Substitution Structures of Multiple Silicon-Containing Species by Chirped Pulse FTMW Spectroscopy Nathan A. Seifert, Simon Lobsiger, Brooks H. Pate University of Virginia Gamil A. Guirgis, Jason S. Overby College of Charleston James. R. Durig University of Missouri, Kansas City

Motivating question: How can we make the process of spectral fitting in MW spectroscopy efficient and easy, so the technique is available to even those not trained in rotational spectroscopy? Autofit – Automated triples fitter (Front end for SPCAT/SPFIT 1 ) (see additionally Steber et al., TC ; Shipman, et al., RH ) Introduction Anecdote: On a good day, a Pate lab group member can check a given triple for goodness of fit once every two to three minutes using JB95. Every day, Autofit can fit approximately >250 triples in a single second. Fitting a rigid rotor spectra is a purely mechanical task, so why not automate it? The beauty in rotational spectroscopy, after all, is in the analysis. Goal: Using CP-FTMW spectroscopy to assign a standard molecular structure with minimal sample usage and ~1 day of experimental effort To solve this, we have developed: (Steve Shipman of New College of Florida and Ian Finneran of Caltech did most of the programming work, so our eternal gratitude to them!) 1. H. M. Pickett, J. Mol. Spectrosc., 148 (1991), 371.

Autofit Program Flow Enter guess rotational constants & dipole moments ( ab initio predictions) SPCAT Choose three transitions to fit Are transitions sufficiently linearly independent to fit A, B & C? No Yes Choose N additional transitions for checking fit correctness Input two column peakpick of target broadband spectrum Input window size Δν: For checking triples with expt. freqs within ±Δν Split full triples lists into M subarrays M cores M SPFIT instances Fit each A/B/C triple with N + 3 transitions INPUT STAGE CHOICE STAGE PROCESS STAGE Sort triples fits by OMC, collate & output Written in Python; works on any platform with a Python interpreter Only requires compatible SPCAT/SPFIT binaries Easily scalable to multiple CPU cores Technical features:

CPU used: Intel Core i7-3770S (3.1 GHz / 3.9 GHz turbo) 4 physical cores (8 logical w/ hyperthreading) 8MB L3 cache, 4 x 256KB L2 cache $300 on newegg.com (as of May 2013) Some performance observations: Invariant to RAM availability Strong dependence on CPU cache & cache bandwidth Optimal performance would likely be found using a high performance CPU with a good cache, like the Intel Xeon or AMD Opteron series Autofit Benchmarking

Motivating Examples CP-FTMW spectra for all systems taken with the new upgrades at GHz (Similar to 2-8 GHz improvements seen in the slides of Cristobal’s talk, TH10). Spectra range from 120k to 500k averages (one afternoon max. of averaging) 5 molecules chosen – in collaboration with Gamil Guirgis of College of Charleston: CH 3 SiHFNCO 1-cyanosilyl-cyclopropane 1,1,3,3-tetra(fluoro/hydro)-1,3-disilacyclopentane 1-isocyanato-1-silacyclohexane (results also available for 1-isocyanatosilyl-cyclopropane) Autofit philosophy: Initial guess rotational constants/dipoles from MP2/ g(d,p) structures Chose the 3 fit transitions from typical strong features that can fit A, B & C well 5-7 additional strong transitions for checking fit, typically between 7-12 GHz Peakpick cutoff made to include all lines ca. >4:1 signal to noise ratio ~500 MHz window for all autofit runs In all cases, a single Autofit run detected the parent species as well as all isotopologues detectable in natural abundance

CH 3 SiHFNCO – Autofit Results A (MHz)B (MHz)C (MHz)OMC (kHz)Assignment No match CH N 13 CO Si NS No match No match NS NS Si No match N 13 CO NS No match N N (rough) Top 16 fits from Autofit output: Note: OMCs unusually high due to A/E splitting and hyperfine 6.14 million triples (7 hrs, 8 cores) Fit transitions: One a-type K a = 0 (B+C) Two b-type transitions (A; asymmetry) 120,000 avg. spectrum (1 hr)

CH 3 SiHFNCO – Final Results MP g(d,p) Expt. A (MHz) (45) B (MHz) (39) C (MHz) (39) Δ J (kHz) (33) Δ JK (kHz) (14) Δ K (kHz) (10) δ J (kHz) (13) δ K (kHz) (19) 3/2 χ aa (MHz) (11) ¼(χ bb -χ cc ) (MHz) (18) V 3 (cm -1 )436480(19) F (GHz)[158]155.7(66) θ (deg) (10) N lines σ fit (kHz) Parent Species Results 29 Si 30 Si 13 CH 3 N 13 CO 15 N A (MHz) (46) (60) (51) (50) (28) B (MHz) (68) (11) (43) (42) (44) C (MHz) (61) (10) (37) (35) (38) N lines σ fit (kHz) Isotopologue Results (Other parameters held fixed) MP2/ g(d,p) structure overlaying r s coordinates A/E states + quad fit simultaneously with XIAM Structural trends consistent with CH 3 SiF 2 NCO results (2012 MH08) V 3 decreases with additional halogenation ; r[H 3 C-Si] decreases by 0.3 Å Difluoro V 3 : 446(5) cm -1 ; CH 3 SiF 2 Cl V 3 : 463(3) cm -1 Similar bend in –NCO: <[Si-N-C] = 159.8(9)° (F 2 ); 152.6(18)° (HF)

