1. FTIR MATRIX AND DFT STUDY OF THE VIBRATIONAL SPECTRUM OF CYCLIC ScC3
R.E. Kinzer, Jr., C. M. L. Rittby, W. R. M. Graham
Department of Physics and Astronomy
Texas Christian University
Fort Worth, TX 76129
63rd International Symposium on Molecular Spectroscopy
The Ohio State University
16-20 June 2008

2. Motivation: Astrophysical
Transition-metal carbide clusters may potentially be observed in circumstellar shells.
Over 140 molecules have been observed in circumstellar shells or the interstellar medium. (Cologne Database, April 2008)
Carbon clusters are observed in circumstellar shells.
C3, C4, C5
Molecules bearing carbon chains are observed.
CCCN, HC4N, C5N, HC9N

3. Motivation: Astrophysical
Numerous metal-bearing molecules have been observed in circumstellar shells.
MgCN, AlNC, FeO
Transition-metals have been observed in F- to M-type circumstellar shells.
Mn, Fe, Cd
Molecules bearing transition-metals observed in spectra of M-type stars.
TiO, VO, FeH, CrH
Few studies have measured transition-metal carbide spectra.
More specific than “near” or “surrounding”?

4. Motivation: Metallocarbohedrenes
Building blocks: M8C12+ (M = Ti, V, Zr, Hf, Nb) metallocarbohedrene (“metcar”).
Other large metal-carbide molecules have been observed; MC2 seems to serve as a building block.
How do smaller transition-metal carbides (ScC2, ScC3, 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.

5. Previous Experimental ScCn Research
Photoelectron spectroscopy (PES) and density functional theory (DFT) study of MC2 and MC3 (M = Sc, V, Cr, Mn, Fe, Co, Ni) clusters. (Li and Wang, 1999/2000)
PES results
ScC2: 670 ± 40 cm-1 metal-carbon stretch
ScC3: 560 ± 30 cm-1 ground state metal-carbon stretch
600 ± 50 cm-1 excited electronic state (+10 kcal/mol)
DFT-B3LYP results
ScC3 structure is fanlike
ScC3 ground electronic state is doublet

6. Previous Theoretical Research
All investigations predicted fanlike ground state.
DFT investigations (Li and Wang 2000; Roszak 2002; Redondo 2006)
fanlike 2A2 electronic ground state
fanlike 4B1 state ~6-10 kcal/mol higher
Linear geometries > 20 kcal/mol above ground state
CASSCF - CASPT2 (Hendrickx and Clima 2004)
fanlike 2A2 ground state (4B1, +9 kcal/mol)
MRSDCI, MRSDCI+Q (Roszak et al. 2002)
fanlike 4B1 ground state (2A2, kcal/mol)
As Roszak et al. dismiss the CASSCF calculation results, we need not mention them here.

7. DFT Theoretical Simulations
Simulations using GAUSSIAN 03 suite
B3LYP functional with 6-311G(3df, 3pd) basis set
Fanlike geometries, 2A2 and 4B1 electronic states modeled
13C isotopic shift spectra simulated for both electronic states for comparison to measured isotopic shifts
Determined molecular structure, species and vibrational modes

8. DFT Simulations Fanlike (C2v) Cβ Cα Cα Sc B3LYP/ 6-311G (3df,3pd) 2A2
Frequency
(cm-1)
Intensity (km/mol)
ν1(a1)
1256 4
1277 10
ν2(a1)
792 2
638 26
ν3(a1)
601 80
532 89
ν4(b1)
415 70
455 8
ν5(b2)
1523 9
1517 18
ν6(b2)
368 23
363 25
560 ± 30 cm-1
(PES)
1.32 Å
1.31 Å Cβ
Fanlike
(C2v) Cα Cα
2.05
2.19
2.05
2.15 Sc

9. 2A2 Vibrational Modes a) ν1(a1) b) ν2(a1) c) ν3(a1) d) ν4(b1)Sc
1256 cm-1
4 km/mol
792 cm-1
2 km/mol
601 cm-1
80 km/mol
d) ν4(b1)
e) ν5(b2)
f) ν6(b2) Sc
(as seen in plane of molecule)
415 cm-1
70 km/mol
1523 cm-1
9 km/mol
368 cm-1
23 km/mol

