Resonance Raman spectra of metallooctaethylporphyrin cation radicals with a1u and a2u orbital character

Dongho Kim, Lisa A. Miller, Gopa Rakhit, Thomas G. Spiro

Research output: Contribution to journalArticle

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Abstract

Resonance Raman (RR) spectra are reported for radical cations of MIIOEP (OEP = octaethylporphyrin; M = Zn, Mg, Ni, Cu) and for the perchlorate and bromide salts of [CoIIIOEP]+, produced by electrochemical and/or chemical oxidation. The enhancement patterns obtained with B (4067 Å) and Q (5682 Å) band excitation (except M = Zn and Mg, for which fluorescence overwhelms the Q band RR spectra) permitted assignment of most of the porphyrin skeletal modes above 1000 cm-1, and frequency shifts relative to the unoxidized porphyrins have been cataloged. Those radicals, magnesium(II), zinc(II), and cobalt(III) bromides, whose EPR spectra have been assigned on the basis of electron removal from an a1u molecular oribtal show a common pattern of shifts, while those with a2u-type EPR spectra, nickel(II) and cobalt(III) perchlorates, show a different pattern. The CuII radical, whose visible absorption spectrum resembles that of other a2u radicals, likewise has an a2u RR frequency shift pattern. The largest shifts are shown by the most prominent RR bands in the B-band-excited spectra, v4 (CaN breathing, primarily) and v2 (CbCb breathing, primarily). These shift up (+14 to +21 cm-1) and down (-19 to -38 cm-1), respectively, for the a1u radicals, but down (-12 to -22 cm-1) and up (+20 to +23 cm-1) for the a2u radicals. Thus an identification of the radical type is quite straightforward in B-band-excited RR spectra. The directions of the shifts, however, are opposite to those expected from simple orbital symmetry arguments, since the a2u molecular orbital is bonding with respect to the Cb-Cb bonds and antibonding with respect to the Ca-N bonds, and vice versa for the a1u orbital. This discrepancy invites further examination via vibrational and electronic calculations.

Original languageEnglish
Pages (from-to)3320-3325
Number of pages6
JournalJournal of Physical Chemistry
Volume90
Issue number15
Publication statusPublished - 1986 Dec 1

Fingerprint

Cations
Raman scattering
Positive ions
Raman spectra
cations
orbitals
Porphyrins
Cobalt
Bromides
Paramagnetic resonance
shift
Perchlorates
breathing
perchlorates
porphyrins
frequency shift
bromides
cobalt
Molecular orbitals
Nickel

All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry

Cite this

Kim, Dongho ; Miller, Lisa A. ; Rakhit, Gopa ; Spiro, Thomas G. / Resonance Raman spectra of metallooctaethylporphyrin cation radicals with a1u and a2u orbital character. In: Journal of Physical Chemistry. 1986 ; Vol. 90, No. 15. pp. 3320-3325.
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abstract = "Resonance Raman (RR) spectra are reported for radical cations of MIIOEP (OEP = octaethylporphyrin; M = Zn, Mg, Ni, Cu) and for the perchlorate and bromide salts of [CoIIIOEP]+, produced by electrochemical and/or chemical oxidation. The enhancement patterns obtained with B (4067 {\AA}) and Q (5682 {\AA}) band excitation (except M = Zn and Mg, for which fluorescence overwhelms the Q band RR spectra) permitted assignment of most of the porphyrin skeletal modes above 1000 cm-1, and frequency shifts relative to the unoxidized porphyrins have been cataloged. Those radicals, magnesium(II), zinc(II), and cobalt(III) bromides, whose EPR spectra have been assigned on the basis of electron removal from an a1u molecular oribtal show a common pattern of shifts, while those with a2u-type EPR spectra, nickel(II) and cobalt(III) perchlorates, show a different pattern. The CuII radical, whose visible absorption spectrum resembles that of other a2u radicals, likewise has an a2u RR frequency shift pattern. The largest shifts are shown by the most prominent RR bands in the B-band-excited spectra, v4 (CaN breathing, primarily) and v2 (CbCb breathing, primarily). These shift up (+14 to +21 cm-1) and down (-19 to -38 cm-1), respectively, for the a1u radicals, but down (-12 to -22 cm-1) and up (+20 to +23 cm-1) for the a2u radicals. Thus an identification of the radical type is quite straightforward in B-band-excited RR spectra. The directions of the shifts, however, are opposite to those expected from simple orbital symmetry arguments, since the a2u molecular orbital is bonding with respect to the Cb-Cb bonds and antibonding with respect to the Ca-N bonds, and vice versa for the a1u orbital. This discrepancy invites further examination via vibrational and electronic calculations.",
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Resonance Raman spectra of metallooctaethylporphyrin cation radicals with a1u and a2u orbital character. / Kim, Dongho; Miller, Lisa A.; Rakhit, Gopa; Spiro, Thomas G.

