Oxygen-containing gas-phase diatomic trications and tetracations: ReOz+, NbOz+ and HfOz+ (z = 3, 4)

Abstract

Three oxygen-containing gas-phase diatomic trications ReO(3+), NbO(3+) and HfO(3+) as well as the diatomic tetracation NbO(4+) have been observed by mass spectrometry at non-integer m/z values. These unusual triply charged molecular ion species, together with the corresponding diatomic dications ReO(2+), NbO(2+) and HfO(2+), were produced by energetic, high-current oxygen ((16)O(-)) ion beam sputtering of rhenium, niobium and hafnium metal samples, respectively, whose surfaces were dynamically oxidized by oxygen primary ion incorporation. In addition, NbO(z+) (z≤ 4) were generated by intense femtosecond laser excitation and photofragmentation (Coulomb explosion) of Nb(x)O(y) clusters and were detected through Time-of-Flight Mass Spectrometry (TOF). Our experimental results confirm previous reports on the detection of NbO(4+), NbO(3+), NbO(2+), HfO(3+) and HfO(2+) with Atom Probe mass spectrometry, whereas ReO(3+) and ReO(2+) apparently had not been observed before. In addition, these multiply charged molecular ions have been studied theoretically for the first time. Ab initio calculations of their electronic structures show that the diatomic trications ReO(3+), NbO(3+) and HfO(3+) are long-lived metastable gas-phase species, with bond lengths of 1.61 Å, 1.62 Å and 1.86 Å, respectively. They present large potential barriers with respect to dissociation of more than 2.7 eV. The corresponding diatomic dications are thermochemically stable molecules with very large dissociation energies (>3.5 eV). Our calculations predict the diatomic tetracation ReO(4+) to be a metastable ion species in the gas phase. We compute a potential barrier toward fragmentation of 0.6 eV; its formation requires a quadruple adiabatic ionization energy of 85.7 eV. Even though our calculations show that NbO(4+) is a weakly bound (dissociation barrier ∼0.1 eV) metastable molecule, it is here identified via linear time-of-flight mass spectrometry.

The bond energy of ReO+: Guided ion-beam and theoretical studies of the reaction of Re+ (7S) with O2

“…87 (Here the energy of the 3 2 is pushed down by interaction with the 1 2 state arising from the same configuration.) In addition, spin-orbit calculations for ReO + ( 3 ) performed at the CASSCF/cc-pV5Z-PP level by Brites et al 85 find A( 3 ) = 1323.4 cm −1 , in good agreement with the 1272.5 cm −1 estimate here. Thus, including these estimated spin-orbit corrections, the 5 −1 → 3 3 excitation energy should be lowered by 0.158 eV.…”

“…They assigned the ground state of ReO + as 3 − (1σ 2 1π 4 1δ 2 2σ 2 ) with unidentified singlet states lying 0.22 (0.68) eV and unidentified quintet states lying 1.08 (0.37) eV higher in energy at the B3LYP (BLYP) levels. Recently, Brites et al 85 performed very high level calculations, multireference configuration interaction (MRCI) using the aug-cc-pV5Z-PP basis set and fully relativistic pseudo potential on Re from Figgen et al 71 They identified the 3 as the ground state with 3 − and 1 + states lying 0.52 and 0.94 eV higher in energy, but do not appear to have considered a 5 state. In our work, the excitation energy of the 3 state is only 0.03 eV at the B3LYP/def2-TZVPP level, such that there is obviously some question as to the true identity of the ground state.…”

A theoretical study of SnF2+, SnCl2+, and SnO2+ and their experimental search

“…Details on such experimental work have been provided in several publications. 6,[12][13][14][15][16][17][18][19][20] We have used prolonged ion surface bombardment (sputtering) of a tin metal foil, of tin (II) fluoride (SnF 2 ) powder as well as of stannous (tin (II)) chloride (SnCl 2 ) powder (the two powder samples were pressed into soft indium metal foil), respectively, with a high-current, energetic oxygen beam (mass-filtered 16 O − , 17 keV, ≥100 nA) to produce the above three gas-phase diatomic dications. The mass spectrometer was operated at maximum instrument transmission, and these diatomic dications were detected at half-integer mass-overcharge (m/z) values in positive ion mass spectra for flight times of about 10 −5 s. The availability of several elemental isotopes helped with their unambiguous identification.…”

“…The possibility of detecting as yet unknown multiply charged ions as thermodynamic stable molecules or as long-lived metastable species is enormous and open-ended, but challenging. The list of diatomic dications appearing as supplementary material to the study by Brites et al 6 represents just a sample of these possibilities.…”

The diatomic dication SiC2+ in the gas phase

“…[12] The experimental finding of Franzreb and Williams was mentioned in the supplementary material (available online) of ref. [13] but data were not shown. Their mass spectra of SiC 2+ are shown in the sup-2 plementary material of the present work for the first time.…”