10. 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
Scandium rod
Carbon rod Ar

11. Experimental Results
Scandium carbide candidates found by comparison of Sc and 12C spectra to 12C spectra.
3 candidate Sc-C absorptions observed which maintained consistent intensity ratios
The ν3(σu)= cm-1 fundamental of C3 dominated spectra, suggesting the three candidates are absorptions of a C3 bearing molecule.
Similar frequencies ( and cm-1) measured for fanlike TiC3, so fanlike ScC3 is a good candidate.
Frequency (cm-1)
Relative Intensity
1478.0
27 %
557.0
100 %
1190.7
5 %
560 ± 30 cm-1 (PES)

12. Shift intensities behave like C3 spectrum
75 / 25 ~ 12C/13C,
after annealing at 30 K
1478.0
● ~ C5 shifts
Shift intensities behave
like C3 spectrum
1468.2
1442.4
1432.4 C5 ν4
Absorbance
1421.3
1457.3 ● ●
b) 2A2 simulation
c) 4B1 simulation
1410
1420
1430
1440
1450
1460
1470
1480
Frequency (cm-1)

13. ν5(b2) fundamental 2A2 4B1 Observed DFT Predicted Scaled Difference
Sc-Cα-Cβ-Cα
νobs
(cm-1)
νDFT
νscaled Δν Sc
1478.0
1522.7
---a
---
1517.1
---b Sc
1468.2
1512.6
0.0
1507.2
1468.4
-0.2 Sc
1442.4
1485.0
1441.4
1.0
1479.5 Sc
1432.4
1474.8
1431.5
0.9
1469.4 Sc
1457.3
1501.0
1456.9
0.4
1495.6
1457.1
0.2 Sc
1421.3
1462.8
1419.9
1.4
1457.4
1419.8
1.5
a The predicted 2A2 shifts scaled by a factor of / =
b The predicted 4B1 shifts scaled by a factor of / =

14. ν5(b2) fundamental Measured at 1478.0 cm-1
Asymmetric carbon stretching mode
Isotopic spectra confirm fanlike geometry
Simulated isotopic spectra for both 2A2 and 4B1 states match measured data well
ν5(b2) data do not allow the elimination of either state

15. Only two isotopic shifts. Similar pattern observed for fanlike TiC3…
75 / 25 ~ 12C/13C,
after annealing at 30 K
557.0
ν3(a1)
550.0
543.0 X
Absorbance
Only two isotopic shifts.
Similar pattern observed for fanlike TiC3…
520
540
Frequency (cm-1)
560
580

16. Cyclic TiC3: 3(a1) fundamental
presented 2006 Astronomical Species & Processes (WH10)
624.3
Absorbance
TiC3
ν3(a1)
616.8
(a) 90% 12C/ 10% 13C rod + Ti rod, 16K
608.4
(b) DFT Simulation
540
560
580
600
620
640
Frequency (cm-1)

17. 4B1 2A2 X 75 / 25 ~ 12C/13C, after annealing at 30 K 557.0 550.0 543.0
550.0
543.0 X
4B1
Absorbance
b) 4B1 simulation
2A2
c) 2A2 simulation
520
540
Frequency (cm-1)
560
580

18. ν3(a1) fundamental 2A2 4B1 Observed DFT Predicted Scaled Difference
Sc-Cα-Cβ-Cα
νobs
(cm-1)
νDFT
νscaled Δν Sc
557.0
601.2
---a
---
531.7
---b Sc
550.0
593.9
550.1
-0.1
526.6
551.6
-1.6 Sc
OLc
601.0
556.9
530.0
555.1 Sc
593.8
525.2 Sc
543.0
586.1
0.0
520.9
545.6
-2.6 Sc
OLd
519.8
544.4
a The predicted 2A2 shifts scaled by a factor of 557.0/601.2 =
b The predicted 4B1 shifts scaled by a factor of 557.0/531.7 =
OL = overlapped
c Shift is overlapped by the fundamental at cm-1 (Sc ).
d Shift is overlapped by the isotopic shift at cm-1 (Sc ).

