Localized and Delocalized States of a Diamine Cation: Resolution of a Controversy.

Marta Gałyńska,Vilhjálmur Ásgeirsson,Ragnar Bjornsson,Hannes Jónsson

doi:10.1021/acs.jpclett.0c03651

Marta Gałyńska, Vilhjálmur Ásgeirsson + Show 2 more

Open Access

https://doi.org/10.1021/acs.jpclett.0c03651

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Abstract

Recent Rydberg spectroscopy measurements of a diamine molecule, N,N′-dimethylpiperazine (DMP), indicate the existence of a localized electronic state as well as a delocalized electronic state. This implies that the cation, DMP+, can similarly have its positive charge either localized on one of the N atoms or delocalized over both. This interpretation of the experiments has, however, been questioned based on coupled cluster calculations. In this article, results of high-level multireference configuration interaction calculations are presented where a localized state of DMP+ is indeed found to be present with an energy barrier separating it from the delocalized state. The energy difference between the two states is in excellent agreement with the experimental estimate. The results presented here, therefore, support the original interpretation of the experiments and illustrate a rare shortcoming of CCSD(T), the “gold standard” of quantum chemistry. These results have implications for the development of density functionals, as most functionals fail to produce the localized state.

Highlights

Dimethylpiperazine (DMP), indicate the existence of a localized electronic state as well as a delocalized electronic state
The radical cation of N,N′-dimethylpiperazine (DMP), see Figure 1, has been identified as an interesting organic mixedvalence molecule for studying lone-pair interactions and delocalization/localization phenomena.[1−7] Experimental studies (EPR/Raman) on DMP+ were originally performed in solution,[1−3] where resonance Raman spectra together with calculations indicated the electronic structure of DMP+ to be most consistent with a C2h-symmetric delocalized state.[1,2]
Density functional theory (DFT) calculations with commonly used density functionals, fail to give a localized state of DMP+, while calculations with a functional where self-interaction error is explicitly removed give results consistent with the Rydberg state experiments.[5]. These DFT results have subsequently led to a debate in the literature, and the existence of a localized state of DMP+ on the potential energy surface (PES) been questioned based on the fact that coupled cluster theory calculations at the CCSD(T) level do not produce a localized state.[6,7]

Summary

Introduction

Dimethylpiperazine (DMP), indicate the existence of a localized electronic state as well as a delocalized electronic state. Using a 78point cut of the energy surface calculated with multireference configuration interaction (FIC-MRCI+Q), we firmly establish the PES of the DMP cation as containing both a localized state and a delocalized state.

Results

Conclusion