Guided ion beam and theoretical study of the reactions of Hf+ with H2, D2, and HD

The kinetic energy dependences of reactions of the third-row transition metal cation Hf+ with H2, D2, and HD were determined using a guided ion beam tandem mass spectrometer. A flow tube ion source produces Hf+ in its 2D (6s25d1) electronic ground state level. Corresponding state-specific reaction cross sections are obtained. The kinetic energy dependences of the cross sections for the endothermic formation of HfH+ and HfD+ are analyzed to give a 0 K bond dissociation energy of D0 (Hf+-H)=2.11±0.08 eV. Quantum chemical calculations at several levels of theory performed here generally overestimate the experimental bond energy but results obtained using the Becke-half-and-half-LYP functional show good agreement. Theory also provides the electronic structures of these species and the reactive potential energy surfaces. Results from the reactions with HD provide insight into the reaction mechanisms and indicates that Hf+ reacts via a statistical mechanism. We also compare this third-row transition metal system with the first-row and second-row congeners, Ti+ and Zr+, and find that Hf+ has a weaker M+-H bond. As most third-row transition metal hydride cation bonds exceed their lighter congeners, this trend is unusual but can be understood using promotion energy arguments.