A Time Resolved Study of Photoinduced Bond Homolysis in Cobalamines and Cobaloximes

A. BUSSANDRI; C. KIARIE; D.M. MARTINO; H. VAN WILLIGEN

Boston, USA, April 27

Conferencia; Forth Annual Northeast Student Chemistry Research Conference; 2002

&amp;amp;lt;!-- /* Font Definitions */ @font-face {font-family:Garamond; panose-1:2 2 4 4 3 3 1 1 8 3; mso-font-charset:0; mso-generic-font-family:roman; mso-font-pitch:variable; mso-font-signature:647 0 0 0 159 0;} /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; text-align:justify; text-indent:36.0pt; line-height:150%; mso-pagination:widow-orphan; tab-stops:36.0pt 57.6pt 108.0pt 144.0pt 208.8pt 252.0pt; font-size:11.0pt; mso-bidi-font-size:10.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman"; mso-ansi-language:EN-US; mso-fareast-language:EN-US;} p.texto, li.texto, div.texto {mso-style-name:texto; margin:0cm; margin-bottom:.0001pt; text-align:justify; text-indent:28.8pt; line-height:17.0pt; mso-pagination:widow-orphan; font-size:12.0pt; mso-bidi-font-size:10.0pt; font-family:Garamond; mso-fareast-font-family:"Times New Roman"; mso-bidi-font-family:"Times New Roman"; mso-ansi-language:ES-TRAD; mso-fareast-language:EN-US;} @page Section1 {size:612.0pt 792.0pt; margin:70.85pt 3.0cm 70.85pt 3.0cm; mso-header-margin:36.0pt; mso-footer-margin:36.0pt; mso-paper-source:0;} div.Section1 {page:Section1;} /* List Definitions */ @list l0 {mso-list-id:2134907769; mso-list-type:simple; mso-list-template-ids:984907656;} @list l0:level1 {mso-level-text:"%1)"; mso-level-tab-stop:none; mso-level-number-position:left; mso-level-legacy:yes; mso-level-legacy-indent:-14.2pt; mso-level-legacy-space:0cm; margin-left:0cm; text-indent:0cm;} ol {margin-bottom:0cm;} ul {margin-bottom:0cm;} --&amp;amp;gt; <!-- /* Font Definitions */ @font-face {font-family:Times; panose-1:2 2 6 3 5 4 5 2 3 4; mso-font-charset:0; mso-generic-font-family:roman; mso-font-pitch:variable; mso-font-signature:536902279 -2147483648 8 0 511 0;} /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; mso-bidi-font-size:10.0pt; font-family:Times; mso-fareast-font-family:Times; mso-bidi-font-family:"Times New Roman"; mso-ansi-language:EN-US; mso-fareast-language:EN-US;} p.MsoBodyText, li.MsoBodyText, div.MsoBodyText {margin:0cm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:14.0pt; mso-bidi-font-size:10.0pt; font-family:"Times New Roman"; mso-fareast-font-family:Times; mso-ansi-language:EN-US; mso-fareast-language:EN-US;} p.MsoBodyText2, li.MsoBodyText2, div.MsoBodyText2 {margin:0cm; margin-bottom:.0001pt; text-align:justify; mso-pagination:widow-orphan; font-size:14.0pt; mso-bidi-font-size:10.0pt; font-family:"Times New Roman"; mso-fareast-font-family:Times; mso-ansi-language:EN-US; mso-fareast-language:EN-US;} @page Section1 {size:612.0pt 792.0pt; margin:70.85pt 3.0cm 70.85pt 3.0cm; mso-header-margin:36.0pt; mso-footer-margin:36.0pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> Fourier Transform Electron Paramagnetic Resonance (FT-EPR) has been used to study free radicals generated by photo-induced homolytic cleavage of the Co-C bond in methyl and adenosyl cobalamines and methyl and ethyl cobaloximes. Spectra of the methyl and adenosyl radicals generated by photo excitation of cobalamines in aqueous solutions show Chemically Induced Dynamic Electron Polarization (CIDEP) produced in precursor radical pairs. This effect can be accounted for in terms of bond cleavage via the cobalamine singlet excited state. The spectrum of CH3• produced by photo-excitation of methyl-cobalamine is wavelength dependent. This confirms earlier observations that bond cleavage can occur via two reaction channels involving distinct transient intermediates. CIDEP patterns in the FT-EPR spectra of methyl and ethyl radicals generated by the photo-excitation of cobaloximes show a striking dependence on solvent (toluene, ethanol, pyridine, water) and, in aqueous solution, on pH. The effect is attributed tentatively to shifts in energy levels of the cobalt complexes induced by changes in axial coordination. Consequently, the spin polarization pattern may be dominated by a contribution from a singlet excited state reaction channel in some solvents, and by a triplet excited state channel in others.