HRnSiH3: A Radon Hydride with Near-Zero Dissociation Energy, Benchmarked Against HRnF

HRnSiH3 is a genuine minimum on the PBE0/ZORA potential energy surface, but at coupled-cluster level it is not meaningfully bound against loss of its atoms. We optimised the C3v structure (H–Rn 2.008 Å, Rn–Si 2.876 Å; all harmonic frequencies real) and computed dissociation energies with a CCSD(T)/def2-TZVP plus MP2(QZ–TZ) composite, using a 60-electron effective core potential on Rn, with counterpoise and zero-point corrections. For HRnSiH3 → Rn + H + SiH3 the result is +1 kcal/mol, with an es mated uncertainty of ±3 kcal/mol, so the molecule is thermoneutral against radical fragmentation. The control molecule HRnF, computed identically, is bound by about 51 kcal/mol against Rn + H + F. Both molecules lie 84–90 kcal/mol above Rn + HX, so metastability toward HX loss is shared and does not separate them. The silyl group therefore does not support the H–Rn bond that stabilises known radon hydrides, and HRnSiH3 could at best be a matrix-isolation target. Spin–orbit coupling and the complete-basis-set limit are not included.