FTIR Matrix Study of Potential Circumstellar Molecules: TiC3

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 TiCn, n2-5. (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
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
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.

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.
a Observation by Wang et al., JPCA 1997.
12
Theoretical Calculations

• Stretching modes of TiC3

?1(a1) 1291.1 cm-1 4 km/mole
?2(a1) 865.5 cm-1 5 km/mole
?3(a1) 696.7 cm-1 64 km/mole
?4(b1) 620.5 cm-1 12 km/mole
?6(b2) 459.8 cm-1 28 km/mole
?5(b2) 1549.4 cm-1 39 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 (500 - 3500 cm-1)
• 0.2 cm-1 resolution

To pump 10-7 Torr or better
To pump 10-3 Torr
Carbon rod
Titanium 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)
1446.6
Absorbance
(b) 90 12C/ 10 13C rod Ti rod, 24K
(48-13-12-12) (48-12-12-13)
(48-12-13-12)
(48-13-13-12) (48-12-13-13)
1473.5
(48-13-12-13)
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.
a Scaling factor of 1484.2/1549.4 0.95792 .
16
?3(a1) fundamental
(48-12-12-12) (48-12-13-12)
624.3
(48-13-12-12) (48-12-12-13) (48-13-13-12) (48-12-1
3-13)
(a) 90 12C/ 10 13C rod Ti rod, 16K
573.8
616.8
(48-13-12-13) (48-13-13-13)
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.
a Scaling factor of 624.3/696.7 0.8961 .
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.
20
?4(b1) fundamental ?

• The ?4 621 cm-1 mode predicted to have 18
  intensity of the ?3 mode 573.8 cm-1 has
  comparable intensity.
• No other possible Ti-C species observed in
  spectrum.
• Lack of isotopic shifts precludes definitive
  assignment to ?4(b1) 573.8 cm-1.

21
Conclusions

• The C2v fan-like isomer in the 1A1
• state is the ground state structure of TiC3.
• The following vibrational modes were observed

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
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