Steven T. Shipman,1 Justin L. Neill,2 Matt T. Muckle,2 Richard D. Suenram,2 and Brooks H. Pate2

Chirped-Pulse Fourier Transform Microwave Spectroscopy of Ethyl 3-Methyl 3-Phenylglycidate

(Strawberry Aldehyde)

1 New College of Florida2 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 – C12O3H14

• 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 18.95 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 SpectrumS/N on largest peak ~2000:1

Spectrum contains: • multiple conformers• isotopomers• decomposition products

1 atm He/Ne2 nozzlesSample at 120 °C929,000 FIDs (20 s)

Threshold S/N # of Peaks

200:1 323

40:1 967

10:1 2472

4:1 6352

3:1 8921

Strawberry Aldehyde Conformers

I

II

III

IVV

Strawberry Aldehyde Rotational ConstantsI II III IV V

A (MHz) 728.09519(7) 1214.72959(29) 723.14164(9) 1330.94914(23) 1460.5288(6)

B (MHz) 628.69162(5) 287.76597(4) 581.41125(6) 293.48257(4) 275.04576(8)

C (MHz) 429.84842(8) 269.46307(4) 421.96311(10) 281.86483(5) 269.61954(7)

J (kHz) 0.0762(5) 0.01579(3) 0.3261(5) 0.01870(4) 0.00869(11)

JK (kHz) -0.0802(20) -0.0628(7) -0.7634(21) -0.0914(6) -0.0236(18)

K (kHz) 0.0674(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 280 221 232 193 124

OMC (kHz) 12.9 10.7 14.5 15.6 10.5

Possible Conformers

1

2

3

4

56

Dihedral 1234: 2 minimaDihedral 2345: 2 minimaDihedral 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)

4000

2000

0 cm-1

Barrier between regions: 120° to 300° : 1220 cm-1

300° 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)

4000

2000

0 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 ConformersI

III

II IV

V

Cis: Orientation of oxygens Trans: Orientation of terminal –CH3

(Structures from B3LYP/6-311++G(d,p) level of theory.)

Matching Theory With Experiment (trans)Method Conformer A (MHz) B (MHz) C (MHz) Dipoles (D)

Experiment II 1214.7 287.8 269.5 A ~ 0, B > C

IV 1330.9 293.5 281.9 A ~ 0, B > C

V 1460.5 275.0 269.6 A ~ 0, B > C

B3LYP II 1214.2 285.3 264.2 A = 0.1, B = 3.1, C = 0.9

6-311++G(d,p) IV 1317.5 289.4 274.6 A = 0.0, B = 3.2, C = 0.3

V 1442.8 272.6 265.1 A = 0.1, B = 2.8, C = 1.2

MP2 II 1215.5 291.8 265.5 A = 0.1, B = 3.7, C = 0.8

6-31G+(d) IV 1337.1 298.2 277.7 A = 0.3, B = 3.7, C = 0.4

V 1459.8 277.8 266.7 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)

Method Conformer A (MHz) B (MHz) C (MHz) Dipoles (D)

Experiment I 728.1 628.7 429.8 A > B, C ~ 0

III 723.1 581.4 422.0 A ~ 0, B < C

B3LYP I 726.6 576.7 403.6 A = 1.5, B = 1.1, C = 0.1

6-311++G(d,p) III 772.8 476.8 374.8 A = 0.7, B = 0.9, C = 3.0

MP2 I 727.0 634.7 434.4 A = 2.0, B = 0.5, C = 0.2

6-31G+(d) III 712.2 604.5 431.2 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 StructuresNeed to verify that calculated structures are correct. Look at carbon backbone!

Can forward predict 13C constants and then search in spectrum.

Procedure:1) Use NS constants from experiment and theory to get scale factors.2) Predict 13C 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, 234315 (2007)

Carbon Backbone AnalysisConformer I – 319 transitions for 12 isotopomersConformer 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

AcetophenoneEthyl 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 = 4948.395 MHz, B = 1677.941 MHz, C = 1273.960 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 conformers24 13C isotopomers4 decomposition products

For this system, it was extremely helpful to have:

Extensive ab initio resultsPreliminary spectrum at 2 – 8 GHzS/N to confirm assignments with 13C in natural abundanceMulti-nozzle, multi-FID setup to reduce sample consumption

Acknowledgements

Current and Former Members of The Pate LabLeo Alvarez

Christoph EtschmaierMatt MuckleJustin Neill

Daniel Zaleski

CollaboratorsRick Suenram

David Pratt

FundingNew College of Florida Start-Up Funding

NSF Chemistry CHE-0616660NSF CRIF:ID CHE-0618755

Relative EnergiesFile # E (cm-1) T (D) D2 D4 Conf #

6 0 2.1 -67 -81 I

8 137 2.3 -61 -168 ─

10 374 3.8 101 80 III

5 387 1.8 -64 84 ─

9 647 3.7 110 179 ─

11 650 3.4 113 -82 ─

File # E (cm-1) T (D) D2 D4 Conf #

3 0 3.3 129 -178 II

2 159 3.0 128 -87 V

4 203 3.2 128 87 IV

18 219 1.7 -65 178 ─

19 390 1.5 -64 87 ─

20 461 1.7 -65 -87 ─

CisMP2 / 6-31+G(d)

