In search of triplet ground state GeCNX germylenes (X = H, F, Cl, and Br): An ab initio and DFT study

Singlet–triplet energy splittings for 24 GeCNX germylenic reactive intermediates are compared and contrasted at 10 levels of theory: B1LYP/6-311++G**, B3LYP/6-311++G**, MP3/6-311+G*, MP3/LANL2DZ, MP2/6-311++G**, MP4(SDTQ)/6-311++G**, QCISD(T)/6-311++G**, CCSD(T)/6-311++G**, G1 and G2 (where X = H, F, Cl, and Br). Singlet (s) and/or triplet (t) GeCNX, are confined to the following three structures: 3-X-2-aza-1-germylacyclopropenylidene (1s-X, 1t-X), [(X-imino)methylene]germylene (2s-X, 2t-X), or X-cyanogermylene (3s-X, 3t-X). For every of these three structures the order of singlet–triplet energy separations (ΔEs–t,X), appears as a function of electro-negativity (F > Cl > Br > H). Fascinatingly, two triplet structures 2t-H and 3t-H appear more stable than their corresponding singlet states. Good linear correlations are found between the LUMO–HOMO energy gaps of the singlet germylenes, and their corresponding singlet–triplet energy separations, calculated at B3LYP/6-311++G**. Relative energies of the divalent 3s-X and/or 3t-X structures are plotted as a function of the divalent bond angle ∠X-Ge-CN, where the singlet and triplet germylenes GeCNH cross at 134°. Little germanium d orbital valence participation is found in the cyclic triplet structures 1t-X. The global minimum, among six GeCNH species, appears to be the singlet cyclic 1s-H. However, the global minima among the other eighteen GeCNX (X≠H) species appear to be the singlet acyclic 3s-X. Among all the calculation methods employed, B3LYP appears as the method of choice.