1. Infrared spectra of carbonyl sulfide-acetylene trimers: OCS-(C2H2)2 & two isomers of (OCS)2-C2H2
Mahin Afshari, M. Dehghany, Jalal N. Oliaee, N. Moazzen-Ahmadi
(Department of Physics and Astronomy, University of Calgary)
A.R.W. McKellar
(Steacie Institute for Molecular Sciences, National Research Council of Canada)
Hi, my name is Mahin. I am going to talk about Infrared spectra of carbonyl sulfide-acetylene trimers. One OCS and 2 C2H2 and two isomers of 2 OCS and 1 C2H2. I would like to thank my supervisor Nasser Moazzen-Ahmadi, my co supervisor Dr. McKellar and my colleges Nader Dehghani and Jalal Norouz Oliaee.

2. Outlines (OCS)2-HCCH trimer with the polar OCS dimer fragment
Review of the microwave study
Infrared spectrum in the region of OCS ν1 fundamental
(OCS)2-HCCH trimer with the nonpolar OCS dimer fragment
Two infrared spectra in the region of OCS ν1 fundamental
Planar (HCCH)2-OCS trimer with Cs symmetry
Review of previous studies
The outline of my talk is as follows: first I will talk about OCS-C2H2 trimer with the polar OCS dimer fragment.
The second trimer is the one with the nonpolar OCS dimer fragment. Since this trimer has no permanent dipole moment there is no microwave study availible and I will directly go to the two infrared spectra we have observed in the region of OCS nu1 fundamental.
The last trimer to be discussed here is the planar trimer with two C2H2 units.
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3. Peebles et al. studied 8 isotopomers of the (OCS)2-C2H2 trimer
Peebles et al. studied 8 isotopomers of the (OCS)2-C2H2 trimer. Their results were consistent with a barrel-shaped structure having a polar OCS dimer fragment and the trimer having C1 symmetry. At that time, there was no direct experimental evidence for the existence of the polar OCS dimer.

4. This slide shows their experimental structure for the trimer and for the polar OCS dimer portion. “The polar OCS dimer unit is clearly not planar with the dihedral angle calculated to be 18 degrees.
The polar OCS dimer portion of the trimer is clearly not planar with the dihedral angle calculated to be 18.0o.
Sean A. Peebles et al., J. Chem. Phys. 111, 10511, 1999.

5. Comparison of the recently observed polar (OCS)2 and the polar (OCS)2 portion of the (OCS)2-C2H2
3.732 Å
3.703 Å
56.4o
63.0o
For comparison, here is the structure of the polar isomer of (OCS)2 as observed by our group a couple of years ago. It is planar with Cs symmetry.
M. Afshari, M. Dehghani, Z. Abusara, N. Moazzen-Ahmadi, and A.R.W. McKellar, J. Chem. Phys. 126, (2007).

6. Infrared spectrum of the polar (OCS)2-C2H2 in the region of the OCS ν1 fundamental
We have observed the infrared band of the same trimer in confirmation of the microwave assignment. The band is relatively weak because we know the polar dimer of OCS is much less abundant than the nonpolar isomer. The assignment is also complicated because of overlapping with two other bands; one which is the band of the polar OCS dimer and the other a trimer which I will talk about in the next couple of slides. The red trace is the experimental spectrum and the blue trace is the simulated one.

7. Infrared bands assigned to the in-phase and out-of-phase vibrations of two OCS monomers in the (OCS)2-C2H2 trimer with nonpolar OCS dimer fragment
2049 band
b-type
2069 band
c-type
Ground state 0
(1)
(1) A
(3)
(2)
(2) B
(4)
(3)
(3) C
(3)
(4)
(3)
We observed two more infrared bands. They were assigned to in-phase and out-of-phase vibrations of two OCS monomers in the OCS-C2H2 trimer with nonpolar OCS dimer fragment having C2 symmetry. Rotational parameters obtained from the fit are shown. We found no need for higher order parameters. One of the bands is purely B-type and the other has only c-type transitions and they both share the same ground state parameters.

8. c b b c a a (OCS)2-C2H2 trimer Nonpolar (OCS)2 b c
Based on the ground state parameters and the type of two bands, we estimated an structure for the OCS-C2H2 trimer with the nonpolar OCS portion as shown here. This structure is compared to the structure of the nonpolar OCS dimer.
Because of C2 symmetry of the trimer the presence of acetylene distorts the dimer in such a way that there will be a dihedral angle for the dimer portion. The experimental evidence for this is the purely b-type band of the in-phase vibrations of two OCS monomers. b c b c a a

