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Photogeneration of Distant Radical Pairs in Aqueous Pyruvic Acid Glasses

Guzmán, Marcelo I. and Colussi, A. J. and Hoffmann, Michael R. (2006) Photogeneration of Distant Radical Pairs in Aqueous Pyruvic Acid Glasses. Journal of Physical Chemistry A, 110 (3). pp. 931-935. ISSN 1089-5639. https://resolver.caltech.edu/CaltechAUTHORS:20150623-154754849

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Abstract

The λ > 300 nm photolysis of h_4- or d_4-pyruvic acid aqueous glasses at 77 K yields identical electron magnetic resonance (EMR) spectra arising from distant (r ≳ 0.5 nm) triplet radical pairs. Spectra comprise:  (1) well-resolved quartets, X, at g ∼ g_e, that closely match the powder spectra of spin pairs interacting across r ∼ 1.0 nm with D ∼ 3.0 mT, E ∼ 0 mT zero field splittings (ZFS), and (2) broad signals, Y, centered at g ∼ 2.07 that display marked g-anisotropy and g-strain, exclude D ≳ 20.0 mT values (i.e., r ≲ 0.5 spin nm separations), and track the temperature dependence of related g ∼ 4 features. These results imply that the n−π* excitation of pyruvic acid, PA, induces long-range electron transfer from the promoted carbonyl chromophore into neighboring carbonyl acceptors, rather than homolysis into contact radical pairs or concerted decarboxylation into a carbene. Since PA is associated into hydrogen-bonded dimers prior to vitrification, X signals arise from radical pairs ensuing intradimer electron transfer to a locked acceptor, while Y signals involve carbonyl groups attached to randomly arranged, disjoint monomers. The ultrafast decarboxylation of primary radical ion pairs, ^3[PA^(+•) PA^(-•)], accounts for the release of CO_2 under cryogenic conditions, the lack of thermal hysteresis displayed by magnetic signals between 10 and 160 K, and averted charge retrotransfer. All EMR signals disappear irreversibly above the onset of ice diffusivity at ∼190 K.


Item Type:Article
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URLURL TypeDescription
http://dx.doi.org/10.1021/jp053449tDOIArticle
http://pubs.acs.org/doi/suppl/10.1021/jp053449tPublisherSupporting Information
ORCID:
AuthorORCID
Guzmán, Marcelo I.0000-0002-6730-7766
Colussi, A. J.0000-0002-3400-4101
Hoffmann, Michael R.0000-0001-6495-1946
Additional Information:© 2006 American Chemical Society. Received: June 24, 2005; In Final Form: November 17, 2005. This work was financed by NSF grant ATM-0228140. A. J. Di Bilio provided valuable assistance with EMR experiments.
Funders:
Funding AgencyGrant Number
NSFATM-0228140
Issue or Number:3
Record Number:CaltechAUTHORS:20150623-154754849
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20150623-154754849
Official Citation:Photogeneration of Distant Radical Pairs in Aqueous Pyruvic Acid Glasses Marcelo I. Guzmán, A. J. Colussi, and Michael R. Hoffmann The Journal of Physical Chemistry A 2006 110 (3), 931-935 DOI: 10.1021/jp053449t
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:58473
Collection:CaltechAUTHORS
Deposited By: George Porter
Deposited On:28 Jul 2015 21:45
Last Modified:15 Jun 2020 19:19

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