Spectroscopy and Infrared Photofragmentation Dynamics of Mixed Ligand Ion-Molecule Complexes: Au(CO)x(N2O)y. J Phys Chem A 2021;125:7266-7277. [PMID: 34433267 DOI: 10.1021/acs.jpca.1c05800]

Infrared spectra and fragmentation dynamics of isotopologue-selective mixed-ligand complexes

Isolated mixed-ligand complexes provide tractable model systems in which to study competitive and cooperative binding effects as well as controlled energy flow. Here, we report spectroscopic and isotopologue-selective infrared photofragmentation dynamics of mixed gas-phase Au(12/13CO)n(N2O)m+ complexes. The rich infrared action spectra, which are reproduced well using simulations of calculated lowest energy structures, clarify previous ambiguities in the assignment of vibrational bands, especially accidental coincidence of CO and N2O bands. The fragmentation dynamics exhibit the same unexpected behaviour as reported previously in which, once CO loss channels are energetically accessible, these dominate the fragmentation branching ratios, despite the much lower binding energy of N2O. We have investigated the dynamics computationally by considering anharmonic couplings between a relevant subset of normal modes involving both ligand stretch and intermolecular modes. Discrepancies between correlated and uncorrelated model fit to the ab initio potential energy curves are quantified using a Boltzmann sampled root mean squared deviation providing insight into efficiency of vibrational energy transfer between high frequency ligand stretches and the softer intermolecular modes which break during fragmentation.

A Combined Infrared and Computational Study of Gas-Phase Mixed-Ligand Rhodium Complexes: Rh(CO)n(N2O)m+ (n = 1-5, m = 1-4)

In this study, mixed carbonyl and nitrous oxide complexes with Rh+ were studied by mass-selective infrared photodissociation spectroscopy in a molecular beam. The infrared spectra, recorded in the region of the CO and N2O N═N stretches, were assigned and interpreted with the aid of simulated spectra of low-energy structural isomers. Clear evidence of an inner coordination shell of four ligands is observed. The observed vibrational structure can be understood on the basis of local mode vibrations in the two ligands. However, there is also evidence of multiple low-lying isomers and cooperative binding effects between the two ligands. In particular, σ donation from directly coordinated nitrous oxide ligands drives more classical carbonyl bonding than has been observed in pure carbonyl complexes. The observed fragmentation branching ratios following resonant infrared absorption are explained by simple statistical and energetic arguments, providing a contrast with those of equivalent Au+ complexes.

An Infrared Study of Gas-Phase Metal Nitrosyl Ion-Molecule Complexes

We present a combined experimental and quantum chemical study of gas-phase group 9 metal nitrosyl complexes, M(NO)_n⁺ (M = Co, Rh, Ir). Experimental infrared photodissociation spectra of mass-selected ion-molecule complexes are presented in the region 1600 cm^-1 to 2000 cm^-1 which includes the NO stretch. These are interpreted by comparison with the simulated spectra of energetically low-lying structures calculated using density functional theory. A mix of linear and nonlinear ligand binding is observed, often within the same complex, and clear evidence of coordination shell closing is observed at n = 4 for Co(NO)_n⁺ and Ir(NO)_n⁺. Calculations of Rh(NO)_n⁺ complexes suggest additional low-lying five-coordinate structures. In all cases, once a second coordination shell is occupied, new spectral features appear which are assigned to (NO)₂ dimer moieties. Further evidence of such motifs comes from differences in the spectra recorded in the dissociation channels corresponding to single and double ligand loss.

An infrared study of CO2 activation by holmium ions, Ho+ and HoO. Phys Chem Chem Phys 2022;24:22716-22723. [PMID: 36106954 DOI: 10.1039/d2cp02862j]

We report a combined experimental and computational study of carbon dioxide activation at gas-phase Ho⁺ and HoO⁺ centres. Infrared action spectra of Ho(CO₂)_n⁺ and [HoO(CO₂)_n]⁺ ion-molecule complexes have been recorded in the spectral region 1700-2400 cm^-1 and assigned by comparison with simulated spectra of energetically low-lying structures determined by density functional theory. Little by way of activation is observed in Ho(CO₂)_n⁺ complexes with CO₂ binding end-on to the Ho⁺ ion. By contrast, all [HoO(CO₂)_n]⁺ complexes n ≥ 3 show unambiguous evidence for formation of a carbonate radical anion moiety, . The signature of this structure, a new vibrational band observed around 1840 cm^-1 for n = 3, continues to red-shift monotonically with each successive CO₂ ligand binding with net charge transfer from the ligand rather than the metal centre.