Stability and structure of the C₂H₆⁺ ion: investigation of the photoelectron spectrum of ethane below 14 eV using ab initio methods

Abstract

Potential surfaces have been calculated for ethane and the three lowest-lying states of its positive ion using ab initio SCF and CI methods. The C₂H₆ ⁺ states treated consist of two ² A_g and a single ² B_g species in the lowest nuclear symmetry considered (C _2h ) and they are found to possess markedly different equilibrium structures both compared to ethane itself and also to one another. The C₂H₆ ⁺ ground state (² A_g ) is a pure ² A _1g species (D _3d symmetry) at large CC distances with optimum CCH angles of 100°, but at shorter R _CC values it becomes a mixture of both ² A _1g and ² E_g character through a large diborane-type (v ₁₂) distortion. The second ² A_g species is complementary to the first and appears to prefer D _3d symmetry both at large CC values, for which it is a ² E_g species, and also at short distances, for which it eventually becomes a ² A _1g state. The ² B_g species correlates with ² E_g in D _3d symmetry but is Jahn-Teller distorted, possessing an R _CC value of 2·75 bohr and an HCH angle of only 90° (ethane values are 2·91 bohr and 109·47°). A Franck-Condon analysis of the calculated potential curves indicates that the spectrum for ionization to the lowest ² A_g species is too diffuse and of wrong characteristic frequency to be attributed to the fairly regular fine structure observed in the 11·5–12·6 eV region of the ethane PES (with average vibrational spacting of 1170 cm^-1). By contrast the analogous results for ² B_g ionization do compare quite well with the latter experimental system, with both the v ₃ (CC stretch) and v ₁₁ (CH₃ deformation) modes showing progressions in the desired frequency range. Finally it is argued that a breakdown of the Born-Oppenheimer Approximation for the ² A_g states in the region of vertical ionization coupled with the fact that one of the species dissociates in the same energy region is responsible for the much more irregular and broad appearance of the PES observed in the 12·5–14·0 eV region.