Steven T. Shipman, 1 Justin L. Neill, 2 Matt T. Muckle, 2 Richard D. Suenram, 2 and Brooks H. Pate 2 Chirped-Pulse Fourier Transform Microwave Spectroscopy.

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Presentation transcript:

Steven T. Shipman, 1 Justin L. Neill, 2 Matt T. Muckle, 2 Richard D. Suenram, 2 and Brooks H. Pate 2 Chirped-Pulse Fourier Transform Microwave Spectroscopy of Ethyl 3-Methyl 3-Phenylglycidate (Strawberry Aldehyde) 1 New College of Florida 2 University of Virginia

Rotational Spectroscopy of Large Molecules Challenges: Low rotational constants → many transitions More conformers → need more molecules / averaging for same S/N Difficult to get into the gas phase Potentially very expensive ($50 / gram for many biomolecules) Ab initio calculations take much longer Prototype large molecule: Strawberry aldehyde – C 12 O 3 H 14 Workable vapor pressures by heating Relatively rigid structure Cheap! (Food additive…)

Direct Detect Spectrometer Chirped pulse is generated by mixing output of 24 GS/s AWG with GHz oscillator. Detection is direct – no mixers! No “image peak” problem. Collect 10 FIDs per valve pulse. 0.6 Hz at FID duration of 20  s. (21,600 FIDs per hour) All data is from a 929,000 FID spectrum taken with 2 nozzles (4 – 5 days of data collection).

Strawberry Aldehyde Spectrum S/N on largest peak ~2000:1 Spectrum contains: multiple conformers isotopomers decomposition products 1 atm He/Ne 2 nozzles Sample at 120 °C 929,000 FIDs (20  s) Threshold S/N# of Peaks 200: : : : :18921

Strawberry Aldehyde Conformers I II III IV V

Strawberry Aldehyde Rotational Constants IIIIIIIVV A (MHz) (7) (29) (9) (23) (6) B (MHz) (5) (4) (6) (4) (8) C (MHz) (8) (4) (10) (5) (7)  J (kHz) (5) (3)0.3261(5) (4) (11)  JK (kHz) (20) (7) (21) (6) (18)  K (kHz) (16)0.862(12)0.5728(19)0.958(9)1.49(4)  J (Hz) 25.55(26)3.389(13)119.11(27)3.144(13)0.896(17)  K (Hz) -18.2(12)-325.7(21)-95.2(15)542(4)-1195(12) # lines OMC (kHz)

Possible Conformers Dihedral 1234: 2 minima Dihedral 2345: 2 minima Dihedral 3456: 3 minima 12 for each diastereomer (1 and 2 are chiral) Why only 5, and why not 6? Next: map assigned species onto molecular geometries Would expect cis and trans species to come in pairs…

Relaxed Potential Energy Surface (trans) cm -1 Barrier between regions: 120° to 300° : 1220 cm ° to 120° : 950 cm-1 Intra-region barrier: 170 cm -1 Calculated at B3LYP/6-31+G(d) 15 degree increments (576 geometry optimizations)

Relaxed Potential Energy Surface (cis) cm -1 Barrier between regions: 1450 cm -1 (Roughly symmetric) Intra-region barrier: 185 cm -1 Calculated at B3LYP/6-31+G(d) 15 degree increments (576 geometry optimizations)

Structures of Main Conformers I III II IV V Cis: Orientation of oxygens Trans: Orientation of terminal –CH 3 (Structures from B3LYP/ G(d,p) level of theory.)

Matching Theory With Experiment (trans) MethodConformerA (MHz)B (MHz)C (MHz)Dipoles (D) ExperimentII  A ~ 0,  B >  C IV  A ~ 0,  B >  C V  A ~ 0,  B >  C B3LYPII  A = 0.1,  B = 3.1,  C = G(d,p)IV  A = 0.0,  B = 3.2,  C = 0.3 V  A = 0.1,  B = 2.8,  C = 1.2 MP2II  A = 0.1,  B = 3.7,  C = G+(d)IV  A = 0.3,  B = 3.7,  C = 0.4 V  A = 0.3,  B = 3.4,  C = 0.8 Both B3LYP and MP2 constants and dipoles are in good agreement with data. All trans conformers have similar dipole moments and directions.

