Cryo spectroscopy of N 2 on cationic iron clusters.

Infrared photodissociation (IR-PD) spectra of iron cluster dinitrogen adsorbate complexes [Fe n (N 2 ) 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 N 2 chromophores, a μ 1,end end-on binding motif. Density Functional Theory (DFT) modeling and detailed analysis of n = 13 complexes are consistent with an icosahedral Fe 13 + core structure. The first adsorbate shell closure at (n,m) = (13,12)-as recognized by the accompanying paper on the kinetics of N 2 uptake by cationic iron clusters-comes with extensive IR-PD band broadening resulting from enhanced couplings among adjacent N 2 adsorbates. DFT modeling predicts spin quenching by N 2 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 2 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 2 binding motifs and couplings. In contrast, the (17,7) spectrum of six sharp bands suggests pairwise equivalent N 2 adsorbates. The IR-PD spectra of (18,m) reveal additional features in the 2120-2200 cm -1 region, which we associate with a μ 1,side side-on motif. Some additional features in the (18,m) spectra at high N 2 loads indicate a μ 1,tilt tilted end-on adsorption motif.