Photoelectron Spectroscopy of the Monofluorovinylidene and Difluorovinylidene Anions: The Monofluorovinylidene-Fluoroacetylene Rearrangement

The 351 .l-nm photoelectron spectra of HFC=C:and F2C=C:are reported. Transitions from the anion ground state to the neutral singlet ground state and low-lying triplet states are observed. The electron affinity of difluorovinylidene is 2.255 (6) eV, and the triplet term energy is To(H 3A2 F,CC) = 0.924 (9) eV. In the singlet state of F2C=C:, vibrational progressions are observed at 510 (25), 905 (25), and 1670 ( 2 5 ) cm-I. Vibrational frequencies of 505 ( 2 5 ) and 800 (35) cm-' are observed in the difluorovinylidene anion. The adiabatic electron_ affinity of monofluorovinylidene is 1.718 (6) eV, and the triplet term energies are T0(5 3A" HFCC) = 1.320 (9) eV and-To(b 'A' HFCC) = 1.358 (9) eV. Vibrational fundamentals are ob.served at 205 (30), 960 (30), and 1680 (30) cm-' in the X 'A' state ?f HFC=C:, at 600 (50) and 1100 (100) cm-' in the b 3A' state, and at 485 (30) and 710 (35) cm-' in the negative ion (X 2A'), Ab initio calculations indicate that the normal mode primarily involved in the hydrogen migration to form fluoroacetylene from monofluorovinylidene is the CCF bending mode. By modeling the observed CCF bending mode as an anharmonic oscillator, we estimate the hydrogen isomerization barrier height to be 2 f 1 kcal/mol.