59th Annual Report on Research 2014

Abstract

The dissociation dynamics of energy-selected ions of a variety of compounds have been investigated by Photoelectron Photoion Coincidence (PEPICO) Spectroscopy through the support of the ACS-PRF 51896UNI4 award.  The award has supported several student research projects over the two year funding period.  In the last year, one Hiram Chemistry major traveled to the Swiss Light Source (SLS) in Villigen, Switzerland, three students were supported for summer research experiences, and three students worked on these projects for academic credit.  Our work has focused on 1) the investigation of hydrogen transfer reactions in a series of small hydrocarbons 2) continued analysis on data recorded in previous years, and 3) the continued development of the TPEPICO apparatus at Hiram College.  To date we have investigated Cr(CO)₆, acetone-h6, acetone-d6, acetaldehyde, anisole, anisole-d8, 3-pentanone, and 2-butnaone.  The 3-pentanone, Cr(CO)₆, acetone-h6 and acetone-d6 results are being prepared for submission in the very near future.  The work on Cr(CO)₆, acetone, and the TPEPICO apparatus is described herein.

Research Progress

Chromium hexacarbonyl, Cr(CO)₆

The dissociation dynamics of internal energy selected chromium hexacarbonyl cations, Cr(CO)₆⁺, have been investigated using the imaging photoelectron photoion coincidence (iPEPICO) spectrometer at the Swiss Light Source. All six carbonyl neutral ligands are removed in the 9 – 20 eV energy range. Chromium hexacarbonyl cations are metastable at the onset of the first carbonyl loss reaction on the time scale of the experiment. The broad, asymmetric Cr(CO)₅⁺ fragment ion time-of-flight (TOF) distribution, shown in figure 1, indicate the molecular ion lifetime is on the order of microseconds.  TOF distributions are modeled to obtain an experimental rate curve for the dissociation reaction. Further carbonyl losses were found to be fast at threshold.

The fractional parent and daughter ion abundances as a function of the initial internal energy of the parent ion, i.e. breakdown diagram (figure 2), as well as the dissociation rates for the first CO loss were modeled using the statistical RRKM theory for unimolecular reactions. The excess energy redistribution was also taken into account in a statistical framework.  The 0K dissociation onsets for the six carbonyl-loss channels were determined for the processes leading to Cr(CO)₅⁺, Cr(CO)₄⁺, Cr(CO)₃⁺, Cr(CO)₂⁺, CrCO ⁺ and Cr⁺, respectively. The chromium cation and carbonyl ligand thermochemistry is well known and these onsets connect the iron ion to the other fragment ions as well as to the gas phase neutral Cr(CO)₆.

Acetone and Acetone-d6

Many ionic dissociation reactions proceed to products with no barrier following a simple bond cleavage.  An example is CH₃ loss from acetone cations (eq 1).  However, some unimolecular ionic dissociation reactions involve rearrangements of the molecular structure prior to the loss of the neutral fragment.  An example is the loss of methane from acetone to yield the ketene ion, which proceeds through a hydrogen transfer reaction (eq 2):                                     CH₃COCH₃⁺ → CH₃CO⁺ + CO                                             (1)

CH₃COCH₃⁺ → CH₂CO⁺ + CH₄                                           (2)

We recently reported that the heavy methane loss reaction, −CD₄, is not competitive in the deuterated molecule; only the CD₃ loss channel is observed (figure 3).  This is an indication that the reaction proceeds solely by tunneling.  We have continued our work in modeling these reactions and have 1) determined final onsets for the reaction channels in both heavy and light acetone, 2) modeled the heavy acetone data using the light acetone determined onsets and ab initio kinetic isotope effects, and 3) completed high level ab initio calculations in support of the experimental results.  The results of the model are illustrated in figure 4.  The CD₄ loss channel is not observed experimentally and the model supports this result.  The final results of this work are being compiled and will be submitted for publication shortly.

Threshold Photoelectron Photoion Coincidence Mass Spectrometer

We have made several important improvements to the TPEPICO mass spectrometer over the last funding year.  We have updated all of the tubing, pressure gauges, and valves.  Students have been an integral part in the upgrades and have developed useful skills and knowledge in working with high and low vacuum systems, cooling systems, a variety of vacuum pumps (mechanical, diffusion, and turbomolecular), electrical controls of valves, electrostatic lens elements, and detectors.  Currently, we are writing the data acquisition program and collaborating with the computer science department on various ways to monitor and control the instrument remotely through windows and android based operating systems.  Additionally, we have acquired several computers to run ab initio and density functional theory calculations.  This work has been incorporated into the second semester of the Physical Chemistry curriculum.

Conclusion

We have recorded data on several new molecules using imaging and threshold PEPICO techniques.  We are currently analyzing the data and preparing the work for publication on acetone, deuterated acetone, 3-pentanone, and Cr(CO)₆.  Five different Hiram College undergraduates have conducted experiments at the SLS through three successful proposals for experimental beamtime.  A total of nine Hiram College undergraduates have contributed to these studies over the funding period.  Three of these students are lead authors on the iron carbonyl paper (see the first report for details).  Six others will be authors on the acetone, 3-pentanone, and chromium hexacarbonyl papers.  We will continue to submit proposals to the SLS and hope to continue this research with the support of the American Chemical Society Petroleum Research Fund. 

Broader Impact

The generous funding from the American Chemical Society Petroleum Research Fund has been an extremely important component in attracting students to chemistry and physical chemistry.  Students who have been supported by this grant have continued their education in masters programs at UMass - Amherst, PhD programs at Akron University, and MD/PhD programs at Case Western Reserve University.  Several others are research chemists at a variety of regional and international companies.