Molek K S, Jaeger T D, Duncan M A
Department of Chemistry, University of Georgia, Athens, Georgia 30602-2556, USA.
J Chem Phys. 2005 Oct 8;123(14):144313. doi: 10.1063/1.2050650.
Transition-metal oxide clusters of the form M(n)O(m) (+)(M=V,Nb,Ta) are produced by laser vaporization in a pulsed nozzle cluster source and detected with time-of-flight mass spectrometry. Consistent with earlier work, cluster oxides for each value of n produce only a limited number of stoichiometries, where m>n. The cluster cations are mass selected and photodissociated using the second (532 nm) or third (355 nm) harmonic of a Nd:YAG (yttrium aluminum garnet) laser. All of these clusters require multiphoton conditions for dissociation, consistent with their expected strong bonding. Dissociation occurs by either elimination of oxygen or by fission, repeatedly producing clusters having the same specific stoichiometries. In oxygen elimination, vanadium species tend to lose units of O(2), whereas niobium and tantalum lose O atoms. For each metal increment n, oxygen elimination proceeds until a terminal stoichiometry is reached. Clusters having this stoichiometry do not eliminate more oxygen, but rather undergo fission, producing smaller M(n)O(m) (+) species. The smaller clusters produced as fission products represent the corresponding terminal stoichiometries for those smaller n values. The terminal stoichiometries identified are the same for V, Nb, and Ta oxide cluster cations. This behavior suggests that these clusters have stable bonding networks at their core, but additional excess oxygen at their periphery. These combined results determine that M(2)O(4) (+), M(3)O(7) (+), M(4)O(9) (+), M(5)O(12) (+), M(6)O(14) (+), and M(7)O(17) (+) have the greatest stability for V, Nb, and Ta oxide clusters.
通过脉冲喷嘴团簇源中的激光蒸发产生了M(n)O(m) (+)形式的过渡金属氧化物团簇（M = V、Nb、Ta），并采用飞行时间质谱法进行检测。与早期工作一致，对于每个n值的团簇氧化物，仅产生有限数量的化学计量比，其中m > n。使用Nd:YAG（钇铝石榴石）激光器的二次谐波（532 nm）或三次谐波（355 nm）对团簇阳离子进行质量选择和光解离。所有这些团簇都需要多光子条件才能解离，这与其预期的强键合一致。解离通过消除氧或裂变发生，反复产生具有相同特定化学计量比的团簇。在氧消除过程中，钒物种倾向于失去O(2)单元，而铌和钽则失去O原子。对于每个金属增量n，氧消除一直进行到达到最终化学计量比。具有这种化学计量比的团簇不会再消除更多的氧，而是会发生裂变，产生更小的M(n)O(m) (+)物种。作为裂变产物产生的较小团簇代表了那些较小n值对应的最终化学计量比。所确定的最终化学计量比对V、Nb和Ta氧化物团簇阳离子是相同的。这种行为表明这些团簇在其核心具有稳定的键合网络，但在其外围有额外的过量氧。这些综合结果确定M(2)O(4) (+)、M(3)O(7) (+)、M(4)O(9) (+)、M(5)O(12) (+)、M(6)O(14) (+)和M(7)O(17) (+)对于V型、Nb型和Ta型氧化物团簇具有最大稳定性。
