1. Chirped-Pulse Broadband Microwave Spectra and Structures of the OCS Trimer and Tetramer Luca Evangelisti, Cristobal Perez, Nathan A. Seifert, Brooks H. Pate University of Virginia Mehdi Dehghany, Nasser Moazzen-Ahmadi University of Calgary A. R. W. McKellar NRC

2. Motivation Broadband rotational spectroscopy serves as an excellent way to explore a shallow, complex potential energy surface Intensities correlate well with energy orderings of a given non-covalent complex Can tune/filter what minima get populated by choice of buffer gas – for example: (H 2 O) 6 : use of Ar reveals only one hexamer configuration, whereas Ne reveals three. [see 2012 RH02 or: Perez et al., Science, 336, 897 (2012)] (OCS) 2 (see next slide): McKellar and coworkers 1 (FTIR), and later Minei and Novick 2 (FTMW) detect polar dimer configuration by switching to He in lieu of Ar Using computer aids such as AUTOFIT for assignment can help provide constraints for ab initio searches

3. Introduction: A short microwave recap OCS dimer Polar dimer first observed in IR by McKellar and coworkers (2007) 1 Microwave observation made by Minei & Novick later in 2007 2 +0.2 kcal mol -1 (~70 cm -1 ) [Sahu 2013, CCSD(T)/CBS)] 3 +0.156 kcal mol -1 (54.6 cm -1 ) with ZPE corrections [Brown et al. 2012, (OCS) 2 PES, CCSD(T)-F12b/VTZ-F12] 4 OCS trimer First observed by Connelly, Bauder, Chisholm & Howard (1996) 5 Some isotopic species observed, partial r 0 structure determined by Peebles & Kuczkowski (1999) 6 Antiparallel barrel structure

4. Introduction So what about a parallel OCS trimer? Antiparallel (prev. observed)Parallel Energies (cm -1 )Dipoles (rel. monomer) CCSD(T)/CBS Pairwise potential μa/μb/μc (B2PLYP-D/aug-cc-pVTZ) Antiparallel -1480 [0]-1564 [0]0.8 / 0.1 / 0.6 Parallel -1577 [+97] -1676 [+112] 2.1 / 0.0 / 2.0 ~+100 cm -1 relative energy, strong dipoles – shouldn’t be hard to detect the parallel trimer using CP-FTMW! (B2PLYP-D/aug-cc-pVTZ structures from Sahu et al. )

5. Experimental CP-FTMW: 3-9 GHz band measured in 2 segments: 3-6 GHz: 7.8 million averages 6-9 GHz: 8.9 million averages Dynamic range: ~11000:1 ~10500 lines at S:N ≥ 3:1 1% OCS in Neon, 3.5 atm backing pressure (OCS) 2 very weak in Ne spectrum, but stronger with He as backing gas Similar results seen with Ar/He in Minei & Novick study

6. r 0 structure from Peebles & Kuczkowski B2PLYP-D/aug-cc-pVTZ r e structure from Sahu, et al. Pairwise potential derived from CCSD(T)-F12b/VTZ-F12 parameterized (OCS) 2 PES from Brown, et al. Purple spheres  Kraitchman r s determination from this study Results: Antiparallel trimer Observed original antiparallel trimer with sufficient sensitivity to detect all 34 S, 13 C and 18 O isotopologues in natural abundance Chiral --- Bauder and coworkers detect tunneling splitting in cavity microwave spectrum (too narrow to resolve in CP-FTMW)

7. B2PLYP-D/aug-cc-pVTZ r e structure from Sahu, et al. Pairwise potential derived from CCSD(T)-F12b/VTZ-F12 parameterized (OCS) 2 PES from Brown, et al. Results: Parallel trimer A / MHz 853.63991(159) D J /kHz 0.5339(124) d 1 /kHz -0.1575(78) B 721.64632(113) D JK 0.731(77) d2d2 -0.0287(32) C 503.18830(105)N lines 92RMS / kHz3.6 First detection, with sufficient sensitivity for 34 S / 13 C

8. Results: Tetramer Detected species with constants consistent with tetramer: A / MHz 611.32965(80) D J /kHz 0.05417(93) d 1 /kHz -1.59(69) B 315.42238(33) D JK 0.1694(32) d2d2 -0.87(32) C 308.46549(32) DKDK 0.0881(136) N lines 245RMS /kHz3.3 Problems! Structures from Sahu et al. (right) were not consistent with observed species Some relief: Using AUTOFIT, the full set of 34 S and 13 C isotopologues were assigned for the tetramer candidate species. The question remains: Can one build a candidate structure using only unsigned Kraitchman coordinates? A683467641554 B338444342384 C282342273299 ΔE (kcal mol-1) -7.33-7.19-7.01-6.99 CCSD(T)/CBS binding energies McKellar’s pairwise potential structures initially giving consistent constants but wrong monomer orientations

