Chirped-Pulse Broadband Microwave Spectraand Structures of the OCS Trimer and Tetramer

Luca Evangelisti, Cristobal Perez, Nathan A. Seifert, Brooks H. PateUniversity of Virginia

Mehdi Dehghany, Nasser Moazzen-Ahmadi University of Calgary

A. R. W. McKellarNRC

Motivation

• Broadband rotational spectroscopy serves as an excellent wayto explore a shallow, complex potential energy surface

• Intensities correlate well with energy orderings of a givennon-covalent complex

• Can tune/filter what minima get populated by choice of buffer gas – for example:

• (H2O)6: use of Ar reveals only onehexamer configuration, whereas Ne reveals three. [see 2012 RH02 or: Perez et al., Science, 336, 897 (2012)]

• (OCS)2 (see next slide): McKellar and coworkers1 (FTIR),and later Minei and Novick2 (FTMW) detectpolar dimer configuration by switching to He in lieu of Ar

• Using computer aids such as AUTOFIT for assignmentcan 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 20072

• +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 r0 structure determinedby 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 -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’tbe hard to detect the parallel trimer using CP-FTMW!

(B2PLYP-D/aug-cc-pVTZ structuresfrom 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 withHe as backing gas• Similar results seen withAr/He in Minei & Novick study

r0 structure fromPeebles & Kuczkowski

B2PLYP-D/aug-cc-pVTZre structure from Sahu, et al.

Pairwise potential derived fromCCSD(T)-F12b/VTZ-F12parameterized (OCS)2 PESfrom Brown, et al.

Purple spheres Kraitchman rs determination from this study

Results: Antiparallel trimer

• Observed original antiparallel trimer with sufficient sensitivityto detect all 34S, 13C and 18O isotopologues in natural abundance

• Chiral --- Bauder and coworkers detect tunneling splittingin cavity microwave spectrum (too narrow to resolve in CP-FTMW)

B2PLYP-D/aug-cc-pVTZre structure from Sahu, et al.

Pairwise potential derived fromCCSD(T)-F12b/VTZ-F12parameterized (OCS)2 PESfrom Brown, et al.

Results: Parallel trimer

A / MHz853.63991(159

)DJ /kHz 0.5339(124) d1 /kHz -0.1575(78)

B721.64632(113

)DJK 0.731(77) d2 -0.0287(32)

C503.18830(105

)Nlines 92 RMS / kHz 3.6

• First detection, with sufficient sensitivity for 34S / 13C

Results: Tetramer

• Detected species with constants consistent with tetramer:

A / MHz611.32965(80

)DJ /kHz 0.05417(93) d1 /kHz -1.59(69)

B315.42238(33

)DJK 0.1694(32) d2 -0.87(32)

C308.46549(32

)DK 0.0881(136)

Nlines 245 RMS /kHz 3.3• Problems!

• Structures from Sahu et al. (right) were not consistentwith observed species

• Some relief: Using AUTOFIT,the full set of 34S and 13C isotopologues were assigned for the tetramer candidate species.The question remains: Can one build a candidate

structure using only unsigned Kraitchman coordinates?

A 683 467 641 554

B 338 444 342 384

C 282 342 273 299

ΔE (kcal

mol-1)

-7.33 -7.19 -7.01 -6.99CCSD(T)/CBS binding energies

• McKellar’s pairwisepotential structures initiallygiving consistent constantsbut wrong monomer orientations

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 besuch that r(CS) is consistent with OCS monomer. The pairingshould 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 3.92605 0.00000 0.11634

C 2.8445 1.09097 0.17052

O 2.0309 1.91170 0.21128

• 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 independentpossibility of being in any of the 8 octants.

Result: 83 = 2048 possible structures [we get thefirst monomer for free, since we fix it to (+++)]

One caveat:

• One set of Kraitchman coordinates for a CS combinationhas an imaginary coordinate along the b axis:

rs 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 generatereasonable CS bond length

Therefore, due to this sign ambiguity, there are FOUR candidate structures that satisfy the geometric construction.

A 614.9842

B 317.6737

C 309.6543σfit (MHz)

0.217

ΔCOM (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

• σfit : RMS residual between predicted scaled isotopologue constants forcandidate structure and experimental isotopologue fits• ΔCOM : average coordinate shift of candidate structure to principal axis

Candidate structures

Expt. Constants: (611.32965(80), 315.42238(33), 308.46549(32))

Kraitchman vs. Best-Fit Candidate Structure

Kraitchman vs. re-optimized pair potential structure

Kraitchman vs. re-optimized M06-2X/6-311++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) ofexistence 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 constraintsenable 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 forisotopic data guide for ab initio calculations

We are NOWHERE close to revealing all ourOCS spectrum has to offer…

Cut spectrum, 3-9 GHz

• ~1600 lines identified• 7100 lines remaining> 3:1 S:N

References1. 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 thankthe National Science Foundation for funding, through the Major Research Instrumentation program, award # CHE-0960074