1. MEASURING CONFORMATIONAL ENERGY DIFFERENCES USING PULSED-JET MICROWAVE SPECTROSCOPY
CAMERON M FUNDERBURK, SYDNEY A GASTER, TIFFANY R TAYLOR, GORDON G BROWN
Department of Science and Mathematics
Coker College
Hartsville, SC
TH05

2. Spectroscopists

3. Microwave Spectroscopists
Frequency
Narrow line-width
Small Error
Intensity
Question: Can we use experimental intensities more effectively?

4. Conformational Energy Differences
pre-Balle/Flygare1 pulsed jet cavity
static cell w/ accurate relative intensities
experimental energy differences often reported
1Balle, T.J., Flygare, W.H., Rev. Sci. Instrum. 1981, 52 (1), 33–45.

5. Conformational Energy Differences
post Balle/Flygare1 pulsed jet (and rise of computing power)
cooling in pulsed jet
use of a cavity – relative intensities not accurate
experimental relative intensities seldom reported
1Balle, T.J., Flygare, W.H., Rev. Sci. Instrum. 1981, 52 (1), 33–45.

6. Conformational Energy Differences
Chirped-pulse microwave spectroscopy
provides accurate relative intensities2
do relative intensities agree with energy differences between conformers?
2Gordon G. Brown, Brian C. Dian, Kevin O. Douglass, Scott M. Geyer, Steven T. Shipman, and Brooks H. Pate, Rev. Sci. Instrum. 79 (2008)

7. Hypothesis:
Relative intensities are proportional to the conformer populations present before the expansion occurs (if we use He gas).
We can use relative intensities to measure experimental relative energies between conformers.

8. Example: Ethylperoxyl radical
intermediate in ethanol combustion
two conformers, gauche and trans
conformation energies determine
most likely reaction path
reaction kinetics

9. Relaxation of conformers in pulsed valves3
3R.S. Ruoff, T.D. Klots, T. Emilsson, and H.S. Gutowsky, J. Chem. Phys. 93, (1990)

10. Relaxation of conformers in pulsed valves3
“When helium was used as the carrier gas all of the conformers showed little if any evidence of conformer relaxation…
That is, the signal intensities were consistent with the equilibrium ratio of concentrations (Keq) predicted be the energy difference (ΔE) between conformers at T~298K,
Keq = f exp(-ΔE/RT) ”1
f = ratio of numbers of equivalent forms (e.g. 2 gauche vs. 1 trans)
3R.S. Ruoff, T.D. Klots, T. Emilsson, and H.S. Gutowsky, J. Chem. Phys. 93, (1990)

11. Relaxation of conformers in pulsed valves3
3R.S. Ruoff, T.D. Klots, T. Emilsson, and H.S. Gutowsky, J. Chem. Phys. 93, (1990)

12. Coker College CP-FTMW Spectrometer4
4Miranda Smith, Brandon D. Short, April M. Ruthven, K. Michelle Thomas, Michael J. Hang, Gordon G. Brown,  J. Mol. Spectr., 307, (2015)

13. Coker College CP-FTMW Spectrometer

14. Target Molecules propanal cis gauche
3R.S. Ruoff, T.D. Klots, T. Emilsson, and H.S. Gutowsky, J. Chem. Phys. 93, (1990)

15. Intensity Ratio Predictions
ethyl formate (gauche/trans)
propanal (gauche/cis)
predicted equilibrium ratio in valve (T = 296K)
1.63
0.43
predicted transition intensities in jet (1K)
0.86
0.52
Instrument response
2.0
1.5
overall predicted intensity ratio
2.8
0.34

16. Preliminary experiments: ethyl formate
transition: 202 – 101
each trial: 100 avgs
Predicted ratio = 2.8
Expt. 1 ratio = 0.15
set carrier frequency to ν – 200 MHz
(10762 MHz and
13859 MHz)
trans
10962 MHz
gauche
14059 MHz
ratio (g/t) 1
21.36
3.84
0.18 2
23.26
3.62
0.16 3
28.84
2.41
0.08 4
23.64
3.22
0.14 5
21.48
3.94 6
21.63
3.77
0.17 7
25.85
4.75 8
27.13
2.42
0.09 9
25.92
3.57 10
26.21
3.38
0.13
avg
24.53
3.49
0.15
std dev
2.62
0.70
0.04

17. Preliminary experiments: ethyl formate
trans
10962 MHz
gauche
14059 MHz
ratio (g/t) 1
8.83
4.42
0.50 2
9.15
4.29
0.47 3
8.84 4
8.72
4.44
0.51 5
9.53
4.53
0.48 6
9.18
4.57 7
9.75
4.73 8
10.19
4.75 9
9.36
4.74 10
10.35
4.99
avg
9.39
4.59
0.49
std dev
0.57
0.21
0.02
Experiment 2:
repeat Experiment 1 on different day
transition: 202 – 101
each trial: 100 avgs
Predicted ratio = 2.8
Expt. 1 ratio = 0.15
Expt. 2 ratio = 0.49
set carrier frequency to ν – 200 MHz
(10762 MHz and
13859 MHz)

