First Observation of a Vibrational Fundamental of SiC 6 Si Trapped in Solid Ar T.H. Lê, C.M.L. Rittby and W.R.M. Graham Department of Physics and Astronomy Molecular Physics Lab Texas Christian University, Fort Worth, TX 68 th International Symposium on Molecular Spectroscopy The Ohio State University June 18, 2013

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 6 Si?

Previous studies on SiC 6 Si Jiang et al. (2002)  DFT B3LYP/6-311G* –Most intense mode, 5 (σ u ) = 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. 3 Si–C11.721C2–C C1–C21.277C3–C

Laser ablation and matrix isolation Compress powder mixtures of 30% Si, 70% 12 C (4.5 × 10 5 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 cm -1 at 0.2 cm -1 resolution. Ar flow FTIR (MCT detector) Mirror Nd:YAG 1064 nm pulsed laser Rod Quartz window CsI window Isotopic shift data Enrich rods with 10% 13 C. 4 Pump

Unidentified band at cm Si 30%/60% 12 C/10% 13 C rod Graphite rod Si 30%/70% 12 C rod x x CnCn x x x Si n C m x – appear in Si- 12 C expt. Frequency (cm -1 ) Absorbance x

Unidentified band at cm Intensities of isotopomer bands relative to the fundamental are twice what we would expect for isotopomers that are singly-substituted with inequivalent C atoms 3 pairs of equivalent carbon atoms Simplest candidate is SiC 6 Si. Si 30%/60% 12 C/10% 13 C rod x x x – appear in Si- 12 C expt. Frequency (cm -1 ) Absorbance ~20%

x x ν 4 ( σ u ) =1952.7cm -1 of C 6 * cm C shift spectrum 13 C shift spectrum for cm -1 compared to ν 4 (σ u ) of C oo o o o o o Frequency (cm -1 ) Absorbance o – related to another species  ν = 2.3  ν = 1.3  ν = 0.6 x – appear in Si- 12 C expt. * Scaled by / o

Vibration BVWN5/ cc-pVDZ  (cm -1 ) Intensity (km/mol) 1(σg)1(σg) 2(σg)2(σg) 3(σg)3(σg) 9380 4(σg)4(σg) 3410 5(σu)5(σu) 6(σu)6(σu) 7(σu)7(σu) DFT predicted modes for SiC 6 Si Vibration BVWN5/ cc-pVDZ  (cm -1 ) Intensity (km/mol) 1(σg)1(σg) 2(σg)2(σg) 3(σg)3(σg) 9380 4(σg)4(σg) 3410 5(σu)5(σu) 6(σu)6(σu) 7(σu)7(σu)

Mode Mixing Single 13 C substitutions break symmetry of molecule Vibrational modes of the same symmetry will mix (couple) Example: 5(σg)5(σg) 1(σu)1(σu) 5(σ)5(σ) 1(σ)1(σ) 9

Perturbation approximation for isotopomer bands Single 13 C isotopic substitutions can be considered as mass perturbations. When 2 modes are well separated, non- degenerate first order approximations are reasonable. 10 Equation for approximating singly-substituted isotopomer bands

Isotopic and mixing perturbation matrix 11 5(σ)5(σ) 1(σ)1(σ)

BVWN5/cc-pVDZ  mixing 13 C shifts for ν 4 (σ u ) = cm -1 of C 6 compared to DFT predictions Frequency (cm -1 ) Absorbance BVWN5/cc-pVDZ  no mixing 13 C shifts for ν 4 (σ u ) of C o – related to another species o o oo o o o

C shifts for x x BVWN5/cc-pVDZ  no mixing BVWN5/cc-pVDZ  mixing 13 Frequency (cm -1 ) Absorbance 13 C shifts for cm -1 compared to SiC 6 Si DFT predictions x – appear in Si- 12 C expt.

Comparison of observed 13 C isotopomer bands to DFT predictions BVWN5/cc-pVDZ Isotopomerobserved obs (cm -1 ) mixing  scaled  sc diff  - ν obs no mixing  scaled  sc diff  - ν obs … … … … … …

Conclusions A previously unidentified absorption at cm -1 has been observed in the laser ablation of Si/C rods The observed 13 C shift pattern indicates it belongs to a Si-bearing molecule with three pairs of equivalent C atoms – linear SiC 6 Si is the simplest candidate. Similarities between the 13 C shift patterns of 1848 cm -1, and the ν 4 ( σ u ) = cm -1 mode of C 6 support the assignment of cm -1 to a molecule with a C 6 chain. DFT predictions for singly substituted isotopomers without mode mixing are in good agreement with observed data (  < 1.7 cm -1 ). The ν 5 ( σ u ) fundamental of SiC 6 Si has been identified for the first time at cm

Acknowledgements TCU Research and Creative Activities Fund The Graduate Student Travel Grant Program TCU Graduate Student Senate W.M. Keck Foundation 16

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). 17

Questions? 18