1-isocyanato-1-silacyclohexane – Autofit Results Equatorial ΔE = 414 cm -1 Axial ΔE = 0 cm -1 Rank A (MHz)B (MHz)C (MHz)OMC (kHz)Assignment N NS Si C C Si C CN Axial -, selected Autofit results (with sorted rankings) Rank A (MHz)B (MHz)C (MHz)OMC (kHz)Assignment NS Si Si CN C C C N Equa -, selected Autofit results (with sorted rankings) 6.13 million triples (10 hrs, 4 cores) Fit transitions: Three a-types; K a = 0, K a = 1, K a = 2 Little A dependence, so error on A is high 5.03 million triples (~9 hrs, 4 cores) Fit transitions: Three a-types; K a = 0, K a = 1, K a = 2 320,000 avg. spectrum (2.8 hrs)

Open issue: MP2 (as well as DFT) Severely underestimates NCO bending angle (X aa discrepencies are a real tell!) 1-isocyanato-1-silacyclohexane – Results MP g(d,p) Expt. A (MHz) (44) B (MHz) (20) C (MHz) (17) Δ J (kHz) (94) Δ JK (kHz) (45) Δ K (kHz) (155) δ J (kHz) (82) δ K (kHz) --[0] 3/2 χ aa (MHz) (33) ¼(χ bb -χ cc ) (MHz) (13) N lines σ fit (kHz) MP g(d,p) Expt. A (MHz) (26) B (MHz) (18) C (MHz) (18) Δ J (kHz) (61) Δ JK (kHz) (52) Δ K (kHz) --[0] δ J (kHz) --[0] δ K (kHz) (80) 3/2 χ aa (MHz) (80) ¼(χ bb -χ cc ) (MHz) [0] N lines σ fit (kHz)

Conclusions The Future of Autofit: On the technical side: CPU scaling via the cloud is cheap and perhaps even trivial! A quick back of the envelope calculation for large scaling on Amazon EC2 clusters: Assume 50 Hz/core; 20 EC2 logical cores/instance for $0.580/hr Maximum of 20 instances at once, so 400 logical cores for $11.60/hr Effective compute speed of 20 kHz, so 500 seconds to fit 10 7 triples  $1.60 per autofit run! Autofit is a fast and efficient way to quickly assign broadband spectra for both parent species and isotopologues for the purpose of molecular identification or structural determination. Orders of magnitude faster than routine, manual spectra fitting Helps enable structure determination via CP-FTMW become a routine activity With new CP-FTMW spectra approaching line densities of over >1 MHz -1, visual pattern recognition for weakly abundant species is nearly impossible Automated optimization for choosing a frequency window size with respect to typical ab initio error on A/B/C Automated choosing of 3 fit transitions based on optimal linear independence to fit A/B/C A graphical interface where autofit results are integrated into a JB95/PGOPHER/AABS-like interactive fitting program

Acknowledgements Thanks to the NSF for funding: MRI-R2, Award CHE Thanks for your time! Pate Group Brooks Pate Cristobal Perez Simon Lobsiger Luca Evangelisti Brent Harris Amanda Steber Nathan Seifert Shameless plug – Autofit is freely available at git repository: git clone git://github.com/pategroup/bband_scripts.git Works in Windows, but even easier to setup in x86/x86-64 Linux!

CH 3 SiHFNCO – Final Results SpeciesV 3 (cm -1 )R[H 3 C-Si] (Å) Citation (CH 3 ) 2 SiH (2) L. Pierce, J. Chem. Phys. 34 (1961) 498. CH 3 SiH 2 Cl (5) R. H. Schwendeman, G.D. Jacobs, J. Chem. Phys. 36 (1962) CH 3 SiHCl J. R. Durig, C. W. Hawley, J. Chem. Phys. 59 (1973) 1. CH 3 SiCl M. A. Qtaitat, et al., Spectrochim. Acta A, 50 (1994) 621. CH 3 SiH 2 F545(1)1.849(5) L.C. Krisher, L. Pierce, J. Chem. Phys. 32 (1960) CH 3 SiHF 2 439(10)1.840(1) L.C. Krisher, L. Pierce, J. Chem. Phys. 32 (1960) CH 3 SiF (9)1.812(14) J.R. Durig, Y.S. Li, C.C. Tong, J. Mol. Struct. 14 (1972) 255. CH 3 SiF 2 NCO446(5)1.814(5) G.A. Guirgis, et al. J. Phys. Chem. A. 116 (2012) CH 3 SiF 2 Cl468(3)1.814(1) N. A. Seifert, et al. J. Mol. Struct (2012) 222. CH 3 SiHFNCO480(19)1.843(7) This work How does it compare to other species in the series?