Cryo kinetics of N2 adsorption onto bimetallic rhodium-iron clusters in isolation.

We report the N2 cryo adsorption kinetics of selected gas phase mixed rhodium-iron clusters [RhiFej]+ in the range of i = 3-8 and j = 3-8 in 26 K He buffer gas by the use of a cryo tandem RF-hexapole trap-Fourier transform ion cyclotron resonance mass spectrometer. From kinetic data and fits, we extract relative rate constants for each N2 adsorption step and possible desorption steps. We find significant trends in adsorption behavior, which reveal adsorption limits, intermittent adsorption limits, and equilibrium reactions. For those steps, which are in equilibrium, we determine the Gibbs free energies. We conclude on likely ligand shell reorganization and some weakly bound N2 ligands for clusters where multiple N2 adsorbates are in equilibrium. The relative rate constants are transferred to absolute rate constants, which are slightly smaller than the collision rate constants calculated by the average dipole orientation (Langevin) theory. The calculated sticking probabilities increase, in general, with the size of the clusters and decrease with the level of N2 adsorption, in particular, when reaching an adsorption/desorption equilibrium. We receive further evidence on cluster size dependent properties, such as cluster geometries and metal atom distributions within the clusters through the accompanying spectroscopic and computational study on the equiatomic i = j clusters [Klein et al., J. Chem. Phys. 156, 014302 (2022)].

Cryo infrared spectroscopy of N2 adsorption onto bimetallic rhodium-iron clusters in isolation.

We investigated the N2 adsorption behavior of bimetallic rhodium-iron cluster cations [RhiFej(N2)m]+ by means of InfraRed MultiplePhotoDissociation (IR-MPD) spectroscopy in comparison with density functional theory (DFT) modeling. This approach allows us to refine our kinetic results [Ehrhard et al., J. Chem. Phys. (in press)] to enhance our conclusions. We focus on a selection of cluster adsorbate complexes within the ranges of i = j = 3-8 and m = 1-10. For i = j = 3, 4, DFT suggests alloy structures in the case of i = j = 4 of high (D2d) symmetry: Rh-Fe bonds are preferred instead of Fe-Fe bonds or Rh-Rh bonds. N2 adsorption and IR-MPD studies reveal strong evidence for preferential adsorption to Rh sites and mere secondary adsorption to Fe. In some cases, we observe adsorption isomers. With the help of modeling the cluster adsorbate complex [Rh3Fe3(N2)7]+, we find clear evidence that the position of IR bands allows for an element specific assignment of an adsorption site. We transfer these findings to the [Rh4Fe4(N2)m]+ cluster adsorbate complex where the first four N2 molecules are exclusively adsorbed to the Rh atoms. The spectra of the larger adsorbates reveal N2 adsorption onto the Fe atoms. Thus, the spectroscopic findings are well interpreted for the smaller clusters in terms of computed structures, and both compare well to those of our accompanying kinetic study [Ehrhard et al., J. Chem. Phys. (in press)]. In contrast to our previous studies of bare rhodium clusters, the present investigations do not provide any indication for a spin quench in [RhiFej(N2)m]+ upon stepwise N2 adsorption.

Kinetics of stepwise nitrogen adsorption by size-selected iron cluster cations: Evidence for size-dependent nitrogen phobia.

We present a study of stepwise cryogenic N2 adsorption on size-selected Fen + (n = 8-20) clusters within a hexapole collision cell held at T = 21-28 K. The stoichiometries of the observed adsorption limits and the kinetic fits of stepwise N2 uptake reveal cluster size-dependent variations that characterize four structural regions. Exploratory density functional theory studies support tentative structural assignment in terms of icosahedral, hexagonal antiprismatic, and closely packed structural motifs. There are three particularly noteworthy cases, Fe13 + with a peculiar metastable adsorption limit, Fe17 + with unprecedented nitrogen phobia (inefficient N2 adsorption), and Fe18 + with an isomeric mixture that undergoes relaxation upon considerable N2 uptake.

Cryo spectroscopy of N2 on cationic iron clusters.

Infrared photodissociation (IR-PD) spectra of iron cluster dinitrogen adsorbate complexes [Fen(N2)m]+ for n = 8-20 reveal slightly redshifted IR active bands in the region of 2200-2340 cm-1. These bands mostly relate to stretching vibrations of end-on coordinated N2 chromophores, a µ1,end end-on binding motif. Density Functional Theory (DFT) modeling and detailed analysis of n = 13 complexes are consistent with an icosahedral Fe13 + core structure. The first adsorbate shell closure at (n,m) = (13,12)-as recognized by the accompanying paper on the kinetics of N2 uptake by cationic iron clusters-comes with extensive IR-PD band broadening resulting from enhanced couplings among adjacent N2 adsorbates. DFT modeling predicts spin quenching by N2 adsorption as evidenced by the shift of the computed spin minima among possible spin states (spin valleys). The IR-PD spectrum of (17,1) surprisingly reveals an absence of any structure but efficient non-resonant fragmentation, which might indicate some weakly bound (roaming) N2 adsorbate. The multiple and broad bands of (17,m) for all other cases than (17,1) and (17,7) indicate a high degree of variation in N2 binding motifs and couplings. In contrast, the (17,7) spectrum of six sharp bands suggests pairwise equivalent N2 adsorbates. The IR-PD spectra of (18,m) reveal additional features in the 2120-2200 cm-1 region, which we associate with a µ1,side side-on motif. Some additional features in the (18,m) spectra at high N2 loads indicate a µ1,tilt tilted end-on adsorption motif.

