Mass-selective two-color resonant two-photon ionization (2C-R2PI), UV/UV hole-burning, and infrared (IR) depletion spectra of supersonic jet-cooled 2-pyridone·(H₂O)_n clusters with n = 1–4 have been measured to investigate the local hydration patterns around 2-pyridone (2PY) as a function of cluster size. As shown by others, the IR frequencies of the OH and NH stretches of the n = 1, 2 clusters are characteristic of water wires stretching from the NH to the C═O group of 2PY. We identify two isomers (3A and 3B) of the n = 3 cluster in the 2C-R2PI spectrum and separate them by IR/UV and UV/UV hole-burning techniques. Isomer 3A exhibits a three-membered water wire, extending the n = 1, 2 structural motif. Isomer 3B exhibits bifurcated water wires with the first H₂O donating to two waters that form H-bonds to the C═O group. This increases the H-bond strength between the NH group of 2PY and the proximal H₂O molecule, lowering the NH stretch to ∼2800 cm^–1. The n = 4 cluster is also bifurcated with two water wires between the bifurcating H₂O and the C═O group. The cluster-selective IR spectra are complemented with density-functional calculations using the PW91, B3LYP, B97-D, and M06-2X functionals, where the latter two include long-range dispersive interactions, and with the ab initio correlated SCS-CC2 method. The calculated IR spectra provide firm assignments of the structures of the n = 1–4 cluster structures and allow us to understand the evolution of individual H-bond strengths with increasing cluster size.

中文翻译：

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2-Pyridone·（H ₂ O）_n，n = 1-4簇中从水线到分叉氢键网络的过渡

超声速射流冷却的2-吡啶酮·（H ₂ O）_{n n}簇的n =的质量选择双色共振双光子电离（2C-R2PI），UV / UV空穴燃烧和红外（IR）耗尽光谱已经测量了1-4，以研究2-吡啶酮（2PY）周围的局部水合作用随团簇大小的变化。如其他人所示，n = 1、2个簇的OH和NH片段的IR频率是从NH到2PY的C = O基团延伸的水线的特征。我们确定了2C-R2PI光谱中n = 3簇的两个异构体（3A和3B），并通过IR / UV和UV / UV空穴燃烧技术将它们分开。异构体3A具有三元水线，延伸了n= 1、2个结构主题。异构体3B表现出分叉的水线，其中第一个H ₂ O贡献给两个与C = O基团形成H键的水。这样可以增加2PY的NH基团与近端的H ₂ O分子之间的H键强度，从而将NH的延伸降低至〜2800 cm ^–1。所述Ñ = 4簇也分叉与分叉h的两个水电线₂ O与C = O基团。使用PW91，B3LYP，B97-D和M06-2X官能团进行密度-官能团计算，可对簇选择红外光谱进行补充，其中后两个包括长距离色散相互作用，并且具有从头算起相关的SCS-CC2方法。计算得出的红外光谱提供了结构的牢固分配n = 1-4团簇结构，使我们能够理解随着团簇尺寸的增加，各个氢键强度的演变。