OCS trimer and tetramer: Calculated structures and infrared spectra Luca Evangelisti, Cristobal Perez, Nathan A. Siefert, Brooks H. Pate Department of Chemistry, University of Virginia M. Dehghany, N. Moazzen-Ahmadi Department of Physics and Astronomy, University of Calgary A.R.W. McKellar National Research Council of Canada The known OCS dimers Nonpolar dimer Randall, Wilkie, Howard, Muenter, Mol. Phys. 69, 839 (1990) Polar dimer Afshari, Dehghani, Abusara, Moazzen-Ahmadi, McKellar, J. Chem. Phys. 126, 071102 (2007) The known OCS trimer (OCS)3 Microwave: Connelly, Bauder, Chisholm, Howard, Mol. Phys. 88, 915 (1996)

Peebles, Kuczkowski, J. Phys. Chem. A 103, 6344 (1999) Infrared: Afshari, Dehghani, Abusara, Moazzen-Ahmadi, McKellar, J. Chem. Phys. 127, 144310 (2007) The present work has three parts 1) Cluster structure calculations: based on a recent high-level ab initio OCS intermolecular. 2) Infrared spectra from Calgary: pulsed supersonic slit-jet expansion (very dilute OCS in He), tunable IR laser probe, 1 region (~2060 cm-1). 3) Broad-band chirped-pulse microwave spectra from Virginia (3 9 GHz). OCS Cluster Structures from ab initio Pair Potential Thanks! to Richard Dawes (Missouri-Rolla) for subroutines to evaluate their potential surface [Brown, Wang, Dawes and Carrington, J. Chem. Phys. 136, 134306 (2012)]. Simply(!) find global and local minima assuming pairwise additivity, starting with random structures and going downhill (using the Powell method).

Sounds easy, but remember its a 14-dimensional space for OCS tetramer. The number of isomers increases very rapidly with cluster size. There are (at least!) 20 local minima for the tetramer within 100 cm-1 of the global minimum at -2773 cm-1 Isomer P4-1 P4-2 P4-3 P4-4 P4-5 P4-6 P4-7 P4-8 P4-9 P4-10 P4-11 P4-12 P4-13 P4-14 P4-15 P4-16 P4-17 P4-18

P4-19 P4-20 Energy / Symmetry cm-1 -2773 -2771 -2762 -2762 -2757 -2756 -2753 -2745 -2738 -2717 -2713 -2711 -2699 -2698 -2697 -2683 -2682 -2680 -2676 -2674 C1 S2 Cs C1

C1 C1 C1 C1 Cs Cs C2v Cs C1 C1 C1 C1 C1 C1 S2 Cs Rotational constant / MHz A 635 714 673 684 666 668 608 725 490 471 638

654 569 469 533 552 508 570 645 552 B 316 341 352 342 356 334 314 325 434 464 358 349 351 448 382 377 373 335 348 391

C 309 291 296 283 290 319 306 301 336 356 331 316 322 351 340 308 338 328 277 302 Dipole (monomer units) a 1.7 0.0 0.0 0.0 0.1

0.3 0.2 0.9 0.1 0.4 0.0 0.0 0.9 1.3 0.4 0.2 0.3 1.0 0.0 0.0 b 0.9 0.0 1.6 0.0 1.7 0.5 0.8 0.4 0.0 0.0 0.0 0.4 0.6 0.8

0.2 1.3 0.2 1.1 0.0 1.2 c 0.1 0.0 1.1 0.1 1.0 0.2 0.8 1.6 0.2 1.9 0.0 0.2 0.0 1.3 0.3 1.5 0.3 0.1 0.0 1.5 Here are the first four calculated minima

a c P4-1 C1 P4-2 S2 P4-3 Cs P4-4 C1 a b 0 cm-1 2 cm-1 11 cm-1 11 cm-1 relative to the global minimum at -2773 cm-1 And the next four calculated minima a c P4-5

C1 P4-6 C1 P4-7 C1 P4-8 C1 a b 16 cm-1 17 cm-1 20 cm-1 relative to the global minimum 28 cm-1 OCS Cluster Structures from ab initio Pair Potential Note that pair-wise additivity ignores possible manybody non-additive effects.

There are also recent direct ab initio OCS trimer and tetramer calculations which automatically include nonadditive effects, but of course they at a lower level of theory [Sahu, Singh, Gadre, J. Phys. Chem. A 117, 10964 (2013)]. In both cases, the structures are classical potential minima. This ignores quantum effects (zero-point motion), which can be significant for weakly-bound clusters. (OCS)4 Infrared Spectra For a number of years, we observed possible IR bands of (OCS)4, but the situation was confused. Finally, one such band was briefly mentioned in our 2013 review paper. This was the clearest of our OCS tetramer bands (OCS)4 0 Like many microwavers, Pates lab often uses OCS as a test molecule (many isotopes, simple spectrum, ). with one 34S substitution (in natural abundance) 4

2 2 3 3 5 6 7 He wondered whether our suspected infrared OCS tetramer might also appear in their deep scans of jetexpanded OCS/He mixtures. 4965 4970 4975 4980 J=8-7 0 Ka = 4 (OCS)4 normal isotopologue

Yes, it does!! 2 2 5 Well, actually, its not quite that simple 33 6 7 4980 4985 4990 Frequency / MHz 4995 5000 Its not that simple because the microwave tetramer is not responsible for the published 2057.6 cm-1 band, but rather for two unpublished infrared bands, at 2048 and 2061 cm-1 (one shown here, the B-values agree

to <0.1 MHz). x0.1 2061.0 2061.2 Wavenumber / cm 2061.4 -1 2061.6 (OCS)4 As seen in the previous talk, the microwave tetramer is an asymmetric rotor with no symmetry. The experimental structure agrees well (but not perfectly) with our most stable calculated tetramer, P4-1, shown here. A B C Obs 611.330 315.422 308.465

Calc 635 MHz 316 309 But what about the other infrared band, which does not quite agree?? It corresponds to the 7th most stable isomer, with a relative energy of 20 cm-1. It does not appear in the MW spectrum (its calculated dipole moment is much smaller). Looks a bit like the other tetramer, right?? Main difference: the top monomer is now at the other end of the trimer barrel. Obs A 600 (B+C)/2 303.96 (B-C) <7 Calc 608 MHz 310 8 P4-1

P4-7 (CS2)4 observed Col 1 vs tetramerplt-1 P(6) R(4) simulated P(4) R(2) P(2) CPL 570, 12 (2013) 1551.3 R(0) 1551.4 1551.5 1551.6 Wavenumber / cm-1

x0.1 (OCS)4 2061.0 2061.2 2061.4 2061.6 -1 Wavenumber / cm simulated sum (N2O)4 [S4] observed simulated (N2O)4 JCP 134, 074310 (2010) simulated (N2O) 3 2163.00 2163.05

Wavenumber / cm -1 2163.10 (N2O)4 Prolate (D2d symmetry) Oblate (S4 symmetry) Conclusions First spectroscopic observation of OCS tetramers. One isomer is well-characterized from MW spectrum, and also observed by two IR bands. Another isomer observed by one IR band may correspond to calculated isomer P4-7.. Amazing broad-band microwave spectra, has many! lines still unassigned. The direct ab initio calculations of the Gadre group (Kanpur) are in good general agreement with our pairwise calculations. But, significantly, they missed 16 of the 20 lowest energy tetramers, including the lowest 3.