Diken Eric G, Hammer Nathan I, Johnson Mark A, Christie Richard A, Jordan Kenneth D
Sterling Chemistry Laboratory, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, USA.
J Chem Phys. 2005 Oct 22;123(16):164309. doi: 10.1063/1.2074487.
Vibrational predissociation spectra are reported for size-selected NH4+ (H2O)n clusters (n=5-22) in the 2500-3900 cm(-1) region. We concentrate on the sharp free OH stretching bands to deduce the local H-bonding configurations of water molecules on the cluster surface. As in the spectra of the protonated water clusters, the free OH bands in NH4+ (H2O)n evolve from a quartet at small sizes (n<7), to a doublet around n=9, and then to a single peak at the n=20 magic number cluster, before the doublet re-emerges at larger sizes. This spectral simplification at the magic number cluster mirrors that found earlier in the H+(H2O)n clusters. We characterize the likely structures at play for the n=19 and 20 clusters with electronic structure calculations. The most stable form of the n=20 cluster is predicted to have a surface-solvated NH4+ ion that lies considerably lower in energy than isomers with the NH4+ in the interior.
报道了尺寸选择的NH₄⁺(H₂O)ₙ团簇（n = 5 - 22）在2500 - 3900 cm⁻¹区域的振动预解离光谱。我们专注于尖锐的游离OH伸缩带，以推断团簇表面水分子的局部氢键构型。与质子化水团簇的光谱一样，NH₄⁺(H₂O)ₙ中的游离OH带从小尺寸（n < 7）时的四重峰演变为n = 9左右的双峰，然后在n = 20的幻数团簇处变为单峰，之后在更大尺寸时双峰再次出现。幻数团簇处的这种光谱简化反映了早期在H⁺(H₂O)ₙ团簇中发现的情况。我们通过电子结构计算表征了n = 19和20团簇可能存在的结构。预计n = 20团簇最稳定的形式是表面溶剂化的NH₄⁺离子，其能量比NH₄⁺在内部的异构体低得多。
