Theoretical Study of the Thermal Dissociation Mechanism of AH4 (A = Si, Ge, Sn, Pb)

Stereoinversion of tetrahedral p-block element hydrides

The potential energy surfaces of 15 tetrahedral p-block element hydrides were screened on the multireference level. It was addressed whether stereoinversion competes against other reactions, such as reductive H₂-elimination or hydride loss, and if so, along which pathway the stereomutation occurs. Importantly, stereoinversion transition structures for the ammonium cation (C_4v) and the tetrahydridoborate anion (C_s) were identified for the first time. Revisiting methane's C_s symmetric inversion transition structure with the mHEAT+ protocol revealed an activation enthalpy for stereoinversion, in contrast to all earlier studies, which is 5 kJ mol^-1 below the C-H bond dissociation enthalpy. Square planar structures were identified lowest in energy only for the inversion of AlH₄ ^-, but a novel stepwise C_s-inversion was discovered for SiH₄ or PH₄ ⁺. Overall, the present contribution delineates essentials of the potential energy surfaces of p-block element hydrides, while structure-energy relations offer design principles for the synthetically emerging field of structurally constrained compounds.

Predicting viable isomers of [X,C,N] and [H,X,C,N] (X = Sn, Pb)

Metal cyanide/isocyanide and hydrometal cyanide/isocyanide compounds are key metal-carriers in interstellar space. Lighter group 14 elements (X = C/Si/Ge) cyanides/isocyanides and hydrocyanides/hydroisocyanides have been studied theoretically and experimentally. However, no reports are available on the analogues of tin (Sn) and lead (Pb). In this work, we carried out the first theoretical study on the structures and stabilities of [X,C,N] and [H,X,C,N] (X = Sn/Pb) at the CCSD(T)/def2-QZVPP//B3LYP/def2-QZVPP level. Comparisons were made with the lower analogues (X = C/Si/Ge) concerning the structural, energetic and bonding properties. Significantly different from that of c-C2N, a dative-bonded valence structure of c-XCN for heavier X was revealed for the first time, which can account for the rather worse kinetic stability of cyclic [X,C,N] for heavier X = Si/Ge. A unique kind of agostic bonding was found within three isomers of [H,Pb,C,N], whereas it is absent for X = C/Si/Ge/Sn. The computed structural and spectroscopic data could aid future laboratory and astrophysical detection of the [X,C,N] and [H,X,C,N] (X = Sn/Pb) isomers.