Spectroscopic data for triplet isotopomers H-C-C-C-H, H-C-13-C-C-H, and H-C-C-13-C-H are consistent with computational predictions for a symmetric structure in which the terminal carbons are equivalent (C or C-2v) and are inconsistent with a planar (C-s) structure in which they are not. Experimentally observed C-13 isotope shifts in the IR spectra and C-13 hyperfine coupling constants in the EPR spectra exhibit good agreement with values predicted by theory for a C-2 structure. The C-13 hyperfine coupling constants also provide an independent experimental estimate for the bond angles in the molecule. The isotope-dependence of the zero-field splitting parameters reveals the influence of molecular motion in modulating the values of these parameters. The interpretation of motional effects provides a basis for rationalizing the anomalously low E value, which had previously been interpreted in terms of an axially symmetric (D-infinity h) structure. Computational studies involving Natural Bond Orbital and Natural Resonance Theory analyses provide insight into the spin densities and the complex electronic structure of this reactive intermediate.