1. Rotational spectra of C2D4-H2S, C2D4-D2S, C2D4-HDS and 13CH2CH2-H2S complexes: Molecular symmetry group analysis
Mausumi Goswami and E. Arunan
Inorganic and Physical Chemistry Department
Indian Institute of Science
Bangalore INDIA
TA 05, 63rd International Symposium on Molecular Spectroscopy, Columbus, 2008

2. Acknowledgements L. Narasimhan, S. T. Manju Prof. P.C.Mathias, IISc
Prof. Brooks Pate, Matt T. Muckle and Justin L. Neill Virginia
Funds
Indian Institute of Science,
Department of Science and Technology, India
Indo-French Centre for Promotion of Advanced Research

3. Outline Introduction to C2H4-H2S rotational spectrum
Spectra for deutereated and 13CH2CH2 isotopomers
Molecular symmetry group analysis
Tunneling splittings for various isotopomers
Large amplitude motions predicted by ab initio calculations
Need for IR-MW double resonance experiments
Conclusions

4. Large amplitude motions in C2H4-H2S
C2H4-H2S rotational spectrum was first reported in the 59th OSU meeting in 2004!
Isotopic substitution and ab initio calculations revealed the global minimum!
Every rotational transition was split in to four, as two doublets of smaller (<1 MHz) and larger splitting (about 10 MHz, in J = 23 region)
H2S, D2S and H234S isotopomers also gave the four line spectrum
HDS showed only a doublet with smaller splitting! It also showed quadrupole hyperfine structure due to D.

5. MP2 level predictions Constant (MHz) 6-31 G* 6-311 ++G** Expt. A 22884
22945
25961(34) B
1967
2000
(2) C
1858
1917
(2)
Look at the fantastic agreement for B and C, especially with the lower basis set? Why isn’t A agreeing? Off by 3 GHz?
Experimental A for the complex is very close to the C of C2H4

6. Fitted parameters for the four states of C2H4-H2SL1 L2 U1 U2
A (MHz)
25961(34)
25968(42)
26189(55)
26204(61)
B (MHz)
(2)
(2))
(3)
(3)
C (MHz)
(2)
(2)
(3)
(3)
d1(kHz)
-0.80(2)
-0.82(2)
-0.74(3)
-0.76(4)
d2(kHz)
-0.21(2)
-0.22(2)
-0.24(3)
DJ (kHz)
14.30(2)
14.31(2)
13.26(3)
13.28(3)
DJK (MHz)
1.0587(2)
1.0575(3)
0.9691(4)
0.9684(4)
Sd (kHz)
4.4
5.6
7.1
8.1
All independently fitted! None of the
Tunneling motions change the dipole moment

7. J=01 spectrum of C2H4-HDS showing D quadrupole coupling, Doppler and tunneling doublets 12 lines per transition!
C6H6-HDS had 110 kHz
as well
eQq = 110 kHz
Projection of OD bond
onto the a axis is similar
26°

8. Splitting observed for H2S, HDS and D2S
Isotop-
Omer
Smaller
MHz
Larger
H2S
0.14
1.67
H234S
0.12
1.33
HDS
0.035 -
D2S
3.11
SH bond in H2S
moves as C2H4
rotates
Deuterium bond
is stronger than
Hydrogen bond
Chem. Phys. Lett. 2004, 393, 22-27

9. Observed four line pattern
(B+C)/2 plotted for the four states observed, note
the intensity differences

10. Molecular Symmetry Group analysis for C2H4-H2S
Motions considered:
1)Rotation of C2H4 about its ‘a’ axis
2)Rotation of H2S about its ‘b’ axis
3) Both 1 and 2 together
E, (13)(24), (56), (13)(24)(56), E*, (13)(24)*, (56)*, (13)(24)(56)*
Molecular symmetry group: G8

11. Fourfold degeneracy of rotational-torsional wave function
Effect of Permutation-inversion operations a b c
Fourfold degeneracy of rotational-torsional wave function

12. G8 character table G8 E
(13)(24)
(56)
(13)(24)(56) E*
(13)(24)*
(56)*
(13)(24)(56)*
A1’ 1
A1” -1
A2’
A2”
B1’
B1”
B2’
B2”
Symmetry of the total wave function for C2H4-H2S needs to be A2’ or A2”

13. Symmetry of the rotational-torsional wave function
Characters of the reducible representation generated by the rotational-torsional wave function for the four distinct configuration: E
(13)(24)
(56)
(13)(24)(56) E*
(13)(24)*
(56)*
(13(24)(56)*
A’ 4
A” -4
A’ = A1’+ A2’+ B1” + B2”
A” = A1”+ A2” + B1’+ B2’

14. K=0, J= A’ = A1’+ A2’ + B1” + B2”
K = 0, J=odd A” = A1” + A2” + B1’+ B2’
J=even A’ = A1’+ A2’ + B1” + B2”
K  0 K= K+ , J=odd A” = A1” + A2”+ B1’+ B2’
J= even A’ = A1’ + A2’ + B1”+ B2”
K= K- , J=odd A’ = A1’ + A2’ + B1”+ B2”
J= even A” = A1” + A2”+ B1’+ B2’