9. A new band at 2058.85 cm-1 which contains only
a-type transitions
We have observed a band at cm-1 which contains only a-type transitions. The rotational parameters obtained from the fit are shown here. They clearly show a planar structure and the band fits the Cs symmetry.
By keeping in mind that this band belong to a species which contains OCS and C2H2, we searched in the literature and found out that....
Δ=h/8π2c(1/C-1/B-1/A)= (1/C-1/B-1/A) uA2
A (MHz)
B (MHz)
C (MHz)
Δ (uÅ2)
Ground state
2689.7(7.1)
791.6(2)
612.9(2)
-1.75
Excited state
2682.1(7.1)
612.5(2)

10. Sordo and Valdes have done an ab initio calculations on OCS-(C2H2)2 trimer and found 4 minima.

11. A(MHz) B(MHz) C(MHz) Δ(uÅ2) MI 1540.3 1468.3 1002.5 -168.18 MII 2065.0
942.6
647.2
-0.020
MIII
2869.0
797.4
624.0
-0.033
MIV
3609.7
857.0
692.6
-0.029
TSI
1646.5
1466.6
1036.7
ORIENT
1746.9
1506.2
1088.8
Peeble’s exp.
(10)
(9)
(10)
Present work
2689.7(7.1)
791.6(2)
612.9(2)
-1.75
These are the structurse for thoes four minima. Among them structure MIII is the most stable isomer. For two reasons we assigned the observed band to this structure. First, the experimental parameters match very well those for MIII. Second, we only observe a-type transitions.

12. 21o c a b ×
This slide explains the second reason. The inertial axes for the planar structure MIII is shown here. The OCS monomer axis makes an angle of 21 with the “a” axis. If there was a b-type component it would be 8 to 10 times weaker. Because we don’t see a b-type component, it is likely that this angle is even smaller (10 to 15 degrees).
The angle between the “a” inertial axis and the OCS monomer axis is 55 in the structure MII and 21.4 in the structure MIII. Since our system is sensitive enough to show the angle of 55 but not 21, we are certain that the observed purely a-type band belongs to the structure MIII.
MIII

13. There is also a microwave study of an isomer of OCS-(C2H2)2 with parameters very different from ours and those calculated by Sordo et al.

14. Summary C1 C2 Cs

15. The end

18. 55 ᵒ c a b ×
MII

19. Comparing with the (OCS)3b
This case is very similar to the OCS trimer where we have observed three individual bands with the same ground state parameters. In that case, the nonpolar dimer portion of the trimer is determined to be planar with a small angle between two monomer axes which results in a weak a-type band for the in-phase vibration of the two OCS monomer.
vibration
(OCS)3
Transition type
(OCS)2-C2H2
OCS on top
2047
a-type
in-phase
2053
2049
b-type
out-of-phase
2077
c- & a-type
2069
c-type

20. Since there was a problem in finding the corresponding transitions for the other isotopomers, exact determination of the structure was failed. In order to find an estimated structure for this isomer, they compared their experimental results with four structures predicted from semi-emperical model OREIENT (structures number 1 to 4 in order of increasing the stability). Since all three selection rules were observed for four isotopomers, the planar structures (number 1 and 4) were easily eliminated. The two remaining structures are very close in energy. And despite the fact that their rotational constants are quite different from the experimental values, Peebles et al.vsuggested the structure 3 to be the most suitable structure.

21. Experimental setup TDL Gas Supply Jet Controller TDL Controller
IR Detectors
Jet
Trigger
TDL
Laser
Sweep
Trigger
Jet Signal
Timer
Controller
DAQ
Trigger
Experiments were carried out at the University of Calgary using a supersonic jet and long-path IR spectrometer and a tunable diode laser probe. Three detectors are used, one for the jet, one for reference gas and a third for etalon signal. The jet is controlled by Iota one. Two cards are used; a 12 bit DAQ card which digitizes the signal from the detectors and a timer controller card which synchronizes the data acquisition and opening of the jet. All the electronics are computer controlled, the software we are using is Labview.
DAQ
Card
Reference Cell
Monochromator
Etalon

22. Timing sequence of data acquisition
Current ramp to
TDL from L5830
Trigger from L5830
to timer controller
DAQ Card records
@ speed of 4MHz
Background
Jet signal
The timing sequence is illustrated on this slide. In order to record a spectrum, the laser is rapidly scanned across a certain wavenumber range (≈0.5–1 cm−1) at a rate of 1 KHz. The trigger from the laser controller is used to record the data and the opening of the jet. Four laser scans are recorded, the first and last scans are used to remove the background due to the laser radiation. The dummy scans are discarded. d1
Jet trigger to Iota valve
from CTR05
Actual jet opening (approx.) d2

23. Conclusion Background Jet signal
This is the computer screen for the data acquisition system. The two traces on the top are the background and the signal, respectively. The next trace is the signal after the background is removed. The averaged trace is shown next and the four traces at the bottom are the background, the two dummy traces and the signal. The averaged signals for reference and etalon are shown on the left hand side.
Background
Jet signal