In: Journal of Physical Chemistry, Vol. 90, No. 15, 01.12.1986, p. 3320-3325.

Research output: Contribution to journalArticle

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T1 - Resonance Raman spectra of metallooctaethylporphyrin cation radicals with a1u and a2u orbital character

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AU - Miller, Lisa A.

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N2 - Resonance Raman (RR) spectra are reported for radical cations of MIIOEP (OEP = octaethylporphyrin; M = Zn, Mg, Ni, Cu) and for the perchlorate and bromide salts of [CoIIIOEP]+, produced by electrochemical and/or chemical oxidation. The enhancement patterns obtained with B (4067 Å) and Q (5682 Å) band excitation (except M = Zn and Mg, for which fluorescence overwhelms the Q band RR spectra) permitted assignment of most of the porphyrin skeletal modes above 1000 cm-1, and frequency shifts relative to the unoxidized porphyrins have been cataloged. Those radicals, magnesium(II), zinc(II), and cobalt(III) bromides, whose EPR spectra have been assigned on the basis of electron removal from an a1u molecular oribtal show a common pattern of shifts, while those with a2u-type EPR spectra, nickel(II) and cobalt(III) perchlorates, show a different pattern. The CuII radical, whose visible absorption spectrum resembles that of other a2u radicals, likewise has an a2u RR frequency shift pattern. The largest shifts are shown by the most prominent RR bands in the B-band-excited spectra, v4 (CaN breathing, primarily) and v2 (CbCb breathing, primarily). These shift up (+14 to +21 cm-1) and down (-19 to -38 cm-1), respectively, for the a1u radicals, but down (-12 to -22 cm-1) and up (+20 to +23 cm-1) for the a2u radicals. Thus an identification of the radical type is quite straightforward in B-band-excited RR spectra. The directions of the shifts, however, are opposite to those expected from simple orbital symmetry arguments, since the a2u molecular orbital is bonding with respect to the Cb-Cb bonds and antibonding with respect to the Ca-N bonds, and vice versa for the a1u orbital. This discrepancy invites further examination via vibrational and electronic calculations.

AB - Resonance Raman (RR) spectra are reported for radical cations of MIIOEP (OEP = octaethylporphyrin; M = Zn, Mg, Ni, Cu) and for the perchlorate and bromide salts of [CoIIIOEP]+, produced by electrochemical and/or chemical oxidation. The enhancement patterns obtained with B (4067 Å) and Q (5682 Å) band excitation (except M = Zn and Mg, for which fluorescence overwhelms the Q band RR spectra) permitted assignment of most of the porphyrin skeletal modes above 1000 cm-1, and frequency shifts relative to the unoxidized porphyrins have been cataloged. Those radicals, magnesium(II), zinc(II), and cobalt(III) bromides, whose EPR spectra have been assigned on the basis of electron removal from an a1u molecular oribtal show a common pattern of shifts, while those with a2u-type EPR spectra, nickel(II) and cobalt(III) perchlorates, show a different pattern. The CuII radical, whose visible absorption spectrum resembles that of other a2u radicals, likewise has an a2u RR frequency shift pattern. The largest shifts are shown by the most prominent RR bands in the B-band-excited spectra, v4 (CaN breathing, primarily) and v2 (CbCb breathing, primarily). These shift up (+14 to +21 cm-1) and down (-19 to -38 cm-1), respectively, for the a1u radicals, but down (-12 to -22 cm-1) and up (+20 to +23 cm-1) for the a2u radicals. Thus an identification of the radical type is quite straightforward in B-band-excited RR spectra. The directions of the shifts, however, are opposite to those expected from simple orbital symmetry arguments, since the a2u molecular orbital is bonding with respect to the Cb-Cb bonds and antibonding with respect to the Ca-N bonds, and vice versa for the a1u orbital. This discrepancy invites further examination via vibrational and electronic calculations.

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