Jing Aling, Szalewicz Krzysztof, van der Avoird Ad
Department of Physics and Astronomy, University of Delaware, Newark, DE, 19716, USA.
Theoretical Chemistry, Institute for Molecules and Materials, Radboud University, Heyendaalseweg 135, 6525, AJ, Nijmegen, The Netherlands.
Nat Commun. 2022 Mar 18;13(1):1470. doi: 10.1038/s41467-022-28862-z.
In the 1980s, Nelson, Fraser, and Klemperer (NFK) published an experimentally derived structure of the ammonia dimer dramatically different from the structure determined computationally, which led these authors to the question "Does ammonia hydrogen bond?". This question has not yet been answered satisfactorily. To answer it, we have developed an ab initio potential energy surface (PES) for this dimer at the limits of the current computational capabilities and performed essentially exact six-dimensional calculations of the vibration-rotation-tunneling (VRT) spectra of NH-NH and ND-ND, obtaining an unprecedented agreement with experimental spectra. In agreement with other recent electronic structure calculations, the global minimum on the PES is in a substantially bent hydrogen-bonded configuration. Since the bottom of the PES is exceptionally flat, the dimer is extremely fluxional and the probability of finding it in configurations that are not hydrogen bonded is high. Nevertheless, the probability of hydrogen-bonded configurations is large enough to consider the ammonia dimer to be hydrogen bonded. We also show that NFK's inference that the ammonia dimer is nearly rigid actually results from unusual cancellations between quantum effects that generate differences in spectra of different isotopologues.
20世纪80年代，尼尔森、弗雷泽和克莱姆珀勒（NFK）发表了氨二聚体的实验推导结构，该结构与通过计算确定的结构有显著差异，这使得这些作者提出了“氨会形成氢键吗？”这一问题。这个问题尚未得到令人满意的答案。为了回答这个问题，我们在当前计算能力的极限下，为该二聚体开发了一个从头算势能面（PES），并对NH-NH和ND-ND的振动-转动-隧穿（VRT）光谱进行了基本精确的六维计算，与实验光谱取得了前所未有的一致。与其他近期的电子结构计算结果一致，PES上的全局最小值处于一个显著弯曲的氢键构型。由于PES的底部异常平坦，二聚体极具流动性，处于非氢键构型的概率很高。然而，氢键构型的概率足够大，足以认为氨二聚体是通过氢键结合的。我们还表明，NFK关于氨二聚体几乎是刚性的推断实际上是由量子效应之间不寻常的抵消导致的，这些抵消产生了不同同位素体光谱的差异。
