Negative ion photodetachment has been shown to be a novel method of probing the transition states of benchmark unimolecular and bimolecular reactions. The underlying concept is that if a stable negative ion has a geometry similar to the neutral transition state for a chemical reaction, then photodetachment of the anion will access the transition state, and the resulting photoelectron spectrum can reveal resolved vibrational structure characteristic of the transition state region. Early examples include using the photoelectron spectroscopy of the vinylidene anion, H2CCˉ, to probe the dynamics of neutral vinylidene isomerization to acetylene (HCCH). There have also been numerous studies of several bimolecular hydrogen transfer reactions via photodetachment of the appropriate hydrogen bonded negative ion, such as photodetaching FH2ˉ to investigate the transition state of the F + H2 reaction. The higher resolution offered initially by anion ZEKE spectroscopy and, more recently, cryo-SEVI, have revealed additional vibrational features not seen in conventional photoelectron spectra, most notably the spectroscopic observation of reactive resonances in the I + HI and F + H2 reactions via photodetachment of IHIˉ and FH2ˉ. Entrance channels for the Cl + H2 and F + CH4 reactions have also been probed. Here, recent results are described in which cryo-SEVI spectra of H2CCˉ and FNH3ˉ are used to probe the transition states for the unimolecular H2CC→HCCH isomerization reaction and the bimolecular reaction F + NH3→HF + NH2. Vinylidene, H2CC, is the simplest carbene and as such is of considerable interest in organic chemistry. As shown schematically in Figure , it lies 1.925 eV above acetylene and can isomerize to HCCH over a small (0.13 eV) barrier. The isomerization reaction is the simplest 1,2-hydrogen shift and is thus a benchmark chemical reaction. This consideration has motivated many experimental 12, 107-109 and theoretical studies of how the vibrational energy levels of H2CC and HCCH are coupled to one another. One approach to this problem has been to map out the excited vibrational energy levels of HCCH that lie above the H2CC minimum and search for perturbations with H2CC levels. A more direct approach, developed by Lineberger, 12 makes use of the fact that that H2CCˉ is the stable anionic form of H2CC, and that photodetachment of this species directly accesses the neutral H2CC well, corresponding to the 𝑋 ̃ 1 A1 state of vinylidene. The resulting photoelectron spectrum shows resolved vibrational structure. Remarkably, the peaks in the singlet band are notably broader than those seen upon photodetachment to the higher-lying 𝑎 ̃ 3 B2 triplet state of H2CC that is known to have a much larger barrier to isomerization. On this basis, the broadening of the peaks in the singlet spectrum was attributed to isomerization to acetylene on a sub-picosecond time scale. This result motivated multiple investigations of H2CCˉ in our laboratory and in Canberra to map out photodetachment to both single and triplet states of H2CC with high resolution and to explore vibrational autodetachment in H2CCˉ. Below, results are described in which cryo-SEVI combined with quantum dynamics calculations probes how (or if) H2CC vibrations mix with high-lying HCCH vibrational states.