Waveguide Chirped-Pulse Fourier Transform Microwave Spectra of 2-Propanethiol

Bri Gordon

Steven T. Shipman

New College of Florida

Astrochemistry of Alkylthiols

• Few organosulfur compounds have been observed in the ISM.

• Methanethiol has been observed in Sagittarius B2.*

*Linke, R.A.; Frerking, M. A.; Thaddeus, P.; Astrophysical Journal 234, L139 (1979).

• Larger alkylthiols may be present; existing fits should be extended in order to search for them in the ISM.

Chirped-Pulse Spectrometer

1. Generate pulse

2. Amplify pulse

3. Send pulse to waveguide

4. Amplify molecular emission

5. Record, average emission in the time domain

6. Fourier transform the emission

Chirped Pulse

(Linear Frequency Sweep)

1

1)

2)3)

4)

5), 6)

1

1

1

Senior Thesis Research

Worked with three previously studied small alkylthiols.

High vapor pressures and strong dipoles:

• Ethanethiol

• 1-propanethiol

• 2-propanethiol

g-EtSH0 cm-1

g'-EtSH0 cm-1

t-EtSH213.3 cm-1

TG-1-PrSH0 cm-1

TG’-1-PrSH0 cm-1

TT-1-PrSH249.5 cm-1

t-2-PrSH0 cm-1

g-2-PrSH41.3 cm-1

g’-2-PrSH41.3 cm-1DFT B3LYP / 6-311++G(d,p)

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Tunneling interactions in EtSH

The thiol H-atom tunneling through the potential barrier

Thiol Potential Energy Scan in EtSH

*Nakagawa, J., Kuwada, K., Hayashi, M. Bull. Chem. Soc. Jpn. 49, 3420 (1976).

**DFT B3LYP/6-311++G(d,p)

• The tunneling induces symmetric and antisymmetric states.

• c - type transitions between these states are split by as much as 3.5 GHz.

trans

trans

gauche'

gauche

Previous* Ab Initio**

V2 -135 (27) -152.28 (18)V3 456.4 (20) 451.68 (17)

Tunneling interactions in 2-PrSH

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trans transgauche' gauche

Thiol Potential Energy Scan in 2-PrSH

Previous* Ab Initio**V2 - 47.74 (14.4)

V3 657.54489.44 (13.4)

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*Griffiths, J. H. J. Mol.Spec., 56, 257 (1975).

b - type transitions between states show tunneling splitting

**DFT B3LYP/6-311++G(d,p)

2-Propanethiol

1

- Experimental - Simulated trans - Simulated

gauche

964 peaks above the 3:1 signal to noise threshold

2 million averages, 7 mTorr, -20° C, 4 μs Fid (D) trans*

µa 1.75

µb 0.004

µc 0.38

µtot 1.80

(D) gauche*

µa 1.52

µb 0.70

µc 0.70

µtot 1.81

*DFT B3LYP / 6-311++G(d,p)

2-Propanethiol

1

- Experimental - Simulated trans - Simulated

gauche

964 peaks above the 3:1 signal to noise threshold

2 million averages, 7 mTorr, -20° C, 4 μs Fid (D) trans*

µa 1.75

µb 0.004

µc 0.38

µtot 1.80

(D) gauche*

µa 1.52

µb 0.70

µc 0.70

µtot 1.81

*DFT B3LYP / 6-311++G(d,p)

2-Propanethiol: trans Conformer Expanded View

1 - Experimental - Simulated trans  

Previous* Current

Lines Fit 12 126Fit RMS (kHz) 50 69

Max J 4 80

*Griffiths, J. H.; J. Mol.Spec., 56, 257 (1975).

# of Lines

a – type 78

b – type -

c – type 48

2-Propanethiol: gauche Conformer Expanded View

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*Griffiths, J. H. ; J. Mol.Spec., 56, 257 (1975).

- Experimental - Simulated

gauche

Previous* Current

Lines Fit 55 265 Fit RMS (kHz) 50 112

Max J 5 42

# of Lines Max Splitting (MHz)

a – type 96 107.6b – type 73 1124.6c – type 96 91.7

g-2-PrSH Splitting

Transition Frequency (MHz) Splitting (MHz)7 4 3 ← 7 3 4 (+ -) 22229.60

1050.017 4 3 ← 7 3 4 (- +) 23279.61

gauche transAb

Initio*Current

Ab Initio*

Current

A (MHz)  7822.81 7873.82 (12) 7831.547892.410

(8)

B (MHz)  4417.69 4528.44 (11) 4319.544414.444

(7)

C (MHz)  3106.01 3172.567 (8) 3096.803158.072

(7)∆v

(MHz) - 562.259 (21) - -

Fa (MHz) - 60.321 (29) - -

Gc (MHz) - 8.19 (22) - -

Fc (MHz) - 145.33 (132) - -

DJ (kHz) 1.5 1.454 (35) 1.3 1.3 (fixed)DJK (kHz) 2.4 2.344 (62) 2.7 2.61 (12) DK (kHz) 2.6 2.97 (16) 2.4 2.77 (33)dJ (kHz) 0.44 0.453 (4) 0.37 0.366 (7)dK (kHz) 3.3 3.390 (47) 2.8 2.997 (92)ФKJ (HZ) - - - 0.0481 (7)

Lines Fit 265 126RMS (kHz)

112 69

Max J 42 80

g-2-PrSH Hamiltonian:H� = H�+++ H�-- + H�+- • H�++ = A+ Ĵ�a2 + B+Ĵ�b2 + C+Ĵ�c2• H�-- = A–Ĵ�a2 + B–Ĵ�b2 + C–Ĵ�c2 + ∆ν• H�+- = Fa (Ĵ�bĴ�c + Ĵ�cĴ�b) + Gc (iĴ�c) + Fc (Ĵ�aĴ�b + Ĵ�bĴ�a) * DFT B3LYP/6-311++G(d,p)

Results

Conclusions and Future Work

• 391 (41%) of the 964 peaks have been assigned to the trans and gauche ground state normal species.

• Next spectra to fit are vibrationally excited states and 34S species

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Acknowledgments

• Professor Shipman• Ben Rooks• Maria Phillips• Christian Metzger• Ian Finneran• Ben Reinhold• New College of

Florida• The National Science

Foundation

This material is based upon work supported by the National Science Foundation Division of Chemistry under Grant No.1111101 (co-funded by MPS/CHE and the Division of Astronomical Sciences).

The Galactic Centre and Sagittarius B2, from the European Southern Observatory http://www.eso.org/public/images/eso0924e/

The Galactic Centre and Sagittarius B2, from the European Southern Observatory http://www.eso.org/public/images/eso0924e/

Laboratory improvements: 18-26.5 GHz Spectrometer

• Our previous frequency range was 8.7 - 18.3 GHz

• A new circuit was built to extend it to 26.5 GHz (left).

2-Propanethiol

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964 peaks above the 3:1 signal to noise threshold

2 million averages, 7 mTorr, -20° C, 4 μs Fid