Infrared spectra of OCS-C 6 H 6, OCS-C 6 H 6 -He and OCS-C 6 H 6 -Ne van der Waals Complexes M. Dehghany, J. Norooz Oliaee, Mahin Afshari, N. Moazzen-Ahmadi Department of Physics and Astronomy, University of Calgary, Canada A. R. W. McKellar Steacie Institute for Molecular Sciences, National Research Council of Canada

Outline Previous work on OCS-C 6 H 6 complex Results and analysis in infrared region OCS-C 6 H 6 and OCS- 13 C 12 C 5 H 6 OCS-C 6 H 6 -He and OCS-C 6 H 6 -Ne OC 34 S-C 6 H 6 and OC 34 S-C 6 H 6 -He Summary 2/12

OCS-C 6 H 6 : Previous work Microwave study by Dahmen et al (1995). 3/12 U. Dahmen, H. Dreizler, and W. Stahl, Ber. Bunsenges. Phys. Chem. 99, 434 (1995). C 6 symmetry axis Isotopologues: OCS-C 6 D 6 18 OCS-C 6 H 6 OCS-C 6 H 5 D Sulfur is adjacent to the Benzene plane A symmetric rotor Hamiltonian

OCS-C 6 H 6 : Our results in infrared region 4/12 Dilute gas mixture: OCS ~ 0.2% C 6 H 6 ~ 0.2% Helium ~ 99.6% Rotational T~2.5 K parallel band cm cm -1 red-shifted with respect to OCS 1 band origin S-bonded configuration center of mass separation of 4.42 Å A complex = C benzene = 2844 MHz

Two series of weaker lines 5/12 B=605.8 MHz B=602.3 MHz B=531.3 MHz

First series: OCS- 13 C 12 C 5 H 6 in its natural abundance 6/12 B=605.8 MHz B=602.3 MHz B=531.3 MHz Shifted down by just around cm -1 with respect to the normal isotopologue Relative intensity of about 6.6% compared to the normal isotopologue Assigned to single 13 C-substitution in Benzene (green trace) Slightly asymmetric in fact The blue atom denotes the single 13 C substituted atom 3.5 MHz

OCS-C 6 H 6 -He side view 7/12 B=605.8 MHz OCS-C 6 H 6 B=602.3 MHz OCS- 13 C 12 C 5 H 6 B=531.3 MHz OCS-C 6 H 6 -He Shifted down by cm -1 with respect to the normal isotopologue Similar appearance to OCS-C 6 H 6 ? Large difference in B rotational constant ? O atom adjacent to benzene ?  Solution: On-axis helium atom to the other side of the benzene plane  The blue atom denotes Helium Second series: A puzzle!

8/12 Experiment with Neon Gas mixture: Neon ~ 1% + OCS ~ 0.2% C 6 H 6 ~ 0.2% Helium ~ 98.6% B ~ 378 MHz OCS-C 6 H 6 -Ne OCS-C 6 H 6 -Ar ? The blue atom denotes Neon OCS-C 6 H 6 -He/Ne side view

9/12 OC 34 S-C 6 H 6 and OC 34 S-C 6 H 6 -He Gas mixture: OC 32 S+OC 34 S ~ 0.33% C 6 H 6 ~ 0.66% Helium ~ 99% Red-shifted by 0.75 cm -1 with respect to the normal isotopologue Isotope shift of the B-value is consistent with the known S-bonded structure

10/12 Fitted molecular parameters OCS-C 6 H 6 a,c OCS- 13 C 12 C 5 H 6 b OCS-C 6 H 6 -HeOCS-C 6 H 6 -NeOC 34 S-C 6 H 6 c OC 34 S-C 6 H 6 -He 0 / cm (1) (1) (1) (1) (1) (1) B' / MHz (3) (53) (70) (77) (18) (60) D J ' / kHz0.2600(3) (12) B" / MHz (51) (69) (73) (19) (57) D J " / kHz (12) In all cases, A' = A" = 2844 MHz. a Ground state parameters B" and D J " fixed at the indicated microwave values. † b D J " and D J ' fixed at the indicated microwave values for OCS-C 6 H 6. † c With D JK " = D JK ' = kHz. † † U. Dahmen, H. Dreizler, and W. Stahl, Ber. Bunsenges. Phys. Chem. 99, 434 (1995).

11/12 Intermolecular separations ComplexDefinition of the bond lengthR (Å)R (Å) † C 6 H 6 -OCSR(C 6 H 6 – OCS) C 6 H 6 -OCSR(C 6 H 6 – S) C 6 H 6 -OCS-HeR(C 6 H 6 – He) C 6 H 6 -OCS-NeR(C 6 H 6 – Ne) C 6 H 6 and OCS geometries are fixed at their monomer values. For C 6 H 6 –OCS–He and C 6 H 6 –OCS–Ne, the C 6 H 6 to OCS distance is fixed at the indicated value for C 6 H 6 -OCS. † U. Dahmen, H. Dreizler, and W. Stahl, Ber. Bunsenges. Phys. Chem. 99, 434 (1995). The blue atom denotes rare gas atoms He/Ne OCS-C 6 H 6 -He/Ne side view

12/12 Summary Infrared spectrum of the OCS-C 6 H 6 complex is studied in 1 fundamental region of OCS. The observed vibrational shift is cm -1. An S-bonded structure with the OCS monomer located along the benzene C 6 symmetry axis was obtained. This is in agreement with the previous microwave study by Dahmen et al (1995). Similar bands were observed and studied for the isotopologues OCS- 13 C 12 C 5 H 6 and OC 34 S-C 6 H 6 and the trimers OCS-C 6 H 6 -He and OCS-C 6 H 6 - Ne. Our attempt to observe a similar band for OCS-C 6 H 6 -Ar has failed which might mean that the most stable form of this complex is an asymmetric structure. Symmetric molecular structures tend to give stronger spectral lines and simpler spectral patterns which are easier to observe and analyze. Thus we were able to recognize even the weak bands of OCS-C 6 H 6 -He and OCS- 13 C 12 C 5 H 6. Thank you for your attention