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Title: Optical spectra of the silicon-terminated carbon chain radicals SiC{sub n}H (n = 3,4,5)

Abstract

The gas-phase optical spectra of three silicon-terminated carbon chain radicals, SiC{sub n}H (n = 3 − 5), formed in a jet-cooled discharge of silane and acetylene, have been investigated by resonant two-color two-photon ionization and laser-induced fluorescence/dispersed fluorescence. Analysis of the spectra was facilitated by calculations performed using equation-of-motion coupled cluster methods. For SiC{sub 3}H and SiC{sub 5}H, the observed transitions are well-described as excitations from a {sup 2}Π ground state to a {sup 2}Σ state, in which vibronic coupling, likely involving a higher-lying Π state with a very large predicted f-value (close to unity), is persistent. The lowest {sup 2}Σ states of both species are characterized by a rare silicon triple bond, which was identified previously [T. C. Smith, H. Y. Li, D. J. Clouthier, C. T. Kingston, and A. J. Merer, J. Chem. Phys. 112, 3662 (2000)] in the lowest {sup 2}Σ state of SiCH. Although a strong Π − Π transition is predicted for SiC{sub 4}H, the observed spectrum near 505 nm more likely corresponds to excitation to a relatively dark Σ state which is vibronically coupled to a nearby Π state. In contrast to the chains with an odd number of carbon atoms, which exhibit relatively sharpmore » spectral features and lifetimes in the 10–100 ns range, SiC{sub 4}H shows intrinsically broadened spectral features consistent with a ∼100 fs lifetime, and a subsequent long-lived decay (>50 μs) which we ascribe to mixing with a nearby quartet state arising from the same electronic configuration. The spin-orbit coupling constants for both SiC{sub 3}H and SiC{sub 5}H radicals were determined to be approximately 64 cm{sup −1}, similar to that of SiCH (69.8 cm{sup −1}), suggesting that the unpaired electron in these species is localized on the silicon atom. Motivated by the new optical work, the rotational spectrum of linear SiC{sub 3}H was detected by cavity Fourier-transform microwave spectroscopy in the 13–34 GHz range. Each rotational transition from the {sup 2}Π{sub 3/2} ground state exhibits well-resolved Λ-doubling and hyperfine structure; the derived rotational constant of B = 2.605 GHz is in excellent agreement with our calculations.« less

Authors:
; ;  [1];  [2]; ;  [3]
  1. Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138 (United States)
  2. (United States)
  3. Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061 (United States)
Publication Date:
OSTI Identifier:
22419939
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 141; Journal Issue: 4; Other Information: (c) 2014 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ACETYLENE; CARBON; FLUORESCENCE; FOURIER TRANSFORMATION; GROUND STATES; L-S COUPLING; SILICON; SILICON CARBIDES; SPECTRA

