Chirped-Pulse Fourier Transform Microwave
Spectroscopy of Ethyl 3-Methyl 3-Phenylglycidate
(Strawberry Aldehyde)
Steven T. Shipman,1 Justin L. Neill,2 Matt T. Muckle,2 Richard D. Suenram,2
and Brooks H. Pate2
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 – 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 ms.
(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
1 atm He/Ne
2 nozzles
Sample at 120 °C
929,000 FIDs (20 ms)
S/N on largest peak ~2000:1
Spectrum contains:
• multiple conformers
• isotopomers
• decomposition products
Threshold S/N # of Peaks
200:1
323
40:1
967
10:1
2472
4:1
6352
3:1
8921
Strawberry Aldehyde Conformers
V
IV
III
II
I
Strawberry Aldehyde Rotational Constants
I
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)
DJ (kHz)
0.0762(5)
0.01579(3)
0.3261(5)
0.01870(4)
0.00869(11)
DJK (kHz)
-0.0802(20)
-0.0628(7)
-0.7634(21)
-0.0914(6)
-0.0236(18)
DK (kHz)
0.0674(16)
0.862(12)
0.5728(19)
0.958(9)
1.49(4)
dJ (Hz)
25.55(26)
3.389(13)
119.11(27)
3.144(13)
0.896(17)
dK (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
Next: map assigned species
onto molecular geometries
Dihedral 1234: 2 minima
Dihedral 2345: 2 minima
Dihedral 3456: 3 minima
1
2
12 for each diastereomer
(1 and 2 are chiral)
3
4
6
5
Why only 5, and why not 6?
Would expect cis and trans
species to come in pairs…
Relaxed Potential Energy Surface (trans)
0 cm-1
2000
4000
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)
0 cm-1
2000
4000
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
IV
II
III
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)
II
1214.7
287.8
269.5
mA ~ 0, mB > mC
IV
1330.9
293.5
281.9
mA ~ 0, mB > mC
V
1460.5
275.0
269.6
mA ~ 0, mB > mC
B3LYP
II
1214.2
285.3
264.2
mA = 0.1, mB = 3.1, mC = 0.9
6-311++G(d,p)
IV
1317.5
289.4
274.6
mA = 0.0, mB = 3.2, mC = 0.3
V
1442.8
272.6
265.1
mA = 0.1, mB = 2.8, mC = 1.2
MP2
II
1215.5
291.8
265.5
mA = 0.1, mB = 3.7, mC = 0.8
6-31G+(d)
IV
1337.1
298.2
277.7
mA = 0.3, mB = 3.7, mC = 0.4
V
1459.8
277.8
266.7
mA = 0.3, mB = 3.4, mC = 0.8
Experiment
Dipoles (D)
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)
I
728.1
628.7
429.8
mA > mB, mC ~ 0
III
723.1
581.4
422.0
mA ~ 0, mB < mC
B3LYP
I
726.6
576.7
403.6
mA = 1.5, mB = 1.1, mC = 0.1
6-311++G(d,p)
III
772.8
476.8
374.8
mA = 0.7, mB = 0.9, mC = 3.0
MP2
I
727.0
634.7
434.4
mA = 2.0, mB = 0.5, mC = 0.2
6-31G+(d)
III
712.2
604.5
431.2
mA = 0.0, mB = 1.3, mC = 3.5
Experiment
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 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 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.
0.41 Å
0.09 Å
0.16 Å
I (B3LYP)
I (MP2)
I (M05-2X)
0.11 Å
0.14 Å
II (B3LYP)
II (MP2)
Decomposition Products
Ethyl Formate (gauche and trans)
Acetophenone
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
Have assigned:
Would like to have:
5 dominant conformers
24 13C isotopomers
4 decomposition products
Electronic spectrum (Pratt group)
Stark effect data (2 – 8 GHz?)
Identity of the mystery molecule!
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 13C 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-0616660
NSF CRIF:ID CHE-0618755
Relative Energies
File # E (cm-1) mT (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
─
Cis
MP2 / 6-31+G(d)
Relative energies are NOT
zero-point corrected
File # E (cm-1) mT (D)
Trans
B3LYP / 6-311++G(d,p)
Relative energies ARE
zero-point corrected
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
─
KRA Table O’ Numbers (Conf I, cis)
Experiment
MP2
M05-2X
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
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
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
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
2.29
C25 0.73
2.86
0.19
0.49
2.87 -0.30 0.59
─
Avg error / C:
MP2: 0.16 Å
M05-2X: 0.09 Å
-0.92 -0.76 -0.01 -0.95 -0.73 0.00
1.16 -2.47 0.74
0.56
1.15
0.13 -1.77 0.03
1.74
0.54
2.85 -0.28
Exp’t good to 3
decimal places.
Only showing 2
for space!
KRA Table O’ Numbers (Conf I, cis)
Experiment
MP2
B3LYP
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
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
1.14
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
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
C25 0.73
2.86
0.19
0.49
2.87 -0.30 1.59
─
Avg error / C:
MP2: 0.16 Å
DFT: 0.41 Å
-0.92 -0.76 -0.01 -1.17 -0.52 0.01
1.16 -2.35 1.28
0.56
2.46
1.82
0.05
0.51
2.79 -0.28
Exp’t good to 3
decimal places.
Only showing 2
for space!
KRA Table O’ Numbers (Conf II, trans)
Experiment
MP2
B3LYP
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
0.35
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
C13 0.47
0.43
0.63 -0.65 0.38
C15 0.20
1.52
1.59
C18 2.06
0.49
0.21 -2.09 0.50
C22 4.04
0.74
0.14 -4.08 -0.74 -0.09 -4.09 -0.75 -0.10
C25 4.50
2.19
─
0.16
1.03
2.59
0.86 -0.29 0.48
0.87 -0.27
0.59 -0.65 0.39
1.53 -1.61 0.24
1.02
0.58
1.58 -1.59
0.19 -2.09 0.52
0.17
-4.55 -2.16 0.12 -4.58 -2.16 0.16
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 C12H14O3
I
III
II
IV
1 nozzle, heated to 120°C