Construction of a 480 MHz Chirped-Pulse Fourier-Transform Microwave Spectrometer: The Rotational Spectra of Divinyl Silane and 3,3-Difluoropentane Daniel A. Obenchain, Amanda L. Steber, Ashley A. Osthoff, Rebecca A. Peebles, Sean A. Peebles Department of Chemistry, Eastern Illinois University, 600 Lincoln Avenue, Charleston, IL Charles J. Wurrey Department of Chemistry, University of Missouri-Kansas City, Kansas City, MO Gamil A. Guirgis Department of Chemistry and Biochemistry, The College of Charleston, Charleston, SC

Goals Looking to construct a broadband microwave spectrometer – Full broadband 1 and SACI 2 type instruments are preferred, but are beyond the price range of research groups at smaller institutions. Need a cost effective spectrometer with improved bandwidth over a Balle-Flygare 3 instrument Determine the limits the instrument 1 G.G. Brown, B.C. Dian, K.O. Douglass, S.M. Geyer, S.T. Shipman, B.H. Pate, Rev. Sci. Instrum. 79 (2008) G.S. Grubbs, C.T. Dewberry, K.C. Etchison, K.E. Kerr, S.A. Cooke, Rev. Sci. Instrum. 78 (2007) T.J. Balle, W.H. Flygare, Rev. Sci. Instrum. 52 (1981) 33. 2

Introduction Chirped-Pulse Instrument – Adapting to smaller bandwidth Two recent molecules – 3,3-Difluoropentane – Divinyl Silane – Rotational constants – Dipole moments – Structure Performance of the instrument will be discussed 3

Schematic of Microwave Circuit AFG3251 Arbitrary Function Generator DC-240 MHz HP8673G MW Synthesizer GHz Amp RF amp Tektronix TDS5054B 500 MHz Oscilloscope Vacuum chamber LNA Molecular expansion Chirp 4

480 MHz Chirped Pulse Fourier Transform Microwave (CP-FTMW) Spectrometer 5 10’’

Timing sequence LNA protection (~1  s) Gas pulse (~0.5 ms) Generate chirp (1  s) Detect FID (20  s) MW amp pulse (1  s) 240 MHz Tektronix AFG MHz Tektronix TDS 5054B Oscilloscope Iota One Pulsed Valve DriverHP 8673G GHz MW source 6 Quantum Composers QC9614+ SRS DG-535

OCS (carbonyl sulfide) J = 1 ← 0 transition Signal intensity ~50 mV (actually not that great, but did improve significantly, to over 1000 mV) 6/9/2009 7

Rotational Spectra Assignment 5 heavy atom backbone structures – Diethyldifluorosilane – Diethylsilane – Diethylgermane Predicted dipole moments – 3,3-Difluoropentane (first assigned on new instrument) µ total = D – Divinyl Silane µ total = D 8

Analyzing the Spectra Spectrum folding in reduced bandwidth gives rise to more challenges 5 MHz center frequency shift – Line frequency directly on oscilloscope – LabVIEW program to sort spectra Wastes sample and spectra, 475 MHz bandwidth Allows for compiled spectrum to be produced from 23 spectra in 7-10 hours 240 MHz step method – LabVIEW program compares multiple spectra offset by 240 MHz to determine absolute frequencies of transitions Accurately determine absolute frequencies Also allows for compiled spectrum to be produced from 46 spectra in about hours 9

3,3-Difluoropentane ab initio Structures and Energies anti-gauche+100 cm -1 anti-anti +18 cm -1 gauche-gauche 0 cm -1 gauche-gauche′+ 914 cm -1 MP2/ G (2d,2p) level of theory

3,3-Difluoropentane 0.1% in 2 atm He/Ne 150-2,500 shot average 10 Hz repetition rate 11

3,3-Difluoropentane 12 Frequency offset from center frequency (MHz) Intensity (V)

Rotational Constants gauche-gauche Spectroscopic Parameter ab initioNormal 13 C 1 (5) 13 C 2 (4) 13 C 3 A (MHz) (13) (6) (6)) (7) B (MHz) (11) (14) (14) (14) C (MHz) (5) (4) (4) (10) N20999 anti-gauche Spectroscopic Parameter ab initioNormal 13 C 1 13 C 2 13 C 3 13 C 4 13 C 5 A (MHz) (7) (6) (7) (8) (7) (7) B (MHz) (5) (6) (9) (9) (7) (10) C (MHz) (4) (6) (4) (5) (4) (5) N anti-anti Spectroscopic Parameter ab initioNormal A (MHz) (10) B (MHz) (7) C (MHz) (6) N

We believe internal rotation of terminal methyl groups causes fine splitting in observed rotational transitions for anti-anti conformer anti-anti conformer 2 21 ← ←1 11 Internal Rotation 1 MHz

