Theoretical characterization of pentazole anion with metal counter ions. Calculated and experimental N-15 shifts of aryldiazonium, -azide and -pentazole systems

Theoretical studies of proposed structures for NaN5, KN5, Mg(N-5)(2), Ca(N- 5)(2), and Zn(N-5)(2) metal complexed pentazole anions have been carried ou t with the RHF, MP2, MCSCF, and DFT theoretical methods. Additional DFT cal culations were performed on MgN5Cl, CaN5Cl, and ZnN5Cl pentazoles. The stru ctures considered are unidentate I, bidentate II, and metallocene-like III. For Mg, Na, K, and Ca pentazoles at every level of theory, II is the most energetically favoured, followed by I, then III. Complex I is preferred wit h Zn complexes due to favourable d orbital interactions. For double ring co mplexes only II (I for Zn) with perpendicular rings has all positive vibrat ional frequencies. For single ring complexes, both II (I for Zn) and III ha ve all positive vibrations. Structure I (II for Zn) is a transition state s tructure for metal ion rotation around the ring (E-a 5-10 kcal mol(-1)). N atom chemical shifts relative to NH3 and nitromethane were calculated for e ach species using the lowest energy configuration and the B3LYP//6-311++G(2 d,p) method on the B3LYP//6-31G(d) optimised geometry. Additional calculati ons were done for 1-arylpentazoles, 1-arylpentazene, aryl azides, and aryld iazonium ions. Calculated N-15 NMR shifts were within 20 ppm of experiment. Time dependent B3LYP/6-31G(d) and B3LYP/6-311+G(d) calculations were perfo rmed on all stable species. All (1)(pi,pi) transitions were calculated to b e below 180 nm, while the (1)(n,pi) transitions were below 210 nm. The lowe st energy transitions are from the lone pairs to the empty metal s orbital. For Mg and Zn these transitions are at similar to 220 nm. For Na, Ca and K the transitions are considerably lower in energy, similar to 250 nm.