Electron Paramagnetic Resonance of Triplet Alternant Methylenes. Propargylene and Homologs

The electron paramagnetic resonance of a number of alternant methylene compounds has been observed, and zero‐field splitting parameters have been obtained. The results are $$\begin{array}{c}\text{D=0.6276±0.0002\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\hspace{0.17em}\mathrm{and}\text{\hspace{0.17em}E=0.0000±0.0008\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\hspace{0.17em}\mathrm{for}\hspace{0.17em}\mathrm{H}{–}_{\uparrow }{\mathrm{C}}^{\uparrow }–\mathrm{C}\equiv \mathrm{C}–\mathrm{H},\\ \text{D=0.6263±0.0002\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\hspace{0.17em}\mathrm{and}\text{\hspace{0.17em}E=0.0000±0.0008\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\hspace{0.17em}\mathrm{for}\hspace{0.17em}\mathrm{H}{–}_{\uparrow }{\mathrm{C}}^{\uparrow }–\mathrm{C}\equiv \mathrm{C}–{\mathrm{CH}}_3,\\ \text{D=0.5413±0.0004\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\hspace{0.17em}\mathrm{and}\text{\hspace{0.17em}E=0.0035±0.0003\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\hspace{0.17em}\mathrm{for}\hspace{0.17em}\mathrm{H}{–}_{\uparrow }{\mathrm{C}}^{\uparrow }–\mathrm{C}\equiv \mathrm{C}–{\mathrm{C}}_6{\mathrm{H}}_5,\\ \text{D=0.6087±0.0001\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\hspace{0.17em}\mathrm{and}\text{\hspace{0.17em}E=0.0000±0.0004\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\hspace{0.17em}\mathrm{for}\hspace{0.17em}\mathrm{H}{–}_{\uparrow }{\mathrm{C}}^{\uparrow }–\mathrm{C}\equiv \mathrm{C}–\mathrm{C}\equiv \mathrm{C}–{\mathrm{CH}}_3,\\ \text{D=0.6055±0.0001\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\hspace{0.17em}\mathrm{and}\text{\hspace{0.17em}E=0.0000±0.0004\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\hspace{0.17em}\mathrm{for}\hspace{0.17em}\mathrm{H}{–}_{\uparrow }{\mathrm{C}}^{\uparrow }–\mathrm{C}\equiv \mathrm{C}–\mathrm{C}\equiv \mathrm{C}–\mathrm{C}({\mathrm{CH}}_3{)}_3,\\ \text{D=0.5530±0.0002\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\hspace{0.17em}\mathrm{and}\text{\hspace{0.17em}E=0.0000±0.0006\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\hspace{0.17em}\mathrm{for}\hspace{0.17em}\mathrm{H}{–}_{\uparrow }{\mathrm{C}}^{\uparrow }–\mathrm{C}\equiv \mathrm{C}–\mathrm{C}\equiv \mathrm{C}–{\mathrm{C}}_6{\mathrm{H}}_5,\end{array}$$ and $$\text{D=0.8629±0.0003\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\hspace{0.17em}\mathrm{and}\text{\hspace{0.17em}E=0.0000±0.0010\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\hspace{0.17em}\mathrm{for}\hspace{0.17em}\mathrm{H}{–}_{\uparrow }{\mathrm{C}}^{\uparrow }–\mathrm{C}\equiv \mathrm{N}.$$ The interpretation of the data is based upon the observed spin‐density distribution in the alternant free radicals having structures similar to the above triplet molecules. Thus, propargylene, H–_↑C^↑–C≡C–H, is discussed in terms of a superposition of two π‐electron systems having the same spin‐density distribution as in the allyl radical. Both positive and negative triplet spin densities are required to account for the observed magnitude of D. The lower values of D obtained for the...