1. Fourier transform microwave spectra of n-butanol and isobutanol
Taigo Uzuyama,1 Yoshiyuki Kawashima,1 and Eizi Hirota2
Kanagawa Institute of Technology1
The Graduate University for Advanced Studies2

2. Introduction isobutanethiol
G-g form
G-g’ form
T-g form
3 isomers among the 5 expected stable isomers were found.
　Yugo Tanaka, et. al, 　
n-butanethiol f2 q f1
G-g’-g’
T-g-g’
G-g’-g
G-t-g
G-t-g’
T-t-g
T-g-g
7 isomers among the 14 expected stable isomers were found.
　Yugo Tanaka, et. al, 　
None of trans form about the C-S bond for n-butanethiol and isobutanethiol was found.

3. Our aim X C4 C3 C2 C1 X C4 C3 C2 C1
X= oxygen or sulfur atom
In order to obtain information on stable conformers and internal motions of n-butanol and isobutanol comparing with their thiol, we have analyzed their rotational spectra observed by a FTMW spectrometer.
Detection, identification, and characterization of the rotational isomers of n-butanol and isobutanol
Comparison of experimental results with ab initio calculations
Comparison of the spectra observed using Ar and Ne as a carrier gas
Comparison of the stability of rotational isomers of butanol and isobutanol with their thiols.

4. 5 expected conformers of isobutanol
isobutanol (2-methyl-1-propanol ) f θ
trans
T-g
T-t
trans
gauche
gauche’ θ f
gauche
G-g
G-g’
G-t

5. 14 expected conformers of butanol① ② O f2 q C2 q C1 f1
T-t-g
trans
gauche
gauche’ ① ② ④ ⑤ ⑥ ⑦ ⑧ ⑨ ⑩ ⑪ ⑫ ⑬ ⑭ ③
T-t-t C4 C3 ③ ④ ⑤ f1 f2
T-g-t
T-g-g
T-g-g’ ⑥ ⑦ ⑧
G-t-t
G-t-g
G-t-g’ ⑨ ⑩ ⑪
G-g-t
G-g-g
G-g-g’ ⑫ ⑬ ⑭
G-g’-t
G-g’-g
G-g’-g’

6. Spectra of 7 conformation isomers Spectra of 3 conformation isomers
The previous works
Conformational study of 1-butanol by the combined use of vibrational spectroscopy
Spectra of 7 conformation isomers
Ar-Matrix
n-butanol IR
Keiichi Ohno, et. al, J. Phys. Chem. 1994,
98, 6924.
liquid
Study of rotational isomerism of iso-butanol by microwave spectroscopy
Spectra of 3 conformation isomers
iso-butanol MW
Abdurakhmanov, A. A, et. Al, 1989, Abstr.

7. Instrument ： Fourier transform microwave spectrometer
　　　　　　　　　　　　　　　　　　Experimental
Instrument ： Fourier transform microwave spectrometer
Sample ： isobutanol or n-butanol
Backing pressure : 1.0~3.0 atm　　　　
Carrier gas 　　 : Ar or Ne Shots ： 20~50
Frequency region : 7～24 GHz Step： 0.25 MHz
Vacuum chamber
Mirror (fixed)
Molecular beam injection nozzle MW MW
Reservoir
heater
Mirror (mobile)
(33~38 ºC)
Diffusion pump
Rotary pump

8. Observed rotational spectra of isobutanol diluted with Ar or Ne
7000
Set1　b- and c-type transitions
(a-type transitions were not observed) Ar
Frequency / MHz Ne
7000
Frequency / MHz

9. Observed rotational spectra of isobutanol diluted with Ar or Ne
Set4: a-type transitions
J=2-1
Frequency / MHz Ne
Set2 and Set3 and Set4: a-type transitions
J=3-2
J=2-1
Frequency / MHz

10. Assignment of the rotational isomers of isobutanol
Experimental
set1
set2
set3
set4
A / MHz
B / MHz
C / MHz
 /uÅ2
N(a-type)
N(b-type)
N(c-type)
(1)
(5)
(4) 15 23
(22)
(13)
(9) 11 7 24
(4)
(2)
(1) 10 4
(1)
(2)
(1) 9 12
MP2/6-311G++(d,p)
G-t
G-g
G-g’
T-t
T-g
a / D
b / D
c / D
DE/ cm-1
7651
3551
2682
-19.90
0.03
1.42
0.92
0.0
7565
3497
2661
-21.37
1.35
0.72
1.24
21.3
7564
3512
2645
-19.63
1.66
0.83
0.80
116.6
6244
4046
3237
-49.72
0.65
0.00
1.65
63.1
6230
3971
3170
-48.95
1.25
1.32
0.78
169.8

11. Observed rotational spectra of n-butanol diluted with Ar or Ne
Set1 Ar
Set2
Set3
13000
14000
15000
16000
Frequency / MHz
Set1 Ne
Set4
Set5
Set6
Set3
Set2
13000
16000
Frequency / MHz
6 sets of the 14 expected isomers were found for n-butanol.

