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

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

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.

35 Cl 35 Cl 12 C 32 Sv = Cl 37 Cl 12 C 32 Sv = Cl 35 Cl 12 C 32 Sv 3 = Cl 35 Cl 12 C 32 Sv 6 = Cl 37 Cl 12 C 32 Sv 3 = Cl 37 Cl 12 C 32 Sv 6 = Cl 37 Cl 12 C 32 Sv = Cl 35 Cl 12 C 32 Sv 4 = Cl 35 Cl 12 C 32 Sv 2 = Cl 37 Cl 12 C 32 Sv 4 = Cl 37 Cl 12 C 32 Sv 2 = Cl 35 Cl 12 C 34 Sv = Cl 37 Cl 12 C 34 Sv = Cl 37 Cl 12 C 32 Sv 3 = Cl 37 Cl 12 C 32 Sv 6 = 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!

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

Previously we studied the 2 and 4 bands

Experimental Conditions Bruker IFS 125HR FTS helium cooled Ge:Cu detector 9.4 m OPD (~ cm -1, unapodized) (observed widths were Doppler-limited, about cm -1 at 820 cm ‑ 1 to 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 = Torr and path = 72 m, pressure = 0.10 Torr

1 band Overview 0 = cm -1 ( 35 Cl 2 CS) 0 = 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 ″).

5 band Overview 0 = cm -1 ( 35 Cl 2 CS) 0 = 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 ″).

Molecular parameters for 35 Cl 2 CS (cm -1 units) ν 1 ν 5 Ground state ν0ν (1) (1) 0.0 A (33) (13) B (56) (12) C (16) (11) KK (49) (23)  JK (67) (22) JJ (19) (48) KK (11) (67) JJ (96) (24)

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

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

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 , but not ?) unassignable band

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

Conclusions 1, 5, 2 4, and are now in good shape for 35 Cl 2 CS and 35 Cl 37 ClCS (though has unidentified perturbations). 3, 6, and “ ” 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.