62nd Annual Report on Research 2017

In the first year of the project, initial testing of the hypothesis has been completed. The results of these experimental tests are reported below as well as the future direction of the project. The proposed project was based on the following two-pronged hypothesis referring to using pulsed-valve chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy to measure multiple conformers of the same compound:

Project Hypothesis:

· Relative intensities are proportional to the conformer populations present before the expansion occurs (if He is used as the carrier gas).

· (Therefore), relative intensities can be used to measure experimental relative energies between conformers.

The above hypothesis was based on the findings of Ruoff et al. described in their seminal paper on the relaxation of conformers in supersonic jets. In this paper, they reported that “When helium was used as the carrier gas all of the conformers showed little if any evidence of conformer relaxation.”¹ Therefore, we proposed using relative intensities to measure experimental relative energies.

Initial Findings

In preliminary experiments on several systems in our proposal, we have not been able to reproduce Ruoff’s finding that conformer relaxation is not observed when using helium as a carrier gas. Furthermore, obtaining reproducible results with a pulsed valve has proved challenging.

For example, we have performed several experiments on ethyl formate, a compound studied in the Ruoff paper. Ethyl formate has two stable conformers, trans and gauche, both of which are observable in the pulsed-valve CP-FTMW spectrum. Using our experimental conditions, we estimate that the signal intensity of the high-energy gauche conformer should be 2.8 times that of the low-energy trans conformer. This estimate is based on their relative populations in the room temperature valve (using a literature value of 42 cm^-1 for the energy difference between the two conformers), the relative intensities of their respective microwave signals (using literature values of 1.85 D and 1.44 D for the μ_a dipole moment components or the trans and gauche conformer, respectively), and the relative instrumental response of the Coker spectrometer at the two experimental frequencies. The ethyl formate experimental results are summarized in Table 1 below. Note that in this table, each “experiment” is the average of 10 trials, each of which is the average of 100 free induction decay (FID) measurements. As an example, in experiment 1, we recorded the signal intensity of the 1_01­-0₀₀ transition of each conformer after our instrument did 100 FID averages, and we repeated that measurement 10 times. The average ratio from all ten trials was 0.15 (gauche/trans); that particular data set had a standard deviation of 0.04 (not shown on table). Over several days, 16 similar experiments were performed, with an average ratio of 0.30 and a standard deviation of 0.20. We have two problems. First, our results are not reproducible (as evidenced by the high standard deviation). Second, they are not close to the prediction on which our hypothesis is based. We suspect that conformer relaxation is to blame for the experimental ratios being much lower than the predicted ones.

We have done similar experiments on several other systems. In each case, the same two problems were encountered. The progress of this project was reported at the International Symposium on Molecular Spectroscopy, where many of the world’s experts on microwave spectroscopy were in attendance (it is the premier conference on molecular and microwave spectroscopy). The feedback we received was that most spectroscopists were not surprised by our results. Indeed, in other presentations at the same conference, the observation of conformer relaxation (with helium as a carrier gas) was reported.

Table 1 Results of relative intensities of trans and gauche conformers of ethyl formate over several experiments.

Future Work

While we have encountered roadblocks, we believe that the future direction of the project is clear. First, while initial experiments compared the signal intensities of individual transitions of conformers, future experiments will include the entire spectrum. In other words, the intensities of many transitions of the two conformers will be measured, and a single ratio between the two conformers will be determined from all of the measured intensities. We believe this will help with the first problem of reproducibility. Second, we will measure signal intensities on Coker’s static-cell CP-FTMW spectrometer. In this spectrometer, the pulsed valve is eliminated; therefore, we can be confident that the relative conformer populations conform to prediction based on the cell temperature. This should address the second problem of conformer relaxation, and it was mentioned in the original proposal as part of the overall plan.

¹R.S. Ruoff, T.D. Klots, T. Emilsson, and H.S. Gutowsky, J. Chem. Phys. 93, (1990) 3142-3150.