Observation of the Infrared Spectrum of SiC5

T. H. Le and W. R. M. Graham

Molecular Physics LabTexas Christian University

66th International Symposium on Molecular SpectroscopyThe Ohio State University

June 20, 2011

2

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 SiC5? Most observed SiCn are linear (e.g. SiC4 , SiC7, SiC9)

3

Previous observation of SiC5

McCarthy et al. (2000)– SiC5 made using silane and diacetylene

– Rotational spectrum detected using microwave spectroscopy

4

DFT-B3LYP/cc-pVDZ calculations: other isomers > 50 kcal/mol above linear

Linear ground state

Other theoretical calculations agree:

Gomei et al. (1997) MP4/6-31G*[END+] Hunsicker et al. (1996) DFT-LSDA with MDJiang et al. (2001) DFT-B3LYP/6-311G* Lan et al. (2009) DFT-B3LYP/aug-cc-pVTZ

Other isomers of SiC5

Cyclic

Ring tail

∆E = 62 kcal/mol

∆E = 65

∆E = 58

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Experimental procedureMatrix isolation

Laser ablate 5% Si/ 95% C rods

Trap molecular vapor on a gold mirror at < 20 K

Record FTIR spectra on BOMEM DA3 from 400 - 3000 cm-1 at resolution

0.2 cm-1.

Quartz window

Direction of Ar flow

FTIR (MCT detector)

CsI window

Mirror

Nd:YAG1064 nm pulsed laser

Sintered rod

Isotopic shift data Rods enriched with 10% 13C. 29Si (4.7%) and 30Si (3.1%) occur in

natural abundance.

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SimCn candidates

920 925 930 935 940

unidentified Cn band

graphite rod

936.9

935.2

Frequency (cm-1)

Ab

sorb

ance

933.5

sintered 5% Si / 95% 12C rod

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Five 13C shifts

Intensity ~10% of 936.9 cm-1

5 inequivalent carbon sites

Likely candidate: linear SiC5

920 925 930 935 940

Isotopic shift spectrum (10% 13C)936.9

Frequency (cm-1)

Ab

sorb

ance

926.3 926.9 928.9 933.6 934.5Cn

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DFT predicted fundamentals: linear SiC5

Vibrational mode

B3LYP/cc-pVDZDFT (cm-1)

Infrared intensity (km/mol)

ν1(σ) 2116 458

ν2(σ) 1966 508

ν3(σ) 1547 2

ν4(σ) 959 46

ν5(σ) 495 18

ν6(π) 649 1

Close to unidentified absorption 936.9 cm-1

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936.9

926.3

926.9928.9

933.6

934.5

920 925 930 935 940 Frequency (cm-1)

Ab

sorb

ance

Isotopic shift spectrum (10% 13C)Single substitutions

B3LYP/cc-pVDZ

sintered 5% Si / 10% C13 rod

Cn

28-12-12-12-12-13

28-13-12-12-12-12

28-12-13-12-12-12

28-12-12-13-12-12

28-12-12-12-13-12

28-12-12-12-12-12

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935.2933.5

920 925 930 935 940 Frequency (cm-1)

Ab

sorb

ance

Si isotopic shifts

B3LYP/cc-pVDZ

sintered 5% Si / 95% C12 rodCn

28-12-12-12-12-12

29-12-12-12-12-1230-12-12-12-12-12

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Quantitative ComparisonDifference

obs - sc (cm-1)

---

0.0

0.2

-0.8

-1.4

1.5

0.8

-0.6

a Results of the B3LYP/cc-pVDZ calculation scaled by a factor of 936.9/959.9.

Isotopomer Si-C-C-C-C-C

28-12-12-12-12-12

29-12-12-12-12-12

30-12-12-12-12-12

28-13-12-12-12-12

28-12-13-12-12-12

28-12-12-13-12-12

28-12-12-12-13-12

28-12-12-12-12-13

Observedobs (cm-1)

936.9

935.2

933.5

926.9

928.9

933.6

934.5

926.3

B3LYP/cc-pVDZ DFT (cm-1)

959.9

958.2

956.6

948.8

950.3

958.1

958.3

948.5

Scaledªsc (cm-1)

936.9

935.2

933.7

926.1

927.5

935.1

935.4

925.8

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Conclusions

The observed isotopic shifts are in very good agreement with the DFT predictions.

The ν4(σ) mode, an asymmetric stretch of linear SiC5,

trapped in solid Ar has been observed, and identified at 936.9 ± 0.2 cm−1.

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Acknowledgements

The Welch Foundation

TCU Research and Creative Activities Fund

The Graduate Student Travel Grant Program

W.M. Keck Foundation for the Bomem spectrometer

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Works Cited

M. C. McCarthy, A.J. Apponi, C.A. Gottlieb, and P. Thaddeus, Astrophys. J.

538, 766 (2000).

S. Hunsicker and R. O. Jones, J. Chem. Phys. 105, 12 (1996).

M. Gomei, R. Kishi, A. Nakajima, S. Iwata, and K. Kaya, J. Chem. Phys.

107, 23 (1997)

Z. Jiang, X. Xu, H. Wu, F. Zhang, and Z. Jin, J. Mol. Struc. 589, 103 (2002)

Y.-Z. Lan and Y.-L. Feng, J. Chem. Phys. 131, 054509 (2009)