To Be or Not To Be Protonated: cyclo-N5- in Crystal and Solvent

Wei Chen; Zhiqiang Liu; Yinghe Zhao; Xianfeng Yi; Zhongfang Chen; Anmin Zheng

doi:10.1021/acs.jpclett.8b02841

To Be or Not To Be Protonated: cyclo-N₅^- in Crystal and Solvent

J Phys Chem Lett. 2018 Dec 20;9(24):7137-7145. doi: 10.1021/acs.jpclett.8b02841. Epub 2018 Dec 13.

Authors

Wei Chen^{1

2}, Zhiqiang Liu¹, Yinghe Zhao³, Xianfeng Yi¹, Zhongfang Chen³, Anmin Zheng¹

Affiliations

¹ State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, National Center for Magnetic Resonance in Wuhan , Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences , Wuhan 430071 , P.R. China.
² University of Chinese Academy of Sciences , Beijing 100049 , P.R. China.
³ Department of Chemistry , University of Puerto Rico , Rio Piedras Campus, San Juan , Puerto Rico 00931 , United States.

PMID: 30395479
DOI: 10.1021/acs.jpclett.8b02841

Abstract

Pentazole (HN₅) and its anion ( cyclo-N₅^-) have been elusive for nearly a century because of the unstable N₅ ring. Recently, Zhang et al. reported the first synthesis and characterization of the pentazolate anion cyclo-N₅^- in (N₅)₆(H₃O)₃(NH₄)₄Cl salt at ambient conditions ( Science 2017, 355, 374 ). However, whether the cyclo-N₅^- in (N₅)₆(H₃O)₃(NH₄)₄Cl salt is protonated or not has been debated ( Huang and Xu, Science, 2018, 359, eaao3672 ; Jiang et al. Science, 2018, 359, aas8953 ). Herein, we employed ab initio molecular dynamics (AIMD) simulations, which can well present the dynamic behavior at realistic experimental conditions, to examine the potential protonated state of cyclo-N₅^- in both crystal and dimethyl sulfoxide (DMSO) solvent. Our simulations revealed that the protonation reaction of (N₅)₆(H₃O)₃(NH₄)₄Cl → (N₅)₅(N₅H)(H₂O)(H₃O)₂(NH₄)₄Cl is thermodynamically spontaneous according to Δ G < 0, and the small energy barrier of 12.6 kJ/mol is not enough to prevent the partial protonation of cyclo-N₅^- due to the temperature effect; consequently, both deprotonated and protonated cyclo-N₅^- exist in the crystal. In comparison, the DMSO solvent effect can remarkably reduce the difference of proton affinities among cyclo-N₅^-, H₂O, and NH₃, and the temperature effect can finally break these hydrogen bonds and lead to the deprotonated cyclo-N₅^- in DMSO solvent. Our AIMD simulations reconcile the recent controversy.