1. Fourier transform microwave spectrum of isobutyl mercaptan Kanagawa Institute of Technology 1 and The Graduate University for Advanced Studies 2 Yugo Tanaka, 1 Akinori Sato, 1 Yoshiyuki Kawashima, 1 and Eizi Hirota 2

2. (1)To study the observation and stability of various rotational isomers of isobutyl mercaptan (2)To study the tunneling effect of equivalent isomers Aim of the present investigation IBSH T form t formg(1) form g(2) form G form  TtTg(1),Tg(2)GtGg(1)Gg(2) Tg  Tt : isobutyl mercaptan (CH 3 ) 2 CHCH 2 SH

3. Instrument : Fourier transform microwave spectrometer Sample : IBSH 0.5% diluted with Ar Backing pressure : 1.0 atm Shots : 20 Frequency region : 3.7~25 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

4. 5 25 Frequency / GHz Observed rotational spectra of IBSH 5~25 GHz C–C bond G form a –type transition C–C bond T form a –type transition J=2 ← 1 J=3 ← 2 J=5 ← 4J=4 ← 3 J=5 ← 4 J=6 ← 5

5. a-type transition, J=3 ← 2, of the gauche form 10500 ~ 12500 MHz 1050012500 Frequency / MHz 3 13 – 2 12 3 03 – 2 02 3 12 – 2 11 3 22 – 2 21 3 21 – 2 20 3 13 – 2 12 3 03 – 2 02 3 12 – 2 11 3 22 – 2 21 3 21 – 2 20 Strong intensity : G1 Weak intensity : G2 J=3 ← 2

6. 13000 14500 Frequency / MHz 3 13 – 2 12 3 03 – 2 02 a-type transition, J=3 ← 2, of the trans form 3 12 – 2 11 3 22 – 2 21 3 21 – 2 20 13000 ~ 14500 MHz J=3 ← 2

7. b-type transitions of the trans form 16500 Frequency / MHz 17000 16500~17000 MHz 4 31 – 4 22 3 30 – 3 21 3 31 – 3 22 4 32 – 4 23 observed splitting of b-type transitions, 13 MHz

8. The tunneling splittings of the torsional motion of the S-H group are observed because two rotational conformers exist in the T1 form. G1G1G2G2 T1T1 A / MHz B / MHz C / MHz N (a-type) N (b-type) N (c-type) 7354.8971 (7) 2109.24699 (32) 1777.08849 (30) 7354.5803 (8) 2110.84667 (31) 1764.24693 (31) 5690.2593 (16) 2456.5132 (13) 2175.7228 (12) 30 27 23 30 18 19 21 13 – Observation of the 13 MHz splitting for the b-type transitions of T1 species Molecular constants of G1,G2 and T1 no b-type transition was observed for G2 we will discuss later on this observation

9. antisymmetry state Tunneling splittings of the S-H group in IBSH c b wave functions of the torsional motion of the S-H group 1 1 1 A’ A’’,,, 0 00 1 01 1 11 1 10 2 02 2 12 2 11 0 00 1 01 1 11 1 10 2 02 2 12 2 11 a-type transitions b-type transitions c-type transitions ∆E symmetry state ss aa sa the molecule has a symmetry plane on the a-c plane of IBSH. So we can define two species on the reflection of the ( a-c ) plane, that is, the symmetric and antisymmetric groups A’ and A’’ the dipole moments of μ a and μ c are symmetric for the operation of the reflection on the ( a-c ) plane and, however, the dipole moment of μ b is antisymmetric. the allowed transitions for a-type and c-type transitions occur between the same symmetric states of the torsional motion of the S-H group. the allowed transitions for the b-type transitions occur between the different symmetric states. the splittings of these b-type transitions occur at the separation of twice ∆ E.

10. 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0306090120150180210240270300 330360 a-type b-type c-type Relative intensities of the gauche forms of IBSH Gg(1) G t Gg(2) Relative intensity = G1= G2 Dihedral angle bond moment μ (C-S)=0.90 D μ (S-H)=0.68 D Intensity μ 2 ∝  degrees we can explain the relative intensities of the two gauche forms using the bond moments of μ( C-S) and μ(S-H). the relative intensities are proportional to the square of the dipole moment. the two gauche forms are located at 60 and 300 degrees. the a-type and c-type transitions for both conformers are expected to observe the spectra In the case of G2 conformer its relative intensity is expected to be very small. therefore this reason is why we couldn’t observe b-type transition of the G2 conformer.

11. Potential energy surface of IBSH, basis sets: MP2/6-31G(d,p)  degrees Tg Gg(1) Gg(2) Gg(1) Gg(2) C–S C–C  degrees the x-axis is the angle.  around the C-S bond and the y-axis is the angle,  around the C-C bond. the blue color becomes a stable energy and the red one show unstable. the most stable form is located here, that is the Gg(1) conformer and second stable form is the Gg(2). the third stable is located as to be Tg form which there are two equivalent rotational isomers. so we will look at the cross section of the potential energy surface at the gauche and trans forms.

