1. Infrared spectra of complexes containing acetylene-d2 Clément Lauzin, J. Norooz Oliaee, N. Moazzen-Ahmadi Department of Physics and Astronomy University of Calgary A.R.W. McKellar Steacie Institute for Molecular Sciences National Research Council of Canada

2. TDL Jet Trigger Ref. Gas 12 bit DAQ Card Timer Controller Card (CTR05) Laser Sweep Trigger DAQ Trigger Gas Supply Jet Signal Jet Controller (Iota One) Jet Controller IR Detectors TDL Controller (L5830) Etalon Monochromator pulsed supersonic jet / tunable diode laser apparatus at The University of Calgary

3. R.E. Miller, P.F. Vohralik, and R.O. Watts, J. Chem. Phys. 80, 5453 (1984). D. Prichard, J.S. Muenter, and B.J. Howard, Chem. Phys. Lett. 135, 9 (1987). G.W. Bryant, R.O. Watts, and D.F. Eggers, J. Chem. Soc. Faraday Trans. II 84, 1443 (1988). D.G. Prichard, R.N. Nandi, and J.S. Muenter, J. Chem. Phys. 89, 115 (1988). G.T. Fraser, R.D. Suenram, F.J. Lovas, A.S. Pine, J.T. Hougen, W.J. Lafferty, and J.S. Muenter, J. Chem. Phys. 89, 6028 (1988). Y. Ohshima, Y. Matsumoto, M. Takami, and K. Kuchitsu, Chem. Phys. Lett. 147, 1 (1988); 152, 116 (1988). K. Matsumura, F.J. Lovas, and R.D. Suenram, J. Mol. Spectrosc. 150, 576 (1991). K. Didriche, C. Lauzin, T. Foldes, X. de Ghellick D’Elseghem Vaenerwijck, and M. Herman, Mol. Phys. 108, 2155 (2010). Acetylene dimer A landmark example in microwave and IR spectroscopy of van der Waals complexes.

4. Acetylene Dimer The possibility of tunneling between equivalent T-shaped configurations splits each rotational level into three states, which are labelled A 1 +, B 1 +, and E +. J, Ka, KcJ, Ka, Kc B1+B1+ A1+A1+ E+E+ (HCCH) 2, ground state: B 1 + - A 1 + = 2207 MHz (DCCD) 2, ground state: B 1 + - A 1 + = 424 MHz

5. Acetylene Dimer We expect two IR bands, each of which has three components A 1 +, B 1 +, and E +. Parallel band (  K a = 0), associated with stretch of this “stem” monomer Perpendicular band (  K a = 1), associated with stretch of this “top” monomer a - axis b - axis 3 asymmetric C-H stretch fundamental HCCH: 3295 cm -1 (and 3282 cm -1, Fermi resonance) DCCD: 2439 cm -1

6. Acetylene dimer Nuclear spin statistics Ka, Kc (HCCH) 2 (DCCD) 2 A1+A1+ B1+B1+ E+E+ A1+A1+ B1+B1+ E+E+ ee 103211518 eo 013152118 oe 6336318 oo 3633618

7. Overview of (DCCD) 2 spectrum Our laser coverage was limited: we observe these 2 important sub-bands, but miss higher K values. We also miss the parallel band, which probably(?) lies at a lower frequency.

8. (DCCD) 2 This plot shows the Q- and R- branches of the K = 1  0 sub-band The selection rules are: A 1 +  B 1 + E +  E + So there are actually 3 bands, as shown.

9. T 0 / cm -1 A or  A / MHz B or  B / MHz C or  C / MHz A 1 + Ground 0.026026.1752.0131624.354 B 1 + Ground 0.014149426006.1751.4821624.269 E + Ground X26007.1751.7481624.303 A 1 + Excited 2435.7990(1)+585.41(253)+0.818(86)+0.238(63) B 1 + Excited 2435.8037(1)+479.77(241)+1.220(102)+0.095(52) E + Excited X + 2435.7917(1) +613.19(647)+0.582(249)+0.352(154) (DCCD) 2 parameters Ground state parameters are from: Matsumura, Lovas, and Suenram, J. Mol. Spectrosc. 150, 576 (1991). X  0.12 cm -1, but the precise value is not known. Note the large change in A value!

10. (DCCD) 2 parameters The excited state tunneling splitting (B 1 + - A 1 + ) turns out to be 141 MHz, considerably smaller than the ground state value of 424 MHz. Comparable ( 3 perpendicular band) values for (HCCH) 2 are 2207 and 572 MHz. The excited state splittings are reduced because tunneling is inhibited by the need to exchange the vibrational excitation when the “top” and “stem” monomers interchange. The (DCCD) 2 spectrum exhibits small perturbations affecting low-J levels of the upper state E + component (examples shown on the right). But these are much less extensive than the mammoth perturbations noted in the IR spectrum of (HCCH) 2.

11. We also observe the mixed dimer HCCH – DCCD Almost exactly the same location as (DCCD) 2. It’s a perpendicular band, so we know DCCD has the “top” position, and HCCH the “stem” position. First observation of this isomer, which has a slightly higher energy than the form with DCCD as the “stem” of the T.

12. This is the linear complex DCCD- NN, which has been studied in the microwave region by: Legon, Wallwork, and Fowler, Chem. Phys. Lett. 184, 175 (1991). Air in our gas sample ?!?!? We also observe a “mystery” spectrum (with B" = 1427 MHz)

13. We observe another “mystery” spectrum in a different region

14. 2418 cm -1 mystery spectrum There are two simple bands here, with B" = 2419 and 2513 MHz. The first B"-value agrees with that of C 2 D 2 -D 2 O.* The second could be that of C 2 D 2 -HDO (not previously studied). But where is the K = 1 – 1 sub-band of C 2 D 2 -D 2 O? * Peterson and Klemperer, J. Chem. Phys. 81, 3842 (1984).

15. Conclusions The (DCCD) 2 IR spectrum ( 3 ) is much less perturbed than that of (HCCH) 2. The large increase in A-value in going from the ground to excited state is difficult to understand. Is it possible that the accepted (indirect) microwave A-values for acetylene dimers do not reflect the true ground state K = 0 to 1 energy spacings? A wider laser scan to observe more perpendicular subbands would give us a handle on this elusive ground state A-value. Might also allow observation of the parallel band. Unfortunately, lasers in this region (2430 cm -1 ) are not so readily available these days.