### Quantum-Chemical ab initio Calculations on Heteroaromatics (C_{5}H_{5}X, X=B, Al, Ga, In, Tl)

### Stefanie Mersmann^{A}, Halima Mouhib^{A}, Emma Persoon
^{A}, Matthias Baldofski^{A}, Yuekui Wang^{B}, Gerhard Raabe^{A}

^{A}Institut für Organische Chemie, RWTH Aachen University, Landoltweg 1, D-52074 Aachen, Germany

^{B}Key Laboratory of Chemical Biology and Molecular Engineering of the Education Ministry, Institute of Molecular Science, Shanxi University, Taiyuan, Shanxi 030006, P.R.China

**Abstract:**

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 (C_{5}H_{5}X) 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(T_{1}-S_{0})) 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 η^{5} 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 η^{5} 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.

[1] Gerhard Raabe, Matthias Baldowski, Quantum-Chemical Ab Initio Calculations on Borabenzene (C_{5}H_{5}B) 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.

[2] Stefanie Mersmann, Halima Mouhib, Matthias Baldowski, Gerhard Raabe, Quantum-Chemical Ab Initio Calculations on Ala (C_{5}H_{5}Al) and Galabenzene (C_{5}H_{5}Ga). Z. Naturforsch. A 2014, 69, 349-359.

[3] Emma Persoon, Yuekui Wang, Gerhard Raabe, Quantum-Chemical Ab Initio Calculations on Inda- and Thallabenzene (C_{5}H_{5}In and C_{5}H_{5}Tl) and their Structural Isomers η^{5}-C_{5}H_{5}In and η^{5}-C_{5}H_{5}Tl. Aust. J. Chem. 2018, 71, 102-110.