Cryo spectroscopy of N on cationic iron clusters.

Infrared photodissociation (IR-PD) spectra of iron cluster dinitrogen adsorbate complexes [Fe(N)] for n = 8-20 reveal slightly redshifted IR active bands in the region of 2200-2340 cm. These bands mostly relate to stretching vibrations of end-on coordinated N chromophores, a μ end-on binding motif. Density Functional Theory (DFT) modeling and detailed analysis of n = 13 complexes are consistent with an icosahedral Fe core structure. The first adsorbate shell closure at (n,m) = (13,12)-as recognized by the accompanying paper on the kinetics of N uptake by cationic iron clusters-comes with extensive IR-PD band broadening resulting from enhanced couplings among adjacent N adsorbates. DFT modeling predicts spin quenching by N 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) N 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 N binding motifs and couplings. In contrast, the (17,7) spectrum of six sharp bands suggests pairwise equivalent N adsorbates. The IR-PD spectra of (18,m) reveal additional features in the 2120-2200 cm region, which we associate with a μ side-on motif. Some additional features in the (18,m) spectra at high N loads indicate a μ tilted end-on adsorption motif.