Citation Formats

Kokkin, D. L., Reilly, N. J., McCarthy, M. C., E-mail: mccarthy@cfa.harvard.edu, School of Engineering and Applied Sciences, Harvard University, 29 Oxford St., Cambridge, Massachusetts 02138, Fortenberry, R. C., and Crawford, T. D. Optical spectra of the silicon-terminated carbon chain radicals SiC{sub n}H (n = 3,4,5). United States: N. p., 2014. Web. doi:10.1063/1.4883521.
Kokkin, D. L., Reilly, N. J., McCarthy, M. C., E-mail: mccarthy@cfa.harvard.edu, School of Engineering and Applied Sciences, Harvard University, 29 Oxford St., Cambridge, Massachusetts 02138, Fortenberry, R. C., & Crawford, T. D. Optical spectra of the silicon-terminated carbon chain radicals SiC{sub n}H (n = 3,4,5). United States. doi:10.1063/1.4883521.
Kokkin, D. L., Reilly, N. J., McCarthy, M. C., E-mail: mccarthy@cfa.harvard.edu, School of Engineering and Applied Sciences, Harvard University, 29 Oxford St., Cambridge, Massachusetts 02138, Fortenberry, R. C., and Crawford, T. D. Mon . "Optical spectra of the silicon-terminated carbon chain radicals SiC{sub n}H (n = 3,4,5)". United States. doi:10.1063/1.4883521.
@article{osti_22419939,
title = {Optical spectra of the silicon-terminated carbon chain radicals SiC{sub n}H (n = 3,4,5)},
author = {Kokkin, D. L. and Reilly, N. J. and McCarthy, M. C., E-mail: mccarthy@cfa.harvard.edu and School of Engineering and Applied Sciences, Harvard University, 29 Oxford St., Cambridge, Massachusetts 02138 and Fortenberry, R. C. and Crawford, T. D.},
abstractNote = {The gas-phase optical spectra of three silicon-terminated carbon chain radicals, SiC{sub n}H (n = 3 − 5), formed in a jet-cooled discharge of silane and acetylene, have been investigated by resonant two-color two-photon ionization and laser-induced fluorescence/dispersed fluorescence. Analysis of the spectra was facilitated by calculations performed using equation-of-motion coupled cluster methods. For SiC{sub 3}H and SiC{sub 5}H, the observed transitions are well-described as excitations from a {sup 2}Π ground state to a {sup 2}Σ state, in which vibronic coupling, likely involving a higher-lying Π state with a very large predicted f-value (close to unity), is persistent. The lowest {sup 2}Σ states of both species are characterized by a rare silicon triple bond, which was identified previously [T. C. Smith, H. Y. Li, D. J. Clouthier, C. T. Kingston, and A. J. Merer, J. Chem. Phys. 112, 3662 (2000)] in the lowest {sup 2}Σ state of SiCH. Although a strong Π − Π transition is predicted for SiC{sub 4}H, the observed spectrum near 505 nm more likely corresponds to excitation to a relatively dark Σ state which is vibronically coupled to a nearby Π state. In contrast to the chains with an odd number of carbon atoms, which exhibit relatively sharp spectral features and lifetimes in the 10–100 ns range, SiC{sub 4}H shows intrinsically broadened spectral features consistent with a ∼100 fs lifetime, and a subsequent long-lived decay (>50 μs) which we ascribe to mixing with a nearby quartet state arising from the same electronic configuration. The spin-orbit coupling constants for both SiC{sub 3}H and SiC{sub 5}H radicals were determined to be approximately 64 cm{sup −1}, similar to that of SiCH (69.8 cm{sup −1}), suggesting that the unpaired electron in these species is localized on the silicon atom. Motivated by the new optical work, the rotational spectrum of linear SiC{sub 3}H was detected by cavity Fourier-transform microwave spectroscopy in the 13–34 GHz range. Each rotational transition from the {sup 2}Π{sub 3/2} ground state exhibits well-resolved Λ-doubling and hyperfine structure; the derived rotational constant of B = 2.605 GHz is in excellent agreement with our calculations.},
doi = {10.1063/1.4883521},
journal = {Journal of Chemical Physics},
number = 4,
volume = 141,
place = {United States},
year = {Mon Jul 28 00:00:00 EDT 2014},
month = {Mon Jul 28 00:00:00 EDT 2014}
}
  • Radiolysis of the mixed crystals of binary n-alkanes has been studied by ESR spectroscopy to understand the role of defects in the radical formation in crystalline organic compounds. Highly efficient (10-25%) and selective formation of solute radicals depending on the carbon-chain difference between solute and matrix molecules was found in the C{sub m}H{sub 2m+2} (1 mol %)/C{sub 10}D{sub 22} mixed crystals (m = 7-12) irradiated at 77 K. The terminal type of radical C{sup {center dot}}H{sub 2}CH{sub 2}CH{sub 2}- (I) is formed only in the mixed crystals with shorter solute molecules than the matrix (m < 10) and is themore » only solute radical in the case of m {le} 8. Penultimate type, CH{sub 3}C{sup {center dot}}HCH{sub 2}- (II), and interior type, -CH{sub 2}C{sup {center dot}}HCH{sub 2}- (III), of radicals are formed selectively in the case of m {ge} 10. These efficient and selective formations were observed neither in the mixed crystals irradiated at 4 K nor in the protiated-alkane matrix (C{sub 10}H{sub 22}) irradiated at 77 or 4 K. These results suggest that the D-atom reaction in the molecular layer boundary regions plays an important role in the solute radical formation. The details of this process are as follows. Deuterium atoms radiolytically produced from the matrix molecules can diffuse at 77 K, and some of them escape to the boundary regions. The escaped D atoms can migrate for some distance in the boundary regions (with looser molecular packings than those in the bulk regions) and efficiently abstract a H atom from a protiated solute molecule via a tunneling reaction with a large isotope effect. In the case of m {le} 8, the escaped D atoms can react with only the terminal CH{sub 3} groups of the solute molecule in the boundary regions, forming selectively radical I.« less
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