title: A C-C bonded phenoxyl radical dimer with a zero bond dissociation free energy.
abstract: The 2,6-di-tert-butyl-4-methoxyphenoxyl radical is shown to dimerize in solution and in the solid state. The X-ray crystal structure of the dimer, the first for a para-coupled phenoxyl radical, revealed a bond length of 1.6055(23) Å for the C4-C4a bond. This is significantly longer than typical C-C bonds. Solution equilibrium studies using both optical and IR spectroscopies showed that the Keq for dissociation is 1.3 ± 0.2 M at 20 °C, indicating a C-C bond dissociation free energy of -0.15 ± 0.1 kcal mol(-1). Van't Hoff analysis gave an exceptionally small bond dissociation enthalpy (BDE) of 6.1 ± 0.5 kcal mol(-1). To our knowledge, this is the smallest BDE measured for a C-C bond. This very weak bond shows a large deviation from the correlation of C-C bond lengths and strengths, but the computed force constant follows Badger's rule.
authors: Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States.
authors: Jessica M
authors: JM
authors: Wittman
authors: Rebecca
authors: R
authors: Hayoun
authors: Werner
authors: W
authors: Kaminsky
authors: Michael K
authors: MK
authors: Coggins
authors: James M
authors: JM
authors: Mayer
mesh_terms: D019281:Dimerization; D008958:Models, Molecular; D015394:Molecular Structure; D010636:Phenols / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D010636:Phenols; C042329:phenoxy radical
references: Angew Chem Int Ed Engl. 2000 Dec 15;39(24):4607-4609
references: 11169686
references: Org Lett. 2004 Jul 22;6(15):2579-82
references: 15255695
references: J Phys Chem A. 2005 Dec 15;109(49):11261-7
references: 16331910
references: J Am Chem Soc. 2012 Dec 19;134(50):20352-64
references: 23181620
references: Angew Chem Int Ed Engl. 2013 Jan 7;52(2):589-92
references: 23172626
references: J Phys Chem A. 2010 Jan 21;114(2):1161-8
references: 20039628
references: J Phys Chem A. 2005 Dec 15;109(49):11198-205
references: 16331903
references: J Org Chem. 2006 Jan 20;71(2):520-6
references: 16408959
references: Chem Commun (Camb). 2006 Oct 1;(37):3894-6
references: 17268662
references: J Phys Chem A. 2012 Dec 20;116(50):12249-59
references: 23176252
references: Science. 2012 Jun 8;336(6086):1298-301
references: 22679095
references: J Phys Chem B. 2012 Apr 26;116(16):4760-8
references: 22475051
references: Coord Chem Rev. 2013 Jan 15;257(2):528-540
references: 23264696
references: J Am Chem Soc. 2006 Feb 15;128(6):1902-6
references: 16464091
references: Nature. 2011 Sep 14;477(7364):308-11
references: 21921913
references: J Am Chem Soc. 2009 Mar 18;131(10):3428-9
references: 19275250
references: J Am Chem Soc. 2012 Aug 22;134(33):13641-50
references: 22835264
references: Chem Soc Rev. 2010 Jun;39(6):2106-19
references: 20514720
references: Chem Commun (Camb). 2008 Jan 14;(2):256-8
references: 18092105
references: J Org Chem. 2002 Nov 15;67(23):8002-9
references: 12423127