Structure of triplet propynylidene (HCCCH) as probed by IR, UV/vis, and EPR spectroscopy of isotopomers.

Spectroscopic data for triplet isotopomers H-C-C-C-H, H-(13)C-C-C-H, and H-C-(13)C-C-H are consistent with computational predictions for a symmetric structure in which the terminal carbons are equivalent (C(2) or C(2v)) and are inconsistent with a planar (C(s)) structure in which they are not. Experimentally observed (13)C isotope shifts in the IR spectra and (13)C hyperfine coupling constants in the EPR spectra exhibit good agreement with values predicted by theory for a C(2) structure. The (13)C 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.