Construction of a 480 MHz Chirped-Pulse Fourier-Transform

Microwave Spectrometer: The Rotational Spectra of Divinyl Silane and

3,3-DifluoropentaneDaniel 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 61920

Charles J. WurreyDepartment of Chemistry, University of Missouri-Kansas City,

Kansas City, MO 64110

Gamil A. GuirgisDepartment of Chemistry and Biochemistry,

The College of Charleston, Charleston, SC 29424 1

Goals

• Looking to construct a broadband microwave spectrometer– Full broadband1 and SACI2 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-Flygare3 instrument

• Determine the limits the instrument

1G.G. Brown, B.C. Dian, K.O. Douglass, S.M. Geyer, S.T. Shipman, B.H. Pate, Rev. Sci. Instrum. 79 (2008) 053103.2G.S. Grubbs, C.T. Dewberry, K.C. Etchison, K.E. Kerr, S.A. Cooke, Rev. Sci. Instrum. 78 (2007) 096106.3 T.J. Balle, W.H. Flygare, Rev. Sci. Instrum. 52 (1981) 33.

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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

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Schematic of Microwave Circuit

AFG3251Arbitrary Function

GeneratorDC-240 MHz

HP8673G MW Synthesizer

2.0-26.0 GHz

Amp

RFamp

Tektronix TDS5054B500 MHz

Oscilloscope

Vacuum chamber

LNAMolecular expansion

Chirp

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

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10’’

Timing sequence

LNA protection (~1 ms)

Gas pulse (~0.5 ms)

Generate chirp (1 ms)

Detect FID (20 ms)

MW amp pulse (1 ms)

240 MHz Tektronix AFG 3251

500 MHz Tektronix TDS 5054B Oscilloscope

Iota One Pulsed Valve Driver

HP 8673G 2.0-26.0 GHz MW source

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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)

6/9/20097

Rotational Spectra Assignment

• 5 heavy atom backbone structures– Diethyldifluorosilane– Diethylsilane– Diethylgermane

• Predicted dipole moments– 3,3-Difluoropentane (first assigned on new

instrument)• µtotal = 2.3-2.4 D

– Divinyl Silane• µtotal = 0.6-0.8 D

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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 16-20 hours

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3,3-Difluoropentaneab initio Structures and Energies

anti-gauche +100 cm -1

anti-anti +18 cm-1gauche-gauche 0 cm-1

gauche-gauche′ + 914 cm-1

MP2/6-311++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

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3,3-Difluoropentane

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Frequency offset from center frequency (MHz)

Inte

nsity

(V)

Rotational Constantsgauche-gaucheSpectroscopic

Parameter ab initio Normal 13C1 (5)13C2 (4)

13C3

A (MHz) 2788.3 2798.1353(13) 2773.4743(6) 2786.0966(6)) 2795.8353(7)B (MHz) 2398.2 2354.5078(11) 2315.7444(14) 2336.3378(14) 2354.7230(14)C (MHz) 1793.2 1772.9048(5) 1741.1095(4) 1765.6990(4) 1772.0261(10)

N 20 9 9 9

anti-gaucheSpectroscopic

Parameter ab initio Normal 13C113C2

13C313C4

13C5

A (MHz) 3689.1 3682.6421(7) 3663.7431(6) 3653.2711(7) 3681.1440(8) 3673.5124(7) 3662.7815(7)B (MHz) 1921.2 1905.1784(5) 1869.4311(6) 1898.9857(9) 1905.2960(9) 1888.0973(7) 1867.6371(10)C (MHz) 1709.4 1694.5378(4) 1662.3049(6) 1686.0120(4) 1694.2877(5) 1682.0220(4) 1660.6872(5)

N 41 9 8 8 8 9

anti-antiSpectroscopic

Parameter ab initio Normal

A (MHz) 4829.3 4819.3323(10)B (MHz) 1677.8 1667.2385(7)C (MHz) 1676.3 1661.1272(6)

N 14

12

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5

15

24

3

We believe internal rotation of terminal methyl groups causes fine splitting

in observed rotational transitions for anti-anti conformer

anti-anti conformer

221←110 220←111

Internal Rotation

1 MHz

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Structural Determination

gauche-gauche

anti-gaucheBond length (Å) Bond angle (°) Dihedral angle (°)

C1-C2 1.527(5)

