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Chirality of and gear motion in isopropyl methyl sulfide:
Fourier transform microwave study
Yoshiyuki Kawashima, Keisuke Sakieda, and Eizi Hirota*
Kanagawa Institute of Technology The Graduate University for Advanced Studies*
6/24/2010
RH03
Chiral molecule
eg. propylene oxide (PO)
Second chiral molecule
eg. isopropanol (IPOH)
( R ) and (S) of PO
gauche and gauche’ of IPOH
Gear MotionPitzer and his collaborators devised to explain the thermodynamical data of some
organic molecules. J.E.Kilpatrick and K.S.Pitzer, J. Res. Natl. Bur. Stand., 37 (1946) 16
cis-2-butene: CH3CH=CHCH3
Kondo et al. concluded the CH3 conformation which was notcompatible with the gear motion for cis-2-butene by a MW spectroscopy.
S. Kondo, Y. Sakurai, E. Hirota and Y. Morino, J. Mol. Spectrosc., 34 (1970) 231
(HCCH)2
IR and MW spectra of interconversion tunneling in acetylene dimer Y. Ohshima, Y. Matsumoto, M. Takami, and K. Kuchitsu, Chem. Phys. Lett., 27 (1988) 1 G. T. Fraser, R. D. Suenram, F. J. Lovas, A. S. Pine, J. T. Hougen, W. J. Lafferty, and J. S. Muenter, J. Chem. Phys., 89
(1988) 6028.
(a) gauche form (b) trans form
(CH3)2CHOCH3, IPME
Gauche form was found by Nakagawa et.al., however, trans form has not been observed. V3 = 603.3 cm-1
J. Mol. Struct. 112 (1984) 201
Our aim
(CH3)2CHSCH3, IPMS
0
200
400
600
800
1000
1200
1400
1600
0 60 120 180 240 300 360
Potential function of IPMSab initio calculation MP2/6-311G(d,p)
transgauche gauche
cm-1
HC-SC deg
μa = 0.3 Dμb = 1.5 Dμc = 1.0 D
μa = 1.6 Dμb = 0.0 Dμc = 0.9 D
V3(S-CH3)
= 500~550 cm-1
V3(isop-CH3)≈1300 cm-1
Instrument: Fourier transform microwave spectrometer
Sample : 0.5% IPMS diluted with Ar
Backing pressure : 1~2 atm Shots : 20
Frequency region : 5~23 GHz Step : 0.25 MHz
MW
sample
Supersonic molecular beam injection
Experimental
Vacuum chamber
Mirror (mobile)
Diffusion pump
Rotary pump
Mirror (fixed)
Molecular beam injection nozzle
10000 15000 20000Frequency/MHz
Spectrum of IPMS
8500-10500 MHz expand
8500 9000 100009500 10500
4 22-4
13
Gauche formb-type transition Q-branch
5 23-5
14
3 21-3
12
6 24-6
15
2 20-2
11
Frequency/MHz
Spectra of IPMS
10000 15000 20000Frequency/MHz
Gauche form
Trans form
10000 15000 20000Frequency/MHz
5000
a-typeJ=2←1
a-type J=3←2
a-typeJ=4←3
Observed Splittings of gauche and trans of IPMS
250
8980.8 8981.6
150
15631.5 15632.3 14371.4 14372.200 0
250
Frequency/MHz
a-type b-type c-type
312-211
523-514
Frequency/MHz Frequency/MHz
Splittings of internal rotation of CH3 group
Gauche form
16344.3 16345.10
15
0
50
18090.1 18090.9
221-111
211-101
Frequency/MHzFrequency/MHz
Trans form
312-211
a-type c-type
A
E EE
E
E
A
A
AA ?
