Photoelectron spectroscopy of the allenyl ion CH2=C=CH−

We have studied the photodetachment spectra of CH/sub 2/=C=CH/sup -/, CD/sub 2/=C=CH/sup -/, and CH/sub 2/=C=CD/sup -/ using deuterium-labelled methylacetylenes and allene and pricussors. By comparing the photoelectron spectra of these selectively labeled species, we conclude that the ion has an allenyl (rather than propargyl) structure. The electron affinities (EA) of a set of propargyl radicals are as follows: EA(CH/sub 2/C=CH) = 0.893 +- 0.025 eV, EA(CD/sub 2/C=CH) = 0.907 +- 0.023 eV, and EA(CH/sub 2/C=CD) = 0.88 +- 0.15 eV. A single active vibration is observed in the photoelectron spectra of CH/sub 2/=C=CH/sup -/ and CD/sub 2/=C=CH/sup -/. This mode has a frequency of 510 cm/sup -1/ and is assigned as an out-of-plane bend of the acetylenic hydrogen of the propargyl radical (either CH/sub 2/C=CH or CD/sub 2/C=CH). The gas-phase acidity of allene and methylacetylene are reported as ..delta..H/sup 0//sub acid/ (H-CH/sub 2/CCH) = 382.3 +- 1.2 kcal/mol and ..delta..H/sup 0//sub acid/ (H-CH=C=CH/sub 2/) = 380.7 +- 1.2 kcal/mol. The ..delta..H/sub f//sup 0//sub 298/ (CH/sub 2/=C=CH) is 59.4 +- 1.2 kcal/mol.