Strategies for Complex Mixture Strategies for Complex Mixture Analysis in Broadband Microwave Analysis in Broadband Microwave SpectroscopySpectroscopy

Amanda L. Steber, Justin L. Neill, Matt T. Muckle, and Brooks H. PateDepartment of Chemistry, University of Virginia, Charlottesville, VA 22904

D.F. PlusquellicBiophysics Group, Physics Laboratory, NIST, Gaithersburg, MD 20899-8441

V. Lattanzi, S. Spezzano, and M.C. McCarthyHarvard-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 SpectraBroadband SpectraDense spectra

◦Parent species◦Isotopologues◦Multiple conformers◦Clusters◦Contaminants

Many lines at the same intensityValuable information (substitution

structures and clusters)

Extracting Overlapping Extracting Overlapping SpectraSpectra

Difficulty in assigning species by pattern matching

Solution: Broadband MW-MW Solution: Broadband MW-MW Double ResonanceDouble Resonance

Masakazu Nakajima, Yoshihiro Sumiyoshi, and Yasuki Endo, Rev. Sci. Instrum. 73, 165 (2002), DOI:10.1063/1.1426230

Formic Acid TrimerFormic Acid Trimer

7000 8000 9000 10000 11000 12000 13000 14000 15000-50

0

50

100P

erce

nt M

odu

latio

n

Frequency (MHz)

919 -818 9286.7374928 -919 9491.729710010 -919 9598.4936 919 -808 10364.770911011 -10110 10760.784110110 -909 11180.830011111 -10110 11312.2940

Monitored 10110 -919 10301.3515

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 Problems/ Weaknesses

Assigning Weak Species and Assigning Weak Species and Tunneling SplittingTunneling Splitting

Tunneling doubletNeeded to find c-type

transitionsShifted due to tunneling

of the water

Formic Acid Trimer and Formic Acid Trimer and WaterWater

Monitored 643-532 transition at 11766.7552 MHz and scanned up from 9000 MHz to find the 532-422

Found the shift to be ~180 MHz from predicted frequency

Then assigned c-types in the broadband

C D

Search for 532-422 transition for Lower State

Search for 532-422 transition for Upper State

Formic Acid Trimer and Formic Acid Trimer and WaterWater

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 Automatic Spectra ExtractionExtraction

HexanalHexanal*11 conformers and 12 13C

assignments have been removedTotal 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 Autofitting Program ProcedureProcedure24 other possible conformational

structures for 1-hexanalPick the next lowest energy

structureInput ab initio rotational constantsProgram generates a predicted

spectrumPick three transitions to make the

triplet lists, as well as set frequency window and intensity threshold

6000 8000 10000 12000 14000 16000 18000 20000-0.005

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045

0.050

0.055

0.060

0.065

Inte

nsi

ty (

mV

)

Frequency (MHz)

Autofitting Program Autofitting Program ProcedureProcedureHave 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 initio Autofit Experimental

A (MHz) 5369.19 5382.1385439.258(10

)

B (MHz) 1120.87 1098.7181098.7497(1

7)

C (MHz) 1070.05 1050.3281050.3568(1

7) DJ (kHz) -- -- 0.657(12)DJK (kHz) -- -- 9.24(21)dJ (kHz) -- -- 0.101(10)

Final Fit for Twelfth Final Fit for Twelfth ConformerConformer

29 Lines with a RMS of 20.731 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 ProgramChallenges 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

OverviewOverview

AcknowledgementsAcknowledgementsNSF Chemistry CHE-0616660NSF CRIF:ID CHE-0618755Kevin DouglassAnthony Remijan