Matching Theory With Experiment (cis) MethodConformerA (MHz)B (MHz)C (MHz)Dipoles (D) ExperimentI  A >  B,  C ~ 0 III  A ~ 0,  B <  C B3LYPI  A = 1.5,  B = 1.1,  C = G(d,p)III  A = 0.7,  B = 0.9,  C = 3.0 MP2I  A = 2.0,  B = 0.5,  C = G+(d)III  A = 0.0,  B = 1.3,  C = 3.5 B3LYP constants are terrible! Dipoles are also bad. MP2 is closer. Constants are similar, so match is on basis of dipole direction.

Confirming Calculated Structures Need to verify that calculated structures are correct. Look at carbon backbone! Can forward predict 13 C constants and then search in spectrum. Procedure: 1) Use NS constants from experiment and theory to get scale factors. 2) Predict 13 C constants, use same scale factor. 3) Use prediction as a starting point for the assignment. Conformer I: MP2 (blue) and B3LYP (grey) B3LYP does not handle dispersion interactions well. 1,2 M05-2X (DFT) calculations underway… 1) Y. Zhao and D.G. Truhlar, J. Chem. Theory Comput. 3, 289 (2007) 2) V. A. Shubert et al., J. Chem. Phys. 127, (2007)

Carbon Backbone Analysis Conformer I – 319 transitions for 12 isotopomers Conformer II – 347 transitions for 12 isotopomers Data analyzed with the KRA program; Numbers are average deviation per C between theory and experiment. I (B3LYP) II (MP2) I (MP2) II (B3LYP) 0.41 Å 0.16 Å 0.14 Å 0.11 Å I (M05-2X) 0.09 Å

Decomposition Products Acetophenone Ethyl Formate (gauche and trans) Also see ethanol, ethyl glycolate, and a mystery species. Mystery species is NOT: anisole, ethylbenzene, styrene, phenol, benzyl alcohol, or benzaldehyde. Constants of A = MHz, B = MHz, C = MHz. All assigned transitions are a-type.

Residuals Still a lot of peaks left! Have only assigned about 20% of the peaks in the original spectrum. Increasingly difficult to work as more and more peaks are removed from the spectrum.

Summary and Future Work Would like to have: Electronic spectrum (Pratt group) Stark effect data (2 – 8 GHz?) Identity of the mystery molecule! Have assigned: 5 dominant conformers C isotopomers 4 decomposition products For this system, it was extremely helpful to have: Extensive ab initio results Preliminary spectrum at 2 – 8 GHz S/N to confirm assignments with 13 C in natural abundance Multi-nozzle, multi-FID setup to reduce sample consumption

Acknowledgements Current and Former Members of The Pate Lab Leo Alvarez Christoph Etschmaier Matt Muckle Justin Neill Daniel Zaleski Collaborators Rick Suenram David Pratt Funding New College of Florida Start-Up Funding NSF Chemistry CHE NSF CRIF:ID CHE

Relative Energies File #E (cm -1 )  T (D) D2D4Conf # I ─ III ─ ─ ─ File #E (cm -1 )  T (D) D2D4Conf # II V IV ─ ─ ─ Cis MP2 / 6-31+G(d) Relative energies are NOT zero-point corrected Trans B3LYP / G(d,p) Relative energies ARE zero-point corrected

KRA Table O’ Numbers (Conf I, cis) a (Å)b (Å)c (Å)a (Å)b (Å)c (Å)a (Å)b (Å)c (Å) C C C ─ C C C ─ C12─1.78─ C ─ C C C C ExperimentMP2M05-2X Avg error / C: MP2: 0.16 Å M05-2X: 0.09 Å Exp’t good to 3 decimal places. Only showing 2 for space!

KRA Table O’ Numbers (Conf I, cis) a (Å)b (Å)c (Å)a (Å)b (Å)c (Å)a (Å)b (Å)c (Å) C C C ─ C C C ─ C12─1.78─ C ─ C C C C ExperimentMP2B3LYP Avg error / C: MP2: 0.16 Å DFT: 0.41 Å Exp’t good to 3 decimal places. Only showing 2 for space!

KRA Table O’ Numbers (Conf II, trans) a (Å)b (Å)c (Å)a (Å)b (Å)c (Å)a (Å)b (Å)c (Å) C C C ─ C C C C C C C C C ─ ExperimentMP2B3LYP Avg error / C: MP2: 0.11 Å DFT: 0.14 Å Exp’t good to 3 decimal places. Only showing 2 for space!

2 – 8.5 GHz Spectrum of C 12 H 14 O 3 I II IIIIV 1 nozzle, heated to 120°C