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

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

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 kcal mol -1 (~70 cm -1 ) [Sahu 2013, CCSD(T)/CBS)] 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

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 [0]-1564 [0]0.8 / 0.1 / 0.6 Parallel [+97] [+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. )

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

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)

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 (159) D J /kHz (124) d 1 /kHz (78) B (113) D JK 0.731(77) d2d (32) C (105)N lines 92RMS / kHz3.6 First detection, with sufficient sensitivity for 34 S / 13 C

Results: Tetramer Detected species with constants consistent with tetramer: A / MHz (80) D J /kHz (93) d 1 /kHz -1.59(69) B (33) D JK (32) d2d (32) C (32) DKDK (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? A B C ΔE (kcal mol-1) CCSD(T)/CBS binding energies McKellar’s pairwise potential structures initially giving consistent constants but wrong monomer orientations

What we know: C and S unsigned Kraitchman coordinates r(CS) = (92) Å, r(CO) = (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: S C O 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 (+++)]

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| (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.

A B C σ fit (MHz) Δ COM (Å) 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: ( (80), (33), (32) )

Kraitchman vs. Best-Fit Candidate Structure

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

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

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

References 1.M. Afshari, M. Dehghani, Z. Abusara, N. Moazzen-Ahmadi, A. R. W. McKellar, J. Chem. Phys., 126, (2007). 2.A. J. Minei, S. E. Novick, J. Chem. Phys. 126, (2007). 3.N. Sahu, G. Singh, S. R. Gadre, J. Phys. Chem. A, 117, (2013). 4.J. Brown, X.-G. Wang, R. Dawes, T. Carrington, J. Chem. Phys., 136, (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