19. ν3(a1) fundamental Measured at 557.0 cm-1
significant overlap of isotopic shifts (similar to TiC3)
most intense ScC3 absorption
Symmetric metal-carbon stretch
Shift pattern confirms 2A2 ground state and not 4B1
Confirms the 560 ± 30 cm-1 vibration measured in previous PES study

20. ν1(a1) 75 / 25 ~ 12C/13C, after annealing at 30 K C6 1190.7 ν5
● ~ C6 shifts
ν1(a1)
1182.8
1171.6 ● ●
1163.0 ● ● ●
Absorbance
b) 2A2 simulation
c) 4B1 simulation
1150
1160
1170
1180
1190
1200
Frequency (cm-1)

21. ν1(a1) fundamental 2A2 4B1 Observed DFT Predicted Scaled Difference
Sc-Cα-Cβ-Cα
νobs
(cm-1)
νDFT
νscaled Δν Sc
1190.7
1256.4
---a
---
1277.3
---b Sc
1171.6
1235.9
1171.3
0.3
1171.2
0.4 Sc
1182.8
1247.4
1182.2
0.6
1267.9
1181.9
0.9 Sc
1163.0
1226.3
1162.2
0.8
1246.5
1162.0
1.0 Sc
1217.1
1153.5
1237.1
1153.2 Sc
1207.1
1144.0
1227.1
1143.9
a The predicted 2A2 shifts scaled by a factor of / =
b The predicted 4B1 shifts scaled by a factor of / =

22. ν1(a1) fundamental Measured at 1190.7 cm-1
three isotopic shifts measured
consistent intensity ratio to other ScC3 bands
Symmetric carbon stretching mode
Shift spectra match 2A2 and 4B1 states well
ν3(a1) shift spectra eliminated quartet state!

23. Conclusions
Cyclic ScC3 (fanlike, C2v) in 2A2 electronic ground state observed
FTIR 13C isotopic shift spectra
DFT simulated isotopic spectra
Three vibrational fundamentals measured
DFT Calculated
(cm-1)
FTIR Observed
PES Observed
ν1(a1)
1256
1190.7
ν3(a1)
601
557.0
560 ± 30
ν5(b2)
1534
1478.0
This research is to be submitted to J. Chem. Phys.

24. Acknowledgements The Welch Foundation
TCU Research and Creative Activities Fund
W.M. Keck Foundation for the Bomem spectrometer

25. References ScmCn Experimental / Theoretical papers
X. Li and L. S. Wang, J. Chem. Phys. 111, 8389 (1999).
L. S. Wang and X. Li, J. Chem. Phys. 112, 3602 (2000).
V. M. Rayón, P. Redondo, C. Barrientos, A. Largo, Chem. Eur. J. 12, 6963 (2006).
P. Redondo, C. Barrientos, and A. Largo, J. Phys. Chem. A 110, 4057 (2006).
S. Roszak, D. Majumdar, and K. Balasubramanian, J. Chem. Phys. 116, (2002).
M. F. A. Hendrickx and S. Clima, Chem. Phys. Lett. 388, 284 (2004).
P. Redondo, C. Barrientos, and A. Largo, J. Phys. Chem. A, 109, 8594 (2005).
Additional Transition-Metal Carbide Papers
R. E. Kinzer, Jr., C. M. L. Rittby and W. R. M. Graham, J. Chem. Phys. 125, (2006).
S. A. Bates, C. M. L. Rittby and W. R. M. Graham, J. Chem. Phys. 125, (2006).
S. A. Bates, C. M. L. Rittby and W. R. M. Graham, J. Chem. Phys. 127, (2007).
R. E. Kinzer, Jr., C. M. L. Rittby and W. R. M. Graham, J. Chem. Phys., 128, (2008).
S. A. Bates, C. M. L. Rittby and W. R. M. Graham, J. Chem. Phys., in press.