First Observation of a Vibrational Fundamental of SiC6Si Trapped in Solid Ar

T.H. Lê, C.M.L. Rittby and W.R.M. Graham

Department of Physics and AstronomyMolecular Physics Lab

Texas Christian University, Fort Worth, TX

68th International Symposium on Molecular SpectroscopyThe Ohio State University

June 18, 2013

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SimCn identified in matrix studiesBlue – Si Red – C

SiC2

SiC7

SiC9

SiC4Si

Si2C

Si2C2

Si3C

Si3C2

T-shaped Linear Cyclic

Bent

SiC4

SiC3Si

Structure of SiC6Si?

Previous studies on SiC6SiJiang et al. (2002) DFT B3LYP/6-311G* – Most intense mode, 5(σu) = 2072 cm-1

Calculated bond lengths (Å):

First reported mass spectroscopy observation: Kaiser et al. (2010) ablated a Si rod with acetylene carrier gas to simulate meteor vaporization in Titan’s atmosphere.– Obtained photoionization energy = 8.5 0.1 eV.

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Si–C1 1.721 C2–C3 1.289C1–C2 1.277 C3–C4 1.268

2 3 4 5 6 1

Laser ablation and matrix isolationCompress powder mixtures of 30% Si, 70% 12C (4.5 × 105 kPa) to form soft rods.

Laser ablate rods and trap vapor on a gold mirror at < 20 K.

Record FTIR spectra on BOMEM DA3 in the range 400 - 3000 cm-1 at 0.2 cm-1 resolution.

Ar flow

FTIR (MCT detector)

Mirror

Nd:YAG1064 nm pulsed laser

Rod

Quartz window

CsI window

Isotopic shift dataEnrich rods with 10% 13C.

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Pump

Unidentified band at 1848.2 cm-1

1810 1815 1820 1825 1830 1835 1840 1845 1850

Si 30%/60%12C/10%13C rod

Graphite rod

Si 30%/70%12C rod

1824.0 1840.2 1844.3

x x

Cn

x x

x SinCm

1848.2

5

x – appear in Si-12C expt.

Frequency (cm-1)

Abs

orba

nce

x

Unidentified band at 1848.2 cm-1

1810 1815 1820 1825 1830 1835 1840 1845 1850

Intensities of isotopomer bands relative to the fundamental are twice what we would expect for isotopomers that are singly-substituted with inequivalent C atoms3 pairs of equivalent carbon atoms

Simplest candidate is SiC6Si.

Si 30%/60%12C/10%13C rod

1824.0 1840.2 1844.3

x x

1848.2

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x – appear in Si-12C expt.

Frequency (cm-1)

Abs

orba

nce

~20% ~20% ~20%

1824.0 1840.2 1844.3

1810 1815 1820 1825 1830 1835 1840 1845 1850

xx

ν4(σu) =1952.7cm-1 of C6*

1848.2 cm-1 13C shift spectrum

13C shift spectrum for 1848.2 cm-1 compared to ν4(σu) of C6

1848.2

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o o o o o o

o

Frequency (cm-1)

Abs

orba

nce

o – related to another species

ν = 2.3ν = 1.3 ν = 0.6

x – appear in Si-12C expt.

* Scaled by 1848.2 / 1952.7

o1826.31838.9 1843.7

VibrationBVWN5/cc-pVDZ (cm-1)

Intensity(km/mol)

1(σg) 2008 02(σg) 1776 03(σg) 938 04(σg) 341 05(σu) 2010 2666(σu) 1369 967(σu) 592 22

DFT predicted modes for SiC6Si

VibrationBVWN5/cc-pVDZ (cm-1)

Intensity(km/mol)

1(σg) 2008 02(σg) 1776 03(σg) 938 04(σg) 341 05(σu) 2010 2666(σu) 1369 967(σu) 592 22

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Mode Mixing

2 3 4 5 6 1

2 3 4 5 6 1

Single 13C substitutions break symmetry of moleculeVibrational modes of the same symmetry will mix (couple)Example:

2 3 4 5 6 1

2 3 4 5 6 1

5(σg)

1(σu)

5(σ)

1(σ)

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212

13

12isotopomer

m um

Perturbation approximation for isotopomer bands

Single 13C isotopic substitutions can be considered as mass perturbations.When 2 modes are well separated, non-degenerate first order approximations are reasonable.

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Equation for approximating singly-substituted isotopomer bands

Isotopic and mixing perturbation matrix

2 1 51 12 121

13 13

25 1 12 5 125

13 13

1 (1) 1 (1) (5)2 2

1 (5) (1) 1 (5)2 2

n n n

n n n

m mu u um m

m mu u um m

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2(5)u

2(1)u

5(σ)

1(σ)

BVWN5/cc-pVDZ mixing

13C shifts for ν4(σu) = 1952.7 cm-1 of C6 compared to DFT predictions

1915 1920 1925 1930 1935 1940 1945 1950 1955

12Frequency (cm-1)

Abs

orba

nce

BVWN5/cc-pVDZ no mixing

13C shifts for ν4(σu) of C6

1952.7

o – related to another species

o o o o o

o

o

1810 1815 1820 1825 1830 1835 1840 1845 1850

13C shifts for 1848.2

x x

BVWN5/cc-pVDZ no mixing

BVWN5/cc-pVDZ mixing

13Frequency (cm-1)

Abs

orba

nce

13C shifts for 1848.2 cm-1

compared to SiC6Si DFT predictions1848.2

x – appear in Si-12C expt.

Comparison of observed 13C isotopomer bands to DFT predictions

BVWN5/cc-pVDZ

Isotopomer observedobs (cm-1)

mixing

scaledsc

diff - νobs

no mixing

scaled

sc diff

- νobs

28 12 12 12… 1848.2 2009.9 1848.2 … 2009.9 1848.2 …28 12 12 13… 1844.3 2009.6 1847.9 3.6 2003.9 1842.7 -1.628 13 12 12… 1840.2 2008.0 1846.5 6.3 2002.4 1841.3 1.128 12 13 12… 1824.0 1982.2 1822.7 -1.3 1984.7 1825.0 1.0

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ConclusionsA previously unidentified absorption at 1848.2 cm-1 has been observed in the laser ablation of Si/C rods

The observed 13C shift pattern indicates it belongs to a Si-bearing molecule with three pairs of equivalent C atoms – linear SiC6Si is the simplest candidate.

Similarities between the 13C shift patterns of 1848 cm-1, and the ν4(σu) = 1952.7 cm-1 mode of C6 support the assignment of 1848.2 cm-1 to a molecule with a C6 chain.

DFT predictions for singly substituted isotopomers without mode mixing are in good agreement with observed data ( < 1.7 cm-1).

The ν5(σu) fundamental of SiC6Si has been identified for the first time at 1848.2 cm-1.

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Acknowledgements

TCU Research and Creative Activities FundThe Graduate Student Travel Grant ProgramTCU Graduate Student SenateW.M. Keck Foundation

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Works Cited

R.I. Kaiser, P. Maksyutenko, C. Ennis, F. Zhang, X. Gu, S.P. Krishtal, A.M. Mebel, O. Kostkoc and M. Ahmed, Farad. Discuss. 147, 429 (2010).Z. Jiang, X. Xu, H. Wu, F. Zhang, Z. Jin, Theochem-J. Mol. Struct. 103, 589 (2002).

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Questions?

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