Millimeter-wave spectroscopy of CrC (X 3Σ-) and CrCCH (X ⊂ 6 Σ +): Examining the chromium-carbon bond

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Abstract

Pure rotational spectroscopy of the CrC (X3Σ-) and CrCCH (X⊂ 6Σ+) radicals has been conducted using millimeter/sub-millimeter direct absorption methods in the frequency range 225-585 GHz. These species were created in an AC discharge of Cr(CO)6 and either methane or acetylene, diluted in argon. Spectra of the CrCCD were also recorded for the first time using deuterated acetylene as the carbon precursor. Seven rotational transitions of CrC were measured, each consisting of three widely spaced, fine structure components, arising from spin-spin and spin-rotation interactions. Eleven rotational transitions were recorded for CrCCH and five for CrCCD; each transition in these cases was composed of a distinct fine structure sextet. These measurements confirm the respective 3Σ- and 6Σ+ ground electronic states of these radicals, as indicated from optical studies. The data were analyzed using a Hund's case (b) Hamiltonian, and rotational, spin-spin, and spin-rotation constants have been accurately determined for all three species. The spectroscopic parameters for CrC were significantly revised from previous optical work, while those for CrCCH are in excellent agreement; completely new constants were established for CrCCD. The chromium-carbon bond length for CrC was calculated to be 1.631 Å, while that in CrCCH was found to be rCr - C = 1.993 Å - significantly longer. This result suggests that a single Cr - C bond is present in CrCCH, preserving the acetylenic structure of the ligand, while a triple bond exists in CrC. Analysis of the spin constants suggests that CrC has a nearby excited 1Σ+ state lying ∼16 900 cm-1 higher in energy, and CrCCH has a 6Π excited state with E ∼ 4800 cm-1.

Original languageEnglish (US)
Article number184308
JournalThe Journal of Chemical Physics
Volume144
Issue number18
DOIs
StatePublished - May 14 2016

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Acetylene
Chromium
Millimeter waves
millimeter waves
chromium
Carbon
Spectroscopy
Hamiltonians
Argon
carbon
Methane
Bond length
Electronic states
Carbon Monoxide
Excited states
spectroscopy
Ligands
acetylene
fine structure
preserving

ASJC Scopus subject areas

  • Physics and Astronomy(all)
  • Physical and Theoretical Chemistry

Cite this

Millimeter-wave spectroscopy of CrC (X 3Σ-) and CrCCH (X ⊂ 6 Σ +) : Examining the chromium-carbon bond. / Min, J.; Ziurys, Lucy M.

In: The Journal of Chemical Physics, Vol. 144, No. 18, 184308, 14.05.2016.

Research output: Contribution to journalArticle

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abstract = "Pure rotational spectroscopy of the CrC (X3Σ-) and CrCCH (X⊂ 6Σ+) radicals has been conducted using millimeter/sub-millimeter direct absorption methods in the frequency range 225-585 GHz. These species were created in an AC discharge of Cr(CO)6 and either methane or acetylene, diluted in argon. Spectra of the CrCCD were also recorded for the first time using deuterated acetylene as the carbon precursor. Seven rotational transitions of CrC were measured, each consisting of three widely spaced, fine structure components, arising from spin-spin and spin-rotation interactions. Eleven rotational transitions were recorded for CrCCH and five for CrCCD; each transition in these cases was composed of a distinct fine structure sextet. These measurements confirm the respective 3Σ- and 6Σ+ ground electronic states of these radicals, as indicated from optical studies. The data were analyzed using a Hund's case (b) Hamiltonian, and rotational, spin-spin, and spin-rotation constants have been accurately determined for all three species. The spectroscopic parameters for CrC were significantly revised from previous optical work, while those for CrCCH are in excellent agreement; completely new constants were established for CrCCD. The chromium-carbon bond length for CrC was calculated to be 1.631 {\AA}, while that in CrCCH was found to be rCr - C = 1.993 {\AA} - significantly longer. This result suggests that a single Cr - C bond is present in CrCCH, preserving the acetylenic structure of the ligand, while a triple bond exists in CrC. Analysis of the spin constants suggests that CrC has a nearby excited 1Σ+ state lying ∼16 900 cm-1 higher in energy, and CrCCH has a 6Π excited state with E ∼ 4800 cm-1.",
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