Gas-Phase Reactions of Atomic Lanthanide Cations with Ammonia: Room-Temperature Kinetics and Periodicity in Reactivity

Abstract

Reactions of (14) atomic lanthanide cations (excluding Pm(+)) with ammonia have been surveyed in the gas phase by using an inductively coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer to measure rate coefficients and product distributions in He at 0.35 +/- 0.01 Torr and 295 +/- 2 K. Primary reaction channels were observed corresponding to H(2) elimination with formation of the protonated lanthanum nitride and NH(3) addition. H(2) elimination was seen only in the reactions with La(+), Ce(+), Gd(+), and Tb(+) and occurs with these ions exclusively. NH(3) addition was seen with Pr(+), Nd(+), Sm(+), Eu(+), Dy(+), Ho(+), Er(+), Tm(+), Yb(+), and Lu(+). Higher-order sequential addition of up to five NH(3) molecules was observed with the Ln(+)(NH(3)) and LnNH(+) ions. The reaction efficiency of the primary reactions is seen to decrease as the energy required to promote an electron to make two non-f electrons available for bonding increases. The periodic trend in reaction efficiency along the lanthanide series matches quite closely the periodic trend in the electron-promotion energy required to achieve a d(1)s(1) or d(2) excited electronic configuration in the lanthanide cation. With La(+), Ce(+), Gd(+), and Tb(+), the electrostatic attraction between the atomic lanthanide cation and ammonia is sufficiently strong to provide enough energy to achieve electron promotion and to overcome any barriers to subsequent N-H bond insertion and H(2) loss, but this is not the case with the other lanthanide cations with which collisional stabilization of the intermediate adduct ion, with or without insertion of Ln(+), predominates.