1. Rotational Spectra and Structure of PhenylacetyleneH2S complex
Presented by VIJAYANAND CHANDRASEKARAN
Mausumi Goswami and E. Arunan
Department of Inorganic and Physical Chemistry, Indian Institute of Science,
Bangalore
64th International Symposium on Molecular Spectroscopy, June 22-26, 2009

2. Why Study?
Phenylacetylene is a multi functional molecule: has different potential hydrogen bonding sites
Phenyl ring  cloud as H-bond acceptor
Acetylene  cloud as H-bond acceptor
Acetylenic C-H group has hydrogen bond donor
However, PhenylacetyleneH2O had a structure resembling none of the above. The structure of the complex was cyclic having the water hydrogen donating its hydrogen to the acetylenic  cloud forming O-H and the ortho C-H group of the phenylacetylene donating the hydrogen to the oxygen atom forming C-HO hydrogen bond
Do H2O and H2S form similar hydrogen bonds with phenylacetylene?
H2O and H2S form similar structure with ethylene and benzene whereas fluorobenzene H2O and fluorobenzene HCl have different structures.
The study of phenylacetylene with H2O and H2S can help to understand the differences/similarities of the interaction of the first row and second row hydrides with multifunctional organic molecule.

3. Ab initio studies at MP2(full)/aug-cc-pVDZ
3.102Å
2.742 Å b
2.446Å
Binding energy = -2.7 kcals/mol
Binding energy = -6.0 kcals/mol
An attempt to put H2S over both the acetylene and benzene  cloud lead to structure b
Binding energy = -4.2 kcals/mol
2.527Å c
Structure b is the global minima as indicated by the calculations

4. Ab initio derived rotational constants
A (MHz)
5374
1279
2209
B (MHz)
313
1176
652
C(MHz)
296
782
506
Well separated rotational constants: the structure can be determined unambiguously by microwave spectroscopy.

5. Search and assignment
A search for the global minima was started as guided by the ab initio prediction.
The prediction shows the difference between 404505, 414515, 414505 and 404515 quartet and the 505606, 515616, 515606 and 505616 is 1.4 GHz and the difference is almost constant as we go the successively higher J
Search was started at ~ MHz for 404505, 414515, 414505 and 404515 transitions. Several transitions were observed. A search near the frequency range 1.4 GHz above these lines yielded many transitions and corresponding lines were also observed at successive higher and lower J.
However, the higher J lines could not be fitted as K=0/1 quartet and eventually they turned out to be K=2/3 quartet. Finally 84 lines, each split due to tunneling, could be assigned for the parent isotopomer.

6. 634725 and 635724 transitions of phenylacetyleneH2S complex collected using Helium as carrier gas
Two states were observed
Stronger
Weaker

10. Both a and b-dipole transitions
Both a and b-dipole transitions
Two independent progressions that could be fitted to a semi-rigid rotor Hamiltonian.

11. Fitted parameters of the phenylacetyleneH2S complex and its isotopomers
C8H5D
HSD
D2S
I(strong)
II(Weak)
I(Strong)
A(MHz)
(7)
(7)
1203.4(1)
(2)
(2)
(7)
(4)
B(MHz)
(6)
(6)
1097.6(3)
(1)
(2)
(7)
(4)
C(MHz)
(6)
(8)
(7)
(8)
(7)
(5)
(3)
J (kHz)
1.6(5)
4.2(4)
1.6
3(1)
-2(1)
3.7(2)
3.1(1)
JK(kHz)
23(1)
17(1) 23
-4(3)
0.6(4)
-5.4(6)
-1.9(2)
k(kHz)
-27.0(3)
-32(1)
27.0
11(1)
20(1)
12.3(7)
7.6(4)
1(kHz)
0.7(2)
2.0(2)
0.7
1.6(6)
-0.9(7)
1.72(9)
1.41(5)
2(kHz)
13.1(2)
14.4(3)
13.1
2.1(4)
-0.6(4)
2.8(2)
3.41(9)
SD(kHz)
6.8
10.6
7.5
3.2
5.9
2.3
# (no of fitted lines) 83 91 21 71 66 41 46
Only H2S spectrum shows doubling. D2S does not show and it is likely that the splitting gets too small to be resolved. HDS does not show doubling as we do not have the indistinguishable nuclei any more.
The least change in the A rotational constant in the 34S isotope indicates that S atom is close to the ‘a’ principal axis of the complex.
The abnormal rise in the ‘A’ rotational constant in HDS and D2S isotopomer indicates that these atoms are very close the ‘a’ principal axis of the complex and is effected by the large amplitude motion

12. Structure of phenylacetyleneH2S complexb C
Experiment
A (MHz)
5374
1279
2209
(7)
B (MHz)
313
1176
652
(6)
C(MHz)
296
782
506
(6) a
The rotational constants are close to the structure b b c

