The electronic states of but-2-yne studied by VUV absorption, near-threshold electron energy-loss spectroscopy and ab initio configuration interaction methods

The photoabsorption spectrum of but-2-yne in the range 5.5-11 eV (225-110 nm) has been recorded using a synchrotron radiation source. The spectrum is dominated by three d-type Rydberg series, converging to the first ionisation energy (IE) (p^-1, 9.562 eV). Origins of the p3d members are 7.841, 7.977 and 8.018 eV, respectively. Transitions of low intensity, arising from excitation of the p3s state (origin, ~6.35 eV) and two p3p Rydberg states (7.38 and 7.51 eV, respectively) have also been identified in the spectrum. Near-threshold electron energy-loss spectra reveal valence excited triplet states at about 5.2 and 5.8 eV, respectively. Electronic excitation energies for valence and Rydberg-type states have been computed using ab initio multi-reference multi-root CI methods. These studies used a triple zeta + polarisation basis set, augmented by diffuse (Rydberg) orbitals, to generate the theoretical singlet and triplet energy manifolds. The correlation of theory and experiment shows the nature of the more intense Rydberg state types, and identification of the main valence and Rydberg bands. Calculated energies for Rydberg states are close to those expected, and there is generally a good correlation between the theoretical and experimental envelopes. It was possible to generate singlet Rydberg states which relate to the 5-lowest IEs of but-2-yne; furthermore, the separation of these sequences shows that the IE order (under D_3h symmetry) is: 2 e^' < 5 a₁^' < 1 e^? < 1 e^' < 4 a₂^?, also supported by direct calculation of the IEs by CI. The lowest valence singlet states are pp^*, optically forbidden, and calculated to lie near 7.3 and 7.6 eV. The states which contribute strongly to the observed spectrum are ps^*(¹ E^' + ¹ A₂^?) near 7.9 eV having 2 e^' 6 a₁^{' *} excitation, followed by several pp^* and ps^* states (¹ E^' + ¹ A₂^?) between 10.0 and 10.5 eV; an ¹E' antisymmetric combination(2e'2e? - 2e'2e?) is by far the strongest in intensity. A further group of symmetry-allowed valence states are calculated to lie near 12.3 and 12.9 eV. The two lowest triplet states, both of E' symmetry (pp^*), have vertical excitation energies of 5.7 and 6.2 eV, but are strongly bent with a trans-CCCC unit (C_S and C_2h). The theoretical work confirms that, on intensity grounds, valence excited states do not contribute significantly to the spectrum. CI calculations of the ionic states give the ionisation energy sequence (D_{3 h}): ² E^' < ² A₁^' < ² E^? < ² E^' < ² A₂^?. Adiabatic structures for the first cation, two triplets, and a singlet (C_2h) were obtained; these show shortening of C-C, and lengthening of C{triple bond, long}C, in a trans-CCCC, as is found with ethyne. © 2007 Elsevier B.V. All rights reserved.