18. Preliminary experiments: ethyl formate
repeat Experiment 2 on same day with different carrier frequencies
transition: 202 – 101
each trial: 100 avgs
Predicted ratio = 2.8
Expt. 1 ratio = 0.15
Expt. 2 ratio = 0.49
Expt. 3 ratio = 0.79
set carrier frequency to ν – 300 MHz
(10662 MHz and
13759 MHz)
trans
10962 MHz
gauche
14059 MHz
ratio (g/t) 1
8.16
6.47
0.79 2
8.04
6.30
0.78 3
7.65
6.76
0.88 4
8.42
6.31
0.75 5
8.20
6.52 6
7.75
6.29
0.81 7
8.66
6.22
0.72 8
7.94
6.51
0.82 9
8.18
6.50 10
8.34
avg
8.13
6.44
std dev
0.30
0.16
0.04

19. Preliminary experiments: ethyl formate
trans
10962 MHz
gauche
14059 MHz
ratio (g/t) 1
24.53
3.49
0.15 2
9.39
4.59
0.49 3
8.13
6.44
0.79 4
7.45
1.12 5
19.09
3.42
0.18 6
17.89
2.99
0.17 7
17.06
3.13 8
17.02
2.94 9
12.65
3.51
0.28 10
12.25
3.44 11
19.47
2.45
0.13 12
7.66
5.48
0.71 13
4.20
1.08
0.26 14
11.70
3.08 15
11.00
3.34
0.30 16
11.42
3.14
0.27
avg
13.18
3.35
std dev
1.34
0.20
Further Experiments:
transition: 202 – 101
Predicted ratio = 2.8

20. Preliminary experiments: propanal
transition: 101 – 000
each trial: 500 avgs
Predicted ratio = 0.34
Expt. 1 ratio = 0.036
set carrier frequency to MHz and
8000 MHz
Room Temp (296K)
cis
10493 MHz
gauche
8463 MHz
ratio (g/t) 1
2.51
0.086
0.034 2
2.63
0.084
0.032 3
2.41
0.090
0.037 4
2.432
0.094
0.039 5
2.386
0.096
0.040 6
2.394
0.092 7
3.981
0.100
0.025 8
3.492
0.029 9
2.372
0.105
0.044 10
2.59
0.112
0.043
avg
2.720
0.036
std dev
0.555
0.009
0.006

21. Preliminary experiments: propanal
transition: 101 – 000
each trial: 500 avgs
Predicted ratio = 0.34 (296K)
Expt. 1 ratio = (296K)
Predicted ratio = 0.44 (354K)
Expt. 2 ratio = (354K)
set carrier frequency to MHz and
8000 MHz
Heated nozzle (354K)
cis
10493 MHz
gauche
8463 MHz
ratio (g/t) 1
2.211
0.1889
0.085 2
2.142
0.1842
0.086 3
2.228
0.1912 4
2.347
0.1965
0.084 5
2.348
0.1758
0.075 6
2.567
0.1789
0.070 7
2.443
0.1968
0.081 8
2.445
0.1911
0.078 9
2.47
0.1779
0.072 10
2.519
0.1897
avg
2.372
0.187
0.079
std dev
0.142
0.008
0.006

22. Preliminary experiments: propanal
transition: 101 – 000
each trial: 500 avgs
Predicted ratio = 0.34 (296K)
Expt. 1 ratio = (296K)
Predicted ratio = 0.44 (354K)
Expt. 2 ratio = (354K)
Expt. 3 ratio = (296K)
set carrier frequency to MHz and
8000 MHz
Room Temp (296K)
cis
10493 MHz
gauche
8463 MHz
ratio (g/t) 1
1.697
0.1093
0.064 2
1.709
0.1139
0.067 3
1.695
0.1219
0.072 4
1.597
0.1321
0.083 5
1.765
0.1361
0.077 6
1.658
0.1277 7
1.644
0.1268 8
1.71
0.1373
0.080 9
1.72
0.1524
0.089 10
1.618
0.1344
avg
1.681
0.129
std dev
0.051
0.012
0.008

23. Experimental consistency (?)
all experimental parameters same
valve timing
valve shims
backing pressure
same tank of chemical/He mix
microwave pulse power

24. Conclusion Future Work
It is difficult to reproduce signal intensities with a pulsed jet.
We seem to observe conformer relaxation despite using helium as a carrier gas.
Future Work
Compare relative intensities of conformer spectra, not individual transitions.

25. Acknowledgements
American Chemical Society – Petroleum Research Fund (56530-UR6)
SCICU Student-Faculty Research Program
Coker College

27. Preliminary experiments: isopropyl alcohol
transition: 101 – 000
each experiment:
10 trials of 100 avgs each
set carrier frequency to MHz – measured both transitions with same measurements
Experiment date
trans
13254 MHz
gauche
13407 MHz
ratio (g/t)
3/14
0.64
1.30
2.03
3/15
0.43
1.15
2.67
0.91
1.39
1.53
avg
2.08
std dev
0.57