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

Spectroscopists

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

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.

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.

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) 053103.

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.

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

Relaxation of conformers in pulsed valves3 3R.S. Ruoff, T.D. Klots, T. Emilsson, and H.S. Gutowsky, J. Chem. Phys. 93, (1990) 3142-3150.

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) 3142-3150.

Relaxation of conformers in pulsed valves3 3R.S. Ruoff, T.D. Klots, T. Emilsson, and H.S. Gutowsky, J. Chem. Phys. 93, (1990) 3142-3150.

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) 49-53.

Coker College CP-FTMW Spectrometer

Target Molecules propanal cis gauche 3R.S. Ruoff, T.D. Klots, T. Emilsson, and H.S. Gutowsky, J. Chem. Phys. 93, (1990) 3142-3150.

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

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

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)

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

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

Preliminary experiments: propanal transition: 101 – 000 each trial: 500 avgs Predicted ratio = 0.34 Expt. 1 ratio = 0.036 set carrier frequency to 10000 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

Preliminary experiments: propanal transition: 101 – 000 each trial: 500 avgs Predicted ratio = 0.34 (296K) Expt. 1 ratio = 0.036 (296K) Predicted ratio = 0.44 (354K) Expt. 2 ratio = 0.079 (354K) set carrier frequency to 10000 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

Preliminary experiments: propanal transition: 101 – 000 each trial: 500 avgs Predicted ratio = 0.34 (296K) Expt. 1 ratio = 0.036 (296K) Predicted ratio = 0.44 (354K) Expt. 2 ratio = 0.079 (354K) Expt. 3 ratio = 0.077 (296K) set carrier frequency to 10000 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

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

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.

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

Preliminary experiments: isopropyl alcohol transition: 101 – 000 each experiment: 10 trials of 100 avgs each set carrier frequency to 13000 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