Quantum-Chemical ab initio Calculations on Ala- (C5H5Al) and Gallabenzene (C5H5Ga)


Stefanie Mersmann, Halima Mouhib, Matthias Baldofski, and Gerhard Raabe*


Institut für Organische Chemie, RWTH Aachen University, Landoltweg 1, D-52074 Aachen, Germany



Abstract:

Conventional quantum-chemical ab initio and time-dependent density functional theory (TD-DFT) calculations employing various basis sets were used to study the spatial as well as the electronic structure of ala- (C5H5Al) and gallabenzene (C5H5Ga), which might be of interest for example as ligands in organometallic chemistry. The energetically lowest closed-shell singlet states of both compounds were found to have a non-planar structure of Cs symmetry with C-X-C bond angles of about 116° (MP2/6-311++G**)and 125° (CCSD/aug-cc-pVDZ). At approximately 103°, the corresponding angles of the energetically lowest triplets are significantly smaller. The lowest triplet state of alabenzene is also non-planar (Cs) at the MP2 level while optimization with the CCSD and the CASPT2 methods resulted in planar structures with C2v symmetry. The corresponding state of gallabenzene has also C2v symmetry at all levels of optimization. The relative stabilities of the energetically lowest closed-shell singlets and the lowest triplet (ΔE(T1-S0)) states are small and their signs even strongly depend on the applied method. However, according to the highest levels of theory applied in this study the singlet states of the title compounds are slightly lower in energy than the corresponding triplets with singlet/triplet gaps between about 0.5 and 5.8 kcal/mol in favour of the closed-shell singlet states. The majority of the applied methods gave a slightly smaller splitting for the ala- than for the gallacompound. Independent of the applied method (TD-DFT/B3LYP/6-311++G(3df,3pd)//MP2/6-311++G** or SAC-CI/6-31++G(3df,3pd)//MP2/6-311++G**) the general shapes of the calculated UV/VIS spectral curves are quite similar for the energetically lowest singlet states of ala- and gallabenzene, and the same is true for the calculated spectra of the normal modes. The calculated UV/VIS spectra of C5H5Al and C5H5Ga are featured by long wavelength bands of moderate intensity around 900 nm at the TD-DFT level and between 1300 and 1500 nm with the SAC-CI method. According to the results obtained with both methods these bands are predominantly due to HOMO(π)→LUMO(σ*) transitions. The results of isodesmic bond separation reactions for the singlet states indicate some degree of stabilization due to delocalization in both of the title compounds. With our best values between 29 and 32 kcal/mol this stabilization appears to be only slightly less than the previously reported value for borabenzene (∼38 kcal/mol, C5H5B).