Department of Chemistry, University of Massachusetts Amherst, Amherst, MA 01003, USA.
Phys Chem Chem Phys. 2011 Nov 7;13(41):18347-54. doi: 10.1039/c1cp21586h. Epub 2011 Sep 6.
The microsolvation of cobalt and nickel dications by acetonitrile and water is studied by measuring photofragment spectra at 355, 532 and 560-660 nm. Ions are produced by electrospray, thermalized in an ion trap and mass selected by time of flight. The photodissociation yield, products and their branching ratios depend on the metal, cluster size and composition. Proton transfer is only observed in water-containing clusters and is enhanced with increasing water content. Also, nickel-containing clusters are more likely to undergo charge reduction than those with cobalt. The homogeneous clusters with acetonitrile M(2+)(CH(3)CN)(n) (n = 3 and 4) dissociate by simple solvent loss; n = 2 clusters dissociate by electron transfer. Mixed acetonitrile/water clusters display more interesting dissociation dynamics. Again, larger clusters (n = 3 and 4) show simple solvent loss. Water loss is substantially favored over acetonitrile loss, which is understandable because acetonitrile is a stronger ligand due to its higher dipole moment and polarizability. Proton transfer, forming H(+)(CH(3)CN), is observed as a minor channel for M(2+)(CH(3)CN)(2)(H(2)O)(2) and M(2+)(CH(3)CN)(2)(H(2)O) but is not seen in M(2+)(CH(3)CN)(3)(H(2)O). Studies of deuterated clusters confirm that water acts as the proton donor. We previously observed proton loss as the major channel for photolysis of M(2+)(H(2)O)(4). Measurements of the photodissociation yield reveal that four-coordinate Co(2+) clusters dissociate more readily than Ni(2+) clusters whereas for the three-coordinate clusters, dissociation is more efficient for Ni(2+) clusters. For the two-coordinate clusters, dissociation is via electron transfer and the yield is low for both metals. Calculations of reaction energetics, dissociation barriers, and the positions of excited electronic states complement the experimental work. Proton transfer in photolysis of Co(2+)(CH(3)CN)(2)(H(2)O) is calculated to occur via a (CH(3)CN)Co(2+)-OH(-)-H(+)(NCCH(3)) salt-bridge transition state, reducing kinetic energy release in the dissociation.
通过测量 355、532 和 560-660nm 处的光碎片光谱，研究了乙腈和水对钴和镍二价阳离子的微溶剂化作用。离子通过电喷雾产生，在离子阱中热化，并通过飞行时间进行质量选择。光解产物的产率、产物及其分支比取决于金属、团簇大小和组成。只有在含有水的团簇中才观察到质子转移，并且随着含水量的增加而增强。此外，含镍的团簇比含钴的团簇更容易发生电荷还原。含有乙腈的均相团簇 M(2+)(CH(3)CN)(n)（n = 3 和 4）通过简单的溶剂损失而解离；n = 2 团簇通过电子转移而解离。混合乙腈/水团簇显示出更有趣的解离动力学。同样，较大的团簇（n = 3 和 4）显示简单的溶剂损失。水的损失大大优于乙腈的损失，这是可以理解的，因为乙腈由于其较高的偶极矩和极化率而成为更强的配体。质子转移，形成 H(+)(CH(3)CN)，作为 M(2+)(CH(3)CN)(2)(H(2)O)(2)和 M(2+)(CH(3)CN)(2)(H(2)O)的次要通道而被观察到，但在 M(2+)(CH(3)CN)(3)(H(2)O)中未观察到。氘代团簇的研究证实水是质子供体。我们之前观察到质子损失是 M(2+)(H(2)O)(4)光解的主要通道。光解产物产率的测量表明，四配位的 Co(2+)团簇比 Ni(2+)团簇更容易解离，而对于三配位的团簇，Ni(2+)团簇的解离效率更高。对于二配位的团簇，解离是通过电子转移进行的，两种金属的产率都很低。反应能量学、解离势垒和激发电子态的位置计算补充了实验工作。Co(2+)(CH(3)CN)(2)(H(2)O)光解中的质子转移通过 (CH(3)CN)Co(2+)-OH(-)-H(+)(NCCH(3))盐桥过渡态计算，降低了解离中的动能释放。
