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.