Why is hafnium so unreactive?: Experimental and theoretical studies of the reaction of Hf + with methane

Abstract

The reaction of atomic hafnium cations with CH 4 and CD 4 is studied using a guided ion beam tandem mass spectrometer. In contrast to most third-row transition metal ions, the dehydrogenation reaction to form HfCH 2 + + H 2 is endothermic. At higher energies, other products, HfCH +, HfCH 3 +, and HfH +, the latter being the predominant species, are observed. Implicit in the behavior of the cross sections for HfH +, HfCH 2 +, and HfCH 3 + is a H Hf + CH 3 intermediate. Modeling of the endothermic cross sections provides for 0 K bond dissociation energies (in eV) of D 0(Hf + CH) = 5.10 ± 0.15, D 0(Hf + CH 2) = 4.37 ± 0.07, D 0(Hf + CH 3) = 2.12 ± 0.26, and D 0(Hf + H) = 1.97 ± 0.11. These experimental bond energies are in good agreement with density functional calculations at the B3LYP/HW+/6–311 ++ G(3 df,3 p) level of theory. Theoretical calculations reveal the mechanism of the reaction and illustrate the geometric and electronic structures of the individual products and intermediates. Unlike its first and second-row congeners, which have quartet ground states and must change spin to dehydrogenate methane, Hf + retains its ground state doublet configuration throughout the dehydrogenation reaction, demonstrating that spin-restrictions are not responsible for the relatively low reactivity of Hf +. Instead, this can be attributed to the unfavorable doubly occupied 6 s orbital in the 2D ground state of Hf +.