Electron resonance spectra of some radicals derived from acetylenes and nitriles

Abstract

The electron resonance spectra of a series of radicals derived by hydrogen abstraction from substrates containing carbon-carbon or carbon-nitrogen triple bonds have been studied in fluid solutions. The radicals "CH~CN, 9 CH2C -CH and "CH2C -C" CHa which have a linear heavy-atom framework show interesting temperature dependent line-width variations. These are attributed to spin-rotational interactions arising from anisotropic molecular reorientation. It is suggested that the motion about the long axis of these radicals is inertial in character.The equilibrium geometries and hyperfine coupling constants of the propargyl and cyanomethyl radicals have been estimated from INDO-LCAO-SCF calculations.

Crystalline Monomeric Allenyl/Propargyl Radical

“…Even more sterically protected propargyl radicals such as IV are known to spontaneously give rise to the propargyl/propargyl and propargyl/allenyl dimers Va and Vb , respectively . However, this class of radicals has been detected by EPR, but only in argon matrices below 100 K . Yet again, even the introduction of an amino group does not allow for the isolation of radicals of type VI .…”

Spin Delocalization by Triple-Bonded Functionalities in Propargyl and Heteropropargyl Radicals, Assessed from the EPR-Spectral D Parameter of 1,3-Cyclopentanediyl Triplet Diradicals

“…In the allyl radical, the odd electron is equally distributed between the equivalent methylene terminals through conjugation. In contrast, in the propargyl-allenyl mesomerism (Figure ), the unpaired electron prefers to populate the propargyl rather than the allenic structure, because in the latter the spin would have to reside at the less favored vinyl-type radical site . This is also manifested by the hyperfine coupling constants ( a H ) in Figure , which are 14.2 G for the allyl 22c and 18.9 G for the propargyl 4 radicals.…”

“…For the cyanomethyl radical, the observed coupling constant was measured to be a H = 20.8 G (the INDO calculated value is a H = 20.0 G), which clearly indicates a higher spin localization at the methylene carbon atom than in the propargyl case. This is expected since the electronegative nitrogen atom will resist spin delocalization to populate the heteroallenic structure …”

Homolytic reactions of ligated boranes. Part 7. An electron spin resonance study of the t-butyl isocyanide–boryl radical

“…The pattern of stronger lines can be analysed as a "€3 quartet of 1 : 3 :4: 3: 1 quintets, arising from accidentally equal coupling to one 14N nucleus and to two equivalent protons, and we attribute this spectrum to the isocyanide-boryl radical (1) [equations (3) and ( 4 the basis of the small temperature dependence of r, we can exclude thermally induced p-scission of (1) [equation (6)] as the 0 B U ' N C ~H , -BU'. + NCBH, (6) major source of But', provided that the A factor for this fragmentation is 'normal' (i.e. close to s-1).18,19 Support for this conclusion was obtained by photolysis of bis(cyclopropy1formyl) peroxide (0.25~) and TBIB ( 0 .…”