CO2 dimer: Five intermolecular vibrations observed via infrared combination bands Jalal Norooz Oliaee, Mehdi Dehghany, Mojtaba Rezaei, Nasser Moazzen-Ahmadi Department of Physics and Astronomy University of Calgary A.R.W. McKellar National Research Council of Canada

TDLS 2009, Zermatt, Switzerland pulsed supersonic jet / tunable laser apparatus at The University of Calgary Jet Controller (Iota One) Jet Trigger Jet Controller Gas Supply IR Detectors TDL Jet Signal Laser Controller Laser Sweep Trigger Ref. Gas Timer Controller Card (CTR05) DAQ Trigger 12 bit DAQ Card Etalon 2

CO2 dimer was one of the very first van der Waals complexes to be observed by (truly) high-resolution IR spectroscopy

as well as the dimer spectrum in the 2350 cm-1 region (3) This CO2 dimer spectrum in the 3700 cm-1 region (1 + 3 / 22 + 3) was assigned a few years later Jucks, Huang, Dayton, Miller, and Lafferty, J. Chem. Phys. 86, 4341 (1987). Jucks, Huang, Miller, Fraser, Pine, and Lafferty, J. Chem. Phys. 88, 2185 (1988). as well as the dimer spectrum in the 2350 cm-1 region (3) Walsh, England, Dyke, and Howard, Chem. Phys. Lett. 142, 265 (1987).

Here are two older works of significance here There are many calculations of CO2 intermolecular forces (empirical, ab initio, …) Here are two older works of significance here

The current work was guided with help from new calculations by Xiao-Gang Wang, Tucker Carrington, and Richard Dawes C2h point group Intermolecular mode Symmetry Chen and Light (2000) (cm-1) Dawes et al. (2016) (cm-1) Geared bend (in-plane) Bu 19.7 20.64 Torsion (out-of-plane) Au 22.6 24.44 2 x geared bend Ag 29.5 32.34 vdW stretch 40.8 46.07 Anti-geared bend (in-plane) 90 92.30

Symmetry is important! There are two intramolecular dimer modes in the 3 region: Ag = forbidden out-of-phase vibration of the CO2 monomers (2346.8 cm-1?) Bu = allowed in-phase vibration of the CO2 monomers (2350.771 cm-1) Intermolecular mode Symmetry Combined with Ag intramolecular mode Combined with Bu intramolecular mode Geared bend (in-plane) Bu Ag Torsion (out-of-plane) Au Bg 2 x geared bend vdW stretch Anti-geared bend (in-plane) Only Au and Bu states have allowed transitions (from the dimer ground state)

Symmetry is broken in the mixed dimer, giving new information A few years ago, we studied (13CO2)2 and 12CO2 – 13CO2 and also searched for dimer combination bands in the 3 region Symmetry is broken in the mixed dimer, giving new information

Combination bands tell us about the interesting low frequency intermolecular vibrations In 2010, we analyzed one combination band. But its vibrational assignment was not completely certain. The new Wang calculations predict the (2 x geared bend) combination band at 2350.771 + 32.34 = 2383.11 cm-1. So this is certainly the correct assignment for our previously observed 2383.280 cm-1 band.

We have long been puzzled by this mystery CO2 cluster band - Mol. Phys. 108, 2195 (2010) By laboriously checking CO2 dimer ground state combination differences, we have finally figured it out!

There are two dimer combination bands here There are two dimer combination bands here. Their upper states have a strong Coriolis interaction, which can be fitted very well. Geared bend Torsion 0 2369.0203(1) 2370.0031(1) A 0.30738(27) 0.29817(26) B 0.051857(48) 0.053578(45) C 0.0438905(40) 0.0456009(40) Ga 0.37489(32) Gb 0.07949(33)

Bu Ag Au Bg Intermolecular mode Symmetry Combined with Ag intramolecular mode  2346.76 cm-1 Combined with Bu intramolecular mode = 2350.771 cm-1 Geared bend (in-plane) Bu Ag Torsion (out-of-plane) Au Bg 2 x geared bend vdW stretch Anti-geared bend (in-plane)

Another mystery region, and here, too, we found many CO2 dimer ground state combination differences. There are two bands here with origins as indicated, a stronger one at about 2443.02 and a weak one at 2441.72 cm-1. Both have Bu symmetry, and both are very perturbed.

The stronger band (2443.02 cm-1) is almost certainly the anti-geared bend combination. Intermolecular mode Symmetry Combined with Ag intramolecular mode  2346.76 cm-1 Combined with Bu intramolecular mode = 2350.771 cm-1 Geared bend (in-plane) Bu Ag Torsion (out-of-plane) Au Bg 2 x geared bend vdW stretch Anti-geared bend (in-plane) The weaker one (2441.72 cm-1) is probably a ‘background’ state which gains intensity from the stronger.

Theory and experiment agree very well!! Note of course that the experiment involves the dimer in the excited (CO2 3) state, while the calculations are for the ground state. Intermolecular mode Symmetry Theory Chen & Light (2000) Theory Wang et al. (2016) Present experiment Geared bend (in-plane) Bu 19.7 20.64 22.26 Torsion (out-of-plane) Au 22.6 24.44 23.24 2 x geared bend Ag 29.5 32.34 31.509 vdW stretch 40.8 46.07 ? Anti-geared bend (in-plane) 90 92.23 92.25 Interestingly, simple empirical estimates of 32 cm-1 for the stretch and 90 cm-1 for the anti-geared bend were made in 1988, based on centrifugal distortion constants.

Conclusions We have observed three of the four intermolecular (van der Waals) modes of the CO2 dimer by means of IR combination bands in the CO2 3 region, plus an intermolecular overtone. The experimental frequencies agree very well with a new ab initio calculation by Wang et al. The geared bend and torsional modes near 23 cm-1 are highly mixed by Coriolis interactions which can be fitted very well. The anti-geared bend mode at 92 cm-1 is highly perturbed in ways that defy analysis. This could be (partly?) due to tunneling shifts (which have not previously been observed for CO2 dimer).