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High resolution far-infrared spectra of thiophosgene with a synchrotron source: The 1, 5, 2 4 and 2 + 2 6 bands A.R.W. McKellar National Research Council.

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Presentation on theme: "High resolution far-infrared spectra of thiophosgene with a synchrotron source: The 1, 5, 2 4 and 2 + 2 6 bands A.R.W. McKellar National Research Council."— Presentation transcript:

1 High resolution far-infrared spectra of thiophosgene with a synchrotron source: The 1, 5, 2 4 and 2 + 2 6 bands A.R.W. McKellar National Research Council of Canada, Ottawa B.E. Billinghurst Canadian Light Source, University of Saskatchewan, Saskatoon

2 S Cl Thiophosgene is a formaldehyde analog Planar, C 2v symmetry Near-oblate ‘accidental’ symmetric top A ~ 3557 MHz B ~ 3464 MHz C ~ 1753 MHz The axes ‘switch’ with isotopic substitution: For 35 Cl 2 CS, the S-C symmetry axis is the a-inertial axis For 37 Cl 2 CS, the S-C symmetry axis is the b-inertial axis

3 Thiophosgene is very popular for studying IVR, intersystem crossing, etc. It’s one of the best-characterized of all molecules in terms of vibrational level structure. But there are no previous high-resolution IR studies.

4 35 Cl 35 Cl 12 C 32 Sv = 00.287 35 Cl 37 Cl 12 C 32 Sv = 00.187 35 Cl 35 Cl 12 C 32 Sv 3 = 10.069 35 Cl 35 Cl 12 C 32 Sv 6 = 10.066 35 Cl 37 Cl 12 C 32 Sv 3 = 10.045 35 Cl 37 Cl 12 C 32 Sv 6 = 10.043 37 Cl 37 Cl 12 C 32 Sv = 00.036 35 Cl 35 Cl 12 C 32 Sv 4 = 10.029 35 Cl 35 Cl 12 C 32 Sv 2 = 10.025 35 Cl 37 Cl 12 C 32 Sv 4 = 10.019 35 Cl 37 Cl 12 C 32 Sv 2 = 10.016 35 Cl 35 Cl 12 C 34 Sv = 00.013 35 Cl 37 Cl 12 C 34 Sv = 00.008 37 Cl 37 Cl 12 C 32 Sv 3 = 10.007 37 Cl 37 Cl 12 C 32 Sv 6 = 10.007 Etc!!! IR spectra are difficult due to Congestion! Relatively heavy molecule Isotopic dilution Hot band dilution The ground state of the principal isotope has less than one third of the total population at room temperature!

5 Microwave study Carpenter, Rimmer, Smith, & Whiffen J. Chem. Soc. Faraday Trans. 2 71, 1752-1762 (1975) First complete vibrational assignment Frenzel, Blick, Bennett, & Niedenzu J. Chem. Phys. 53, 199-204 (1969) 3 C-Cl bend 292 cm -1 6 C-Cl rock 300(?) 4 out of plane bend 471 2 C-Cl sym stretch 504 5 C-Cl asym stretch 820 1 C-S stretch 1139

6 Previously we studied the 2 and 4 bands

7 Experimental Conditions Bruker IFS 125HR FTS helium cooled Ge:Cu detector 9.4 m OPD (~0.00064 cm -1, unapodized) (observed widths were Doppler-limited, about 0.0008 cm -1 at 820 cm ‑ 1 to 0.0012 cm -1 at 1140 cm ‑ 1 ) 2 m multi-pass cell at T = 230 K (about as low as you can go with Cl 2 CS)! path = 32 m, pressure = 0.002 Torr and path = 72 m, pressure = 0.10 Torr

8 1 band Overview 0 = 1138.837 cm -1 ( 35 Cl 2 CS) 0 = 1138.603 cm -1 ( 35 Cl 37 ClCS) Detail clusters have a spacing of about 2C, and equal values of (2J – K c ). The numbers indicate (2J″ – K c ″).

9 5 band Overview 0 = 819.614 cm -1 ( 35 Cl 2 CS) 0 = 816.165 cm -1 ( 35 Cl 37 ClCS) Detail clusters have a spacing of about 2C, and equal values of (2J – K c ). The numbers indicate (2J″ – K c ″).

10 Molecular parameters for 35 Cl 2 CS (cm -1 units) ν 1 ν 5 Ground state ν0ν0 1138.8372(1)819.6142(1) 0.0 A 0.118401423(33) 0.11838145(13) 0.118659052 B 0.115051918(56) 0.11525443(12) 0.115550298 C 0.058306778(16) 0.058436834(11) 0.058477122 KK 0.41659(49) 0.4650(23) 0.433533  JK -0.07293(67)-0.0934(22)-0.089395 JJ 0.21355(19) 0.19457(48) 0.217084 KK 0.18341(11) 0.18159(67) 0.179959 JJ 0.092178(96) 0.09386(24) 0.092798

11 We also observed and fitted the 2 4 overtone at 941.954 cm -1. It is about 3 – 5 times weaker than 4 (and 500 times weaker than 5 ). This shows the central Q-branch region.

12 We also observed and fitted the 2 + 2 6 combination band at 1103.576 cm -1, just below 1 (which was 25 - 30 times stronger). Perturbations were noted, and the fit quality was not great.

13 Recall the 5 origins are 820 and 816 cm -1. But look: a 2 nd band at about 795 cm -1 ! It is assigned in the literature as 2 + 6, presumed to be stealing intensity from 5 by Fermi-type resonance. We could not assign anything in this 795 band!!! (Why can we assign 2 + 2 6, but not 2 + 6 ?) unassignable band

14 Overview of the weak and chaotic spectrum in the ( 3, 6 ) region, which has completely baffled us (so far)! 3 is evidently centered at 292.8 (for 35 Cl 2 CS). But where is 6 ? Is the problem here related to the problem with the supposed 2 + 6 band seen earlier?

15 Conclusions 1, 5, 2 4, and 2 + 2 6 are now in good shape for 35 Cl 2 CS and 35 Cl 37 ClCS (though 2 + 2 6 has unidentified perturbations). 3, 6, and “ 2 + 6 ” are still in bad shape, and we are almost ready to give up! How else can we attack 3, 6 ? IR hot bands ? We have tried! Pure rotational spectrum ! Difficult due to isotopic dilution, and also large hfs!! Carpenter, Rimmer, Smith, & Whiffen (Newcastle) did a great job in 1975! Now we need to extend it to the lowest excited vibrational states, 3 and 6. Best approach might be broad-band mm-wave spectrum.


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