1. Methylstyrenes – Microwave Spectroscopy
cis-β-Methylstyrene
trans-β-Methylstyrene
Objectives
Assign the rotational spectra
Compare the potential barriers to methyl internal rotation
RANIL M. GURUSINGHE, MICHAEL TUBERGEN
Department of Chemistry and Biochemistry, Kent State University, OH.

2. Methylstyrenes – Internal Rotation: Microwave Spectroscopic Study
Fitting Strategy
Rotational constants estimated
Geometry optimization at ωB97XD/ G(d,p)
Barrier height estimated
Relaxed potential scan at ωB97XD/ G(d,p)
A components were fitted
RRFIT, ZFAP Programs
A and E components fitted simultaneously
XIAM Program
Methylstyrenes – Internal Rotation: Microwave Spectroscopic Study

3. Equilibrium Structures
α-Methylstyrene
A (MHz)
B (MHz)
C (MHz)
μb=0.33 D > μc=0.15 D > μa=0.02 D
cis-β-Methylstyrene
A (MHz)
B (MHz)
C (MHz)
μa=0.27 D > μb=0.24 D > μc=0.17 D
trans-β-Methylstyrene
A (MHz)
B (MHz)
C (MHz)
μa=-0.51 D > μb=0.12 D > μc=-0.04 D
Methylstyrenes – Internal Rotation: Microwave Spectroscopic Study

4. Relaxed Potential Scans at ωB97XD/6-311++G(d,p)
Barrier estimation for trans-β-Methylstyrene
α-Methylstyrene
cis-β-Methylstyrene
trans-β-Methylstyrene
Barrier(cm-1)
665.7
704.0
654.9
Methylstyrenes – Internal Rotation: Microwave Spectroscopic Study

5. Microwave Rotational Spectra
Mini-Cavity FTMW Spectrometer
Range : – 22.0 GHz
Resolution : 2.4 kHz
Carrier Gas : 30% He : 70% Ne
Backing pressure : 1.5 atm
Methylstyrenes – Internal Rotation: Microwave Spectroscopic Study

6. Global Fit: cis-β-Methylstyrene
Parameter
XIAM
ωB97XD
A (MHz)
(3)
B (MHz)
(8)
C (MHz)
(4)
ΔJ (kHz)
0.1378(4) -
ΔJK (kHz)
-0.234(3)
ΔK (kHz)
2.178(23)
δJ (kHz)
(16)
δK (kHz)
0.949(12) N
144
Δνrms (kHz)
1.866
Parameter
XIAM
ωB97XD
V3 (cm-1)
714(7)
704.0
F0 (GHz)
Fixed 152.5
151.9
Iα (uA2)
Fixed 3.31
3.33
ε (rad)
0.49(5)
0.47
< (i,a)
80.40(7)
710
< (i,b)
300(2) -
< (i,c)
620(2)
μa > μb >μc
Methylstyrenes – Internal Rotation: Microwave Spectroscopic Study

7. Global Fit: trans-β-Methylstyrene
Parameter
XIAM
ωB97XD
A (MHz)
(16)
B (MHz)
(10)
C (MHz)
(9)
ΔJ (kHz)
(18) -
ΔJK (kHz)
0.1354(10)
ΔK (kHz)
0.7(3)
δJ (kHz)
(15)
δK (kHz)
0.083(19) N
170
Δνrms (kHz)
1.491
Parameter
XIAM
ωB97XD
V3 (cm-1)
578(3)
654.9
F0 (GHz)
159.9(8)
151.6
Iα (uA2)
3.161(16)
3.33
ε (rad)
Fixed 0.00
0.00
< (i,a)
13.20(9)
130
< (i,b)
76.80(9) -
< (i,c)
90.00(<0)
μa> μb> μc
Methylstyrenes – Internal Rotation: Microwave Spectroscopic Study

8. α-Methylstyrene : Preliminary Fits
Parameter
ωB97XD/
G(d,p)
ZFAP Fit
[A component]
XIAM
[Global Fit]
A (MHz)
(8)
(9)
B (MHz)
(5)
(22)
C (MHz)
(5)
(3)
ΔJ (kHz) -
0.061(6)
0.0658(10)
ΔJK (kHz)
(15)
-0.068(14)
ΔK (kHz)
0.65(9)
0.45(8)
δJ (kHz)
0.0085(6)
0.0061(5)
δK (kHz)
0.57(4)
0.72(3)
V3 (cm-1)
665.7
603 (8)
ε (rad)
0.44
Fixed 0.44
< (i,a)
59.50
50.30(9) N 39 69
Δνrms (kHz)
4.835
6.381
Only the transitions with Ka= 0,1 were fitted
μb> μc > μa
Methylstyrenes – Internal Rotation: Microwave Spectroscopic Study

9. Methylstyrenes – Internal Rotation: Microwave Spectroscopic Study
Transitions with Ka = 2, 3
4 lines ??
4 lines ??
3 lines ??
Methylstyrenes – Internal Rotation: Microwave Spectroscopic Study

10. Propenyl/Benzene Internal Rotation
Relaxed potential scan at ωB97XD/ G(d,p)
Barrier Heights = cm-1, cm-1
Methylstyrenes – Internal Rotation: Microwave Spectroscopic Study

11. Conclusion
Assigned the microwave rotational spectra and determined the potential barrier to internal rotation of cis-β-Methylstyrene, trans-β-Methylstyrene.
Preliminary XIAM fit of α-Methylstyrene can not fit Ka = 2,3 transitions
Additional fitting in α-Methylstyrene is probably due to the large amplitude motion of the propenyl (CH2-CH-CH3 )/ Benzene.
A (MHZ)
B (MHz)
C (MHz)
Exp. V3(cm-1)
Calc. V3(cm-1)
cis-β-Methylstyrene
(3)
(8)
(4)
714(7)
701.25
trans-β-Methylstyrene
(16)
(10)
(9)
578(3)
654.90
Methylstyrenes – Internal Rotation: Microwave Spectroscopic Study

12. Methylstyrenes – Internal Rotation: Microwave Spectroscopic Study
Acknowledgement
Ohio Supercomputer center
Kent State University
Methylstyrenes – Internal Rotation: Microwave Spectroscopic Study