15 Structural Determination gauche-gauche anti-gauche Bond length (Å)Bond angle (°)Dihedral angle (°) C 1 -C (5) C 1 -C 2 -C (3) C 1 -C 2 -C 3 -C (2) 1.539(21)111.1(36)176.7(10) C 2 -C (3) C 2 -C 3 -C (3) C 2 -C 3 -C 4 -C (5) 1.503(52)114.4(38)64.9(30) C 3 -C (2) C 3 -C 4 -C (3) 1.554(14)112.7(32) C 4 -C (6) r 0 fit r 0 fit r s fit r s fit 1.532(44) Bond length (Å)Bond angle (°)Dihedral angle (°) C 1 -C (6)C 1 -C 2 -C (2)C 1 -C 2 -C 3 -C 4 ±57.9(3 ) C 4 -C (10)C 3 -C 4 -C (10)C 2 -C 3 -C 4 -C 5±57.2(10) C 2 -C (3) C 2 -C 3 -C (4) C 3 -C (10)118.5(10)

Divinyl Silane Dipole moment predictions – µ total = D 0.4% in 2 atm He/Ne 5,000 shot average 4 Hz repetition rate 16

Divinyl Silane ab initio Structures Conformer I +0 cm -1 Conformer II cm -1 Conformer III +141 cm MP2/ G(2d,2p) level of theory

Intensity (V) Frequency (MHz) Compiled Broadband Spectrum 18 Conformer I Conformer II

Divinyl Silane Rotational Constants Conformer II Spectroscopic Parameterab initioNormal 29 Si 30 Si A (MHz) (19) (11) (16) B (MHz) (13) (7) (6) C (MHz) (10) (5) (4) N1777 Conformer I Spectroscopic Parameterab initioNormal 29 Si 30 Si A (MHz) (26) (22) (9) B (MHz) (7) (20) (9) C (MHz) (8) (12) (6) N1566

Divinyl Silane & 3,3-Difluoropentane Dipole Moments Conformer IConformer II µ a (D)00.01(1) µ b (D)0.6138(7)0.715(15) µ c (D)00.02(2) µ total (D)0.6138(7)0.715(15) (21)2.4186(40)μ total (D) (19)0μ c (D) (20)2.4186(40)μ b (D) (29)0μ a (D) anti-gauchegauche-gaucheDipole Component 3,3-Difluoropentane Divinyl Silane Measured Stark shifts on Balle-Flygare 1 cavity instrument at EIU 1 T.J. Balle, W.H. Flygare, Rev. Sci. Instrum. 52 (1981) 33.

3,3-Difluoropentane & Divinyl Silane Rotational transition assignment for 3 conformers of 3,3-Difluoropentane and 2 conformers of Divinyl Silane – Structural determinations – Dipole moments – Conformer analysis 21

480 MHz CP-FTMW Construction of a 480 MHz Chirped-Pulse Fourier Transform Microwave Spectrometer Observable 13 C isotopologues and weakly- bound complex transitions in 150 gas pulses Compiled broadband spectrum ( GHz) in 16 hours – 5,000 shot average 22 7/2-5/2 9/2-7/2 7/2-5/2 9/2-7/2 CH 35 ClF 2 -H 2 O Upper state Lower state

480 MHz CP-FTMW Broadband frequencies are reproducible on cavity instrument – 4 kHz for strong transitions (S/N>5:1) – 6 kHz less intense transitions (S/N<5:1) Line widths – kHz (FWHM) Resolution – 80 kHz minimum peak separation 23

480 MHz CP-FTMW Unable to achieve multiple chirps per gas pulse – Limitation of our arbitrary function generator (AFG 3251) 2 Nozzles (General Valve Series 9) – Increases S/N by factor of ≈3 in initial tests 24

25 Intensity varies at different points in the spectrum – Center frequency – Frequency in the chirp Intensity Variation Intensity (V) Frequency Offset (MHz) MHz /2-1/2 CH 81 BrF MHz /2-1/2 CH 79 BrF MHz /2-1/2 CH 81 BrF MHz MHz

Additions to the CP-FTMW Ion source – Pulsed-discharge nozzle o-benzyne 1 Halogenated benzene derivatives 2 – Electron gun (future project) Laser ablation – Pt and Pd containing species 26 1 S.G. Kukolich, C. Tanjaroon, M.C. McCarthy, P. Thaddeus, J. Chem Phys. 119 (2003) G.H. Sutter, H Dreizler, Zeitschrift fur Naturforschung. A, A journal of physical science. 56 (2001) 425

Acknowledgements Prof. Brooks Pate, University of Virginia – Justin Neill Prof. Steve Cooke, University of North Texas – Smitty Grubbs NSF Research at Undergraduate Institutions CHE

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