Gas-phase infrared spectroscopy and multidimensional quantum calculations of the protonated ammonia dimer N2H7+
Gas-phase infrared spectroscopy and multidimensional quantum calculations of the protonated ammonia dimer N2H7+
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DOI：
10.1002/anie.200702607
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发表时间：
2007-01-01
影响因子：
16.6
通讯作者：
Kuehn, Oliver
中科院分区：
文献类型：
--
作者：
Asmis, Knut R.;Yang, Yonggang;Kuehn, Oliver
理解水和冰中异常高的质子传导率以及水介导的质子跨生物膜转移的挑战引发了质子化水网络的大量工作。[1，2]描述这些快速波动网络的重要限制结构是Zundel（H5 O2+）和Eigen（H9 O 4+）阳离子，其特征在于在凝聚相中具有宽但不同的红外（IR）吸收光谱。[3]有趣的是，孤立的Zundel阳离子的红外特征[4-6]，其中一个质子被两个水分子强烈束缚并平均共享，直到最近才被完全阐明。[7]在含有强氢键的系统中，X··· H+··· X质子转移模式（X=闭壳层原子或分子）的振动频率与自由的X·H伸缩的值相比发生了显著的红移;[8，9]它经常发生在1000 cm ↑ [2]以下。[10]直到最近，这个光谱区域一直无法使用可调谐桌面激光器，因为这些激光器的功率不足以在孤立系统上进行作用光谱学。对所产生的谱带模式进行理论分析可能更具挑战性。这种强氢键的显著非谐特征需要对振动能级结构进行高度精确的多维量子处理，这推动了当前计算机的极限。[11]涉及氨的强H键[12]受到的关注比它们的水类似物少得多，它们在氨转运蛋白中的作用对氮代谢至关重要，只是在分子水平上才变得可用。[13]气相红外光谱N2 H7+，Zundel阳离子类似物质子化氨簇（NH 4+）（NH3）n，迄今已被限制在2600 cm-1以上的范围。[14]对于更大的系统（n= 3-9），已经报道了使用线性可调谐CO2激光器[15]或自由电子激光器在更长波长下的光解离光谱。[16这些研究表明，虽然n> 1的簇优选形成被氨分子溶剂化的铵离子，但质子化的氨二聚体是独特的，因为它采用D3 d平衡几何结构，其中过量的质子在两个氨分子之间均等地共享。在计算上，（NH 4+）（NH3）n团簇的结构和光谱特性主要是在谐波近似下（见参考文献[18，19]和其中的参考文献）或使用N2 H7+中N···· H+··· N共享质子伸缩振动的有效一维量子模型[20]研究的。虽然与Zundel阳离子等电子，但N2 H7+（见图1a）由于质子转移的经典势垒的高度不同而呈现出概念上不同的情况。具体来说，共享质子的潜力，
The challenge of understanding the unusually high proton conductivity in water and in ice as well as water-mediated proton transfer across biomembranes has triggered considerable work on protonated water networks.[1, 2] Important limiting structures in describing these rapidly fluctuating networks are the Zundel (H5O2+) and Eigen (H9O4+) cations, which are characterized by broad but distinct infrared (IR) absorption spectra in the condensed phase.[3] Interestingly, the infrared signature of the isolated Zundel cation,[4–6] in which a proton is strongly bound and equally shared by two water molecules, has only very recently been fully elucidated.[7] The vibrational frequency of the X··· H+··· X proton-transfer mode (X= closed-shell atom or molecule) in systems containing strong hydrogen bonds is dramatically red-shifted from the value of the free XÀH stretch;[8, 9] it often occurs below 1000 cmÀ1.[10] Until recently, this spectral region has been inaccessible to tunable tabletop lasers, because these lasers were not sufficiently powerful to carry out action spectroscopy on the isolated systems. Theoretical analysis of the resulting band patterns may be even more challenging. The pronounced anharmonic character of such strong hydrogen bonds requires highly accurate, multidimensional quantum treatment of the vibrational-level structure that pushes the limits of current computers.[11] Strong H-bonds involving ammonia [12] have received much less attention than their water analogues, and their role in ammonia transport proteins that are vital to nitrogen metabolism is just becoming accessible at a molecular level.[13] Gas-phase IR spectroscopy of N2H7+, the Zundel cation analogue in protonated ammonia clusters (NH4+)(NH3) n, has to date been restricted to the range above 2600 cmÀ1.[14] For larger systems (n= 3–9), photodissociation spectra at longer wavelengths have been reported using a line-tunable CO2 laser [15] or a free electron laser.[16, 17] These studies have shown that while the n> 1 clusters prefer to form an ammonium ion solvated by ammonia molecules, the protonated ammonia dimer is unique in that it adopts a D3d equilibrium geometry, with the excess proton shared equally between two ammonia molecules. Computationally, the structure and spectroscopic characteristics of (NH4+)(NH3) n clusters have been studied mainly within the harmonic approximation (see references [18, 19] and references therein) or using an effective onedimensional quantum model [20] of the N··· H+··· N sharedproton stretching vibration in N2H7+. Although isoelectronic to the Zundel cation, N2H7+(see Figure 1 a) presents a conceptually different scenario owing to the differing heights of the classical barriers for proton transfer. Specifically, the shared-proton potential for the