International Symposium on Molecular Spectroscopy 71 st Meeting

International Symposium on Molecular Spectroscopy, 71 st Meeting Champaign-Urbana, June 20 -24, 2016 Large molecules structures by Broadband Fourier Transform Rotational Spectroscopy Luca Evangelistsi 1, Nathan Seifert 2, Lorenzo Spada 1, Brooks H. Pate Department of Chemistry University of Virginia 1 Dipartmento di Chimica “Giacomo Ciamician” Bologna 2 Department of Chemistry, University of Alberta http: //faculty. virginia. edu/bpate-lab/

Rotational Spectroscopy of Single Molecules The Problem from the Physical Chemistry Perspective: Modern computational chemistry produces structures and conformational potential energy surfaces that provide accurate estimates of the spectroscopic constants and relative energies. Is there a reason to perform the measurement? The Dream of Analytical Chemistry: A rapid spectroscopy technique where molecules can be unambiguously identified (especially in a mixture) by direct comparison of experimental spectroscopic constants to theoretical estimates. Library-free detection (the ability to perform analysis without a previously prepared, high purity reference sample)

Major Challenge: Working with molecules of interest in chemistry Large Molecule Rotational Spectroscopy (Pulsed Jet) • Cedrol C 15 H 26 O MW 222 Monomer A 801 B 505 C 382 -1 Q 12, 431 Linear decrease in frequency of peak intensity (fixed T)

Simulated Rotational Spectra as a Function of Molecular Size Solketal C 6 H 12 O 3 Simon Lobsiger, Cristobal Perez, Luca Evangelisti, Kevin K. Lehmann, Brooks H. Pate, “Molecular Structure and Chirality Detection by Fourier Transform Microwave Spectroscopy”, J. Phys. Chem. Lett. 6, 196 -200 (2015). Ambroxide C 16 H 28 O Target size for applications in pharmaceuticals A-type rotational spectra; equal dipole moment SPCAT Simulations at T = 1. 5 K

Low Frequency (2 -8 GHz) Chirped-Pulse Fourier Transform Microwave Spectrometer General Spectral Properties: Measurement Bandwidth: FWHM Resolution: Transitions in a Spectrum: RMS Frequency Error in Fit: 6000 MHz 60 k. Hz (105 data channels) 20 -200 (0. 02 -0. 2% of range) 6 -10 k. Hz (~10% of FWHM) C. Perez, S. Lobsiger, N. A. Seifert, D. P. Zaleski, B. Temelso, G. C. Shields, Z. Kisiel, B. H. Pate, Chem. Phys. Lett. 571, 1 (2013).

Sample in gas phase… Laser ablation Heatable nozzle - Quite expensive - Cheap - Need expertise - Easy to handle - Safety - Safe

Large Molecule Test Set Small Drug Pharmaceuticals and Lipinski’s Rule of Five - Its molecular weight is less than 500 Da. -Octanol Partition Less than Five (Polar) - The number of groups in the molecule that can donate hydrogen atoms to hydrogen bonds (usually the sum of hydroxyl and amine groups in a drug molecule) is less than 5. - Less than 5 rotatable bonds* *Drug discovery: Chemical beauty contest Paul Leeson Nature 481, 455– 456 Nootkatone C 15 H 22 O MW 218 MP 35 o. C Cedrol C 15 H 26 O MW 222 BP 273 o. C Ambroxide C 16 H 28 O MW 236 MP 75 o. C Sclareolide C 16 H 22 O 2 MW 250 MP 125 o. C DHAA C 15 H 24 O 2 MW 236 MP 75 o. C

Ambroxide (C 16 H 28 O): Heavy Atom Structure from the Analysis of Isotopologue Rotational Spectra 1000: 1 1 M FID Averages

Molecular Structure from Isotopic Substitution Structure Information through Principal Moments of Inertia: Measure: A, B, C “normal species” A, B, C singly substituted isotopomer From : ΔIa, ΔIb, ΔIc Obtain: (|Ra|, |Rb|, |Rc|) J. Kraitchman, Am. J. Phys. 21, 17 (1953). Kraitchman Analysis 1) Build up the molecular structure “atom-by-atom” 2) No model assumptions required (but guidance on sign is helpful) 3) A single answer for a single data set 4) Other methods for refinement Structures of Phenol Dimer and Trimer from Isotopes in Natural Abundance Phys. Chem. Phys. , 2013, 15, 11468 Alberto Lesarri, Valladolid Reproducibility and Repeatability

Large Molecule Test Set Nootkatone C 15 H 22 O MW 218 MP 35 o. C Cedrol C 15 H 26 O MW 222 BP 273 o. C Ambroxide C 16 H 28 O MW 236 MP 75 o. C Sclareolide C 16 H 22 O 2 MW 250 MP 125 o. C DHAA C 15 H 24 O 2 MW 236 MP 75 o. C

Comparison of Experimental and Theoretical Structures Ambroxide EXP HF Basis set MP 2 B 2 PLYPD 3 6 -311++G** A / MHz 860. 37 859. 45 866. 13 862. 35 B / MHz 363. 29 361. 95 365. 42 363. 71 C / MHz 313. 27 312. 31 315. 03 313. 47 -1. 2, 0. 6 -1. 2, 1. 1, 0. 7 µa, µb, µc HF: MP 2: D 3: Rtheory > RKRA Rtheory < RKRA Rtheory ~ RKRA Btheory < Bexp Btheory > Bexp Btheory ~ Bexp

Rotational spectroscopy VS X-ray structures rs structure for Sclareolide Absolute configuration: -Optical rotatory dispersion -Vibrational circular dichroism -Proton NMR (chiral shift reagent) -X-ray crystallography X-ray structure for Sclareolide

Direct Analysis of Diastereomer Ratios by Molecular Rotational Spectroscopy • Identification of diastereomers without the need for a known, pure sample • Sensitivity of the structures to epimerization for large molecules • Conformational flexibility • Accuracy of theoretical structures and rotational constants (and dipole moment)

Conclusions 1) Accurate structures (0. 02 A for coordinates) from B 2 PLYP D 3 Calculations BROOKS PATE WK 04 DHAA 2) For molecules with MW 200 -250 Da (15 -20 heavy atoms): S/N Ratio 300 -1000: 1 (all things being equal: 30 -100: 1 up to 500 Da) Cedrol C 15 H 26 O MW 222 Monomer Dimer (32 Heavy Atoms!) A 801 A 298 MHz B 505 B 68 MHz C 382 C 64 MHz MW 444

Acknowledgements Ø Pate Group Ø Bright. Spec Ø NSF Ø MC-IOF 328405 Ø RSC