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FTIR Matrix Study of Potential Circumstellar Molecules: TiC3

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1 FTIR Matrix Study of Potential Circumstellar Molecules: TiC3
R.E. Kinzer, Jr., C. M. L. Rittby, W. R. M. Graham Department of Physics and Astronomy Texas Christian University Fort Worth, TX 76129 61st International Symposium on Molecular Spectroscopy The Ohio State University 19-23 June 2006

2 Astrophysical Potential
Carbon chains (e.g. C3, C5) and molecules containing carbon chains have been detected in interstellar space and circumstellar shells. (Hinkle, Science 1988; Bernath, Science 1989) Molecules containing transition-metals have been detected in stars. TiO is a signature of M-type stars. The presence of TiC crystals and Ti bonded to fullerenes in post-AGB stars has been considered. (Duncan, Science 2000; Kimura, ApJ 2005) See also WG05 on CrC3 (Bates, previous Matrix/Condensed phase session)

3 Metallocarbohedrenes
Castleman et al. reported the Ti8C12+ metallocarbohedrene (“metcar”). (Science 1992) Other large metal-carbide molecules have also been observed; TiC2 seems to serve as a building block. (Castleman, JPC 1992) How do smaller transition-metal carbides (TiC2, TiC3, etc.) combine to form larger metcars, and what are their structures? Photoelectron spectroscopy and theoretical studies of smaller transition-metal carbides have attempted to address this question.

4 Photoelectron Spectroscopy (PES) Study
Wang et al. observed vibrationally resolved spectra of TiCnˉ, n=2-5. (JPCA 1997) Only TiC2 had any previous theoretical study; it was predicted to have cyclic C2v structure. (Rheddy & Khanna, JPC 1994) Structures proposed based on comparisons to theoretical studies of LaCn and YCn. Cyclic structures were predicted for all molecules considered.

5 Photoelectron Spectroscopy (PES) Study
TiC2 560 ± 50 cm-1 TiC3 650 ± 30 cm-1 TiC4 440 ± 40 cm-1 TiC5 240 ± 60 cm-1 Wang et al., JPCA 1997.

6 Theoretical DFT Study Sumathi & Hendrickx Density Functional Theory (DFT) study using B3LYP functional for TiC2, TiC3, TiC4, Ti2C2, Ti2C3. (CPL 1998; JPCA 1998, JPCA 1999) Vibrational frequencies calculated for singlet, triplet, and quintet states of several isomers. Intensities of the modes not reported. Isomers of TiC3 considered

7 Theoretical DFT Studies
Relative energies for various isomers and electronic states of TiC3 Relative energy in kcal/mol 100.0 80.0 60.0 40.0 20.0 0.0 fan kite linear Non- planar Exo-Ti Exo-C VI VII I II VIII IV III 27.4 7.1 38.2 36.0 21.5 41.2 38.9 28.9 97.2 67.9 55.1 54.5 86.8 59.8 50.6 ~76 q t s Bond lengths (Å) for singlet, triplet (), and quintet isomers [ ].

8 Theoretical DFT Study The 1A1 state, C2v fan-like isomer is the ground state structure. 465.3 6(b2) 1531.4 5(b2) 591.2 4(b1) 686.5 3(a1) 833.6 2(a1) 1281.3 1(a1) DFT Calculated (cm-1) Vibrational mode 650 ± 30 PES Observeda C Ti a Observation by Wang et al., JPCA 1997.

9 Strategy Fourier Transform Infrared (FTIR)
measurements of vibrational frequencies 13C isotopic shifts for clusters trapped in Ar at ~10 K. Density Functional Theory (DFT) simulations vibrational frequencies and intensities calculated for main 12C frequencies and 13C isotopic shifts comparison of DFT simulations with observed frequencies and isotopic shifts determine molecular structure, species and vibrational modes.

10 Theoretical Calculations
Calculations using Gaussian 03 program suite DFT calculations using B3LYP/6-311G(3df,3pd) functional Calculated frequencies for the C2v singlet structure are in good agreement with Sumathi & Hendrickx. 13C isotopic shifts calculated for comparison to experimental results.

11 Theoretical Calculations
DFT B3LYP/6-311G(3df,3pd) predicted vibrational frequencies (cm-1) and infrared intensities (km/mole) for the fan-shaped (C2v) isomer (singlet) of TiC3. 460 6(b2) 1549 5(b2) 621 4(b1) 697 3(a1) 866 2(a1) 1291 1(a1) DFT Calculated (cm-1) Vibrational mode 650 ± 30 PES Observeda 28 39 12 64 5 4 Infrared Intensity (km/mole) C C Ti a Observation by Wang et al., JPCA 1997.

12 Theoretical Calculations
Stretching modes of TiC3 1(a1) ~ cm-1 ~ 4 km/mole 2(a1) ~ cm-1 ~ 5 km/mole 3(a1) ~ cm-1 ~ 64 km/mole 4(b1) ~ cm-1 ~ 12 km/mole 5(b2) ~ cm-1 ~ 39 km/mole 6(b2) ~ cm-1 ~ 28 km/mole

13 Experimental Apparatus
Nd-YAG 1064 nm pulsed laser Laser focusing lens CsI window Quartz window Gold mirror ~ 10K Bomem DA3.16 Fourier Transform Spectrometer • KBr beam splitter • liquid N2 cooled MCT detector ( cm-1) 0.2 cm-1 resolution To pump 10-7 Torr or better To pump 10-3 Torr Titanium rod Carbon rod see also WG04 on GeC5Ge (Gonzalez, previous Matrix/Condensed phase session) Ar

