Fries Daniela V, Klein Matthias P, Straßner Annika, Huber Maximilian E, Niedner-Schatteburg Gereon
Department of Chemistry and State Research Center OPTIMAS, Rheinland-Pfälzische Technische Universität (RPTU) Kaiserslautern-Landau, 67663 Kaiserslautern, Germany.
J Chem Phys. 2023 Oct 28;159(16). doi: 10.1063/5.0157218.
We present an IR-PD study of tantalum cluster adsorbate complexes [Tan(N2)m]+, abbreviated (n,m), n = 5-8. We utilize infrared spectroscopy of isolated and size selected clusters as prepared and characterized by a cryogenic tandem ion trap setup, and we augment our experiments with quantum chemical simulations at the level of density functional theory. The cluster adsorbate complexes (n,m) reveal vibrational bands above 2000 cm-1, which indicate end-on coordinated μ1-N2 oscillators, and bands below 2000 cm-1, which indicate side-on μ2-κN:κN,N coordinated ones. We observe a general increase in spectral complexity and an inhomogeneous broadening, mainly towards the red, at certain points of N2 loading m, which originates from an increasingly higher amount of double and triple N2 coordination at Ta sites, eventually at all of them. Other than the small tantalum clusters Tan+, n = 2-4, the IR-PD spectra of the initial N2 adsorbate species (n,1), n = 5-8, provide strong evidence for a lack of spontaneous N2 cleavage. Spontaneous N2 cleavage by Tan+, n = 5-8, seems suppressed. Therefore, the ability of a small Ta cluster to cleave dinitrogen disappears with one more tantalum core atom. The study of stepwise N2 adsorption on size selected Tan+, n = 5-8 clusters revealed adsorption limits m(max) of [Tan(N2)m]+ that are independent of cluster size within this size range. Cryo-adsorption kinetics at 26 K allowed for kinetic fits to consecutive N2 adsorption steps, and the fits revealed significant N2 desorption rates upon higher N2 loads, and the cluster adsorbate complexes eventually reached equilibrium. Some enhanced N2 desorption rates point towards likely adsorbate shell reorganization, and there is also some evidence for the coexistence of isomeric cluster adsorbate complexes.
我们展示了对钽簇吸附物络合物[Tan(N2)m]+（简称为(n,m)，n = 5 - 8）的红外光解离（IR-PD）研究。我们利用通过低温串联离子阱装置制备和表征的孤立且尺寸选择的簇的红外光谱，并通过密度泛函理论水平的量子化学模拟来补充我们的实验。簇吸附物络合物(n,m)在2000 cm-1以上显示出振动带，这表明是端接配位的μ1-N2振子，而在2000 cm-1以下的带则表明是侧接μ2-κN:κN,N配位的振子。我们观察到在特定的N2负载量m点，光谱复杂性普遍增加且出现非均匀展宽，主要是向红移，这源于Ta位点上双配位和三配位N2的数量越来越多，最终所有位点都如此。除了小钽簇Tan+（n = 2 - 4）外，初始N2吸附物物种(n,1)（n = 5 - 8）的IR-PD光谱有力地证明了不存在自发的N2裂解。n = 5 - 8的Tan+引发的自发N2裂解似乎受到抑制。因此，随着再多一个钽核心原子，小Ta簇裂解二氮的能力就消失了。对尺寸选择的n = 5 - 8的Tan+簇上逐步N2吸附的研究表明，[Tan(N2)m]+的吸附极限m(max)在此尺寸范围内与簇大小无关。26 K下的低温吸附动力学允许对连续N2吸附步骤进行动力学拟合，拟合结果显示在较高N2负载量时存在显著的N2解吸速率，并且簇吸附物络合物最终达到平衡。一些增强的N2解吸速率表明可能存在吸附物壳层重组，并且也有一些证据表明存在异构簇吸附物络合物的共存。
