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Closed-Shell Molecules That Ionize More Readily Than Cesium

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Science  06 Dec 2002:
Vol. 298, Issue 5600, pp. 1971-1974
DOI: 10.1126/science.1078721

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Abstract

We report a class of molecules with extremely low ionization enthalpies, one member of which has been determined to have a gas-phase ionization energy (onset, 3.51 electron volts) lower than that of the cesium atom (which has the lowest gas-phase ionization energy of the elements) or of any other known closed-shell molecule or neutral transient species reported. The molecules are dimetal complexes with the general formula M2(hpp)4 (where M is Cr, Mo, or W, and hpp is the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine), structurally characterized in the solid state, spectroscopically characterized in the gas phase, and modeled with theoretical computations. The low-energy ionization of each molecule corresponds to the removal of an electron from the delta bonding orbital of the quadruple metal-metal bond, and a strong interaction of this orbital with a filled orbital on the hpp ligands largely accounts for the low ionization energies.

  • * To whom correspondence should be addressed. E-mail: cotton{at}tamu.edu (F.A.C.); jiandegush{at}go.com (J.G.); dlichten{at}email.arizona.edu (D.L.L.); murillo{at}tamu.edu (C.A.M.)

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