1. Fourier Transform Infrared Spectral
Investigation of the 6 Band of cyclic-C3H2
Pradeep R Varadwaj, Ryuji Fujimori, Kentarou Kawaguchi
Molecular Spectroscopy Laboratory,
Department of Chemistry,
Faculty of Sciences
Okayama University, Japan
FB11, page-290
65th Meeting – June 21-25, 2010
International Symposium on Molecular Spectroscopy

2. A species of interstellar interest
c-C3H2 molecule:
A species of interstellar interest
Interesting because that
it is highly reactive
importance in organic chemistry
Its occurrence in the interstellar medium ( diffuse clouds,
Cold quiescent clouds, star forming regions etc.)
The major formation reaction of c-C3H2 is the dissociative recombination of c-C3H3+
C3H3+ + e- → C3H2 + H
c-C3H3+ is a product of a long chain of carbon chemistry that occurs in the ISM.

3. Studies so far
First experimental matrix isolation infrared detection was done by Reisenauer et al. in 1984 Bands at 1279, 1063, 888, and 789 cm-1 were confirmed.
Astronomical detection (P. Thaddeus et al Astrophys. J. Lett. 299, L63 (1985))
Microwave study: Bogey et. al CPL 126 (1985); GHz region (studies of 13C and D isotopomers leads to molecular structure)
Stark measurement: Saito et al CPL 140 (1987): determination of  (3.43D)
Millimeter-wave study: GHz region (ground and several excited states, R. Mollaaghababa et al, JCP 99 （1993))
6. High-resolution study: Hirahara et al JCP 95 (1991) ; cm-1 region; confirming the band at (1) cm-1
Ab initio /DFT results suggest that the band at 789 cm-1 should be observable. The band Intensity is about 2.4 times less intense than that observed at 1277 cm-1.
Motivation:
An accurate determination of the centrifugal distortion constants
To assign the band and that to determine the origin of this band.

4. Molecular Constants of c-C3H2 in the ground
and four vibrationally excited states
R. Mollaaghababa, C. A. Gottlieb, J. M. Vrtilek, P Thaddeus
J. Chem. Phys. 99 (1993) 890

5. The calculated FTIR spectrum of
the 6 band of c-C3H2
B1 symmetry
In phase out-of-pane
CH vibrations
R-branch
P-branch
Q-branch
C-type spectrum

6. Experimental Arrangements
Production of c-C3H2

7. An Observed FTIR Spectrum of c-C3H2 in the region 775.4-776.5 cm-1

8. The FTIR spectrum of c-C3H2

9. Calculated spectrum
Subtracted spectrum
A comparison

10. A part of the FTIR spectrum of c-C3H2
Q(1-14)
Q(29)
Q(16)
Q(27)
Q(21)
Q(25)
Q(23)
Q(19)
Q(5)
Q(7)
Q(6)
Q(8)
Q(13)
Q(11)
Q(9)
Q(10)
Ka,Kc = 0,1
Ka,Kc = 1,2
A part of the FTIR spectrum of c-C3H2

11. Few Q-branch lines strongly & distinctly stand out in the spectrum
J Ka Kc J' Ka' Kc' Calc Obs. Diff.

12. Ro-vibrational Parameters derived from a simultaneous fit
This work
Millimeter-wave dataa A B C
J  103
JK  103
K  103
J  103
K  103
HJ  103
HJK 106
HKJ 106
HK  106
h1  106
h2  106
h3  106
0 (uÅ2)
v6 (cm-1) N
(49)
(49)
(4)
209.66(44)
-409.1(3)
-511.4(40)
100.61(22)
40.06(8)
0.00
37.1(19)
-150(11)
112(15)
0.499(33)
-0.88c
1.22c
(7)
357(332+25)
(61)
(75)
(10)
(3)
(5)
(14)
(1)
41.313(4)
6.37b
-14.7b
10.08b
0.477b
-0.88b
1.22b
(46)
--- 28
a Mollaaghababa et al; J. Chem. Phys. 99 (1993)
b Fixed to the ground state values

13. Summary
Cyclic-C3H2 has been produced through microwave discharge of Allene (25 mTorr) in Ar (30 mTorr)
The FTIR high-resolution absorption spectrum has been observed and assigned.
A simultaneous fitting of the assigned transitions of this work along with the rotational transitions from a previous study resulted in an accurate determination of the ro-vibrational constants.
A good agreement is found between the results of this work and those reported by R. Mollaaghababa et al J. Chem. Phys. 99 (1993) 890 except higher order distortion constants.
The band origin of the 6 band is found to be at cm-1
Acknowledgement
Thanks Japan Society for the promotion of Science (JSPS) for funding and scholarship

14. 9 stable isomers of the
Singlet and triplet
c-C3H2
Mohajeri et al.
J. Mol. Struct: Theochem
820 (2007) 65-73