15. Symmetry of the nuclear spin wave function
Characters of the reducible representation for the nuclear spin functions: E
(13)(24)
(56)
(13)(24)(56) E*
(13)(24)*
(56)*
(13(24)(56)*
NS 64 16 32 8
NS = 30A1’+ 10A2’ + 6B1’ +18 B2’

16. Nuclear spin degeneracy of the rotational-torsional sublevels
Rotational-torsional sublevel symmetry
Nuclear Spin function symmetry
Total wave function symmetry
Spin statistical weights
A1’
A2’ 10 30
B1”
B2’
A2 ” 18
B2”
B1’ 6
A1”
A2”

17. Spin statistical weight for C2H4-H2S and the isotopomers
Level symmetry
C2H4-H2S
C2H4-D2S
C2D4-H2S
C2D4-D2S
A1’
10 (31)
60(74) 45
270
A2’
30(67)
30(45)
135
B2”
6(16)
36(46) 36
216
B1”
18 (38)
18(33)
108 U2 L2 U1 L1
Get the C2D4-H2S spectrum! Shouldn’t have been
difficult, but we had to wait for the 63rd OSU meeting
for reasons beyond our control!

18. What can we learn from C2D4-H2S data?
Confirmation about the smaller splitting! If it is due to C2H4 tunneling, it should disappear!
How will the smaller splitting change with H2S substitution?
Larger splitting should not change significantly!

19. C2D4-H2S spectrum Transitions L1 L2 L3 L4 Observed freq. (MHz) Res.
(kHz)
Observed freq. (MHz)
-14.2
2.7
-12.0
8.2
2.1
-0.9
-3.5
-2.1
-0.2
-11.3
3.3
-2.5
7.7
7.0
7.5
-3.8
1.0
4.0
2.4
8.8
2.8
2.5
-2.7
1.3
-6.6
0.4
-1.2
0.8
-0.4
-0.8
-6.4
3.5
-3.6

20. C2D4-D2S spectrum Transitions L1 L2 L3 L4 Observed freq. (MHz) Res.
(kHz)
Observed freq. (MHz)
15.9
20.6
56.5
59.4
-1.3
1.6
-7.0
-3.7
8.0
-6.6
-17.0
-22.4
-5.6
-2.9
-27.4
-38.3
1.5
-1.7
8.3
4.4
-21.9
-13.2
-17.9
-4.1
-3.8
-8.1
-7.7
-1.0
-0.5
0.5
-2.7
-1.4
12.4
12.0
21.9
14.3

21. C2D4-HDS spectrum
Transitions L1 L2
Observed freq.
(MHz)
Res.
(kHz)
-18.5
-20.0
80.1
81.9
-61.7
-57.1
16.6
19.0
-81.7
-85.0
65.1
60.6
-3.1
-4.2
21.2
22.8
-18.0
-16.9

22. Tunneling splitting (MHz)
Complex L-U
C2H4-H2S
C2H4-H234S
C2H4-HDS
C2H4-D2S
C2D4-H2S
C2D4-HDS
C2D4-D2S
13CCH4-H2S

23. What do we understand now?
Both motions leading to doubling of the rotational lines involve C2H4 and H2S, though the smaller splitting is dominated by tunneling of hydrogens in C2H4!
Observation of all the four lines in case of 13CH2-12CH2---H2S confirms that the rotation of C2H4 about its ‘a’ axis (C-C bond) is the possible candidate.
Increase in larger splitting for D2S, compared to H2S!?
Will IR-MW double resonance help? We asked Brooks Pate and the next talk will answer some of these questiosns!

24. Large amplitude motions?
What does ab initio calculations tell us about these large amplitude motions?
Frequency calculations

25. Large amplitude motions of C2H4-H2S
Frequency (cm-1) 39 50 78 89
176
279

26. Large amplitude motions of C2H4-H2S
Frequency (cm-1) 39 50 78 89
176
279
39 cm-1

27. Large amplitude motions of C2H4-H2S
Frequency (cm-1) 39 50 78 89
176
279
50 cm-1

28. Large amplitude motions of C2H4-H2S
Frequency (cm-1) 39 50 78 89
176
279
78 cm-1

29. Large amplitude motions of C2H4-H2S
Frequency (cm-1) 39 50 78 89
176
279
89 cm-1

30. Large amplitude motions of C2H4-H2S
Frequency (cm-1) 39 50 78 89
176
279
176 cm-1

31. Large amplitude motions of C2H4-H2S
Frequency (cm-1) 39 50 78 89
176
279
279 cm-1

32. Conclusions
C2D4-H2S/D2S/HDS and 13CH2-CH2-H2S rotational spectra have been identified and assigned.
Large amplitude motions of C2H4 and H2S lead to a quartet of lines for every transition, with a smaller (few kHz) and a larger (1-2 MHz) splitting.
Large of amplitude motions of the two molecules are coupled.

33. Thanks for listening

34. Potential energy surfaces for various motions of C2H4 and H2S in the complex at MP2(full)Aug-cc-pVDZ level

35. Intensity (Arbitrary unit)
C2H4-H2S Spectrum
Frequency (MHz)
Intensity (Arbitrary unit)
C2H4-Ar
C2H4-H2S
C2H4-H2O
Internal motion of C2H4
Internal motion of both C2H4 and H2S
Internal motion of H2O
Why are these intensities different?