12.  E/ cm -1 Potential energy of the gauche form estimated by ab initio MO calculation [MP2/6-31G(d,p)] Gg(1)Gg(2) Gg(1) = 0 cm -1 Gg(2)=32 cm -1  degrees  = 60º the most stable form is  =60º,that is Gg(1). the second stable form is near 290º,that is Gg(2). Gg(2) is higher by 40 cm -1 than Gg(1). we couldn’t detect the Gauche-trans form which is higher by 300 cm -1

13. Potential energy of the trans form estimated by ab initio MO calculation [MP2/6-31G(d,p)]  E/ cm -1 Tg(1) Tg(2) Tg(1) = 125 cm -1 Tg(2)=125 cm -1  = 180º  degrees the third stable forms are near 75º and 285º, that are two equivalent forms, Tg, whichis higher by 130 cm -1 than Gg(1). we couldn’t detect the Trans-trans form which is higher by 450 cm -1

14. r s structure of the Gg(1) form of IBSH Gg(1) s a s b s c D 1.85976-0.34119-1.32656 13 C1 0.16259-0.690310.35235 13 C2 -0.961340.0-0.32753 13 C3 -1.109541.458430.11426 13 C4 -2.24890-0.78351-0.03796 34 S 1.85151-0.01799-0.02918 / Å Gg(1) r s ab initio r s - r s ' deviation r(SH) 1.3371.3360.0011.301-0.036 r(SC1) 1.8581.8200.0371.838-0.020 r(C1C2) 1.4841.531-0.0471.5030.019 r(C2C3) 1.5311.5280.0031.5400.009 r(C2C4) 1.5351.5310.0041.528-0.006 ∠ SC1C2 115.2114.90.3115.20.010 ∠ C1C2C3 112.6 111.8 0.8111.5-1.110 ∠ C1C2C4 108.2109.4-1.2109.10.920 ∠ HC1S 96.895.41.498.61.890 / Å

15. r s structure of the Gg(2) form of IBSH Gg(2) s a s b s c D 1.765841.106030.58670 13 C1 0.16571-0.679990.35745 13 C2 -0.963270.0-0.32968 13 C3 -1.124271.457490.10218 13 C4 -2.24536-0.79192-0.02514 34 S 1.85893-0.04517-0.09114 Gg(2) r s ab initio r s - r s ' deviation r(SH) 1.3391.3350.0041.3680.029 r(SC1) 1.8591.8200.0391.847-0.012 r(C1C2) 1.4861.530-0.0441.5040.018 r(C2C3) 1.5291.5280.0011.5380.009 r(C2C4) 1.5371.5310.0061.529-0.008 ∠ SC1C2 116.1115.50.5115.0-1.060 ∠ C1C2C3 112.7112.00.7111.6-1.110 ∠ C1C2C4 107.8109.1-1.3108.91.060 ∠ HC1S 96.296.6-0.495.4-0.760

16. Summary Three stable rotational isomers in IBSH were found. The torsional motion of SH group was observed. The seven isotopomers for the Gg(1) and Gg(2) forms were assigned and their r s structures were obtained. Tg form Gg(1) form Gg(2) form

18. Rotary Pump CD 3 OD, CD 3 OH (CH 3 ) 2 CHCH 2 SH + CD 3 OD (CH 3 ) 2 CHCH 2 SD + CD 3 OH the observation of isotopomers of IBSH IBSH + CD 3 OD

19. G2 34 S 10800 frequency / MHz 11200 the observation of isotopomers of IBSH G1 3 13 – 2 12 G2 3 13 – 2 12 G1 34 S G1 13 C1 G1 13 C2 G1 13 C3 G1 13 C4 G2 13 C1 G2 13 C2 G2 13 C3 G2 13 C4

20. Gt Gg(1)Gg(2) TtTg

21. Molecular constants of G1,G2 and T1 G1 G2T1 7354.8971 (7)7354.5803 (8)5690.2604 (14) 2109.24700 (31)2110.84667 (31)2456.5095 (12) 1777.08850 (29)1764.24694 (31)2175.7245 (12) 0.2623 (43)0.2686 (40)0.676 (14) 2.840 (20)3.119 (10)3.79 (7) 2.49 (9)-0.35 (11)-2.03 (12) 0.0603 (29)0.0503 (34)0.018 (8) 30 19 27-21 231813 A / MHz B C Δ J / kHz Δ JK / kHz Δ K δ J N (a-type) N (b N (c

22. Potential energy of the trans form estimated by ab initio MO calculation [MP2/6-31G(d,p)]  E/ cm -1 Tg(1) Tg(2) Tg(1) = 125 cm -1 Tg(2)=125 cm -1  = 180º  degrees