A LABORATORY AND THEORETICAL INVESTIGATION OF THE SILICON

SULFUR MOLECULES H2SiS AND Si2S.MICHAEL C. MCCARTHY1, PATRICK THADDEUS1, HARSHAL GUPTA2,

SVEN THORWIRTH3, JÜRGEN GAUSS4 and FRANÇOIS SHINDO1

1Harvard-Smithsonian Center for Astrophysics, Cambridge MA, USA.2Institute for Theoretical Chemistry, University of Texas, Austin, USA3Max-Planck-Institut für Radioastronomie, Bonn, Germany4Institut für Physikalische Chemie, Johannes-Gutenberg-University, Mainz, Germany

MotivationsAstronomical interest:

• Significant fraction of molecules with Si or S in circumstellar shells ex: 1/3 of the 37 molecules in IRC+10216

• Closely related in composition with SiS: Importance of SiS in the photochemistry of IRC+10216 High abundance: seven isotopes including 29Si34S and 30Si34S Rotational transitions high J (20-19) vibrational excited levels (v=3) maser action

But little information on other molecules containing both Si and S

Motivations

Laboratory work on similar systems:

•Extensive spectroscopy on the three isovalent molecules of silanethione (H2SiS): Formaldehyde (H2CO), H2CS, silanone (H2SiO)

• rotational spectrum of radical HSiS detected in glow discharge cell Brown et al., J. Mol. Struct. 413-414, 537(1993)

Theoretical studies on both H2SiS and Si2S fairly polar (H2SiS μ~3D) Most stable isomeric form Lai et al., Inter. J. Quant. Chem. 82, 14 (2001) Davy and Schaefer, Chem. Phys. Lett. 255, 171 (1996)

This work

Joint experimental and theoretical study of H2SiS and Si2S

• Lab investigation: Detection of the rotational spectra by FTM spectroscopy (5-43GHz)

Rotational spectra of many isotopes (~10 species) Rotational spectra from several vibrational levels H2SiS in mmw band (200-376GHz) in glow discharge Derive precise rotational and centrifugal distortion constants

• Theoretical investigation with coupled-cluster (CC) methods: Vibrational energies of GS, vibration-interaction constants

Empirical and theoretical structures

The SearchBoth planar, prolate asymmetric-top, closed-shell molecules with C2ν symmetry

• Si2S Two Si atoms: Bose-Einstein statistics

b-type transitions (KaKc=ee or oo) Fundamental transition 11,1→00,0 at 11.4 GHz

• H2SiS

Two equivalent H: ortho-para statistics Triplet structure split by ~750 MHz

a-type transitions Fundamental transition 10,1→00,0 at 15.7 GHz

Centimeter band investigation

• Discharge: SiH4+H2S diluted in Ne, 1000 V • 90 individual scans of 0.4 MHz, 2h

• FTM spectrometer 5-43 GHz, supersonic beam with T of 1-3 K

Intense lines observed

For Si2S :• 13 transitions (J=5, Ka=3)• 3 Isotopes (Si34SSi, 29SiSSi, 30SiSSi)• 8 spectroscopic constants, rms~few kHz

For H2SiS :• 4 transitions because B+C~ 15GHz• Measurement of H2Si33S hfs (33S, 0.76%) Additional study in mm (200-376GHz)•58 transitions in mm up to J=23 and Ka=5• 10 spectroscopic constants, rms~10 kHz

Vibrational excitation

• Intense vibrational satellites for several rotational lines of H2SiS (and normal species) in Ne

• Quenched by H2

• Determination of the vibration-rotation constants αi

B and αiC

Bv-B0=-vαiB

=> ν6 mode (b2, SiH2 in plane rock) calculated at 618 cm-1

(MHz) α6A α6

B α6C

Experimental -12.82 5.67

Theory* 84605.474 -14.050 6.919

*fc-CCSD(T)/cc-pV(Q+d)Z

rSi-S (Å) rH-Si (Å) HSiH˚

Best estimate 1.9371 1.4743 110.23

reemp 1.9357 1.4735 110.33

rSi-S (Å) rSi-Si (Å) SiSSi˚

Best estimate 2.1307 2.3756 67.76

reemp 2.1301 2.3744 67.74

•Best estimate: fc-CCSD(T)/cc-pV∞Z + Δcore/cc-pCVQZ +ΔT/cc-pVTZ + ΔQ/cc-pVDZ• re

emp: r0 after zero-point vibrational correction in fc-CCSD(T)/cc-pV(Q+d)Z level theory

Structures derived from this work

Si=S Si-Si dimer

re (Å) 1.929 2.246

• Comparison with known bond lengths

Conclusion

•Extensive experimental and theoretical study of H2SiS and Si2S

•H2SiS μ =2.67 D Si2S μ=0.66 D

• Derived spectroscopic constants more than adequate for astronomical search and detection

•Intense lines of both species observed in laboratory: Si2O, cis and trans-HSiSH

H2SiS in the millimeter band

• Freq. modulated MW spectrometer, discharge cell • range of measurements 230-377GHz

• Complete observations in cm band • 58 transitions up to J=23 and Ka=5

• Best fit: 10 spectroscopic constants, rms~10 kHz

• Intensity ratio SiS/H2SiS~1000•Discharge: SiH4+H2S+Ar, 150 mA •Ptot=55 mTorr, T=-120˚C•Integration time 23s/MHz

(v=0)14-13

160,16-150,15

Si2S/isotopes spectroscopic constants

Si2S/isotopes spectroscopic constants

H2SiS/isotopes spectroscopic constants

H2SiS/isotopes spectroscopic constants

Vibrational energies

Vibration-rotation interaction