Gas-phase reactions of nitric oxide with atomic lanthanide cations: Room-temperature kinetics and periodicity in reactivity

Abstract

An inductively coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer has been employed in a systematic survey of the room-temperature kinetics of reactions of NO with 13 atomic lanthanide cations from Ce⁺ to Lu⁺ (excluding Pm⁺). The atomic ions are produced at ca. 5500 K in an ICP source and are allowed to decay radiatively and to thermalize by collisions with Ar and He atoms prior to reaction in helium buffer gas at 0.35 ± 0.01 Torr and 295 ± 2 K. All lanthanide cations were observed to exhibit some reactivity towards NO, almost exclusively resulting in LnO⁺ formation. Reactions were observed that are both first and second order in NO. Periodic trends in the measured reaction efficiencies for direct exothermic O atom transfer correlate with trends in the energy required to promote an electron in Ln⁺ to achieve either a d¹s¹ or a d² excited electronic configuration in which two non-f-electrons are available for bonding. No such correlation is apparent for the remaining reactions.

Introduction

Our recent laboratory measurements of reactions of atomic lanthanide cations with the oxygen-containing gases N₂O and O₂ have shown that the unique electronic configurations of lanthanide cations lead to unique trends in the kinetics of oxidation of these cations across the periodic table [1]. As early as 1988, Schilling and Beauchamp [2] had proposed that electron promotion from the 4fⁿ6s¹ ground state to the 4fⁿ⁻¹5d¹6s¹ excited state was required for these cations to be effective in CH and CC bond activation and insertion and this has been borne out by the plethora of investigations into lanthanide hydrocarbon ion chemistry that followed [3]. Our results with N₂O have shown that electron promotion also is required (to provide two unpaired non-f-electrons) for bonding with atomic oxygen in the formation of LnO⁺ in reactions of lanthanide cations with N₂O [1]. Triplet oxygen in comparison reacts more readily, apparently by using one or more of the unpaired electrons to initiate binding. The electron promotion that is clearly required with N₂O gives rise to a periodic dependence of the efficiency of O-atom transfer on the electron promotion energy and intriguing Arrhenius-like dependencies of the efficiency on the electron promotion energy which exhibit different characteristic temperatures for the early and late lanthanide cations. A recent bonding configuration analysis by Gibson [4] suggests that two unpaired 5d valence electrons rather than a 5d and a 6s electron affect the bonding between the metal centre and the oxygen atom in lanthanide oxide cations. The variations in the promotion energies required to achieve either 5d² or 5d¹6s¹ excitation are qualitatively similar across the lanthanides and so give rise to similar predictions of the periodic and Arrhenius-like dependencies of the efficiencies of O-atom transfer on the electron promotion energy.

Here we report results of reactions of lanthanide cations with another oxygen gas, nitric oxide. We shall see that the radical nature and high bond energy of NO gives rise to new kinetic insight into the lanthanide cation chemistry of oxygen-containing gases.

We present a room-temperature survey of gas-phase reactions of NO with all the lanthanide cations (except Pm⁺ which does not have a stable isotope) as part of a more extensive study of the reactions of atomic cations with NO [5], [6]. We are aware of only one previous investigation of lanthanide cation reactions with NO. Ion beam studies in 1988 [2] provided cross-sections of 4.8 and 16 Å² at a centre-of-mass energy of approximately 0.25 eV for the reactions of Gd⁺ and Pr⁺ with NO to produce GdO⁺ and PrO⁺, respectively. There appear to be no previous room-temperature studies of the kinetics of any of the 13 lanthanide–cation reactions with NO reported here (other than our study of the reaction with La⁺ [6]).