Molecular parameters of gauche and trans of IPMSExperimental gauche trans
A /MHz 6039.36375(6) 5105.38888(11)
B /MHz 2777.50750(4) 2774.42333(9)
C /MHz 2128.157503(4) 2578.42201(8)
N(a-type) 38 32
N(b-type) 148 -
N(c-type) 100 131
V3/cm-1
σkHz
601.64(7) 2.0
559.00(11) 3.2
ab initio Calculation
MP2/6-311++g(d,p) B3LYP/6-311++g(d,p)
gauche trans gauche trans
A /MHz 6049 5100 5988 5072
B /MHz 2804 2788 2697 2697
C /MHz 2136 2599 2084 2511
μa /D -0.27 1.58 0.27 1.47
μb /D 1.53 0 1.37 0
μc /D 0.97 0.94 0.99 -0.89
V3/cm-1 518 566 507 521
ΔE /cm-1 0 192 0 229
E
1
1 -1
1 a
bc
a
sA’
A’’
eeeo oe
oo
Cs E
1
1 -1
1 a
bc
a
sA’
A’’
eeeo oe
oo
E
1
1 -1
1 a
bc
a
sA’
A’’
eeeo oe
oo
Cs Selection rule: A’ ↔ A’’
s ↔ s and a ↔ a ; a-type and c-type transitions
s ↔ a; b-type transition
A’
eos,A’’ oes,
ees, oos, eoa, oea,
eea, ooa,
A’
eos,A’’ oes,
ees, oos, eoa, oea,
eea, ooa,
g’ g
gauche
sa
)'+(2
1= ggs
)'-(2
1= gga
Analysis for gauche of IPMS
FiQ
FiQ
iQF
iQF
0
0
0
0
F: the first-order internal-rotation term
Δ: the tunneling splitting
δ: the K-type doubling
iQ: the interaction term between the s and a states: -iQKa
2222
22
)(state, For the
)(,0 state, For the
QFEE
QEFA
s state a state +K -K +K -K
E state
s 322
s 321a 321
a 322
s 3,3(-) a 3,3(-)a 3,3(+)s
3,3(+)
s 322
s 321a 321
a 322
s 331a 331
a 330s 330
A stateGauche of IPMS
A stateA stateE state E state
0
20
40
60
80
17555.8 17556.6/ MHz0
20
40
60
17563.2 17564.0/ MHz
331-321 330-321
/ MHz
0
10
20
30
40
50
17554.0 17554.8
a-a allowed
s-s allowed
a-a forbidden
s-s forbidden
0
20
40
60
80
100
/ MHz17564.5 17565.3
s-a forbidden
a-s forbidden
s-a allowed
a-s allowed
a-a
s-s
s-a
a-s
Analysis for gauche of IPMS
FiQ
FiQ
iQF
iQF
0
0
0
0
F: -3.66 MHz
δ: 7.471 MHz for J = 3 and 51.74 MHz for J = 4
A state E state
Δ: around 0.001 MHz 0.001~0.002 MHz
iQ: 6.47 MHz 9.05 MHz
The difference in iQ between the A and E states is probably due to the CH3 internal rotation, which, when being the E state, contributes to the Coriolis interaction term –iQKa.
Observed Splittings of gauche and trans of IPMS
250
8980.8 8981.6
150
15631.5 15632.3 14371.4 14372.200 0
250
Frequency/MHz
a-type b-type c-type
312-211
523-514
Frequency/MHz Frequency/MHz
Gauche form
16344.3 16345.10
15
0
50
18090.1 18090.9
221-111
211-101
Frequency/MHzFrequency/MHz
Trans form
312-211
a-type c-type
A
E EE
E
E
A
A
AA ?
E state
322
321
3,3(-)
3,3(+)
322
321
331
330
A state
Trans of IPMS
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
12089 12090 12091 12092 12093 12094 12095 12096 12097
A state
Allowed transition of E state
Forbidden transition of E state
0
5
10
15
12092.7 12093.5/ MHz
0
5
10
12090.4 12091.2/ MHz
0
5
10
15
12095.3 12096.1/ MHz
331 - 321
Sat ISat ISat II Sat II
Sat
36 cm-1 for trans68 cm-1 for gauche
C(isop) - S torsion
The lowest vibrational modes of trans and gauche of IPMS We need to treat this rotational isomer
as a system which consists of one methyl of S-CH3 and two methyl groups (CH3)2CH and internally rotating about the C(isop)-S bond.
This system belongs to a group G54, which Ohashi and Hougen discussed in some detail. J. Mol. Spectrosc. 211 (2002) 119.
The S-CH3 internal rotation, when coupled in gear-like fashion with those of the two CH3 in (CH3)2CH group, will yield three substrates each for the A and E states: A1, A1+, A1- and E1, E1+, E1-, and we suspect that A1+ and A1- are merged into the A satellite and E1+ and E1- correspond to sat I and II, respectively. The above-mentioned ab initio calculation reveals the C(isop)-S tortional frequency to be 36 cm-1 around the trans, compared with 68 cm-1 at the gauche conformation.
angle of the C(isop) – S torsion
Potential functions for the C(isop) – S torsion in gauche and trans of IPMS
C(isop) - S torsionCH3 - S internal rotation isop C - S - CH3 bending
isop anti-gear CH3 internal rotation isop gear CH3 internal rotation
36 cm-1 for trans68 cm-1 for gauche
175 cm-1
164 cm-1
240 cm-1
238 cm-1
246 cm-1
217 cm-1267 cm-1
270 cm-1
Vibrational modes of trans of IPMS
Summary
1. The existence of two rotational isomers gauche and trans was established for IPMS.
2. Some gauche spectra were found split, which were explained by the tunneling between the two equivalent forms and were thus of great significance for the racemization process in the chiral gauche.
3. Trans showed unique satellite spectra, which were ascribed to the excited state of the C(isop) – S torsion perturbed by the “gear” type motion between the CH3 groups in the isopropyl radical and the CH3 directly bonded to the S atom.
Future works
1. Analyze the satellite spectra of the trans
of IPMS
2. Reinvestigate the spectrum of IPME.
Acknowledgements
We thank Dr. Yoshihiro Osamura for his advise in performing ab initio calculation.