C1-C2-C3

113.2(3) C1-C2-C3-C4

177.7(2) 1.539(21) 111.1(36) 176.7(10)

C2-C3 1.517(3)

C2-C3-C4 117.0(3)

C2-C3-C4-C5 60.8(5)

1.503(52) 114.4(38) 64.9(30)

C3-C4 1.515(2)

C3-C4-C5 114.0(3)

1.554(14) 112.7(32)

C4-C5 1.530(6) r0 fit rs fit 1.532(44)

Bond length (Å) Bond angle (°) Dihedral angle (°)

C1-C2 1.520(6) C1-C2-C3 114.2(2) C1-C2-C3-C4 ±57.9(3)

C4-C5 1.515(10) C3-C4-C5 114.4(10) C2-C3-C4-C5 ±57.2(10)

C2-C3 1.514(3) C2-C3-C4

117.4(4)

C3-C4 1.506(10) 118.5(10) 1

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4

5

Divinyl Silane

• Dipole moment predictions– µtotal = 0.6-0.8 D

• 0.4% in 2 atm He/Ne• 5,000 shot average• 4 Hz repetition rate

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Divinyl Silane ab initio Structures

Conformer I +0 cm-1 Conformer II +20.0 cm-1

Conformer III +141 cm-117

MP2/6-311++G(2d,2p) level of theory

Inte

nsity

(V)

Frequency (MHz)

Compiled Broadband Spectrum

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Conformer I

Conformer II

Divinyl Silane Rotational Constants

Conformer IISpectroscopic

Parameter ab initio Normal 29Si 30SiA (MHz) 6556.8 6744.2603(19) 6668.1961(11) 6595.7002(16)B (MHz) 2529.9 2488.4083(13) 2488.4253(7) 2488.4393(6)C (MHz) 1986.9 1971.0427(10) 1964.4666(5) 1958.1001(4)

N 17 7 7

Conformer ISpectroscopic

Parameter ab initio Normal 29Si 30SiA (MHz) 11864.2 12068.8575(26) 11946.0374(22) 11831.5402(9)B (MHz) 1829.1 1831.5263(7) 1831.5502(20) 1831.5696(9)C (MHz) 1742.7 1744.4679(8) 1741.9247(12) 1739.4503(6)

N 15 6 6

Divinyl Silane & 3,3-Difluoropentane Dipole Moments

Conformer I Conformer IIµa (D) 0 0.01(1)µb (D) 0.6138(7) 0.715(15)µc (D) 0 0.02(2)

µtotal (D) 0.6138(7) 0.715(15)

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2.2769(21)2.4186(40)μtotal (D)1.1826(19)0μc (D)1.7286(20)2.4186(40)μb (D)0.8933(29)0μa (D)anti-gauchegauche-gaucheDipole Component

3,3-Difluoropentane

Divinyl Silane

• Measured Stark shifts on Balle-Flygare1 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

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480 MHz CP-FTMW

• Construction of a 480 MHz Chirped-Pulse Fourier Transform Microwave Spectrometer

• Observable 13C isotopologues and weakly-bound complex transitions in 150 gas pulses

• Compiled broadband spectrum (7.0-18.0 GHz) in 16 hours– 5,000 shot average

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7/2-5/29/2-7/2

7/2-5/2

9/2-7/2

CH35ClF2-H2O303-202

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– 120-140 kHz (FWHM)

• Resolution – 80 kHz minimum peak separation

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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

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• Intensity varies at different points in the spectrum– Center frequency– Frequency in the chirp

Intensity VariationIn

tens

ity (

V)

Frequency Offset (MHz)

13086.23 MHz

110-000 3/2-1/2

CH81BrF2

13054.88 MHz

110-000 1/2-1/2

CH79BrF2

13035.77 MHz

110-000 1/2-1/2

CH81BrF2

13145 MHz 13155 MHz

Additions to the CP-FTMW

• Ion source– Pulsed-discharge nozzle

• o-benzyne1

• Halogenated benzene derivatives2

– Electron gun (future project)• Laser ablation

– Pt and Pd containing species

Product

Reactant/Reagent

HF

F

261S.G. Kukolich, C. Tanjaroon, M.C. McCarthy, P. Thaddeus, J. Chem Phys. 119 (2003) 4353

2G.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-0809387

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