No abstract

No abstract

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 D=0.6276±0.0002 cm−1 and E=0.0000±0.0008 cm−1 for H–↑C↑–C≡C–H,D=0.6263±0.0002 cm−1 and E=0.0000±0.0008 cm−1 for H–↑C↑–C≡C–CH3,D=0.5413±0.0004 cm−1 and E=0.0035±0.0003 cm−1 for H–↑C↑–C≡C–C6H5,D=0.6087±0.0001 cm−1 and E=0.0000±0.0004 cm−1 for H–↑C↑–C≡C–C≡C–CH3,D=0.6055±0.0001 cm−1 and E=0.0000±0.0004 cm−1 for H–↑C↑–C≡C–C≡C–C(CH3)3,D=0.5530±0.0002 cm−1 and E=0.0000±0.0006 cm−1 for H–↑C↑–C≡C–C≡C–C6H5,and D=0.8629±0.0003 cm−1 and E=0.0000±0.0010 cm−1 for H–↑C↑–C≡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 phenyl-substituted alternant methylenes reflect the delocalization of one of the two triplet electrons into a π orbital of the phenyl group. This effect is more important in the case of phenylpropargylene where a nonzero value of E was obtained. The linear geometry of the alternant methylene chain is responsible for the cases where zero values of E were obtained. The trends in D and trends in linewidths are given qualitative explanations. The discussion assumes that all of the molecules studied have a spectroscopic splitting factor equal to the free-electron value.

No abstract

No abstract