Relative energies are NOT zero-point corrected

TransB3LYP / 6-311++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)

C1 2.48 0.72 1.26 -2.52 0.56 -1.26 -2.51 0.62 -1.25

C2 1.50 0.15 1.23 -1.46 -0.35 -1.24 -1.48 -0.32 -1.22

C3 0.97 0.69 ─ -0.92 -0.76 -0.01 -0.95 -0.73 0.00

C4 1.47 0.14 1.17 -1.43 -0.26 1.19 -1.44 -0.19 1.19

C5 2.47 0.83 1.14 -2.49 0.65 1.16 -2.47 0.74 1.15

C6 3.01 1.19 ─ -3.04 1.08 -0.06 -3.00 1.15 -0.06

C12 ─ 1.78 ─ 0.19 -1.77 0.01 0.13 -1.77 0.03

C13 1.52 1.39 ─ 1.60 -1.31 -0.05 1.55 -1.36 -0.04

C15 0.42 3.14 0.43 -0.15 -3.15 -0.48 -0.26 -3.14 -0.46

C18 1.92 0.02 0.22 1.91 0.14 -0.25 1.90 0.08 -0.26

C22 1.86 2.23 0.51 1.62 2.33 0.56 1.74 2.29 0.54

C25 0.73 2.86 0.19 0.49 2.87 -0.30 0.59 2.85 -0.28

Experiment MP2 M05-2X

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)

C1 2.48 0.72 1.26 -2.52 0.56 -1.26 -2.51 1.08 -1.26

C2 1.50 0.15 1.23 -1.46 -0.35 -1.24 -1.68 -0.04 -1.21

C3 0.97 0.69 ─ -0.92 -0.76 -0.01 -1.17 -0.52 0.01

C4 1.47 0.14 1.17 -1.43 -0.26 1.19 -1.51 0.17 1.19

C5 2.47 0.83 1.14 -2.49 0.65 1.16 -2.35 1.28 1.14

C6 3.01 1.19 ─ -3.04 1.08 -0.06 -2.85 1.74 -0.08

C12 ─ 1.78 ─ 0.19 -1.77 0.01 -0.31 1.73 0.05

C13 1.52 1.39 ─ 1.60 -1.31 -0.05 1.18 -1.64 -0.03

C15 0.42 3.14 0.43 -0.15 -3.15 -0.48 -0.97 -3.02 -0.40

C18 1.92 0.02 0.22 1.91 0.14 -0.25 1.91 -0.36 -0.29

C22 1.86 2.23 0.51 1.62 2.33 0.56 2.46 1.82 0.51

C25 0.73 2.86 0.19 0.49 2.87 -0.30 1.59 2.79 -0.28

Experiment MP2 B3LYP

Avg error / C:MP2: 0.16 ÅDFT: 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)

C1 3.57 1.16 1.07 3.49 -1.31 -1.02 3.47 -1.38 -0.98

C2 2.36 0.48 1.22 2.26 -0.65 -1.17 2.26 -0.72 -1.12

C3 1.77 0.19 ─ 1.77 0.17 -0.15 1.80 0.15 -0.12

C4 2.46 0.18 1.10 2.51 0.31 1.03 2.59 0.35 1.02

C5 3.68 0.46 1.24 3.73 -0.34 1.18 3.80 -0.31 1.16

C6 4.24 1.15 0.16 4.22 -1.16 0.16 4.25 -1.18 0.16

C12 0.39 0.88 0.15 0.45 0.86 -0.29 0.48 0.87 -0.27

C13 0.47 0.43 0.63 -0.65 0.38 0.59 -0.65 0.39 0.58

C15 0.20 1.52 1.59 0.16 1.53 -1.61 0.24 1.58 -1.59

C18 2.06 0.49 0.21 -2.09 0.50 0.19 -2.09 0.52 0.17

C22 4.04 0.74 0.14 -4.08 -0.74 -0.09 -4.09 -0.75 -0.10

C25 4.50 2.19 ─ -4.55 -2.16 0.12 -4.58 -2.16 0.16

Experiment MP2 B3LYP

Avg error / C:MP2: 0.11 ÅDFT: 0.14 Å

Exp’t good to 3 decimal places. Only showing 2 for space!

2 – 8.5 GHz Spectrum of C12H14O3

I II

III IV

1 nozzle, heated to 120°C