Long-Term Monitoring of the Internal Energy Distribution of Isolated Cluster Systems.

A method is presented to monitor the internal energy distribution of cluster anions via delayed electron detachment by pulsed photoexcitation and demonstrated on Co_{4}^{-} in an electrostatic ion beam trap. In a cryogenic operation, we calibrate the detachment delay to internal energy. By laser frequency scans, at room temperature, we reconstruct the time-dependent internal energy distribution of the clusters. The mean energies of ensembles from a cold and a hot ion source both approach thermal equilibrium. Our data yield a radiative emission law and the absorptivity of the cluster for thermal radiation.

Probing cluster surface morphology by cryo kinetics of N2 on cationic nickel clusters.

We present the stepwise N2 adsorption kinetics of size selected Nin+ (n = 5-20) clusters at 26 K as obtained by a hybrid tandem ion trap instrument. Pseudo-first-order kinetic fits confirm consecutive adsorption steps without evidence of cluster isomers and up to adsorption limits, which scale with the cluster size. The reaction rates for the initial N2 adsorption increase smoothly with the cluster size and similar to hard sphere cluster modeling. The isothermal kinetics allow for the tentative elucidation of cluster surface morphologies and for their classification into highly symmetrical clusters with all smooth surfaces, small clusters with rough surfaces, and large clusters with partially rough and smooth surface areas. The parallel characterization of the vibrational spectroscopy of some cluster adsorbate complexes supports and refines the achieved conclusions and is published back to back with this contribution [S. Dillinger, J. Mohrbach, and G. Niedner-Schatteburg, J. Chem. Phys. 147, 184305 (2017)]. These two studies elucidate the adsorbate to cluster interaction, and they confirm and specify the sometimes considerable structural fluxionality of finite and curved metal surfaces in high detail. This work precedes further studies along the present lines of thought.

Probing cluster surface morphology by cryo spectroscopy of N2 on cationic nickel clusters.

We present the cryogenic (26 K) IR spectra of selected [Nin(N2)m]+ (n = 5-20, m = 1 - mmax), which strongly reveal n- and m-dependent features in the N2 stretching region, in conjunction with density functional theory modeling of some of these findings. The observed spectral features allow us to refine the kinetic classification [cf. J. Mohrbach, S. Dillinger, and G. Niedner-Schatteburg, J. Chem. Phys. 147, 184304 (2017)] and to define four classes of structure related surface adsorption behavior: Class (1) of Ni6+, Ni13+, and Ni19+ are highly symmetrical clusters with all smooth surfaces of equally coordinated Ni atoms that entertain stepwise N2 adsorption up to stoichiometric N2:Nisurface saturation. Class (2) of Ni12+ and Ni18+ are highly symmetrical clusters minus one. Their relaxed smooth surfaces reorganize by enhanced N2 uptake toward some low coordinated Ni surface atoms with double N2 occupation. Class (3) of Ni5+ and Ni7+ through Ni11+ are small clusters of rough surfaces with low coordinated Ni surface atoms, and some reveal semi-internal Ni atoms of high next-neighbor coordination. Surface reorganization upon N2 uptake turns rough into rough surface by Ni atom migration and turns octahedral based structures into pentagonal bipyramidal structures. Class (4) of Ni14+ through Ni17+ and Ni20+ are large clusters with rough and smooth surface areas. They possess smooth icosahedral surfaces with some proximate capping atom(s) on one hemisphere of the icosahedron with the other one largely unaffected.

Vibrational blue shift of coordinated N2 in [Fe3O(OAc)6(N2)n]+: "non-classical" dinitrogen complexes.

We present "non-classical" dinitrogen Fe(iii) oxo acetate complexes in vacuo utilizing Infrared Photodissociation (IR-PD) at cryo temperatures. The IR-PD spectra reveal a blue shift of the N2 stretching vibration frequencies in the complexes. Density Functional Theory (DFT) calculations confirm the experiments and indicate strengthened N-N bonds due to pronounced σ bonding and a lack of π back donation.

Infrared spectroscopy of N2 adsorption on size selected cobalt cluster cations in isolation.

We report IR active N2 stretching frequencies in isolated and size selected cobalt cluster nitrogen adsorbate complexes, [Con(N2)1](+) as recorded by virtue of InfraRed Photon Dissociation (IRPD) spectroscopy. The observed frequencies of the [Con(N2)1](+) complexes (n = 8-17) are significantly redshifted (2180 to 2290 cm(-1)) with respect to the IR inactive vibrations of free N2 (2359 cm(-1)). These bands are assigned to a µ1 head-on type of coordination of the N2 to the cobalt cluster surface, revealing remarkable cluster size dependent features to interpret.