Ditechnetium Heptoxide Revisited: Solid-State, Gas-Phase, and Theoretical Studies

Ditechnetium heptoxide was synthesized from the oxidation of TcO2 with O-2 at 450 degrees C and characterized by single-crystal X-ray diffraction, electron-impact mass spectrometry (EI-MS), and theoretical methods. Refinement of the structure at 100 K indicates that Tc2O7 crystallizes as a molecular solid in the orthorhombic space group Pbca [a = 7.312(3) angstrom, b = 5.562(2) angstrom, c = 13.707(5) angstrom, and V = 557.5(3) angstrom(3)]. The Tc2O7 molecule can be described as corner-sharing TcO4 tetrahedron [Tc---Tc = 3.698(1) angstrom and Tc-O-Bri-Tc = 180.0 degrees]. The EI-MS spectrum of Tc2O7 consists of both mononuclear and dinuclear species. The main dinuclear species in the gas-phase are Tc2O7 (100%) and Tc2O5 (56%), while the main mononuclear species are TcO3 (33.9%) and TcO2 (42.8%). The difference in the relative intensities of the M2O5 (M = Tc, Re) fragments (1.7% for Re) indicates that these group 7 elements exhibit different gas-phase chemistry. The solid-state structure of Tc2O7 was investigated by density functional theory methods. The optimized structure of the Tc2O7 molecule is in good agreement with the experimental one. Simulations indicate that the more favorable geometry for the Tc2O7 molecule in the gas-phase is bent (Tc-O-Bri-Tc = 156.5 degrees), while a linear geometry (Tc-O-Bri-Tc = 180.0 degrees) is favored in the solid-state.