Formation of ionic complexes in cryogenic matrices: A case study using co-deposition of Cu− with rare gas cations in solid argon

Abstract

Matrix isolation spectra have been obtained for ionic species formed from a beam of mass-selected ions, with a coincident beam of externally generated counter-ions used to provide charge balance. Infrared spectra were obtained for copper carbonyl complexes formed following deposition of Cu(-) ions with rare-gas counter-cations into CO-doped argon matrices. Both anionic and neutral copper carbonyl complexes Cu(CO)(n)(q) (n = 1-3; q = 0, -1) were observed in the spectra, with peak positions corresponding to previously reported assignments; new partially resolved bands appearing in the range 1830-1845 cm(-1) are assigned to larger [Cu(CO)3●(CO)n](-) aggregates, having additional CO ligands in the second solvation shell. The experimental geometry ensures that all Cu-centers initially arrive at the matrix as anions, so the relative abundance of anionic relative to neutral complexes is much higher than in previous studies employing alternative methods for ion deposition; this allows for monitoring of electron-transfer processes between anions and cations in the matrix. Comparison of time-dependent vs. temperature-dependent trends reveals that there are two distinct mechanisms by which the population of anionic complexes is converted into neutral complexes: short-range electron transfer between a cation-anion pair following diffusion, and long-range electron transfer involving photodetachment of an electron from the anion into the conduction band of solid argon, resulting in eventual recombination of the electron with a cation in a remote matrix site. The spectra also show a marked dependence on the deposition temperature and dopant concentration, in that 100-fold higher CO concentrations were required during deposition with the sample window at 10 K compared to that used at 20 K, in order to obtain a similar distribution of copper carbonyl complexes. Furthermore, although no carbonyl complexes are observed initially when low concentrations of CO are used at 10 K, upon warming the matrix to 15 K, the neutral di- and tricarbonyl peaks appear abruptly, which is attributed to fast diffusion of CO stimulated by the energy released upon short-range electron-transfer between Cu(-):counter-cation pairs.

Chemical Reactions Triggered Using Electrons Photodetached from “Clean” Distributions of Anions Deposited in Cryogenic Matrices via Counterion Codeposition

“…Figure A shows an improved version of the experiment where the matrix deposition was conducted under completely darkened conditions at 10 K. (Full experimental details are given in Supporting Information.) The expected bands for the anionic mono-, di-, and tricarbonyl complexes are observed at 1733, 1780, and 1829 cm –1 , respectively, as in previous studies, , but now the neutral copper carbonyl peaks are conspicuously absent from the spectrum. (The peak at 1774 cm –1 was also previously assigned to an anionic dicarbonyl species …”

“…The expected bands for the anionic mono-, di-, and tricarbonyl complexes are observed at 1733, 1780, and 1829 cm –1 , respectively, as in previous studies, , but now the neutral copper carbonyl peaks are conspicuously absent from the spectrum. (The peak at 1774 cm –1 was also previously assigned to an anionic dicarbonyl species …”

“…The coupling of a vibrational spectroscopy matrix isolation apparatus to the output of a mass spectrometer to enable structural characterization of arbitrary ions of interest has long been recognized as an important goal in instrumental analytical and physical chemistry . Recent results from our group demonstrated the feasibility of using simultaneous deposition of equal currents of anions and cations for just such a purpose . Anionic copper carbonyl complexes in sufficient number densities for FTIR spectroscopic characterization were produced by simultaneously directing low-energy beams of monatomic copper anions (Cu – ) and rare gas cations (Ar + or Kr + ) into a CO-doped argon matrix .…”

“…Recent results from our group demonstrated the feasibility of using simultaneous deposition of equal currents of anions and cations for just such a purpose . Anionic copper carbonyl complexes in sufficient number densities for FTIR spectroscopic characterization were produced by simultaneously directing low-energy beams of monatomic copper anions (Cu – ) and rare gas cations (Ar + or Kr + ) into a CO-doped argon matrix . That study demonstrated proof-of-concept for the technique but still was not an ideally “clean” source of matrix isolated ions because neutral copper carbonyls were also produced during deposition.…”

Soft‐ and reactive landing of ions onto surfaces: Concepts and applications

“…Methods of ionization fall broadly into two categories, either continuous or pulsed. The most prevalent continuous ionization sources used for ion soft landing include electron impact ionization (EI) (Pradeep et al, ; Biesecker et al, ; Wijesundara et al, ; Bottcher et al, ), electrospray ionization (ESI) (Feng et al, ; Ouyang et al, ; Alvarez et al, ; Volny & Turecek, ; Mazzei et al, ; Hamann et al, ; Hauptmann et al, ), direct current (DC) or radiofrequency (RF) magnetron sputtering combined with gas aggregation (Haberland et al, , , ; Barnes et al, ; Pratontep et al, ; Tanemura et al, ; Lim et al, ; Duffe et al, ; Watanabe & Isomura, ; Gracia‐Pinilla et al, ; Nielsen et al, ; Wepasnick et al, ; Hartmann et al, ; Ludwig & Moore, ; Yin et al, ), high energy ion sputtering (Lapack et al, ; Harbich et al, ; Dong et al, ; Bromann et al, ; Fedrigo et al, ; Schaffner et al, ; O'Shea et al, ; Yamaguchi et al, ; Lau et al, ), and gas condensation/aggregation (GC) (Patil et al, ; Goldby et al, ; Yoon et al, ; Baker et al, ). Soft landing has also been accomplished using various pulsed ionization methods including matrix‐assisted laser desorption ionization (MALDI) (Rader et al, ), laser ablation/vaporization (Honea et al, ; Messerli et al, ; Pauwels et al, ; Klingeler et al, ; Heiz & Bullock, ; Melinon et al, ; Kemper et al, ; Mitsui et al, ; Winans et al, ; Cattaneo et al, ; Kaden et al, ; Davila et al, ; Tournus et al, ; Wepasnick et al, ; Woodward et al, ), the pulsed arc cluster ion source (PACIS) (Siekmann et al,…”