9. What we know: C and S unsigned Kraitchman coordinates r(CS) = 1.56484(92) Å, r(CO) = 1.15638(113) Å [OCS monomer] 7 All C-S pairs are unique (we observe 4 sets)  relative signs between each pair of C & S coordinates must be such that r(CS) is consistent with OCS monomer. The pairing should be one-to-one with the monomer constraints Algorithm: Build up monomer by monomer. We start with a pair that has the same relative C/S signs, build a monomer using the r(CO) constraint, and force this into the (+++) octant: S3.926050.000000.11634 C2.84451.090970.17052 O2.03091.911700.21128 Add next monomer. This monomer can be in any of the 8 octants wrt. the first monomer. Continue to build up to 4 monomers, each with the independent possibility of being in any of the 8 octants. Result: 8 3 = 2048 possible structures [we get the first monomer for free, since we fix it to (+++)]

10. One caveat: One set of Kraitchman coordinates for a CS combination has an imaginary coordinate along the b axis: rsrs S(3) C(2) |a| 3.92605(40) 2.664(42) |b| [0] 1.456(78) |c| 0.116(13) 0.29(40) Therefore, the relative signs of the b coordinate is ambiguous. Four possible sign combinations The relative |b| sign can be +/– Additional consequence that |c| can be relative +/- and generate reasonable CS bond length Therefore, due to this sign ambiguity, there are FOUR candidate structures that satisfy the geometric construction.

11. A 614.9842 B 317.6737 C 309.6543 σ fit (MHz) 0.217 Δ COM (Å) 0.013 613.9524 317.4038 309.4475 0.254 0.0078 621.9364 318.302 308.2825 0.378 0.052 618.8134 318.1013 309.9487 0.285 0.057 σ fit : RMS residual between predicted scaled isotopologue constants for candidate structure and experimental isotopologue fits Δ COM : average coordinate shift of candidate structure to principal axis Candidate structures Expt. Constants: (611.32965(80), 315.42238(33), 308.46549(32) )

12. Kraitchman vs. Best-Fit Candidate Structure

13. Kraitchman vs. re-optimized pair potential structure Kraitchman vs. re-optimized M06-2X/6-311++g(d,p) structure Hindsight is always 20/20…

14. Tetramer = trimer + monomer? Blue carbon monomers  tetramer Brown carbon monomers  trimer #1 (overlaid via “oxygen-up” monomer) tetramer + trimer enantiomer tetramer enantiomer + trimer Trimer chirality is locked in tetramer complex Calculations unclear (also no experimental detection) of existence of tetramer with opposite trimer chirality

15. Conclusion Take home points on (OCS) 4 detection and elucidation: Structure determination while blind: Clusters are a good case study : monomer constraints enable independent determination of cluster geometry The obvious: rotational spectroscopy excels at finding global (and sometimes local) minima on a potential energy surface Elucidation of a spectrum, with broadband sensitivity sufficient for isotopic data  guide for ab initio calculations We are NOWHERE close to revealing all our OCS spectrum has to offer… Cut spectrum, 3-9 GHz ~1600 lines identified 7100 lines remaining > 3:1 S:N

16. References 1.M. Afshari, M. Dehghani, Z. Abusara, N. Moazzen-Ahmadi, A. R. W. McKellar, J. Chem. Phys., 126, 071102 (2007). 2.A. J. Minei, S. E. Novick, J. Chem. Phys. 126, 101101 (2007). 3.N. Sahu, G. Singh, S. R. Gadre, J. Phys. Chem. A, 117, 10964 (2013). 4.J. Brown, X.-G. Wang, R. Dawes, T. Carrington, J. Chem. Phys., 136, 134306 (2012). 5.J. P. Connelly, A. Bauder, A. Chisholm, B. J. Howard, Mol. Phys. 88, 915 (1996). 6.R. A. Peebles, Robert L. Kuczkowski, J. Phys. Chem. A, 103, 6344 (1999). 7.J. K. G. Watson, A. Roytburg, W. Ulrich, J. Mol. Spectrosc., 196, 102 ( 1999 ). Thanks for your time! Questions? The authors at University of Virginia would like to thank the National Science Foundation for funding, through the Major Research Instrumentation program, award # CHE-0960074