Helium Chemistry: Theoretical Predictions and Experimental Challenge

Quantum mechanical investigations at the MP4(SDTQ)/6-31 lG(2df,2pd)//MP2/6-31G(d,p) + ZPE level of theory show that helium is capable of forming strong bonds with carbon in cations and that even a neutral molecule containing He (HeBeO) can be thermodynamically stable in its ground state. The electronic state of a binding partner is crucially important for the bond strength and bond length of the He bond. He₂C²⁺has a rather long (1.605 A) He-C atomic distance in its 1A1 ground state, but a much shorter bond (1.170 A) is found in the 3B1 excited state. The shortest He-C bonds (1.080-1.085 A) are found in the 2⁺(47 t) <states of HeCC²⁺, HeCCHe²⁺, and HeCC⁺. The bond dissociation energies of the dications in these electronic states yielding neutral He and a cationic fragment are predicted to be as high as 89.9 kcal/mol for HeCC²⁺. Helium compounds are best understood as donor-acceptor molecules consisting of He as electron donor and the respective acceptor fragment. Strong helium bonds are formed when a binding partner (acceptor) provides low-lying empty a orbitals (a-holes). Electronegative elements such as fluorine or oxygen are not suitable for binding He due to their highly filled valence shells. More promising candidates should provide empty orbitals which are still capable of attracting the low-lying Is electrons of the poor electron donor He. The stability of HeBeO is confirmed by CASSCF calculations with a 6-31G(d,p) basis set and an active space of all 14 electrons in 11 orbitals. The structures and energies of the helium compounds are rationalized by molecular orbital arguments and by analysis of the electron density and its associated Laplace field. The strongly bound helium ions are characterized by covalent semipolar He-C bonds, whereas the weaker bonds in some structures are caused by electrostatic interactions between closed-shell systems. The impact of our study on experiment, especially interstellar chemistry, is discussed.