1. 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. 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 )

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

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 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

5. 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 400 - 3000 cm -1 at resolution 0.2 cm -1. 5 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. 6 Si m C n candidates 920 925 930 935 940 unidentified C n band graphite rod 936.9 935.2 Frequency (cm -1 ) Absorbance 933.5 sintered 5% Si / 95% 12 C rod

7. Five 13 C shifts Intensity ~ 10% of 936.9 cm -1 5 inequivalent carbon sites Likely candidate: linear SiC 5 7 920 925 930 935 940 Isotopic shift spectrum (10% 13 C) 936.9 Frequency (cm -1 ) Absorbance 926.3926.9 928.9 933.6 934.5 CnCn

8. DFT predicted fundamentals: linear SiC 5 8 Vibrational mode B3LYP/cc-pVDZ DFT (cm -1 ) Infrared intensity (km/mol) ν1(σ)ν1(σ) 2116458 ν2(σ)ν2(σ) 1966508 ν3(σ)ν3(σ) 15472 ν4(σ)ν4(σ) 95946 ν5(σ)ν5(σ) 49518 ν6(π)ν6(π) 6491 Close to unidentified absorption 936.9 cm -1

9. 936.9 926.3 926.9 928.9 933.6 934.5 9 920 925 930 935 940 Frequency (cm -1 ) Absorbance Isotopic shift spectrum (10% 13 C) Single substitutions B3LYP/cc-pVDZ sintered 5% Si / 10% C 13 rod CnCn 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

10. 935.2 933.5 10 920 925 930 935 940 Frequency (cm -1 ) Absorbance Si isotopic shifts B3LYP/cc-pVDZ sintered 5% Si / 95% C 12 rod CnCn 28-12-12-12-12-12 29-12-12-12-12-12 30-12-12-12-12-12

11. Quantitative Comparison Difference obs - sc ( cm -1 ) --- 0.0 0.2 -0.8 -1.4 1.5 0.8 -0.6 11 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 Observed obs (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 Scaledª sc ( cm -1 ) 936.9 935.2 933.7 926.1 927.5 935.1 935.4 925.8

12. 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 936.9 ± 0.2 cm −1.

13. 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

14. 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) 14