Strategies for Complex Mixture Analysis in Broadband Microwave Spectroscopy Amanda L. Steber, Justin L. Neill, Matt T. Muckle, and Brooks H. Pate Department of Chemistry, University of Virginia, Charlottesville, VA D.F. Plusquellic Biophysics Group, Physics Laboratory, NIST, Gaithersburg, MD V. Lattanzi, S. Spezzano, and M.C. McCarthy Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, and School of Engineering and Applied Sciences, Harvard University, 29 Oxford St., Cambridge, MA 02138

Broadband Spectra Dense spectra ◦ Parent species ◦ Isotopologues ◦ Multiple conformers ◦ Clusters ◦ Contaminants Many lines at the same intensity Valuable information (substitution structures and clusters)

Extracting Overlapping Spectra Difficulty in assigning species by pattern matching

Solution: Broadband MW-MW Double Resonance Masakazu Nakajima, Yoshihiro Sumiyoshi, and Yasuki Endo, Rev. Sci. Instrum. 73, 165 (2002), DOI: /

Formic Acid Trimer Monitored Kevin O. Douglass made the initial assignment of formic acid trimer.

Too many averages needed to see weak species Takes a significant amount of time ~50 hours How do we get around this problem? Problems/ Weaknesses

Assigning Weak Species and Tunneling Splitting

Tunneling doublet Needed to find c-type transitions Shifted due to tunneling of the water Formic Acid Trimer and Water Monitored transition at MHz and scanned up from 9000 MHz to find the Found the shift to be ~180 MHz from predicted frequency Then assigned c-types in the broadband CD

Search for transition for Lower State Search for transition for Upper State Formic Acid Trimer and Water

Autofitting program developed in conjunction with D. Plusquellic Based on the success of electronic structure theory and high frequency precision of microwave spectroscopy Used to determine if there is a candidate structure present ◦ Quick approach Automatic Spectra Extraction

Hexanal *11 conformers and C assignments have been removed Total of 918 lines have been cut *The assignment of these conformers as well as the ab initio calculations were done by R.D. Suenram, A. Lesarri, S.T. Shipman, G.G. Brown, L.-H. Xu, and B.H. Pate. This work has not yet been published.

Autofitting Program Procedure 24 other possible conformational structures for 1-hexanal Pick the next lowest energy structure Input ab initio rotational constants Program generates a predicted spectrum Pick three transitions to make the triplet lists, as well as set frequency window and intensity threshold

Autofitting Program Procedure Have all possible candidate assignments from frequency window Tests triplets and looks for other transitions that hit a line in the spectrum From a hit we get new rotational constants as well as the frequencies of the lines

Ab initioAutofitExperimental A (MHz) (10) B (MHz) (17) C (MHz) (17) D J (kHz) (12) D JK (kHz) (21) d J (kHz) (10) Final Fit for Twelfth Conformer 29 Lines with a RMS of kHz

Lines may be absent because they may have blended with cut lines thus making it hard to determine right triplet 50,000 triplets can be scanned in an hour Good for isotopomers but with more complex Hamiltonians, computational cost would increase significantly. Challenges to Program

Dense spectra from reactive chemistry and discharge are hard to assign Use broadband MW-MW double resonance to analyze the more intense lines, but doesn’t work well with weak species Can use cavity MW-MW DR to analyze the weak species or species split by tunneling To help these two techniques and aid in faster assignments, programs are being developed to extract spectra automatically Overview

Acknowledgements NSF Chemistry CHE NSF CRIF:ID CHE Kevin Douglass Anthony Remijan