Karthikeyan S, Singh Jiten N, Park Mina, Kumar Rajesh, Kim Kwang S
Center for Superfunctional Materials, Department of Chemistry, Pohang University of Science and Technology, San 31, Hyojadong, Namgu, Pohang 790-784, Korea.
J Chem Phys. 2008 Jun 28;128(24):244304. doi: 10.1063/1.2943671.
Important structural isomers of NH(4) (+)(H(2)O)(n=4,6) have been studied by using density functional theory, Moller-Plesset second order perturbation theory, and coupled-cluster theory with single, double, and perturbative triple excitations [CCSD(T)]. The zero-point energy (ZPE) correction to the complete basis set limit of the CCSD(T) binding energies and free energies is necessary to identify the low energy structures for NH(4) (+)(H(2)O)(n=4,6) because otherwise wrong structures could be assigned for the most probable structures. For NH(4) (+)(H(2)O)(6), the cage-type structure, which is more stable than the previously reported open structure before the ZPE correction, turns out to be less stable after the ZPE correction. In first principles Car-Parrinello molecular dynamics simulations around 100 K, the combined power spectrum of three lowest energy isomers of NH(4) (+)(H(2)O)(4) and two lowest energy isomers of NH(4) (+)(H(2)O)(6) explains each experimental IR spectrum.
利用密度泛函理论、莫勒-普莱斯二阶微扰理论以及含单、双和微扰三重激发的耦合簇理论[CCSD(T)]，对NH(4) (+)(H(2)O)(n = 4,6)的重要结构异构体进行了研究。对于NH(4) (+)(H(2)O)(n = 4,6)，确定其低能量结构时，必须对CCSD(T)结合能和自由能的完全基组极限进行零点能(ZPE)校正，否则可能会为最可能的结构指定错误的结构。对于NH(4) (+)(H(2)O)(6)，笼型结构在ZPE校正之前比先前报道的开放结构更稳定，但在ZPE校正之后却变得不太稳定。在约100 K的第一性原理卡-帕里尼罗分子动力学模拟中，NH(4) (+)(H(2)O)(4)的三个最低能量异构体和NH(4) (+)(H(2)O)(6)的两个最低能量异构体的组合功率谱解释了每个实验红外光谱。