14 n5(b2) fundamental 1484.2 (a) 90% 12C/ 10% 13C rod + Ti rod, 9K C5(n4)
3.4 1484.2 (a) 90% 12C/ 10% 13C rod + Ti rod, 9K C5(n4) 1446.6 Absorbance 3.4 3.4 3.4 (b) 90% 12C/ 10% 13C rod + Ti rod, 24K ( ) ( ) ( ) ( ) ( ) 1473.5 1450.9 ( ) 1439.9 1461.4 in C spectrum (c) DFT Simulation 1430 1440 1450 1460 1470 1480 1490 Frequency (cm-1)

15 5(b2) fundamental Comparison of observed vibrational frequencies (cm-1) of the 5(b2) mode with 13C isotopomers and B3LYP/6-311G(3df,3pd) calculations. -- 1426.0 1488.6 0.7 1460.7 1524.9 1461.4 0.9 1439.0 1502.2 1439.9 0.8 1450.1 1513.8 1450.9 0.2 1473.3 1538.0 1473.5 0.0 1484.2 1549.4 Δ SC DFT Ti-C-C-C  Difference Scaled a B3LYP/ 6-311G(3df,3pd) Observed Isotopomer a Scaling factor of / =

16 3(a1) fundamental 624.3 (a) 90% 12C/ 10% 13C rod + Ti rod, 16K 573.8
( ) ( ) ( ) ( ) ( ) ( ) 624.3 (a) 90% 12C/ 10% 13C rod + Ti rod, 16K 573.8 ( ) ( ) 616.8 Absorbance 608.4 (b) DFT Simulation 540 560 580 600 620 640 Frequency (cm-1)

17 3(a1) fundamental Comparison of observed vibrational frequencies (cm-1) of the 3(a1) mode with 13C isotopomers and B3LYP/6-311G(3df,3pd) calculations. -- 608.4 679.0 -0.1 608.5 679.1 0.2 616.6 688.1 616.8 624.1 696.5 overlapped 0.1 616.7 688.2 0.0 624.3 696.7 Δ SC DFT Ti-C-C-C  Difference Scaled a B3LYP/ 6-311G(3df,3pd) Observed Isotopomer a Scaling factor of 624.3/696.7 =

18 4(b1) fundamental ? 3(a1) 624.3 (a) 90% 12C/ 10% 13C rod + Ti rod, 16K 573.8 616.8 Absorbance 608.4 (b) DFT Simulation 540 560 580 600 620 640 Frequency (cm-1)

19 4(b1) fundamental ? DFT B3LYP/6-311G(3df,3pd) predicted vibrational frequencies (cm-1) and infrared intensities (km/mole) for the fan-shaped (C2v) isomer (singlet) of TiC3. 460 6(b2) 1549 5(b2) 621 4(b1) 697 3(a1) 866 2(a1) 1291 1(a1) DFT Calculated (cm-1) Vibrational mode 1484.2 573.8? 624.3 FTIR Observed 28 39 12 64 5 4 Infrared Intensity (km/mole) C C Ti

20 4(b1) fundamental ? The 4 ~ 621 cm-1 mode predicted to have ~18% intensity of the 3 mode; cm-1 has comparable intensity. No other possible Ti-C species observed in spectrum. Lack of isotopic shifts precludes definitive assignment to 4(b1) ~ cm-1.

21 Conclusions The C2v ‘fan-like’ isomer in the 1A1
Ti The C2v ‘fan-like’ isomer in the 1A1 state is the ground state structure of TiC3. The following vibrational modes were observed 1549 5(b2) 621 4(b1) 697 3(a1) DFT Calculated (cm-1) Vibrational mode 650 ± 30 PES Observedb 1484.2 573.8? 624.3 FTIR Observeda a Uncertainty of ± 0.2 cm-1 in FTIR observed. b Observation by Wang et al., JPCA 1997.

22 Acknowledgements The Welch Foundation
TCU Research and Creative Activities Fund in support of this research W.M. Keck Foundation for the Bomem spectrometer

23 References K.H. Hinkle, J.J. Keady, P.F. Bernath, Science 241, 1319 (1988). P.F. Bernath, K.H. Hinkle, J.J. Keady, Science 244, 562 (1989). G. von Helden, A.G.G.M. Tielens, D. van Heijnsbergen, M.A. Duncan, S. Hony, L.B.F.M. Waters, G. Meijer, Science 288, 313 (2000). Y. Kimura, J.A. Nuth III, F.T. Ferguson, Astrophysical J. 632, L159 (2005). B.C. Guo, K.P. Kerns, A.W. Castleman, Jr. Science 255, 1411 (1992). S. Wei, B.C. Guo, J. Purnell, S. Buzza, A.W. Castleman, Jr. J. Phys. Chem. 96, 4166 (1992). X.B. Wang, C.D. Ding, L.S. Wang, J. Phys. Chem. A 101, 7699 (1997). B.V. Reddy and S.N. Khanna, J. Phys. Chem. 98, 9446 (1994). R. Sumathi and M. Hendrickx, Chem. Phys. Lett. 287, 496 (1998). R. Sumathi and M. Hendrickx, J. Phys. Chem. A 102, 4883 (1998). R. Sumathi and M. Hendrickx, J. Phys. Chem. A 103, 585 (1999). R. Sumathi and M. Hendrickx, J. Phys. Chem. A 102, 7308 (1998).

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