12. Assignment of rotational isomers of n-butanol
Frequency / MHz O
C3H7 H
Tunneling splitting was observed for a-type transitions of the sixth isomer T-t-g.
set1
set2
set3
set4
set5
Experimental
set6
A / MHz
(81)
(10)
(16)
(31)
(53)
18715(49)
B / MHz
(14)
(34)
(34)
(52)
(13)
(37)
C / MHz
(14)
(31)
(26)
(51)
(12)
(37)
 / uÅ2
-22.82
-22.44
-12.87
-22.23
-24.15
-13.51
N(a-type) 18 16 12 19 18 15
N(b-type) 21 10 14 － 3 --
N(c-type) 15 5 – 7 5 --
MP2
G(d,p)
T-g-t
G-t-g
T-t-t
T-g-g’
T-g-g
T-t-g
A / MHz
12487
12240
18711
12652
12324
18520
B / MHz
2394
2357
1983
2338
2350
1965
C / MHz
2202
2157
1877
2155
2181
1866
 / uÅ2
-22.10
-21.42
-12.62　
-21.64
-24.45
-13.62
a / D
0.97
1.37
-0.11
-1.78
0.78
1.69
b / D
1.11
1.40
-1.84
0.25
1.30
-0.12
c / D
0.98
0.82
0.00
1.06
-1.09
1.21
DE/ cm-1
291
150
123 66
121

13. G-g-g’ G-g-g’ G-g-g G-g-t G-g-t G-g-g G-t-t G-t-t G-t-g’ T-g-t G-g’-g
Comparison of stability of various rotational isomers for n-butanol and n-butanethiol
732
* For n-butanol, five conformers were of T-form and one was of G-form.
G-g-g’
G-g-g’
1059
570
G-g-g
G-g-t
1026
498
The gauche’ conformation about the CC-CO axis is stabilized by a hydrogen bond between the hydroxyl oxygen and the hydrogen of the C(3) position.
G-g-t
G-g-g
953
352
G-t-t
G-t-t
481
347
G-t-g’
T-g-t
344
DE/ cm-1
292
G-g’-g
T-t-t
320
favorable
291
G-t-g
G-g’-t
237 C1 C1 C2 C2
150
-0.327
T-t-t
G-t-g’
213
2.56
147 C4 C3 C4 C3
G-g’-g’
G-t-g
165
0.015
123
T-g-g’
G-g’-g
134
121
Calculated distance /Å between the H and O atoms.
T-t-g
T-g-g’
128 66
T-g-g
G-g’-g’
116 62
natural bond orbital charge distribution
G-g’-t
T-g-g 96
T-g-t
T-t-g
* No C-S trans form was found for n-butanethiol.
n-butanol
n-butanethiol

14. Interpretation of the calculated results
2.69 C3 C2
2.74 C1 C4 C3
2.60 C3 C1 C1
2.71
2.55
2.61
T-g-g’
T-g-g
T-g-t
The calculated distances between O and H(C2) and O and H(C3) of the T-g-t form are Å and 2.55 Å , which are close to the sum of the van der Waals radii of oxygen and hydrogen atoms; 1.4 Å Å = 2.6 Å.
Calculated distance /Å between the H and O atoms.

15. Comparison of stability of various rotational isomers of isobutanol with isobutanethiol
*No C-S trans forms was found for isobutanethiol.
471
/ cm-1
T-t form
*All the expected gauche forms of isobutanol were found.
DE/ cm-1
170
*The preference of the gauche conformation in isobutanol has been explained by the electrostatic interaction between the hydroxyl oxygen and the hydrogens of the methyls in the isopropyl group.
/ cm-1
277
T-g form
/ cm-1
G-t form
116
G-g’ form
/ cm-1
T-g form
126 63
/ cm-1
T-t form
/ cm-1
G-g’ form 21 32
G-g form
/ cm-1
/ cm-1
G-g form
/ cm-1
/ cm-1
G-t form
isobutanol
isobutanethiol

16. Interpretation of the calculated results
The calculated distances between O and H of the T-t, G-t, and G-g forms are shown below, which are close to the sum of the van der Waals radii of oxygen and hydrogen atoms: that is, 1.4 Å+ 1.2 Å =2.6 Å.
G-t form
2.52
T-g form
2.74
2.64
T-t form
2.58
G-g’ form
2.69
G-g form
2.58
: Calculated distance between the H and O atoms. (Å)

17. K. N. Houk, et. al, J. Am.Chem. Soc., 1993
Interpretation of the calculated results for NBO
0.0152
favorable
unfavorable
T-t form
T-g form
0.0159
favorable
0.0074
0.0157
G-t form
favorable
G-g form
favorable
unfavorable
G-g’ form
0.0098
0.0001
:natural bond orbital charge distribution
Only one induced dipole exists in the G-t, G-g, T-g conformer.　In the T-t conformer there are two induced dipoles. The repulsive steric interactions dominate in the T-t conformer.
G-g’ conformer has not induced dipole.
K. N. Houk, et. al, J. Am.Chem. Soc., 1993
115, 4170

18. Comparison of stability of various rotational isomers for isobutanol
G-t form
G-g form
T-t form
G-g’ form
T-g form
0 / cm-1
21 / cm-1
63 / cm-1
116 / cm-1
170 / cm-1
: experimental result
Potential energy of the trans form / MP G++(d,p)
A large tunneling splitting of the T-g form of isobutanol is expected from the potential energy surface calculated by ab initio MO calculation.
T-g
T-g
T-t
DE/ cm-1
It is not easy to assign the rotational
spectrum of the T-g form.
f / degrees

19. Summary
1. Four rotational isomers of the five were found for isobutanol.
2. A trans forms was detected for isobutanol , in sharp contrast with isobutanethiol , for which none of rotational isomers exits in trans.
3. Considerable differences were found between the spectra using Ar and Ne as a carrier gas.
4. Six rotational isomers of the 14 expected isomers were found for n-butanol.
5. All trans forms of n-butanol were found.
6. It is not easy to assign the rotational transition of the fifth isomer of isobutanol because of large tunneling splitting expected due to OH group.