13/2
F=11/2
F= 9/2
F= 7/2
J=5
J=6
F=15/2
F=13/2
F=11/2
F= 9/2
F= 7/2
4.7 MHz
F→F
2I components  3
F→F-1
2I-1 components  2
Very weak intensity
2I+1 components  4 possibilities
Very strong intensity
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8. ::: 1-bromobutane ::: Spectrum assignment
Br isotopic ratio 81Br: %
79Br: %
(a) ga conformer
(b) aa conformer
(c) gg conformer
79Br
81Br
(d) Experimental (7.5 ~ 18 GHz)
by JB95
(a) ga conformer μa=1.621 D, μb=1.257 D, μc=0.216 D a-type & b-type transition assigned
(b) aa conformer μa=2.179 D, μb=0.744 D, μc=0.000 D only a-type transition assigned
(c) gg conformer μa=1.705 D, μb=1.041 D, μc=0.478 D only a-type transition assigned
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9. ::: 1-bromobutane ::: Rotational constants
calc: ab initio at MP2/6-311+G(2d,2p) ga aa gg
79Br
81Br
A (MHz, calc)
B (MHz, calc)
C (MHz, calc)
A (MHz)
(7)
(6)
(9)
(1)
(2)
(8)
B (MHz)
(1)
(9)
(6)
(6)
(4)
(4)
C (MHz)
(8)
(8)
(7)
(6)
(4)
(4)
DJ (kHz)
0.3171(8)
0.3116(6)
0.0634(3)
(3)
1.5084(3)
1.4786(2)
DJK (kHz)
-5.639(1)
-5.675(9)
-3.091(1)
-3.004(9)
(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)
(1)
132.69(2)
χbb-χcc (MHz)
(6)
(6)
(1)
(1)
(5)
(7)
χab (MHz)
394.70(4)
329.92(4)
(1)
(2)
(3)
-280.1(4)
χac (MHz)
-72.26(2)
-59.81(2)
(3)
186.5(6)
χbc (MHz)
(2)
-74.04(1)
(8)
(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
-0.996
-0.955
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10. ::: 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
μtot (D)
2.309
2.303
0.006 N 49
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11. ::: 2-bromobutane ::: Structure & ab initio calculationG+ A G-
ΔE (kJ/mol)
MP2/6-311+G(2d,2p)
0.00
2.08
2.67
Dihedral angle
Calculated angle G+
+60°
+63.80° A
180°
174.57° G-
-60°
-67.03°
2-bromobutane ab initio calculation result with MP2/6-311+G(2d,2p)
Previous study
This study
Conformer
ΔE (kJ/mol) G+
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., , (2001)
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12. ::: 2-bromobutane ::: Spectrum assignment
Br isotopic ratio 81Br: %
79Br: %
(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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13. ::: 2-bromobutane ::: Rotational constants
calc: ab initio at MP2/6-311+G(2d,2p) G+ A G-
79Br
81Br
A (MHz, calc)
B (MHz, calc)
C (MHz, calc)
A (MHz)
(2)
(2)
(3)
(2)
(1)
(1)
B (MHz)
(2)
(2)
(4)
(3)
(1)
(9)
C (MHz)
(1)
(1)
(3)
(3)
(6)
(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)
(2)
(2)
(9)
(9)
(1)
(1)
χbb-χcc (MHz)
(3)
(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)
(2)
(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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14. ::: 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
G+ 79Br
G+ 81Br
5/2-5/2
-0.01
7/2-7/2
11/2-9/2
0.01
0.00
9/2-7/2
5/2-3/2
7/2-5/2
9/2-9/2
ΔF = 0
ΔF = 0
(Δν = 150 MHz)
ΔF = +1
(Δν = 10 MHz)
ΔF = 0
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15. ::: 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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16. 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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17. 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
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