Highly correlated single- and multi-reference wavefunction methods are used for calculations of excitation energies upon aurosubstitution in acetylene and ethylene. Lowering of excitation energies of diauroacetylene and tetraauroethylene compared with their parent acetylene and ethylene molecules is demonstrated.

Abstract

We present CASPT2 calculations of vertical excitation energies for low-lying singlet and triplet states of auroderivatives of acetylene and ethylene representing small model aurocarbons. Data are supplemented by CCSD(T) results for triplet states. All four considered species, namely linear C₂Au₂ molecule and three conformers of the C₂Au₄ molecule—Au₂C₂Au₂ (tetraauroethylene, the analog of the parent ethylene molecule) and σ$$ \sigma $$– and π$$ \pi $$–adducts of the Au₂ molecule with the auroacetylene exhibit considerable lowering of low-lying excitation energies when compared with their parent acetylene and ethylene molecules. Singlet and triplet excitation energies of diauroacetylene drop by 57% and 48%, respectively, and of tetraauroethylene, by 68% and 56% when compared with their respective parent molecules. Even more is lowered the singlet–triplet energy gap. We stress the importance of the dynamical correlation in CASPT2 calculations and discuss problems with selection of the appropriate active space in aurocarbons.