13. A (MHz) B (MHz) C(MHz) C8H6 monomer parent C8H5D(14) C8H5H2S
(Stronger state)
C8H5D(14)H2S(stronger state)
A (MHz)
(7)
(2)
B (MHz)
(6)
(1)
C(MHz)
(6)
(8)
The A rotational constant of the parent and C8H5DH2S complexes are close the ‘c’ rotational constant of the phenylacetylene monomer which means
H2S is located above the phenylacetylene plane.
Pbb (88.9 amu Å2) of the PhA monomer is very close to the Pcc(87.1 amu Å2) of the complex. The average geometry of the complex is Cs symmetric. a b

14. Substitution analysis
The distance of the substituted atom from the center of mass can be obtained using Kraitchman substitution formula
|a| (Å)
|b| (Å)
|c| (Å) S
2.69
0.93
0.21
H(?)
3.37 i
1.02
1.26
0.34
H15
0.87
4.11
S (ab initio)
2.624
0.413
0.004
H15 (ab initio)
1.402
4.146
0.0
H16 (ab initio)
-1.691
1.391
H17 (ab initio)
-1.674
-0.550
-0.001 a b
The two hydrogen atoms of the H2S unit are effected by the large amplitude motion and the substitution analysis on them produce unreasonable values. This may be because the vibrationally averaged geometry is different in the deutereated structure. The imaginary co-ordinate of H15 is because it is in the ab inertial plane of the complex

15. Distance (R) of the H2S center of mass from the phenylactylene center of mass in the complex
Assumption: H2S is bound symmetrically to the phenylacetylene
Parallel axis theorem: 1 2
From relation 1, R has been found to be 3.73 Å and from relation 2 gives R as 3.75 Å

16. Where is the Sulphur?
3.67 Å (from Kraitchman substitution co-ordinates)
Å (from the phenylacetylene monomer experimental geometry)
3.74 Å (using parallel axis theorem)
However, these distances indicate that the S atom is away from the center of mass of the phenylacetylene monomer towards the acetylenic group which is contradictory to the conclusion that S is located very close to the ‘c’ principal axis of the monomer.
The H2S may be undergoing a sliding motion over the ring which produce these ambiguities. This may be the reason for the rising of the ‘A’ rotational constant of the HDS and D2S isotopomer compared to the H2S isotopomer as they sample different geometry along this co-ordinate

17. Shift of the ligand atom from the phenyl ring center in different complexes
Shift from the phenyl ring center (Å)
PhenylacetyleneAr (towards acetylene)
(Dreizler)
BenzonitrileAr Towards cyano group)
(Dreizler)
FluorobenzeneAr
(towards F atom)
0.297 (Neusser, Kuczkowski)
Fluorobenzene HCl (towards the para carbon atom)
0.162 (Kuczkowski)
The only conclusion that can be drawn is that S atom is shifted towards the acetylenic group and the structure is analogous to the phenylacetyleneAr and benzonitrile Ar complexes.
Ab initio potential energy scans at MP2/aug-cc-pVDZ level show that at the minimum energy configuration, S is shifted by 0.4 Å from the center of mass of the phenylacetylene monomer towards the acetylene group

18. Is there any interaction with the acetylenic  bond?
No bond critical point between the hydrogen and the acetylene  cloud!

19. Tunneling motion of H2S moiety in the complex
Observation of only one state in phenylacetyleneHDS indicates that the two hydrogen atoms of H2S are getting exchanged during the motion. Rotation of H2S about the C2 symmetric axis is the most probable mechanism
407 cm-1
The barrier for this rotation at MP2/aug-cc-pVDZ level is 407 cm-1 and the zero pint energy along this mode is 63 cm-1
The motion is hindered. This supports the quenching of the splitting observed in phenylacetyleneD2S complex.

20. Fluorobenzene H2O forms a planar structure whereas fluorobenzene HCl forms a  bonde structure.
PhenylacetyleneH2O forms cyclic planar structure with O-H(acetylenic) and C-HO hydrogen bond. (63rd meeting, arunan and co-workers)
PhenylacetyleneCH3OH forms a  bonded structure with O-H and C-H (acetylenic) hydrogen bond. PhenylacetyleneNH3 forms C-HN hydrogen bond whereas Phenylacetylene NH2Me forms a  bonded struture. The authors called it as a methyl group induced change in structure. (Sedlak et. al. J. Phys. Chem. A 2009)
This study shows that Phenylacetylene H2S froms a  bonded structure.
Thus, if the molecule is bulky, it seems that it would prefer to go above the plane and cannot be accommodated in the plane due to steric repulsion.

21. Conclusion
PhenylacetyleneH2S has the hydrogen sulphide located over the phenylacetylene  cloud.
H2S isotopomers show doubling of the transitions due to tunneling. Neither HDS nor D2S show doubling.
The H2S is shifted away from the phenyl ring center towards the acetylene group.
The transitions are split due to the interchange of equivalent hydrogens of H2S through a C2 rotation of H2S about its symmetric axis.
The bulkier second row hydrides prefer to be on the top of the ring rather than to be in the plane.

22. Acknowledgement
We thank the Indo-French centre for promotion of advanced research, Council for Scientific and Industrial Research (CSIR) for financial support , DST and IISc for the infrastructure, S.T. Manju and L. Narasimhan for the helps in the experiments. MG acknowledges CSIR research fellowships.