Malloum Alhadji, Fifen Jean Jules, Dhaouadi Zoubeida, Engo Serge Guy Nana, Jaidane Nejm-Eddine
Department of Physics, Faculty of Science, University of Ngaoundere, Ngaoundere 454, Cameroon.
Laboratoire de Spectroscopie Atomique Moléculaire et Applications, Faculté des Sciences de Tunis, Université de Tunis El Manar, Campus Universitaire 1060, Tunis, Tunisie.
Phys Chem Chem Phys. 2015 Nov 21;17(43):29226-42. doi: 10.1039/c5cp03374h.
A hydrogen bond network in ammonia clusters plays a key role in understanding the properties of species embedded in ammonia. This network is dictated by the structures of neutral ammonia clusters. In this work, structures of neutral ammonia clusters (NH3)n(=2-10) have been studied at M06-2X/6-31++G(d,p) and MP2/6-31++g(d,p) levels of theory. The analysis of the relative stabilities of various hydrogen bond types has also been studied and vibrational spectroscopy of the ammonia pentamer and decamer is investigated. We noted that M06-2X provides lower electronic energies, greater binding energies and higher structural resolution than MP2. We also noted that at the M06-2X level of theory, the binding energy converges to the experimental vaporization enthalpy faster than that at the MP2 level of theory. As a result, it is found that the M06-2X functional could be more suitable than the MP2 ab initio method in the description of structures and energies of ammonia clusters. However, we found that the electronic energy differences obtained at both levels of computation follow a linear relation with n (number of ammonia molecules in a cluster). As far as the structures of ammonia clusters are concerned, we proposed new "significant" isomers that have not been reported previously. The most remarkable is the global minimum electronic energy structure of the ammonia hexamer, which has an inversion centre and confirms experimental observation. Moreover, we reported the relative stabilities of neutral ammonia clusters for temperatures ranging from 25 to 400 K. The stability of isomers changes with the increase of the temperature. As a result, the branched and less bonded isomers are the most favored at high temperatures and disfavored at low temperatures, while compact and symmetric isomers dominate the population of clusters at low temperatures. In fine, from this work, the global minimum energy structures of ammonia clusters are known for the first time at a given temperature (T ∼ 0-400 K) and at a reliable computational level of theory.
氨簇中的氢键网络在理解嵌入氨中的物种性质方面起着关键作用。该网络由中性氨簇的结构决定。在这项工作中，已在M06 - 2X/6 - 31++G(d,p)和MP2/6 - 31++g(d,p)理论水平上研究了中性氨簇(NH₃)ₙ(n = 2 - 10)的结构。还研究了各种氢键类型的相对稳定性分析，并对氨五聚体和十聚体的振动光谱进行了研究。我们注意到，与MP2相比，M06 - 2X提供了更低的电子能量、更大的结合能和更高的结构分辨率。我们还注意到，在M06 - 2X理论水平下，结合能比在MP2理论水平下更快地收敛到实验汽化焓。结果发现，在描述氨簇的结构和能量方面，M06 - 2X泛函可能比MP2从头算方法更合适。然而，我们发现，在两个计算水平上获得的电子能量差与n（簇中氨分子的数量）呈线性关系。就氨簇的结构而言，我们提出了以前未报道过的新的“重要”异构体。最显著的是氨六聚体的全局最小电子能量结构，它有一个反演中心并证实了实验观察结果。此外，我们报道了温度范围为25至400 K时中性氨簇的相对稳定性。异构体的稳定性随温度升高而变化。结果，支化且键合较少的异构体在高温下最受青睐，在低温下不受青睐，而紧凑且对称的异构体在低温下主导簇的群体。总之，通过这项工作，首次在给定温度（T ∼ 0 - 400 K）和可靠的理论计算水平下知道了氨簇的全局最小能量结构。
