Vibronic spectrum and structure of the trans-bent acetylene radical anion

We report the first observation of the vibronic absorption spectrum of the trans-bent acetylene radical anion, produced in 3-methylpentane matrices by γ irradiation at 77 K. Two optical absorption bands are observed at 300–360 nm and in the region λ<420 nm. The former (strong) and the latter (weak) bands correspond respectively to electronic transitions to the Ψ2(Bu) and Ψ1(Au) excited states, specifically to the vertical transitions ‖pπy*(C≡C)〉←‖pπy(C≡C)〉 and ‖pπy*(C≡C)〉←‖πz(C≡C)〉 from the ground state ΨG(Ag) having an in-plane pseudo π* type singly occupied molecular orbital: ‖pπy*(C≡C)〉. For the 12C2H2− anion, these bands are accompanied by a single-vibrational progression of 1300–1240 (±20) and ∼1150 (±60) cm−1, respectively. These progressions are assigned to the (v←0) transitions of C≡C stretching modes and to H–C≡C–H bending vibrational modes by comparison with the results for 13C2H2− and 12C2D2− anions. The appearance of only a single-vibrational mode is reasonably well understood, from a molecular orbital calculation, as a consequence of the above electronic transitions. The frequency of the Ψ2(Bu) excited anion is much smaller than the frequency of the acetylene molecule νC≡C=1974 cm−1 and that of the ground state of the cis-bent anion moiety in the reported [Li+...C2H−2] complex 1655 cm−1. Our results indicate that the C≡C bond is greatly weakened by an addition of an excess electron to the antibonding orbital and by electronic excitation. The vibronic structure of the anion is discussed in conjunction with the results of an electron spin resonance study.