Conventional quantum-chemical ab initio (MP2, CISD, CCSD, CCSD(T), BD, BD(T), QCIDS, QCISD(T), CASSCF, SAC-CI) and time-dependent density functional theory (TD-DFT) calculations (B3LYP, CAM-B3LYP) employing various basis sets were used to study the spatial as well as the electronic structure of heteroaromatics (C5H5X) with X = B, Al, Ga, In, and Tl. Core orbitals were included in the calculations on bora-, ala- and gallabenzene, however, Wood-Boring quasi-relativistic effective core potentials were used for the inda- and for the thalla compound [1, 2, 3].
Except for borabenzene where we found a significant singlet-triplet splitting (ΔE(T1-S0)) of 28 – 46 kcal/mol in favor of the lowest singlet state, these splittings are much smaller for X = Al, Ga, In, and Tl. While the calculated total energies obtained at the highest levels of theory applied in our study are between 0.5 and 5.8 kcal/mol lower for the closed shell singlet than for the lowest triplet states of ala- and gallabenzene, it appears that inda- and thallabenzene have triplet ground states with energies of 0.9 to 2.1 kcal/mol below the lowest singlet state.
Calculations were also performed for the η5 structural isomers of the title compounds (CpX, Cp = cyclopentadienyl). While the six-membered ring was found to be energetically about 30 kcal/mol below its η5 structural isomer for X = B the order of stability is reversed for the other compounds, where CpAl and CpGa are about 70 kcal/mol and CpIn and CpTl up to 102 kcal/mol more stable than the corresponding hetero aromatics.
 Gerhard Raabe, Matthias Baldowski, Quantum-Chemical Ab Initio Calculations on Borabenzene (C5H5B) and its Adducts with Ne, Ar, Kr, and N2. Could Free Borabenzene be Observed in Rare Gas Matrices? Aust. J. Chem. 2011, 64, 957-964.
 Stefanie Mersmann, Halima Mouhib, Matthias Baldowski, Gerhard Raabe, Quantum-Chemical Ab Initio Calculations on Ala (C5H5Al) and Galabenzene (C5H5Ga). Z. Naturforsch. A 2014, 69, 349-359.
 Emma Persoon, Yuekui Wang, Gerhard Raabe, Quantum-Chemical Ab Initio Calculations on Inda- and Thallabenzene (C5H5In and C5H5Tl) and their Structural Isomers η5-C5H5In and η5-C5H5Tl. Aust. J. Chem. 2018, 71, 102-110.