Observation of the Infrared Spectrum of SiC 5 T. H. Le and W. R. M. Graham Molecular Physics Lab Texas Christian University 66 th International Symposium on Molecular Spectroscopy The Ohio State University June 20, 2011

2 Si m C n identified in matrix studies Blue – SiRed – C SiC 2 SiC 7 SiC 9 SiC 4 Si Si 2 C Si 2 C 2 Si 3 C Si 3 C 2 T-shaped Linear Cyclic Bent SiC 4 SiC 3 Si Structure of SiC 5 ? Most observed SiC n are linear (e.g. SiC 4, SiC 7, SiC 9 )

Previous observation of SiC 5 McCarthy et al. (2000) –SiC 5 made using silane and diacetylene –Rotational spectrum detected using microwave spectroscopy 3

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 MD Jiang et al. (2001)DFT-B3LYP/6-311G* Lan et al. (2009)DFT-B3LYP/aug-cc-pVTZ Other isomers of SiC 5 4 Cyclic Ring tail ∆E = 62 kcal/mol ∆E = 65 ∆E = 58

Experimental procedure Matrix 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 cm -1 at resolution 0.2 cm Quartz window Direction of Ar flow FTIR (MCT detector) CsI window Mirror Nd:YAG 1064 nm pulsed laser Sintered rod Isotopic shift data Rods enriched with 10% 13 C. 29 Si (4.7%) and 30 Si (3.1%) occur in natural abundance.

6 Si m C n candidates unidentified C n band graphite rod Frequency (cm -1 ) Absorbance sintered 5% Si / 95% 12 C rod

Five 13 C shifts Intensity ~ 10% of cm -1 5 inequivalent carbon sites Likely candidate: linear SiC Isotopic shift spectrum (10% 13 C) Frequency (cm -1 ) Absorbance CnCn

DFT predicted fundamentals: linear SiC 5 8 Vibrational mode B3LYP/cc-pVDZ DFT (cm -1 ) Infrared intensity (km/mol) ν1(σ)ν1(σ) ν2(σ)ν2(σ) ν3(σ)ν3(σ) ν4(σ)ν4(σ) ν5(σ)ν5(σ) ν6(π)ν6(π) 6491 Close to unidentified absorption cm -1

Frequency (cm -1 ) Absorbance Isotopic shift spectrum (10% 13 C) Single substitutions B3LYP/cc-pVDZ sintered 5% Si / 10% C 13 rod CnCn

Frequency (cm -1 ) Absorbance Si isotopic shifts B3LYP/cc-pVDZ sintered 5% Si / 95% C 12 rod CnCn

Quantitative Comparison Difference obs - sc ( cm -1 ) a Results of the B3LYP/cc-pVDZ calculation scaled by a factor of 936.9/ Isotopomer Si-C-C-C-C-C Observed obs (cm -1 ) B3LYP/cc- pVDZ DFT (cm -1 ) Scaledª sc ( cm -1 )

Conclusions 12 The observed isotopic shifts are in very good agreement with the DFT predictions. The ν 4 ( σ ) mode, an asymmetric stretch of linear SiC 5, trapped in solid Ar has been observed, and identified at ± 0.2 cm −1.

Acknowledgements The Welch Foundation TCU Research and Creative Activities Fund The Graduate Student Travel Grant Program W.M. Keck Foundation for the Bomem spectrometer 13

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, (2009) 14